Organosilicon Chemistry: 2. Plenary Lectures Presented at the Second International Symposium on Organosilicon Chemistry

247 Pages·1969·7.555 MB·English

UNION INTERNATIONALE DE
CHIMIE PURE ET APPLIQUΙE
DIVISION DE CHIMIE ORGANIQUE
et
UNIVERSITΙ DE BORDEAUX
FACULTΙ DES SCIENCES
CHIMIE
ORGANOSILIGIQUE
2
Conferences pléniéres presentees au
DEUXIΙME SYMPOSIUM INTERNATIONAL SUR LA
CHIMIE DES COMPOSES ORGANIQUES DU SILICIUM
Bordeaux, France
9-12juillet 1968
LONDRES
BUTTERWORTHS
INTERNATIONAL UNION OF
PURE AND APPLIED CHEMISTRY
ORGANIC CHEMISTRY DIVISION
in conjunction with
THE UNIVERSITY OF BORDEAUX
FACULTY OF SCIENCE
ORGANOSILICON
CHEMISTRY
2
Plenary L·clures presented at the
SECOND INTERNATIONAL SYMPOSIUM ON
ORGANOSILICON CHEMISTRY
held in Bordeaux, France
9-12 July 1968
LONDON
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The contents of this hook appear in
Pure and Applied Chemistry, Vol. 19 Nos. 3-4 (1969)
Suggested U.D.G. number: 547.245
©
International Union of Pure and Applied Chemistry
1969
Standard Book Number 408 89471 7
Printed in Great Britain by Page Bros (Norwich) Ltd., Norwich
PREFACE
Le deuxiιme Symposium International sur la Chimie des Composes
Organiques du Silicium s'est tenu α Bordeaux du 9 au 12 juillet 1968 sous
les auspices de TUnion Internationale de Chimie Pure et Appliquιe et a
ιtι organise par le Laboratoire de Chimie Organique de la Facultι des
Sciences de Bordeaux. Ce deuxiιme Symposium a rιuni 383 participants
provenant de 18 pays. II ιtait place sous la Prιsidence d'Honneur du
Professeur H. NORMANT Membre de l'Acadιmie des Sciences, les
Professeurs R. CALAS et J. VALADE ιtaient respectivement President et
Secretaire General du Comitι dOrganisation.
Les seances ont eu Heu α la Facultι des Sciences de Bordeaux et ont permis
de presenter 13 conferences plιniιres et 92 communications rιparties en
3 sections
Un Comitι de Coordination du Symposium comprenant les Professeurs
Κ. Α. ANDRIANOV (URSS), V. BAZANT (Tchιcoslovaquie), A. BROOK
(Canada), R. CALAS (France), C. EABORN (Grande-Bretagne), J. VALADE
(France), U. WANNAGAT (RFA), R. WEST (USA), s'est rιuni pour
envisager Tavenir de ces reunions. Le 3ιme Symposium sur la Chimie des
Composes Organiques du Silicium aura heu en 1972 α MADISON (USA);
le Professeur R. WEST a bien voulu accepter d'en assurer l'organisation.
Professeur J. VALADE
Facultι des Sciences de Bordeaux
Bordeaux.
Ao٧t 1968
vil
ANIONIC REARRANGEMENTS OF
ORGANOSILICON COMPOUNDS
ROBERT WESTf
Department of Chemistry
University of Wisconsin^ Madison, Wisconsin, U.S.A.
In comparison to the rearrangements of carbonium ions so long known and
well studied in organic chemistry, anionic rearrangements have been much
less thoroughly explored^. Recently, we have found that certain anionic
rearrangements proceed with unprecedented speed when the migrating
group is an organosilicon moiety. The realization of the very high mobility
of silyl substituents in anionic rearrangements opens a new area of organo-
metallic chemistry, and at the same time offers a highly useful technique
for the study of anionic rearrangements generally.
Types of anionic rearrangements
We shall be concerned initially and principally with 1,2-anionic rearrange­
ments, in which the migrating group moves from one atom to an adjacent
bonded atom bearing a negative charge :
R—X—Y- o > -X—Y—R (1)
Most of the best-known anionic rearrangements are of this 1,2 type. Rear­
rangements involving migration to more remote anionic sites, sometimes
classed together as "Smiles Rearrangements",^ will be dealt with briefly
later.
