The OzoneHydrosilane Reaction: A Mechanistic Study

6 The Ozone-Hydrosilane Reaction: A Mechanistic Study

Downloaded by CORNELL UNIV on July 27, 2016 | http://pubs.acs.org Publication Date: June 1, 1972 | doi: 10.1021/ba-1972-0112.ch006

LEONARD SPIALTER, LEROY PAZDERNIK, WILLIAM A. SWANSIGER, GLEN R. BUELL, FREEBURGER

STANLEY BERNSTEIN, and MICHAEL E.

Chemical Research Laboratory, Aerospace Research Laboratories, Wright-Patterson AFB, Ohio 45433

The reaction

of a hydrosilane

quantitative

conversion

moiety.

The

mechanism

elucidated.

It involves

ozone (acting

3

silanol.

·OΗ

radical

Extensive of

mono-,

di-,

bond

rapid,

the

Si-OH

has now

been

silicon

of

atom,

fol

attack by the

bound

and decomposition

into

recombine

concerning properties and

to

complexation

the

electrophilic hydrogen,

other structure-dependent number

with

pair which

data

Si-H

of this conversion

as a nucleophile)

ozone upon the hydridic R Si·

the

a fast, reversible

lowed by rate-determining a

with ozone results in the

of

the

to produce

relative

in the

trihydrosilanes

ozonation are

the

rates

and of a

presented.

* T p h e use of o z o n e as a n o x i d a n t f o r o r g a n i c substrates has b e e n k n o w n A

f o r s e v e r a l decades, a n d the s c o p e a n d m e c h a n i s m ( s ) of these o x i d a

tions h a v e e x p e r i e n c e d c o n s i d e r a b l e i n v e s t i g a t i o n (1,

2).

H o w e v e r , it

was n o t u n t i l 1962 t h a t the i n i t i a l r e p o r t of the o z o n e o x i d a t i o n t i o n ) of organosilanes a p p e a r e d

(3).

(ozona

I n 1965, i t w a s d i s c o v e r e d t h a t

ozone w o u l d rapidly, cleanly, a n d quantitatively convert an S i - Η moiety to the c o r r e s p o n d i n g s i l a n o l ( S i - O H ) (4).

S u b s e q u e n t i n v e s t i g a t i o n s of

this r e a c t i o n e l u c i d a t e d its s c o p e a n d r e s u l t e d i n suggested m e c h a n i s t i c p a t h w a y s (5-11).

A l t h o u g h the o z o n a t i o n of the C - H b o n d is r e l a t i v e l y

s l o w a n d y i e l d s , i n g e n e r a l , a m i x t u r e of p r o d u c t s ( I , 12-15),

that of the

S i — H b o n d is r a p i d a n d affords o n l y s i l a n o l . T h e p r e s e n t p a p e r describes a r a t i o n a l m e c h a n i s m f o r t h e o z o n a t i o n of the S i — H b o n d .

D u r i n g the

i n v e s t i g a t i o n , c o n s i d e r a b l e d a t a c o n c e r n i n g s u b s t i t u e n t e l e c t r o n i c effects o n s i l i c o n w e r e a c c u m u l a t e d a n d are d e s c r i b e d .

Their implications i n

o r g a n o s i l i c o n c h e m i s t r y are also d i s c u s s e d . 65

Bailey; Ozone Reactions with Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1972.

