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Neighboring-Group Effects and Rearrangements in Reactions of Cyclopropylmethyl Cyclobutyl and Homoallyl Systems.

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Neighboring-Group Effects and Rearrangements in Reactions of
Cyclopropylmethyl, Cyclobutyl, and Homoallyl Systems
The neighboring-group effectproduced by a double bond in the homoallylposition or by a
cyclopropane or cyclobutane ring leads to interesting rearrangements, which are discussed in this paper mainly from the point of view of mechanism, but also from
that of preparative work. The question of the postulated non-classical carbonium ions
acquires a special significance in this connection.
The unusual reactions of cyclopropylmethyl, cyclobutyl, and homoallyl compounds, which are frequently
accompanied by rearrangement of one of these
systems into another (homoallyl rearrangement), have
attracted considerable attention in recent years. While
the earlier literature contains only a few observations
on these rearrangements [la], the recent growth of
interest in the mechanisms of the reactions of carbonium ions and in the bonding in small rings has led to
numerous investigations in this field [I].
The cyclopropylmethyl, cyclobutyl, and homoallyl
esters, which can form carbonium ions, are solvolysed
10 to 1014 times as rapidly as the open-chain or saturated compounds used for comparison. Moreover, the
solvolysis is often accompanied by complete rearrangement. These reactions are therefore particularly
good examples of neighboring-group effects. For
example, the three-membered ring not onIy lowers
the energy required, so increasing the reaction rate,
but also causes modification of the structure by the
formation of new bonds. The decrease in energy due
to charge delocalization in ionic intermediates that
are incapable of resonance led to the assumption
that non-classical ions occur [li, 21. Finally, these
rearrangements are also becoming increasingly important in preparative work.
Priv.-Doz. Dr. M. Hanack and Dipl.-Chem. H.-J. Schneider
Chemisches Institut der Universitat
Wilhelmstr. 31, 74 Tubingen (Germany)
[l] For a detailed review, see: [a] M. Hanack and H . 4 . Schneider,
Fortschr. chem. Forsch., in press; for summaries of special
topics, see: [b] J. A. Berson in P . de Mayo: Molecular Rearrangements. Interscience, New York. 1963, Vol. I, p. 111;
[c] R . Breslow, ibid. Voi. I, p. 233; [d] N. L. Wendler, ibid. Vol. 11,
p. 1075; [el B. Capon, Quart. Rev. 18, 45 (1964); [f] M . Y. Lukina, Russ. Chem. Rev. 32, 635 (1963); [g] E. Vogel, Angew.
Chem. 72, 4 (1960); [h] E. Vogel, Fortschr. chem. Forsch. 3, 430
(1955); [i] S. Winstein, Experientia SuppI. 11, 137 (1955); [k] A.
Streitwieser, j r . : Solvolytic Displacement Reactions. McGrawHill, New York 1962; [l] cf. also P. D . Bartlett: NoncIassical
Ions. Benjamin, New York 1965.
[21 G. D . Surgenf, Quart. Rev. 20, 301 (1966).
[3] K. L. Servis and J. D . Roberts, J. h e r . chem. SOC.86, 3173
141 M. Hanack, S. Kang, J. Ha’ner, and K . Gorler, Liebigs Ann.
Chem. 690,98 (1965).
151 E. Renk and J. D. Roberts, J. Amer. chem. SOC.83,878 (1961).
Direction of Rearrangement and Reaction
The strong influence of the reaction conditions on the
direction of the homoallyl rearrangement is due to the
difference in the energies of the compounds concerned,
which are appreciable in most cases.
In kinetically controlled reactions the ratio of the
products depends on their rates of formation, and
hence also on the stability and structure of the intermediates. Thus the solvolysis of homoallyl esters ( I ) ,
X = OSOzR or Hal, generally leads to the isomeric
cyclic systems (2) and (3), X = OR‘, which are about
10 kcal/mole more strained in the ground state, but
which are more stable than the open-chain systems
as ionic intermediates.
kinetic control
A necessary condition for the rearrangement is the
production of a carbonium ion by an SN1 reaction.
For example, homoallyl p-toluenesulfonate ( I ) , X =
OTs, which tends to undergo SN2 reactions, gives the
rearrangement products (2) and ( 3 ) , X = -0-CH=O,
only in the weakly nucleophilic but strongly ionizing
(ionizing power Y = 2.1 Ilk]) formic acid [31; for the
same reason, less rearrangement is observed on
alcoholysis than on acetolysis or hydrolysis 141.
The formation of a carbonium ion by an SN1 reaction
is also possible on deaniination of homoallylamines
( I ) , X = NH2. Nevertheless, the quantity of cyclization
products obtained is generally very small; hydride
shifts [4,51 take place in addition to retention of
structure. As will be shown later, the double bond
assists the release of the acid residue during solvolysis,
so that the resulting carbonium ion already has the
optimum conformation for the subsequent ring
closure. On the other hand, the carbonium ion formed
by the removal of nitrogen from a diazonium ion
probably adopts the thermodynamically more stable
extended conformation, which is unfavorable for the
rearrangement, and as a “hot” carbonium ion, it can
Angew. Chem. internat. Edit.
Vol. 6 (1967) No. 8
readily provide the energy required for a hydride
shift [6,71.
Jn thermodynamically controlled rearrangements, the
initial products can react again until a reversible
equilibrium has been established, as in the reaction if
cyclic alcohols (2) and (3), X = OH, with strong
acids, zinc chloride, thionyl chloride, or phosphorus
halides [8-101. The conversion into the stable, predominantly trans homoallyl system is more or less
complete, depending o n the reaction conditions [ I l l .
When the dicyclopropylcarbinol (4) is treated with HBr, the
substituted ring is opened"21 to give the homoallyl system
(5). whose carbonium ion is particularly strongly stabilized
by substituents [131.
The acid-catalysed ring cleavage of cyclopropylcarbinols has
been investigated by numerous Russian authors [la,141, and
has been used in particular by Julia et al. in a n elegant preparation of open-chain isoprenoids "51.
because the primary effect of the neighboring-group
is frequently to displace the reaction mechanism from
S,2 toward S,1 1171. This effect can be determined e.g.
by kinetic measurements in different solvents 1181.
The ratio of rate constants kunsaturated/ksaturated
for unsubstituted homoallyl p-toluenesulfonate ( I ) , X = OTs, in
formic acid is only 3.7 171, and so corresponds to an unusually
small neighboring-group effect for a homoallyl compound.
On the other hand, if there were no neighboring-group effect,
the inductive effect of the double bond should slow the reaction by a factor of 1 0 2 to lo3 [191.
For the same reason, the cyclopropyl group should have a
retarding (not accelerating [201) effect; however, cyclopropylmethyl p-toluenesulfonate is acetolysed -106 times as rapidly
as the comparable isobutyl p-toluenesulfonate [211. The rate
of hydrolysis of cyclopropylmethyl chloiide (Z), X = CI, i s
even 40 times as high as that of methylallyl chloridersl. Even
in the presence of sodium alkoxide, the alcoholysis of cyclopropylmethyl benzenesulfonate (2), X = OS02C&, is first
order, and proceeds without any additional increase in rate [221.
A second and third three-membered ring each make an additional contribution of the same magnitude to the stabilization
of the carbonium ion, as was concluded by Hart et al. from
the increase in the hydrolysis constants of the p-nitrobenzoates (6) to (9) [231.
Kinetic Neighboring-Group Effects
The earliest and most important indication of the
formation of non-classical cations from cyclopropylmethyl, cyclobutyl, and homoallyl compounds was
given during the determination of the kinetic neighboring-group effect, which is defined as the ratio of
the rate of solvolysis of a compound containing a
neighboring group to that of a structurally similar
compound with no neighboring group 1161. The
magnitude of this ratio depends, not only on the
substitution and stereochemistry of the system, but
also on the solvents used. The neighboring-group
effect of a homoallyl sulfonate can be less than 1 in
ethanoli41, but about 102 in formic acidl71. This is
The ratio of the rate constants for the acetolysis of cyclobutyl
and cyclohexyl p-toluenesulfonates is 11 (241, and the ratio for
the hydrolysis of the chlorides is 401251. On the other hand,
from the total strain energy (I strain), one would expect the
formation of an sp2 state with a C-C angle of 120' to be
much more difficult for the four-membered than for the sixmembered ring 1261. The particularly clear conflict between
the reactivity of the cyclobutyl compound and the classical
theory also led H. C. Brown to postulate a different structure
for the carbonium ion in this case [241.