The classical examples of 1,2 anionic rearrangement are the Wittig
rearrangement^.* involving migration from oxygen to carbon:
RLi
PhCHaOR PhCHOR .Ph—CH—O- Li+ (2)
Li+
R
And the Stevens rearrangement,^ in which the substituent migrates from
nitrogen to carbon:
O
+ OH- -A—CH—:
ArCOCHaNMea > Ar- NMes ArCOCHNMez (3)
Δ L I
Me
t Work described here was supported by the Air Force Office of Scientific Research (SRC )
O.A.R., U.S.A.F., Grant No. AF-AFOSR 1061-66.
291
P.A.C.—Β
ROBERT WEST
These and similar rearrangements in which the atoms X and Y in Eq. 1
are of different kind will be called heter o atomic.
Recently, 1,2 anionic rearrangements have been observed in which the
atoms X and Y are of the same kind (though they may be differently sub­
stituted). These we will term homoatomic 1,2 anionic rearrangements. At
present just two examples are known: the ethane rearrangement, discovered
independently by Grovenstein^.^ and by Zimmerman and his co-workers^»^
in 1957:
Μ -
AraC—GHaCl ArgC—GHg ArgG—GHgAr (4)
And the hydrazine rearrangement, observed by Bailey and West in 1964:^0
_N—Ν Ν—Ν, (5)
Η 'S¡ 'si Si'
(Si = triorganosilyl group)
The silylhydrazine rearrangement: origins
The history of the anionic rearrangement of silylhydrazines provides an
excellent example of the role of chance, coincidence, and even unrecognized
error in scientific discovery. In 1964, Robert Bailey of our laboratories set
out to prepare N,N'-bis(trimethylsilyl)hydrazine (I), as a starting material
for oxidation to the disilyldiazene, MesSi—N=N—SiMes. (Gompounds
of the latter type were then unknown but have recently been synthesized
by a quite different route^^^.) The synthesis of compound I from trimethyl-
chlorosilane and hydrazine was reported in 195711, a part of the now classic
work of Wannagat and his students in the area of silicon-nitrogen
chemistryi2:
3N2H4 + 2Me3SiGl MeaSiNH—NHSiMea + 2N2H5+GI- (6)
(I)
The disilylhydrazine was reported to have the Ν,Ν' structure (I) as shown.
We found that the product, after careful distillation through a platinum spinning
band distilling column, showed only one Ν—Η resonance and one Si—G//3
resonance in the n.m.r. spectrum, indicating that it was a single isomer.
In the course of characterizing the disilylhydrazine, it was treated with
two equivalents of n-butyllithium and then with two equivalents of methyl
iodide, in order to convert it to the symmetrical dimethyl derivative. The
product distilled over a relatively narrow boiling range, but gas chromato­
graphic analysis showed it to be a nearly 1:1 mixture of two compounds,
identified as the N,N and N,N'-isomersio:
GH3
l.)2RLi I
MeaSiNH—NHSiMea > MesSiN—NSiMea
2.)2CH3l I
GHs
+ (Me3Si)2N—N(GH3)2 (7)
292
ANIONIC REARRANGEMENTS OF ORGANOSILICON COMPOUNDS
It seemed, then, that a new rearrangement had been observed!^. However,
we learned at this point by private correspondence with Professor Wannagat
that he now beheved the disilylhydrazine product of equation (6), originally
thought to be pure I, was actually a mixture of the Ν,Ν' and N,N isomers,
I and II. Although our analytical sample after purification had been
isomerically pure, we nevertheless repeated the preparation according to
reaction (6), this time distilling the product rapidly in glass apparatus. Under
these conditions the bis (trimethylsilyl)hydrazine is indeed a mixture of
nearly equal amounts of the two possible isomers!
