Silicon - reaction mechanisms annual survey covering the year 1973

265

SLLICON - REACTIONKECEANISMS ANNUALsURvEY

Omar

cnvFmNcTEIEYEAR1973

W. Steward

Department of Chemistry,

Duqueane University,

Pittsburgh. Pennsylvania 15219 (U.S.A.)

COhTENTS

Introducriou

265

Substitution

266

Addition-Elimination

275

Rearrangement

281

Redistribution

284

Free Radical

285

Insertion

289

Decamposltion

291

Polymerization

293

Direct Process

294

References

296

IXTRODUCTION Tbie survey brings together the information publiehed In 1973 wbicb relate8 to organosilicon suageat mechanisms The swey

based

only

reaction mechanisms. on

All papers which

reaction producca are not Included.

is divided into eectlons baaed on reaction types; in come wea

vbere overlapplng

OCCUI~, both sections should be consulted.

section, papers on related areas are grouted together

Within each

as much am poemible.

266 SWSTlTUTION Ab -the mode of model

quantum

Fnitio formation

chezucal

and stability

calculations

indicate

tetrahedron Eorming the

of

phase

reactants.

Effects

The

a Si(V)

been

.

solvolysis

isotope

studies

of

of

effects

Ustng

hydride

that

are

have been

methanolysis

of

r-ins

is 16.93

of

kcal/mol.

more stable on the

stability gas

2.3 has

ratio,

triphenylsilane

been

Investigated.

MT/M), have

transition

essentially

the

kcal/mol.

Positive-ion

dlscussed.

product

of

8.6

substituents

hydrides

and

on a face

energy

reported.

five-coordinate

bond

ion

an activation

also

rate

the

the

organosilicon

for

been

state

the

reported.

(1)

in which

intact.

I

cH30-si--U 1)

‘...U--O-CH3 I CE3OS

\

I

(1)

has

the

SiH5-

electronegative

on the

suggest

silicon-hydrogen

of

species

intermediate

r The

study

to

-ployed

intermediate.L

E- --P

vith

in monosilane

nethoxide-catalyzed

the

proceeds

bipyramidal

the

f

attack

a trigooal

reactions

Solvent

that

silane

and geometry of

These

of

have

system

SiH4

than

techniques

reductive been

shown

alcoholysis to occur

of in

dimethylindenylsilane

a stepvise

manner.

5

catalyzed

by Pd-C

4

267

ROB

o-$) I ’/

\

Pd-C H

Si&k2H

H

&Me 20R

ROH Pd-C H

The

alcoholysis

ralative

SiMeZH

oE silanrs

rates

are

kOB

asyunnetric

has

been

active

> %SiH2

observed.

The hydrides This

rhodium

reaction ailyl

vinyl

series

of

ethers.

(Ph3P).-,RhC1 in

the

9

The

rates

11

The

process

Referencesp. 296

of

(X = F: p values are

yield

presence

following

acidCl,

of

been

-0.57

H, For

in

terms

of

in

are

reported.

catalyzes

to yield

the

determined.

acid-

org=osllicon

thioether

of

Me, MeO) the

compF?l;v~s

an optically

thioetbers

and base-catalyzed

Rr.

discussed

rjlth

ketones

stability

has

either

by

srlyl

enolizable

by rhodium

complex.

OE a silyl

hydrolytic

the

induced

mercaptans

to

ulth

be

catalyzed

rhodium

various

trialkoxysilaoethiols

(e-XC6k14)tie2.SIOPh

catalyzed

by

may

active

of

triorganosilaoes

The

reported.

asymmetry

an optically

complex

Of

The

> !k$.iOH

diorganoa -ilanes

of

reactitities

catalyzed

6

(Ph3P)3fiC1.

> %SiH

The

7

or

by

reported:

alcoholysls

alcohol

catalyzed

> EtOH > Me2CHOH

RSLH~

The

is

thiol

8

the

triorganogroup

in

a

10 alcoholysis

absolute and +0.19

bimolecular

of

ethanol

have

for

base-

the

mechanisms.

A

been

pentacoordinate reaction

slllcon

while

this

base-catalyzed In

intermediate

type

of

rate

of

follous:

foilowing

is

the

alcobolysis

obtainment

of

considered

acid-catalyzed

unimportant

for

the

He3SiOMe + ROH

7

equlllbrium

varies

The solvolytic

cleavage

in

ferrocenyloethyl-

and polarograpbic

mecbanisn

for

ferrocenylmethylsilanes.

the

of

the

13,14 studies

iroo(III)

with

the

nature

silicon-carbon

cited

bond 15

m-carboranylare

of

R as

12

in

has

been

, and alkynylsllanes.

support

chloride-catalyzed

of

the

metbaoolyais

of

13

CH2SiMe3 Fe

the

oE alkoxytrimethylsilanes,

> PhMeCH > PbMe2C.

rate

for

reaction.

PhCX2 > Ph2CH

investigated Relative

proposed

intermediate

Me3SiOR + HeOE

the

is

ca2s*3 + Fe(III)

-

*

Fe+

-

Fe(II)

0

(2)

. _o cEi2

Fe

-0 (3)

+

Ke3SiOMe

+ + B

16

269

Cf$OHe (4)+MaOH

Ln

second

a

+

Fe

B

14

paper,

the

iron(IIL)

chloride-catalyzed

[a-(trimethylsilyl)benzyl]ferrocene ferrocene cient the

is

Fe(III) reactant

radical of

reported is

present

and product

coupling

the

to

proposed

and proceed

as

and

reaction

is

products radical

the

the

is

observed.15

EtjSi(CX)nH cleavage group.

observed.

aqueous

parallels

tenuates

the

studies

indicate

Further

studies

at

or

reaction

of

the

butyroacetate rat ion, donate,

the

with reaction

ethyl

configuration.

lithium with

benzoacetate The

folloving

16

acid

The of

a limiting

reported

of

on the

the

at

investigated.

inversion ethyl

of

learning

leads

derivatives

intermediates

to of

and benzoylacetone are

groups

for

of active

solvolyeis

with Phile

the

and ethyl inversion

of

acetylacetone,

leads

at-

n = 8-10.

configuration.

acetoacetate

hydride

of

optically

17

bond,

and

stereochemistry

substitution

been

Si-H

rate

value

to

sllyl

the

reaching

aluminum ths

of

cleavage

Et3Si(C:C)nSiEt3

reported.

of

in support

solvolytic

by H(C!C)n

has

derivatives

are

electron-withdrawal

been

leads

of

a mixture

of

cleavage

conjugate

Rucleophilic

probably silyl

been

the

that

have

usual

iosuffi-

chloride.

observations

solvolysis

of

compounds

methanol

These

the

has

of

if

or

hydrogen

On methanolysis

to

n increases,

silicon.

