Oxidation of Organic Compounds

---

67 Type II Photosensitized Oxygenation Reactions KLAUS GOLLNICK

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University of Arizona, Tucson,

Ariz.

a

In Type II photo-oxygenation reactions, singlet oxygen is produced by an energy transfer process from the electronically excited light absorber. Various classes of compounds, such as polycyclic aromatic hydrocarbons, cyclic 1,3-dienes, and furans as well as olefins containing allylic hydrogens are suitable substrates for the reaction with singlet oxygen. Stereoelectronic effects exerted by olefins on the reactions with O are dealt with, and the mechanism of O formation is discussed. Preliminary results on O production and its reaction with 2,5-dimethylfuran as a function of the triplet energy of (π,π )- and (n,π )-sensitizers are reported. 1

1

2

2

1

2

*

*

p h o t o - o x y g e n a t i o n reactions of o r g a n i c c o m p o u n d s , A , i n s o l u t i o n , fry/(sensitizer) A + 0

> A0

2

2

a f f o r d i n g a d d i t i o n p r o d u c t s ( A 0 ) , m a y o c c u r as d i r e c t or i n d i r e c t (sensi­ 2

t i z e d ) p h o t o - o x y g e n a t i o n reactions, d e p e n d i n g o n w h e t h e r A or a m o l e ­ cule other t h a n A — i . e . , a p h o t o s e n s i t i z e r — a b s o r b s the l i g h t . ( O n l y w h e n the e x c i t i n g photons possess w a v e l e n g t h s shorter t h a n 2000 A . m u s t ab­ sorption b y 0

be c o n s i d e r e d . )

2

F u r t h e r m o r e , d e p e n d i n g o n w h e t h e r free

r a d i c a l s or o n l y e l e c t r o n i c a l l y e x c i t e d molecules are i n v o l v e d as inter­ mediates, T y p e I processes m a y be d i s t i n g u i s h e d f r o m T y p e II processes (17). T h e b e n z o p h e n o n e - s e n s i t i z e d p h o t o - o x y g e n a t i o n of 2 - p r o p a n o l

(53)

m a y be c o n s i d e r e d as a t y p i c a l e x a m p l e of a T y p e I process, i n w h i c h the e l e c t r o n i c a l l y e x c i t e d b e n z o p h e n o n e initiates a free r a d i c a l o x i d a t i o n b y a b s t r a c t i n g a h y d r o g e n a t o m f r o m 2 - p r o p a n o l . T h e i n i t i a t i o n is t h e n followed by 0

2

a d d i t i o n to the 2 - h y d r o x y i s o p r o p y l r a d i c a l a n d r e a c t i o n

On leave of absence from the Max-Planck-Institut für Kohlenforschung, Abteilung Strahlenchemie, Mülheim an der Ruhr, Germany. a

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

67.

GOLLNICK

Type II

79

Oxygenations

w i t h a h y d r o g e n d o n o r , w h i c h m a y be either another 2 - p r o p a n o l m o l e ­ c u l e i n a c h a i n - p r o p a g a t i n g step or the b e n z o p h e n o n e k e t y l r a d i c a l i n a t e r m i n a t i o n step. T h u s , the o v e r - a l l r e a c t i o n is hy/benzophenone CH3CHOHCH3 + 0

• CH C(OH)CH

2

3

3

I OOH T h e T y p e I I p h o t o - o x y g e n a t i o n reactions w i t h w h i c h w e are c o n ­ c e r n e d o c c u r b y a c o m p l e t e l y different m e c h a n i s m . T h e m a i n feature of these reactions is that a n " a c t i v a t e d o x y g e n " is f o r m e d d u r i n g the r e a c t i o n , w h i c h c a n react stereoselectively

with

certain substrates to g i v e

the

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a d d i t i o n p r o d u c t s . T o d a y , w e h a v e reason to b e l i e v e that the " a c t i v a t e d o x y g e n " is the e x c i t e d singlet o x y g e n ,

1

0 . 2

P o l y c y c l i c a r o m a t i c h y d r o c a r b o n s s u c h as anthracenes,

tetracenes,

a n d pentacenes, as w e l l as c y c l o p e n t a d i e n e s , c y c l o h e x a - l , 3 - d i e n e s , c y c l o h e p t a - l , 3 - d i e n e s , a n d furans, h a v e b e e n f o u n d to b e s u i t a b l e d i e n e systems to w h i c h the singlet o x y g e n adds as a d i e n o p h i l e i n a 1 , 4 - c y c l o a d d i t i o n r e a c t i o n . T h u s , e n d o p e r o x i d e s ( t r a n s a n n u l a r p e r o x i d e s ) a n d , i n the case of furans, ozonides of the c o r r e s p o n d i n g c y c l o b u t a d i e n e s are the p r i m a r i l y p r o d u c e d , m o r e or less stable a d d i t i o n p r o d u c t s (2, 21,

22).

