Free Radical Aspects of Photooxidation

2 Free Radical Aspects of Photooxidation PAUL D. BARTLETT

Downloaded by LOUISIANA STATE UNIV on August 28, 2014 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0069.ch002

Texas C h r i s t i a n U n i v e r s i t y , F o r t W o r t h , TX 76129

Photo-oxidation i s defined, for our present purpose, as oxidation by molecular oxygen under the influence of l i g h t . The process may r e s u l t i n any product more oxidized than the s t a r t i n g material; and it may o r i g i n a t e i n the absorption of l i g h t by the oxygen, by the substrate, or by a t h i r d molecule (a " s e n s i t i z e r " ) whose function may be to transfer e x c i t a t i o n energy or to involve the reactants i n s h o r t - l i v e d r e a c t i v e complexes or covalent intermed i a t e s . Many investigators of photo-oxidation may have entertained the hope of discovering a common mechanistic thread underlying all the kinds of photo-oxidation; but the current mood i s more one of appreciation of the v e r s a t i l i t y of Nature, who does not reserve her s u b t l e t i e s only f o r complex b i o l o g i c a l systems, but lavishes them on even some of the smallest molecules that we have. In p r i n c i p l e , any compound that can be thermally oxidized can be oxidized by some a p p l i c a t i o n , d i r e c t or i n d i r e c t , of l i g h t . But the things that are unique about photo-oxidation are seen mainly with unsaturated s t a r t i n g materials, and it i s with them that the hope of s o r t i n g out mechanisms reaches a maximum. Included i n such photo-oxidations are reactions leading to three general r e s u l t s : 1. The 0-0 bond remains i n t a c t i n the product, which may be an a l l y l i c hydroperoxide (1) (1), a dioxetane (2) (2), or an "endoperoxide" (3) (3), depending on the nature of the s t a r t i n g material.

©

0-8412-0421-7/78/47-069-015$05.00/0

In Organic Free Radicals; Pryor, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

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ORGANIC F R E E RADICALS

Η ^

C H OÇ—(j) C H OC—0 H 2

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2. The p r o d u c t c o n t a i n s o n l y one o f t h e atoms o f t h e oxygen m o l e c u l e , as i n t h e c a s e o f an e p o x i d e (4) ( 4 ) , o r a s u l f o x i d e (5) (5, 6) C H fi

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3. Cleavage p r o d u c t s o r rearrangement p r o d u c t s a r e found, o f t e n a t t r i b u t a b l e t o an i n i t i a l h y d r o p e r o x i d e (7) o r p e r o x i d e (8) u n d e r g o i n g f u r t h e r r e a c t i o n under t h e i n f l u e n c e o f l i g h t o r t h e r m a l l y ( 9 ) : ,00H HX



2.

BARTLETT

Free Radical Aspects of Photooxidation

The Development o f M e c h a n i s t i c

17

Criteria

Not o n l y i s t h e r e a r a m i f i c a t i o n o f p r o d u c t s from any p a r t i c u l a r p h o t o - o x i d a t i o n method, but t h e r e a r e some c l e a r cases where one and the same product may be formed by two o r more mechanisms. The study o f t h i s f i e l d has t h e r e f o r e demanded the development o f some c r i t e r i a f o r a t l e a s t a s s i g n i n g observed o x i d a t i o n s to mechanistic c a t e g o r i e s . G e n e r a l c r i t e r i a f o r s i n g l e t oxygen: 1. The f o r m a t i o n o f s i n g l e t oxygen r e q u i r e s the i n p u t o f energy, and a photo-system g e n e r a t i n g s i n g l e t oxygen ceases t o o p e r a t e when t h e l i g h t i s t u r n e d o f f . I t thus d i f f e r s from a c h a i n r e a c t i o n o p e r a t i n g under t h e r m a l i n i t i a t i o n . 2. Quenchers, t y p i f i e d by β-carotene and by l , 4 - d i a z a ( 2 . 2 . 2 ) b i c y c l o o c t a n e (DABCO), p o w e r f u l l y i n h i b i t r e a c t i o n s i n which s i n g l e t oxygen i s the r e actant, while normally l e a v i n g f r e e - r a d i c a l c h a i n r e a c t i o n s and g r o u n d - s t a t e thermal p r o c e s s e s u n a f f e c ted.



