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Anchimerically Accelerated Bond Homolyses.

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Volume 18 + Number 3
March 1979
Pages 173-238
International Edition in English
Anchimerically Accelerated Bond Homolyses
By Manfred T. Reetz[*]
Radical reactions of the type R'-a-b-Rz-+'a-b-R'
+.Rz are feasible if the new bond
formed between b and R' is considerably stronger than the old bond between a and R'.
Furthermore, both the radical fragments formed must be resonance stabilized. An example
of this reaction type is the thermolysis of benzyl trimethylsilylmethyl ethers, in which the
trimethylsilyl group (with empty orbitals) migrates to the oxygen atom (with lone pairs). Assumption of a cyclic reactive intermediate with pentacoordinated silicon explains the observed
intramolecular nature and the negative entropy of activation of such homolyses.
1. Introduction
In organic chemistry substituent effects are generally transmitted through bonds or through space. However, substituents
can also influence reactions by bonding with the reaction
center, thereby stabilizing transition states or intermediates"].
This effect is called neighboring group participation, and may,
but need not, result in rearranged products. If the neighboring
group stabilizes the transition state in the rate determining
step, one speaks of ctnchimeric[**l accelerationrz! Once the
phenomenon of anchimeric participation had been shown
to be general in the field of carbocation chemistry, as evidenced
by numerous examples, the search for analogous effects in
radical processes began in the fifties. It turned out that a
number of unim01ecular~~~
homolytic reactions failed to show
anchimeric effects, in spite of the fact that the neighboring
group in corresponding cationic systems exerts a pronounced
accelerating effect.
For example, in his studies concerning the thermal decomposition of azo compounds ( I ) , Ouerberger et a/.[41excluded
the participation of phenyl groups in the transition state of
radical formation according to (2). The generation of rearranged products was explained by a secondary phenyl group
migration (3) + ( 4 ) .
2
[*] Do/. Dr. M. T. Reetz
Fachbereich Chemie der Universitlt
Lahnberge. D-3550 Marburg (Germany)
Present address: lnstitut fur Organische Chemie und Biochemie der
Universitit,
Gerhard-I)omagk-Strasse I , D-5300 Bonn (Germany)
[**I The term "anchimcric" (Greek ar~chinear, m w o s part) was coined by
Wirisfeir~[2].
Radical decomposition reactions of other azo compounds
also show no appreciable anchimeric effects" - 'I. Among them
are the thermolyses of bornyl and norbornyl derivatives ( 5 t r )
and ( 5 b), respectively, studied by Brrson rt
and Riicliurdr
rt ai.[61.
173
0 Vcrlag Chemie, GmbH, 0-6940 Weinlierm, 1979
0570-O833/79/0202-O170 S Ol.OO/O
R'
4!+.
.........
R2
2
2
having low-lying empty orbitals and/or lone pairs. Such a
system was described by LefJler et a!.["], and was generalized
and mechanistically studied by Martin['*1 in an impressive
series of papers. It was observed that peresters of type (8)
as well as corresponding dibenzoyl peroxides[' 1 undergo
homolytic decomposition to ( 9 ) at different rates. depending
upon the nature of the substituent X (Table 1).
&
R2
( a ) , R' = CH3, R 2 = H; ( b ) , R' = H , R2
=
CH3
According to Bartlett et a!.[81,thermal decomposition of
the tert-butyl peroxynorbornanecarboxylates (6) and (7) proceeds at normal rates (kexo/krndo
= 4.1). Again, the norbornyl
framework induces no nonclassical effects. The search for
anchimeric acceleration in the thermal decomposition of other
peroxyesters as well as in other homolytic processes also
remained fruitless1" 9 - 1 3 ] .
Tahle 1. Kinetic data [ I X ] of the radical fragmentation of terr-butyl perbenzoates (81. Chlorobenaene as solvent.
a
h
C
d
e
'
X
k,,,
AH +
[kcal!mol]
AS
[e.~.]
C6HSS
CH=C(C6H&
I
C(CH,),
H
6.5 x 10'
150
X0
2.4
1 .0
23.0
26.3
28.0
34.4
34. I
- 3.4
-
5.0
0.8
+ 12.5
10.0
-
+
In particular, sulfur gives rise to an enormous anchimeric
effect (see Table 1). Martin postulates a transition state ( 1 0 )
for the rate determining step of the fragmentation of ( 8 a )
in which sulfur coordinates with oxygen, thereby weakening
the 0-0 bond. Resonance forms ( 1 0 ) and ( 1 I ) were proposed to describe the transition state. They explain the radical
reaction path and the pronounced dependence of the fragmentation rate on solvent polarity. It is not quite clear whether
( 1 1 ) is also an intermediateon the way to ( 1 2 ) [ 1 9201.
, However,
there is no doubt that radicals are indeed generated, as shown
by trapping
and ESR studies[*"
Why d o carbenium ions and radicals behave differently?
