Organosilicon Chemistry - Advances in Chemistry (ACS Publications)

1 Organosilicon Chemistry

Downloaded by FORDHAM UNIV on January 13, 2013 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0224.ch001

A Brief Overview

Thomas J. Barton1 and Philip Boudjouk2 Chemistry Department and Ames Laboratory (Department of Energy), Iowa State University, Ames, IA 50011 2 Chemistry Department, North Dakota State University, Fargo, ND 58105 1

The chapter gives a brief overview of organosilicon chemistry and serves as an introduction to the rest of the material in the volume.

ο R G A N O S I L I C O N C H E M I S T R Y is a rich stratum under the long-plowed fields of organic chemistry and contains largely untapped veins that are interlinked to a wide variety of chemical, physical, and engineering disciplines. Our collective experience requires that we warn the reader that the forthcoming material may well prove to be addictive! The reader is also warned that simple extrapolation from organic chemistry to organosilicon chemistry is a dangerous predictive tool. As expected from their proximity in the periodic chart, carbon and silicon often display close similarities in structures and reactions, but it is the numerous differences that are most interesting and important. In this chapter, we will be skimming the rough topology of organosilicon chemistry, and thus, we will give only the peaks of the highest mountains and completely miss the valleys and often exclude the smaller hills. Our purpose is not to be exhaustive but simply to prepare the reader for some of the material that lies ahead in this volume, and we hope to whet your interest in what we believe is a most fascinating and important subject.

0065-2393/90/0224-0003$12.00/0 © 1990 American Chemical Society

In Silicon-Based Polymer Science; Zeigler, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

4

SILICON-BASED POLYMER SCIENCE: A COMPREHENSIVE RESOURCE

Nomenclature Organosilicon compounds can always be named by the oxa-aza convention as organic compounds containing silicon atoms substituted for carbons in the longest continuous chain. For example, (CH 3 ) 3 Si-CH 2 -Si(CH3) 2 -CH 2 -Si(CH3)3


2,2,4,4,6,6-hexamethyl-2,4,6-trisilaheptane However, this method is rarely used, because at least for relatively uncomplicated molecules, the compounds are easier to name as derivatives of silane, SiH 4 . For example, Me 2 SiCl 2

dichlorodimethylsilane

Me 3 SiSiMe 3

hexamethyldisilane

If the silicon group is to be named as a substituent, the radicals are named as follows: H3Si-

silyl

H2SiC

silylene

H3Si-SiH2-

disilanyl

H2SiO-

siloxy

H3SiNH-

silylamino

H3SiOSiH2-

disiloxanyl

H3SiOSiH20-

disiloxanoxy

For example: Et 3 Si-CH 2 -Œ-CT 2 CH3 OSiMe3 l-triethylsilyl-2-trimethylsiloxybutane Cyclosilanes can be named as illustrated by the following examples: Ph2 ^

Λ

Ph2Si

hexaphenylcyclotrisilane or hexaphenylcyclopropasilane SiPh 2


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BARTON & BOUDJOUK

Organosilicon

Chemistry: A Brief Overview

5

M&2 M^Si'

SiMea

dodecamethylcyclohexasilane

SiMe2


Hydroxy derivatives are named analogously to alcohols by using the -ol ending: Me 3 SiOH

trimethylsilanol

Me 2 Si(OH) 2

dimethylsilane diol

Me 3 Si-0-SiMe 2 OH

pentamethyldisiloxanol

Ph 3 SiONa

sodium triphenylsilanolate

Substituents other than hydroxyl are named exactly as they are in stand ard organic nomenclature. An exception is hydrogen, because Η bonded to Si is a functional atom. Therefore, it is often useful to refer to organosilicon hydrides. The term organosilane does not indicate the presence of an Si-H bond. CH3CH2SiH2SiH3

ethyldisilane

Me 3 SiNH 2

aminotrimethylsilane

Silazanes [H 3 Si(NHSiH 2 ) n NHSiH 3 ] are called disilazane, trisilazane, etc., depending on the number of silicons. For example, Me3Si-NH2-Si-NH2-SiMe3 Μβ2 1,1,1,3,3,5,5,5-octamethyltrisilazane Siloxanes are named in a similar fashion: H 3 Si(OSiH 2 ) n OSiH 3

η = 0, disiloxane η = 4, hexasiloxane


6


The naming of eyclosiloxanes is very simple, as shown by the following example; Me2 I