18


3. On t h e o t h e r hand, r e a c t i o n s depending on s i n g l e t oxygen do n o t respond s p e c i f i c a l l y t o c h a i n i n h i b i t o r s , such as hydroquinones o r s t a b l e f r e e r a d i c a l s which s t r o n g l y i n t e r f e r e w i t h thermal c h a i n reactions. 4. C e r t a i n compounds, such as t e t r a m e t h y l e t h y l e n e o r d i p h e n y l i s o b e n z o f u r a n , which a r e v e r y r e a c t i v e t o s i n g l e t oxygen, may s e r v e as i n d i c a t o r s o f i t s p r e s e n c e when i n t r o d u c e d i n t o a p h o t o - o x i d a t i o n where t h e p a r t i c i p a t i o n o f s i n g l e t oxygen i s s u s pected. The absence o f t h e normal s i n g l e t oxygen r e a c t i o n p r o d u c t s o f t h e s e agents i s a g e n e r a l l y r e l i a b l e c r i t e r i o n o f the absence o f t h e e x c i t e d species. 5. The a b i l i t y o f s i n g l e t oxygen t o a t t a c k a double bond i s a s e n s i t i v e f u n c t i o n o f e l e c t r o n a v a i l a b i l i t y i n t h e p i system b e i n g a t t a c k e d . Some photo-oxidations a c t i v e l y involve electron-poor double bonds which a r e u n r e a c t i v e toward s i n g l e t oxygen; t h i s c a n be a s t r a i g h t f o r w a r d m e c h a n i s t i c criterion. S p e c i a l C r i t e r i a f o r C e r t a i n Kinds o f Oxidations. For a r e a c t i o n l e a d i n g t o hydroperoxides, a u s e f u l c r i t e r i o n o f mechanism i s t h e p r o d u c t d i s t r i b u t i o n when 1,2-dimethylcyclohexene i soxidized by t h a t method. A u t o x i d a t i o n under i n i t i a t i o n by f r e e r a d i c a l s leads to i n t r o d u c t i o n o f the hydroperoxy group a t t h e 3 - p o s i t i o n t o t h e e x t e n t o f 54% (10), t h i s mode o f r e a c t i o n b e i n g u n d e t e c t a b l y s m a l l when e i t h e r t h e r m a l l y - o r p h o t o - g e n e r a t e d singlet oxygen i s t h e r e a c t a n t . C r i t e r i a t h a t have been used i n t h e case o f dioxetane formation include s t e r e o s p e c i f i c i t y o f the r e a c t i o n , comparison o f r e a c t i v i t i e s w i t h t h o s e toward known s i n g l e t oxygen s o u r c e s , and t h e presence o r absence o f t e r m i n a t i o n p r o d u c t s from any p o s s i b l e chain-carrying species. A p p l i c a t i o n o f such c r i t e r i a as t h e s e has shown t h a t among p h o t o - o x i d a t i o n s r e t a i n i n g t h e 0-0 bond, h y d r o p e r o x i d e s c a n be formed e i t h e r from s i n g l e t oxygen o r from r a d i c a l c h a i n r e a c t i o n s , w h i l e t h e r e i s a growing number o f c a s e s o f d i o x e t a n e s o r endop e r o x i d e s b e i n g a p p a r e n t l y formed i n n o n - s i n g l e t oxygen r e a c t i o n s . The o x i d a t i o n o f 2 - p h e n y l n o r b o r nene i n the dark gave p r o d u c t s which c o u l d have i n v o l v e d d i o x e t a n e f o r m a t i o n , b u t c o u l d a l s o be account e d f o r i n p a r t by a Mayo c h a i n . (11)


2.

BARTLETT


The


19

M e c h a n i s t i c Problem i n P h o t o - e p o x i d a t i o n .

E p o x i d e s c a n be produced s t e r e o s p e c i f i c a l l y from o l e f i n s by compounds so c o n s t i t u t e d t h a t they can c o n c e r t e d l y d e l i v e r an 0 atom and l e a v e a s t a b l e molecule behind. P e r o x y a c i d s a r e prime examples o f t h i s c a p a b i l i t y . (12) S u b s t i t u e n t e f f e c t s on t h i s k i n d o f c o n c e r t e d , s t e r e o s p e c i f i c epoxidation i n d i c a t e that the peroxya c i d f u n c t i o n s as an e l e c t r o p h i l e ; e l e c t r o n - d o n a t i n g groups a c t i v a t e t h e o l e f i n w h i l e e l e c t r o n - w i t h d r a w i n g groups s t r o n g l y d e a c t i v a t e i t . When t h e e l e c t r o n f l o w a t the t r a n s i t i o n s t a t e i s r e v e r s e d by u s i n g e l e c t r o n - p o o r o l e f i n s and peroxy a n i o n s f o r e p o x i d a t i o n , t h e r e a c t i o n i s s t e p w i s e and s t e r e o - e q u i l i brating and b o t h c i s - and t r a n s - o l e f i n s (e.g., benzalacetophenone) y i e l d t r a n s - e p o x i d e . (13, 14) What might appear t h e s i m p l e s t model o f a concerted epoxidation, the a d d i t i o n o f a f r e e gasphase oxygen atom t o an o l e f i n , proves t o be i n f a c t a s t e p w i s e p r o c e s s as a r e s u l t o f t h e t r i p l e t c h a r a c t e r o f g r o u n d - s t a t e oxygen atoms. (15)