0-sllyl-$-dicarbonyl water

acidity

as

substitution

addition

methanol

rapidly

extended

palladium-catalyzed,

The base-catalyzed in

The

in

is

methanolic

intermediates.

bond,

SF-Ccarb

However,

above.

time

with

1,7-bis(dFmetbylsLlyl)-g-carborane, of

mathanolysis

[E-methyl-a-(trimethylsilyl)benzyl]-

indicated

treated

are

H+

to retention

proposed:

configudisthyl of

\d

--si

AlH3- --H

retention

The a-mercurated of

explain

of

optically

and a-stannylated

the

at

silicon.

I

v4

the

In 0-silyl-substituted 19

ways

resemble

The alkenyloxy alkoxy

.p3Si*X

Lnversion nucleophile

compounds at

sllicon involved

thiosilanes

proceeds

state

X = Cl,

Br,

with

with

(5)

is

SA,

SSFYe5).

inversion

proposed

to

of

cleavage

formed

enols, groups

the

are

poor

by

of

the

leaving

the

sillcon-

above

reaction,

groups

and in

many

groups.

Scereocbemical-rate of

and

compounds

So;

stereochemistry

oxygen

reported.

/‘R

I

___x

(5)

paper,

halo-

transition

I

I

In a second

is

The

(M = Eg,

qqI-__c-_%=q-J___si

bond

active

carbocyl 18

sterrochetistry

[

0.

inversion

reaction

configuration

---

law

with

i-Bu$UH

is

correlated

in

the

correlations in

ether

vith

transition

are

given

and n-hexane.

the state.

number

of

for 20

the

reduction

Retention

molecules

of

the

or

*/ / / x, \

R3Si

\

R2 0 ..

\

\

/

\

271

/ lllR2

H

R,*

i

__

Si

__

H’ [

S&Si

SN2-Si

A comparison following

two systems

in has

the

StereochEmistry

been

of substitution

reporLed.21

Ph

Ph

I*

BC-si2

in the

X

b9 \

/

1

\

/

(6)

(7)

While substitution at silicon in (7) may proceed with retention or inversion, In (6) only retention is observed.

The results

are

explained

In terms of the effect of angle strain on displacement at silicon. Kinetic, isotopic and stereochemical studies on the reaction of R3Six with

organolichium and Crignard reagents in ether or THF solvents

are consistent with an SRi-Sl mechanism in which the Si(V) complex formed

in

the

rate-determining

22

step.

A highly stereospecific

is

method for reduction of triorganosilyl

derivatives with Grignard reagents having B-hydrogens has been developed. LL,Si*X

+

BMgBr

X = We,

Retention reactivity

of of

Ni(II)

ether

R3Si*H

=r

F, Cl; R = Et, n-Pr, i-Pr, n-Bu, i-Bu configuration the

Refereacesp.296

at

sFlicon

Grignard reagents

is

observed,

and

the

order

parallels the order observed

of

for

Lhe

23.24

272 exchange reaction of the corresponding deuteriosilane.

hydrogen-deuterim

Other stereochemical studies on the reaction of organometalllc given

reagents with optically active triorganosilyl derivatives are Ni(II) ether)

R3Si*H + R'HgX

belov:

(Ref. 25)

Me2CuLi

ether

>

(Ref. 26)

$Si*He

X = 8, OUe, F. Cl, 0-Menthyl '6%

R3SI*Li + R3Si*C1

AD optically

active

R2Sl*C1

P

triorganosilylketene

P'~h~%

has been synthesized.

R3SI*CICOEt

The stereochesietry

(Ref. 27)

R3Si*Si*RJ

125-1300>

28

R9Si*CLi=C=0

of the formation and cleavage of silicon-

platinum and silicon-mercury bonds haa been studied.

29,29a

The complexes

[PtX(Si*R3)(PHe2Ph),], vhere X = Cl, Br, I. and [PtR(Sl*R3)(PPh3)2] vere used in these studies.

The absolute configuration of (+)-trans-

[PtCl{SiMe(l-C10H7) Ph](PMe2Ph)2] has been determined by X-ray diffraction vhich enables the stereochemistry configuration,

to be assigned.

R3Si*H + c&-PtCl2(P&2Ph)2

of the folloving reaction,

retention of

30

+

The work reported on optically-active

tram-[PtC1(Si*R3)(PMe,Ph)g]

silylmercuriala suggeats

that the configuration of sillcon is predominently retained in the forsraiion and cleavage reactions studied. 29a Some nev chlral org._nosilanes have been partially resolved by a lAnetic method:

phaoylethyImethylsFlane.

mesicylpbenylmethylsi_lane

ad

banzylphenylmethylsilane.

phenylisopropylmethylsilane.

indications that the observed stereochemistry

There are

for some reactions vith these

systems may differ from that reported in the literature. that the presence of a naphthyl group at the asymetric lead to a different transition state topology.

31

It

is

suggested

reaction center may

273 The kinetics of the reduction of triorganochlorosilanes (hydrosilyl)methyl and -ethyl Crigrmrd reagents are reported. 32 simplest

with The

scheme to explain the third-order kinetics, first order In silane

and second order in Grignard reagent, is given below:

Br

Cl'

~.__ 1 I I

Ph3SiC1 + Me2Si(H)CH2HgBr _3

?h3SI

B

I I SFMe2CH2MgBr

H ,

Ph.,SiR + Ue2Si(H)CE2SiMe2CR2MgBr

The reactions of ethyl bFs(triphenyls~lgl)dluminus and peroxides have been studied.

with mercury compounds

33 DMSO, DMFA and

The replacePent of various ligaads, EDIPA. DKM, TBP, from the complexes by pyrldine appears

RoSiC14_,. mL, where R = He and Ph and m = 1 and 2,

to proceed by a dissociative process.

34

Reactivity studies at bridgehead organosilicon compounds, 1-chloro-1-ailabicyclo[2.2.l]beptane 1,3,5,7-tetrasilaadamantane, compared Host

of

with the

explained of

with

an acyclic results

ti

terms

conflgurat

In

displacement

mechanism

are

reported

and 35,36

which

at

silicon

proceeds

with

are

best

retention

ion.

Fluoride ion catalyzes alcohols

reagents

tris(trimethylsilylmethyl)chlorosilane.

nucleopbllic

an %2-Si

1-chloro-3,5.7-trimethyl-

aucleophilic

analog,

for of

and

anhydrous

Lhe basicity Reference8p. 296

of

fluoride

the

acetooitrile.37 ion

in

this

reaction

of

trlalkylsilanes

The proposed mechanism solvent.

with emphasizes

F-

ROB +

RO- + R;SiH

The by

imidazole.

-I- ROH

reaction

38

sod

Evidence

is

an

trans.-silylation

The

studied

for

ehe

influence of

on

trimethylchlorosilane

yields

trkethylsilyl

of

the

42

by tJ?+RGethods.

HCl and H20.

