Evidence for a New Mechanism of Ozonolysis - ACS Publications

63 Evidence for a New Mechanism

Downloaded by SUNY STONY BROOK on October 7, 2014 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch063

of Ozonolysis PAUL R. STORY, JOHN B. OLSON

CLYDE

E.

BISHOP,

JOHN

R.

BURGESS,

and

The University of Georgia, Athens, G a . 30601 R. W .

MURRAY

and R.

D.

YOUSSEFYEH

B e l l Telephone Laboratories, Inc., M u r r a y Hill, N. J.

Generation of oxygen-18 labeled ozonides followed by location of the isotopic label using reductive techniques has served to substantiate a new mechanism of ozonolysis which was proposed to account for the dependence of ozonide cis/trans ratios on olefin geometry. The new mechanism requires fragmentation of the molozonide to produce some aldehyde and zwitterion but further requires that ozonide may also be formed by the reaction of molozonide and aldehyde.

T a r g e l y b a s e d o n o u r finding that t h e cis/trans ratios of cross ozonides ^

(10) f o r m e d f r o m u n s y m m e t r i c a l olefins d e p e n d e d o n olefin geometry

( I I , 1 2 ) , w e h a v e p r o p o s e d a n e w m e c h a n i s m of ozonolysis w h i c h takes a c c o u n t of this effect (14).

T h e n e w m e c h a n i s m , w h i c h considers o n l y

a l i m i t e d t y p e of olefin, n a m e l y t r a n s - d i s u b s t i t u t e d a n d r e l a t i v e l y u n h i n d e r e d cis olefins, differs s i g n i f i c a n t l y f r o m the g e n e r a l l y Criegee mechanism

accepted

(1,5).

I n o u r v i e w , cross o z o n i d e s m a y b e f o r m e d b y t h e t y p i c a l sequence b e l o w i n w h i c h a l d e h y d e ( 4 ) , p r o d u c e d i n t h e reaction, reacts w i t h m o l o z o n i d e ( 2 ) i n a n a l d e h y d e interchange r e a c t i o n to y i e l d o z o n i d e ( 5 ) . As demonstrated previously, the molozonide-aldehyde interchange mech a n i s m (12, 14), f r o m c o n s i d e r a t i o n of steric interactions, c o r r e c t l y p r e dicts that cis-olefin w i l l generate r e l a t i v e l y m o r e c i s - o z o n i d e t h a n w i l l t h e c o r r e s p o n d i n g trans-olefin. T h i s i n t e r p r e t a t i o n does n o t refer t o t h e absolute values of the cis/trans ratios. 46 In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.


63.

STORY

ET

AL.

New

5a

Mechanism

47

of Ozonolysis

5ab

Figure 1.

Proposed mechanism of

ozonolysis

A c c o r d i n g to the C r i e g e e m e c h a n i s m , o z o n i d e is f o r m e d b y c o m b i n a t i o n of a z w i t t e r i o n ( 3 )

and an aldehyde ( 4 ) .

O u r mechanism

not d i s c a r d the c o n c e p t of the C r i e g e e z w i t t e r i o n .

3b

5a

American Chemical Society Library 1155 16th St,

N.W.

In Oxidation WasMnfton, of Organic Compounds; Mayo, F.; D.C 20031 Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

does

48

OXIDATION

OF ORGANIC

COMPOUNDS

III


E s s e n t i a l l y , w e h a v e p r o p o s e d that o z o n i d e ( 5 ) is f o r m e d n o t o n l y b y the C r i e g e e m e c h a n i s m b u t also b y r e a c t i o n of the m o l o z o n i d e w i t h a l d e h y d e . T h u s , a c o m p e t i t i o n exists b e t w e e n m o l o z o n i d e f r a g m e n t a t i o n a n d m o l o z o n i d e r e a c t i o n w i t h a l d e h y d e . I n a d d i t i o n , o z o n i d e m a y also b e f o r m e d b y t h e r e a c t i o n of m o l o z o n i d e w i t h z w i t t e r i o n f o l l o w e d b y regeneration of a n e w z w i t t e r i o n ( R e a c t i o n 2 ) . A s y e t w e h a v e n o e v i -

(2)

+ o 3a

5a

dence b e a r i n g o n this p o s s i b i l i t y ; this r e a c t i o n does n o t result i n i n c o r p o r a t i o n of oxygen-18 l a b e l , a n d i n this s t u d y it cannot b e differentiated f r o m c o m b i n a t i o n of z w i t t e r i o n w i t h u n l a b e l e d a l d e h y d e .