Non-Ckssical Carbonium Ions
[6] M . S. Silver, M . C . Caserio, H. E. Rice, and J. D . Roberts,
J. Amer. chem. SOC.83, 3671 (1961).
[7] K . L . Servis and J. D . Roberts, J. Amer. chem. SOC.87, 1331
[ 8 ] J . D . Roberts and R . H . Mazur, J. Amer. chem. SOC.73,2509
[9] M . C . Caserio, W . H. Graham, and J. D . Roberts, Tetrahedron 1 1 , 171 (1960).
[lo] M . Hanack and H. Eggensperger, Liebigs Ann. Chem. 663,
31 (1963).
[ll] See, e.g., M . Julia, S. Julia, and J . Amaudric du Chaffaut,
Bull. SOC.chm. France 1960, 1735; M . Julia, S. Julia, and S. Y.
Tchen, ibid. 1961, 1849; M . Julia, S . Julia, T . S . Yu, and C . Neuville, ibid. 1960, 1381.
1121 M . Hanack and H . Eggensperger, Chem. Ber. 96, 1259
[13] See, e.g., H . M . Walborsky and L . Plonsker, J. Amer. chem.
SOC.83, 2138 (1961); H . M . Walborsky and J . F. Pendleton, ibid.
82, 1405 (1960).
[14] T . A . Favorskaya, K . A . Konopova, and M . I. Titov, 2. obSE.
Chim. 29, 2894 (1959), and earlier papers.
[15] For example, M . Julia, Ind. chim. belge 1961, 995; further
work cited in [la].
[16] W . Lwowski, Angew. Chem. 70,483 (1958).
Angew. Chem. internat. Edit. J Vof.6 (1967) J No. 8
According to classical views, ionic intermediates
may be stabilized in two fundamentally different ways.
Steric factors can increase the reaction rate, parti[17] M. Hanack and H.-J. Schneider, Tetrahedron 20, 1863
[I 81 H. Schneider-Bernlohr, H . 4 . Schneider, and M. Hanack,
Tetrahedron Letters 1967, 1425.
[19] Cf., e.g., S. Winstein, H. M . Walborsky, and K . Schreiber,
J. Amer. chem. Sac. 72, 5795 (1950).
[20] E. S . Could: Mechanismus und Struktur in den organischen
Chemie. Verlag Chemie, Weinheim 1962, p. 706.
1211 D. D . Roberts, J. org. Chemistry 29, 294 (1964); 30, 23
[22] C . G. Bergstrom and S. Siegel, J. Amer. chem. SOC.74, 145
[23] H . Hart and P. A . Law, J. Amer. chem. SOC.86,1957 (1964).
[24] H. C . Brown and G. Ham, J. Amer. chem. SOC.78, 2735
[25] J. D . Roberrs and V. C . Chambers, J. Amer. chem. SOC.73,
5034 (1951).
1261 H . C . Brown, R . S . Fletcher, and R. B. Johannesen, J. Amer.
chem. SOC. 73, 212 (1951).
cularly of cyclic compounds, by allowing the total
strain energy (I strain)[27l to become large in the
ground state and small in the ionic state. The obvious
idea of a gain in energy by relief of the high strain in
the three-membered ring during the solvolysis of
cyclopropylmethyl compounds is refuted, infer a h ,
by the fact that the three-membered ring is generally
found to have remained intact in the reaction products [231. The second classical means of stabilization
of carbonium ions is distribution of the charge concentrated on the C atom with the aid of x electrons
in the conjugation position. This resonance (10) which
is characterized by a pure x overlap, is ideally achieved
in allyl and benzyl cations and related systems.
Non-classical ions, on the other hand, are relatively
stable intermediates, which exhibit charge delocalization involving electron pairs having a high cs
characterrlll or more distant double bonds that
cannot enter into resonance [2,281. The bonds concerned are partially broken, so that the C atoms can
accept part of the charge. This is indicated in the
formulae by dotted bonds.
Non-classical carbonium ions have been postulated on the
basis of various ideas. Electron acceptors such as protons or
metal cations can form stable complexes, not only with lone
pairs of electrons o n suitable donors (e.g. NH3), but also
with the pairs of x electrons in olefins. The covalent bond in
such x-complexes, which is distributed over three centers,
was represented by Dewar as shown in formula (11); the
interaction of a carbonium ion with a double bond gives a
typical electron-deficient compound, and is represented in a
similar manner [291. The bonding results from the overlap of
the bonding x orbitals with the vacant acceptor orbital I301
[(12) and ( I S a ) ] [311.
Winstein and other authors 1321 formulated bridged
ions (13) without invoking double bonds (13), an
example being the norbornyl cation [11,21, (14) = (15).
Ions such as (14) can also be formulated as x complexes by formal cleavage at their weakest point to
form a double bond suitable for complex formation
[e.g. C(l)-C(6) in (14) + (Id)]. According to this
view, the cyclopropylmethyl, cyclobutyl, and homoallyl cations would contain a vinyl group (17) capable
of accepting and simultaneously back-donating x
electrons 1291.
[27] M . Hanack: Conformation Theory. Academic Press, New
York 1965, p. 167.
1281 Cf., however, H. C. Brown, K . J. Morgan, and F. J. Chloupek, J. Amer. chem. SOC.87, 2137 (1965), and further literature
there; see also H. C. Brown: ,,The Transition State,” Special
Publ. No. 16, Chemical Society, London, 1962; cf. also W .
Huckel, I. prakt. Chem. 28, 27 (1965).
1291 M . J. S.Dewar and A . P. Marchand, Ann. Rev. Phys. Chem.
16, 321 (1965).
[30] M. J. S . Dewar, Bull. SOC.chim. France 18, C71 (1951).
1311 A. Sfreifwieserj r . : Molecular Orbital Theory for Organic
Chemists. Wiley, New York 1961.
1321 S. Winstein, B. K. Morse, E. Grunwald, K . C . Schreiber,
and J. Corse, J. Amer. chem. SOC.74, 1113 (1952).
Walsh has proposed an explanation of the three-center bond
that does not involve extensive cleavage of the molecule.
According to this explanation, an acceptor can be coordinately bound, not only by free pairs of electrons, but also by
pairs of bonding electrons, though only on condition that the
bond formed by these electrons is weakc331. For example, the
weak B-H bond interacts with the p orbital of a second
boron atom to give a filled molecular orbital linking the three
atoms B,H,B, in the boron hydrides 1331. Apart from the system involving x electrons, the non-classical ions described so
far all have one or more weakened C-C bond close to the
carbonium ion (cyclopropylmethyl, cyclobutyl, 2-norbornyl,
and cyclobutylmethyl cations). These bonds can make their
electrons available to the carbonium ion for the formation of
a coordinate multicenter bond (13). The non-classical ions
would thus provide the connection observed in many systems
between an increase in reaction rate and a tendency toward
On the basis of the high rates of reaction and rearrangement
in a number of Wagner-Meerwein rearrangements, IngoId and
Hughes postulated the formation of a type of non-classical
ion, which is described as a “synartetic” ion (13) 1341. This is
a carbonium ion (which is isoelectronic with a boron atom)
that is not stabilized in any other way than by entering into
an energy-reducing interaction with the cs electrons of a C-C
bond. This interaction removes the energy barrier between
the classical ions and permits “resonance” [34,361 between
them, as was also suggested by Winstein[351. The concept of
non-classical carbonium ions has been definitely rejected by
some authors. The concepts developed by H . C. Brown etaI.[**l
are, in essence, opposed to the participation of 5 bonds in the
ionization. The rearrangements of systems having the structure (14) are explained by means of rapid equilibria between
classical cations in which kinetics, and composition of product, are controlled by steric effects.