Apart from possible effects during the formation of carbenium
ions and radicals, Wagner-Meerwein rearrangements proceed
much more rapidly than analogous radical 1,2-sigmatropic
A simple explanation
shifts. The latter are in fact
based on MO theory was published by Zimmerman et ~ 1 . ~ ' ~ ~
in 1961 and later refined by Jaffe and Urchin['61. The cyclic
transition state is treated as a three-center bond with two,
three, or four electrons, which gives rise to appreciable energy
differences for carbenium ions, radicals and carbanions, respectively (see Fig. 1). Therefore, the participation of a neighboring group in a bonding manner during the formation of
radicals will usually be of no energetic advantage.
c)
I
4
$6"5
it
w
I
u
U
it
c
(12)
Fig. 1 . M O scheme for I,2-sigmatropic shifts in carbocations, radicals, and
carbanions.
2. Thermolysis of ovtho-Substituted tevt-Butyl Peroxybenzoates
The above results suggest that appreciable anchimeric effects
may at best be found in compounds that contain hetero atoms
174
+
6-C(CH3)3
::
3. Thermolysis of Alkyl Silylmethyl Ethers
The reactions studied by Martin involve neither three-center
transition states nor complete migration of the assisting group
to the radical reaction center. In contrast, our own studies
started from a completely different standpoint. We speculated
that anchimerically accelerated homolyses of the type
( I 3 j + ( I 4 ) may be possible if the new bond b-R' is considerAiigen,. Chetn. Inr. Ed. Engl. 18.173-180 ( 1 9 7 9 )
ably stronger than the old bond a-R',
generated are resonance stabilized.
and if the radicals
This working hypothesis evolved in I974 from the observation that the dyotropic rearrangement["] of allyl silylmethyl
ethers ( 1 5 ) + ( 1 6 ) (Path A), which proceeds with allyl inversion, is accompanied by a radical reaction[23.241. Thermolysis
(160--190"C)affords 80% of ( 1 6 ) and 20% ofthe "wrong" isomer ( 1 7). Formation of the latter can be explained by postulating the formation of allyl radicals. The overall yield of rearrangement product drops to 60% in the presence of a radical
trapping agent. Importantly, it then contains 97 % of (16).
Under these conditions the greater portion of the suspected
allyl radicals are trapped.
,C H = C D z
CHz
of the rearrangement ( 1 8 ) + (29) were studied (Table 2). First
order kinetics were found in all cases. The yield of rearrangement products is 85-98 %.
Table 2. Kinetic data of radical fragmentation of alkyl silylmethyl ethers
(18) to d o x y compounds (19) at 185°C. Solvent: benzene.
R'
R2
C6H5
CsH5
C6Hs
I-Naphthyl
2,T-Biphen ylylene
2,Z'-Biphenylylene
2,2'-Biphenylylene
2,2'-Biphenylylene
2,2'-Biphenylylene
2,2-Biphenylylene
H
C6H5
R3
k,,,
k 0.05
0.3
0.9
1 .o
1.5
2.2
0.2
0.3
-
<10-4
Within the fluorenyl series (R' + R Z= 2,2'-biphenylylene) the
benzyl and furfuryl ethers react with comparable rates, whereas
the methyl ether ( 1 8 h ) is completely stable under the reaction
conditions. This reflects the relative ability of the substituents
R3 to function as a radical leaving group. Whereas (substituted)
ben~yl['~],a l l ~ l [and
~ ~ ]furfuryl radicals[301 (20), (21 ), and
(22), respectively, are resonance stabilized, this fails to apply
to the high energy methyl radical (23)[29J.
R'
+
R 2 = 2,Z'-Biphenylylene
( 1 5 ) and similar derivatives (R' = R2= C6H5; R' = C6H5,
RZ=CloH8) were found to rearrange with astonishingly low
activation energies (E, z 33 kcal/mol). Thus the hypothesis of
allyl radical formation appeared rather dubious, since the
homolysis of C-0 bonds, generating resonance stabilized
radicals usually requires ca. 50 k c a l / m ~ l [ ~ ~ ] :
For these reasons, a simple radical cleavage of the C-0
bond of ( 1 5 ) (Path B) was excluded[241. In contrast, a
mechanism was proposed in which the silyl group provides
anchimeric assistance by migrating toward oxygen and initiating radical fragmentation (Path C). The formation of the very
strong Si-0 bond (110-130 kcal/mol)[Z61as well as the
generation of resonance stabilized radicals correspond to the
general scheme ( 1 3 ) + ( 1 4 ) . In order to study these unusual
and intruiging effects more closely, a number of alkyl silylmethyl ethers were synthesized which are incapable of allyl
inversion (e.g. benzyl ethers)[271.
