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Effects of Through-Bond Interaction.

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[ 161 B.
Zlil/c,r.
.M. 1. Brlluc-uriw. and S /3ruckw\iwi. Anal. Chem. 44. 19x3
( 1977).
[ 171 C H . Huinuiiii a n d M: ~ ' i r / . s t k ~I / .i . Electroanal. Chem. 32, 459 ( I97 I ).
[ I 8 3 M: J ..llhrry and S. B r u c h , r e t i i . Trans. Faraday Soc. 62. 1915, 1920
(I966).
[ 191 G .
Tkiiithorn.
Dissertation. IJniversitit Bonn 1969.
/~ikuiii~iru
Ii. / c i / , u i i i i i r ( i . C i : , V I ~ J ~;ind
W . E. l i w y e r . I ZSth N;itional
Meeting. T h e Electrochem. Soc.. Nen York 1969 (Extcnded Ahatracta p.
1.311 I
IXX).
1321 F. G m ;itid K J L<,rr<,r.%. Ph!\ ('hem N F 77. 317 11972)
[ 3 3 ] 4. K N K d d . , I!. G ~ w ~ l i c i i tand
.
J . O'.$I. 50d,ri\, _I Electroanal.
Chciii. 8. 406 (1964).
[20] H . H Buiirr. L). L. Siiiirh. and P. J . Elriiiq J Amer Chem. Soc. 87,
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Tensametry. Wiley. New York 1963.
[ 3 5 ] J L. Or0 and D. J . DrSiiirf. J . Electrochen?. Soc I l 3 . 1258 (1966).
[36] 7 K t r i t o i i u . Electro;tnal. Chem 1. I57 (1966).
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[ 2 5 ] J . U: Srr-ojek a n d T Kuiwnu. J. Electroanal. Chem. 16, 471 (1968).
[26] W N . Huiiscii, 7:
Soc. X X . 1062 (1966).
K u i ~ o i i o ,a n d
K . .4. O.\feri-ouiiq. 1. Amer. Chem.
[27] B. S. Poiis, J . S. Morrcoii, L. 0.Wiizsfrowi, and H . B. ,\4ur/,, Jr.. Anal.
Chem. 3Y. 685 (1967).
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paper 308 (Extended Abstracts p. 195).
[29] A . H . Rrud a n d €. Ewyer. Appl. O p t 7. 451 (1968).
[30] W J . Pltrrh. Chem. Ing. Tech. 44, 221 (1977).
[iX]
N . 4 . Bulu\hocx a n d I : E . K u z u r i w r . Elcctroanal Chem. 3. 135 (1969).
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~ W ~ L W ! Ya n d
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[42] J . Hrirhmtiri a n d CC L i d > f t d i . t o be published
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1971. Angew. Chem. XI. 662 (1969): Angew. Chem. internill. Edit. 8 . 673
( 1969)
[44] ,%f. F / e i ~ c l i i i i ~ i i i iP.
i . J . Hriidru. a n d 4 .I
M < Q u t / / u i i . .I. C. S. Chcm.
Coniiii. 1973. 80.
[J5] C . (;L~III\. E . B i i d t w . S. S r w i w i . 7: Biwqiiiui-h. xnd K . Sirghohii. C o m m .
of the Uppsala Universtcy UUIP-817. April 1973.
Effects of Through-Bond Interaction
By Rolf Gleiter[*I
The effect of through-bond interaction is derived for 1,4-butanediyl as an example, and changes
caused by this effect in the ground state of the system are demonstrated. Spectroscopic methods
for the detection of the effect are discussed, and its chemical consequences are illustrated
by a number of examples.
1. Introduction
Until a few years ago the interaction of lone pairs of electrons
and of R orbitals situated more than 3a apart was assumed
to be very small. Examples of this situation are found in
1,4-diazabicyclo[2.2.2]octane (1 ), p-benzoquinone (2), anritricycI0[4.2.0.O~.~]octa-3,7-diene
( 3 ) ; and pyrazine ( 4 ) . The
assumption that these interactions are very small became
the subject of critical examination following calculations by
Hoffmann et a!. for the three isomeric diazabenzenes['] and
didehydrobenzenesf2! These calculations were stimulated by
experiments by Rees and S t ~ r r [on
~ lcycloadditions to 1,S-didehydronaphthalene (1,8-naphthalenediyl) and by investigations
by McKinney and Geske14] on the radical-cation ( 1 ) +.