O l e f i n s c o n t a i n i n g at least one a l l y l i c h y d r o g e n are s u i t a b l e substrates a n d are of s p e c i a l i m p o r t a n c e a n d interest w i t h r e g a r d to the i n t r i n s i c m e c h a n i s m i n v o l v e d i n their reactions w i t h singlet o x y g e n . A l l y l i c h y d r o ­ peroxides are f o r m e d , b u t the m e c h a n i s m of their f o r m a t i o n is c l e a r l y d i s t i n c t f r o m that b y w h i c h a l l y l i c h y d r o p e r o x i d e s are p r o d u c e d i n ther­ m a l or photochemically initiated

(see

example

above

for a T y p e

I

p r o c e s s ) a u t o x i d a t i o n reactions. T h i s has u n e q u i v o c a l l y b e e n s h o w n w i t h o p t i c a l l y active l i m o n e n e as a substrate,

w h i c h gives rise to different

p r o d u c t s i n free r a d i c a l a n d T y p e I I p h o t o - o x y g e n a t i o n reactions 57,

(22,

61). W h i l e , f o r e x a m p l e , the t h e r m a l a u t o x i d a t i o n r e a c t i o n of ( + ) - l i m o ­

nene ( 1 )

proceeds as a free r a d i c a l c h a i n r e a c t i o n (61),

the photosensi­

t i z e d o x y g e n a t i o n of 1 occurs a c c o r d i n g to the scheme s h o w n at the t o p of the next p a g e (51,

52).

P h e n o m e n o l o g i c a l l y , this r e a c t i o n m a y be d e s c r i b e d as o c c u r r i n g i n three steps: ( 1 ) A t t a c h m e n t of o x y g e n to one of the c a r b o n atoms of the d o u b l e b o n d ( i n the s c h e m e to C i ) . ( 2 ) S h i f t of the d o u b l e b o n d to the a l l y l i c p o s i t i o n ( C , C ) ( 3 ) M i g r a t i o n of the a l l y l i c h y d r o g e n to the t e r m i n u s of the p e r o x y group. 2

3

M e c h a n i s t i c a l l y , h o w e v e r , the r e a c t i o n p r o b a b l y takes p l a c e c o n c e r t e d f a s h i o n i n v o l v i n g a c y c l i c s i x - m e m b e r e d t r a n s i t i o n state 22,45,56,57).

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

in a (13,

80

OXIDATION

hv/sensitizer/0

OF

ORGANIC

2

COMPOUNDS

1

III

l I C2=C3"

Ci

I OOH unidentified product

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13%

10%

(2) reduction

X.„

l(+)

H

1

/'

(a-)H

CH

I

*

Y y

M

la 1) sens./h /0 . (2) reduction v

2

T h e f o l l o w i n g results are i n f a v o r of the a s s u m e d c o n c e r t e d r e a c t i o n : (1) O n l y those a l l y l i c h y d r o g e n s are u s e d i n the r e a c t i o n w h i c h are c i s - o r i e n t e d w i t h respect to the o x y g e n attack o n the d o u b l e b o n d carbons, as w a s d e m o n s t r a t e d w i t h 7«-D- a n d 7/?-D-cholesterol (45) affording 5a-hydroperoxy-A -cholesten-3/?-ol (58). 6

(2) T r i a l k y l s u b s t i t u t e d ethylenes s u c h as l i m o n e n e (57) a n d 1m e t h y l c y c l o h e x e n e (50) g i v e rise to ratios of t e r t i a r y - s e c o n d a r y h y d r o ­ peroxides of a b o u t 44 to 56, w h i l e o p e n - c h a i n olefins s u c h as t r i m e t h y l ethylene, l , l - d i m e t h y l - 2 - e t h y l e t h y l e n e , 2,6-dimethyl-2-octene, m y r c e n e , 0 - c i t r o n e l l o l , l i n a l o o l , a n d l , l - d i m e t h y l - 2 - b e n z y l e t h y l e n e g i v e ratios of t e r t i a r y - s e c o n d a r y h y d r o p e r o x i d e s b e t w e e n 54 to 46 a n d 60 to 40 (31, 43, 47, 60, 63, 66). Since there is n o h y d r o g e n a b s t r a c t i o n p r i o r to o x y g e n a d d i t i o n to one of the d o u b l e b o n d carbons, this a d d i t i o n m u s t b e t h e first step if a m u l t i s t e p r e a c t i o n takes p l a c e . W h a t e v e r the s o - f o r m e d inter­ mediates m a y be, h o w e v e r , d i r a d i c a l species s u c h as 8 a or 8 b, or i o n i c species s u c h as 9 a or 9 b [the latter has b e e n suggested b> some authors (37, 67)], secondary h y d r o p e r o x i d e s s h o u l d be p r o d u c e d almost e x c l u ­ s i v e l y f r o m the n o n s t e r i c a l l y h i n d e r e d olefins since i n the case of the p e r o x y intermediates, 8 a a n d 9a, the most stable ( t e r t i a r y ) a l k y l r a d i c a l or c a r b o n i u m i o n , r e s p e c t i v e l y , s h o u l d b e f o r m e d , a n d i n the case of the

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

67.