Experimental M a s s s p e c t r a l analyses w e r e p e r f o r m e d b y G o l l u b A n a l y t i c a l L a b s . , B e r k e l e y H e i g h t s , N . J., a n d b y M o r g a n - S c h a f f e r L a b o r a t o r i e s , M o n t r e a l , Quebec, Canada. A c e t a l d e h y d e - O . U s i n g s t a n d a r d v a c u u m l i n e techniques, 4.4 grams ( 1 0 0 m m o l e s ) of a c e t a l d e h y d e ( E a s t m a n W h i t e L a b e l ) a n d 2.0 grams (100 m m o l e s ) of 9 7 . 2 % oxygen-18 w a t e r ( Y E D A R e s e a r c h ) w e r e transferred to a r e a c t i o n vessel, w h i c h h a d been sealed t o t h e v a c u u m system, a n d a l l o w e d to s t a n d at r o o m t e m p e r a t u r e f o r 36 hours. A s s u m i n g c o m p l e t e exchange after this t i m e (4), t h e m i x t u r e w a s c o o l e d to 0 ° C , a n d t h e a c e t a l d e h y d e was d i s t i l l e d into a vessel c o n t a i n i n g about 2 grams of a n h y d r o u s s o d i u m sulfate. A f t e r s t a n d i n g at r o o m t e m p e r a t u r e f o r 1 hour, the acetaldehyde was redistilled through a 6 m m . X 8 i n c h c o l u m n of a n h y d r o u s s o d i u m sulfate i n t o a r e m o v a b l e container. M a s s spectral analysis gave a n oxygen-18 assay of 4 7 . 8 % . Y i e l d of l a b e l e d a c e t a l d e h y d e was 4.1 grams ( 8 9 % ). ls

Cis/traws-Methyl Isopropyl Ozonide— O ( 5 a b ) . A s o l u t i o n c o n t a i n i n g 4.04 grams (88 m m o l e s ) of a c e t a l d e h y d e - O , 14.5 grams ( 1 3 0 m m o l e s ) of frans-diisopropylethylene ( 6 ) ( B a k e r ) , a n d 75 m l . pentane ls

l s

In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

63.

STORY E T

AL.

New

Mechanism

of

49

Ozonolysis

( F i s h e r S p e c t r o g r a d e ) was o z o n i z e d i n a d r y i c e - a c e t o n e b a t h to 9 0 % c o m p l e t i o n . F o l l o w i n g ozonolysis, the r e a c t i o n vessel w a s s t o p p e r e d a n d b r o u g h t to r o o m temperature. P a r t of the solvent w a s r e m o v e d at l o w t e m p e r a t u r e o n a r o t a r y evaporator. T h e r e m a i n i n g l i q u i d s w e r e sepa r a t e d a n d p u r i f i e d b y G P C u s i n g a n 8 ft. X 3/4 i n c h c y a n o s i l i c o n e ( 2 0 % ) c o l u m n . P r o d u c t s i s o l a t e d w e r e 2.0 grams (15.2 m m o l e s , 1 7 % b a s e d o n a c e t a l d e h y d e - 0 ) of cis-trans-methyl isopropyl o z o n i d e - O (5ab) and 6.4 grams (40 m m o l e s ) of c i s / t r a n s - d i i s o p r o p y l o z o n i d e ( 5 a ) . T h e G P C retention times a n d N M R spectra of b o t h ozonides w e r e i d e n t i c a l to those of a u t h e n t i c samples (12). O z o n i d e ( 5 a b ) was also c h e c k e d f o r p u r i t y u s i n g a 12 ft. X 1/4 i n c h fluorosilicone ( 1 0 % ) c o l u m n . C a r e w a s t a k e n not to fractionate the i s o t o p i c a l l y l a b e l e d p r o d u c t d u r i n g G P C isolation. Downloaded by SUNY STONY BROOK on October 7, 2014 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch063