I

MeoSi

^Si — M e


OMe methoxypentamethylcyclotrisiloxane A special shorthand method, the "General Electric siloxane notation", is commonly used for methylsiloxanes. In this notation, the groups are ab breviated as follows: Me I -OSiOI Me

Me3SiO-

M

I

Me I -O-Si-OΟ

D

Τ

I Ο I -O-Si-OI Ο I Q

For example: Me 3 Si-0-SiMe 3

MM

Me 3 Si(OSiMe 2 ) 8 OSiMe 3

MD8M

(Me3SiO)3Si-0-Si-0-Si(OSiMe3)2 Μβ2 Me

M 3 QDTM 2

Μβ2 M^Si Ο x

Si-0

SiMe2

D4

The Nature(s) of Silicon Bonding Most organosilanes are tetrahedral about silicon, which is consistent with the use of sp3-hybridized orbitals on silicon. R /{^

R

Si3s 2 3p 2 3cf


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7

Organosilicon Chemistry: A Brief Overview


However, in contrast to carbon, numerous compounds are known in which Si is penta- or hexacoordinated.

The formation of higher coordinated silicon is often explained in terms of d-orbital involvement (3d 2 in trigonal bipyramidal complexes or 3d z and d 2_ 2 in octahedral complexes). However, it has long been recognized that it is equally possible to describe the bonding without 3d orbitals by using three-center molecular orbitals. No compelling data are available for either bonding possibility, but it is of interest to note that if d orbitals are to be used, some contraction must occur before they can be effectively hybridized with the s and ρ orbitals. Indeed, in keeping with this requirement, higher coordinated Si compounds are most easily formed from the halides, and the ease of formation and complex stability both increase with increasing elec tronegativity (I< Br < Cl < F). Thus, z

x

z

y

SiCl 4 + 2 N ^ ~ \

SiCl4«2NC5H5

but

MeSiCl 3 +

^

—> no reaction

( p—d)7t

Bonding. Experimental Si-X bond lengths are always shorte than the sum of the respective covalent radii. The Si-C bond length is brought very close to the experimental value if polar effects from the elec tronegativity differences are included, but Si-X bonds for more electroIn Silicon-Based Polymer Science; Zeigler, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

8


negative Xs are still predicted to be too long, as shown by the following bond length data: Si-O in (H 3 Si) 2 0

1.634 A (experimental) 1.77 Â (calculated)

Si-F in SiF 4

1.553 À (experimental)


1.71 A (calculated) The most frequently used rationalization for these differences is (p-d)n bonding, that is, the donation of lone-pair electrons from X into the vacant 3d orbitals of silicon. Thus, the electronegative atom X would both contract the d orbitals and transfer electrons to create additional bonding. This concept has been used to rationalize a number of unique structural features such as the planarity about nitrogen in (H 3 Si) 3 N. H,Siι — Si-X *^->Si=X

120° N

N—SiHa

H3Si Acceptance of (p-d)ii bonding in silicon compounds has not been uni versal and is now being challenged frequently by theoretical studies. The question seems to be down to {p-d)ix bonding versus polar effects, either of which can explain most of the data. Whereas ab initio calculations have better reproduced experimental bond lengths and dipole moments by in clusion of d functions in the Si basis set, this better reproduction may simply be the result of compensation for an inadequate s-p basis set. Of particular importance to this volume is the Si-O bond. In addition to the previously mentioned difference in calculated and experimental bond lengths, replacement of the carbons in dimethyl ether with silicons also produces dramatic changes in bond angles, and this effect has also been attributed to ρ -» d back bonding. H3CjO ^ΛΉ3 111.5°

H 3 Si—Ο WXCH3 120.6°

H 3 Si—Ο ^>\iH3 3 144.P

Once again, however, ab initio calculations reproduced both bond angles and lengths without the use of d orbitals. It was concluded that, evidently, the Si-O bond is much more polar than estimated from electronegativities. Thus, both angles and lengths can be explained by coulombic repulsions.