Homoallyl Resonance and Bicyclobutonium Ions
The i-steroid rearrangement [Id], which has been
known for some time, is stereospecific in that only the
3P-cholesteryl derivatives (18) cyclize to give 6Pcyclocholestanyl derivatives (19), while n o 3a compounds [Id] are formed on solvolysis. Winstein et ai.
found that the cyclopropylmethyl system (19), X = C1,
is solvolysed 107.4 times as rapidly as the homoallyl
system ( I S ) , X = C1; the hydrolysis of (19) is only
about 10 times as fast as that of the stereoisomeric 6a
product, despite the less favorable position of the
1331 A . D . Walsh, J. chem. SOC.(London) 1947, 89.
[34] C . K . Ingold: Structure and Mechanism in Organic Chemistry. Cornell University Press, New York 1953, p. 511, 519.
[351 Cf. 1321 as well as S. Winstein and H. J. Lucas, J. Amer.
chem. SOC.61, 1576 (1939).
[36] C. K. Ingold, J. chem. SOC.(London) 1953,2845.
Angew. Chem. i n t e r a t . Edit. / Vol. 6 (1967) 1 No. 8
three-membered ring with respect to the leaving
substituent in the latter case [371. Winstein interprets
the rearrangements and its kinetics on the basis of a
common non-classical intermediate (22), in which the
charge is delocalized by “homoallyl resonance”
(20) f-t ( 2 1 ) [381.
Fig. 2. Geometry of the bicyclobutonium ion [38 b].
i 18/,3
Fig. 1. Homoally1 resonance in the i-steroid rearrangement.
It can be deduced from Figure 1 that
[(IS), X
CI] = R x P
where R is the ratio of the rate constants k(isj, x = ci/
k(Ig), x = CI and P is the ratio of the products [(19), X =
OH]/[(18), X = OH] after hydrolysis. (However, a similar
relationship can be deduced if several ionic intermediates are
postulated (see p. 670).) The calculated difference between the
strain energies of (19) and of (18), AFo = 9 kcal/mole, shows
the importance of the energy of the ground state in the kinetics and direction of homoallyl rearrangements [371.
According to Winstein and Simonetta, the mixed n, m overlap[311 of the vacant p orbital with the first double-bond
orbital in the homoallyl cation (23) should result in an
energy gain of 6 kcal/mole [3ga]. The principle of such calculations involves the estimation of the delocalization energies
(DE) and strain energies (SE) for several positions of the
atoms by an LCAO-MO procedure. The arrangement in
which the difference (DE-SE) is greatest is the most probable
arrangement; the greatest difference (DE-SE) is thus the
most probable energy gain.
However, Dewar and Coulson [291 have pointed out that since
the different repulsive forces between electrons and between
nuclei are disregarded, a single MO calculation of geometrically different systems is practically impossible.
Nevertheless, mention should be made of the recalculation of
the homoallyl cation (23) by the same method by Roberts and
Howden. This calculation gave a (DE-SE) value of only
2.8 kcal/mole for simple 1,3 overlap, but 11.4 kcal/mole when
a 1,4 interaction was also taken into account. The optimum
geometry of the cation is shown in Fig. 2 [38bl.
I371 S. Winstein and E. M . Kosower, J. Amer. chem. SOC. 81,
4399 (1959).
[38] [a] S. Winstein and M . Simonet?a, J. Amer. chem. SOC.76,
18 (1954); [b] see also M . E. H. Howden and J . D . Roberts,
Tetrahedron 19, Suppl. 2, 423 (1963).
Angew. Chem. internat. Edit. I Vol. 6 (1967) No. 8
A structure in which both a 1,3 and a 1,4 interaction
are assumed is the bicyclobutonium ion postulated by
Roberts. On solvolysis and deamination of simple
homoallyl, cyclopropylmethyl, and cyclobutyl derivatives, the reaction products were repeatedly found
t o occur in the same ratio 181. This observation together
with the fact that the reaction of labeled compounds
resulted in substantial migration of the isotope [391, led
to the assumption that all the isomers give the same
ionic intermediate. The reaction of [a-*4C]-cyclopropylmethylamine (24) gave a non-uniform distribution
of the 14C, see (25) [401; this rules out the possibility of
the fully symmetrical intermediate (26) (tricyclobutonium ion)[39]. On the other hand, this 14C distribution and many substituent effects would be
compatible with the existence of three pyramidal bicyclobutonium ions (27a) to (27c), which are converted rapidly, but at different rates, into one another [401. Each bicyclobutonium ion yields cyclopropylmethyl, cyclobutyl, and homoallyl products, which
differ only in the position of the 14C atom.
Bisected CycIopropylmethyI Ions and Isomeric
Carbonium Ions
While the kinetic behavior of the compounds discussed gives a clear indication of the formation of
non-classical ions, the structure and number of these
intermediates can be deduced only from a series of
further observations, some of which contradict the
bicyclobutonium ion model.
In non-classical structures such as (13) and (27), substitution
can take place only from the unbridged side, and will therefore be stereospecific. However, the rearrangement of cyclic
homoallyl compounds does not yield sterically uniform
products [17,41J. The racemization that accompanies the
I391 J. D . Robertsand R . H . Mazur, J. Amer. chem. SOC.73,3542
[40] R . H. Mazur, W . N . White, D . A . Semenow, C . C. Lee,
M . S. Silver, and J . D . Roberts, J. Amer. chem. SOC.81, 4390
[41] M . Hanack and W . Keberle, Chem. Ber. 96, 2937 (1963).
solvolysis of optically active cyclopropylmethyl derivatives [42J
and the fact that the rates of racemization and of solvolysis
are the same1431 show that the solvent can attack from both
sides of the cyclopropylmethyl cation.
A product expected on the basis of a common bridged intermediate may not be formed if a carbonium ion corresponding
to the product is particularly unstable [171.
Methyl groups on cyclopropylmethyl p-nitrobenzoates
increase the rate of hydrolysis, even when they are attached
to the C atom that is uncharged in the bicyclobutoniurn ions
(270) to ( 2 7 ~ ) .This means that the charge must in fact be
delocalized over all the C atomsr441.
According to the models discussed so far, the dissociation
products of the homoallyl, cyclopropylmethy1, and cyclobutyl compounds are identical; the common intermediate
means, not only ease of isomerization, but also a decrease in
the ionization energies of all the compounds involved, owing
to the delocalization of the charge. However, many homoallyl
compounds exhibit little or no kinetic neighboring-group
effect, though they undergo substantial cyclization. Conversely, it is often found that the very systems that d o not
rearrange have very high rates of solvolysis.
Hanack and Schneider explained these observations by
assuming separate but rapidly isomerizing carbonium
ions [171. Accordingly, for example, a homoallyl ester
dissociates relatively slowly to give the homoallyl
cation, in which there is generally little delocalization.
Owing to its high energy content, this cation quickly
isomerizes into the cyclopropylmethyl and cyclobutyl
cations, which experience greater non-classical stabilization.
*I i
Reaction coordinate +
Fig. 3. Energy relationships in the solvolysis of homoallyl esters.
Thus in the solvolysis of homoallyl esters, the composition of
the product depends on the relative stabilities of the isomeric
ions; however, it also depends on the rates of substitution of
the ions by the solvent and on the ratio of the isomerization
energy to the substitution energy (Fig. 3). The rearrangement
of a n ion, the rate of which depends on the energyof the
non-classically stabilized transition state (bicyclobutonium
ion), competes with substitution by the solvent. The various
activation energies can also be deduced mathematically. The
relationship obtained is similar to that presented on p.