1 8

l s

L i t h i u m A l u m i n u m H y d r i d e R e d u c t i o n o f cis/trans—Methyl Isop r o p y l O z o n i d e - 0 ( 5 a b ) . A s o l u t i o n of 1.67 grams (12.7 m m o l e s ) of 5ab i n 25 m l . of a n h y d r o u s ether was a d d e d d r o p w i s e w i t h s t i r r i n g to a s l u r r y c o n s i s t i n g of 0.75 grams (19.7 m m o l e s ) of l i t h i u m a l u m i n u m h y d r i d e a n d ether. F o l l o w i n g a d d i t i o n , the r e a c t i o n m i x t u r e w a s s t i r r e d for 30 m i n u t e s at r o o m t e m p e r a t u r e a n d t h e n h e a t e d u n d e r reflux f o r 15 m i n u t e s . M o i s t s o d i u m sulfate w a s a d d e d after the excess h y d r i d e w a s d e s t r o y e d b y a d d i n g w a t e r c a u t i o u s l y . T h e clear ether l a y e r w a s de c a n t e d , a n d the r e s i d u a l solids w e r e w a s h e d several times w i t h ether. T h e c o m b i n e d extracts w e r e d r i e d over s o d i u m sulfate. E t h e r w a s re m o v e d c a r e f u l l y b y d i s t i l l a t i o n , a n d the r e m a i n i n g l i q u i d w a s separated a n d p u r i f i e d b y G P C u s i n g a 10 ft. X 3/8 i n c h U C O N n o n p o l a r ( 2 0 % ) c o l u m n . P a r t of the e t h y l a l c o h o l (225 m g . , 3 8 % ) a n d the i s o b u t y l alco h o l (450 m g . , 4 8 % ) c o l l e c t e d w a s transferred b y v a c u u m l i n e t e c h n i q u e into b r e a k s e a l a m p u l e s a n d sealed for mass s p e c t r a l analysis. E t h y l a l c o h o l - 0 assay: 2 5 . 4 % oxygen-18; i s o b u t y l a l c o h o l assay: 7.6% oxygen-18. 1 8

1 8

R e a c t i o n o f 5ab w i t h M e t h y l l i t h i u m . O Z O N I D E ( 5 a b ) F O R M E T H Y L REDUCTION. T h i s substance was p r e p a r e d u s i n g 4 0 . 6 1 % 0 w a t e r ( Y E D A ) to generate a c e t a l d e h y d e c o n t a i n i n g 2 1 . 0 5 % O a c c o r d i n g to the p r o c e d u r e d e s c r i b e d above. LITHIUM

1

8

l s

A s o l u t i o n of 110 m g . (0.83 m m o l e s ) of 5 a b i n ether was a d d e d s l o w l y to a s o l u t i o n of m e t h y l l i t h i u m ( 1 0 % excess, F o o t e C h e m i c a l ) i n ether. T h e h i g h l y e x o t h e r m i c r e a c t i o n was c o o l e d i n a r o o m t e m p e r a t u r e w a t e r b a t h . M e t h a n e (39 m l . ) , ether v a p o r , a n d p o s s i b l y c a r b o n d i o x i d e w e r e c o l l e c t e d [theoretical f o r p r o t o n abstraction r e d u c t i o n : 19 m l . of m e t h a n e ] . A f t e r a d d i t i o n of o z o n i d e w a s c o m p l e t e , the r e a c t i o n w a s w o r k e d u p i n the same m a n n e r as the l i t h i u m a l u m i n u m h y d r i d e r e d u c t i o n . G P C analysis of the c r u d e m i x t u r e r e v e a l e d i s o p r o p y l a l c o h o l (9) (~60% b y G P C standard) and 3-methyl-2-butanol ( 1 0 ) ( ~ 6 0 % ) . M e t h a n o l is n o r m a l l y p r o d u c e d i n a p p r o x i m a t e l y t h e same y i e l d (,—60% ) as 9 a n d 10. W e w e r e u n a b l e to collect a sufficient q u a n t i t y f r o m the l a b e l i n g e x p e r i m e n t for mass spectral analysis. P r o d u c t i d e n t i f i c a t i o n w a s b a s e d o n G P C r e t e n t i o n times a n d b y c o m p a r i s o n of i n f r a r e d spectra w i t h those of a u t h e n t i c c o m p o u n d s . M a s s s p e c t r a l results w e r e as f o l l o w s : i s o p r o p y l a l c o h o l - 0 assay: 1 1 . 8 8 % oxygen-18; 3 - m e t h y l - 2 - b u t a n o l ( 1 0 ) assay: 2 . 4 5 % . 1 8


50

OXIDATION OF ORGANIC COMPOUNDS

Til


Results and Discussion C o m p a r i s o n of the t w o m e c h a n i s m s u n d e r c o n s i d e r a t i o n reveals that the a l d e h y d e o x y g e n is i n c o r p o r a t e d into o z o n i d e i n different w a y s ; the C r i e g e e m e c h a n i s m y i e l d s o z o n i d e w h i c h finds the a l d e h y d e o x y g e n at the ether b r i d g e ( O - l ) , w h i l e o u r m e c h a n i s m places the a l d e h y d e o x y g e n i n the p e r o x i d e b r i d g e ( 0 - 3 ) . T h i s suggested, of course, t h a t a c r i t i c a l test of the p r o p o s e d m e c h a n i s m w o u l d be p r o v i d e d b y generating o z o n i d e f r o m oxygen-18 l a b e l e d a l d e h y d e a n d d e t e r m i n i n g the fate of the isotope. ( T h e c o n c l u s i o n that the o z o n i d e f r o m the C r i e g e e m e c h a n i s m finds the a l d e h y d e o x y g e n at the ether b r i d g e is m a d e a s s u m i n g that the z w i t t e r i o n cannot a d d to the a l d e h y d e c a r b o n y l 1,3 t h r o u g h the oxygens as t h o u g h i t possessed the s t r u c t u r e :