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Organosilicon Chemistry: A Brief

Overview

9


δ* Most recently, an 1 7 0 NMR study of quadrupole coupling constants in sil icates has been interpreted as strong evidence for (p—d)ir bonding between silicon and oxygen. This question will be subject to considerable scrutiny for some time to come. Another important system for which (p-d)iï bonding has been assumed, but calculations deny, is the α-silyl anion, R 3 Si-CH 2 ~, which is unusually stable and easy to form. Current thinking is that α-carbanion stabilization is due to the high polarizability of Si and the presence of low-lying σ * orbitals. (CH3)4Si \

d/4

(CH 3 ) 4 C \

J/4

*~^>

(CH 3 ) 3 Si-CH 2 Li

THF/TMEDA m

v

0 / 0

ù

> no reaction

THF/TMEDA

In the previous reactions, T H F is tetrahydrofuran, and T M E D A is tetramethylenediamine. or-Tr Conjugation. An extremely important aspect of silicon bonding is hyperconjugation or σ-ττ conjugation. Hyperconjugative interaction of the Si-C bond with various ττ systems is well documented in allylic silanes by a variety of spectroscopic methods. A particularly telling demonstration is the dramatic lowering of the ionization potential (IP) of 1 (in which hyper conjugation is possible) and 2 (in which the^ Si-C bond is locked in the ττ nodal plane).

^SiMe2

1, IP = 8.13 eV

2, IP = 8.42 eV

Of equal importance and probably of more pertinence to the subject of this volume is the ability of the Si-Si bond to hyperconjugatively interact with TT systems. In the case of Ar-SiR 3 , hyperconjugative electron release by Si-R is possibly masked by the stronger electron acceptance by Si through (p-djiT bonding. However, for the Ar-SiR 2 SiR 3 system, much greater elec-


10


tron donation is expected because of the higher polarizability of the Si-Si bond relative to that of Si-C, as clearly seen in the following examples: Me3Si-CH=CH2 λm _ax 202 nm

Me3Si-SiMe2-CH=CH3 A

max 2 2 3

n

m


Si—SiMe3

A

max For example, Si ς:

f?


• CH3ca

AICI3

/ —

- j p j .

(Y

^

(82%)

Over the past few years, the debate over the origin of the β-silicon effect on carbocations has narrowed to one of the relative magnitudes of inductive and hyperconjugative factors. Theory and experiment are finally in agree ment that hyperconjugation is by far the dominant factor—29 kcal/mol cal culated to be from β-stabilization (!) versus 9 kcal/mol from induction and polarization. The realization of these effects is dramatically revealed in the SN1 solvolyses of the conformationally locked cyclohexyl trifluoroacetates (OTFA) (3-5). The relative solvolysis rates at 25 ° C for compounds 3-5 are 1, 4 Χ 104, and 2.4 Χ 1012, respectively. Compound 4 cannot attain the necessary anti-coplanar relationship of the Si-C and C - O bonds, which is present in 5 and required for full hyperconjugative interaction with the cation formed as the C - O bond suffers heterolysis.

OTFA

Both a- and β-silyl radicals are stabilized relative to the all-carbon sys tems. Although these stabilizations are sufficiently large to control a great deal of chemistry, their magnitudes (probably ~3 kcal/mol) are far less than for the analogous cations. The relative reactivities for H" abstraction by the ierf-butoxy radical are as follows: 4.2

1.0

22

5.5

Et^C—CH£ — CH3 Et^Si—CH2—CH3 The energies of various silicon bonds are given in Table I. The experimental S i - Η bond strengths are most interesting in that they are remarkably insensitive to substitution on Si, with the dramatic exception


12


Table I. Silicon Bond Energies Bond

Si-H

Compound

Bond Energy (kcal/mol)

H 3 Si-H Me3Si-H

90.3 90.3 91.3 88.2 85.3 79.0 88.2 89.4 (74.8)* 74 80.5 (63.0)fe 113 96 77 111 160 128 100 99

CI3S1-H

Si-C


Si-Si Si-X

PhSiH2-H Me3SiSiH2-Hfl (Me3Si)3Si-Hfl H 3 Si-CH 3 Me 3 Si-CH 3 H 3 Si-SiH 3 Me3Si-SiMe3 Me3Si-Cl Me3Si-Br Me3Si-I ClaSi-Cl F 3 Si-F Me3Si-OH Me3Si-NHMe Me3Si-SBu

"Data were obtained by a photoacoustic technique. ^Values in parentheses were derived from ΔΗ{ (enthalpy of fusion) data obtained by mass spectroscopy and probably have some in herent error.