t f

Without t h e i n d i c a t e d c o l l i s i o n a l d e a c t i v a t i o n " ( c . d . ) , t h e e p o x i d e and c a r b o n y l isomers a r e l a r g e l y r e p l a c e d i n t h e p r o d u c t by complex m i x t u r e s o f c l e a vage p r o d u c t s . The p r o d u c t f i g u r e s o f C v e t a n o v i c e x t r a p o l a t e d t o i n f i n i t e p r e s s u r e , a l l o w an e s t i m a t e (16) t h a t t h e e n e r g y - r i c h c i s - b i r a d i c a l r o t a t e s t o t h e trans-conformâtion 2.9 times as f a s t as i t u n d e r goes r i n g c l o s u r e t o t h e e p o x i d e . Although t h i s c o r r e s p o n d s t o a r e t e n t i o n index o f 2.1, which i s q u i t e normal f o r condensed-phase c y c l o a d d i t i o n s v i a b i r a d i c a l s , t h e gas phase systems cannot be p e r f e c t models o f r e a c t i o n s i n s o l u t i o n because o f t h e h i g h v i b r a t i o n a l e x c i t a t i o n which i n t h i s c a s e l e a d s t o m i g r a t i o n o f hydrogen and m e t h y l i n amounts comparable to t h e amount o f r i n g c l o s u r e . Thus t h e d i s t i n c t i o n between c o n c e r t e d and



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ORGANIC F R E E

RADICALS

s t e p w i s e e p o x i d a t i o n s i s not a m a t t e r o f n e u t r a l vs. i o n i c r e a c t a n t s , nor even o f s p i n - p a i r e d v s . t r i p l e t systems, but r a t h e r has t o do w i t h t h e a v a i l a b i l i t y o f a t r a n s i t i o n s t a t e a l l o w i n g s i m u l t a n e o u s bonding o f the incoming oxygen atom t o both carbon atoms o f t h e double bond. The c r i t e r i a f o r such a c o n c e r t e d mechanism a r e not u n d e r s t o o d on a l e v e l a l l o w i n g prediction. A c l e a r l y s t e p w i s e mechanism f o r epoxide f o r m a t i o n was demonstrated i n d e t a i l by Mayo and c o workers (18, 19). I t was shown t h a t i n a c h a i n c o p o l y m e r i z a t i o n o f s t y r e n e and m o l e c u l a r oxygen, any p o l y m e r i c r a d i c a l t e r m i n a t i n g i n a s t y r e n e u n i t has t h e p o s s i b i l i t y o f c l e a v i n g o f f an epoxide m o l e c u l e from t h e end o f the c h a i n : R RO

(OOCH CHC H ) 2

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In the c a s e o f s t y r e n e , because o f r a p i d β-fission o f t h e r e m a i n i n g c h a i n , e p o x i d e i s a minor p r o d u c t , r e a c h i n g i t s maximum y i e l d i n comparison t o a l d e h y d i c f i s s i o n p r o d u c t s a t a low oxygen p r e s s u r e a s s u r i n g a s i g n i f i c a n t l i f e t i m e t o the s t y r e n e - t e r m i n a t e d radical. An example was r e c e n t l y found i n t h e c a s e o f p h o t o - o x i d a t i o n o f 9-methoxymethylenefluorene (8) where t h e c l e a v a g e o f such a r a d i c a l t o epoxide i s so f a v o r e d over a d d i t i o n o f oxygen t h a t comparable amounts o f e p o x i d e and f l u o r e n o n e a r e a t t a i n e d a t and above room temperature (20) (Scheme 1 ) :

^ ^ X - C R

2

8 In a c h a i n r e a c t i o n i n v o l v i n g peroxy r a d i c a l s , c o u p l i n g of these r a d i c a l s i s a very r a p i d process and can be t h e p r i n c i p a l mechanism o f c h a i n termina tion, i n t h e c a s e o f methoxymethylene f l u o r e n e , however, peroxy r a d i c a l c o u p l i n g i n normal c o u r s e does not l e a d t o t e r m i n a t i o n , but t o f l u o r e n o n e and a c h l o r i n e atom t o c o n t i n u e t h e c h a i n (Scheme 2).


2.