T h e r e is no e v i d e n c e i n d i c a t i n g that the z w i t t e r i o n reacts i n this fashion, a n d it w o u l d not account f o r the d e p e n d e n c e of cross o z o n i d e stereoi s o m e r i c ratios o n olefin geometry. W e have experiments u n d e r w a y to c h e c k this p o s s i b i l i t y , h o w e v e r . ) T h e l a b e l i n g experiment has b e e n a c c o m p l i s h e d u s i n g the trans-diisop r o p y l e t h y l e n e ( 6 ) — a c e t a l d e h y d e - 0 ( 7 ) system o u t l i n e d i n F i g u r e 2. L a b e l e d o z o n i d e ( 5 a b ) w a s p r e p a r e d b y o z o n i z i n g to 9 0 % c o m p l e t i o n a pentane s o l u t i o n c o n t a i n i n g J r a n s - d i i s o p r o p y l e t h y l e n e ( 6 ) a n d acetaldeh y d e - 0 ( 7 ) . T h e m e t h y l i s o p r o p y l o z o n i d e ( 5 a b ) was i s o l a t e d as b e f o r e (11, 12) as a cis/trans m i x t u r e i n 1 7 % y i e l d b a s e d o n acetaldehyde. 1 8

1 8

L o c a t i o n of the isotopic l a b e l i n the p r o d u c t o z o n i d e ( 5 a b ) w a s determined b y two independent methods: (1) lithium a l u m i n u m hydride r e d u c t i o n of 5ab to e t h y l a l c o h o l a n d i s o b u t y l a l c o h o l , ( 2 ) r e d u c t i o n w i t h m e t h y l l i t h i u m . O z o n i d e ( 5 a b ) for l i t h i u m a l u m i n u m h y d r i d e r e d u c t i o n was p r e p a r e d u s i n g a c e t a l d e h y d e c o n t a i n i n g 4 7 . 8 % oxygen-18 b y mass spectral analysis. M e t h y l i s o p r o p y l o z o n i d e ( 5 a b ) w a s r e d u c e d c l e a n l y a n d q u a n t i t a t i v e l y to e t h y l a l c o h o l a n d i s o b u t y l a l c o h o l . M a s s spectral analysis r e v e a l e d that the e t h y l a l c o h o l c o n t a i n e d 2 5 . 4 % oxygen-18, a n d isobutyl alcohol contained 7.6%. U n f o r t u n a t e l y , the m e c h a n i s m of h y d r i d e r e d u c t i o n of ozonides is not k n o w n . H o w e v e r , w e c a n consider the most reasonable possibilities, a l l of w h i c h p l a c e the greatest p o r t i o n of t h e l a b e l i n the p e r o x i d e b r i d g e at oxygen-3.


63.

STORY

New

ET A L .

Mechanism

of

l

CH3CHO

_

\


>1 .

51

Ozonolysis

.V

0\

>

0

oT

0

, 4

0—0

1

5ab

5ab

Figure 2.

Isotopic labeling

scheme

If w e assume t h e m e c h a n i s m u s u a l l y w r i t t e n ( 7 ) , b y a n a l o g y t o t h e r e d u c t i o n of peroxides, w h e r e t h e o x y g e n most l i k e l y to b e lost is f r o m the p e r o x i d e b r i d g e [ M e c h a n i s m A ] , w e c a n c o n c l u d e that 1 5 . 2 %

of t h e

o z o n i d e molecules c o n t a i n e d oxygen-18 i n t h e ether b r i d g e ( O - l ) a n d that 3 2 . 6 %

c o n t a i n e d l a b e l at oxygen-3.

U n d e r these circumstances a n d (25.4%)

O—O LiAltL

(47.8%

1 8

CH,CH

2

1 8

OH

(7.6%) +

\>

0)

5ab


CH i«OH 2

52

OXIDATION OF ORGANIC COMPOUNDS

A l

HI

-

/IX

Mechanism A


p r e s u m i n g that the h y d r i d e r e d u c t i o n is n o t seriously affected sterically b y the substituents, the e t h y l a l c o h o l s h o u l d c o n t a i n a p p r o x i m a t e l y

24%

oxygen-18 regardless of the i s o t o p i c d i s t r i b u t i o n b e t w e e n positions 1 a n d 3 i n the o z o n i d e ( 5 a b ) .

