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63 Evidence for a New Mechanism
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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.
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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
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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
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
50
OXIDATION OF ORGANIC COMPOUNDS
Til
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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.
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
63.
STORY
New
ET A L .
Mechanism
of
l
CH3CHO
_
\
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>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
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
CH i«OH 2
52
OXIDATION OF ORGANIC COMPOUNDS
A l
HI
-
/IX
Mechanism A
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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
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
63.
STORY
ET
H -
AL.
New
Mechanism
Al —
of
-
53
Ozonolysis
^
\
,/,
/
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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
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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
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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
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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 -
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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 .
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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.
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
are its
