Robert West University of Wisconsin Madison. WI 53706 Thomas J. Barton Iowa State University Ames. IA 5001 1
I
Organosilicon Chemistry
I ""
Stereochemistry and Reaction Mechanisms As mentioned in Part I organosilicon compounds are usually tetracoordinated with tetrahedral geometry about silicon. Even so, several hundred compounds are now known in which silicon is penta- or hexacoordinated. Higher coordinated compounds of silicon are most easily formed from its halogenides, with the ease of formation, and stability of the complexes increasing in the predictable order I < Br < CI < F,and the more the better. Thus, for example, S i c 4 readily forms with pyridine a complex, SiC142 C5H5N, which is stable a t room~t&nl,erature,while MeSif& dws not appear to react at all. A few examples of cornpuunds containing silicon with coordination greater than four are shown here.
SF:(but mt S)JL'?
Even for tetracoordinate silicon there exists the nossibilitv for a dramatic stereochemical contrast to carbon.*~uantum mechanical calculations reveal that the enerm difference between tetrahedral and planar SiHa is much'iess than for CHd (37). Additional electronenative, a-donor substituents should lower the tetrahedral-planar energy (e.g; 0 ~ a n NR3 d difference. In fact there are different preferred hybridizations for planar CH4 and SiH4. The HOMO is a p type orbital for alanar CH4, while this orbital is the LUMO for planar SiH4. 'rbrse facts present the possibility for spontaneous racemipation of chiral silicon cmnpuunds, hut such a phenomenon has never been observed. Before 1958, i t was not even known whether substitution reactions at silicon were ever stereosoecific. We owe much of our present knowledge of the stereochemistry of organosilicon comvounds 10 the sssternatic studies oi 1.eo Sommer (38). Mosi of this work was carried out with the chiral system, a-naphthylphenylmethylsilyl-, which we will denote as R3Si*-. ~~~
M-
w--
-&
In striking contrast to substitutions a t carbon, many of the substitutions a t silicon were found to proceed with retention of configuration. Thus, chlorination of a silyl hydride proceeds with complete retention, while reduction of the resulting chloride affords the product of inversion. 334 1 Journal of Chemical Education
Table 2. Slereochemlstry of Several Subrtltutlon Reactions &six RsSiCl RISiOCHs RsSiOAc R3SIH R&iH RISiCl RISiCl RISiCl R3SiOAc
Reagent LiAlH. LiAlH,
LiAIH, CIS
R'OKIR'OH H20/Et20 C2HSLi
SiOKIxyi MeOH
Product
Stereochem.
SiH SiH SiH Sic1 SiOR' SiOH SiEt SiOSi SiOMe
In". Ret. In". Ret. Ret.
-
hv.
Ret. Inv. Inv.
(+)R3SiH2 (-)RaSiCL LIAI& (-)R&H (Retention) (Inversion) Displacement reactions a t silicon which proceed with inversion of configuration undoubtedly proceed through a process analogous to S Ndisplacements. ~ However, for silicon the process can either involve only a pentacoordinate transition state, or a pentacoordinate intermediate (39). The greater reactivity of silicon (as compared with carbon) can be attributed to the lessened steric strain of the valence-expanded transition state due to longer bonds and perhaps to stabilization by 3d orbital participation.
Ransition state leading to inversion sue = nveleophile X =leaving group
Retention of configuration can be explained through the so-called SN~-Si mechanism involving a cyclic transition state. The actual transition state could either be a tetragonal pyramid or a trigonal hipyramid.
The several examples given in Table 2 should serve to demonstrate hoth the stereospecific and seemingly capricious nature of these substitutions. Thus, it would appear difficult to predict, except on an empirical basis, the stereochemical outcome of a displacement reaction on silicon. Indeed, the group of Robert Corriu in France has established that the stereochemical outcome of . nucleophilic displacement a t silicon is a function of a t least three factors each of which is discussed below. ~~~~~
~
Nature of Leaving Group The stereochemistry depends closely upon the nature of the leaving group as illustrated by the followingreactions with methyllithium.