66

OZONE REACTIONS W I T H ORGANIC COMPOUNDS


Experimental Materials. Solvents u s e d i n c l u d e d hexane, c a r b o n t e t r a c h l o r i d e , a n d m e t h y l e n e c h l o r i d e , a l l of s p e c t r o q u a l i t y a n d f u r t h e r p u r i f i e d b y s a t u r a tion w i t h ozone followed b y nitrogen p u r g i n g a n d distillation. F o r ozone g e n e r a t i o n , o x y g e n gas, d r i e d b y passage t h r o u g h a d r y ice-acetone c o l d t r a p , was i n t r o d u c e d i n t o a W e l s b a c h m o d e l T - 2 3 l a b o r a t o r y ozonator o p e r a t e d at 10 volts a n d 7 p s i g . T h e effluent gas, a b o u t 4 % ozone i n o x y g e n , w a s passed t h r o u g h a n adjustable s t r e a m splitter for flow c o n t r o l a n d t h e n b u b b l e d t h r o u g h a saturator c o n t a i n i n g the solvent also b e i n g u s e d i n the r e a c t i o n system. ( F o r the experiments w i t h oxygen-free ozone, the o z o n e was first s e l e c t i v e l y a d s o r b e d f r o m the ozonator effluent s t r e a m o n t o s i l i c a g e l at — 77 ° C a n d t h e n d e s o r b e d b y w a r m i n g a n d e l u t i o n b y a d r y a r g o n stream. ) T h e h y d r o s i l a n e reagents u s e d , w h e r e a v a i l a b l e , w e r e o b t a i n e d f r o m c o m m e r c i a l s u p p l i e r s s u c h as P i e r c e C h e m i c a l ( R o c k f o r d , 111.), P e n i n s u l a r C h e m Research (Gainesville, F l a . ) , and Matheson, Coleman and Bell ( N o r w o o d , O h i o ) . O t h e r s w e r e s y n t h e s i z e d b y the l i t h i u m a l u m i n u m h y d r i d e or d e u t e r i d e ( A l f a C h e m i c a l , B e v e r l y , M a s s . ) r e d u c t i o n of a p p r o p r i a t e o r g a n i c c h l o r o - or fluorosilane, either p u r c h a s e d f r o m the a b o v e - m e n t i o n e d sources or s y n t h e s i z e d b y c o n v e n t i o n a l routes i n v o l v i n g o r g a n o m a g n e s i u m or - l i t h i u m condensations w i t h halosilanes. A l l s u c h c o m p o u n d s w e r e at least 9 8 % p u r e as d e t e r m i n e d b y v a p o r - p h a s e c h r o m a t o g r a p h y ( V P C ) . S t r u c t u r a l i d e n t i t y was e s t a b l i s h e d b y m o l e c u l a r analysis, i n f r a r e d , a n d N M R s p e c t r o s c o p y w h e r e necessary. A new compound synthesized was 2,2'-biphenylenylsilane, b y L i A l H r e d u c t i o n of 2 , 2 - b i p h e n y l e n y l d i f l u o r o s i l a n e ( p r o v i d e d b y A n d e r son C h e m i c a l D i v i s i o n , Stauffer C h e m i c a l C o . , A d r i a n , M i c h . ) w i t h m.p. 3 6 . 6 ° C , b . p . 8 6 ° C (0.31 m m . ) . Anal C a l c d for S i C H : S i , 15.41; C , 79.06; H , 5.53. F o u n d ( G a l b r a i t h M i c r o a n a l y t i c a l L a b , K n o x v i l l e , T e n n . ) : S i 15.12; C , 78.92; H , 5.61. Ozone Competition Reaction Procedures. I n t h e r e l a t i v e rate studies, t h e s o l v e n t - s a t u r a t e d ozone—oxygen s t r e a m was passed into a glass b u b b l e r r e a c t o r vessel c h a r g e d w i t h 4 m l of a b o u t 4 X 1 0 " M c o n c e n t r a t i o n of e a c h of the t w o silanes to b e c o m p e t i t i v e l y o z o n i z e d as w e l l as of a n inert s a t u r a t e d a l i p h a t i c h y d r o c a r b o n to f u n c t i o n as i n t e r n a l s t a n d a r d . ( F o r e x a m p l e , n - u n d e c a n e w a s u s e d for the t r i b u t y l s i l a n e / t r i h e x y l s i l a n e s t u d y . ) T h e effluent f r o m the r e a c t o r passed t h r o u g h a s o l v e n t - f i l l e d b u b b l e c o u n t e r to v i s u a l i z e t h e flow. T h e i n l e t s t r e a m splitter m e n t i o n e d e a r l i e r w a s a d j u s t e d to a l l o w 2 - 4 h o u r s for e a c h run's c o m p l e t i o n , as d e t e r m i n e d b y experience. T h e t e m p e r a t u r e w a s c o n t r o l l e d at 0 ° C i n b o t h the saturator a n d the reactor b y a h ice b a t h . T o f o l l o w the course of the r e a c t i o n , s m a l l (0.1 m l ) a l i q u o t s w e r e p e r i o d i c a l l y r e m o v e d a n d a n a l y z e d b y gas c h r o m a t o g r a p h y o n a 150-ft 0.1 i n c h i d c a p i l l a r y c o l u m n c o a t e d w i t h D o w - C o r n i n g D C - 5 5 0 silicone. P e a k areas w e r e d e t e r m i n e d a n d c o n v e r t e d to c o n c e n t r a t i o n values b y reference to the area of the i n t e r n a l s t a n d a r d p e a k . F r o m the u n r e a c t e d f r a c t i o n ( S i ) ^/ ( S i ) , of e a c h p a i r e d c o m p e t i n g silane S i at s a m p l i n g t i m e t was c o m p u t e d the r e l a t i v e r a t e constant fc i, the r a t i o of the c o r r e s p o n d i n g first o r d e r ( i n s i l a n e ) rate constants, b y the e a r l i e r r e p o r t e d ( 5 , 11) equation: 4