Type II

GOLLNICK

81

Oxygenations

p e r e p o x y intermediates, 8b a n d 9b, the a t t a c k i n g r a d i c a l or c a t i o n is m o r e strongly b o n d e d to the c a r b o n w i t h the smallest n u m b e r of a l k y l s u b s t i t u ents (94). O b v i o u s l y , there is not m u c h d i s c r i m i n a t i o n b e t w e e n the t w o d o u b l e b o n d c a r b o n atoms, a n d the slight d e v i a t i o n s f r o m 1 to 1 ratios i n a l l these cases are p r o b a b l y d u e to stereochemical rather t h a n electronic effects exerted b y the olefins o n the r e a c t i o n w i t h singlet o x y g e n . O O—O

O

H

— c—c—1 -

c—c—c —

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H

III

8a

8b

oO—O

1

H

O

H

/+»

I

III

III 9a

9b

( 3 ) A c c o r d i n g to the results o b t a i n e d i n c o n f o r m a t i o n a l analysis, l i m o n e n e ( i n s o l u t i o n at r o o m t e m p e r a t u r e ) assumes a h a l f - c h a i r c o n f o r ­ m a t i o n w i t h the i s o p r o p e n y l g r o u p i n a n e q u a t o r i a l p o s i t i o n (6, 34) (see F o r m u l a l a ) . A s the e n h a n c e d f o r m a t i o n of 2 as c o m p a r e d w i t h 3 a n d the 1 to 1 ratio of 6 a n d 7 s h o w , there is no steric h i n d r a n c e exerted b y the e q u a t o r i a l side c h a i n o n a n o x y g e n attack at C i or C . T h e r e f o r e , the stereoselective r e a c t i o n of l i m o n e n e w i t h s i n g l e t o x y g e n as r e v e a l e d b y the p r o d u c t d i s t r i b u t i o n must be c a u s e d b y a n e n h a n c e d r e a c t i v i t y of the q u a s i - a x i a l ( a ' ) h y d r o g e n s at C a n d C as c o m p a r e d w i t h the corre­ s p o n d i n g q u a s i - e q u a t o r i a l (e ) h y d r o g e n s . F u r t h e r m o r e , the 1 to 1 r a t i o of 6 a n d 7 a n d t h e i r e n h a n c e d p r o d u c t i o n as c o m p a r e d w i t h the f o r m a t i o n of 4 a n d 5 m u s t b e caused b y the f a c t that the C — H b o n d of the m e t h y l g r o u p c a n a p p r o a c h ( o n b o t h sides of the l i m o n e n e m o l e c u l e ) the p e r ­ p e n d i c u l a r a r r a n g e m e n t w i t h respect to the d o u b l e b o n d p l a n e , necessary f o r d e v e l o p i n g the n e w d o u b l e b o n d , even better t h a n a q u a s i - a x i a l r i n g a l l y l h y d r o g e n . T h e i n c r e a s e d r e a c t i v i t y of a l l y l i c h y d r o g e n s , — C H > q u a s i - a x i a l > q u a s i - e q u a t o r i a l , has b e e n f o u n d to be a g e n e r a l p h e n o m e ­ n o n (17, 45, 57). 2

3

6

f

3

( 4 ) T h e t e r t i a r y h y d r o p e r o x i d e s f o r m e d f r o m the o p e n - c h a i n olefins m e n t i o n e d above a l l c o n t a i n trans-substituted d o u b l e b o n d s (63, 66). A s one c a n see f r o m m o d e l s , the most stable c o n f o r m a t i o n s of these olefins ( g e n e r a l f o r m u l a 10) are those i n w h i c h one of the a l l y l i c h y d r o g e n s at C is e c l i p s e d w i t h the d o u b l e b o n d ( l 0 a , b ) . R e a c t i o n w i t h the other a l l y l i c h y d r o g e n m u s t therefore give rise to trans-substituted d o u b l e b o n d s . T h e c o n f o r m a t i o n a l i s o m e r 10c, w h i c h w o u l d g i v e rise to ciss u b s t i t u t e d d o u b l e b o n d s b y a concerted r e a c t i o n , is e x p e c t e d to b e h i g h l y 3

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

82

OXIDATION

OF ORGANIC

COMPOUNDS

H I

u n f a v o r e d because of strong steric r e p u l s i o n b e t w e e n t h e R g r o u p a n d the m e t h y l g r o u p at C i w h i c h is cis to this R g r o u p . T h e f o r m a t i o n of intermediates s u c h as 8 or 9 i n a m u l t i s t e p r e a c t i o n w o u l d also e x p l a i n the o c c u r r e n c e of the t h e r m o d y n a m i c a l l y m o r e stable trans-substituted d o u b l e b o n d s i n t h e tertiary h y d r o p e r o x i d e s . H o w e v e r , t h e n e a r l y e x c l u ­ sive f o r m a t i o n of t h e s e c o n d a r y h y d r o p e r o x i d e , 15, f r o m l , l - d i m e t h y l - 2 i s o p r o p y l e t h y l e n e , 13 (22,63, 66), supports t h e a s s u m p t i o n of a c o n c e r t e d r e a c t i o n : T h e most stable c o n f o r m a t i o n of 13 is 13a, i n w h i c h the a l l y l i c h y d r o g e n at C n e e d e d f o r t h e f o r m a t i o n of the t e r t i a r y h y d r o p e r o x i d e , 14, is e c l i p s e d w i t h t h e d o u b l e b o n d , the most u n f a v o r a b l e p o s i t i o n a n a l l y l i c h y d r o g e n c a n assume f o r a c o n c e r t e d reaction w i t h o x y g e n . T h e r e ­ fore, the p r o d u c t i o n of 14 s h o u l d b e almost suppressed. O n t h e other h a n d , i f t h e tertiary h y d r o p e r o x i d e s ( 1 1 ) w e r e f o r m e d b y a m u l t i s t e p r e a c t i o n , one w o u l d expect a reasonable a m o u n t of 14 to b e f o r m e d f r o m 13 b y the same m e c h a n i s m .

