Type II Photosensitized Oxygenation Reactions - Advances in

67 Type II Photosensitized Oxygenation Reactions KLAUS GOLLNICK

Downloaded by MONASH UNIV on June 17, 2013 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch067

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


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


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


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 —

III Downloaded by MONASH UNIV on June 17, 2013 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch067

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


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 .


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%



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 -


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


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-


II

or d i -


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 .


OXIDATION

O F ORGANIC

T a b l e I.


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


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


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

%


OXIDATION OF ORGANIC COMPOUNDS

HI


Table I.


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

±


C-2

2

2.45 ( M ) 2.45(R)

—


1.75(M) 2.18(R)

90

OXIDATION

OF

ORGANIC

COMPOUNDS—III

Table I.

Moles/L.

Compound


25

0.0177

0.17
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


—

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


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


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.

Literature Cited

(1) (2) (3) (4) (5)

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

Acharya, S. P., Tetrahedron Letters 1966, 4117. Arbuzov, Y. A., Russ. Chem. Rev. 34, 558 (1965). Arnold, S. J., Ogryzlo, E. A., Witzke, H., J. Chem. Phys. 40, 1769 (1964). Bader, L. W., Ogryzlo, E. A., Discussions Faraday Soc. 37, 46 (1964). Bowen, E. J., "The Photochemistry of Aromatic Hydrocarbon Solutions," in "Advances in Photochemistry," W. A. Noyes, Jr., G. S. Hammond, and J. N. Pitts, Jr., eds., Vol. I, p. 23, Interscience, Wiley, New York, London, Sidney, 1963. Brewster, J. H., J. Am. Chem. Soc. 81, 5493 (1959). Browne, R. J., Ogryzlo, E. A., Proc. Chem. Soc. London 1964, 177. Calvert, J. G., Pitts, J. N., Jr., "Photochemistry," Wiley, New York, Lon don, Sidney, 1966. Corey, E. J., Taylor, W. C., J. Am. Chem. Soc. 86, 3881 (1964). Eisfeld, W., Dissertation, University of Göttingen, 1965. Foote, C. S., Cheng, H., private communication. Foote, C. S., Wexler, S., J. Am. Chem. Soc. 86, 3879, 3880 (1964). Foote, C. S., Wexler, S., Ando, W., Tetrahedron Letters 1965, 4111. Foote, C. S., Wuesthoff, M. T., Wexler, S., Burstain, I. G., Denny, R., Schenck, G. O., Schulte-Elte, K. H., Tetrahedron 23, 2583 (1967). Gaffron, H., Biochem. Z. 264, 251 (1933). Gaffron, H., Z. Physik. Chem. B37, 437 (1937). Gollnick, K., Advan. Photochem., in press. Gollnick, K., Dörhöfer, G., unpublished manuscript. Gollnick, K., Schade, G., Tetrahedron Letters 1966, 2335. Gollnick, K., Schade, G., unpublished manuscript. Gollnick, K., Schenck, G. O., "1,4-Cycloaddition Reactions: the DielsAlder Reaction in Heterocyclic Syntheses," J. Hamer, ed., Chap. X, p. 255, Academic Press, New York, 1967. Gollnick, K., Schenck, G. O., Pure Appl. Chem. 9, 507 (1964). Gollnick, K., Schroeter, S., Ohloff, G., Schade, G., Schenck, G. O., Liebigs Ann. Chem. 687, 14 (1965). Hammond, G. S., Leermakers, P. A., J. Am. Chem. Soc. 84, 207 (1962).

(25) Higgins, R., Foote, C. S., Cheng, H., ADVAN. CHEM. SER. 77, 102

(1968).


100

(26) (27) (28) (29) (30) (31) (32) (33) (34)


(35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45) (46) (47) (48) (49) (50) (51) (52) (53) (54) (55) (56) (57) (58) (59) (60) (61) (62) (63) (64) (65) (66) (67) (68)