[*] Prof. Dr. R. Gleiter
Institut fur Organische Chemie der Technischen Hochschule
61 Darmstadt, Petersenstrasse 15 (Germany)
696
The results of these calculations wilI be illustrated for the
three diazabenzenes (diazines) pyrazine ( 4 ) . pyrimidine ( 5 ) ,
and pyridazine (6). Ifthe two orbitals indicated on the nitrogen
atoms in ( 4 ) , I S ) , and (6) are denoted by n, and nb, we
can write down the two non-normalized, but symmetryadapted linear combinations
n + = (n, + nh)
n - = (n, - nh)
which are shown schematically in Figure 1. The n + linear
combination is symmetric (S) and the n- linear combination
antisymmetric (A) with respect to the mirror plane m.
If the interaction between n, and nb is very small, n + and
n - should have approximately equal energies (Fig. 2, center).
If the interaction is large, on the other hand, the energy
A n g c n . ('hfm. intrrnut Edit. f ./'/I
13 ( 1 9 7 4 ) / No. I /
15)
14 ;
This energy differencedepends on the overlap integral (n ,In4),
which is small because of the spatial separation of the two
centers (about 3 A).
16J
m
m
0
(7)
,
U
ln.l-&ln.J
pT7y
E
[eVl
-1 6 7
0 80
0 99
Fig I . Schcmatic representation ofthe linear combinations of the non-bonding
orhitiils n,, and n a for thc diarabenienes ( 4 ) - - ( 6 )
levels belonging to n + and n - should be very different (Fig.
2, right and left). In the latter case there are two conceivable
possibilities: Either n + is lower on the energy scale than
n (Fig. 2. right) or uice L'('I'SU (Fig. 2, left).
However, relatively strong splitting can be achieved for n +
and n - through the interaction with the G and o* orbitals
of the C2-C3 bond. As can be seen from Figure 3, the
G orbital is symmetric (S) and the (J* orbital antisymmetric
( A ) with respect to the mirror plane m in (7). The interaction
of the other orbitals in ( 7 ) with n + and n - may be disregarded
to a first approximation~'~l[*l.
--111A
w
-_
--- --
A((T*)
.'"k92" /'
v
'-,:y*
"t_t"
-<
S(n+,,
interaction
II
,
,
f
l
\
\rI
r\
~
'\
interaction
T
.=-
%z.=
A
,/
'\,
~
'\\\
ir2-
big 7. Variation of the cncrgy levcls of n
intcractioii bctucen ii,, and nh.
.
a n d n~ a s
ii
fiinction of the
Surprisingly. the EH (extended Huckel)
for the
diazabenzenes and the didehydrobenzenes not only indicated
a large difference (about 1.5eV) for the compounds of the
type ( 4 ) , but also showed that n + is above n - in this case,
whereas n + is below n - for compounds of the types ( 5 )
and {6).Thecalculated orbital energy differencesE(n-)-&(n +)
are given at the bottom of Figure I .
The discovery that the n + level can be either lower or higher
than the n - level suggested the existence of two interaction
mechanisms, i. P . a purely spatial interaction['] (through space),
i n which the n level falls below the n level, and an interaction
mechanism in which the o skeleton (through bond) leads.
under certain conditions[**],to a reversal of this "normal"
sequence.
+
The purely spatial interaction isdescribed indetail in textbooks
o n chemical bonding. The effect of through-bond interaction
will be discussed below for the case of 1,4-butanediyl (7).
For a detailed theoretical treatment the reader is referred
to the Ii tera ture["l.
Just as for the n orbitals of the diazabenzenes ( 4 ) , ( 5 ) , and
( 6 ) . we can form the two symmetry-adapted linear combinations n + ( S ) = ( n t + n 4 )and n-(A)=(nl-n4) for the orbitals
n , and n A shown on the centers C i and C4 in (7). If one
considers only the spatial interaction, the energy difference
between n + and n - in ( 7 ) should be small (Fig. 3, left).