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3

( 5 ) M e t h y l groups s u c h as t h e C - m e t h y l groups of a-pinene, 16 (54), A - c a r e n e , 18 (23,62), A - c a r e n e , 22, a n d t h e A - c a r e n e s , 25 a n d 2 7 (19), or the a n g u l a r m e t h y l groups a t t a c h e d to C i i n c e r t a i n steroids (10, 45, 46, 55, 58) m a y c o m p l e t e l y s h i e l d t h e d o u b l e b o n d against a n attack of the singlet o x y g e n . T h u s , steric s h i e l d i n g effects i n a d d i t i o n to c o n f o r m a ­ t i o n a l effects exerted b y t h e a l l y l i c h y d r o g e n s cause t h e stereoselectivity 8

3

4

2

0

H

R 10

10b

10a

H 10c H

OOH

OOH

11

12

54-60%

46-40%

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

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67.

GOLLNICK

16

Type II

83

Oxygenations

16a

17

of T y p e I I p h o t o - o x y g e n a t i o n reactions. I n t e r e s t i n g l y e n o u g h , the o b ­ s e r v e d p r o d u c t d i s t r i b u t i o n f r o m A - c a r e n e was e x p l a i n e d (23) b y assum­ i n g that of the t w o b o a t c o n f o r m a t i o n s , 18a a n d 18b, o n l y the closed-boat f o r m , 18 b, takes p a r t i n the h y d r o p e r o x i d a t i o n r e a c t i o n since o n l y if this is the case, the p r o d u c t ratios of 19 to ( 2 0 + 2 1 ) a n d of 2 0 to 21 s h o u l d be 1 to 1. T h e attack of o x y g e n o n the a-side of 18 at C a n d C s h o u l d f o l l o w a statistical p a t t e r n a n d the a-hydrogens at C a n d C s h o u l d b e as a v a i l a b l e as the a l l y l i c h y d r o g e n s of the C - m e t h y l g r o u p . O n the other h a n d , a ^-attack o n 1 8 b s h o u l d be c o m p l e t e l y p r e v e n t e d b y the C - m e t h y l g r o u p . R e c e n t l y , it was s h o w n that A - c a r e n e exists to about 9 3 % i n the closed-boat c o n f o r m a t i o n ( 1 8 b ) at 2 0 ° C . ( I ) at w h i c h the p h o t o - o x y g e n a t i o n r e a c t i o n was c a r r i e d out. 3

3

2

4

5

1 0

8

3

I n p r e p a r a t i v e o r g a n i c c h e m i s t r y , m u c h use has b e e n m a d e of the stereoselective T y p e II p h o t o - o x y g e n a t i o n r e a c t i o n since the p r i m a r i l y p r o d u c e d a l l y l i c h y d r o p e r o x i d e s c a n be r e d u c e d u n d e r r e t e n t i o n of c o n ­ figuration. H o w e v e r , d e p e n d i n g o n the n a t u r e of the a l l y l i c h y d r o p e r -

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

84 oxides

OXIDATION

and

the

particular

reaction

OF

conditions

ORGANIC

applied

COMPOUNDS

IEE

(temperature,

concentrations of the substrates, solvents, e t c . ) , secondary reactions

may

occur—e.g., rearrangements of a l l y l i c tertiary h y d r o p e r o x i d e s to secondary a l l y l i c h y d r o p e r o x i d e s , e l i m i n a t i o n of w a t e r f r o m secondary a l l y l i c h y d r o ­ peroxides to y i e l d «,/?-unsaturated ketones, a n d f r a g m e n t a t i o n of a l l y l i c h y d r o p e r o x i d e s to c a r b o n y l c o m p o u n d s .

F u r t h e r m o r e , T y p e I processes

(free radical chain autoxidation reactions) reactions,

s u b s t i t u t e d olefins) serve as substrates

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may accompany

especially w h e n r e l a t i v e l y u n r e a c t i v e

olefins

(17).

19

20

21

50%

27%

23%

22

23

24

18%

82%

Type

(mono-

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

II

or d i -

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67.

Type II

GOLLNICK

Oxygenations

85

27 Generally,

tetraalkyl-substituted double bonds react w i t h

singlet

o x y g e n at faster rates t h a n d o t r i a l k y l - s u b s t i t u t e d d o u b l e b o n d s , w h i c h i n t u r n react faster t h a n d i a l k y l - s u b s t i t u t e d ones ( 2 2 , 50, 63, 66). A c c o r d ­ i n g to the r e a c t i o n sequence o f T y p e I I p h o t o - o x y g e n a t i o n reactions g i v e n b e l o w , t h e r e a c t i o n o f a substrate w i t h singlet o x y g e n ( S t e p 9 ) competes w i t h t h e spontaneous d e a c t i v a t i o n of singlet o x y g e n ( S t e p 8 ) . F r o m t h e v a r i a t i o n o f the rate o f o x y g e n c o n s u m p t i o n w i t h the c o n c e n t r a t i o n o f the substrate, k /k 8

9

c a n b e d e t e r m i n e d f o r v a r i o u s substrates.