BARTLETT

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10 Downloaded by LOUISIANA STATE UNIV on August 28, 2014 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0069.ch002

25


11

1Λ formed from 3Ό by a t t a c k o f t r i p l e t oxygen. A l though 1JL, as a s i n g l e t s p e c i e s , might c y c l i z e t o 1 £ which c o u l d be the same k i n d o f one-O-donor as a c h e l a t e d p e r a c i d , y e t !L1 might a l s o behave as a b i r a d i c a l i n r e a c t i n g w i t h an o l e f i n w i t h the end r e s u l t o f d o n a t i n g a s i n g l e oxygen atom t o form the o b s e r v e d epoxide.

R11

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14 The r e m a i n i n g s p e c i e s , 1J3 and !L4, a r e a l t e r n a t e forms o f the c a r o n y l o x i d e o f the ketone. I f the C r i e g e e s t r u c t u r e 13, were formed i n the p r o c e s s , i t would mean t h a t t h e epoxide oxygen came not from t h e oxygen m o l e c u l e but from the k e t o oxygen of the s e n s i t i z e r . By e i t h e r o f these paths any s e n s i t i z e r k e t o group which becomes r e g e n e r a t e d must be e i t h e r c o m p l e t e l y exchanged ( i n the case o f 13) o r h a l f exchanged ( i n the c a s e o f 14) w i t h the oxygen from 0 . Dr. Johannes B e c h e r e r has t e s t e d t h i s p o s s i b i l i t y o f b i r a d i c a l involvement i n the p h o t o - e p o x i d a t i o n by c a r r y i n g o u t the r e a c t i o n w i t h c o m p l e t e l y ] 8 Q - l a b e l e d m o l e c u l a r oxygen, and u n l a b e l e d b e n z i l ana b i a c e t y l . Small samples i n benezene c o n t a i n i n g 2


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3 6

0.33 m i l l i m o l e s o f 0 and 0.20 m i l l i m o l e s each o f benzil o r b i a c e t y l and norbornene were i r r a d i a t e d through pyrex w i t h a 450-W. Hanovia mercury vapor lamp. Under i d e n t i c a l c o n d i t i o n s , b e n z i l on 15minute i r r a d i a t i o n underwent about 5% i s o t o p i c 0exchange i n the absence o f norbornene and 10% i n i t s presence. In the same l e n g t h o f time the n o r bornene was c o n v e r t e d 70% i n t o norbornene o x i d e whose oxygen was 93% i * o and 7% * 0. A result like t h i s c o u l d not have been produced by any o b v i o u s v a r i a n t o f the mechanism d e p i c t e d above, but r a t h e r r e q u i r e s a r e a c t i o n p a t h by which the oxygen o f 0 i s conveyed i n t o the epoxide w i t h o u t e v e r h a v i n g been c o v a l e n t l y a t t a c h e d t o the c a r b o n y l group o f the s e n s i t i z e r . The r e s u l t s w i t h b i a c e t y l a r e e q u a l l y emphatic. In the absence o f o l e f i n b i a c e t y l i s r a p i d l y des t r o y e d by l i g h t and oxygen: on i r r a d i a t i o n f o r 15 minutes the y e l l o w c o l o r o f the a l p h a - d i k e t o n e had c o m p l e t e l y faded. The s m a l l amount o f b i a c e t y l r e m a i n i n g showed 42% i n c o r p o r a t i o n o f i s o t o p i c oxygen, s t i l l s h o r t o f the c a l c u l a t e d 62.5% f o r e q u i l i b r a t i o n i n c i d e n t a l t o the p h o t o - d e s t r u c t i o n . In t h e p r e s e n c e o f e q u i m o l a r norbornene a f t e r t h e same i r r a d i a t i o n , the much l a r g e r amount o f r e c o v e r e d d i k e t o n e was o n l y 8% exchanged, w h i l e t h e r e was a 90% c o n v e r s i o n t o norbornene o x i d e whose oxygen was 93% 0. 2