1 2

1 0

2

0

re


6.

spiALTER E T A L .

The

Table I.

Ozone-Ηydrosilane

Relative R a t e / S a *

Silane

&rel

T r i s ( p e r h y d r o - l - n a p h t h y 1) s i l a n e Tricyclohexylsilane tert-Butyldicyclohexylsilane Trihexylsilane Tributylsilane Triethylsilane 3 , 3 , 3 - T r i f l u o r o p r o p y l d i m e t h y lsilane D i c h l o r o m e t h y l d i m e t h y lsilane Tris(3,3,3-trifluoropropyl)silane Phenyldimethylsilane Diisopropylsilane Chloromethyldimethylsilane Diphenylmethylsilane Triphenylsilane Dibutylsilane Triethoxysilane Phenylmethylsilane 2 , 2 - D i p h e n y l e n y lsilane Diphenylsilane Cyclohexylsilane Dibenzylsilane H e x y lsilane Phenylsilane


c

Correlation 4

Σσ*

l0g fc l

Λ

re

378 236 226 115 100 84 51.7 41.7 35.6 30.7 28.5 23.6 23.0 23.0 21.7 19.0 12.0 9.6 7.9 3.65 3.31 3.04 2.10

67

Reaction

4

Σσ*
d)U

b a c k d o n a t i o n f r o m the substituent i n t o the v a c a n t 3d o r b i t a l s of s i l i c o n (21,

r e s u l t i n g i n p a r t i a l c a n c e l l a t i o n of the i n d u c t i v e e l e c t r o n - w i t h

22),

d r a w i n g p o w e r of the g r o u p .


I n this research, since the silanes i n c l u d e d m o n o - , d i - , a n d t r i h y d r o species, the s u m of all four σ* values w a s t a k e n as i n d i c a t i v e of the t o t a l e l e c t r o n i c e n v i r o n m e n t of s i l i c o n . W h e n t h e l o g a r i t h m s of t h e r e l a t i v e 4

rates ( l o g excellent

fc i) re

w e r e p l o t t e d against 2 σ *

(Table

I and Figure 1),

c o r r e l a t i o n was o b s e r v e d for m o s t substituents

(vide

an

infra).

L e a s t - s q u a r e s analysis of the d a t a afforded E q u a t i o n 1. log A w =

-1.2513 Σα* +

Substituents w h o s e s t a n d a r d T a f t removed

o| -4

2.2166

σ*

(1)

values p r o d u c e d

points

far

f r o m the p l o t of F i g u r e 1 i n c l u d e d p h e n y l , c h l o r o m e t h y l , d i -

I

I

0

.4

I 4

.8

I

1.2

Σσ* Figure

1.

Hydrosilane-ozone

rate/Σσ*

correlation


I

1.6

6.

The Ozone-Ηydrosïlane

SPIALTER E T A L .

Table II.

