A C T A U N I V E R S I T A T I S L O D Z I E N S I S FOLIA C H I MI CA 10, 1993 W o j c i e c h J. Kin ar t* S I N G L E T O X Y G E N O X I D A T I O N OF ( E ) - 2 , 5 - D I M E T H Y L - 3 - H E X E N E A solution of (E)-2,5-dimethyl-3-hexene (1_) in d i c h l o r o m e t h a n e c o n t a i n i n g t e t r a p h e n y l p o r p h i n e as a p h o t o s e n s i t i z e r was illu m in ated , u nd er an o x y ge n a t m o s ph e re , with a s o d i u m lamp, and 3 - h y d r o p e r o x y - 2 ,5 - d i m e t h y l - 4 - h e x e n e (£) was o b t a i ne d . It p a r t i a l l y r e a r r a n g e d in CDCl^, to give a m i x t u r e of h y d r o p e r o x i d e (2) and ( E ) - 2 - h y d r o p e r o x y - 2 , 5 - d i m e t h y l - 3 - h e x e n e (3), in the ratio 2 : 3.
U n s a t u r a t e d c o m p o u n d s react with the s i n gl et o x y g en to give some a l l y l - t y p e h y d r o p e r o x i d e s [ l ] . In n o n - p o l a r so l ve nt s , these h y d r o p e r o x i d e s may then u nd e r g o the a l l yl ic r e a r r a n g e m e n t to g iv e the is o me ri c h y d r o p e r o x i d e s [2] (Sc heme 1) with s t r u c t u r e s i m i l a r to p r o d u c t s of the re a ct io n of u n s a t u r a t e d c o m p o u n d s w it h the tri plet oxygen.
I N T R O D U C T I O N
Sch em e 1
0
2 D .
Si nce only a l i m i t e d n u m be r of e x a m p l e s of such the r e a r r a n g e m e n t has as yet been r e p o r t e d [2] and c a . one half of them is r e l a t e d to the c o n g e n e r s of c h o l e s t e r o l , I have been a n x io us to give f u r th er few e x a m p l e s of this r e a r r a n g e m e n t , w h i c h m ig ht h av e been h el pful for some fut ure m e c h a n i s t i c i n v e s t i g a t i o n s , p a r t i c u l a r y in a c y c l i s sys tems , since it a pp e a r s that in r e l at ed r e a r r a n g e m e n t s of (3-acyloxyalkyl rad ical s, d i f f e r e n t m e c h a n i s m may apply d e p e n d i n g on w h e t h e r the alkyl m o i e ty is c y c l ic or acyclic.
These r e a c t i o n s are, g e n e r a l l y fai rl y s e n s i t i v e to i n h i b i t i o n by p he nols , and to a c c e l e r a t i o n by light and by ind uc ed free rad ical , and they are a c c e p t e d to fol lo w a ra d ical chain m e c h a n i s m i n v o l v i n g the i n t e r m e d i a t e a l l y l p e r o x y l ra d ic al s [3], as it is s h o wn in S ch eme 2.
S ch em e 2
00' '00
I n v e s t i g a t i o n s on the a u t o x y d a t i o n of a cy c l i c olefins, u n l i k e to those d e a l i n g with c y c l ic olefins, have been i mp eded by the u n a v a i l a b i l i t y of the pure h y d r o p e r o x i d e p r o d u c t s and, c o n s e q u e n t l y , by the lack of a r e l i a b l e i n f o r m a t i o n on their r e a c t i v i t y [2]. The aim of the p r e s e n t study was to o b t a i n a few new h y d r o p e r o x i d e s from the o x i d a t i o n of ( E ) - 2 , 5 - d i m e t h y l - 3 - -hexene, and to s tu dy the p o s s i b l e m e c h a n i s m of their r e a r r a n g e ment .
E X P E R I M E N T A L
N.m.r. s p e ct ra we re r e c o r d e d on a V a r i an XL 200 i n s t r um e nt , u s i n g C DC lj as a s o l v e n t and Me^Si as an i nt e r n a l sta ndar d. (E ) - 2 ,5 - D i m e t h y 1 - 3 - h e x e n e was a c o m m e r c i a l s am ple ( Al d ric h); and it was c h e c k e d for its p u r it y by the n.m.r. s pe c t r o m e t r y .
O xy g en was b u b b l e d for 6 h t h r o u g h a s o l u t i o n of the alk ene (1 g) and t e t r a p h e n y l p o r p h i n e (0.03 g) in d i c h l o r o m e t h a n e (400 c m 3 ) in a w a t e r - c o o l e d flask, w h i c h was i l l u m i n a t e d a 400 W
s o d iu m lamp. The s ol v e n t was r e m ov ed u nd er r e d u c e d pr e ss ur e , and the p ro d u c t was p u r i f i e d by a s ta tic d i s t i l a t i o n b e l o w 30°C, fo l lo we d by c h r o m a t o g r a p h y on sil ic a gel (ethyl a c e t a t e - l i g h t p e t r o l e u m (1 : 10 v/v) was us ed as eluant).