Since kg r e ­

m a i n s constant u n d e r s i m i l a r r e a c t i o n c o n d i t i o n s , r e l a t i v e reactivities o f the substrates t o w a r d s singlet o x y g e n c a n b e o b t a i n e d (56). I n T a b l e I, r e l a t i v e free energies o f a c t i v a t i o n , A A F * , for the r e a c t i o n o f singlet o x y g e n at C i a n d C o f different olefins a r e g i v e n , c a l c u l a t e d f r o m e x p e r i m e n t a l 2

k /k 8

9

values w i t h t e t r a m e t h y l e t h y l e n e

(TME)

( 2 8 ) as t h e reference

compound: A A F * = RTlnfc

rel

(T = 2 9 3 K . , reaction temperature) 0

(1)

with —

( ^ / ^ . T M E )

.substr. /

Psubstr.

%

P T M E

M

E

^substr.

i n w h i c h p is t h e factor w h i c h accounts f o r t h e p r o d u c t d i s t r i b u t i o n a n d n is t h e n u m b e r o f e q u i v a l e n t t r a n s i t i o n states l e a d i n g to a p a r t i c u l a r product.

I n o r d e r to d e t e r m i n e n , t h e e q u i v a l e n c e of o n e r i n g - a l l y l i c

h y d r o g e n to three C H - a l l y l i c h y d r o g e n s i n a c o n c e r t e d r e a c t i o n has b e e n 3

assumed.

Furthermore, no distinction between

quasi-axial a n d quasi-

e q u a t o r i a l h y d r o g e n s was m a d e .

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

OXIDATION

O F ORGANIC

T a b l e I.

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2

Relative

TME

^8/^-9,8ubstr.

}

Compound

1

COMPOUNDS

Moles/L.

^8/^9,8

28

0.003

1.00

30

0.055

0.055 0.045 0.07

a

b

33

0.030

0.100

36

1.20

0.0025 0.0020° 0.002 0.0082 b

c

40

42

26.0

0.18

0.00012

0.0166

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

m

Free

67.

Type II

GOLLNICK

87

Oxygenations

E n e r g i e s of A c t i v a t i o n

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Reaction at

AAF*, Kcal./Mole, Reaction at

Pi

n

p

n

0.50

4

0.50

4

0

0

0.54

2

0.46

4

1.25

1.75

0.44 ( M ) 0.06(R)

2 2

0.44 ( M ) 0.06(R)

2 2

1.01(M) 2.17(R)

1.01(M) 2.17(R)

0.45

2

0.40 ( M ) 0.15(R)

2 2

3.15

3.20 ( M ) 3.79(H)

0.50

2

0.50

2

4.86

4.86

0.55

2

0.45

4

1.93

2.44

t

2

%

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

OXIDATION OF ORGANIC COMPOUNDS

HI

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Table I.

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

67.

89

Type II Oxygenations

GOLLNICK

Continued R

E

A

C

T

I

O

N

A

AAF$,

T

C-1

C-2

0.05

2

0.95

4

4.48

3.17

—

-

1.00

4

—

3.83

—

-

0.94

1

—

3.43

0.50

1

0.25(M)

1

2.04

0.25 ( R )

1

0.80 ( M ) 0.20(R)

2 1

—

-

2

n

C-1

n

t

p

Kcal./Mole,

Reaction at

p

±

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C-2

2

2.45 ( M ) 2.45(R)

—

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

1.75(M) 2.18(R)

90

OXIDATION

OF

ORGANIC

COMPOUNDS—III

Table I.

Moles/L.

Compound

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25

0.0177

0.17<

AAFJ calculated from experimental data taken from Schenck and Gollnick "Obtained from photosensitized oxygenation (11, 25). Obtained from H 0 / N a O C l experiments (11, 25). Calculated from data given by Kopecky and Reich (37). b

2

(56).

2

c

Since o n l y r e l a t i v e rate constants c o u l d be m e a s u r e d so far a n d no a c t i v a t i o n energies for the c o m p o u n d s of T a b l e I h a v e b e e n d e t e r m i n e d yet, the c o n t r i b u t i o n s of A A H * a n d A A S * to a p a r t i c u l a r A A F * c a n n o t b e separated.

H o w e v e r , differences i n A A F * o b t a i n e d f o r 28 a n d 3 0 as w e l l

as f o r a n o x y g e n attack o n C - 1 of 33, 36, a n d 4 0 f o r m i n g the c y c l o h e x e n e d e r i v a t i v e s 34, 37, a n d 4 1 , r e s p e c t i v e l y , are p r o b a b l y m a i n l y d u e to d i f ­ ferences i n A H * . O n the other h a n d , differences i n A A F * o b t a i n e d f o r the f o r m a t i o n of 3 4 a n d 35 f r o m 33 a n d of 37, 38, a n d 39 f r o m 36 seem to b e d u e m a i n l y to differences i n A S * . T h u s , there seems to b e a m a r k e d de­ crease i n A H * w i t h a n i n c r e a s i n g n u m b e r of m e t h y l groups d i r e c t l y attached

to a d o u b l e b o n d c a r b o n , i n d i c a t i n g the

e l e c t r o p h i l i c i t y of

OOH 28

30

29

OOH

OOH

31

32

54%

46%

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

67.