of silyl substitution. At this time it is unclear why successive silyl substitution progressively weakens the S i - Η bond. Many of the various silicon bonds have strengthened considerably over the past 10-20 years! It seems that they are now leveling off, but different techniques continue to produce different numbers. The Si-Si bond is quite different from the C - C bond and actually re sembles more the C = C bond in its chemistry and properties. For example, disilanes readily undergo electrophilic cleavage by the same reagents that add to olefins by cleavage of the IT bond. Me 3 Si-SiMe 3 Me 3 Si-SiMe 3

2Me3SiBr PhC

° 3 H > Me 3 Si-0-SiMe 3

The Si-Si bond provided thefirstexample of a σ-bond donor in the formation of charge-transfer complexes with T C N E (tetracyanoethylene). Conversely, the Si-Si bond can act as an electron acceptor to form disilanyl radical anions. w o.o.w , χ Me 3 SiSiMe 3 + (CN)2C = C(CN)2

[Me 3 Si-SiMe 3 ] + e > [(NC)2C = C(CN) 2 ]-

(Xcr = 417 nm) Me 3 SiSiMe 3 —U [Me3SiSiMe3]^ K +


1.

BARTON & BOUDJOUK


13

Perhaps most striking is the demonstration that the system (-Si-)n is electronically "conjugated". An intense UV absorption results from a σ —>' σ* (or σ —» 3dir) transition, which shifts position bathochromically with increased silicon catenation (Table II). These examples show that the X m a x quickly approaches a limiting value with increasing chain length. Thus, alkylsilane polymers absorb at 305-320 nm. Table II. Bathochromic Shift in Silicon Catenation


Silane

kmax (nm)

ca. 200 215 234 260 285 291 293

Me 3 Si-SiMe 3 Me(Me 2 Si) 3 Me Me(Me 2 Si) 4 Me Me(Me 2 Si) 6 Me Me(Me2Si)i2Me Me(Me 2 Si) 1 8 Me Me(Me 2 Si) 2 4 Me

Making and Breaking of Bonds to Silicon The Rochow Process. Rochow found that alkyl and aryl halides react directly with silicon when their vapors contacted silicon at elevated tem peratures to produce complex mixtures of organosilicon halides. The reaction is promoted by a wide variety of metals from both the main group and the transition series, but the most efficient catalyst is copper. The most studied reaction of this type is the reaction between methyl chloride and silicon to give dimethyldichlorosilane and methyltrichlorosilane. Dimethyldichlorosilane is major feedstock silane for methylsilicon polymers. Me 2 SiCl 2 (bp 70 °C; 30-800%) MeSiCl 3 (bp 66 ° C ; 10-40%) MeCl +

S i ^ U

Me 3 SiCl(bp 57.7 °C) SiCl 4 (bp 57.6 °C) MeSiHCl 2 (bp 40.7 °C) SiCl 3 H (bp 31.8 °C)

Me 2 SiCl 2

-Me 2 SiO(SiMe 2 ) n OSiMe 2 -

Since the "direct reaction" produces all substitution possibilities of methylchlorosilanes, it is, indeed, remarkable that selectivity for the most desired Me 2 SiCl 2 can be >90%! The mechanism of the direct reaction has not been fully elucidated, but evidence points to the formation of a Si-Cu intermetallic compound that more readily polarizes the C - C l bond than either silicon or


14


copper alone to generate highly reactive silicon subchlorides and methyl radicals. The following scheme is a reasonable pathway to tetrasubstituted silanes: MeCl + (Si-Cu) - » [MeCuCl] - » Me' + CuCl CuCl + Si - * [SiCl] + Cu


[Si]Cl + Me' - » MeSiCl -> Me.SiCL^ (x = 1-3)

Hydrosilation. The second most important method for preparing organosilanes on a large scale is the addition of hydrosilanes across a car bon-carbon double bond. The reaction is quite general and applies to a wide variety of substituted alkenes, dienes, and alkynes. R*Si — H

. , . catalyst

R*S: 3Si\

^

R

*

X

"T^

These reactions, which are catalyzed by a broad spectrum of agents including pure and supported metals, metal salts, bases, ultraviolet light, and free radical initiators, can give high yields of product at less than 100 °C and often at room temperature. Typically, homogeneous catalysts are used, the most efficient of which is chloroplatinic acid, H 2 PtCl 6 , known as Speier's catalyst. As an example, a ΙΟ" 6 M concentration of this catalyst relative to silane can produce quantitative hydrosilation of terminal alkenes within minutes. Unfortunately, the catalyst is not recoverable, and on an industrial scale, this loss adds significantly to the cost of production (~$0.12/lb or $0.26/kg). Often, hydrosilation of dienes is better accom plished with palladium [e.g., Pd(PPh3)4] catalysis.