The

initial

concentration

and

inversely

rate

the of

path

39

intermediate,

ad

the

in

by N,N-diphenyl-N-

and silver

OF Me SiOMe with 3

reaction uas

is

found

catalyzed

to be

proportional

or nercury

salts

41

Sic1

4

have

been

by MeOH, Et3S10H,

proportional

eo the

hydrolysis

2-benrothiazolyl-

dinitromethanenit~onats.

reaction

The

reaction

the

trinitromerhane

and by

kinetics

of

catalyzed

character

2-inidazolylsilaoes

of

is

formation

qucleophllic

high

trmmethylsilylarion

erinitronethane The

important

+ Ii2 + RO-

hexamethyldisilazane

presented

reactioos

trim+zthylsilylurea oi

with

The

has

silanes.40

ROSiR;

thiols

oE

RO- + HF

-

I-(tr~thylsilyl)~idazole. imino-nitrogen

--)

squara

to

of

the

the Sic1

catalyst 4

concentration. Extensive and

relative

rates

organosilicon

relative

constants

Studies

substituted 44

director.

reported

type

in

the

ortho

results

effects.

reported.

and para

the

He3Si-

errplained

in

electrophilic of

The cleavage

active

by

reduction

product.

with

lithium

X = Cl,

is

The

favored.

are

and dimethylfluorois

a poor

competing

of

of

cobalt

active

perfluorophenylsilicon

of

cr-naphthyl

the in

aluminum

F;

Einductive are

45

substitution.

optically

of

iododesilylation

complexes

a-naphtbylperfluorophenylpheoylmethylsilane followed

where

group

terms

distribution

bronination

oE trimethylsilyl

indicate

isomer

positions and

Silacyclopenradiene

46

3-nxn ’

65

bromodesilylation

are

on the

iron-catalyzed

C H SiMe

the

for

reported

and

of

The preparation is

iodine-

benzene,

to undergo

compounds

be=

on ehe nitration

These

and n-bonding

have

the

Bromination

rate

reported. sLly1

of

compounds

43

n = o-3.

studies

group

a BrCl-chloroform

hydride

yields

from solution

an optically

275

A few studies have been reported on substitution at Steric

than silicon in organosilicon compounds.

atoms

other

and electronic effects are

discussed for the metal-hydrogen exchange reactloo of trimechylsllyl substituted methanes. silanes are reported.

47.48 49

The modes of lithiation of vlnyltrimrthyl-

Nucleophilic

silicon in bls(trimethylsily1) By fast reac:ion

substitution at both oxygen and

peroxide has been observed.

50

techniques the rates of metalatlon of

9-methylfluorene were determined: Ph2SiCs > Ph2SFRb :* Ph_,SiK > Ph5SiNa With the otception of Ph3Sfil, the

> Ph3SrLi.51

reaccivrty

1s determined

by the ion-pair Coulombic binding energy.

ADDITION-ELIMlNATiON Probably the most cotanon addition reaction encountered

in

organosilicon chemistry is the addition of silicon hydrides to unsaturated The use of phosphioe-nickel

compounds.

complexes to catalyze

silylation oE olefins has been extensiveJy studied. studies,

the mechani6m

lylatioa

reaction.

of Scheme

1 ha6 been

52-54

Based on these ior

prOpO6ed

the hydro-

the hgdrosi-

Tha rols of metal-a-aJ_lylic intermediates in the pallaaium- and placlnumcatalyzed addition of trimethylsiloxy-substituted

silicon hydrides to

A mechanism involving a plarioum-

8-methallyl chloride is discussed."

diene-silane complex is proposed to explain the stereochemistry of the addition of methyldichlorosilane

to

pentadienes.

Kinetic and structure-orientation on the platinum-catalyzed alkynes. of

58,59

1-nonene

and

Rate

studies have been conducted

addition of silicon hydrides to alkenes =' and

constants

and activation

2-methyl-1-pentene

energies

by MeSiW.12,

(catalyst, R,PtClb) were determined.

ReIareoceSp 296

56

for

the

hydrosilylation

EtSIHC12 and Et2SiHC1

With alkynes, the reaction (solvent,

277 ion

on the

terms

of

addition

a mechanism

carbonyl

study

phenyl

of

of

of

cnreacted

been

addition

of

compounds

has

the

ketones

The

elimination

of

reaction compounds

au&de For

has the

anion

been

used

to add

.

r.

a

;

(10)

o-

_

[

lithium

vinylsFlanes72 in

terms

of

reaction

of

65

reported.

been

;c -

are

I

to

the CO

the

The

addition-

reagents

to

examethylpbosphorto carbonyl

cerbonyl

intermediates

to

47,66,67

discussed.

groups

68-70

compounds. radical-

compounds,

proposed.

1

_

c = c-o

1

(10)

(9)

Similarly,

the

On llthlation

organometallic

has

II

reagents

u-lithiovinylsilanes

the

is

a,+unsaturated

I

to

involvinn

compound may add

by

trinethylsilyl

of

and diradical

;c -

mechanism

tr~thylchlorosilane/magnasium/h

1,4_disilylation

(9)

situ

anhydrides

olefins

A

thiols

attention.

of

silicon-containing

to yield

The system,

l,&addition

acid

37

silicon.

and organometallic

considerable

--in

the

proposed.

vinyllithium

prepared

with

is

metals

received

49

a-llthiovinylsilanes

carbonyl

complex

to

orgaaosilicon

of

in

explained

ion

at

two-step

A

is

fluoride

substitution

64

reported.

alkali

of

addition

intermediate

vfnylsllane.

a.S-unsaturated

addition

amine-catalyzed

has

vinylsilanes,

the

to benzaldehyde

by nuclopbilic

an amine-thiol

Tbe unsaturated

involving

the

isocyanate

formation

triorganosilanes

followed

group

kinetic

of

in ether

solvents

and ketones73 radical-anion

yielding

solvent

system,

a.S-unsaturated

radical-anion

with

products

intermediates.

hexamethylphospboramide silane,

reacts

71

diarylacetylenes,

which

can be

accounted

for

Losing a l,Z-dimethoxyethanea potassium intermediates

mirror, bave

and an

been

detected

by ESR.74 Catalytic

systems

based

on Pd(0)

and Pt(II)

have

been

employed

278 to

add

trlmethylsilanol

proposed

75

to butadiene.

co explain

the

Metal-olefin

addition-dimerizaticm

intermediates which

reaction

are

yields

l-trimethylstloxy-Z.7-octadiene. Addition phosphines

with

for

the

silicon

with

addition

of

syaretry

adducts

has

The

reaction

of

of

aud of

reaction

of

silyl-

(trimethylsilylmethylene)-

triorgauosilyl

implications

H2Sa1 and RSeCla2

allowed

azides

are

also

with

discussed

to unsaturated

studled.

The

compared

been

been

deduced

observed.

on the

basis

a3 of

of

The high

triwthylsLlylcyclopeutadieue

to

ouch

addition

higher

the

demonstrated. g5

a4S + T?S additions

have

Ring-openiog

chloride

the

organo-

various

stereochemistry resolution

with

dienophiles of

the

NMR studies.

various

1,3-dienes

a4

reported.

ylldes.

for

compounds.