O u r e x p e r i m e n t a l v a l u e of 2 5 . 4 %

f a v o r a b l y a n d adds c r e d e n c e to this i n t e r p r e t a t i o n . the o z o n i d e of 1 , 2 - d i m e t h y l c y c l o p e n t e n e ether b r i d g e , b u t c o n t a i n i n g o n l y 0 . 3 % h y d r i d e r e d u c t i o n , a l l the

s p e c i f i c a l l y l a b e l e d i n the

(6)

oxygen-18.

label was retained

O n lithium aluminum

i n the

because of the v e r y l o w i n c o r p o r a t i o n of oxygen-18, c o u l d be i n v o l v e d (3)

compares

[ W e have synthesized

diol.

However,

considerable

error

].

T h e same c o n c l u s i o n r e g a r d i n g i s o t o p i c d i s t r i b u t i o n is r e a c h e d i f w e assume a r e d u c t i o n m e c h a n i s m s i m i l a r to the scheme p r o p o s e d b y R i e c h e (13)

f o r h y d r o l y s i s of ozonides [ M e c h a n i s m B J

— A l

Mechanism B

O

O

H

If, h o w e v e r , o x y g e n loss o n h y d r i d e r e d u c t i o n is p u r e l y statistical as i l l u s t r a t e d b y M e c h a n i s m C , the i s o t o p i c d i s t r i b u t i o n w o u l d b e a l t e r e d b u t w o u l d s t i l l p l a c e m o r e t h a n h a l f the i s o t o p i c l a b e l at oxygen-3. U n d e r


63.

STORY

ET

H -

AL.

New

Mechanism

Al —

of

-

53

Ozonolysis

^

\

,/,

/


5ab

o

+

+

O — A l ^

-

—( \A\

—

Mechanism C these c o n d i t i o n s o n e - t h i r d of the o x y g e n i n the i s o b u t y l a l c o h o l r e d u c t i o n p r o d u c t w o u l d arise f r o m the ether b r i d g e ( O - l ) 7.6%

or 22.8%

i n d i c a t i n g that 3

X

of the o z o n i d e molecules m u s t h a v e c o n t a i n e d l a b e l at

this p o s i t i o n ; 2 5 % , then, c o n t a i n e d l a b e l at oxygen-3.

T h e estimate

of

isotopic d i s t r i b u t i o n p r o v i d e d b y M e c h a n i s m C is c o n s i d e r e d the l o w e r l i m i t for i s o t o p i c l a b e l at oxygen-3. C e r t a i n l y , a n y conclusions d r a w n o n the basis of the h y d r i d e r e d u c t i o n alone are s o m e w h a t tenuous. however.

T w o points are w o r t h c o n s i d e r i n g ,

( 1 ) I n the absence of large steric effects o n the h y d r i d e r e d u c

t i o n , the p o s s i b i l i t y that a l l of the l a b e l o r i g i n a t e d at oxygen-1 i n the ozonide (5ab)

is e l i m i n a t e d b y the v a l u e o b t a i n e d f o r the i s o t o p i c l a b e l

i n e t h y l a l c o h o l , 2 5 . 4 % , a n d b y the large difference i n a m o u n t of isotope c o n t a i n e d i n the t w o a l c o h o l p r o d u c t s .

(2)

A n y steric effect o n

the

r e d u c t i o n s h o u l d be s m a l l . I n a d d i t i o n , the effect s h o u l d be i n the d i r e c t i o n to r e m o v e a n excess of oxygen-3; the cis o z o n i d e s h o u l d e x h i b i t n o m e a s u r a b l e steric effect since attack w o u l d o c c u r p r e f e r e n t i a l l y o n t h e side of the r i n g a w a y f r o m the substituents.

T h e trans isomer, o n the

other h a n d , s h o u l d suffer h y d r i d e attack cis to the m e t h y l substituent a n d as far a w a y f r o m the substituent as possible, thus l e a d i n g to a p r e d o m i nant

loss of oxygen-3

w h e t h e r r e d u c t i o n takes p l a c e

b y a d d i t i o n of

h y d r i d e to o x y g e n or to c a r b o n . W e c o n c l u d e , then, that the o z o n i d e ( 5 a b )

is f o r m e d b y t w o c o m

p e t i n g b u t c o m p l i m e n t a r y r e a c t i o n paths, a n d u n d e r the p a r t i c u l a r c o n d i t i o n s [ a d d e d a l d e h y d e ] of this experiment, t r a n s l a t i o n of the d a t a to


54

OXIDATION

100%

oxygen-18

OF ORGANIC

COMPOUNDS

H I

[considering reduction Mechanisms A or B only] re

veals t h a t , — ' 3 0 % of 5ab is f o r m e d via t h e C r i e g e e z w i t t e r i o n m e c h a n i s m a n d — 7 0 % of 5 a b a c c o r d i n g to t h e m o l o z o n i d e - a l d e h y d e i n t e r c h a n g e mechanism

(14).