N e w asi* Values

β l-C

-.26 -.01 -.01 .01 .05 .09 .18

H -»

1 0

i r

F3C—CH2—CH2—

C1 CH2,2'-Biphenylenyl Phenyl C H 0C1CH 2

2

5

2

69

Reaction

σ*'

σ* —as»*

0.32 1.94

0.33 1.95

0.60 1.35 1.05

0.55 1.26 .87

e

d e

T h i s paper. M i x t u r e of 1-decalinyl isomers o b t a i n e d b y c a t a l y t i c hydrogénation of 1 - n a p h t h y l on silcon (δ). T h i s value is p r o b a b l y well b e h a v e d a n d equal to σ*. I n exact agreement w i t h value of A t t r i d g e . S u b s t a n t i a l l y different from calculated value of 0.75 for C H 0 - , whose standard σ* is 1.35 (see text). ' S t a n d a r d T a f t values (20, 29). α


6

c

d

e

3

chloromethyl,

ethoxy,

a n d 3,3,-trifluoropropyl.

These

points

may

be

b r o u g h t i n t o c o r r e l a t i o n b y r e d e f i n i n g the substituent constant as a i * , S

w h o s e v a l u e m a y b e d e d u c e d f r o m fc i. T a b l e I I lists the v a l u e s of re

σ$*

thus d e r i v e d . T h e factor w h i c h these substituents h a v e i n c o m m o n is the presence of u n s h a r e d e l e c t r o n p a i r s or Π-electrons close to the s i l i c o n atom. T h e v a l u e of σ ΐ* for p h e n y l , 0.05, δ

agrees w i t h t h a t c a l c u l a t e d f o r

p h e n y l f r o m the S i - Η s t r e t c h i n g f r e q u e n c y t w e e n the latter a n d 0.60, a t t r i b u t e d to (p —> d)li

This discrepany

(21).

be

the " n o r m a l " σ* for p h e n y l , has also b e e n

b a c k b o n d i n g (21, 23-25).

H o w e v e r , the h e r e -

o b s e r v e d a i * for ethoxy, —0.09, does n o t agree w i t h the v a l u e of S

0.75

d e r i v e d f r o m V Î-H stretch for m e t h o x y a l t h o u g h t h e i r σ* values are q u i t e S

similar—1.35

a n d 1.45,

respectively.

I n d e e d , a l l of the " e l e c t r o n - r i c h "

substituents i n d i c a t e a n e a r c a n c e l l a t i o n of i n d u c t i v e a n d

mesomeric

effects. H o w e v e r , alternate explanations of the s o - c a l l e d (p - » d)U d o n a t i o n exist (26, Improved

back

27).

u n d e r s t a n d i n g a n d treatment of

the p h e n y l case

was

d e v e l o p e d f r o m the r e c o g n i t i o n that the d e r i v e d a i * ( C H - ) is almost 6

S

5

identical w i t h the "standard" H a m m e t t σ value, a parameter containing both mesomeric

and inductive contributions.

Since p u b l i s h e d data on

the fc ei of o z o n a t i o n of s u b s t i t u t e d p h e n y l d i m e t h y l s i l a n e s w e r e a v a i l a b l e r

(11),

a n d i n c l u d e d the

common

point, phenyldimethylsilane,

simple

m a t h e m a t i c a l m a n i p u l a t i o n c o u l d b r i n g these a n d the present d a t a o n to t h e same scale. W h e n the H a m m e t t σ f o r t h e s u b s t i t u t e d p h e n y l s was 4

u s e d i n the 2σ* c a l c u l a t i o n , a l l of the c o m p o u n d s c o r r e l a t e d w e l l w i t h E q u a t i o n 1. A c o m p a r i s o n of the f o u n d a * ( X - C H - ) a n d t h e i r H a m m e t t σ values ( 2 9 ) appears i n T a b l e I I I . S i

6

4


70

OZONE REACTIONS W I T H

ORGANIC

COMPOUNDS

I n essence, t h e n , t h e e l e c t r o n i c e n v i r o n m e n t of the s i l i c o n c a n d e s c r i b e d b y a substituent v a l u e ,

w h i c h is ( a )