MesSiCl
N2H4 > (Me3Si)2N—NH2 + MesSiNH—NHSiMea (8)
(Π) (I)
We were able to separate I and II by preparative gas chromatography,
and to show that the separated compounds both gave the same 1:1 mixture
of dimethylated isomers upon treatment with w-butyllithium and methyl
iodide:
l)2RLi l)2RLi
(Me3Si)2N—NH2 > (Me3Si)2N—NMe2 < (Me3SiNH)2
(II) 2)2CH3l 2)2CH3l (I)
+
(Me3SiNGH3)2 (9)
Rearrangement does, therefore, take place with either I or II. But why
had our original, carefully purified sample consisted only of one isomer?
By coincidence, we had chosen the very purification conditions which led
to the selective decomposition of the Ν,Ν' isomer—namely, heating with
platinum in the spinning band distillation column^^. Thus our original
sample was actually pure II, rather than I as we had assumed from the
literature reportsH-i^.j
In order to avoid the troublesome separation of isomers I and II, for
further study of the anionic rearrangement reaction N,N'-bis(trimethylsilyl)
methylhydrazine. III, was employed. This compound can also be made
following directions given by Wannagat and Liehr^^ from methylhydrazine
and trimethylchlorosilane :
3MeNH—NH2 + 2Me3SiCl -> Me3SiN—NHSiMeg + 2N3H5+GI- (10)
GH3
(III)
t Earlier workers^^-ie had assigned the Ν,Ν' structure I to the bis-silylhydrazine product,
based on chemical reactions with reagents such as phenyl isocyanate, assumed to take place
at the Ν—Η bond. These fail to distinguish between the isomers, because in fact the Si—^N
bond in the disilylhydrazines is about equally as reactive as the Ν—Η bond. All chemical
methods of structure proof for silylhydrazine are ambiguous for this reason. The correct
assignment of isomeric structure was only successful when physical methods (n.m.r. and i.r.
spectroscopy) were used.i^'i'^
293
ROBERT WEST
The proton n.m.r. spectrum of the product of equation 10, shown in Figure 1,
indicates that it is the isometrically pure Ν,Ν' isomer. III. Note that two
equal Si—G//3 resonances are observed, corresponding to the two different
kinds of silylmethyl protons. In the N,N isomer IV, all silylmethyl groups
are equivalent and only one Si—C//3 resonance is found {Figure 1).
(MegSiJo NNHMe
10
MeaSiNHNSiMeg
file
10
ppm( r )
Figure 1. Proton n.m.r. spectra of compounds III and IV, 20% v/v in carbon tetrachloride,
with cyclohexane (r = 8-56) as internal standard.
When either III, or IV, are treated with one equivalent of n-butylhthium
and then one equivalent of methyl iodide, the same mixture of dimethyl
bis(trimethylsilyl) isomers is produced^^:
294
ANIONIC REARRANGEMENTS OF ORGANOSILIGON GOMPOUNDS
MeaSiN—NHSiMea > (Me3SiNGH3)2 J^^^ (Me3Si)2NNHGH3
j 2)CH8l 2)CH8l
GH3 +
(III) (Me3Si)2N—N(GH3)2 (IV) (11)
These products are the same as those formed in reaction (9), and once
again about equal amounts are formed. If either III or IV are lithiated and
then treated with pyrrole rather than methyl iodide, reprotonation takes
place and a nearly equimolar mixture of III and IV is obtained. This
reaction provides a means for synthesis of IV, which can be separated from
the mixture by gas chromatography^^:
l.)RLi
(III) >(III) + (IV) (12)
2.)pyrrole 46% 54%
At this point, it was found that complete lithiation and reprotonation are
unnecessary. If III or IV is simply treated with a trace of n-BuLi, it is rapidly
converted to a similar mixture of III and IV:
trace trace
(III) ^MegSiN—NHSiMea + (Me3Si)2N—NHCH3 < (IV) (13)
I
RLi RLÍ
CH3
(III) 50%) (IV) 50%)
This finding is of great significance, for most previously known anionic
rearrangements require one full equivalent of base, and are usually even
then quite slow. In the silylhydrazine rearrangement, then, we have an
example of a catalytic anionic rearrangement^^.