6

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Ion R a d i c a l s i n P h o t o - o x i d a t i o n These experiments h a v i n g e l i m i n a t e d p a r t i c i p a t i o n o f b o t h t h e b i r a d i c a l JA, hypothetical p e r a c i d a n a l o g J2,, t h e r e remains a l a r g e a r e a where t h e r e a r e p e r s i s t e n t i n d i c a t i o n s o f involvement o f odd-electron species i n o x i d a t i o n processes. Single e l e c t r o n t r a n s f e r s o c c u r i n many o f the same s i t u a t i o n s i n which f r e e r a d i c a l i n i t i a t i o n and photos e n s i t i z a t i o n occur. There have been c a s e s o f t h i s k i n d where s u p e r o x i d e r a d i c a l i o n , 0 -, has been observed, and some o f i t s i n t e r a c t i o n s w i t h s i n g l e t oxygen have been s t u d i e d . J e f f o r d and Boschung (26) have i n t e r p r e t e d t h e i r e x t e n s i v e experiments on the p h o t o o x i d a t i o n o f b i a d a m a n t y l i d e n e i n terms o f an i n t e r v e n t i o n o f s u p e r o x i d e a n i o n , g e n e r a t e d i n a secondary r e a c t i o n between s i n g l e t oxygen and v a r i o u s grounds t a t e s e n s i t i z e r s , e s p e c i a l l y rose bengal. In a s e r i e s o f s t e p s , t h e r e s u l t i n g RB 'takes an e l e c t r o n a

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from Ad-Ad and t h e r e s u l t i n g c a t i o n r a d i c a l Ad-Ad * r e a c t s w i t h t r i p l e t ground s t a t e oxygen t o g i v e t h e c a t i o n r a d i c a l o f a perepoxide. This species i n t u r n i s c o n v e r t e d by a second s u p e r o x i d e a n i o n i n t o t h e o f t e n - d i s c u s s e d p a i r , epoxide +ozone. The advantage o f t h i s sequence over t h e d i r e c t p r o c e s s v i a p e r e p o x i d e and s i n g l e t oxygen i s t h a t i t p r o v i d e s a p i c t u r e o f t h e s p e c i a l r o l e p l a y e d by each s e n s i t i z e r i n i n f l u e n c i n g t h e r e l a t i v e amounts o f d i o x e t a n e ( s t i l l formed v i a s i n g l e t oxygen and o l e f i n ) and e p o x i d e . The a u t h o r s show t h a t DABCO, as a s i n g l e t oxygen quencher, s t o p s b o t h k i n d s o f p h o t o - o x i d a t i o n , c o n s i s t e n t w i t h t h e view t h a t s i n g l e t oxygen i s a p r e c u r s o r o f t h e e p o x i d i z i n g r e a g e n t itself. S p e c i f i c t o o l s used i n t h e s e experiments i n c l u d e d d i - t - b u t y l - p - c r e s o l as a r a d i c a l i n h i b i t o r t o i n t e r r u p t s e l e c t i v e l y t h e r a d i c a l c h a i n , and t h e use o f p h e n y l g l y o x y l i c a c i d t o r e a c t s p e c i f i c a l l y with superoxide anion (33). Other probes t h a t have been used f o r t h e o c c u r r e n c e o f s u p e r o x i d e a n i o n r a d i c a l (36) i n t h e p r e s e n c e o f s i n g l e t oxygen i n c l u d e s u l f i t e a n i o n (34), s e l e c t i v e l y o x i d i z e d by s u p e r o x i d e , and s u p e r o x i d e dismutase (34, 3 5 ) , w h i c h e f f i c i e n t l y c o n v e r t s s u p e r o x i d e i o n i n t o oxygen and hydrogen peroxide. I n s e e k i n g a framework f o r t h e e p o x i d i z i n g a c t i o n o f t h e a l p h a - d i k e t o n e s as p h o t o s e n s i t i z e r s , i t i s s u g g e s t i v e t h a t t h e s e compounds a r e themselves very e f f e c t i v e e l e c t r o n - a c c e p t o r s , g i v i n g r i s e t o the w e l l c h a r a c t e r i z e d semidiones (37). I n terms o f t h e J e f f o r d - B o s c h u n g framework, we might t h e n s a y t h a t a s e n s i t i z e r whose e x c i t e d s t a t e i s h i g h enough i n e l e c t r o n a f f i n i t y (such as an α-diketone) c a n s i m p l i f y t h e p h o t o - o x i d a t i o n sequence by t a k i n g an e l e c t r o n d i r e c t l y from t h e o l e f i n i c s u b s t r a t e r a t h e r than t r a n s f e r r i n g any e x c i t a t i o n energy t o oxygen, and so c a n i n i t i a t e an e p o x i d a t i o n sequence w i t h o u t i n t e r v e n t i o n o f s i n g l e t oxygen. E p o x i d a t i o n c o u l d be completed i n t h e absence o f b o t h s u p e r o x i d e i o n and s i n g l e t oxygen i f t h e p e r e p o x i d e c a t i o n r a d i c a l , 15, c o u l d r e a c t w i t h t r i p l e t oxygen t o a s p e c i e s (J67~which i s c o n v e r t e d by c o l l i s i o n