σ* i f the substituent is a n a l k y l g r o u p , ( b )

be

e q u a l to the T a f t

e q u a l to the H a m m e t t σ i f

t h e substituent is p h e n y l or s u b s t i t u t e d p h e n y l , a n d ( c )

for substituents

b e a r i n g u n s h a r e d e l e c t r o n p a i r s close t o the s i l i c o n , e a c h σ * m u s t b e 81

individually determined. T h e σ * 8

v a l u e s c o m p u t e d i n this s t u d y a p p e a r

in Table II. Reaction Mechanism. T h e p r o p o s e d m e c h a n i s m for t h e o z o n e - h y d r o silane r e a c t i o n ( 7 )

s h o w n i n E q u a t i o n 2, as d e d u c e d b y a n a l y z i n g a n d

c o r r e l a t i n g d a t a o n r e l a t i v e rates, substituent effects, d e u t e r i u m isotope


effects, l o w t e m p e r a t u r e N M R , a n d u l t r a v i o l e t spectroscopy for a r a n g e of h y d r o s i l a n e s , is a m u l t i s t e p one as f o l l o w s : 0

R Si—H

+

3

p r i m a r y ) exactly

A l t h o u g h this e x p l a n a t i o n s h o u l d n o t

fortuitous

coincidence

required

makes

be it

unattractive. A s e c o n d p o s s i b i l i t y is t h a t the o z o n e forms some k i n d of

complex

w i t h the silane before attack o n the h y d r o g e n . F r o m this c o m p l e x , a l l h y d r o g e n s are e q u a l l y accessible, a n d the d e c o m p o s i t i o n is first o r d e r i n complex.

I n t h e h o p e of o b s e r v i n g s u c h a c o m p l e x a t i o n , the u l t r a v i o l e t

s p e c t r a of o z o n e / s i l a n e m i x t u r e s i n c a r b o n t e t r a c h l o r i d e w e r e e x a m i n e d (33).

A l t h o u g h n o s p e c t r a l b a n d s a t t r i b u t a b l e to a s i l i c o n - o z o n e

complex

w e r e f o u n d , i t w a s o b s e r v e d t h a t any s i l i c o n - c o n t a i n i n g species c a t a l y z e d the d e c o m p o s i t i o n

of ozone.

T h a t is, n o t o n l y t r i e t h y l s i l a n e , b u t t r i -

e t h y l s i l a n o l a n d t e t r a m e t h y l s i l a n e as w e l l , destroy o z o n e i n c a r b o n t e t r a c h l o r i d e . T h i s r e s u l t i n d i c a t e s a n association of the o z o n e w i t h the s i l i c o n a t o m , regardless of t h e f u n c t i o n a l i t y of the s i l i c o n species types e x a m i n e d )

3

( w i t h i n the

a n d c o m p l e t e l y i n d e p e n d e n t of the s i l i c o n substrate's

4


6.

spiALTER E T A L .

The Ozone-Η y drosilane

73

Reaction

a b i l i t y to enter i n t o a subsequent t r a n s f o r m a t i o n r e a c t i o n . I n v i e w of the electropositive n a t u r e of s i l i c o n , the n u c l e o p h i l i c association of ozone w i t h t h e s i l i c o n is the l i k e l y c a n d i d a t e for a s u i t a b l e i n t e r m e d i a t e .

Possible

models for this t y p e of i n t e r a c t i o n r a n g e f r o m one of w e a k electrostatic a t t r a c t i o n , 3, to a f u l l y p e n t a c o o r d i n a t e s i l i c o n species, 4. I n a n a t t e m p t to observe

the c o m p l e x ,

a l o w temperature N M R

i n v e s t i g a t i o n of mixtures of o z o n e w i t h t r i e t h y l s i l a n o l a n d w i t h t e t r a methylsilane were conducted.