Type II

GOLLNICK

91

Oxygenations

Continued Reaction at

AAF*,

C-1

Pt

p

t

2

7

/

w

n

2

1

C-1

C-2

—

1.14

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1.00

, Kcal/Mole,

Reaction at

C-2 n

„

singlet o x y g e n . T h e increase i n A A F * for r i n g - a l l y l i c h y d r o g e n s as c o m ­ p a r e d w i t h C H - a l l y l i c h y d r o g e n s reflects the c o n f o r m a t i o n a l effects of 3

the t w o different h y d r o g e n s o n the r e a c t i o n w i t h o x y g e n .

[ N o distinction

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

92

OXIDATION

OF

ORGANIC

COMPOUNDS

HI

b e t w e e n the t w o different r i n g - a l l y l i c h y d r o g e n s , q u a s i - a x i a l a n d q u a s i e q u a t o r i a l , has b e e n m a d e i n these c a l c u l a t i o n s . H o w e v e r , f r o m the fore­ g o i n g d i s c u s s i o n it s h o u l d b e o b v i o u s that the q u a s i - e q u a t o r i a l

allylic

h y d r o g e n s are m u c h less suitable for the reaction t h a n the q u a s i - a x i a l allylic hydrogens.] Differences i n A A F * f o r an o x y g e n attack o n C - 2 of 30, 4 2 , 13, a n d 45 are p r o b a b l y m a i n l y c a u s e d b y steric s h i e l d i n g of C - 2 b y the i n c r e a s i n g b u l k i n e s s of the a l k y l groups a t t a c h e d to this c a r b o n .

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H

OOH 45

46

T h e c o m p a r i s o n of A A F * values of c o m p o u n d s 36, 16, 18, a n d 2 2 f o r the o x y g e n attack at C - 2 reveals a r e m a r k a b l e difference i n the effect of an

i s o p r o p y l i d e n e g r o u p w h e n attached

1,3- a n d a 1,2- m a n n e r .

to 1 - m e t h y l c y c l o h e x e n e

in a

T h u s , w h i l e this g r o u p i n a - p i n e n e , 16, exerts

o n l y a s m a l l effect o n the r e a c t i v i t y of the d o u b l e b o n d , a c l e a r l y en­ h a n c e d r e a c t i v i t y of the d o u b l e b o n d of observed.

18 a n d e s p e c i a l l y of 2 2 is

T h e r e a c t i v i t y - i n c r e a s i n g effect of the d i m e t h y l c y c l o p r o p a n e

r i n g conjugated to a d o u b l e b o n d becomes even m o r e o b v i o u s w h e n one c o m p a r e s the A A F * values of d s - A - c a r e n e , 2 5 , a n d of cyclohexene, 4 0 . 2

C a r b o n y l groups conjugated to a d o u b l e b o n d a n d a l l y l O H - a n d O O H - g r o u p s decrease the r e a c t i v i t y of the d o u b l e b o n d c o n s i d e r a b l y , at least b y about 1 k c a l . p e r m o l e ( 5 6 ) .

T h u s , i f the p h o t o - o x y g e n a t i o n

leads to h y d r o p e r o x i d e s w i t h d o u b l e b o n d s s u b s t i t u t e d to the same or a l o w e r degree t h a n those of the i n i t i a l olefins, o v e r o x i d a t i o n ( c o n s u m p ­ t i o n of m o r e t h a n one m o l e c u l e of o x y g e n p e r m o l e c u l e of olefin) is easily a v o i d e d b y s t o p p i n g the r e a c t i o n after one m o l e of o x y g e n p e r m o l e of olefin is t a k e n u p . C o n s u m p t i o n of t w o molecules of o x y g e n takes p l a c e w i t h olefins l i k e i s o - T e t r a l i n , 4 7 ( 36, 59)

or n e o a b i e t i c a c i d , 50

since the r e a c t i v i t y - d e c r e a s i n g effect of the O O H g r o u p is

(65),

compensated

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

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67.

GOLLNICK

Type II

93

Oxygenations

50

51

52

b y the h i g h l y r e a c t i v e 1,3-cycIohexadiene tively

s y s t e m of 48 a n d 51,

respec­

(17).

F i n a l l y , it m a y be m e n t i o n e d that c e r t a i n h y d r a z o n e s , t h i o s e m i c a r b a zones, sulfides, sulfoxides, a n d amines h a v e b e e n f o u n d to be substrates i n p h o t o s e n s i t i z e d o x y g e n a t i o n reactions

suitable

(17).