3.1. Structural Prerequisites for Radical Fragmentation[",
In order to ascertain the effect of R', RZ,and R3 on the
rate of decomposition of alkyl silylmethyl ethers, the kinetics
A i ~ y e w .C h m . 1111. Ed.
Engl. 18. 173-180 ( 1 9 7 9 )
The relative rates in relation to the nature of the substituent
at the carbon atom (R' and Rz) are equally revealing. Di-arylsubstituted benzyl ethers (18a)-(28g) react at similar rates.
However, ifone of the arene groups is substituted by hydrogen,
no rearrangement is observed; ( 2 8 i ) is stable between 150
and 195°C. At 230°C competing p-eliminations set in, so
that only a lower limit of the theoretically possible radical
fragmentation rate can be approximated: k 1 1 8 , , / k ( l B<
c)
Assuming the intermediate formation of ketyl radicals (see
ESR experiments, Section 3.4), a resonance effect can be invoked once more. Whereas ( 2 4 ) , ( 2 5 ) , and ( 2 6 ) are stabilized by two aromatic substituents, the potential radical (27)
contains only one group capable of resonance. Therefore,
competing b-eliminations leading to ( 2 8 ) , (29), (30), and
( 3 1 ) dominate.
Although the presence of bulky groups is known to significantly affect the rate of unimolecular h o m ~ l y s e s [ ~steric
'~~~~,
acceleration could be ruled out as the dominating factor in
the above cleavage. The tert-butyl derivative benzyl 9-terfbutyl-9-fluorenyl ether analogous to ( 18 c) remains unchanged
at 195°C for several days. In sharp contrast, the half-life
of the rearrangement (28c) + ( 2 9 c ) under identical conditions amounts to a mere 18 minutes. This underlines the
dramatic influence of the trimethylsilyl group and is in line
with the low activation energy of 32 kcal/mol and the negative
175
(lea)
--
+
i(c H3 )3
O/~-O\S
(18b’
0;19,i(cH3)3
propy1)methyl radicals are in fact formed, they should undergo
rapid rearrangement to 4-butenyl radicals, as described by
K o ~ h i [ ~Thus,
~ ] . two different trapping products are to be
expected. Indeed, methylcyclopropane ( 3 2 ) and 1 -butene (33)
were identified in a ratio of 1 : 74.
R
+ R
= Z,Z’-Biphenylylene
3.4. ESR Experiments[331
H 3 C 0
/
H5Cs,
+
@-Si(CHa)3
(28)
(29)
CHZCBHS
HF-0’
S i ( CH3)3
(18 i)
\
OCH2Si(CH3)3
9
+ O C - H
(30)
(31)
entropy of activation AS* = - 8.6 e. u. for (18c)+(19c). The
entropy factor suggests a reduction in the number of degrees
of freedom in the transition state.
3.2. Crossover Experiments[331
The intermediate formation of ketyl radicals was proved
directly by ESR spectroscopy. Heating ( l a c ) to 180°C in
an ESR spectrometer results in a well resolved spectrum with
uH=3.48, 0.83, 3.78 and 0.83G for the protons H-I, H-2,
H-3, and H-4 of the fluorenyl ring in (26). The values correspond to those of Neurnann et u ~ . [ ~ ~who
I , prepared (26) by
a different route. Under identical conditions the rearrangement
product ( 1 9 c ) does not give rise to ESR signals. This control
experiment again shows that the theoretically possible reverse
reaction is not responsible for radical formation.
180°C
Crossover experiments using labeled compounds show that
the silyl groups migrate 100 % intrumolecularly. On the other
hand, the benzyl groups undergo cu. 2 5 % intermolecular
migration. These observations are also in accord with the
postulated radical mechanism.
3.3. Radical Trapping Experiments[331
If the thermolysis of ( 1 8 ) indeed leads to R3 radicals, it
should be possible to prove this by the use of appropriate
trapping agents. The therrnolysis of ( 1 8 c ) in the presence
of an excess of 9,lO-dihydroanthracene does in fact lead to
toluene (59 %) and anthracene. The thermolysis of independently synthesized rearrangement product (19 c) does not
generate benzyl radicals, i.e. no trace of toluene is observed
under identical conditions.
180°C
119 c i
3.5. Solvent Effects[331
In order to determine the influence of solvent polarity on
the rate of decomposition, kinetic measurements were carried
out in solvents having different ET values (ETvalues are empirical parameters of solvent polarity.) Utilizing (18 c) at 185“C,
the results point to a very small effect: krel (based on benzene)
is 1.4 in o-dichlorobenzene and 2.2 in acetonitrile. The fact
that the activation parameters of (1 8 c) + ( I 9 c ) do not change
significantly in going from benzene to acetonitrile as solvent
is mechanistically significant[231.The intermediacy of ions
or zwitterions is quite unlikely.