669 C17.451.
The elucidation of the structure of cyclopropylmethyl
carbonium ions was aided in particular by the direct
investigation of the ionic intermediates obtained,
[42] M . Vogel and J. D . Roberts, J. Amer. chern. SOC.88, 2262
[43] H . G. Richey and J. M . Richey, J. Amer. chem. SOC.88,
4971 (1966).
[44] [a] P. v. R . Schleyer and G. W. Van Dine, J . Amer. chem.
SOC.88, 2321 (1966); [bl cf., however, [21] and D . D . Roberts,
J. org. Chemistry 31, 2000 (1966).
[45] M. Hanack and H.-J. Schneider, unpublished.
according to Olah [46,471 and Deno [481, by reaction of
the alcohols with strong acids in solution at low
temperatures. The tricyclopropykarbinyl cation corresponding to (9) is even formed in weaker acid than
is required for the triphenylcarbinyl cation 1491, which
has been known for some time. The UV spectra of the
tricyclopropylcarbinyl and cyclopropyldimethylcarbinyl cations show conjugation with the three-membered
ring (A,
= 270 mp, c = 22000[491 or A,,,
= 289
mg, E = 10800[471), whereas the t-butyl cation shows
practically no absorption above 200 mu [501.
The NMR spectra of cyclopropylmethyl cations impressively demonstrate the delocalization of the
positive charge over all the C atoms: the 2 to 3 ppm
shift of the a-proton signal of the three-membered ring
toward lower fields could, in certain circumstances, be
due to the inductive effect of the positive charge, but
this could not explain the larger shift of the (hroton
signals"+6,47,*11.The position of the C"-H signal is
displaced by the positive charge to T = -3.5 in the isopropyl cation, to 7 = 0.2 in the diphenylmethyl cation,
but only to t = 1.9 in the dicyclopropyl cation, since
the charge in the last case is distributed over other
atoms [471. The signal due to the methyl protons in the
cyclopropyldimethylcarbinyl cation (28) differ by
0.54 ppm, indicating [471 that the methyl groups occupy
different positions in a fixed bisected form (28) 1481.
The fact that a bisected form is a necessary condition for
conjugation is also clear, inter alia, from the NMR[471 and
UV spectra [lf352,531 of cyclopropyl ketones (29), olefins,
and nitriles, as well as from the kinetics of the solvolysis of
p-cyclopropylbenzyl systems [541.
The homoallyl carbonium salts obtained by reaction
of I-butenyl or 2-pentenyl bromides with AgBF4 in
nitromethane or methylene chloride rearrange immediately, even at low temperatures, to give the three1461 C . U. Pittman j r . and G. A . Olah, J. Amer. chem. SOC.87,
2998 (1965).
1471 C . U. Pitfmanjr.and G. A. Olah, J. Amer. chem. SOC.87,5123
[48] N . C. Deno, Progr. physical org. Chem. 2, 148 (1964).
1491 N. C . Deno, H . G. Richey j r . , J . S. Liu, D . N. Lincoln, and
J . 0 . Turner, J. Amer. chem. SOC.87, 4533 (1965).
[ S O ] G. A. Olah, E. B. Baker, J. C. Evans, W. S . Tolgyesi, J. S.
McIntyre, and I. J. Bastien, J. Amer. chem. SOC.86, 1360 (1964).
[51] N . C. Deno, H. G. Richey jr., J. S . Liu, J. D . Hodge, J. J .
Houser, and M . J. Wisotaky, J. Amer. chem. SOC.84,2016 (1962).
I521 [a]C. A. Grob and J. Hostynek, Helv. chim. Acta 46, 1676
(1963); [b] G. L. KIoss and H. B. Klinger, J. Amer. chem. SOC.86,
3265 (1965); cf. also A. L. Goodman and R. H. Eastman, ibid. 86,
908 (1964).
[53] [a] N. H. Cromwell and G. V. Hudson, J. Amer. chern. SOC.
75, 872 (1953); [b] E. M . Kosower and M . Ito, Proc. chem. SOC.
(London) 1962, 25.
[54] H. C . Brown and J . D . Cleveland, J. Amer. chem. SOC.88,
2051 (1966); L. B. Jones and V. K. Jones, Tetrahedron Letters
1966, 1493.
Angew. Chem. internat. Edit.
Vol. 6 (1967) No. 8
and four-membered ring systems. The more stable
cyclopropylmethyl and cyclobutyl cations, on the
other hand, isomerize much more slowly, as can be
seen by hydrolysis of the carbonium salts after various
times [55J.
The non-classical interaction in cyclopropylmethyl carbonium ions depends on the fact that the vacant C@orbital is
parallel to the plane of the three-membered ring [If]. The
published theoretical studies on cyclopropane, in which a
wide range of methods was used, all agree that the C-C
bonds have a considerable p component in the plane of the
ring. Walsh formulated cyclopropane as a combinationof three
trigonal spz-hybridized C atoms in which the 2p orbitals are
situated parallel to one another in one planeI561. Coulson
carried out complicated VB calculations
and simpler calculations using the principle of maximum overlap[581, according to which cyclopropane is most stable when the
atomic orbitals that form the C-C bonds are a = 22 to 24
outside the C-C axis, cf. (30). The hybridization parameters
show a particularly small amount of s character. Calculation
of the C-C bonds in cyclopropane by the MO method gave
an angle of CL = 21 to 25 O, and a higher p component for the
atomic orbitals of the carbon [591.
bond. Whereas the ratio kunsaturated/ksaturated
ethanolysis is smaller than, or only slightly greater
than 1, considerably higher rates of solvolysis are
found for the homoallyl compounds in less nucleophilic solvents (acetic acid, formic acid) [4,71. The
neighboring-group effect increases with the number
of substituents on the double bond (Table 1).
The UV spectra of alkyl-substituted olefins show that
the strength of the electrostatic bonding of the x
electrons decreases with the number of alkyl groups [601
(red shift). As can be seen from Fig. 4, an approximately linear relationship exists between the ionization
energies of olefins 1611, found spectroscopically and
by electron or photon impact methods, and the
logarithm of the solvolysis constants (which are
proportional to the activation energy) of corresponding homoallyl compounds in SNl reactions 1621.
The charge delocaiization in the cyclobutyl cation is probably also due to the ring strain. Both VB[s-iJ and MO calculations1591 indicate that the C-C orbitals are situated 5 to
10 outside the ring, and that they have a p character intermediate between that of the three-membered ring and that
of the strain-free bonds.
Simple Substituted Homoallyl, Cyclopropylmethyl,
and Cyclobutyl Compounds
The composition of the solvolysis products and the
kinetic neighboring-group effect of the double bond
have been particularly intensively studied for aliphatic
homoallyl compounds. It can be seen from Table 1
that the neighboring-group effect kunsaturated/
ksaturated(see p. 667) is influenced not only by the
solvent used, but also by the substituent on the double
Table 1. Kinetic neighboring-group effects in the solvolysis of hornoallyl
i I 1
EthanoIysis ~4~
(H,C)zC= CH-(CHz),-X
lrans-cyclopropylCH= CH-(CHz)z-X
I41 [a1
47 5
350 [cl
171 [bl
ca. 770
cu. 165
ca. 350
cu. 16x lo3
Ial For B-naphthylsulfonates.
[bl For p-toluenesulfonates.
Cf. also 1. B. Rogun, 3. org. Chemistry 27, 3910 (1962).
[ 5 5 ] M. Hanaek and H.-J. Sehneider, Angew. Chem. 77, 1023
(1965); Angew. Chem. internat. Edit. 4, 976 (1965).
[56] A . D . Walsh, Trans. Farad. SOC.45, 179 (1949).
[57] C. A. Coulson and W. E. Moffit, Phil. Mag. 40, 1 (1949).
[58] C. A. Coulson and T. M . Goodwin, J. chem. SOC. (London)
1962, 2851.
D. Peters, Tetrahedron 19, 7 539 (1963)
Angew. Chem. internat. Edit.
Vol. 6 (1967),iNo. 8
log k,,i
Fig. 4. Relationship between rate of solvolysis (krel) and the spectroscopically determined ionization potential (IP).