1:l

been

78

Hecbanistic

RSO3C1, a’

to silacyclopeotadienes

as

discussed

and ketenes,77

ketones,

79

chlorides.

The

is

76

a-diketones

triphenylphospborane perfluoroacyl

are

mechanisms

reactions

reactivity

silacyclobutane a5

of

for the

system

The silacyclobutane

small-ring

sllacyclopropane

toward ring

is

cleaved

mild

conditions a7

sllyl)ethylcyclopropane.

in

the

gas

The activated

I ,//; Cl'

_

H’

// /

is phase

complex

(ll)

a

(11)

been

system

reactions

by phosphorus

cleaved

to yieid

have

ring

ring-opening

3,3-Dimethyl-3-silabicyclo[3.l.O]hexane under

systems

by hydrogen

S-(chlorodlmethylis

proposed.

has

279

The addition of silylenes to olsfins and carbenes to unsaturated silicon compounds has been studied. 1,2-dichlorodisilane

The reaction of lithium with

and conjugated diu;es in the solvent. THF, yields

monosilylcyclopentenes.

88

A silylene intermediate is proposed.

The

nuclear recoil technique has been used to generate SiH2 in the presence of buta-1,3-diene.

89

Both

singlet and triplet SiH2 add to butadiene to

yield silacyclopent-3-enc. SiH2 .Si_H2 +

CR2 = CH - CH = CH,, c>

SiH2 :siH2

+

CH2 = CH - CH = CH2 ___3 /

l,l-Dichlorocyclopropane trichlorometbylsilane

derivatives are obtained in good yields

IS thermally

Kinetic evidence Is presented

for

decomposed the

in

iotewdiacy

The addition of :ChCO2Et to trinrthylsilovyalkines esters

probably

proceeds

I

through

Rekenceap.296

intermediate

presence

of

olrfins.

of dichlarocarbenz. which

yields

ti-keto

(12) .‘l

R\C /H2\Hco,,t

Me.pO

The reaction

the

the

when

/

(12)

of bromine and deuterium chloride with

cis-

and

10

280 traua-B-trl.methylsllylstyrene group

is

replaced

at

low

by bromine

92.93

epecificity.

For

are

proposed

to explain carbanion

a cyclic

chlorides

with The

the is

analogous class

of

the

to

of

state

of

is

reported.v7

Two independent

proposed:

a solvent carbooium

classical

Ion

ion

dehydrobromination

the

groups

mechanism

reaction

solvolytic

of

acyl

aqueous and

an important

study

ethanol

of

solvolysis

and methanol

El-ett

conelatioirs

leadiag

to products

and a process

formally

silicon, is

A Wetic

pathways

process

from

halides,

compounds.

in

assisted

be isolated

A bridging

the

for

alkyl

of

G ruwald-Winstein

observed.

stereo-

cerbonium

for

proposed

reaction

organosilicoa

Linear

can

of

cln addition-elimination

alkyl

(Z-chloroethyl)aryldimethylsilanes

been

products

proposed

B-substituted

&elimination

reactions

degree

95,96

CE12=C(OR)OSiEt3. of

a high

trimethylsilyl

stereochemistry.

94

transition

cleavage

with

the

trimethylsilyl-stabilized

trams-2-bromviuyltrimetbylsilane.

involving

in which

additfon

reactions,

Bridging

trimethylsilyl-stabilfned of

products

and deuterium

both

temperatures.

intermediates

gives

involving

the

has are

are non-

(l3).

R3 Si

\

/‘+

‘\

c”2-m2

(13)

Kinetrc of

studies

acid-

and base-catalyzed

2-haloethyltrimethoysilanes

a n-amber

the

on the

of

leaving

pathway. substituted

mechanisms.

group

are

A mechanism carbonium

98

indicate The

nature

that of

the

important

factors

lnvolvieg

rate-determining

ion

is

proposed

in

for

hydrolysis the

process

the

can

substituents

determining

and fragmentation proceed

on silicon

by and

the mechanistic

formation acid-catalyzed

of

a silyldecoimposition

281 of (I-hydroxyethyl)aryldimethplsilanes

In aqueous methanol.

of various reagents to 1-sila-3-cyclopentene

99

The addition

gives g-substituted silicon

compounds which undergo ring-opening reactions through a b-elimiootion process.

100

Elimination reactions also have been reported for the acidcatalyzed cleavage of allylic organoailicon derivatives

70

and the dehalo-

silylation of (o-hslophenyl)trimethylsi_lanes by potassium t-butoxide or tetramathylammoaium

fluoride in aprotic solvents.

101

A benryne inter-

mediate Is proposed for the latter process. The ions formed in the mass spectrometer from 2,2,3,3-tetrafluorocyclobutylsilanes 102 and perfluorocyclobutylsilanes 103 undergo B-elimination reactions yielding olefins; molecular

ions are usually absent.

REXRIUNGEKENT One of the mst

common types of rearrangement of organosilicon

compounds involves tbe migration of a silyl group between two or

more

atoms.

The facile migration of silyl groups from Ccc in nonrigid silyl-olefin ring systems (fluxional behavior) continues to receive temperature PMR probes

of the S-silylcyclopentadiee

pentadieoe systems indicate a 1,2 or random shift of PHR studies are also reported on pentadiene.

105

trls-

atteotion. and

Variable

5-silylmethylcyclo-

the silyl group. 104

and tetrakis(trimethylsilpl)cyclo-

pate and equilibrium data are reported for the thermal

isomsrization of several substituted silylindenes.

106,107

The rate data

suggests that the thermal rearrangement of 5-trimethylsilylbicyclo[2.l.OIpsntane to 3-trimethylsilylcyclopentene intermediate

proceeds

through the diradical

(14), followed by digration of the trirnethylsilyl group. 108

(14)

The kinetics and stereochemistry of the silaallylic and silapropynylic rearrangements are reported.

109

Stereochemical

studies using optically

active compounds and deuterium labeling techniques show that

each

act

of migration is accompanied by inversion of the silicon configuration. Thermolysis of the diazo r;trivatives of a,B-unsaturated

esters gives 110

rise to products which entails the migration oE a trLmethylsily1 group. 1,2- and l,L-trimethylsilyl l,2-diphenylethane

shifts occur when l,l-bis(trimethyleilyl)in hexane. 111

is lithiated with n-butyllithium-Rim-4

O+O migration of sllyl groups has been reported for trimethylsilyl 112,113

esters of nitrool9 acids systcms.l14-U6

and triorganosilyl ethers of 8-dicarbooyl

With the carbonyl derivatives, migration of the silyl group

occurs with retention of stereochemistry.