3.6. Influence of Substituents and Secondary Kinetic Isotope
Effects in the Radical Leaving Group[”, 3 3 , 3 5 1
H3c0
(28)
A similar experiment was performed with the cyclopropyl
methyl ether f l 8 j ) which reacts considerably slower. If (cyclo-
176
In order to examine possible electronic effects in the radical
leaving group, a Hammett study utilizing (substituted) benzyl
ethers (18c)-(18f) was undertaken. The krelvalues lie closely
together (see Table 2). Whereas the application of o constants
leads to a good correlation (p = -0.92), no straight line results
with D+ values (Fig. 2). Thus, radical formation has not
advanced to a significant degree in the transition state, i. e.,
the C-0 bond is not stretched much. The values can be
explained on the basis of a small polar effect‘”’.
Aiigrw.
Chem. l i l t . Ed Eiigl. 18, 173-180 ( 1 9 7 9 )
1+
germyl groups migrate faster than the analogous silyl moieties1381.Apparently the strength of the new bond to oxygen
plays an important role. The Ge-0 bond (ca.85 k ~ a l / m o l [ ~ ~ q
is weaker than the Si-0 bond (110-130 kcal/mo1)[261.
1.8 -
1.6 1.1-
3.9. Mechanism of Radical for ma ti or^[^^.^^'
1.2 -
- 1.0 c
cn
It is clear that the presence of the silyl group induces radical
formation under mild conditions. But how does this neighboring group act? Rate-determining complexation between silicon
and oxygen leading to the generation of a reactive intermediatec41 having cyclic three-membered structure (38 a ) in
which the R3-O bond is weakened [see resonance form
(38b)], is in line with all presently known observations. This
may involve d,-p, interaction. Rapid homolytic decomposition
then leads to the complete formation of the strong Si-0
bond and the generation of resonance stabilized radicals. If
the potential radical is not sufficiently stabilized [for example
in the case of the methyl ether ( 1 8 h ) ] , the complex merely
reverts back to starting material, since an additional energy
barrier would have to be overcome (see Fig. 3).
l
X
NO,
0.6
I
I
-0
I
I
- 6 -L
I
-2
I
I
I
I
I
0
2
L
6
8
\
\
a, d*Fig. 2. Hammett plot for thermolysis of the (substituted) benzyl ethers (18 c )
( l 8 f ) to (19c)---(19f) (see Table 2).
that
The above results are in line with the
the secondary kinetic deuterium isotope effect at 150°C is
rather small: k(18cl/kij8rl=kH/ko=1.04-tO.03.
3.7. Stereochemistry at the Migrating Silyl Groupz4,3 6
Although ally1 ethers react only in part by a radical
mechanism [cf. e. g. ( 1 5 ) ] , the stereochemistry at the silicon
atom was investigated for the total rearrangement process.
For this purpose a complete Walden cycle involving ( 3 4 )
and ( 3 5 ) , which contain an asymmetric silicon atom, was
transversed. It turned out that the silyl group migrates from
carbon to oxygen with >96 % retention of configuration. This
means that not only the concerted rearrangement, but also
the radical rearrangement, proceeds stereospecifically.
R,
,CHzCH=CHz
~
C HzC H=C Hz
\
&*
4
RX-0
;Y
-0,
/I\
si*
/I\
(34)
(35)
Reaction coordinate
-
Fig. 3. Possible reaction profile for thermolysis of the ethers ( 1 X r . ) and
(18h).
\ 56
;Si
=
Methyl(a-naphthy1)phenylsilyl
3.8. Replacement of Silicon by Germanium[371
In order to study the role of the neighboring group more
closely, the (germylmethyl) ether ( 3 6 ) analogous to (18 c)
was synthesized and thermally rearranged to (37). ( 3 6 ) reacts
much slower than the silicon analogue: k ( 3 6 , / k 1 1 8 c , z 4 x
At first glance this seems surprising, since in sigmatropic
1,5 rearrangements involving the cyclopentadienyl framework
Anyew. C'hcm.
lnt.
Ed. Engl. 18.173-180 (19791
The formation of species (38) with pentacoordinated silicon
is not only consistent with the intramolecularity and stereochemistry of silyl group migration, but also with the negative
entropy of activation, the small kinetic deuterium isotope
effect, and the results of the Hammett study. However, the
existence of the reactive intermediate ( 3 8 ) cannot yet be
proved beyond all doubt. Concerted fragmentation of ( I 8)
into radicals does not involve any local energy minimum
along the reaction coordinate. The difference between the
two-step and concerted mechanism is thus rather small. In
both cases the motion of the neighboring group is far advanced
compared to that of the leaving group. The pronounced affinity
of silicon for oxygen is the driving force of the reaction and
is responsible for the relatively low activation energy. The
germanium analogue ( 3 6 ) rearranges slower, since the Ge-0
bond is considerably weaker.