AS,

16



28


w i t h t h e semidione i n t o epoxide and ozone. Thus t h e i n c l u s i o n o f i o n r a d i c a l s among t h e r e a c t i v e i n t e r mediates f o r p h o t o - o x i d a t i o n g r e a t l y widens t h e m e c h a n i s t i c p o s s i b i l i t i e s among which a d e c i s i o n must be made. Another s e n s i t i z e r which might be e x p e c t e d t o g e n e r a t e a r a d i c a l a n i o n on e x c i t a t i o n i n t h e p r e s r ence o f an e l e c t r o n donor i s 9,10-dicyanoanthracene (31). A l t h o u g h Foote and co-workers found t h i s t o be a s e n s i t i z e r f o r o x y g e n a t i o n w i t h o u t any e v i dence o f s i n g l e t oxygen b e i n g i n v o l v e d , y e t i t s a c t i o n i n p r o d u c i n g benzophenone from t e t r a p h e n y l e t h y l e n e s u g g e s t e d t h a t t h e o x i d a t i o n p r o d u c t was a d i o x e t a n e ; no e p o x i d a t i o n was o b s e r v e d . Cation r a d i c a l s a r e s t r o n g l y implicated i n the s e r i e s o f c a t a l y s t s d i s c o v e r e d by B a r t o n and c o workers (38) which, some p h o t o c h e m i c a l l y and some t h e r m a l l y , produce p e r o x i d e s from e r g o s t e r y l a c e t a t e and o t h e r d i e n e s i n methylene c h l o r i d e a t - 7 8 ° . These c a t a l y s t s , i n c l u d i n g Lewis a c i d s , carbonium i o n s , o r aminium i o n r a d i c a l s , a r e i n remarkable c o n t r a s t t o those j u s t mentioned i n t h a t they have n o t been o b s e r v e d t o produce any e p o x i d e s . Dr. M. J . S h a p i r o has a l s o found t h a t B a r t o n ' s c a t i o n r a d i c a l ( p - B r C H ) N ' converts biadamantylidene i n t o d i o x e t a n e and not i n t o e p o x i d e . It i s striking in t h i s c o n n e c t i o n t h a t t h e c a t i o n o i d r a d i c a l complex (CH ) N ZnCl (39) r e a c t s , a l s o t h e r m a l l y , w i t h oxygen and o l e f i n s t o y i e l d , n o t p e r o x i d e s , but e p o x i d e s , as i n f e r r e d from t h e s t r u c t u r e and c o n f i g u r a t i o n o f the aminoalcohols i s o l a t e d . 6

4

3

e

3

2

2

Photo-oxidation of Sulfides D i a l k l s u l f i d e s , l i k e o l e f i n s , can undergo one-0 o r two-0 o x i d a t i o n , w i t h t h e o b v i o u s d i f f e r e n c e t h a t i n a s u l f o n e t h e two oxygen atoms do n o t remain bonded t o each o t h e r . F o o t e and P e t e r s (5) d e v e l o p e d c o n v i n c i n g e v i d e n c e t h a t s i n g l e t oxygen r e a c t s d i r e c t l y w i t h d i e t h y l s u l f i d e t o y i e l d a r e a c t i v e peroxys u l f o x i d e Q 7 ) which i s c a p a b l e o f c o n v e r t i n g

3

Et S + 0 2

:

'2

Et SO + R S0 2

2



2.

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29

another s u l f i d e molecule i n t o s u l f o x i d e , or of r e a r r a n g i n g more s l o w l y i n t o s u l f o n e , i n c o m p e t i t i o n w i t h quenching by d i s s o c i a t i o n i n t o s u l f i d e and t r i p l e t oxygen. Cleavages a t t e n d i n g t h e p h o t o o x i d a t i o n o f b e n z y l a l k y l s u l f i d e s (40) c o u l d a l s o be f o r m u l a t e d as i n v o l v i n g p e r o x y s u l f o x i d e s d e r i v e d from s i n g l e t oxygen. However, the p h o t o - o x i d a t i o n o f s u l f i d e s s e n s i t i z e d by 9,10-dicyanoanthracene (31) a l s o produced s u l f o x i d e s and s u l f o n e s . T h i s o x i d a t i o n was not i n h i b i t e d by β-carotene, and showed a s t r o n g r e v e r s a l o f the r e l a t i v e r e a c t i v i t i e s o f d i p h e n y l and d i e t h y l s u l f i d e s compared t o those i n s i n g l e t oxygen o x i d a t i o n . Another k i n d o f e v i d e n c e f o r the involvement o f i o n r a d i c a l s i n p h o t o - o x i d a t i o n comes from the o b s e r v a t i o n o f c l e a v a g e o f c e r t a i n s u l f i d e s (41) with formation of unoxidized d i s u l f i d e s . Unlike the b e n z y l a l k y l s u l f i d e s (40), d i - t - b u t y l s u l f i d e g i v e s on o x i d a t i o n w i t h s e v e r a l p h o t o s e n s i t i z e r s , i n a d d i t i o n t o t h e s u l f o x i d e and s u l f o n e , amounts o f d i - t - b u t y l d i s u l f i d e v a r y i n g from t r a c e s w i t h r o s e b e n g a l o r methylene b l u e i n methanol t o 97 and 100% i n acetone w i t h r o s e b e n g a l f r e e and bound on polymer beads, r e s p e c t i v e l y . The y i e l d o f the d i s u l f i d e may be a measure o f the r e l a t i v e r a t e a t which the c a t i o n r a d i c a l 18 d i s s o c i a t e d t o the t b u t y l c a t i o n a n d t h e t - b u t y l t h i y l r a d i c a l i n the v a r i o u s media: +. . (CH ) CSC(CH ) -KCH ) C +(CH ) CS --KCH ) CSSC(CH ) +