114,115

Retention of configuration

at sLlicon is observed for the cis-isomer and inversion for

C-O

the

large

bond

raar-raogatits

energy

are of

c~~lllon

in

orgzmosilicon

the silicon-olrlgen

bond.

the

trans-

chdstr)

First-order

due

to

kinetics

are

repor,ed for the thermal rearrangement, Si-C-O-N + N-C-0-Si, of 1-[bis(trifluoromethyl)aminoxyl]alkylsilar~es.

117

The

thermal

decomposition

of organosilicon peroxides, a first-order reaction, yields products consistent

with

118-121 o_xygen.

an

intrarmlecular

migration

of

silicon from carbon to

1-Trimethylsilyl-1-(phenylsulfinyl)ethane

O-trimrthylsilyl-S-pheayl Several C-N

throacetal on heating

to

80°C.

rearranges to

122

rearrangements of silyl groups have been reported.

2-Trimethylsilylpyrrole

is unstable and slowly converts to the N-isomer.

When N-trimethylsilyldibcnzylacetamidine

123

(15) is lithiated with n-butylithiun

the trimethylsilyl group migrates from nitrogen to carbon yielding N-henzyl-N-'[pbenyl(trLmetbyleilyl)]methylacetamidine Tne Process is silane

is

where

R =

a

reversed,

catalyst

(16) + (15), on heating to

for the rearrangement,

He, Et, n-Pr, i-Bu.

125

(16) on alcobolysis. 124 >95O.

TrFmethylbromo-

283 0-N

migration of silyl groups has been observed.

of organosilylhydroxylamines

undergo a 1,2-rearrangement

organosilyl group migrates from oxygen to nitrogen. 126

The anions

in which an Bis(organosilyl)-

nitroride radicals are formed by hydrogen abstraction and radical r-arrangemeot of N.O-bis(organosilyl)hydro.xylamiaes. groups in N.O-bis(organosilyl)hydroxylarmnrs

127

The exchanga of organosilyl

is reported, and the intermediate

(17) is suqgested.l28

The migration of a phenyl group from silicon to carbon occurs when (chloromethyl)phenyldimethylsilane

and potassium acetate 10 anhydroui

acetic acid are heated to 240-250°.12g Two papers report on the Cope rearrangement of organosllicon compounds.

The silicon-Cope rearrangement of cis- and trans-propenyl-

allyldlmethylsilane

is proposed to proceed through the double bonded

silicon intermediate (18).13'

(18)

'ihe siloxy-Cope rearrangement of cis-l-trLmethylsiloxy-l-vinylcyclooct-3ene yields predominantly ring contraction products. are proposed.

131

Diradical intermediates

284

iEDISTRIBllTION Redistribution have been studied.

A

reactions Involving only organosilicon moieties

quantitative study

of

the

redistribution equilibria

of fluorine -vs. methoxy, thiomethyl and dimethylamino groups between 132 Based on these and methyl-substituted silicon moieties is reported. published data, the order of eubstituent groups vith respect to preferento

tial affinity for the least alkylated silicon was determined > me

with vinyldimethylethoxysilane

by tiMR and gas-liquid chromatographic methods. The thermal disproportionation and dlalkylfluorosilanes

135

tion OF phenyl-containing some disproportionatioo The dimer and

trimer

Fncerconvartible.

have been determined

133

siloxanes in the presence of alkaline catalysts.

occurs through migration of the phenyl groups.

species

of

pheoylmetbylcydosilthianes

are

136

thermally

137 the

acid- and base-catalyzed cyclization

kinetic data, a four-centered qechausm

the intermediate

134

During the thermal degrada.

of (w-etho_xysilylalkoxyy)erimeehylsilanes have been determined. the

of

of diorganoalkoxyEluorosilanes

has been observed.

The relative rates of

on

OMe

Tbe constants for the equilibration

2>/H>F>/SMe>Cl>Br.

dimethylchlorosilaoe

be

138

Based

is proposed involving

(19).

(19)

(Chloromethyl)mathylchlcrosilanes

undergo condensation-disproportionationatlon

285

139 reactions with trichlorosilane in the presence of tertiary amines. These redistributions “a-ef

may be associated

moieties

also

have

reactions

been

effects

and steric

between

organosilicon

support

a SRI mechanism

than

Sn-R

bond

pentacovalent between

effects

mercury

of

where

and

silicon

used of

Me2SiXg

bond

MegSiX

and HeF2,

Y = Br,

exchange

of

the

OPh,

the

IVb

free

published.

radicals

Arrhenius

exchange 141

extended.

142

products.

by PMR procedures

NFle2.

143

and bromine

moieties

are

equilibrium

between

discussed. the

the

dimethyl-

PNR methods

condensation

and MeP(0)Y2. where X and Y = Cl,

for

moieties:

The

article

radicals

covering with

the

particular

preparation

OMe.

also

to

144

&nd chemistry

emphasis

on sllyl

reported

on the

studies

by atom transfer

parameters

silar~e.~~~

Relet-ewes

p.296

of

radicals

has

145

have

been

have

been

and chemical measured

for

activation the

reaction

146 with silane and methylsllane and for the reactionof with

been

M&(S)Y2,

and

important

an unstable

unsymmetrical

MeP(O)Yg,

results

bis(perfluoroalkyl)-

measured

between

more

have

with

SHe,

The

reactions

oxygen-halogen

chlorine

exchange

140

is

tbe

moieties

been

exchange

involving

on

compounds favors

the

formation

mechanism

-tLn

have

for

isotope

RADICAL

Mechanistic silyl

and

and non-silicon

compounda.

Studies

fluoride

to study

A review

been

iun

and methylthiophospbonyl been

0-Si

intermediate.

constants

of

determined

a rwo-step

Br. OPh, We;

kinetics

oligomers

group

OS the

stereochemistry,

and alkoxytin

crganosilylmercury

betuzen

X = Cl,

data

or

and mercury(I1)

exchange

been

in which

formation

Equilibrium

FREF.

occurrence

organosilicon

Kinetics,

have

hydrides

silicon

between

studied.

organosillcon,-german

The exchange

have

the

feet”. Redistribution

the

with

The

kinetics

of hydrogen atom abstraction

generation

of

processes. of

methyl

higher from

alkyl

radicals radicals

chloro-

236 silane

by trifluoromethyl radicals have b2an determioed.

148

Decomposition

and ethylsilane produced

kinetics of ch2mically activated dimethylsilane

by reaction of singlet qethylene with methylsilane are reported.

149

Rate constants for various decomposition paths were deduced from the kinetic data.

A free

radical process IS proposed for the reaction of

nitrogen droxide with trxcethylsilane and triethylsilane in which trlorgaoosilyl radicals

are

by

formed by hydrogen extraction

2nd radx2.l intermeorates. 150

nitrogen dioxide

Data are presented which indicate

that branched-chain polysilanyl r2dicaLs generated by hydrogsn extractzon by t-butoxy radicals in benzene solution survive until dimcrization occurs.

151

A radical pathway, rather than a four-centered molecular process

1s suggested for the thermolysis of ethyltrichlorosilane methylsil2ne.