177
Since alternative mechanisms have been discussed in detail
331, only two of the most important possibilities
are mentioned here. Path A involves rate-determining 3-elimination affording carbene (39). Related a-eliminations are
but involve formation of stabilized nucleophilic
carbenes. Path A appears unlikely because trapping experiments
fail to afford carbene add~cts[’~!
and similar systems1441.Such is not the case in the benzhydryl
derivatives.
This effect was not found in radical reactions, where the
benzhydryl and fluorenyl compounds [e.g. (28 a ) and (18 c ) ]
fragment at comparable rates. This is not surprising, since
intermediate (38) has an “escape route”, namely homolytic
fragmentation which does not involve concerted backside
attack. The strikingly different behavior of (18) and ( 4 0 )
in going from fluorenyl to benzhydryl derivatives is not consistent with an jdide or a carbene mechanism. However, it is
in line with the formation of structurally related reactive intermediates (38) and ( 4 2 ) .
It remains to be explained why the rearrangement
(18) + ( 1 9 ) proceeds with such surprisingly high yield, i. e.,
without the formation of dimerization products such as bibenzyl. Relevant is the fact that preferred cross-dimerization is
a common phenomenon in radical chemistry. In our case
a somewhat enhanced concentration of ketyl radicals which
trap just about all the benzyl radials could be the cause.
It is more difficult to disprove the direct ylide mechanism
(Path B), which represents anchimerically accelerated homol3.10. Comparison of Thermal, Electron-Impact Induced, and
ysis, just like the previously discussed mechanism involving
Photolytic Behavior
( 3 8 ) . The difference between the ylide ( 4 0 ) and the three-membered ring ( 3 8 ) has to do with the fact that in the former
Surprisingly, silylmethyl- and germylmethyl ethers (18) and
there is no bonding between silicon and carbon. Is it possible
(36) as well as related compounds show very intense mass
to detect this difference experimentally? Perhaps the formation
spectral peaks at e/m= [M-CH2C6Hs]+[461. Since electronof a genuine oxygen ylide would result in a larger solvent
impact induced cleavages of C-0 bonds with elimination
effect.
of radicals and fixation of charge on the oxygen moiety are
The main counterargument is more subtle. We observed
high energy processes which rarely afford intense fragment
that the dyotropic valence isomerism‘431 of silyl methylsilyl
ions[471,a neighboring group effect was postulated. Migration
ethers (41 a ) + ( 4 4 a ) occurs without radical f ~ r m a t i o n [ ~ ~ , ~ ’ ~ .
of the metal from carbon to oxygen prevents charge localizaThe silyl groups interchange their places strictly intramolecution.
larly at 160-190°C with retention of configuration, whereby
an equilibrium is reached. However, the activation parameters
R,
/cH2C6H5
R\c;q.-6H5
-+ R\@
/c-o\
and solvent effects are almost identical to those of the radical
R;C-0
R/ ?.,
R
X(CH3)3
fragmentation (18) + (19). This suggests a similar transition
X ( CH3)3
X
state, namely rate-determining coordination between silicon
+ H & - H ~
( l a ) , X = Si
and oxygen. It necessarily follows that the energy surface
(36), X = G e
is characterized by a double minimum ( 4 2 ) $ ( 4 3 ) . A
mechanistically significant observation is that the benzhydryl
Indeed, further studies utilizing DAD1 spectra show that
derivative (41 b ) is completely stable under normal thermolthis new type of electron-impact induced ether cleavage occurs
ysis conditions (160--190°C). Only at ca. 230°C does slow
if the transition state ( 4 5 ) is stabilized by anchimeric participasilyl-silyl exchange occur, along with considerable amounts
tion of the silyl or germyl
Relevant and significant
of decomposition products.
is the observation that in case of the tert-butyl analogue
(CD3)3
of ( 1 8 c ) (C in place of Si) the genesis of [M-CH2C6H5]+
Si
does not come about, in spite of the fact that a stabilized
R
R,Y
,, -0 /Si(CD3)3
R\/C-o@
/Si(CD3)3
R$L@
benzyl radical could be generated. Thus, in this case there
R \a/
R
\Si(CH3)3
S i ( C H3 ) 3
Si
is a strict correspondence between thermal and electron-impact
(CH3)3
(43)
induced behavior.
(41)
(42)
These and other experiments confirm the decisive role of
S:l(CD3)3
the silyl and germyl groups and also demonstrate that generation ofions by an alternative migration of the C9-C1O a-bond
of the fluorenyl skeleton to the oxygen, which involves ring
expansion, offers no competition. The observation that hydrogen scrambling processes--which are otherwise very evident
in
the low energy spectra of arenes-do not take place (deuterThe dramatic difference between (41 a ) and ( 4 1 b )
ated ally1 derivatives loose only C3H3D’) also suggests a fast
( k , 4 1 b ) / k 1 4 i , ) ~ 1 0 - ’ ) can only be explained by a steric model.
process facilitated by migration of the adjacent metal
The motion of the silyl group in (42 b ) from oxygen to the
groups[48!
backside of the carbon atom in the reactive intermediates
Thedifferingrates ofthermalfragmentation of ( 1 8 c ) , ( 1 8 h ) ,
( 4 2 ) and ( 4 3 ) is rapid only if the R-substituents do not
and ( 1 8 i) have no quantitative parallels in the mass spectrum.
give rise to steric inhibition, e. g., as in flat fluorenyl derivatives
I?