-
X = CI, Br Inv. The best general relationship seems to be between ease of displacement and the stereochemistry of displacement. Ease of displacement: Br C1> SR F > OMe > H, Stereochemistry: Inv. * Ret.
-
-
Nature of the Attacking - Nucleo~hile The stereurhem~stryis oftrn dependent on the nature of thu allochrny nu(It.oph~IeFor the same lravinggruup, both retention and inversih ran he ohitrved hy changing the nature of the nucleophile. The four reartions ofthe hilyl fluoride shown here servetu demonstrate this dependency.
Corriu has concluded that for a given leaving group: (1) Hard nucleophiles (e.g., alkyl anions), which have localized negative charge, prefer to attack equatorially to give products of retention,
(2) Softer nucleophiles (e.g., henzyl or ally1 anions) with delocalized negative charge, prefer apical attack to give inverted products of substitution,
and (3) hoth processes involve the rate determining formation of a pentacoordinated intermediate. Corriu has given us the rule of thumb that reagents which add to a-enones in a 1,2-fashion will react with silyl compounds with retention, while those reagents preferring 1,4addition (conjugate) favor inversion a t silicon. Nature of the Solvent Predicting the stereochemical outcome of substitution reactions on silicon is made even more difficult when the crucial role of soluent is recoenized. For examnle. chlorosilanes react with methanol withcomplete inveriiod, except when the solvent is HMPA. DMSO or DMF in which cases retention is the outcome (40).This can be explained by a mechanism inwlving an initial, nw.riil,le formation ot'a pmtavwrdinatvd ailicvn through n~a:leup~dic attack ui the wlvmt, fdluwrd hy displacrmcnt oI'~.hloridehy irontnl attack of alcohol.
Perhaps the most glaring absence in the substitution reactions on silicon is a mechanism analogous to that of the S N ~ reactions of carbon. It was once thought that the rapid racemization of trioreanusilicon halides in oolar solvents was due to iuni7ation to a planar silylrnium cation. However, it was discovered that the dielectric constant o i the solvent is less important than the nucleophilicity of the solvent. This, coupled with kinetic evidence, has led to the conclusion that racemization occurs through the reversible formation of a hexacoordinate, octahedral intermediate, rather than siliconhalogen bond heterolysis (41 ).
I
Sol".
2 Sol".
Psol".
Reactlve Intermediates
Silylenes (R,Si:) 'The study of the silicon analogs of carbenes really came alive In 1968, when it was d~scoveredthat readily availdhle methoxyd~silsnesundergo therrnallv-induced alnha-elmination to produce organosilylenes. ~ o d a ythis is the most common method for silylene generation.
However, the thermolysis of methoxydisilanes as a method for silylene production is limited by the fact that silylenes (at least those without bulky suhstituents) readily insert into Si-0 bonds.
Other methods of silylene generation include thermolysis of 7-silanorhornadienes; \../
MB
thermolysis of the only recently available silacyclopropanes 143);
and photolysis of polysilanes. Volume 57. Number 5, May 1980 / 335
The latter reaction has been used for matrix isolation of Me&:, either in argon a t 10°K or in hydrocarbon glasses a t = 450 nm 77OK. Dimethylsilylene is bright yellow, with A,, (4%). More and more it appears that silylenes are similar to carbenes in general chemical behavior. They can insert into the a-bond of olefins;
various photochemical rearrangements (50);
insert into a a-bond of acetylenes (44); MY,./" H.,CC-CCH,
MrZi i
/
Me
lL'\ \
-C
Me
insert into C-H bonds;
and insert into Si-H bonds. Indeed the SiH bond is such an efficient trap that silyl hydrides are frequently employed when silylene detection is an issue. Recent evidence suggests that silylenes can insert into the a-bond of a carbonyl group to form an unstable oxasilacyclopropane intermediate (45). Mr,Si:
elimination from silyl halides or esters; Ii
+PhK4
M ~% ~s,A
The reactions-of dihalosilanes and magnesium, while not truly silylene chemistry, often present a synthetic equivalent. The major examples come from reactions with conjugated dienes. For example, Me\
/Me Si
I
I
CI
CI
r\
Me
0bV
A
+
Me/
0 MI e
(Ref. ( 4 8 ) )
I
M&i=C(SiMe,h
and rearrangement of silyl carbenes.