OXIDATION OF ORGANIC COMPOUNDS III

Hoijtink, G. J., Mol. Phys. 3, 67 (1960). Kautsky, H., Biochem. Z. 291, 271 (1937). Kautsky, H., deBruijn, H., Naturwissenschaften 19, 1403 (1931). Kautsky, H., deBruijn, H., Neuwirth, R., Baumeister, W., Ber. deut. Chem. Ges. 66, 1588 (1933). Kawaoka, K., Khan, A. U., Kearns, D. R., J. Chem. Phys. 46, 1842 (1967). Kenney, R. L., Fisher, G. S., J. Am. Chem. Soc. 81, 4288 (1959). Khan, A. U., Kasha, M., Ibid., 88, 1574 (1966). Khan, A. U., Kasha, M., J. Chem. Phys. 39, 2105 (1963). Klyne, W., "Progress in Stereochemistry," Vol. I, p. 81, Butterworths, London, 1954. Koblitz, W., Schumacher, H. J., Z. Physik. Chem. B35, 11 (1937). Köller, H., Diplomarbeit, University of Göttingen, 1958. Kopecky, K. R., Reich, H. J., Can. J. Chem. 43, 2265 (1965). Lamola, A. A., Herkstroeter, W. G., Dalton, J. C., Hammond, G. S., J. Chem. Phys. 42, 1715 (1965). Livingston, R., "Photochemical Autoxidation," in "Autoxidation and Anti oxidants," W. O. Lundberg, ed., Vol. I, p. 249, Interscience, Wiley, New York, London, Sidney, 1961. Livingston, R., Owens, K. E., J. Am. Chem. Soc. 78, 3301 (1956). Livingston, R., Rao, V. S., J. Phys. Chem. 63, 794 (1959). McKeown, E., Waters, W. A., J. Chem. Soc. London B 1966, 1040. Mertz, C., Diplomarbeit, University of Göttingen, 1958. Murrell, J. N., Mol. Phys. 3, 319 (1960). Nickon, A., Bagli, F., J. Am. Chem. Soc. 83, 1498 (1961). Nickon, A., Mendelson, W. L., Can. J. Chem. 43, 1419 (1965). Ohloff, G., Klein, E., Schenck, G. O., Angew. Chem. 73, 578 (1961). Porter, G., Wright, M. R., Discussions Faraday Soc. 27, 18 (1959). Roberts, J. D., Caserio, M. C., "Basic Principles of Organic Chemistry," Chaps. 7-5, 7-6, W. A. Benjamin, New York, N. Y., 1965. Schenck, G. O., Angew. Chem. 64, 12 (1952). Schenck, G. O., German Pat. 933925 (Dec. 24, 1943). Schenck, G. O., Naturwissenschaften 35, 28 (1948). Schenck, G. O., Becker, H. D., Schulte-Elte, K. H., Krauch, C. H., Chem. Ber. 96, 509 (1963). Schenck, G. O., Eggert, H., Denk, W., Liebigs Ann. Chem. 584, 177 (1953). Schenck, G. O., Eisfeld, W., unpublished manuscript. Schenck, G. O., Gollnick, K., Forschungsber. Land Nordrhein-Westfalen, No. 1256, Westdeutscher Verlag, Koln, Opladen, 1963. Schenck, G. O., Gollnick, K., Buchwald, G., Schroeter, S, Ohloff, G., Liebigs Ann. Chem. 674, 93 (1964). Schenck, G. O., Gollnick, K., Neumiiller, O. A., Ibid., 603, 46 (1957). Schenck, G. O., Koller, H., unpublished manuscript. Schenck, G. O., Mertz, C., unpublished manuscript. Schenck, G. O., Neumiiller, O. A., Ohloff, G., Schroeter, S., Liebigs Ann. Chem. 687, 26 (1965). Schenck, G. O., Schroeter, S., Ohloff, G., Chem. Ind. (London) 1962, 459. Schenck, G. O., Schulte-Elte, K. H., unpublished manuscript. Schonberg, A., Liebigs Ann. Chem. 518, 299 (1935). Schuller, W. H., Lawrence, R. V., /. Am. Chem. Soc. 83, 2563 (1961). Schulte-Elte, K. H., Dissertation, University of Gottingen, 1961. Sharp, D. B., "Abstracts of Papers," 138th Meeting, ACS, Sept. 1960, p. 79P. Singh, I. S., Becker, R. S., J. Am. Chem. Soc. 82, 2083 (1960).


67.

GOLLNICK

Type

II

Oxygenations

(69) Tsubomura, H., Mulliken, R. S., J. Am. Chem. Soc. 82, 5966 (1960). (70) Wilson, T., J. Am. Chem. Soc. 88, 2898 (1966). (71) Winer, A. M., Bayes, K. B., J. Phys. Chem. 70, 302 (1966). December 5, 1967.


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