2
-
d
11
178
A n g m . Chem. I n t . Ed. Engl. 18, 173-180 ( 1 Y 7 Y )
The radical R3 liberated on electron-impact induced decomposition is not necessarily resonance stabilized. Moreover,
just one stabilizing group on the stationary C atom is sufficient
[cf. ( l s i ) ] . The germyl compound shows effects of similar
magnitude although it undergoes thermal fragmentation much
slower than the silicon analogue.
It was of interest whether the above anchimerically accelerated ether cleavages can also be induced photochemically.
Such a process would require specific photochemical excitation
of the C-Si bond. The direct excitation of a certain o-bond
is generally difficult. ( J 8 c ) was chosen as experimental substancerzx1
because the fluorenyl skeleton is a suitable chromophore in which the absorbed light energy can "flow" into
the a-positioned o-bond. Experiment (Hg high-pressure lamp,
Pyrex filter) showed that the C-0 bond, rather than the
C-Si bond, reacts preferentially[281: normal ether cleavage
was detected. The experiments underline the fundamental difference between thermal and photolytic behavior.
4. Thermolysis of Benzyl(silylmethy1)aminesand Benzyl Silylmethyl Thi~ethers[~~,
511
If the hetero atom with a lone pair does in fact play a
crucial role in the radical fragmentation of alkyl silylmethyl
ethers, then the analogous amines should likewise undergo
anchimerically assisted homolyses. Of course, the strength
of the Si-N bond amounts to only ca. 85 kcal/mol[sO].The
radical fragmentation of ( 4 6 ) does indeed proceed more slowly
than that of the corresponding ether: k146,/k(18c,
z 10- '. In
the presence of 9,lO-dihydroanthracene as a radical trapping
agent, 7 6 % of toluene is formed. A mechanism according
to ( 4 6 ) 4( 4 7 ) + (20) is therefore likely[49! In the absence
of radical trapping agents a number of products are formed,
among them bibenzyl(15 %). In its mass spectrum ( 4 6 ) shows
a low-intensity [ M -CH2C6H5]+ peak[48! in contrast to the
oxygen analogue (18 c ) . Hence there exists a parallel between
thermolysis and electron impact.
(47)
R + R = 2,Z'-Biphenylylene
H~CCGH
(28)
~
These results underscore the central role of the heteroatom,
with the strength of the new bond to silicon being of prime
importance for the subsequent reactions. Brook has noted
similar effects in the base-induced sigmatropic silyl migration
in a-silyl alcohols and a-silylamines; the latter rearrange
considerably slower[s01.
The thioether (48) analogous to ( 1 8 c ) was also synthesized
and thermolyzed in order to study possible anchimeric
effects["'. Although the mechanistic studies have not been
completed, it is clear that the reaction ( 4 8 ) -+ ( 4 9 ) is much
slower than (J8c)+(19c): k ~ 4 8 1 1 k ~ l Xx r l ~ 6 Again, this
observation supports the working hypothesis according to
which the strength of the new bond to the neighboring
group is decisive for the rate of radical fragmentation:
Es,-o - Es, -sz30 kcal/m01[~*~~~1.
Angew. Chem. I n t . Ed. Engl. 18. 173-180 (1979)
5. Summary
The rearrangements described in Sections 3 and 4 are the
first known examples of anchimerically accelerated homolyses
involving three-center bonds in the transition state as well
as the migration of the neighboring group to the radical
reaction center. The conjecture that such processes are possible
if a strong new bond is made and if both radical fragments
are resonance stabilized has been confirmed so far. Rate-determining complexation between the neighboring group having
empty orbitals and a heteroatom having a lone pair is presently
a useful working hypothesis. Some degree of correspondence
is observed in many cases between thermal and electron-impact
induced behavior. Further parallels are found in recent studies,
e. g. on thermolysis and behavior of silyl methylacetates in
the mass ~pectrometer'~'.541. It remains to be seen whether
groups other than SiR3 (e.g., phosphorus or boron) are also
capable of anchimeric assistance.
This review w ~ written
~ s
by the author during his stay as
guest professor at the Department of Chemistry, University
of Wisconsin (Madison). I would like to thank the members
of the department for their hospitality and for interesting discussions. Particular thanks are due to my co-workers Dr. M . Kliment, Dr. N . Greg Dip1.-Chem. A . Mnarouji, and M . Plachky.