A: formolyses, X = OTs (after [71); B: acetolyses, X = OTs (after [la]);
C : acetolyses, X = p-N02CsH1S03 - [krel calculated from data given
by P. D . Burtlett and G. D . Surgenf, J. Amer. chem. SOC.87, 1297 (1965)l.
A similar relationship is found for cyclic homoallyl compounds
and other compounds which solvolyse with participation of
a double bond (B and C in Fig. 4.).
Under the conditions of a kinetically controlled SN1
reaction, the substituted homoallyl systems listed in
Table 1 are solvolysed exclusively to cyclopropyl
derivatives formed from more stable carbonium
ions[41. Special mention should again be made of the
difference in the compositions of the solvolysis
products and of the products formed on deamination
of appropriate homoallylamines (see p. 666) [4,63.
If the alkyl group is situated in position 3 on the
double bond (31), the reaction leads exclusively
1601 [a] H. A. Staab: Einfiihrung in die theoretische organische
Chemie. 3rd Edit., Verlag Chemie, Weinheim 1962, p. 307;
[bl F. A. Matsen in W. West: Technique of Organic Chemistry.
Interscience, New York 1956, VoI. VIII, p. 647f.
[611 W. C. Price, R . Bralsford, P. V. Harris, and R . G . Ridley,
Spectrochim. Acta 14, 45 (1959).
1621 H:J. Schneider, Dissertation, Universitat Tiibingen, 1967.
Cyclic a n d Bicyclic Systems
a; - A,,-*
to rearrangement into the cyclobutane compound
(35) [7,631.
In the solvolysis and in the deamination of simple
substituted cyclopropylmethyl and cyclobutyl compounds, one isomer again predominates in the product. Compounds (32) and (34) [211 give (33), whereas
(35) and (37) give (36) 163,641. An explanation for
this [6,631 is that the positive charge in the bicyclobutoniuni ions [cf. (27a) to (27c)l is concentrated on
the substituted C atom.
Cyclopropylmethyl compounds having the structure
(38), R = CH3, X = NH2, rearrange during deamination, mainly with formation of (36), R = CH3,
Y = O H [61. On the other hand, hydrolysis of 2-vinylcyclopropylmethyl p-toluenesulfonate (38), R =
-CH=CH2, X = OTs is accompanied by a cleavage of
the three-membered ring and formation of (39) and
(40). This ring cleavage, which has not been observed
in any other kinetically controlled reactions, is explained by the high stability of the ally1 cation corresponding to (39) and (40) (651.
Cyclopropylmethyl derivatives having the structure
(37), R = CH3 or C6H5, x = P-C02-C&4-N02,
esters of secondary alcohols, are solvolysed particularly rapidly [661, whereas tertiary cyclobutyl esters
(32) react at roughly the same rate as the corresponding t-butyl esters [631. This indicates that the
ionization of (37) proceeds via a non-classical intermediate, and that of (32) via a classical intermediate.
The presence of the strongly electronegative CF3
group in (3.5), R = CF3, X = OTs, and (37), R = CF3,
X = OTs, directs the hydrolysis both toward (35),
R = CF3, X = OH, and (38), R = CF3, X = OH, and
toward the corresponding homoallyl alcohol (Ctrifluoromethyl-3-buten-1 -01) 1671.
[63] E. F. Cox, M. C. Caserio, M . S . Silver, and J. D . Roberts,
J. Amer. chem. SOC. 83,2719 (1961).
[64] J. W. Wilt and D . D . Roberts, J. org. Chemistry 27, 3430
[65] M . Hanack and P . Krause, unpublished.
[66] R . A. Sneen and A . L. Baron, J. Amer. chem. SOC. 83, 614
[67] M . Hanack and H. Meyer, unpublished.
The rearrangements of cyclic homoallyl compounds
and of bicyclic cyclopropylmethyl and cyclobutyl
compounds are particularly interesting, since they
provide a further insight into the mechanism of the
rearrangements. As can be deduced from considerations
of models and from the rules of conformational analysis, the geometry of the compounds taking part in the
rearrangements is partly fixed in such a way that the
ability of the orbitals involved to overlap is considerably altered (see p. 669), with the result that both
the direction of the rearrangement and the kinetics
are affected. The same arguments apply to the carbonium ion formed during the rearrangement, whose
tendency to be formed depends both on the possibility
of delocalization of the positive charge and on the
strain in the system formed (see p. 669). As was
mentioned earlier, a stereochemically, uniform reaction course is of the utmost importance to the assump21
tion that non-classical bridged ions are involved
(see p. 669). The stereochemistry of the homoallyl
rearrangement has become known only by the study
of cyclic compounds.
Rearrangements of Bicyclic Homoallyl
The influence of strain on the direction of rearrangement can be illustrated by the example of a bicyclic
homoallyl compound. The solvolysis of 2-norbornenyl
derivatives (41) leads to nortricyclyl derivatives
(43) [1i368-7o1 not only under the conditions of a kinetically controlled reaction, but also (in contrast to the
examples discussed so far) in thermodynamically controlled reactions. Thus the reaction of bicycloheptadienes[71-731 (42) or norborneols (41), X = OH,
with acids (HCl, HCl/ZnC12, HF) leads to rearrangement and formation of (43), X = C1 or F [70,741.
Schleyer showed by the position of the equilibrium
between norbornene and nortricyclene, that the cyclo-
[681 J . D . Roberts, W. Bennett, and R. Armstrong, J. Amer.
chem. SOC. 72, 3329 (1950).
j691 S. Winstein, M. Shatavsky, C. J. Norton, and R. B. Woodward, J. Amer. chem. SOC. 72, 5795 (1950).
[70] M . Hanack and W. Kaiser, Liebigs Ann. Chem. 657, 12
(1962); cf. also W. Treibs, Naturwissenschaften 49, 255 (1962).
[71] S. Winstein and M . Shatuvsky, J. Amer. chem. SOC. 78, 2819
1721 H. Krieger, Suomen Kemistilehti 3 3 4 183 (1960).
[731 S. J. Cristol, W. K . Seifert, D . W. Johnson, and J . B. Jurale,
J. Amer. chem. SOC. 84, 3918 (1962).
[74] M. Hanack, H. Eggensperger, and R . ffihnle, Liebigs Ann.
Chem. 652, 96 (1962).
Angew. Chem. internat. Edit./ Yol. 6 (1967) / No. 8
propane systems (43) are more stable than their homoallyl isomers (41), both as the carbonium ions and as
the final products, owing to their tricyclic structure [751.
Though both exo- (44) and endo-norbornyl derivatives (47)
rearrange during solvolysis to give the nortricyclyl system
(43) for the reasons given above (689 691, a characteristic difference is found in the rates of solvolysis, owing to the different positions of the substituents in relation to the double
bond. 2-exo-Norbornenyl p-toluenesulfonate (44), X = OTs,
acetolyses at almost the same rate as the saturated 2-exonorbornyl p-toluenesulfonate [69,761, since the arrangement
of the orbitals of (45) favors overlap. On the other hand,
the endo-compound (47), X = OTs, reacts more slowly than
the saturated comparison system, owing to the less favorable
arrangement (48).
ion (SO) showed that the charge is located mainly on
C-5 and C-6, and less on C-7 [80,811.
7-syn-Norbornenyl p-toluenesulfonate (52) is also solvolysed lo4 times as rapidly as the saturated system.
Since the position of the vacant orbital in relation to
the double bond is unfavorable for overlap, the high
rate of solvolysis is assumed to be due to the energetically favorable formation of the allyl cation (53) [821.
The introduction of a second double bond (54),
X = C1, leads to a further increase in the rate of
solvolysis [83.841. Ionization leads to (55), which is
assigned an asymmetric form on the basis of the N M R
spectrum of a solution of 7-norbornadienyl tetrafluoroborate in liquid SO2 [81,851.