[ * ] The siniplc\t interaction of this type is that oftwo H(l s ) orbitals leading to
thc MOs of hqdrogen [h. 71.
[**I
kor
ii
through-bond interaction i-iu an odd number of
Icvcl IS alw.;iys belou thc n . level [Za]
Qualitative rules derived from perturbation
were used to construct the interaction diagram shown in
Figure 3 for the 5 and n levels.
I t can be seen from Figure 3 that n + is destabilized by the
interaction with the G orbital, whereas n - is stabilized by
the interaction with the a* orbital.
Figure 4 shows a comparison of the one-electron orbitals
from an EH calculation for 1,4-butanediyl with and without
through-bond interaction; the MO scheme for a through-bond
interaction is also compared with the n-MO scheme of 1,3bu tad iene.
This diagram leads to three conclusions for a system in which
only the two lowest MOs are occupied:
2. Theory of Through-Bond Interaction
11
Fig. 3 Interaction diagram for the orbitals o f the centers c" and C' and
of the C'-C2 CT bond i n 1.4-butanediyl ( 7 ) .
CT
bonds. the
1 ) The through-bond interaction leads to the reversal of the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital ( L U M O ) compared with a system in which this interaction is absent (Fig. 4, left).
2) The system with through-bond interaction is more stable
than the system without this interaction[**'].
[*] Apart from the C'--C" bond. all other bonds(r.y. C ' - - C ' a n d C'-C')
give symmetric und antisymmetric combinations of CT orbitals and CT*orbitals
I t may be assumed to a first approximation that the position ' - 4 . of the
n . lebel does not change if one symmetric orbital is lower (CT) and one
is higher (n*)
[**I These rules are:
Only orbitals of the same symmetry interact with one another. Interacting
non-degenerate orbitals "repel" each other. i.r. the lower orbital on the
energy scale is stabilized, while the higher one is destabilized.
[***] In our example the total energy of the system is E=Zn,c,. n , is the
occupancy and 6 , i s the orbital energy. In the didnion of ( 7 ) . 1,Cbutanediide
(7)'.
the through-bond interaction would lead to less stabilization than
In ( 7 ) .
697
3) The C2-C3 bond in ( 7 ) is weakened by a through-bond
interaction.
The first two conclusions require no further explanation, but
the third will be briefly interpreted. The predicted bond
alternance(bonds C1-C2 and C3-CJstronger than C2-C3)
can be best understood by comparison of the MO scheme
for (7)(Fig. 4, center) with the K-MOscheme for 1.3-butadiene
(Fig. 4, right). The one-electron MO schemes are identical
as to the symmetry of the orbitals with respect to the mirror
plane m. In both cases an S orbital and an A orbital are
have been confirmed by EH calculations[8.9! Thus as soon
as through-bond interaction predominates, i.e. A is lower
than S, the C'-C" bond is distinctly weakened in comparison
with C'-C' and C3-C".
Conclusions 2 and 3 disappear for a system in which the
three lowest orbitals are occupied. Examples of such systems
would be 1,4-butanediide (7) 2 - and 1,2-diaminoethane.
The model calculations by Hoffmunn et uI.L2"1
that were mentioned in the introduction have also been carried out for
other systems (Fig. 5). The order LUMO-HOMO and the
energy difference between the t w o orbitals are indicated under
the corresponding formulas in Figure 5.
,,'A
t
3. Methods of Detecting Through-Bond Interaction
-%
A
Among the many possible M O descriptions of the bonding
in 1.4-butanediide ( 7 ) 2 ' and in the isoelectronic 1 , M aniinoethane (8).two are outstanding; these are the canonical
MOs shown schematically in Figures 3 and 4 and the equivalent MOs. The latter are approximately localized in the bonds
or lone pairs of electrons (see below). The two descriptions
give the same total energy for the system, but different orbital
energieslbJ[*I.