R3Si-H + ^ \ R
< I I CI Li

/+

-HC1 CI

.

r

Me


26


^SiMe3

SiMe3 Mc^Si-

JS^SiMeBA F Li 1_

z

-LiF

Me2Si=C^

SiMeBu^

(isolable)

and via rearrangement of α-silyl carbenes. N2 " Δ or hu Me3Si-C-SiMe3 •

Me

·· Me 3 Si-OSiMe 3

Me2Si=C

\ SiMe


3

The chemistry of silènes can generally be predicted from known olefin chemistry and from a consideration of the polar nature of the Si = C unit.

R2C-0

4-k

OR D ROD>

R3SiCSiOR' /I A

•Si—α I I 0-0

R3SiOR'

THF I Si

ο,

•

\

(

.s{

s

-Si ι

N=N

Si-

The reactions of silènes should not be viewed merely as laboratory curiosities. The synthetic routes to silènes are often quite efficient, and the reactions usually proceed with excellent yield. Thus, silènes represent important building blocks in organosilicon chemistry. Silènes have been the object of numerous theoretical studies. It now seems generally agreed that the silicon-carbon π bond strength is 3536 kcal/mol or roughly half that of the carbon-carbon IT bond. It should be noted that whereas the inclusion of d orbitals does improve the computed geometry, the IT bond strength is unchanged. Perhaps the most interesting reaction of silènes is one that finds little


1.

BARTON & BOUDJOUK

Organosilicon

Chemistry: A Brief

Overview

27

counterpart in carbon chemistry. Appropriately substituted silènes can isomerize to silylenes by a 1,2-shift of H (or SiMe3).


R

/

Si=CH2 ZTZ* *

R

Si-CH 3 /

Although the rearrangement is almost isothermal, the barrier is a considerable 41 kcal/mol. Although the resonance energy of silabenzene is calculated to be about three-fourths of that of benzene, silabenzenes are extremely reactive and, thus, have been examined only in frozen matrices and in the gas phase. By far the simplest route to silabenzenes is pyrolysis of the 1,4-dihydro derivatives.

Silylenes (R2Si*). Silylenes, which are considerably more stable than the analogous carbenes, have a singlet ground state. With few exceptions, their reactions are analogous to those of carbenes. Silylenes are generated by α eliminations,

Me3SiOMe + :SiMe2

(migrating group can also be Η, X or vinyl) by thermal extrusions, Me

Me


28


Me

X

Me

108 ° C ^SiMe 2

—

1

4

h

*

Me2C=CMe2 + Me2Si:

by photochemical Meextrusions, Me


Mea Me2Si/ ^ S i M e j J I Meî

h u

—•

/^MeTX i^-Si ->H

SiMe2

+ MejSi:

5

^Si Mej and by a variety of isomerizations. Me3Si ^Si=CH2 M

c

SiMe3 •

Si—CH 2 Me

The majority of silylene reactions are insertions that immediately reestablish a tetracoordinated silicon. The most frequently observed insertions are into Si-O,


1.

BARTON & BOUDJOUK

Organosilicon Chemistry: A Brief M e

2

Overview

S i :

•

Me2Si(OMe)2

Meî SiMea OMe OMe

into Si-H, MeoSi


Et3Si-H

•

Et3Si-SiMe2 H

into O - H , Me^Si: — •

ROH

ROSiMe2 H

into C - H (actually quite rare), H3C

H2C

SiMe2

SiMej

I I

Me—Si—CHj

Me—Si—CH 2 H into C = C,

Me2C=CMe2

Me-Si

Si I Me

Me2 Si

M^Si:

/ \ Me2C

CMe2

H—Si— I Me

Η

Me


30



intoC = C - C = C,

and into C - C .

Disilenes (R 2 Si = SiR 2 ). Most of the excitement in disilene chemistry in recent years stems from the discovery that relatively stable, isolable disilenes can be prepared. Disilenes with small substituents are unstable even at very low temperatures, but those with bulky substituents are stable in solution and, in extreme cases, can be isolated as crystalline solids. Examples of the preparation of "stable" disilenes are


BARTON & BOUDJOUK

Organosilicon

Chemistry: A Brief

Overview


1.