T h i s c o n c l u s i o n is c o n f i r m e d b y r e d u c t i o n of t h e same o z o n i d e ( 5 a b ) u s i n g m e t h y l l i t h i u m ( w e t h a n k R . C r i e g e e f o r s u g g e s t i n g this t y p e of r e a c t i o n to locate t h e i s o t o p i c l a b e l ) .

It w a s e x p e c t e d that r e d u c t i o n

w o u l d o c c u r b y p r o t o n r e m o v a l as i l l u s t r a t e d i n F i g u r e 3. H e n c e , t h e fate of t h e o z o n i d e oxygens w o u l d b e u n a m b i g u o u s ; analysis of t h e i s o p r o p y l


a l c o h o l w o u l d p r o v i d e a measure o f t h e oxygen-18 l a b e l at oxygen-3.

Figure 3.

Methyllithium reduction of ozonide by proton removal

T h e r e a c t i o n of m e t h y l l i t h i u m w i t h o z o n i d e ( 5 a b ) w a s extremely r a p i d a n d exothermic, a n d a l t h o u g h m e t h a n e w a s e v o l v e d , the r e a c t i o n appears to p r o c e e d p r i n c i p a l l y b y d i s p l a c e m e n t o n t h e o x y g e n - o x y g e n b o n d b y m e t h i d e to y i e l d i s o p r o p y l a l c o h o l ( 9 ) , 3 - m e t h y l - 2 - b u t a n o l ( 1 0 ) , a n d m e t h y l a l c o h o l . T h e source of t h e m e t h a n e is u n k n o w n at present. H o w e v e r , t h e entire r e a c t i o n is b e i n g i n v e s t i g a t e d . M e t h y l l i t h i u m r e d u c t i o n of ozonides appears

to f o l l o w a

course

s i m i l a r to that of G r i g n a r d r e d u c t i o n . G r e e n w o o d treated d i e t h y l o z o n i d e w i t h isopropyl G r i g n a r d a n d obtained 2-methyl-3-pentanol, isopropyl alcohol, and propane i n good yield ( 9 ) . It is also c o n c e i v a b l e that r e d u c t i o n occurs b y m e t h i d e d i s p l a c e m e n t o n c a r b o n to y i e l d h y d r o p e r o x i d e , w h i c h is s u b s e q u e n t l y r e d u c e d to a l c o h o l ( 9 , 1 0 ) a n d m e t h a n o l . W e cannot at present d i s t i n g u i s h


63.

STORY

ET A L .

New Mechanism

of

55

Ozonolysis

these latter t w o possibilities either b y p r o d u c t analysis o r b y isotope distribution. If r e d u c t i o n of 5ab occurs w i t h d i s p l a c e m e n t o n o x y g e n as s h o w n i n F i g u r e 4, one-half the o x y g e n c o n t a i n e d i n 3 - m e t h y l - 2 - b u t a n o l o r i g i n a t e d at p o s i t i o n 1.

(10)

T h e r e f o r e , the a m o u n t of l a b e l i n t h e ether

b r i d g e is o b t a i n e d b y d o u b l i n g t h e percentage oxygen-18 f o u n d i n 10. Ozonide

(5ab)

prepared

f r o m acetaldehyde

containing

21.05%

oxygen-18 w a s treated w i t h excess m e t h y l l i t h i u m i n a n apparatus w h i c h a l l o w e d c o l l e c t i o n of e v o l v e d gases.

I n e v e r y case, at least 1 m o l e of

m e t h a n e w a s c o l l e c t e d ( s m a l l traces of w a t e r a c c o u n t e d f o r some o f t h e Downloaded by SUNY STONY BROOK on October 7, 2014 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch063

methane).

A n a l y s i s of t h e p r o d u c t m i x t u r e b y G P C r e v e a l e d i s o p r o p y l

alcohol ( 9 ) a n d 3-methyl-2-butanol

(10)

i n approximately equimolar

amounts a n d i n h i g h y i e l d (ca. 6 0 % , b y G P C ) . M a s s spectral analysis of t h e G P C p u r e p r o d u c t s r e v e a l e d that iso propyl

alcohol

(9)

contained

11.88%

oxygen-18;

3-methyl-2-butanol

( 1 0 ) c o n t a i n e d 2 . 4 5 % . I n t e r p r e t e d i n terms of m e t h i d e d i s p l a c e m e n t at o x y g e n ( F i g u r e 4 ) or at c a r b o n [ p r o t o n r e m o v a l ( F i g u r e 3 ) is e x c l u d e d as a major r e a c t i o n p a t h w a y b y p r o d u c t analysis a n d b y the presence of l a b e l i n 1 0 ] , the ether b r i d g e ( o x y g e n - 1 ) oxygen-18.

must have contained 4.90%

T h e r e m a i n d e r , 1 6 . 1 5 % , m u s t h a v e b e e n at oxygen-3.