Our work was generously supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.
Received: March 20. 1978 [A 258 IE]
German version: Angew. Chem. 91, 185 (1979)
B. Capon, S. P. MrManus: Neighboring Group Participation. Vol.
1. Plenum Press, New York 1976.
S. Winstein, C . R . Lindegren, H . Marshall, L. L. Ingraham, J . Am.
Chem. SOC.75, 147 (1953).
Several examples of anchimerically accelerated bimolecular homolyses
are known: P. S. Skell, K. J. Sheu in J. K. Kochi: Free Radicals.
Vol. 11. Wiley, New York 1973, p. 809.
C. G. Overberger, J . Gainer, J . Am. Chem. SOC. 80, 4561 (195%).
J . A. Berson, C. J . Olsen, J . S . Walia, J. Am. Chem. Soc. 84, 3337
( I 962).
J . H i m , C . Riichardt, Tetrahedron Lett. 1970, 3095.
S. Seltzer, S. Scheppele, J. Am. Chem. SOC.90, 358 (1968).
P. D. Barrlett, J . M . McBride, J. Am. Chem. Soc. 87, 1727 (1965).
C . Riichardt, R. Hecht, Chem. Ber. 98, 2460 (1965).
C. Riichardt, R. Hechl, Chem. Ber. 98, 2471 (1965).
L. A. Singer in D. Swernr Organic Peroxides. Vol. I, Wiley Interscience,
New York 1970, p. 265.
C. Ruchardr, H . Traurwein, Chem. Ber. 98, 2478 (1965).
C . Ruchardr, Fortschr. Chem. Forsch. 6, 251 (1966).
Review: J. W Wilt in J. K. Kachi. Free Radicals. Vol. I, Wiley, New
York 1973, p. 333.
H . E. Zimrnerman, A . Zweig, J . Am. Chem. SOC.83, 1196 (1961).
N . F . Phelan, H . H . Jaffk, M . Orrhin, J . Chem. Educ. 44, 626 (1967).
J. E . Lefler, C . C. Pefropoulos, R . D. Faukner, Chem. Ind. (London)
1956, 1238.
Review: J . C. Martin in J . K . Kochi: Free Radicals, Vol. 11, Wiley,
New York 1973, p. 493.
P. Liuant, J . C . Marrin, J . Am. Chem. Soc. 98, 7851 (1976).
W A. Prior, H . W Hendrickson, J . Am. Chem. SOC.97, IS80 (1975).
J . C . Marrin, P. Liuant, M . M . Chau, Abstr. Pap. 174th Nat. Meeting
ACS, Chicago 1977, Orgn. 135.
M . 7: Rrerz, Angew. Chem. 86, 416 (1974); Angew. Chem. Int. Ed.
Engl. 13, 402 (1974).
179
1231 M . 7: Reetz, Chem. Ber. 110, 954 (1977).
[24] M . 7: Reetz, Chem. Ber. 110, 965 (1977).
[25] a) K . Kwart, S . F. Sarner, J . Slutsky, J. Am. Chem. Soc. 96, 5234
(1973); b) M . J . Mobra, E. Ariza, An. R . Soc. Esp. Fis. Quim., Ser.
B, 56, 851 (1960).
[26] A . G. Brook, Ace. Chem. Res. 7, 77 (1974).
[27] M . 7: Rretz, M . Kliment, Tetrahedron Lett. 1975, 797.
1287 M . 7: Rertz, M . Kliment, N . Grrif, Chem. Ber. 111, 1083 (1978).
1291 D. Griller, K . U . Ingold, Ace. Chem. Res. 9, 13 (1976).
[30] Furfuryl radicals have been generated by an alternative method: L.
D . Kispert, R. C. Quijano, C. U . Pittman, J . Org. Chem. 24, 3837 (1971).
[31] W P . Neumann, B. Schroeder, M . Ziebarth, Justus Liebigs Ann. Chem.
1975, 2279.
[32] H.-D. Beckhaus, C . Ruchardt, Chem. Ber. 110, 878 (1977).
[33] M . 7: Reetz, N . Greif, M . Kliment, Chem. Ber. 1 1 1 , 1095 (1978).
1341 J . Kochi, P. J . Krusic, D. R . Eaton, J. Am. Chem. Soc. 91, 1877 (1969).
[35] M . 7: Reetz, unpublished results 1977.
1361 M . 7: Reetz, Tetrahedron Lett. 1976, 817.
[37] M . 7: Reetz, N . G r e f , to be published.
[38] A. l! Kisin, l! A . Korenetisky, N . M . Sergeueti, Y. A . Ustynyuk, J.
Organomet. Chem. 34, 93 (1972).
[39] E. A . Ebsw'orth in A . G. MacDiarmid. The Bond to Carbon. Vol. I,
Part I, Marcel Dekker, New York 1968, p. 46.