31

32



The bimolecular processes of disilenes are rather predictable,

with at least one notable exception:

si=si

—L_*. Ί

IN

r> »

si

71

\

Because H - S i - S i H 3 is nearly isoenergetic with H 2 Si = SiH 2 , it is not obvious which structure would be favored for hexasilabenzene, S i 6 H 6 . Whereas proponents of aromaticity may take heart from the theoretical cal culations that show the hexasilabenzene structure to be 34 kcal/mol more stable than the tris(silylene) form, they will be reminded of how little aro maticity has to do with the magic of six ττ electrons by the revelation that the hexasilaprismane is calculated to be 14 kcal/mol more stable yet.

/ \ HSi

•SiH /H

HSi-

"SiH

most stable

Η ^Si HSi^

SiH

HSi

,SiH

H>SÏ

Si 4

:s

Si H 0

si: H2 least stable


1.

BARTON & BOUDJOUK


33


Silanones (R 2 Si = O). Despite the fact that the past few years have yielded several reports of spectroscopic observation of several silanones in frozen matrices, all we know of silanone chemistry to date comes from the rationalization of reaction product structures in terms of silanone interme diates. The sheer weight of these data, coupled with theoretical calculations, makes it difficult not to believe in the existence of free silanones. However, the reader should be aware that often there exist alternative mechanisms that do not involve the postulated silanone intermediates. Silanones have apparently been produced by a wide variety of reactions, a few of which are shown in the following schemes;

Mes2|i-

Mes2Si=0

hv

Me^Jji

+

Mesî-

~—C-—SiMes2

Mes2Si=C^

\

02

^Si-

I

-o Me \ / Ο

\

S i

Ο

^

Si II Ο

\ /

c

Me / x NSiR3

/

[Me2Si=0] + R3S1N3


34


R2Si

Ο

>—
90% retention of configuration)

α = +33.7° (Np is naphthyl.)

it may be concluded that the rate of inversion of silyl radicals is slow compared with the rate of CI abstraction from C C L . Other reactions include

pf9c=o

—SiR-,

R3Si-0-CIÎ2

1

V

R. ,Si-Ç-C(

RoS'i

Table III. Properties of C and Si Radicals Radical

% s-Character of Unpaired Electron

Bond Angle

Structure

' CH3 • SiH 3

0 21

120° 111°

planar pyramidal


36


Silyl Anions (R3S£~). For years aryl substitution was thought to be necessary for the formation of silyl alkali compounds. This is not the case, and trialkylsilyl alkali metal compounds are now readily available. The most general and convenient method of generation is disilane cleavage. For example: Me 3 Si-SiMe 3

H

^>

Me 4 Si + Me 3 SiLi


In the previous reaction, HMPA is hexamethylphosphoramide. The reactions of R3Si~ are usually similar to those of R 3 C". ?

H

Me 2 C-SiR 3

Ph 2 CHCH 2 SiR :

R 3 SiEt

Trialkylsilyl alkali metal compounds are readily oxidized by a variety of aromatic compounds, ketones, amides, and anhydrides. Thus electron transfer is likely to be the first step of the majority of R 3 Si" reactions.

w 0 . Me 6 Si 2 5

1

NaOMe HMPA

•

naphthalene M^SiNa — • ^

. M^Si

+

Of considerable synthetic potential are alkylpentafluorosilicates: KF

R-SiCl 3

|VsiF J K 2 + 5

Ph

Br

NBS

V

?

f

1

vT\ïf5

Cu(SCN)2 ^

P l

\^

(NBS is iV-bromosuccinimide.)


SCN

1.

BARTON & BOUDJOUK

Organosilicon

37

Chemistry: A Brief Overview

Silylenium Ions (R 3 Si + ). Because silicon is considerably more elec tropositive than carbon, R 3 S i + can be expected to be significantly more stable than R 3 C + . Indeed R 3 S i + is always encountered in the mass spectra of organosilicon compounds, and theoretical calculations support the stability ofR 3 Si + . H 3 S i - C H 2 -> H 2 S i - C H 3 (thus, H 3 S i - C H 2 + is incapable of existence!)