The catalytic silylhydrazine rearrangement
The scope of the catalyzed rearrangement has been systematically
studiedi^-21^ The rearrangement was followed by n.m.r. spectroscopy of the
equilibrating silylhydrazine solution, and by gas chromatography of the
mixture after equilibration, n-butyllithium (0-03 to 0-3 equivalent per equiv
alent of silylhydrazine) was ordinarily used as a catalyst, but any base
strong enough to deprotonate the silylhydrazine will cause rearrangement.
Silylhydrazines which have been studied are listed in Table 1. The known
examples where rearrangement occurs all involve hydrazines with two
triorganosilyl groups, but there is no reason to believe that the rearrangement
is not a completely general reaction for all silylhydrazines which can form
anions. From the data in Table 7, both steric and electronic eflfects can be
seen to shift the position of equilibrium. For example, N,N-bis(¿-butyl-
dimethyIsilyl)hydrazine undergoes 86 per cent rearrangement to the Ν,Ν'
isomer, because of strong steric interference between i-butylsilyl groups
when they are on the same nitrogen atom^i. Phenyl groups strongly stabilize
the isomers bearing a hydrogen on the same nitrogen as the aromatic ring,
295
ROBERT WEST
so that, for example, Ν,Ν'-bis (trimethylsilyl) Phenylhydrazine undergoes
96 per cent rearrangement at equilibrium; and N-trialkylsilyl-N'-phenyl-
hydrazines do not rearrange at alU^. We believe that the effect of phenyl is
largely electronic, due either to (i) reduced opportunity for N-^Si pi-bonding
in the Ph—Ν—Si compounds, or (ii) specific stabilization because of the
presence of the Ph—Ν—Η system, or both.
Table 1. Equilibration of organosilylhydrazines with /-butyllithium in benzene
% Reananged
at equilibration íi, (min)a
(Me3Si)2NNH2 51 < 1
(EtMe2Si)2NNH2 53 < 1
Me3Si(EtMe2Si)NNH2 52 < 1
í-BuMe2SiN(SiMe3)NH2 73 ^2
(í-BuMe2Si)2NNH2 86 4
Me3SiNHN(CH3)SiMe3 50 3-5
EtMe2SiNHN(GH3)SiEtMe2 47 4-5
/-BuMe2SiN(SiMe3)NH]Víe 68b 80
Me3SiNHNHPh 0 —
EtMe2SiNHNHPh 0 —
^BuMeaSiNHNHPh 0 —
Me3SiNHN(Ph)SiMe3 96 600
EtMe2SiNHN(Ph)SiEtMe2 91 900
í-BuMe2SiNHN(Ph)SiMe3 80b 2800C
^BuMe2SiNHNHMe 0 —
a 0-4 Μ Hydrazine, 0 04 Μ n-BuLi.
l> Two different products formed.'^
c At 65°
In the absence of strong steric or electronic constraints, the bis(organo-
silyl) hydrazines and bis(organosilyl)methylhydrazines give essentially equal
amounts of the N,N and Ν,Ν' isomers at equilibriumi^. For these simple
cases, the reaction is therefore thermoneutral, with no enthalpic driving force.
This observation differentiates the silylhydrazine rearrangement from all
other anionic rearrangements, which are thermodynamically driven
reactions proceeding essentially to completion.
The finding that I and II, or III and IV, have identical thermodynamic
stability is quite surprising, for the silyl and methyl substituents might be
expected to stabilize one isomer over the other. The simplest rationalization
of the results is that the organosilyl and methyl substituents have little
influence on the stability of the hydrazines. However, examination of models
shows that there is moderate steric interference between trimethyl- or
ethyldimethylsilyl groups attached to the same nitrogen atom in the 1,1
isomers, which is relieved in the 1,2 compounds. This steric hindrance
might be compensated by increased N->Si pi-bonding in the 1,1 isomers,
which would stabilize the 1,1 bissilyl compounds electronically relative to
the 1,2. This explanation requires that special stabilization is conferred by
the silazane bond (Si—Ν—Si) over and above that of two separate Ν—Si
linkages, or else that there is a *'beta pi-interaction" between the lone pair
of the remote nitrogen atom and the silicon in the (R3Si)2NNH2 isomers.
Neither the nature of the solvent nor the base concentration had any
296