1401 N . G r e g Dissertation, Universitat Marburg 1977.
[41]
[42]
[43]
1441
[45]
[46]
[47]
[48]
[49]
In the thermal rearrangement of u-silyl ketones to vinyl silyl ethers
the intermediacy of reactive pentacoordinated silicon species is likely:
H . Kwart, W E. Barnette, J. Am. Chem. SOC.99, 614 (1977).
A. G . Brook, P . J . Dillon, Can. J. Chem. 47, 4347 (1969).
Definition: M . 7: Reetz, Tetrahedron 29, 2189 (1973); review: M . 7:
Rertz, Adv. Organomet. Chem. 16, 33 (1977).
M . 7: Reetz, M . Kliment, M . Plachky, Chem. Ber. 109, 2716 (1976).
M . 7: Reerz, M . Kliment, M . Plachky, Chem. Ber. 109, 2728 (1976).
H . Schwarz, M . Kliment, M . 7: Reetz, G . Holzmann, Org. Mass. Spectrom.
11, 989 (1976).
H . Schwarz, R. WoKYchiitz, Org. Mass. Spektrom. 11. 773 (1976).
H . Schwarz, M . 7: Reetz, Angew. Chem. 88, 726 (1976); Angew. Chem.
Int. Ed. Engl. 15, 705 (1976); H . Schwarz, C. Wesdemiotis, M. 7: Reetz,
J. Organomet. Chem. 161, 153 (1978).
M . 7: Reetz, A . Maaroufi, to be published.
[SO] A . G . Brook, J . M . Dnsf, J. Am. Chem. SOC.96, 4692 (1974).
[Sl] M . 7: Rretz, M . Kliment, unpublished results 1979.
[52] A. Haas, Angew. Chem. 77, 1066 11965); Angew. Chem. Int. Ed. Engl.
4, 1014 (1965).
[53] M . 7: Reet;, M . Kliment, Tetrahedron Lett. 1975, 2909.
[54] M . 7: Rertz, N . Greif, Angew. Chem. 89, 765 (1977); Angew. Chem.
Int. Ed. Engl. 16, 712 (1977); cf. A . R. Bassindale, A. G. Brook, P. F .
Jones, J . M . Lennon, Can. J . Chem. 53, 332 (1975).
Liquid Column Chromatography with Chemical Derivatizations after
Separation
New analytical
By Georg S c h w e d t [ * ]
Modern liquid column chromatography (high-pressure liquid chromatography, HPLC) has
evolved in the last few years into a highly efficient and versatile separation technique. The
selectivity of an analytical process that depends upon a previous separation step can in
many cases be increased considerably by chemical derivatizations after the separation. In
addition, lower detection limits can be achieved in this way than in detection without derivatization. The physicochemical principles of these combined processes involving chromatographic
separation and chemical derivatization prior to detection (coupling of HPLC and a reaction
detector) are presented and discussed. The state of development is outlined, with a survey
of the more important applications so far described in the literature.
1. Introduction
The essential problems of modern analytical chemistry are
to increase the sensitivity and selectivity and to improve the
detection limits of analytical processes used to detect and
determine trace elements and low contents of organic substances in complex mixtures. During the last two decades
progress in this direction has been made possible mainly
with the help of physics and physical methods of measurement,
namely by novel analytical applications of physical effects
and by improvements in the available apparatus.
While the development of analytical chemistry up to about
1960 was mainly characterized by the analytical application
of chemical reactions and the discovery of new reagents for
[*] Prof. Dr. G. Schwedt
Analytische Chemie, Fachbereich 8 der Gesamthochschule
Adolf-Reichwein-Strasse 2, D-5900 Siegen 21 (Germany)
180
the classical gravimetric, volumetric, colorimetric, and photometric methods, it seems that the most significant part in
further development is nowadays played by physical methods
of measurement, for example spectroscopic methods.
In chromatography too, improvements in the equipment,
above all in the fields of gas and liquid chromatography,
have been responsible for a decisive improvement in separation
performances and hence in selectivity. Technological advances
in the manufacture of column packings with particle diameters
below 10 pm, and improvements in pumping techniques that
enable a constant pulsation-free delivery of the mobile phase
against high pressures, have in the last ten years transformed
classical liquid chromatography into the incomparably more
efficient high-pressure liquid chromatography (HPLC). However, compared to the analytical side of HPLC, the detection
stage has, as yet, not reached the same high level of sophistication as far as sensitivity, detection limits, and selectivity are
concerned. Nevertheless, developments are appearing that
. A I I ~ + w . Chem. Int. Ed. Engl. 18, 180-186 ( 1 9 7 9 )
0 Verlag Chemie, GmbH, 0-6940 Weinheim, 1979
0570-0833/79/020?-0170
S 01.00/0
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