S
A
I
,
+''.,, .,
-,
\
,
'U
S
A
a?~2-~~,-~~~,-'C
71,-' ~ , 0
In the case of ionization (e..y.( 8 ) ' ) or excitation (e.9. (a)*)
or interaction with another system (X)(e.8. ( 8 ) ... X), localized
MOs are no longer suitable for their description.
without
with
t h r o u g h - bond
inieraciion
'4 2 7 L:
Fig. 4 Comparison o f the orbital energies o f a system with a n d wrthotit
through-bond interaction w,ith the P M O S of I.3-bucadienc.
each occupied by two electrons. Because of the occupation
of the lowest A orbital, which has a node between the centers
C' and C3, the contribution of the lowest occupied S orbital
to the bond order is roughly cancelled out. In 1,3-butadiene
the result is a weakening of the K bonding between the centers
C 2and C 3 (single bond) and a strengthening of the x bonding
between the centers C ' and.Czand between C 3and C'(doub1e
bond).
The same is true of (7), since here again only one S orbital
and one A orbital are occupied. These qualitative deductions
t c %
4
5
161eV
AOL6eV
S
5
A
L 23eV
5 071eV
A
026eV
S
006eV
2
133eV
A
If we wish to "detect" the orbital sequence as expected for
the canonical MO description, therefore, we must introduce
a perturbation in the manner indicated above. An indication
of the correctness of the derived orbital schemes should be
obtainable from photoelectron (PE) spectra, ESR spectra,
charge transfer spectra, pK, values, etc.
3.1. PE
n-Orbital splitting of 2.13["' and 1.72eV1121is in fact found
for the compounds ( I ) and ( 4 ) respectively: this agrees well
with semiempiricall'. l Z - ' ' - l h l and ah-initio calculations['31.
By analysis of the vibrational fine structure of the first two
bands in the PE spectrum of ( I ) , it has been shown'"l that
the level corresponding to the first band on ionization belongs
to the n , orbital, and that of the second band to the n orbital. The PE spectrum of ( 4 ) also showed1181
that the n,
level is higher than the n - level. As in the case of the diazines,
through-bond interaction effects can also be detected for the
diazanapht halenes"9!
The prediction[201of a through-bond interaction between the
2p orbitals of r,P-dicarbonyl compounds has also been confirmed by PE spectroscopyL2'.221. As in the examples considered above. it is found experimentally here again that the
n + linear combination is above the n - combination (cf. Fig.
6). Another point that seems interesting is that the splitting
An = I n + - n I is practically independent of the dihedral angle
[*] T h e
Fig 5 Model arrangements of lone pairs of electrons und o bonds. T h e
symmetry of L U M O a n d H O M O and thecalculated energy dlfference between
these two orbitals a r e indicated under each fofmula
698
I** J
t o t a l energy is a n observable. whereas the orbital energy is not
Photoelectron spectroscopy measures transition from the ground state
to the ionired state. Measurements of t h i s type therefore o n l y provide iniorrnation a b o u t how the system reacts to the removal of a n electron.
[**I
4. Consequences for the Ground State and for Reactivity
4.1. Fragmentation
A typical result of the bond weakening predicted as the third
conclusion in Section 2 is heterolytic fragmentation, which
was studied in detail by G w h ijt d i 4 ' I .
.
I
+ a-b
a-b;-c-dj-x
,
Fig. 6. Schematic r e p r o e n t a t i o n of through-bond interaction between the
lincnr combinations n . a n d n in irrriir-glyoxal (symmetr) CX,).
+
c=d
RzN-C3Tiz-C2H,-C'H,-X
Q
One important result of the through-bond interaction concept
is the prediction that n is strongly delocalized. This prediction
isconfirmed by susceptibility measurements on the three diazabenzenes137! H-D exchange reactions on pyridine, the diazabenzenes, and the diazanaphthalene~[~'I,on the other hand,
give no indication of a through-bond interaction. This is also
true of basicity measurements on I-azabicyclo[2.2.2]octane
and ( 1 )'3'1 and of the diazabenzene-catalyzed hydrolysis of
2,4-dinitrophenyl a ~ e t a t e l ~ ~ ~ ~ * * * ~ .