Although directobservation of silenes is just beginning, two silenes have been spectroscopically observed (both by IR and one by UV) in argon matrices (
""9c e,".>\Lz,>,.
156) R m k . D. N.andPeddle,G.J.D.."R4actionsof7,S-Disilabicyclol2.2.2l~ta-2,5-diiiii. Evidence for the Transient Existence o f s Disileno,l)J. Amer Chenl. Soc.. 91.5837 11972). in the 157) Barton, T. J. and Wulff. W. D.. "On the Role of TrimethylsilylmethyI~iIyI~n~ Gsr-Phase Reaction3 of Tetramethyldisilone." J. Amer Chem. Soe. 100, 6236
,..... ,,
160) Dsvir. D. D., Orgonnmetoi. Chem.Reu. A,. 6,283 11970).Agood, butaomewhetdeted TPV~PW"" ~~
~~
ail", aninns.
161) Sakurai. H.,aehapter in"FloeRadiak,'Vol. 2. (Editor: Kuchi. K.) Wiley-Interscience, N.Y., 1973. An excellent review on sllyl radicals. (621 Carriu. R. J. P. and Menner, M., "Tho Silicnnium Inn Question? J. Ononometol. Chsm.. 74.1 119741. Althowh witten in 1973. this excellent review is.,~~~ neumthdeaa. ~~~~~~~, almost completely current. 163) F-"den. R.and Feaenden. J., "Trends in Olganonilieon Biologid Research." Aduon. O w n o m e t d Chem., 18. 275 11980l. A eoneise review of bialogically active si~~
dorotation and s graphical topological approach. 1401 Comu.R. J.P..Dabai. L a n d Manineau,M.,"StermhhmidPmofaftheExiaknce ,~-,
..
~
~
..? J. Or.qen~m~tai. Ch.m.. 64,861 11974). 1421 Three recent reviewiare avsilableon siklenw: (a) Chernyahev. E. A,. Komalenkovs. N. G., and Hsshk1rova.S. A,, Russian ~ h r ~ i ~ . z l 45.913 ~ ~ ~ 1197S):lbJ i ~ & , Gaapm. P. P.,schspter in"Raoctiue Intermediates 1977,' lEditors:Jones, M. Jr, and Moss. R b and ir, Nsfednu 0 M.. ~ ~ . Knlerniko". - ~ , S. P. end 1offe.A. . 1..a . eheotel in J. orronomeroi. C h ~ mLibrary . Series, (Editor: Seyferth, D.I. Vol. 5,1978. (43) Theearly ruorkon silscyclopropsnw has been reviewed by Seyhrth.D."TheElusivc Silacyelopropanes: the Preparation and Propphiesof aLong-sought Classof Olga-
,.
~
340 1 Journal of Chemical Education
.
eomprehensivo. 167) Hudrlik, P. F. end Huddik, A. M.."SameApplicationsofOrganosilieonCompounds to OlsanieSynthesis,'Petrarch Caw- S4. P e t r t r h System. L W i m . PA, 1979, p. 25. 163) Chsn, T.-H., "Alkene Synthesis vie 8-Functionalized Organosilieon Compounds." Acmvnlr. Chem Re*.. 10,442 11977). 169) Pierce.A. E.."Silylation of OrganieCompounds."PiereeChemidCo., Roekford,IL, 1968.A bwk devoted entirely to thi. subject. 170) Klebe. J. F., "Silyl-Proton Exchange Reaefiana." Aeeounrr Chpm. Rea., a, 299 11970).