A t — 57 ° C i n m e t h y l e n e c h l o r i d e , there

was no d i s c e r n i b l e change i n the p r o t o n N M R of either t r i e t h y l s i l a n o l or t e t r a m e t h y l s i l a n e as the o z o n e c o n c e n t r a t i o n of the r e s u l t i n g b l u e


solutions was i n c r e a s e d to the p o i n t of s a t u r a t i o n . T w o possible e x p l a n a tions for these results a r e : ( a ) the c o m p l e x exists i n e q u i b r i u m w i t h the silane a n d ozone, a n d the a c t u a l c o n c e n t r a t i o n of c o m p l e x is too to be d e t e c t e d b y N M R (less t h a n 5%

small

i n this e x p e r i m e n t ) , or ( b )

c h e m i c a l shifts of the silane protons are not a l t e r e d b y

the

complexation.

O f the t w o explanations, the first seems the m o r e reasonable, p a r t i c u l a r l y if the c o m p l e x has m o r e of the character of 4 t h a n of 3. T h e exact n a t u r e of the c o m p l e x is i m p o s s i b l e to d e t e r m i n e at this t i m e , b u t one significant o b s e r v a t i o n is that o p t i c a l l y a c t i v e p e r h y d r o - l - n a p h t h y l p h e n y l m e t h y l s i l a n o l undergoes s l o w r a c e m i z a t i o n u p o n s t a n d i n g at r o o m t e m p e r a t u r e i n a pentane—ozone s o l u t i o n . T h i s is consistent w i t h r e v e r s i b l e c o m p l e x a t i o n to a species h a v i n g some p e n t a c o o r d i n a t e character f r o m w h i c h r a c e m i z a t i o n m a y o c c u r via p s e u d o r o t a t i o n

(34).

T h e s e d a t a suggest that the i n i t i a l step i n t h e r e a c t i o n of w i t h the S i - Η b o n d is the r e v e r s i b l e f o r m a t i o n of a s i l i c o n - o z o n e plex.

ozone com

T h i s cannot be the rate step since ρ w o u l d h a v e to b e p o s i t i v e

( n u c l e o p h i l i c a t t a c k ) a n d no p r i m a r y isotope effect w o u l d b e p r e d i c t e d . T o e l i m i n a t e the s t a t i s t i c a l factor for the d i - a n d t r i h y d r o s i l a n e s , attack b y o z o n e o n the h y d r i d i c p r o t o n f r o m w i t h i n the c o m p l e x m u s t be m u c h m o r e f a v o r a b l e t h a n d i r e c t encounter a n d r e a c t i o n w i t h

uncomplexed

ozone and S i - H . Step(s) B i and B — H y d r o g e n Abstraction. H o w t h e o z o n a t i o n p r o 2

ceeds f r o m t h e c o m p l e x is n o w c o n s i d e r e d . I f d i r e c t σ-bond i n s e r t i o n ( 1 ) is e l i m i n a t e d o n the basis of isotope effects as discussed above, t h e n 5 a n d 6 are the v i a b l e alternatives. T h e t r a n s i t i o n state 5 c o u l d collapse to f o r m the s i l i c o n h y d r o p e r o x i d e 7, w h i l e t r a n s i t i o n state 6 c o u l d collapse to f o r m the s i l i c o n h y d r o t r i o x i d e 8 ( p a t h B i ) ; a l t e r n a t i v e l y , 6 c o u l d collapse d i r e c t l y to i o n o r r a d i c a l p a i r s ( p a t h B ) . 2

T h e transformation

5 - » 7 is n o t m e a n t to suggest that a t o m i c o x y g e n is the other p r o d u c t . S i n c e the r e a c t i o n o r d e r i n o z o n e has n o t b e e n d e t e r m i n e d , the fate of the other o x y g e n a t o m ( s ) is m o o t .


74

OZONE REACTIONS W I T H ORGANIC COMPOUNDS

R Si

H

3

^

0

3

0—0 7

5

•0-0 RsSic' Downloaded by CORNELL UNIV on July 27, 2016 | http://pubs.acs.org Publication Date: June 1, 1972 | doi: 10.1021/ba-1972-0112.ch006

R si00H

\0

R3S1OOOH

R Si(+or«) 8

I f t h e r e a c t i o n is f o u r - c e n t e r e d should be formed.