152

2nd ethyltri-

Thr primary process in each case is the dchydrosilylatioo

reaction. Several studies have been reported on the photolysis of silicon compounds to produce silyl radicals.

Mechanistic pathvays involving

sibyl radicals are proposed for the room-temperature mercury 6(3pl) photosensitization of disilane.

153

have established the nature radxal

of

Kinetic, isotopic 2nd scavenger studies the initiation step.

The trichlorosilyl

has been prepared in.an adamantane matrix by irradiating ethyltri-

chlorosilane.

154

ESR studies indicate that (d-p)Tibonding between silicon

2nd chlorine is importznt.

Mechanistic studies indicate that the trichloro-

silyl radical is an intermediate in the photoinduced reductions of methyl acetate and ethyl methyl ether by trichlorosilane.

155

The photolysis of

acylsilnnes in non-polar solvents yields silyl radicals. 156

In non-

halogenaced sol=enrs, s.,

pr-lmarily

-_r:;rct 9:

r-b

silgl

cyclohexane.

r-_di_cal c.2 -32

the products

rar5x:;l

oT:?3

Photolysis of bis(trimethylsilylmethyl)mercury

GI

result L:e

of

an

+?..~si?a.e. . -

yields products resulting

from reactions involving the radicals, Ue3SiCE12~, Me3SiUltraviolet irradiation

from

2nd

Me-.

157

optically active organosilylmercurlal,

(R3Si*)2Rg. in cyclohexane, givee the disilane, (R3Si*)g, in high yield.2ga

A free least

radical

process

partially

retain

is

their

Trimethylsilyl process

when

such

naphthalene,

as

suggested

radicals

fluorene

free

radical

of

the

bond

in methylsilanes

lodine

and

to calculate

the

considerably

higher

sis

of

found

have

been

used

been

reported.

phase

of

at

200°

OF

trichlorosilane also

to

has

flucrodisilylene, unsaturated

synsgtry the

studied.

been

hydrocarbons

cycloaddition conservation

acetylene

Refezeneeop Z96

molecule

reaction, requires

is

chlorine

The (cf. -

perpendicularly.

of

The

tetrafluorodisilane The

ease

from in

the of

gas

the phase

acetone

thermal

addition

oE ethyltrichlorotriplet

silylene 89

165

principle unit oE addition

with In

of

to

Tetra-

reactions

chloride. the

data

compounds

of

presence

163

cycloaddition

of

has

kinetic

determined

Addition-Elimination).

application tte

the

are

thermoly-

process

the

study

decomposition

trifluoroscetyl

the

organic

been

A kinetic

addition

obtained

atom extraction

have

in

used

energy.

various

reversible.

the

The and

tritiuin

obtarced

of

161

energies

between

The values

dissociation

of

undergoes and

160

with

162

and

a diradicai,

the data

process,

radical

152

observed

164

reaction

to ethylene

ethylene,

been

gas-phase and

determined

dissociation

The kinetics

bond

rates

been

Erom recoil

the

radicals

reaction

a substrate

estimated

reinvestigated.

technique.

the

bond

energy.

trimethylsilyl

that

have

silyl

trichlorosilane

indicates

buta-l,?,-diene

olefin

the

relative

studied,

Si-Si

with

transfer

and Si-Si hme

free-radical

relative

by a cospetitire

photoaddition

silane

by

at

158

estimates.

been

the

of The

been

previous have

reactions

of

dissociation

to calculate

chlorides

gas

than

treated

Si-H

The

kinetics have

bond

radicals

+ ArtI.-

have: bi-en

to be a short-chain,

The

albyl

The

hexamethyldisilane

been

have

Si-El

L-IMP;\is

of

reactions.

trimethylsilane

R3Si”

by an electron

benzoph=one.

energies

studying

159

formed

Me3Si.

by

yields.

in or

dissociation

Bond

abstraction

are

m

hH

the

ConEiguration.

trlmethylsilylsodium

Me3Si - +

Si-H

in which

the

orbital

to approach of

silyl

288

radicals to the carbonpl group is related to the nature of the carbonyl compound, diketones > oxaletes > ketonee I trifluoroacetates > >

acetatf:s.i66

formates

The silyl radicals are more reactive than radicals of the

other group IVb elements of radicals wchanlsm

sbilar

structure,

excluding carbon.

A

is proposed for the addition of triorganosilicon hydrides

to N=N bonds of dialkyl azodicarboxylates

and azobenzene. 167

Bomolytic substitution at silicon, SB2. is proposed for the reactisn cf trimettylsiio,_: radicals r*th trimethyl~Ll:.-1t-but)-1 pnrotide to generate t-butylperoxy radicals.

168

The photo-oxidatioo in THF and

peroxide oxidation of 1,1,2,2-tetramethyl-3,4,5,6-tetraphenyl-l,Z-disila3,4-zyclohexadiene attack

oE a peroxy

radical

is

proposed

hydraperoxide

with

to the cyclic disiloxane are proposed to proceed radical as

at

the

Sf-SI

an intermediate

phenyl

isocyanate.

bond. in

the

169

by

The trlphenylsiloxy

reaction

of

triphenylsilyl

170

Other radicals containing trimethylsilyl groups have been Pyrolysis

reported. reported

to

of Et3SiCH2CE12C020-t-Bu and Et3SiCli2C020~~2Ph

are

form the radicals Et3SiCE2CB2- and Et3SiCB2- respectively.

Unen 2-bromoalkoxysilanes

ruct

171,172

with magnesium in diethyl ether solution,

elimination, coupling and dlsproportionation

products are obtained.

173

A qecbwsm

is proposed in which the Grignard reagent decomposes forming II the radical, Me SiO-t-r-. Tris(trimothylsilyl)hydrazyl and 3

tria(t-butyldimethylsilyl)hydrazyl

radicals have been obtained by electro-

lytic oxidation of the lithium salt of the parent hydrazine and by the ultraviolet photolysis of the parent bydrazine. 174

The photolysis of

bis[bis(rzrimethylsilyl)methyl]tin(II) yields the unusually stable free radical, tris[hls(trimethylsFlyl)methyl]tin(III).175

The overall process

is vlauerl as a photochemical disproportionation. The products of the photolysis of 1,2,4,5-tetrakis(triaethylsilyl)benzene have been determined.

176

It is proposed that the reaction proceeds

vln a syzzaetry allowed concerted process from excited states of 1,2,4,5tetraids(trimethylsilyl)benzene

and/or via the intermediate

(20) vhich also

289

may be involved Fn a symmetry forbidden rearomacization.

:o SiMeg

Me+

H

.

H

-,'/

I/

SiMe3

Stile2

(20)

JiNSERTION

Insertion reactions are commen for silylenes and carbenss. Arrheniua parameters and relative rates of the insertion of silylene, prepared by pyrolysis of disllane, into Si-Si and Si-H bonds are reported.