£ a (activation energy) — 0; Δ £ = 49 kcal/mol (from calculations) Only in 1982 was apparently solid evidence presented for the direct observation of a silylenium ion: (n-PrS)3SiH + Ph 3 C + C 1 0 4 " -> (n-Pr) 3 Si + Cl0 4 - (1982) Ph 3 SiH + P h C + C 1 0 4 - > Ph 3 Si + C10 4 (1986) At the time of this writing, this area remains controversial. On the one hand, we have 1 3 C and L H NMR spectra, along with conductance studies, in support of the presence of free silylenium ions in solution, whereas 2 9 Si and 35C1 NMR data are used to support the claim that Ph 3 SiC10 4 is simply a covalent ester in solution, as it has long been known to be in the solid state.

Molectdar Rearrangements Organosilicon compounds are, in general, quite prone to intramolecular rearrangements. The majority of these rearrangements are actually intra molecular nucleophilic substitutions on silicon and can be viewed as

RssT^X

•

M i

^

•

R 3 Si^

Thus, most rearrangements proceed with retention on silicon. We will, in this section, simply try to provide at least one example each of the most common organosilicon rearrangements.

tions).

1,2 Rearrangements.

Γ Ι ™ R2Ç—Ο

I

M^SiS

/

3

Dyatropic (Two Concurr 14O-170 - C

f33* R 2 C-

M

Ε = 31 kcal A

Î

S L M E


3

38


Me

/

F I fast -Me-C-SiFCl2 •MeC—SiF 2 Cl I Cl2 α

£iCl2

MeC-SiCl3 - — • ρ slow Of

Amonic. Downloaded by FORDHAM UNIV on January 13, 2013 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0224.ch001

OH I ,Na, K, RLi, R3N, etc. R3Si—CR2 • HCR2—OS1R3 (AS* = -35-40 eu; therefore, concerted)

Ο

0J(

Me3Si-C-R

Me3Si-C-R"

I

M^SiO—50

1 5

°

C

is: > 50

•

QAc Si) Cz=C= 3

2

Nv X

SiMe3


42


Between C and C.


(1,3- C to C is the only rearrangement proceeding with inversion on Si)

MesSi

§iMe3 —

\

H

e=C=CH2

*

>.

/

HC^C

M

CH 2

(probably a stepwise process)

1,4 Rearrangements.

(RO)2P

Between C and O.

(RO)2P.

SiMe2

\CHj-CH/ ^ M e

1

(RO)2P-OSiMe2Et

2


1.

BARTON or BOUDJOUK



Between Ο and O.


43

44


Between C and C. Ph-C(SiMe3)3

!(SiMe3)2"-^Me3Si-

:H(SiMe 3 ) 2

•SiMe3


SiMe3

-SiMe3

The fact that Me 3 Si migrates in a 1,5-sigmatropic fashion on the cyclopentadiene ring 106 times faster than does hydrogen is often cited as dramatic evidence for its superior migratory aptitude. However, on the cycloheptatriene ring, 1,5-migration of H is faster than that of Me 3 Si. The explanation lies in the nature of the polar transition state.

Acknowledgment The support of this work by the National Science Foundation is gratefully acknowledged by Barton.

References 1. Onopchenko, Α.; Sabourin, Ε. T. J. Org. Chem. 1987, 52, 4118. 2. Sheldon, J. C.; Hayes, R. N.; Bowie, J. H . J. Am. Chem. Soc. 1984, 106, 7711. 3. Dieters, J. Α.; Holmes, R. R. J. Am. Chem. Soc. 1987, 109, 1692.

Bibliography General Sources for Organosilicon Chemistry. 1. Eaborn, C. Organosilicon Compounds; Xerox Microfilms: Ann Arbor, MI, 1976. This is the original bible of organosilicon chemistry. 2. MacDiarmid, A. G. The Bond to Carbon, Part I; and MacDiarmid, A. G. Organometallic

Compounds

of the Group IV Elements, Vol. 1, Part I. These

excellent comprehensive books are no longer available. I have lost mine, so if you see an extra copy lying around . . . . 3. Comprehensive

Organometallic Chemistry; Pergamon: New York, 1982, Vol. 2.