The higher rate of solvolysis of (44) and the distribution of
14C between positions 2 and 3 in the products obtained on
acetolysis of [2-14C]- or [3-14C]-2-norbornenyl sulfonates
point to the formation of the non-classical cation ( 4 6 a ) (681.
Since the 14C is also found in positions 1 and 7 after solvolysis
with the weakly nucleophilic formic acid, secondary formation of the symmetrical ion ( 4 6 b ) from ( 4 6 a ) is assumed to
take place 169~76aI.On the other hand, the acetolysis of 3-cisdeuterio-2-exo-norbornenylbrosylate also yields 7-deuterated acetate, pointing to primary formation of ( 4 6 6 ) or to
the establishment of a n equilibrium ( 4 6 a ) + ( 4 6 c ) 1771.
The 7-anti-norbornenyl system satisfies the steric requirements for a substantial rs overlap (see p. 669).
Consequently, 7-anti-norbornenyl derivatives (49),
X = OTs or C1, are solvolysed 1011 times as rapidly
as the corresponding 7-norbornyl compounds in acetic
acid "8,791. NMR-spectroscopic investigation of the
1751 P. Y. R. Schieyer, J. Amer. chem. SOC.80, 1700 (1958).
1761 [a] J. D. Roberts, C . C . Lee, and W. H . Saunders jr., J.
Amer. chem. Soc. 77, 3034 (1955); [b] S. Winstein and M .
Shatavsky, ibid. 78, 592 (1956).
1771 S. J . Cristol, T . C. Morril, and R . A . Sanchez, J. Amer.
chem. SOC.88. 3087 (1966).
[78] S. Winstein, M . Shatavsky, C . Norton, and R. B. Woodward, J. Amer. chem. SOC.77, 4183 (1955).
1791 W. G. Woods, R. A . Carboni, and J . D . Roberts, J. Amer.
chem. SOC.78, 5653 (1956).
Angew. Chem. internat. Edit. I VoI. 6 (1967)
1 No. 8
The systems (49) and (54), which are particularly favorable
for charge delocalization, d o not rearrange into the tricyclic derivatives (51) and (56) on solvolysis under the conditions indicated. However, (51) and (56), Y = H, can be
obtained by reaction of ( 4 9 ) , X = OTs [86), or ( 5 4 ) , X = OTs
or C1[86*871,with NaBH4 or LiAIH4. Methanolysis of ( 4 9 ) ,
X = OTs or CI, and (54), X = CI, yields the tricyclic endomethyl ethers (SI), Y = OCHx, and (56), Y = OCH3, if the
strongly nucleophilic sodium methoxide is used in excess [80,88,891.
Kinetic studies and analysis of the products have also
revealed neighboring-group effects in other bicycloheptyl systems having a double bond in the homoallyl
position with respect to the C atom carrying the
functional group [la]. The 7-isopropylidene-2-endonorbornyl system (57) satisfies the stereoelectronic
requirements for the formation of a non-classical ion
(58) ; the acetolysis rate constant is therefore much
higher than that of endo-2-norbornyl p-toluenesulfonate, and the reaction proceeds with retention of configuration. The same is true of the 2-rxo system ( 5 9 ) ,
since delocalization involving the second double bond
is possible in this case with formation of (60). In (61),
on the other hand, overlap with T;-electron orbitals is
geometrically impossible, so that the solvolysis of this
compound is practically no faster than that of exo-21801 M . Brookhart, A . Diaz, and S . Winstein, J. Arner. chem.
SOC.88, 3135 (1966).
[81] H . G. Richey j r . and R . K. Lustgarten, J. Amer. chem. SOC.
88, 3136 (1966).
1821 S. Winsrein and E. T.Stafford, J. Amer. chem. SOC.79, 505
[83] S. Winstein and C . Ordronneau, J. Amer. chem. S O C . 82,
2084 (1960).
1841 P . R. Story and M. Saunders, J . Amer. chem. Soc. 82, 6199
[ 8 5 ] P. R. Story, L. C. Snyder, D . C. Douglas, E. W. Anderson,
and R. L . Kornegay, J. Amer. chem. SOC.85, 3630 (1963).
I861 H. C . Brown and H . M . Bell, J. Amer. chem. SOC.85, 2324
[87] P.R . Story, J. Amer. chern. SOC. 83, 3347 (1961).
1881 H. Tanida, T. Tsuji, and T. Irie, J. Amer. chem. SOC.88, 864
1891 A . Diaz, M . Brookhart, and S . Winstein, J. Amer. chem. SOC.
88, 3133 (1966).
norbornyl p-toluenesulfonate, and leads to the product
(62) of a Wagner-Meerwein rearrangement, such as
would be expected from a 2-norbornyl system; (63)
reacts similarly to give (64) 1901.
Since the position of the double bond in relation to the
positive charge is unfavorable for overlap of the
orbitals, the ion is stabilized by hydride shift. However,
on solvolysis with complex anions, e.g. in the reaction
of 3-cyclopentenyl p-toluenesulfonate (65), X = OTs,
with LiAlH4 or NaBH4, a small quantity of an unstable intermediate can also be intercepted in the form
of bicyclopentane (66), Y = H[92I.
The 3-cyclohexenyl carbonium ion can assume a conformation close to the homoallyl resonance model (see
p. 669), so that the rearrangement should be facilitated.
Hanack et al. not only found that the rate of acetolysis
of 3-cyclohexenyl p-toluenesulfonate (68) was higher
than that of cyclohexyl p-toluenesulfonate, but also
isolated the expected bicyclo[3.1 .O]hexyl compounds
(69) on acetolysis and hydrolysis 141,931.
Kinetic neighboring-group effects and homoallyl rearrangements have also been observed with bicyclo[2.2.2]0ctenyl and
bicyclo[3.2.1]octenyl compounds [la]. The resulting tricyclic
cyclopropane derivatives have a low thermodynamic stability.
Reports on the other related systems have also been published [la].
Rearrangements of Monocyclic Homoallyl
The dependence of the neighboring-group effect on
stereochemistry and on the strain in the systems can
be seen even in simple cyclic homoallyl compounds
and in the cyclopropylmethyl and cyclobutyl derivatives obtained from them.
The lowest homologue of the 3-cycloalkene series, i.e.
the 3-cyclopentene system (65), does not undergo a
homoallyl rearrangement to form the bicyclo[2.1 .O]pentyl system (66). The absence of a neighboringgroup effect of the double bond, which is due to the
planar conformation of the cyclopentene ring, is also
indicated by the low rate of solvolysis of (65), X = Br
or P-naphthylsulfonate (ONs), in comparison with
corresponding cyclopentyl derivatives 162,911. The
investigation of various 3-cyclopentenyl derivatives
(65), X = Br, OTs, or ONs, showed that solvolysis
leads mainly to 2-cyclopentenyl compounds (67) [62,921.
1901 C. H. DePuy, J . A . Ogawa, and J . C. McDaniel, J. Amer.
chern. Soc. 83, 1668 (1961).
[91] P. D. Barrletr and M . C. Rice, J . org. Chemistry 28, 3351
[92] Cf. M. Hanack, Angew. Chem. 77, 624 (1965); Angew.
Chem. internat. Edit. 4, 603 (1965).
The dependence of the three-membeied ring-carbonium ion
interaction on the spatial position of the three-membered ring
is not very pronounced in the bicyclo[3.l.O]hexyl derivatives
(69). cis-Bicyclo[3.l.O]hex-2-y1p-nitrobenzoate (69), X =
is hydrolysed only 1.5 times as rapidly as
the trans isomer[941, and both of these compounds also hydrolysed only slightly more slowly than methylcyclopropylcarbinyl p-nitrobenzoate (37), R = CH3, X = p N O ~ C ~ H ~ Cwhich
O Z , can assume the optimum conformation
for a neighboring-group effect of the three-membered
ringc661. The hydrolysis of (69) is not stereospecific, but
yields a mixture of cis- and trans-bicyclo[3.1.0]hexan-2-ol~941.