177

Similarly, silyleaes xere prepared by pyrolysis oE Si2F6, FSi2H5. ClSi2HS, l.l-F2SiH4, l,l-C12Si2H4 and (MeSiRC1)2.178 The following order of silylene Insertion rates into Si-H bonds was determined: > FSIH I=> SiCl2, SiF2.

The relative

methylsilanes and methylgermanes

rates

oi

Sit3

2

> ClSiH

SiH2 insertion into several

in competition with disilane can be

correlated with steric effects and the hydridic character of the N-H bond. Pyrolysis of trisilane yields the diradicals, SiH2 and Si2H4. 179 of R2Si species into Sl-8 bonds can account phosphine)chlororhodium-catalyzed

for

the

Insertion

tris(triphenyl-

disproportionation

It is proposed that a rhodium-"silenoid" This

of polyhydromonointermediate is formed.

rhodium catalyst also is effective for insertisq R2Si species into

O-H bonds of alcohols.182

The following mechanism is proposed for the

reaction of hydropolysilanes

with elcabols.

179,180

290

I

I -Rb

-KSiZ>

-Si-Si=

/

I

+ ROSiK \ I

The insertion of photochenucally generated wthylphenylsilylene

into the

allylic C-K bonds of cyclohexene is proposed to proceed by the following mechanism.

183

K

PhSiKe

0

I + Ptl&?S1:

4

I b

j_

The formation of hexamethyl-1 ,2-disila-3-cyclobutene tharmolysis of 1,2-dfmathoxytetramethyldisilane dlrrethylacecylene of into

a

1s

propose-d

s~lacyclopropeoe a C-Si

bond. The

to

proceed

intervcdiate

by

followed

a

by the vapor phase

in the presence of mechanism

by

insertion

~ovolviog of

fornation

dimethylsilylene

184

photolysis

of

acylsilanes

in

polar

solvents,

e.g.,

alcohols.

acetic acid, hydrogen cyanide, leads to the isomeric siloxycarbenes. The silxycarbenes of the solvrnt.

156,185

act as nucleophiles and insert into the polar K-X bond The kinetics and stereochemistry of the photolysis of

acyleilanes also are reported.

185

The insertion of carbenes into Si-8 bonds has been atudied. Fluorobromocarbene,

formed by the thermal decomposition oE phenyl(fluoro-

dibromomethyl)mercury,

reacts with triethylsilane

to

yield products which

are attributed to insertion of the carbene into the Si-H bond. compounds of group VI metals,

186

Carbene

(CO)5MC(X)C6Ei4Y, where H = Cr. Ho, W;

X = OHe, NCnH2n(n = 2,4); Y = @Me,

@!e,

ii,e-Cl, react with triorgano-

291 sllanes Si-H

to

yield

products

bond. 187 ‘la8

group

IVb

metal

The

resulting

relative were

chromium

studies

on the

silicon

hydride

attacks

carbene

indicate of

into

that

to

the

trichloromethylsilane

insertion

thermolysis

of

from

xnsertion

Rate

Trimethylsilyl-

and

such

sulfur

trioxide.

cis-

and

as,

these

which

have

linear

been free

insertion

tr-inetbylsilyl With

lg2

energy

the

thermal

for

when

reaction

addition

and

generated

bond.

products

the

from

yields

Si-Cl

co insertion

of

studies

generated

correletlons

products

the

phenyl-

Kinetic

yielding

the

and

and

insertion

and oethylene,

products

190 have

oronolysis were

been

of 191

obtained.

tr~sth~lsrloxuallen7es treated

chlorosulfonate the

from

189

olefins

determxned

various

in which

of

hexachlorodisilane

lead

with

trans-1,3-dimethyl-

in

into

the

complex.

dichlorocarbina,

species

in

experiments,

resulting

bonds.

trimethylsiloxyalkynes

trroxide

agents

of

, with

reactions

and

sulfur

of

constants

triorganosilanes,

yield

reaction

@u-catalysis)

Two other studied.

The

compounds

decomposition

an intermediate with

carbene

a mechanism

phenyl(trichJ.oromethyl)mercury,

f ram diazomethane resulting

is

the

products

derivatives 90

products.

with

and g-C-H

dichlorocarbene

these

the

competition

suggest

from

give

Sl-C(ring)

of

carbene-chromium

reacts

1-o-butyl-1-silacyclobutane the

complur

generated

(bromodichloromethyl)mercury

of by

determined

an activated

Dicblorocarbene,

insertion

reactivities

hydrides

kinetic

from

with

and dioxane

trimethylsiloxyalleoes,

sulfonating containing

rearrangements

ma>

occur.

DECOM.POSITION

a variety etc. one of

The mechanisms

of

of

free

processes:

Decomposition the

above The

decomposition

reactions sections

kinetics

of

radical, not

because the

reactions ionization,

included

It

thermal

was

in

this

may be compluc elimination, section

involving

rearrangement

may appear

deemed more appropriate.

decomposition

of

tetramethylsilane

in

other

in the presence of ammonia forming silicon nitride, Si_,N4. and have been atudied.lg3 7-silanorbornad~enes

products

The rate coaseants for the thernal decomposition of by elinuna:ion

of silylenes have been measured.

194

Stability toward silylene elimination is increased by bulky groups and electron-withdrawing

substituents.

The pyrolysis of carbcn-functional leads to elimination products to silicon.

when the functional group is alpha or beta

The race of the thermal decomposirion of trichloromechyl-

erichlorosilane in the gas phase reported.

silicon compounds, usually

195

The unimolecular

teerachloride

accounts

for

in the presence of ethylene hss been

formation of dichlorocarbene

about

90% of

the

decomposirion

and silicon Th*

reaction.

rates of the thermal decomposition of 1,1,2,2-tetrafluoroethylWifluorosilane and -crimethylsilane are kinetically first order. proposed

mechanfsm

involves

the

formation

of

ehe

196,197

The

Cdifluoromethyl)-

intermediate

fluorocarbene in the rate-determining step which, in the absence of other compound, iswerizes

quantitatively to rrifluoroeehylene.

A

kinetic study

of the gas phase pyrolysis of the three isomeric 1-(trimethylsilylphenyl)ethyl acetates has been reported.

198

The erimethylslyl substituent activates in

all positions according to the sequence, 0 > m > E. of Eaminoethylsilanes Studies ccnpounds have

and their

on fbe

been

either

with

salts

has been studied.

reported.

Kinetic

Fn solution

studies

of

indicate

itself or with the solvent.

200

proceed by a four-centered molecular process or via Pyrolysis of sflylphosphine and germylsilane

yields

accounted for in terms of the titermediates, SiH respectively.

201

199

thermal decomposieion of silicon-functional

of bls(tr~etbylsFlyl)mercury occurs

quaLernary

The thermal stability

First-order

2’

the

thermal

deconposi~ion

a bimolecular

reaction

These reactions wy free radical products

WI, -

which

intermediates. can

be

snd, SiH 2, &E2r

kinetics are observed for the thermal decal-

position of N-Crimethylsilylamides

of carboxylic acids.

data suggest a mechanism involving N -0 silyl group before decomposition.