Chapters on "Organopolysilanes", Carbocyclic Silanes", "Organosilanes", and "Silicon Compounds in Organic Synthesis" (Vol. 7) by R. West, T. J. Barton, D. A. Armitage, and P. D. Magnus, respectively. 4. Bazant, V ; Chvalovsky, V.; Rathousky, J. Organosilicon Compounds. This ex tremely valuable tabulation of pertinent literature references and physical prop erties of every known organosilicon compound has appeared four times since 1965. It is not published or available outside of the Soviet block. I (T. J. B.) have not found a copy of the fourth serial (1980). In Silicon-Based Polymer Science; Zeigler, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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BARTON & BOUDJOUK

Organosilicon

Chemistry: A Brief

Overview

45

5. Silicon Compounds—Register and Review. This reference is actually the catalog of Petrarch Systems (Bartram Road, Bristol, PA 19007), which has the largest commercial offerings of organosilicon compounds. However, much of this book has about as much resemblance to a catalog as the Sears-Roebuck volume does to Kama Sutra. Since 1979, each issue has contained a variety of excellent, timely reviews of various aspects of organosilicon chemistry. Eminently readable!

The Nature of Silicon Bonding. • (ρ-ά)ττ and σ-ττ Bonding 1. Egorochkin, A. N . Russ. Chem. Rev. (Engl. Transi.) 1984, 53(5), 445.


2. Janes, N . ; Oldfield, E. / . Am. Chem. Soc, 1986, JOS, 5743. 3. Kwart, H . ; King, K. ά-Orbitals in the Chemistry of Silicon, Phosphorus,

Sulfur; Springer: New York, 1977.

and

4. Sakurai, H . / . Organomet. Chem. 1980, 200, 261.

• β-Silyl Stabilization of Cations 1. Lambert, J. B.; Wang, G. T.; Finzel, R. B.; Teramura, D. H . / . Am. Chem. Soc. 1987, 109, 7838.

2. Apeloig, Y.; Arad, D. / . Am. Chem. Soc. 1985, 107, 5285.

• α,β-Silyl Stabilization of Radicals 1. Jackson, R. Α.; Ingold, K. U.; Griller, D.; Nazran, A. S. / . Am. Chem. Soc. 1985, 107, 208.

2. Davidson, I. M . T. et al. Organometallics 1987, 6, 644. • Bond Strengths 1. Walsh, R. Acc. Chem. Res. 1981, 14, 246. This is the current bible of silicon bond strengths. 2. Kanabus-Kaminska, J. M . ; Hawari, J. Α.; Griller, D.; Chatgilialoglu, C. / . Am. Chem. Soc. 1987, 109, 5267. The authors use a promising photoacoustic tech nique to get some surprising results.

Hydrosilation. 1. Speier, J. L. Adv. Organomet. Chem. 1979,17, 407. This review of hydrosilation is written by the master himself.

2. Ojima, I. ; Kogure, T. Reviews on Silicon, Germanium, Tin, and Lead Compounds;

Vol. 5, No. 1, 7-66, 1981. This reference gives a review of hydrosilation with metal complexes other than Speier's catalyst.

Reactive Intermediates. 1. Raabe, G.; Michl, J. Chem. Rev. 1985, 85, 419. This is the most recent compre hensive review of ττ-bonded silicon. We understand that an updated version is in the works. 2. Gaspar, P. P. In Reactive Intermediates; Jones, M . ; Moss, R., Eds.; Wiley: New York. Keeping up with silylene chemistry is made easy by these critical compre hensive reviews every two or three years by Gaspar. 3. Sakurai, H . In Free Radicak; Kochi, J., Ed.; Wiley: New York, 1973; Vol. 2. This paper is a truly excellent review of silyl radical chemistry. In Silicon-Based Polymer Science; Zeigler, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

46


4. Wilt, J. W In Reactive Intermediates; Abramovitch, R. Α., Ed.; Plenum: New York, 1983; Vol. 3. This excellent review of sijyl radicals is, to our knowledge, the most recent. 5. Davis, D. D.; Gray, C. E. Organomet. Chem. Rev. A 1970, 6, 283. This article is a good, but obviously dated, review of silyl anions through about 1969. 6. Colvin, E. Silicon in Organic Synthesis; Butterworth: London, 1980; Chapter 11. The article is only six pages but gives a good update on silyl anions.


Molecular Rearrangements. 1. Brook, A. G.; Bassindale, A. R. In Organic Chemistry; DeMayo, P., Ed.; Aca demic: New York, 1980; Vol. 2, Essay No. 9. This article is the only comprehensive review of organosilicon rearrangements and suffers only from being a bit dated and its total exclusion of the rearrangements of organosilicon reactive interme diates. RECEIVED for review May 27, 1988. ACCEPTED revised manuscript March 20,


1989.

Organosilicon Chemistry - Advances in Chemistry (ACS Publications)

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