Because of their greater conformational mobility, 3cycloheptenyl (75), n = 3, and 3-cyclooctenyl derivatives (75), n = 4, can attain almost completely the
geometry required for homoallyl resonance (see
p. 669). As was shown by Cope et af. [951, derivatives of
these systems are solvolysed much more rapidly than
corresponding cyclohexyl compounds, and the solvolysis is accompanied by extensive rearrangement
into the cis-rrans-isomeric bicyclic derivatives (74).
2-Cycloalkenylmethyl compounds with small rings do
not undergo the homoallyl rearrangement.Thus neither
2,3-diphenyl-2-cyclopropenyImethyl (72) 1961 nor 2cyclobutenylmethyl p-toluenesulfonate 1971 (73), n = 1,
X = OTS, rearranges to give the corresponding bicyclic compound; the main products are those formed
by ring expansion, as is shown e.g. by the formation of
the isomeric cyclopentenols (75), n == 1, Y = OH, and
[93] M . Hanack and P. Ch. Krause, unpublished.
[94] M . Hanack and H . Allmendinger, unpublished.
1951 Summary: A. C. Cope, M. M . Martin, and M . A . McKervey,
Quart. Rev. 20, 119 (1966).
1961 R . Breslow, J. Lockhart, and A. Sniall, J. Amer. chem. SOC.
84, 2793 (1962).
[97] M . Hanack and K . Riedlinger, Chem. Ber. 100, 2107 (1967).
Angew. Chem. internat. Edit. / Vol. 6 (1967)
/ No.8
(76), n -- 1 , Y - OH, on hydrolysis of (73), n
X = OTs. The rate of acetolysis of cyclobutenylmethyl
p-toluenesulfonate, which is lower by a factor of
3 than that of cyclobutylmethyl p-toluenesulfonate [97J,
can again be explained by the unfavorable position of
the C atom carrying the functional group in relation
to the double bond.
f 741
( 731
A further increase in ring size in the 2-cycloalkenylmethyl system (73) allows the homoallyl rearrangement to predominate over ring expansion. 2-Cyclopentenylmethyl derivatives ( 7 3 ) , n = 2 , X = OTs or
NH2, rearrange on solvolysis or deamination into the
stereoisomeric bicyclo[3.1 .O]hexyl compounds ( 7 4 ) ,
n 2. The 3-cyclohexenyl derivatives are obtained particularly in weakly nucleophilic solvents 1171. The
neighboring-group effect of the double bond in the
more mobile systems ( 7 3 ) , with n == 2, is also reflected
in the kinetics; (73), n
2, X -= ONs, is acetolysed
10 times as rapidly as the saturated cyclopentylmethyl
naphthylsulfonate 1171.
In niethylenecyclohexylcarbinyl p - toluenesulfonate
( 7 7 ) , the C atom carrying the functional group can
assume the axial position, so that the steric requirements for a homoallyl rearrangement are satisfied.
The hydrolysis of (77) leads not only to ( 7 8 ) but also
to ( 7 9 ) , which is formed from the bicyclo[4.2.0]octyl
cation corresponding to (78) by isomerization into
the spirooctyl cation corresponding to (79)
Another product is assigned the structure (80). To
explain the formation of this product, it is assumed
that the homoallyl rearrangement is accompanied by
ring expansion to form the ion ( S I ) , which then
rearranges into the bicyclo[4.1.1.] system (80).
f 80)
3-Methylenecycloalkyl derivatives, which lead directly t o the
carbonium ion (81), have n o t so far been studied very extensively. It can be seen particularly clearly i n th e case o f t h e
3-methylenecyclobutyl cation (82) th at th e reactivity of a
homoallyl c om poun d is determined by t h e positions o f t h e p
orbitals of t h e double bond a n d o f th e vacant orbital of t h e
carbonium ion. Since these a r e parallel in (82), o overlap is
impossible. Accordingly, t h e solvolysis of 3-methylenecycloAngew. Chem. internat. Edit. / Vol. 6 (1967) / No. 8
butyl bromide is slower by a factor of 160 than that of 3butenyl bromide, an d t h e dedmination of 3-methylenecyclobutylamine proceeds without rearrangement 1981.
The I-cycloalkenylethyl derivatives (83), which are
influenced only slightly by steric effects, are examples
of the few cases in which cyclopropane and cyclobutane compounds are formed simultaneously. As
was shown by Hanack and Schneider 1991, it is possible
in this way to obtain both spirocyclopropane derivatives (84) and condensed cyclobutane derivatives
(85) in good yields. The rearrangement of homoallyl
compounds, which has so far been studied mainly
from the point of view of mechanism, is also of
preparative importance in this case. The hydrolysis of
1-cyclohexenylethyl p-toluenesulfonate ( 8 3 ) , n
X - OTs, gives spiro[2.4]octan-l-ol ( 8 4 ) , n
4, in
50 ”/, yield, and deamination of (83), n == 4, X == NH2,
leads by acid-catalysed secondary rearrangement of
( 8 4 ) , n - 4, to a 50 0; yield of bicyclo[4.2.0]octan-l-ol
(85), n ~- 4.
In addition to I-cyclopentenylethyl derivatives [99,1001,
corresponding medium-sized rings were also subjected to this rearrangement; spiro compounds and bicyclic compounds containing four-membered rings,
some of which had previously been known only as
hydrocarbons, were obtained [1*11.
Condensed Cyclopropylmethyl and Cyclobutyl
These systems, which have been studied mainly by
Wiberg and his co-workers, show particularly clearly
that the formation of a non-classical cation is dependent on certain structural and steric conditions.
Acetolysis of exo-bicycIo[2.2.0]hex-2-yl p-toluenesulfonate (86), n = 2, does not lead torearrangement
into the cyclopropylmethyl system, as is expected of a
cyclobutyl compound; on the contrary, the main
products isolated are exo-bicyclo[2.1 .l]hexyl derivatives (87), X - OTs or OAc [lo*, 1031. The rigid planar
[98] E. F. Kiefer and J . D. Roberts, J. Arner. chem. SOC.84, 784
[99] M . Hanack and H.-J. Schneider, Liebigs Ann. Chern. 686, 8
[loo] W. D. Closson and G. T. Kwiatkowski, Tetrahedron 21,
2719 (1965).
[loll H . Schneider-Bernlohr, R. Huttinger, H.-J. Schneider, and
M . Hanack, unpublished.
[lo21 [a] K. B. Wiberg and A. J. Ashe U I , Tetrahedron Letters
1965, 4245; [b] R. N . McDonald and C . E. Reineke, J. Amer.
chem. SOC.8 7 , 3020 (1965).
[lo31 [a1 K . B. Wibera and A . J. Ashe III, Tetrahedron Letters
1965, 1 5 5 3 ; [b] F. Th.-Bond and L. Scerbo, Tetrahedron Letters
1965, 4255.
fusion of the two four-membered rings prevents the
formation of a bicyclobutonium ion. Similarly, the
isomeric compound (88), n = 2, which is acetolysed
to (89) without rearrangement, cannot react via this
ion. The slightly higher rate of solvolysis of (88) in
comparison with cyclopropylmethyl p-toluenesulfonate
leads to the conclusion that stabilization of the
carbonium ion by the cyclopropane ring is also
possible in this system.
by neighboring-group effect or by steric effects (e.g. in
tertiary esters). The strong temperature-dependence of the
composition of the products indicates that several parallel
reactions are involved [1051.
Allene Derivatives
The allene derivatives (95), which are similar to the
homoallyl compounds, can also cyclize to give cyclopropane derivatives, though a vinyl cation (96) is
involved as an intermediate in this case [1061. Solvolysis
of (95), R = H or alkyl, X = ONs, leads mainly to the
cyclopropyl ketones (97) ; cyclobutyl compounds have
also been obtained from 7 - or 3-substituted allenes [1071.
exo- and endo-BicycIo[3.1 .O]hex-6-yImethyl p-toluenesulfonate (88), n = 3, and (90) are acetolysed at
approximately the same rate, but give different
products [lo)]. While the exo-p-toluenesulfonate (88),
n = 3, gives the same products as the exo-p-toluenesulfonate (86), n = 3 11041, the endo isomer (90) cannot
form the same bicyclobutonium ion as (91). Consequently, acetolysis of (90) and of (91) leads to
different products.