R,N=CH,
+ CHz=CH, + Xo
E
From what has been said so far it is easy to understand
that through-bond interaction can also be detected by electronic spectroscopy. The electronic spectra of y,&unsaturated
ketones and amines show the existence of an interaction
between the two chromophores when the conformation of
the molecule allows an interaction riu the 0
The electronic spectra of the dia~anaphthalenes"'~, of pyrazinel'". 301, ,md
,
of dimethylpyra~ine'~".
3 ' 1 can be plausibly
interpreted by the concept of through-bond interaction. Witho u t this interaction, no splitting would be expected for r c * t i i k
and n * t n s in pyrazine and 1,.l-diazanaphthaIene, whereas
values between 0.9and 0.5eV are found. This interpretation
of the spectra is supported by Lih-inifio'"' and semiempirical
calculations[ 1 6 . 3CI. 33.3a1
3.3. Other Investigations
-----)
This reaction can proceed either by a concerted mechanism
or as a two-step process. In the concerted mechanism the
C2-C3 and C'-X bonds are broken simultaneously. In the
two-step process the C'-X bond first heterolyzes. with formation of the cation R,N-CH,-CH,-?H2.
which is
isoelectronic with ( 7 1 .
3.2. Electronic Spectroscopy
Similar conclusions can be drawn for the a,&dicarbonyl compounds. The observed splitting of the two x * t n transitions
for biacetyl IS 1.36eV1353 h i . and the energy difference between
ns and n., as found from the PE spectra is 1.9eV~*'l[**].
(1)
An example of this is the fragmentation of y-amino-halo compounds into olefins and imonium salts.
~
0 of the two cdrbonyi groups. Thus the splitting is 1.9eV
forbiacetyl ( Q =180"C)and 1.6eVforcamphorquinone(8=O0).
The through-bond interaction between K orbitals in y,&-unsaturated ketones[231and I .4-dienes has been demonstrated by
many cxamplesl"- "1 . in s~*n-tricy~lo[4.2.O.O'~~]octa-3,7diene[Zsl and in hypostrophene (tetracyclo[ 5.3.0.02.".03 . ' "3deca-4,8-diene)12", for example, it leads to reversal of the
orbital sequence (A below S) from the through-space interaction[*].
+ x:
.
A
Fig. I . Corrclation diagram for the fragmentation of 1.4-butanediyl ( 7 ) into
h y I enc m ol ecii Ies
t w o i't
As soon as the through-bond interaction dominates in this
cation, the C"C3 bond should be weakened, and the species
can undergo fragmentation. A necessary condition[91for a
fragmentation is that the orbital A (HOMO) should be lower
than S ( L U M O ) ; then and only then is the fragmentation
(cf. Fig. 7). The steric condition'"" for this i s
generallyi*] the antiperiplanar arrangement of the lone pair.
the C'-C3 CI bond, and the unoccupied 2p orbital o n C ' .
[*] According
lo
a n EH caIcuIalion [42]. the birddicals
["I
This rirbital sequence could be the reason why the compounds d o not
iindcrgo intraniolectilar cyAization on irradiation.
[**I
l h e 5plitting At: o f the t w o z * + n (ransitions as calculated from the
of hi;icctyl a n d 'in estimation [30] of the C o u l o m b
.ind enchangc integr;ils h! thc CNDO 2 method IS - 1.6eV.
r**'1 T h e systems investigated a r e all derived from / 7 ) * - or (Xi T h e perturh;ition tntrodiiccd I \ \ c r y \light in comparison with electronic excitation or
i o n i ~ i i t i o i ipotential
i 0 l l i l i l t 1011
should i i o i undergofragmenrntion, since the HOMO is symmetric with respect
t o the plane ofsymmrtry. However. the steric condition 1411 for a fragmcnration I S satisfied
699
(2)
4.2. Possibilities of Stabilization
I t follows from the second conclusion In Section 2 that the
effect of the through-bond interaction can be used for stabilization. This can be illustrated by predictions on the stabilization
of the phenyl cationlJ4! According to EH calculations, the
aryl cations (9)-(12), for example. should be 0.L-0.5 eV
more stable than the phenyl cation.
gations, which demonstrated the consequences of this new
concept for chemical reactions, it has been mainly photoelectron spectroscopy that has shown that through-bond interaction effects can be very considerable. Besides further examples
of the existence of a through-bond interaction in formal 1.4-diradicals and 1.4-dipoles. in electronic transitions. and in ionization, further investigations on systems derived from ( 7 i 2 - ,
but in which the perturbation does not lead to ionization
or electronic excitation, appear to be particularly interesting.