(5 - »

7),

0

2

(-or-)OH

a silicon hydroperoxide

T h e r e a c t i o n of t r i e t h y l s i l a n e w i t h o z o n e w a s m o n i -

t o r e d b y N M R at — 5 7 ° C .

T h e o n l y species o b s e r v e d u n d e r these c o n d i -

tions w e r e t h e silane a n d the s i l a n o l ; n o e v i d e n c e f o r a h y d r o p e r o x i d e i n t e r m e d i a t e w h i c h m i g h t h a v e b e e n stable at t h a t t e m p e r a t u r e

(35)

w a s detected, a n d c h e m i c a l tests for peroxides p r o v e d n e g a t i v e ( 5 , 6 ) . A s the c o n c e n t r a t i o n of o z o n e w a s i n c r e a s e d f r o m z e r o to s a t u r a t i o n , the s p e c t r u m of the silane c o m p l e t e l y d i s a p p e a r e d w i t h the c o n c u r r e n t a p p e a r a n c e of the s i l a n o l N M R s p e c t r u m . I f the r e a c t i o n is be formed.

five-centered

( 6 - » 8 ) , a silicon hydrotrioxide may

S i n c e there has b e e n n o p r e v i o u s r e p o r t of a s i l i c o n h y d r o -

t r i o x i d e , the s t a b i l i t y of a species s u c h as 8 u n d e r these c o n d i t i o n s c a n o n l y b e e s t i m a t e d . A m o n g c a r b o n analogs, the r e p o r t e d d i a l k y l t r i o x i d e s h a v e o n l y m a r g i n a l s t a b i l i t y at l o w temperatures hydrotrioxides, proposed a n d ethers

(38),

(36, 3 7 ) ,

and alkyl

as i n t e r m e d i a t e s i n the o z o n a t i o n of

d e c o m p o s e at ca.

— 10°C.

alcohols

A d m i t t e d l y , speculative

e x t r a p o l a t i o n b a s e d o n t h e c o m p a r a t i v e stabilities of other types of s i l i c o n a n d c a r b o n analogs suggests t h a t a s i l i c o n h y d r o t r i o x i d e 8, s h o u l d have b e e n o b s e r v a b l e i f i t h a d b e e n present, b u t this r e q u i r e m e n t is d e b a t a b l e . Steps C and D — T h e N a t u r e of the Recombining Fragments. a s s u m p t i o n of the i n t e r m e d i a c y of a

five-center

Upon

t r a n s i t i o n state, the q u e s -

t i o n t h e n arises as to h o w 6 ( o r 8 ) d e c o m p o s e s to the s i l a n o l . T h e b r e a k d o w n c o u l d b e via r a d i c a l or i o n p a i r s . T h e l a r g e p r i m a r y isotope effect suggests a r a d i c a l p a t h w a y ( 1 2 ) . ozonation

of

optically

active

F u r t h e r i n s i g h t w a s g a i n e d f r o m the

perhydro-l-naphthylphenylmethylsilane,

w h i c h y i e l d s the s i l a n o l 1 0 w i t h r e t e n t i o n of c o n f i g u r a t i o n ( E q u a t i o n 3 ) . P r o l o n g e d exposure of 1 0 to o z o n e causes r a c e m i z a t i o n , b u t t h e p r o d u c t


6.

spiALTER E T A L .

The Ozone-Ηydrosilane

75

Reaction

10

i n i t i a l l y has r e t a i n e d its c o n f i g u r a t i o n . P r e s u m a b l y , the silane 9 is also Downloaded by CORNELL UNIV on July 27, 2016 | http://pubs.acs.org Publication Date: June 1, 1972 | doi: 10.1021/ba-1972-0112.ch006

r a c e m i z e d b y o z o n e (via faster.