If

+

9

Figure

4.

Apparent

this scheme

reaction

is correct,

c o n t a i n 8.07 +

10

path for the reaction ozonide (5ab)

of methyllithium

with

isopropyl alcohol should have been f o u n d to

2.45 o r 1 0 . 5 2 % oxygen-18.

O u r e x p e r i m e n t a l v a l u e of

1 1 . 8 8 % gives a d i s c r e p a n c y of 1 . 3 6 % or a b o u t a 1 0 % error w h i c h c a n p r o b a b l y b e a t t r i b u t e d to a s l i g h t steric effect. A steric effect a r g u m e n t


56

OXIDATION

O F ORGANIC

COMPOUNDS

III

is reasonable since d i s p l a c e m e n t at oxygen-4, w h i c h w o u l d p r o d u c e iso p r o p y l a l c o h o l r i c h i n oxygen-3, w o u l d b e f a v o r e d .

It is also

possible

that some o f t h e alcohols w e r e p r o d u c e d b y p r o t o n a b s t r a c t i o n , a n d i n that w a y l e d to a h i g h isotope content i n 9 . I n fact, w e o b t a i n e d a s m a l l a m o u n t of a m a t e r i a l w i t h t h e same G P C r e t e n t i o n t i m e as a u t h e n t i c m e t h y l i s o p r o p y l ketone w h i c h w o u l d b e p r o d u c e d b y h y d r o g e n a b straction ( F i g u r e 3 ) . W e w e r e u n a b l e to isolate sufficient m e t h y l a l c o h o l f r o m the l a b e l i n g e x p e r i m e n t f o r mass s p e c t r a l analysis. F r o m this e x p e r i m e n t w e w o u l d c o n c l u d e that a b o u t 2 5 % o f t h e o z o n i d e w a s f o r m e d a c c o r d i n g to t h e C r i e g e e m e c h a n i s m a n d that t h e Downloaded by SUNY STONY BROOK on October 7, 2014 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch063

remainder,

7 5 % , was formed according

to o u r m o l o z o n i d e - a l d e h y d e

m e c h a n i s m . T h i s result s u b s t a n t i a l l y agrees w i t h that o b t a i n e d b y h y d r i d e reduction. T h e l a b e l i n g experiments, therefore, p r o v i d e strong s u p p o r t f o r t h e mechanism

w e proposed to account

cis/trans ratios o n olefin geometry

for the dependence

(14).

c o n d i t i o n s of a d d e d a l d e h y d e , a p p r o x i m a t e l y 7 0 - 7 5 % (5ab)

of ozonide

I n this p a r t i c u l a r case, u n d e r of t h e o z o n i d e

was, b y a l l indications, formed through the molozonide-aldehyde

reaction.

H o w e v e r , i n a n o r m a l ozonolysis a l d e h y d e

is n o t

present

initially, a n d before the m o l o z o n i d e - a l d e h y d e mechanism c a n become i m p o r t a n t , a sufficient q u a n t i t y of a l d e h y d e m u s t b e p r o d u c e d , p r e s u m a b l y b y fission of t h e m o l o z o n i d e to z w i t t e r i o n a n d a l d e h y d e .

Under

these c o n d i t i o n s i t w o u l d n o t b e s u r p r i s i n g to find t h e n e w m e c h a n i s m somewhat

less i m p o r t a n t t h a n i n t h e present

study.

Once

sufficient

a l d e h y d e is o b t a i n e d i n t h e n o r m a l ozonolysis, p r o d u c t i o n o f z w i t t e r i o n m a y w e l l n e a r l y cease since t h e m o l o z o n i d e - a l d e h y d e r e a c t i o n does n o t d e p l e t e a l d e h y d e c o n c e n t r a t i o n , a n d at sufficiently h i g h a l d e h y d e c o n centrations this r e a c t i o n competes w e l l w i t h m o l o z o n i d e

fission.