On the theoretical side, the study of the interactions that
were disregarded as a first approximation in the model ( 7 )
could lead to new results.
I am uerr gruteful to Pi-ofessoi-Rould Hoifnzcinn und Pr?f&~oiEdgar Heilbronner, with whom I had the pleasure of working
on tkrough-bond interaction effhcts.
In these compounds an interaction between the positive center
(low unoccupied orbital) and the lone pair on the nitrogen
( ( 9 ) . (12)). the Walsh orbitals of the three-membered rings
((101, (i2)).or the n orbitals of the double bonds ( i l l ] ) .
which are perpendicular to the plane of the ring. should lead
to stabilization.
In 5-pyrimidinylnitrene ( 1 3 ) , the singlet state (n') should
bestabilized in relation to the triplet state by the through-bond
interaction between the lone pairs on the ring nitrogen atoms
and the empty 2p orbital on the nitrene nitrogen.
Just as a positive charge can be stabilized by donor groups
(high occupied orbitals), so acceptor groups (low unoccupied
orbitals) can stabilize a carbanion. Examples of this are the
m-nitrophenyl anion ( 1 4 ) [ 4 5 1and the carbene ( 1 5 ) . In the
latter compound, the singlet state ( 0 2 )should be stabilized
in relation to the triplet state (op)by a through-bond interaction.
Received. Ma! 15. 1974 [ A 1 7 ll:]
German version: Angea. Chem. 86. 770 119741
Translated by Express Translation Service. London
[ I ] R. Hoffmunn. J. Chem. Phqs. 40. 2745 (1964): M: 4 h m . 4 ~ ~ ~ l n i l \ i J l l .
a n d R Hiiffiniriin, J. Amer. Chem. Soc Y1, 2590 (1969).
[2] a ) R . Hoffinunn. 3. Imuniiirri. a n d M' J . f l e h r v . J . Anier. Chcm. Soc
YO. 1499 11968). b) R . Hoffmuriii. Accounts Chem Rca. 4. I 11970: c ) R .
Hoffinunn and W D . Srohrer, Spec. Lect. X X l l l r d Int. Congr Pure Appl.
Chem.. Vol. 1. p. 157. Butterworths. London 1971.
[ 3 ] C 'M R w s a n d R. C. Srurr-. Chem. Commun. lY6,S. 193,
141 T M M i K i ~ i n c ~and
j . D H G e d r . J. Amcr Chem Soc. K7. i O l 3 (1965).
[S] R. Hu/Tmirnn. J. Chem Phys 39. 1397 (196.7): R. H ~ f / ~ i i ~ ,and
i i i M. A
Lipscomb. ihid 36. 2 179. 3489 (1962). 1 7 . 2877 11962)
[ 6 ] M: Kulzelnigg, Angew. C'hem. 85. SSI 11973): Angew. Chcm intcrniit
Edit. 12, 546 (1972).
[7] E Heilhronnrr a n d H . BocL Das HMO-Model1 und seine Anwendung.
Val. I . Verlag Chemie. Weinheim 1968. M J S D c i w r T h e Molcccilsr
Orbital Theorq of Organic Chemistry. M c G r a ~ - H i l l New
.
Y d 1969
[ S ] R. Hofinrimi. S. Sii.uniinoihuii. B G Odd/. and K. Glvrrw.
Chem Soc. Y2. 7091 ( I 970).
Amer
[9] R. Gkirer, Ui-D. Siohrer. a n d R Hoffiiiimri. Helv Chim Acta 55. X9.7
1-l
A = Acceptor,
A e g YO2, C K
13,
wv
r;
A
,I7
A
( 1972).