R e t e n t i o n of

t r a t e d (39)

c o m p l e x a t i o n ) , b u t t h e rate of o z o n i z a t i o n is

configuration b y

s i l y l r a d i c a l s has b e e n

whereas the o p t i c a l p r o p e r t i e s ( i n d e e d , the v e r y

of t r i v a l e n t s i l i c o n i u m ions h a v e n o t b e e n established.

demon-

existence)

F o r this reason,

d e c o m p o s i t i o n of 6 ( o r 8 ) t h r o u g h a r a d i c a l p a i r w i t h subsequent, r a p i d r e c o m b i n a t i o n is the m e c h a n i s m w h i c h appears most c o m p a t i b l e

with

the e x p e r i m e n t a l observations. Conclusions W h e n a l l the m e c h a n i s t i c e v i d e n c e is t a k e n i n t o c o n s i d e r a t i o n , the f o l l o w i n g r e a c t i o n sequence appears to best satisfy the d a t a . T h e silane undergoes

reversible complexation

(A)

w i t h the ozone, the

b e i n g present i n o n l y s m a l l concentrations.

complex

T h e rate step t h e n i n v o l v e s

e l e c t r o p h i l i c attack o n the h y d r i d i c h y d r o g e n , p a s s i n g t h r o u g h a

five-

center t r a n s i t i o n state. T h i s m a y d e c o m p o s e to either a s i l y l h y d r o t r i o x i d e ( B i ) or d i r e c t l y to the r a d i c a l p a i r ( B ) . 2

T h e silyl hydrotrioxide, if pres

ent, m u s t d e c o m p o s e r a p i d l y to the r a d i c a l p a i r ( C ) . then r e c o m b i n e s

This radical pair

w i t h r e t e n t i o n of c o n f i g u r a t i o n to afford t h e u l t i m a t e

p r o d u c t , the s i l a n o l ( D ) . W i t h r e g a r d to the

five-center

t r a n s i t i o n state, K e n n e t h W i b e r g ( Y a l e

U n i v e r s i t y ) has k i n d l y p o i n t e d out to us t h a t the m a g n i t u d e of the isotope effect supports the

five-centered

activated complex w h i c h w e have postu

l a t e d . I n the s m a l l e r r i n g complexes, the v i b r a t i o n a l m o d e c o n v e r t e d to the r e a c t i o n c o o r d i n a t e is a b e n d i n g m o d e , w h i c h w o u l d n o t p r o d u c e a l a r g e isotope

effect.

s t r e t c h i n g m o d e s are

I n the

five-centered

converted,

complex,

both bending

a n d the l a r g e isotope

and

effect is

not

F i n a l l y , a referee has suggested t h a t 8 c o u l d d e c a y to p r o d u c t

via

unexpected. a concerted pathway (i)

[p. 7 6 ] .

A l t h o u g h s u c h a p a t h w a y cannot

be

r u l e d out, w e f e e l that t h e l a r g e isotope effect suggests the r a d i c a l p a i r , probably from B . 2

B o t h i a n d the r a d i c a l m e c h a n i s m w o u l d p r o c e e d w i t h

r e t e n t i o n of c o n f i g u r a t i o n .


76

OZONE REACTIONS W I T H ORGANIC

R Si—Ο—0 3

R SiOH + 0 3

COMPOUNDS

2

(i)

Η—0


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6.

SPIALTER

E T AL.

The Ozone-Hydrosilane

Reaction

77

33. Nakagawa, T . W., Andrews, L . J., Keefer, R. M., J. Amer. Chem. Soc. (1960) 82, 269. 34. Muetterties, E. L., Accounts Chem. Res. (1970) 3, 266. 35. Dannley, R. L., Jalics, G.,J.Org. Chem. (1965) 30, 2417. 36. Bartlett, P. D., Guaraldi, G.,J.Amer. Chem. Soc. (1967) 89, 4799. 37. Mill, T., Stringham, R. S., J. Amer. Chem. Soc. (1968) 90, 1064. 38. Murray, R. W., Lumma, W. C., Lin, J. W.-P., J. Amer. Chem. Soc. (1970) 92, 3205. 39. Brook, A. G., Duff, J. M.,J.Amer. Chem. Soc. (1969) 91, 2118. 1971.


R E C E I V E D May 20,


The OzoneHydrosilane Reaction: A Mechanistic Study

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