F o r a l o n g t i m e , the q u e s t i o n of w h e t h e r the o x y g e n a t i n g species i n T y p e I I p h o t o - o x y g e n a t i o n reactions is the e l e c t r o n i c a l l y e x c i t e d singlet o x y g e n (27,

28, 29)

or a c o m p l e x c o m p o s e d of the e l e c t r o n i c a l l y excited

l i g h t absorber a n d o x y g e n (5, 22, 35, 39, 52, 64)

remained open.

Based

o n r e c e n t results that e l e c t r o n i c a l l y e x c i t e d singlet o x y g e n , generated b y n o n p h o t o c h e m i c a l means, reacts w i t h c e r t a i n substrates to f o r m the same p r o d u c t s (9,12,

42, 7 1 ) , a n d d i s p l a y s the same stereoselectivity (13)

the same kinetics (11, 25) reactions

(56, 5 7 ) ,

and

as the " a c t i v a t e d o x y g e n " i n p h o t o - o x y g e n a t i o n

there is b u t l i t t l e d o u b t that singlet o x y g e n is the

o x y g e n a t i n g species i n T y p e II p h o t o - o x y g e n a t i o n reactions.

Further­

m o r e , the relative reactivities of different substrates t o w a r d s the oxy­ g e n a t i n g species w e r e f o u n d to be i n d e p e n d e n t of the n a t u r e of sensitizers a p p l i e d (37,

the

56, 7 0 ) , w h i c h supports the i d e a of a c o m m o n

i n t e r m e d i a t e i n a l l these reactions. E l a b o r a t e k i n e t i c analyses of the p h o t o - o x y g e n a t i o n of 2,5-dimethyIf u r a n (22,

56)

a n d a l l y l t h i o u r e a (40)

sensitized b y xanthene dyes a n d

c h l o r o p h y l l , respectively, a n d of the d i r e c t p h o t o - o x y g e n a t i o n of a n t h r a ­ cene a n d 9 , 1 0 - d i p h e n y l a n t h r a c e n e

(41)

have

revealed

that

Type

II

d i r e c t a n d i n d i r e c t ( s e n s i t i z e d ) p h o t o - o x y g e n a t i o n reactions p r o c e e d w i t h

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

94

OXIDATION

essentially t h e same m e c h a n i s m

O F ORGANIC

COMPOUNDS

a c c o r d i n g to t h e f o l l o w i n g

H I

reaction

sequence: abs.

x

!S +

h

0

l

s

V

• S 1

^ S Q + hy

1

S-

1

(1)

Fluorescence

(2)

Internal conversion

(3)

*S

• Si

Intersystem crossing

(4)

S

• S

Intersystem crossing

(5)

3

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Absorption

1

3

1

0

+ o,

*S

S + O | Energy transfer; singlet / oxygen formation

3

3

+ 0

S

3

4is + iO 0

2

^.2

1

0

0

3

+ A 1

2

1

0

^

)

2

(7) Deactivation of singlet oxygen

2

-> ( A 0 ) o 1

0

(6)

a

Product formation

2

w i t h S = A i n direct, S =

(8)

(9)

photosensitizer, different f r o m A , i n i n d i r e c t

p h o t o - o x y g e n a t i o n reactions;

1

S , Si, and Si = 0

1

singlet g r o u n d state,

3

first excited singlet a n d t r i p l e t state of S, r e s p e c t i v e l y , A 1

0

and ( A 0 ) o 1

=

2

singlet g r o u n d states o f substrate A a n d p r o d u c t A 0 , r e s p e c t i v e l y ; 0

2

t r i p l e t g r o u n d state o x y g e n ( ' % " ) ; * 0

(%

3

2

a n d / o r A ). 1

=

2

e x c i t e d singlet o x y g e n

= +

Singlet o x y g e n replaces t h e l i g h t a b s o r b e r - o x y g e n c o m p l e x

g

g i v e n i n t h e o r i g i n a l papers. O m i t t i n g t h e state s y m b o l s f o r s i m p l i c i t y , t h e q u a n t u m y i e l d of p r o d ­ u c t f o r m a t i o n is g i v e n b y . *

[Q»] A

°

2

~

k

a

+

k

B

+

k

4

(

*7

V

k+k

* [0 ]

+

6

2

6

4

h

* [0 ]

+

7

2

)

\ *.[A] *= + * * [ A ] 8

or _

[A]

4>AO - * X 3

Table

(V,7r*)-Sensitizers

II.

T r i p l e t E n e r g y of

Triplet Energy; Kcal./Mole

84.5 Benzene 67.6 Fluorene Triphenylene 66.6 Biphenyl 65.7 62.2 Phenanthrene 60.9 Naphthalene 59.0 1 -Bromonaphthalene 1 -Iodonaphthalene 58.6 48.7 Pyrene 47.2 1,2-Benzanthracene Anthracene 42.0 Acenaphthylene Eosin 42.4 Erythrosin 42.0 39.4 Rose Bengal Azulene Between 31 and 3 8 29.4 Tetracene

Sensitizers

( n,7r*)-Sensitizers

Triplet Energy; Kcal./Mole

Acetone Propiophenone Xanthone Acetophenone Carbazole Benzophenone Thioxanthone Flavone 2-Naphthylphenylketone* 1-Naphthylphenylketone Fluorenone