[lo] Reviews: H. B o d and B. G. Runi.\r,. Angeu Chem K . 773 (1973):
AngeN Chem. internat Edit. I ? , 734 (1973). D. M: 7 i i r w r - , C. Buirr. 3 .
D. t?ilker. a n d C. R. Briinillc Molecular Photoelectron Spectroscopy. Wile).
Interscience, London 1970. S D Mhrlrj. Chem Rcv 7 / . 195 (1971).
[ I I] P. Bischof, J . A. Hu.~hinul/,E. Hi,i/br(miie-. and I .
Lett. 1969. 4025: Y Huiiiudii. .4 ?- Hiruhoiw. M
Bull. Chem. Soc. J a p 46. 2244 (1973).
4.3. Other Examples
An example of the course of a reaction being influenced by
through-bond interaction is provided by the photolysis of
four-membered ring ketones['? In this reaction the lightinduced ring expansion to form the isomeric tetrahydro-7-furylidene, which is not observed for homologous cycloalkanones.
has been explained by through-bond interaction.
The high stereospecificity of the cycloaddition of enol ethers
to tetracyanoethylene via 1,4-dipole~~"~
is also attributable
to the domination of a through-bond interaction in the 1,4dipole[48!
5. Outlook
The concept of through-bond interaction was recognized six
years ago by Hoffmann et a1.""I. Apart from theoretical investi700
_I
f / ( m w i i ~ / Tetiahedron
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I
Prescriptions and Ingredients for Controlled CC Bond Formation with I New synthetic
Organometailic Reagents
[methods (5)
By Manfred Schlosser[*l
I
I
Merits and drawbacks of known carbon-carbon linking procedures are outlined. Two novel
methods are discussed in some detail : the copper-catalyzed alkylation of Grignard reagents
and reactions with allylpotassium compounds. Both methods provide a very efficient access
to saturated, unsaturated, as well as functionally substituted hydrocarbons and moreover permit
an astonishing degree of regio- and stereoselective control of olefin synthesis.
1. Introduction
"The synthesis of most organic molecules can be broken down
into the problems of preparing the carbon framework and
that of the introduction, modification and (or) removal of
various functional groups""'. If we accept this approach, then
the formation of the carbon skeleton is generally the more
demanding part of the task. Fortunately, we can draw on
a large repertoire of diverse methods for its accomplishment,
though very few are without complications or of universal
application. The development of new methods and the improvement of known ones, and the extension of their scope.
will continue to tax the ingenuity of the synthetic chemist
for a long time to come.
2. Possibilities of CC Bond Formation with Organometallic Reagents
As in the past. organometallic chemistry will continue to
play an important role in the further evolution ofdevelopments
in this field. How can its contribution be defined, and what in[*] Prof. D r M. Schlosser
I n s t i t u t d c Chinlie Organique de I'UniversitC
CH-IOO5 Lausanne. Rue d e la Barre 2 (Switzerland)
A n g t w . Chcrn. intcrnal. Edir.
/ Vul.
13 ( 1 9 7 4 ) / Nu. I /
herent advantages does it offer? Let us first consider the various
types of pertinent reactions. They can be roughly divided into
three groups[2.3 ' :
<C-M
CC-Condensation
tC-X
Addition
>C=X
ZC-M
\
$-c(
\
I
;C-C-X-M
I
R~~arrunyrmcwts
offer a most convenient, but insufficiently
exploited route for linking two carbon atoms. Numerous syntheses of cyclic'". and open-chain[" compounds have ulilized
this principle.
Examples of uddirions of carbanionic species to aldehydes.
ketones. and carbon dioxide are too numerous to list. Generally trouble-free, these reactions are nevertheless occasionally
accompanied by side reactions. especially reduction of the CO
double bond by hydrogenation, particularly when there is massive steric hindrance of the carbonyl group. In contrast. carbon
monoxide combines only reluctantly with organo-alkali (and
alkaline earth) metals to forma mixture ofproducts["; however.
its fixation to acetylenes, olefins. or ally1 halides can be readily
701
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