Rearrangements during Pyrolysis
The direct participation of an allene double bond in
the carbonium ion reaction has been detected by
kinetic studies [106,1081.
Acetylene Derivatives
An interesting and useful cyclization reaction is exhibited by acetylene derivatives having the structure
(981, R = CH3, C2H5, i-C3H7, X = m-S03-C6H4N02.
Solvolysis with a solvent having a low nucleophilic
strength and a high ionizing power leads to practically quantitative rearrangement into cr-alkylcyclobutanones (100) [109,1101.
The rearrangements of cyclopropylmethyl, cyclobutyl,
and homoallyl systems into one another have so far
been considered only in connection with typical
carbonium ion reactions such as solvolysis or deamination. However, they have recently also been observed
( T O A c
in the pyrolysis of esters having the structures (92),
(93), and (94) [1051, which give not only the olefins
of the same structure, but also isomeric hydrocarbons.
These products, as well as the pyrolysis kinetics [1051,show
that the pyrolysis of esters can also proceed via an ionic
mechanism if the formation of a carboniurn ion is facilitated
[lo41 F. F. Nelson, Ph.D. Thesis, University of Wisconsin, 1960.
[lo51 M . Hanack, H. Schneider-Bernfohr, and H . J . Schneider,
Tetrahedron, 23, 2195 (1967).
The solvolysis of phenyl-3-butynyl sulfonates (98),
R=C6H5, X = OSOzR, leads mainly to phenyl cyclopropyl ketone, though other products are also formed.
Ethoxy-3-butynyl p-toluenesulfonate (98), R=C2H50,
X = OTs, partly rearranges on hydrolysis to give ethyl
cyclopropanecarboxylate[1111. The cyclization of (98)
[lo61 M . Hanack and J . Haffner, Tetrahedron Letters 1964,
2191; M . Hanack and J. Haffner, Chern. Ber. 99, 1077 (1966).
[lo71 M . Bertrand, and M . Santelli, C. R. hebd. Seances Acad.
Sci. 259, 2251 (1964).
11081 A. R. Ballentine, R . S . Bly, and S. U. Kuock, Abstracts
152nd Meeting Arner. chem. SOC., New York, Sept. 1966,
Section S .
[lo91 M . Hanack, J . Hfiffner,and I. Heterich, Tetrahedron Letters 1965, 875.
[I101 M . Hanack and I. Herterich, Tetrahedron Letters 1966,
[111] M . Hanack and W. Kaiser, unpublished.
Angew. Chem. internat. Edit.
1 Vol. 6 (1967)1 Nu. 8
(R = alkyl) presumably involves the direct participation
of the triple bond, and, as with the allene derivatives,
proceeds via a vinyl cation (99). This is supported by
the fact that, despite the strong inductive effect of the
triple bond, the rates of solvolysis of the acetylene
derivatives (98), R = alkyl, are only slightly lower
than those of the corresponding saturated compounds.
The formation of the cyclobutanones (100) could
also be explained by addition of the solvent to the
triple bond and subsequent homoallyl rearrangement.
In the presence of Hg2+ salts, which are known to
catalyse the addition to the triple bond, solvolysis of
(98), R = alkyl, leads mainly via ( l o r ) , R = alkyl, to
cyclopropyl ketones (102), R = alkyl. This indicates
that the uncatalysed solvolysis of (98), R = alkyl,
proceeds via a direct participation of the triple
bond [1121.
The present survey of the reactions of cyclopropylmethyl, cyclobutyl, and homoallyl compounds has had
to be confined to selected typical examples. The large
number of articles published even very recently shows
that this field is not yet closed. The present article has
been concerned only with cationic reactions. No
mention has been made of numerous publications e.g.
from the steroid field, or of the interesting rearrangements of carbanions, free radicals, and carbenes [la].
Finally, it should be pointed out that cyclizations
involving a double bond have been observed, not only
with homoallyl compounds, but also with other
unsaturated systems [ * c , 1131.
We are grateful to the Deutsche Forschungsgemeinschaft and to the Fonds der Chemischen Industrie for
their support of our own work, which was carried out by,
Helmut Allmendinger, Heinz Eggensperger, Klaus Giirler, Jiirgen Hafner, Inge Herterich, Rudolf Hiittinger
Sungrong Kang, Wolfgang Keberle, Karl Riedlinger,
Hans-Jorg Schneider, Helga Schneider-Bernlohr, and
Volker Vott.
Received: June 27th, 1966; revised: February 23rd, 1967 [A 586 IEi
German version: Angew. Chem. 79, 709 (1966)
Translated by Express Translation Service, London
[112] M . Hanack, I. Herterich, and V. V f f t t ,unpublished.
[I 131 P. D . Bartlett, Liebigs Ann. Chern. 653, 45 (1962).
Formation and Properties of Carbosilanes [*I
The present paper describes the formation of silicon-carbon compounds (carbosilanes),
the moIecular skeletons of which consist of alternate Si and C atoms, and which can
be obtained by thermal decomposition of Si(CH3)4, CH3SiCI3, (CH3)2SiCIz, and
(CH3)3SiCl. The description of the formation of carbosilanes is followed by a discussion
of the reactions of polychlorinated carbosilanes with organometallic compounds.
1. Synthesis of Carbosilanes by Pyrolysis
Carbosilanes are compounds with alternate Si and C
atoms in their molecular skeletonrz]. They may be
open-chain or cyclic compounds,e.g.(C13Si-CH2)2SiC12
(1,1,1,3,3,5,5,5 - octachloro - 1,3,5 - trisilapentane) or
(1,1,3,3,5,5-hexachloro-1,3,5-trisilacyclohexane). The work described in this article arose
from experiments in which compounds containing SiH
were pyrolysed and made to react with hydrogen
compounds of other elements [31. Such reactions can
I'] Prof. Dr. G. Fritz
Institut fur Anorganische Chemie der Technischen Hochschule
Englerstrasse 11
75 Karlsruhe (Germany)
[I] Lecture at the 151st National Meeting of the American
Chemical Society in Pittsburg (USA) on March 28, 1966, on the
occasion of the presentation of the ACS Frederic Stanley Kipping
Award, 1966.
[2] G . Fritz, J. Grobe, and D. Kummer, Advances inorg. Chem.
Radiochem. 7, 349 (1965).
Angew. Chem. internat. Edit.
1 Vol. 6 (1967) / No. 8
in fact be carried out once H atoms and silyl radicals
have been produced, as is shown by the formation of
numerous organosilicon compounds on reaction of
SiH4 with ethylene 141 and with vinyl chloride fS1 above
400 "C, as well as by the reaction of SiH4 with PH3 to
form H3Si-PH2 and other silylphosphines 161. Since
these and other observations on silicon chemistry
pointed to the possibility of obtaining complicated
silicon compounds from simple compounds by freeradical reactions, we tried to utilize the thermal cleavage
of Si-CH, and C-H groups for the synthesis of
silicon-carbon compounds.
The Si-C bond in simple alkylsilanes is much more
stable to heat than the Si-H bond"]. The thermal
[3] G. Fritz, Fortschr. chem. Forsch. 4 , 459 (1963).
141 G. Fritz, Z . Naturforsch. 7b, 207 (1952).
[5] G. Fritz, Z. Naturforsch. 76, 379, 507 (1952).
161 G. Fritz, Z. Naturforsch. 8b, 776 (1953); Z. anorg. allg. Chem.
280, 332 (1955).
[7] G . Fritz, Z. anorg. allg. Chem. 273, 275 (1953).
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effect, rearrangements, reaction, group, homoallylic, system, cyclobuten, cyclopropylmethylen, neighboring
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