5

6

c

c

e

d

"Triplet energies of ( 7 r , 7 r * ) - and (n,7r*) -sensitizers [taken from ( 8 ) ] . Reacting triplets are probably ( 7 r , 7 r * ) (24). " F r o m (17). F r o m (38). b

3

d

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

—

74.6 74.2 73.6 70.1 68.5 65.5 62.0 59.6 57.5 53.3

98

OXIDATION

-> %

+

^

a n d F -> ( S . . . O ) -> % 3

3

2

O F ORGANIC

+

0

3

COMPOUNDS

H I

0 , are f u n c t i o n s of t h e 2

t r i p l e t energies of the sensitizers or n o t ; because of the m e c h a n i s m g i v e n a b o v e singlet o x y g e n f o r m a t i o n s h o u l d n o t d e p e n d o n the singlet energy b u t m a y d e p e n d o n the t r i p l e t e n e r g y of t h e l i g h t absorber.

[Recently, a

q u a n t u m m e c h a n i c a l t r e a t m e n t of the m e c h a n i s m of q u e n c h i n g of t r i p l e t state m o l e c u l e s b y o x y g e n w a s c a r r i e d o u t ( 3 0 ) . T h e authors, also c o n ­ s i d e r i n g the i n t e r a c t i o n of singlet o x y g e n w i t h singlet g r o u n d state l i g h t absorbers, c o n c l u d e that the ratio of rates f o r t h e t w o c o m p e t i n g p r o c ­ esses is of the o r d e r of 100 to 1000—i.e., t r i p l e t q u e n c h i n g b y o x y g e n s h o u l d almost e x c l u s i v e l y b e a c c o m p a n i e d b y singlet o x y g e n f o r m a t i o n . ] Since a l l t h e d i r e c t a n d s e n s i t i z e d p h o t o - o x y g e n a t i o n reactions

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been

carried out w i t h

" l o w - e n e r g y l i g h t absorbers''

(triplet

have

energies

l o w e r t h a n about 4 5 k c a l . p e r m o l e ) , w e e x a m i n e d a series of sensitizers w i t h t r i p l e t energies f r o m a b o u t 30 to 85 k c a l . p e r m o l e (18).

2,5-Di-

m e t h y l f u r a n , 53, w h i c h reacts w i t h singlet o x y g e n to the o z o n i d e (54) t h a t is i m m e d i a t e l y c o n v e r t e d to a m e t h o x y h y d r o p e r o x i d e , 55, i n t h e presence of m e t h a n o l the

(14,

21),

w a s u s e d as a substrate.

(7r,7r*)- a n d (n,7r*)-sensitizers

I n the presence of

given i n Table II, the dimethylfuran

i n m e t h a n o l i c s o l u t i o n took u p 1 m o l e of 0

2

p e r m o l e of d i m e t h y l f u r a n ,

O 53

O 54

(+CH OH) 3

56

55

a n d the o n l y p r o d u c t f o r m e d w a s the m e t h o x y h y d r o p e r o x i d e w h i c h w a s i d e n t i f i e d b y its m e l t i n g p o i n t , i n f r a r e d s p e c t r u m , a n d h y d r o l y s i s p r o d u c t , 56. F u r t h e r m o r e , i n a l l cases the u n c h a n g e d sensitizer w a s r e c o v e r e d i n

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

67.

GOLLNICK

Type II

Oxygenations

99

a b o u t 9 0 % y i e l d , except f o r anthracene, w h i c h w a s d i m e r i z e d to a b o u t 7 5 % d u r i n g the i r r a d i a t i o n p e r i o d , 1 - i o d o n a p h t h a l e n e w h i c h w a s p h o t o l y z e d , a n d a z u l e n e , a b o u t 1 0 % of w h i c h w a s d e s t r o y e d . I n a d d i t i o n , w e c a r r i e d o u t the t h e r m a l a u t o x i d a t i o n of 2 , 5 - d i m e t h y l f u r a n i n r e f l u x i n g m e t h a n o l a n d i s o l a t e d o n l y a b r o w n i s h , viscous o i l that possesses a d i f ­ ferent i n f r a r e d s p e c t r u m f r o m that of 55. ( T h e m e t h o x y h y d r o p e r o x i d e , 55, is stable u n d e r these c o n d i t i o n s . ) T h e s e results c l e a r l y s h o w that a l l t h e (?r,7r*)- as w e l l as t h e (n,7r*)sensitizers i n d e p e n d e n t of t h e i r t r i p l e t energies transfer their energy to 3

0 , t h e r e b y f o r m i n g ^ o . T h e s e experiments d o n o t answer, h o w e v e r , 2

the q u e s t i o n of w h i c h singlet o x y g e n is i n v o l v e d a n d i f t h e t w o singlet Downloaded by MONASH UNIV on June 17, 2013 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch067

oxygens exert different c h e m i c a l b e h a v i o r . A t present, q u a n t u m y i e l d of 1

0 - f o r m a t i o n as w e l l as p r o d u c t f o r m a t i o n a n d p r o d u c t d i s t r i b u t i o n s 2

w i t h v a r i o u s olefinic substrates as a f u n c t i o n of the sensitizers a p p l i e d are being studied.

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(6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24)

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RECEIVED

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