3

3

3

3

3

3

3

3

3

3

3

3

λ&

The o t h e r p r o d u c t s o f t h e p h o t o - o x i d a t i o n s a r e mix-. t u r e s o f d i - t - b u t y l s u l f o x i d e and d i - t - b u t y l s u f o n e , t h e s e m i x t u r e s becoming s t e a d i l y r i c h e r i n s u l f o n e as the r e a c t i o n p r o g r e s s e s . A l t h o u g h s i n g l e t oxygen g e n e r a t e d t h e r m a l l y was c a p a b l e o f p r o d u c i n g t h e s u l f o x i d e and s u l f o n e , t h e d i s u l f i d e was absent from such t h e r m a l p r o d u c t m i x t u r e s . N e i t h e r was d i s u l f i d e produced w i t h s e n s i t i z e r i n the absence o f oxygen. In p a r a l l e l i r r a d i a t i o n s o f d i - t - b u t y l s u l f i d e , one under oxygen and t h e o t h e r under argon, no c l e a v a g e o r any o t h e r r e a c t i o n i s seen i n the experiment w i t h o u t oxygen. These r e s u l t s show t h a t oxygen p l a y s an e s s e n t i a l p a r t i n g e n e r a t i n g t h e c a t i o n r a d i c a l s , even though d i r e c t a t t a c k o f s i n g l e t oxygen on the s u l f i d e (as shown i n the experiments w i t h c h e m i c a l g e n e r a t i o n ) i s no p a r t



30

ORGANIC F R E E

RADICALS

of the process. This i s further evidence supporting some such multi-step electron transfer process as that proposed by J e f f o r d and Boschung i n some of the epoxidation reactions. Not s u r p r i s i n g l y , although superoxide anion i s implicated i n the genesis of the cation r a d i c a l s , a s o l u t i o n of potassium superoxide-crown ether i n methylene c h l o r i d e was without e f f e c t on d i - t - b u t y l s u l f i d e alone. D i - t - b u t y l s u l f i d e could be converted to d i s u l f i d e i n two hours at room temperature by t r i s - ( p bromophenyl)-aminium fluoborate i n a stream of oxygen, but neither tetrahydrothiophene nor diphenyl s u l f i d e underwent cleavage under these conditions. Conclusions Photo-oxidation has been shown to proceed, not only by concerted reactions with s i n g l e t oxygen, but through stepwise mechanisms i n v o l v i n g neutral free r a d i c a l s and r a d i c a l ions. In some of the l a t t e r cases s i n g l e t oxygen i s implicated i n the genesis of the r a d i c a l ions and t h e i r precursors, and the r e s u l t i n g r a d i c a l reactions compete with the d i r e c t r e a c t i o n of the s i n g l e t oxygen. Radical and c a t i o n - r a d i c a l processes appear t o be e s p e c i a l l y important i n photo-epoxidation.


2.

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31

L i t e r a t u r e Cited 1. 2. 3.


4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

17. 18. 19.