202

The kinetic

rearrangements of the triorgano-

The simultaneous processes of a siloxane-

293 type

degradation

leading

to elimination

of

cyclic monomer and a radical-type

splitting of the Si-C and C-C bonds in the hydrocarbon segmeot followed by .subsequent reactions are proposed for the pyrolysis of poly[alkylenebis(dimethylsiloxane)).203 A number of decomposition Si=C intermediates are proposed: butane,'04 pymlysis

reactions have baen reported for uhich

pbotolysis of l,l-diphenylsilacyclo-

of l,l-dimetbylsilacyclobutane, 205'206pyrolysis of

4_silaspiro[3,3]heptane,

207

and photolysis of trtmethylsilyldiazoacetate.

208

Evidence in support of the proposed Si=C intermediates is based on reJctioos with alcohols, 204,208 aldehydes and ketones, 205 and acetonitrile and its

The unusually high thermal stability of triorganosilylmethyl derivatives of transition metals has been attributed to the absence of E-hydrogens which prevents decomposition

via alkene elimination and the

large steric requirements of the bulky organic groups hindering the vacant coordination sites.20g-212

to

BCCBSS

Thermolysis of the oxido-carbene

salts, (CO)5CrC(ONR4)CH2SiMe3, where B = Me, Et, results in fission of the CR2-C(carbene) bond, yielding tetramethylsilane

and the zwitterionx

carbene

compound, (C0)5CrC(0-)CR9'N+R3.213 A number of studies have been reported on the fragmentation of organosilicon compounds in the mass spectrometer: linear organopolysilanes,

215

2,2-difluorocyclopropyl-

sileoes.216 2.2.3.3-tetrafluorocyclobutylsilanes, silanes,

103

orgsnocyclopolyeilaoes,

silacyclopentanols,

217

102

214

and 3,3-difluoroallylperfluorocyclobutyl-

silacyclopent-3-ones,

218

phenoxyphenyl-

and tolyloxyphenylsilanes, 219 trimethylsiloxy derivatives in tetramethylsiiane,220 and trlmethylsiloxy derivatives of steroids. 221,222

POLYHERIZATTON A kinetic study of the polymerization of monomeric silicic acid in aqueous bydrochioric acid solutions has been carried out, and the simple ReIerescesp. 296

polymeric species isolated by chromatographic

qean3.223

The polymerization

proceeds via the di-, cyclotri-, cyclotetra-, cyclohexa- or doubleThe hydrolytic polycondensation

threering-silicic acad as mtermediates. of nechyltrichlorosilane

ln the presence of a hydrogen chloride acceptor

proceeds with the Eomtion

of soluble polycyclic compounds and is inde-

pendent of the qethyltrichlorosilane solvent is employed. cioa products OP the to

Easter

rate

224

concentration when a water soluble

The formation oE solubla hydrolytic polycondensa-

methyltrlchloroszlanr of

the

under

intralecular

these

condensation

termination of the growing chain.

Idrared

conditions reaction

is

due

which

to

leads

and ebullioscopic studies on

alkali necal silanolates in media of low dielectric constants indicate dimer io-tion.225

These rasults readily e.xplain the observed reaction order

of 0.5 Eor the alkali metal silanolate catalysts in siloxane polymerization reactions. The efr'ectiveness of mecal halides for catalyzing the polymerization of l,l-dimethylsilacyclobutane butane has been evaluated.

226

The

and 1,1.3.3-tetramethyl-1.3-disilacyclostudies

reveal

that polymerization

occurs via cleavage of a ring Si-C bond uxth the formation of a linear polymer. zation.

Kinetics studies are reported for the CuCl-catalyzed polymeriDilatometric methods have

n-butyllichium-initiated methyl-l-silocyclobutane.

been

used

to

study the kinetics of the

polymerization of l,l-dlmethyl- and l-phenyl-l227

The polymerlzatron

first order In silane and one-half

order

in

reaction was found to be

n-butyllitbium

for

S-15%

conversion.

DIRECT PROCESS Several papers have appeared during 1973 which deal with mechanistic aspects of the so-called direct process. occurring

on

the

surface

of

the

A chain

process

contact mass of silicon-copper powder

which proceeds with the participation

of initiators is proposed.

228

The

initiators

of

surface

of

the

the

chain

contact

donors

(CuCl,

Xl).

silane

in

presence

CO the

conclusion

the

mechanism.22g reaction

of

of

that

methyl

the

are

formed of

Studies

proposed

to be SiCLn

by reaction

the

direct

with

the direct of

The

synthesis

the

reduced

of

activity

to

formed

chlorine

led

the

authors

by a cheoisorptlon

contact loss

on the

mass

in

explained

is

or

on the

methyltrichloro-

tolurne

the

initially

products

of

proceeds

surface

leads

intermediate

or

coopounds

chlorine

synthesis

methyldicblorosilane

chlorude

surface.230

sumption

mass

Chlorination

with

silicon

process

beEore of

activity

in

terms

surface

of

the

con-

during

chlori-

nation. The

copper

decomposition

and silicon-copper-calcium

mined.231 ethyl

Addition

chloride,

formation bismuth ethyl

of to

the

using

a

mechanism

and after

234

is

can be

pbenylchlorosilanes

References

p. 296

yield

the

of

of

the

the

235

reported.

and broken to the

on the

surface

chloride has

mixtures been

have

diract

of the

synthesis

chloride

type,

an

by

x-ray

These

of

on the

determined.

indicates

that

(hydrogen

mass

in before

indicate

an intermetallic and

the

catalyst effect

phrnyltrlchlorosilanes

course 237

of

phenylchloro-

The mechanistic

reactions.

236

of

of

electron

reaction

Cu Si. 3

the

233

difEraction,

studies

and

indicated

contact

down during of

of

as

of

microamounts

initiator

SiCln,

deter-

decomposition

sixcures

of

been

mass,

diethyldxhlorosilano.

synthesis catalytic

silicon-

decomposition

contact

inhibits

silicon-copper

amount

direct

the

The addition

involvement

of

on the

masses

on the

mass

chlorobenzeoe

formed

of

ethylene

on the

compounds

related

terms

t.;drogen-hydrogen of

with

with

enhances

and hydrogen

An ucazaination

and ESR is

In

the

chloride

contact

231,232

data

the

holds

tricblorosilane

discussed

of

contact

chlorobenzene

compound CuSSi

of

of

reaction

microscopy

activity

adsorption

and surface

228.

silicide

silicon-copper

silanes

chloride)

calcium

and enhances

ethyl

of

sillcide

ethyldichlorosilane.

chloride

chain

of

the

Analysis

Ref.

products

T-ne effect

the

direct

is of

synthesis

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