Reaction

t e m p e r a t u r e s h o u l d b e i m p o r t a n t i n this c o m p e t i t i o n . W h i l e t h e oxygen-18

l a b e l i n g results d e s c r i b e d here c o n f i r m t h e

m o l o z o n i d e - a l d e h y d e m e c h a n i s m f o r t h e types of olefins c o n s i d e r e d , t h e ozonolysis r e a c t i o n i n general is q u i t e c o m p l e x a n d seems to v a r y w i d e l y d e p e n d i n g e s p e c i a l l y u p o n t h e stereochemistry of t h e olefin. T o s u m u p , the m o l o z o n i d e - a l d e h y d e m e c h a n i s m

(14)

c o n s i d e r e d here appears t o

b e a p p l i c a b l e to a n y i m p o r t a n t degree only to t r a n s - d i s u b s t i t u t e d olefins, r e l a t i v e l y u n h i n d e r e d cis olefins, a n d p e r h a p s to u n h i n d e r e d t e r m i n a l olefins. A s p o i n t e d out, m o r e h i n d e r e d olefins seem to react b y o n e o r m o r e different p a t h w a y s , w h i c h differ most n o t a b l y f r o m t h e present system i n t h e a p p a r e n t

absence of a m o l o z o n i d e i n t e r m e d i a t e

( 2 , 8,

12,14).

Acknowledgments W e t h a n k t h e U . S . P u b l i c H e a l t h Service, N a t i o n a l C e n t e r f o r A i r Pollution Control [Grant #

AP00505-01-02] f o r s u p p o r t o f this i n v e s t i -


63.

STORY

ET

New

AL.

Mechanism

of

57

Ozonolysis

g a t i o n a n d the G e n e r a l R e s e a r c h Office, T h e U n i v e r s i t y of G e o r g i a , f o r p u r c h a s e of the oxygen-18 w a t e r .


Literature Cited

(1) Bailey, P. S., Chem. Rev. 58, 925 (1958). (2) Bailey, P. S., Thompson, J. A., Shoulders, B. A., J. Am. Chem. Soc. 88, 4098 (1966). (3) Bishop, C. E., unpublished work. (4) Byrn, M., Calvin, M., J. Am. Chem. Soc. 88, 1916 (1966). (5) Criegee, R., Rec. Chem. Progr. (Kresge-Hooker Sci. Lib.) 18, 111 (1957). (6) Criegee, R., Lohaus, G., Chem. Ber. 86, 1 (1953). (7) Gaylord, N. G., "Reduction with Complex Metal Hydrides," p. 708, Interscience, New York, 1956. (8) Greenwood, F. L., J. Org. Chem. 29, 1321 (1964). (9) Greenwood, F. L., Haske, B. J., J. Org. Chem. 30, 1276 (1965). (10) Loan, L. D., Murray, R. W., Story, P. R., J. Am. Chem. Soc. 87, 737 (1965). (11) Murray, R. W., Youssefyeh, R. D., Story, P. R., J. Am. Chem. Soc. 88, 3143 (1966). (12) Ibid., 89, 2429 (1967). (13) Rieche, A., Meister, R., Sauthoff, H., Ann. 553, 187 (1942). (14) Story, P. R., Murray, R. W., Youssefyeh, R. H., J. Am. Chem. Soc. 88, 3144 (1966). RECEIVED

A p r i l 8,

1968.

Discussion D . G . M . D i a p e r : R e a c t i o n of m o l o z o n i d e a n d z w i t t e r i o n gives a n e i g h t - m e m b e r e d r i n g w i t h f o u r adjacent oxygens.

N o stable rings w i t h

f o u r adjacent oxygens are k n o w n , a n d c a t e n a t i o n

of f o u r oxygens

not often b e e n i n v o k e d . d i m e r , b u t it lost 0

2

has

M i l a s h a d s u c h a system i n p e r o x y r a d i c a l

at — 3 0 ° C .

Analogous 0

2

loss w o u l d give a six-

m e m b e r e d r i n g p e r o x i d e w i t h that e m b a r r a s s i n g C — C b o n d intact. o—o

P a u l R . S t o r y : I n the first p l a c e w e cannot r u l e out some h o m o l y t i c d e c o m p o s i t i o n of the p r o p o s e d i n t e r m e d i a t e . paring our proposed seven-membered

H o w e v e r , it is w o r t h c o m

r i n g t r i o x i d e w i t h the m o l o z o n i d e

f o r w h i c h a n i o n i c d e c o m p o s i t i o n to z w i t t e r i o n a n d c a r b o n y l is r e a d i l y e n v i s i o n e d ; our i n t e r m e d i a t e a n d the p r o p o s e d i o n i c d e c o m p o s i t i o n q u i t e analogous

to Criegee's

m o l o z o n i d e or p r i m a r y o z o n i d e a n d

decomposition.


are its

Evidence for a New Mechanism of Ozonolysis - ACS Publications

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