Schenck, G.O., and Schulte-Elte, K., Ann. (1958) 618 185. B a r t l e t t , P.D., and Schaap, A.P., J . Am. Chem. Soc. (1970) 92 3223. Schenck, G.O., and Ziegler, K., Naturwissenshaften (1944) 32 57. Shimizu, Ν., and B a r t l e t t , P.D., J . Am. Chem. Soc. (1976) 98 4193. Foote, C.S., and Peters, J.W., J . Am. Chem. Soc. (1971) 93 3795. Foote, C.S., and Peters, J.S., 23 Int. Cong. Pure and Appl. Chem. (1971) 4 129. Kharasch, M.S., and Burt, J.G., J . Org. Chem. (1951) 16 150. Bocke, J . , and Runquist, O., J . Org. Chem. (1968) 33 4285. McCapra, F., and Beheshti, I., J . Chem. Soc. Chem. Comm. (1977) 517. Foote, C.S., Accts. Chem. Res. (1968) 1 104. Jefford, C.W.; Boschung, Α.; and Rimbault, C.G., Helv. Chim. Acta (1976) 59 2542. Swern, D., "Organic Peroxides," V o l . 2, Chapter 5, pp. 466-475; Wiley-Interscience, New York, 197. Lutz, R.E., and Weiss, J.O., J . Am. Chem. Soc. (1955) 77 1814. Curci, R., and Edwards, J.O., in D. Swern, "Or ganic Peroxides," V o l . 1, Chapter 4, p. 245. Cvetanovic, R.J., Adv. Photochem. (1963) 1 117-149. The retention index PQ is defined (17) and its use i n evaluating rate constant r a t i o s i s des cribed by L. K. Montgomery, K. Schueller, and P.D. B a r t l e t t , J . Am. Chem. Soc., 86, 622 (1964). We have recently formulated a more general analysis of stepwise, s t e r e o - e q u i l i b r a t i n g c y c l o additions, containing some derivations not given i n the o r i g i n a l paper, and describing a few applications. This manuscript, prepared on i n v i t a t i o n f o r a review journal, i s a v a i l a b l e to interested readers on request. B a r t l e t t , P.D., 23 I n t l . Cong. Pure and Appl. Chem. (1971) 4 281. Mayo, F.R., J . Am. Chem. Soc. (1958) 80 2465. Mayo, F.R., and M i l l e r , Α.Α., J . Am. Chem. Soc. (1958) 80 2480. d

d


32



20.

B a r t l e t t , P.D., and Landis, M.E., J . Am. Chem. Soc. (1977) 99 3033. 21. B a r t l e t t , P.D., and Ho, M.S., J . Am. Chem. Soc. (1974) 96 627. 22. Jefford, C.W., and Boschung, Α., Helv. Chim. Acta (1974) 95 3381. 23. B a r t l e t t , P.D., Chem. Soc. Reviews (1976) 5 154. 24. Unpublished work of M. J . Shapiro, N. Shimizu, and G.Y. Moltrasio I g l e s i a s . 25. Jefford, C.W., and Boschung, A.F., Tet. Letters (1976) p. 4771. 26. Jefford, C.W., and Boschung, A.F., Helv. Chim. Acta, December, 1977. We thank Professor J e f f o r d f o r a preprint. 27. Schaap, A.P., and Faler, G.R., J . Am. Chem. Soc. (1973) 95 3381. 28. Ho, M.S., Thesis, Harvard University (1974). 29. Dewar, M.J.S.; G r i f f i n , A.C.; T h i e l , W.; and Turchi, I . J . , J . Am. Chem. Soc. (1975) 97 4439. 30. Shapiro, M.J., unpublished work. 31. Eriksen, J . ; Foote, C.S.; and Parker, T.L., J. Am. Chem. Soc. (1977) 99 6455. 32. Frimer, Α.Α.; B a r t l e t t , P.D.; Boschung, A.F.; and Jewett, J.G., J . Am. Chem. Soc. (1977) 99 7977. 33. Jefford, C.W.; Boschung, A.F.; Bolsman, T.A.B.M.; Moriarty, R.M.; and Melnick, B., J . Am. Chem. Soc. (1976) 98 1017. 34. Srinivasan, V.S.; Podolski, D.; Neckers, D.C.; and Westrick, N.J. (1977) Submitted to J . Am. Chem. Soc. We thank Prof. Neckers f o r a preprint. 35. F r i d o v i t c h , Accts. Chem. Res. (1972) 5 321. 36. For a review of superoxide chemistry, see E. Lee-Ruff, Chem. Soc. Revs. (1977) 6 196. 37. Russell, G.A., i n "Radical Ions," ed. by E.T. Kaiser and L. Kevan, Chapter 3, Interscience Publishers, New York, 1968. 38. Barton, D.H.R.; Haynes, R.K.; Leclerc, G.; Mag nus, P.D.; and Menzies, I.D., J . Chem. Soc. (1975) Perkin I, 2055. 39. Michejda, C.J., and Campbell, D.H., J . Am. Chem. Soc., (1976) 98 6728. 40. Corey. E.J., and Ouannes, C., Tet. Letters (1976) 4263. 41. Shapiro, M.J., and Landis, M.E., unpublished work. RECEIVED December 23,

1977.


Free Radical Aspects of Photooxidation

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