R E S UL TS AND D I S C U S S I O N
A s ol u t i o n of ( E ) - 2 , 5 - d i m e t h y l - 3 - h e x e n e ( i ) in d i c h l o r o m e t h a n e c o n t a i n i n g t e t r a p h e n y l p o r p h i n e as p h o t o s e n s i t i z e r was i r r a d i a t e d und er an o x y ge n a t m o s p h e r e by a s o d i um lamp. The h y d r o p e r o x i d e s 3 - h y d r o p e r o x y - 2 ,5 - d i m e t h y 1 - 4 - h e x e n e (2.) and ( E ) - 2 - h y d r o p e r o x y - - 2 ,5 - d i m e t h y 1 - 3 - h e x e n e O ) w h i ch were for med and the c o r r e s p o n d ing a lc o h o l s 3 - h y d r o x y - 2 , 5 - d i m e t h y l - 4 - h e x e n e U ), and (E )-2- - h y d r o x y - 2,5 - d i m e t h y 1 - 3 - h e x e n e (2) o b t a i n e d by the s u b s e q u e n t r e d u c t i o n w it h t r i p h e n y l p h o s p h i n e ( Sc heme 3) we re i d e n t i f i e d by the ^H.n.m.r. s pe c tr o s c o p y .
Sch em e 3
D e t a il s of the s p e ct ra are g iv en b el ow ( c h e m ic a l shi ft s in p .p . m . ): 8 NMR (CDCl-j) (2): 0.934 , 0.968 (6 H, 2 d), 1.62 (1 H, m), 1.734, 1.809 (6 H, 2 d), 4.50 (1 H, dd), 5.40 (1 H, m). S NMR (CDClj) (2): 1-01 (6 H, d), 1.33 (6 H, s), 2.08 (1 H, m), 5.60 (2 H, m). S ( C D C 1 3 ) (4): 0.79, 0.95 (6 H, 2 d), 1.47 (1 H, m), 1.65, 1.70 (6 H, 2 d), 3.86 (1 H, dd), 5.09 (1 H, dm). S ( C D C 1 3 ) (5): 0.98 (6 H, d), 1.26 (6 H, s), 1.91 (1 H, m), 5,53 (2 H, m ) .
C he m ic a l s h i f t s for a l c o h o l s (4.) and (¿) are in a g r e e m e n t with the val ue s r e p o r t e d e a r l i e r [5].
The p h o t o s e n s i t i z e d o x i d a t i o n of (E)-2 , 5 - d i m e t h y 1 - 3 - h e x e n e (_1) has pr o du ce d , alm os t e x c l u s i v e l y , the m o n o h y d r o p e r o x i d e (2), w h i c h was found to be r e a di ly is o la te d and p u r i f i e d by a sim ple c h r o m a t o g r a p h y on s i l ic a gel. The c o m p o u n d (2_) in CDCl-^, r e a r r a n ge d s p o n t a n e o u s l y to give a m i x t u r e of (,2) and (3.) in the r a tio 2 : 3. The r e a c t i o n was, a p p r o x i m a t e l y , of the first order. The m e c h a n i s m of this r e a c t i o n is not known, but in the case of h y d r o p e r o x i d e s d e r i v e d from o le ic acid and c h o l e s t e r o l [6] it
1 R
has been s h o w n . t h a t they do not i n c o r p o r a t e 0 into the p ro duct 1 fl
from an a t m o s p h e r e c o n t a i n i n g 0 2 , and it is b e l i v e d to p r o c e e d by a n o n d i s s o c i a t i v e , p e r h a p s the p e r i c y c l i c m e c h a n i s m w h i ch is a s s u m e d to be a f f e c t e d by s t e ri c eff ects . Again, the e q u l i b r i u m c o m p o s i t i o n of the m i x t u r e of h y d r o p e r o x i d e s w o u ld be det e rm i ne d, p r i n c i p a l l y , by ste ri c effects.
A c k n o w l e d g m e n t
This work was p e r f o r m e d at the C h e m i s t r y D ep a r t m e n t , U n i v e r sity C ol lege , London, and was g e n e r o s l y s u p p o r t e d by an award from the East E u r o p e a n L e v e r h u l m e Fol l ow s hi p. The e n c o u r a g e m e n t and the h e l p f u l a d v i ce of P r o f e s s o r s A lw yn G. Dav ie s and R. S k o w r o ń s k i are g r a t e f u l l y a ck n o w l e d g e d .
R E F E R E N C E S
[1] G. 0. S c h e n c k , 0. A . N e u m u 1 1 e r, W. E i s f e 1 d, Chem. Ber., 618, 202 (1958).
[2] A. A. F r i m e r, Chem. Rev., 22» 5 ^9 (1979).
[3] W. F. B r i 1 1, J. Chem. Soc., P e r k i n Trans., 2, 621 (1984). [4] A . G . D a v i e s , W . 3. K i n a r t, 3. Chem. Res., 22 (1989). [5] S . R a u c h e r , G . A . K 0 0 1 p e , J . Org. Chem., £3, 4252
(1978).
.6] A. L. 3. B e c k w i t h , P. J . D u g g a n , 3. Chem. Soc. Chem. C om mun. , 1000 (1988).
W o j c i e c h J. K i n a rt
U T L E N I A N I E ( E ) - 2 , 5 - O I M E T Y L 0 - 3 - H E K S E N U T L E N EM S I N G L E T O W Y M W w y n ik u f o t o c h e m i c z n i e i n i c j o w a n e g o p r o c e s u u t l e n i a n i a (E)- -2 , 5 - d i m e t y l o - 3 - h e k s e n u o t r z y m a n o 3 - h y d r o p e r o k s y- 2, 5 -d i me ty l o- 4- -heksen. Na p o d s t a w i e b ad ań 1H NMR s t w i e r d z o n o , że ten nowo otrzy many w o d o r o n a d t l e n e k u le ga c z ę ś c i o w e m u s a m o r z u t n e m u p r z e g r u p o w a niu w CDCl j do ( E ) - 2 - h y d r o p e r o k s y - 2 , 5 - d i m e t y l o - 3 - h e k s e n u .
