Use of the Semidione Spin Probe to Study Molecular Rearrangements

Jun 1, 1978 - Abstract. Bicyclic—monocyclic valence isomerization of a radical dianion in the bicyclo[3.1.0]hexanesemidione system has been demonstr...
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23 Use of the Semidione Spin Probe to Study Molecular Rearrangements (1) GLEN A. R U S S E L L , K. S C H M I D T , C . T A N G E R , Ε . G O E T T E R T , M . Y A M A S H I T A , Y. K O S U G I , J. S I D D E N S , and G . S E N A T O R E

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D e p a r t m e n t of C h e m i s t r y , I o w a State U n i v e r s i t y , A m e s , IA 50011

R a d i c a l i o n s d e r i v e d f o r m a l l y b y t h e one e l e c t r o n r e d u c t i o n o f α - d i o n e s are termed 1,2-semidiones.

S p i n d e n s i t y i s about e q u a l l y d i s t r i b u t e d over the f o u r atoms i n the π-system, an e s t i m a t e o f the c a r b o n y l c a r b o n s p i n d e n s i t y b e i n g 0.35 f o r c i s - d i m e t h y l s e m i ­ d i o n e a n d 0.28 f o r c y c l o h e x a n e - l , 2 - s e m i d i o n e (2). This l e a d s t o c o n s i d e r a b l e d o u b l e bond c h a r a c t e r between the c a r b o n y l c a r b o n atoms o f a c i s - s e m i d i o n e i n the r a n g e o f 30%. Use o f the v a l e n c e b o n d s t r u c t u r e 1 e m p h a s i z e s

t h i s double bond c h a r a c t e r and c l a s s i f i e s the 1,2-semid i o n e as an o l e f i n d e r i v a t i v e . This paramagnetic o l e f i n d e r i v a t i v e c a n be o b s e r v e d b y e s r s p e c t r o s c o p y d u r i n g m o l e c u l a r r e o r g a n i z a t i o n s w h i c h may i n t i m a t e l y i n v o l v e the c a r b o n y l c a r b o n atoms. S i n c e the p a r a ­ m a g n e t i c c e n t e r may be i n v o l v e d d i r e c t l y i n t h e r e a c ­ t i o n the semidione group i s n o t r e a l l y a s p i n l a b e l f o r a d o u b l e b o n d , when t h e c a r b o n y l c a r b o n a t o m s a r e d i r e c t l y i n v o l v e d i n the rearrangement reaction. Semidiones e x i s t i n e q u i l i b r i u m w i t h the α - d i o n e a n d t h e e n e d i o l d i a n i o n , e q u a t i o n 2. Low t e m p e r a t u r e s and i o n - p a i r i n g f a v o r the d i a m a g n e t i c components o f

©0-8412-0421-7/78/47-069-376$10.00/0

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

23.

RUSSELL ET AL.

Semidione

Spin

Probe

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2 RC(0.)=C(0")R t RCOCOR + R C ( 0 " ) = C ( 0 " ) R

(2)

e q u a t i o n 2. A c y c l i c semidiones e x i s t as a thermody­ n a m i c m i x t u r e o f t h e c i s a n d t r a n s - i s o m e r s shown i n e q u a t i o n 1, a t l e a s t i n a s t a t i c s y s t e m o f DMSO p l u s a l k a l i metal cation. The e q u i l i b r i u m i s d r a m a t i c a l l y a f f e c t e d b y i o n i c a s s o c i a t i o n ; f o r example w i t h l i t h i u m a s t h e g e g e n i o n o n l y t h e c i s ( l i t h i u m chelated)« i o n p a i r c a n b e o b s e r v e d i n DMSO w h e r e a s w i t h c e s i u m as t h e g e g e n i o n o n l y t h e t r a n s ( f r e e i o n ) - s e m i d i o n e i s o b s e r v e d (3). With sodium o r p o t a s s i u m m i x t u r e s of the c i s and t r a n s - s p e c i e s a r e observed. Pertur­ b a t i o n o f e q u i l i b r i u m (2), f o r e x a m p l e b y UV i r r a d i a ­ t i o n does n o t a f f e c t t h e o b s e r v e d c i s / t r a n s - r a t i o . T h i s i s n o t because t h e c i s and t r a n s - s e m i d i o n e a r e r e a d i l y i n t e r c o n v e r t e d b y r o t a t i o n about t h e p a r t i a l double bond. Instead the further equilibrium of Scheme 1 c a n s e r v e t o e s t a b l i s h a t h e r m o d y n a m i c r a t i o of t h e two i s o m e r s . Scheme Ι­ Ο" I

R-C-C-R + R-C=C-R

88

%

-h

oR-C=C-R + R-C=C-R

R-C-k-R + R-C=C-R

»

1

1

0 -0 0The s e m i d i o n e g r o u p more n e a r l y f u l f i l l s t h e d e f i n i t i o n o f a s p i n l a b e l when t h e c o n f o r m a t i o n a l e q u i l i b r i a o f the cycloalkane-1^2-semidiones a r e con­ sidered. Thus y b y e s r s p e c t r o s c o p y t h e ΔΗ* f o r r i n g f l i p o f t h i s cyclohexene d e r i v a t i v e has been measured as 4 k c a l / m o l e w h i c h c o r r e s p o n d s t o a m a g n e t i c c o ­ a l e s c e n c e t e m p e r a t u r e o f -85° (4). This value can be c o m p a r e d w i t h a A F * o f 5.3 k c a l / m o l e o b s e r v e d f o r

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

ORGANIC F R E E RADICALS

378

r i n g i n v e r s i o n o f c i s - 3 , 3 ^ 4>5,6 6-d -eyelohexene i n b r o m o t r i f l u o r o m e t h a n e b y p.m.r. ( c o a l e s e n c e tempera­ t u r e = - 1 6 5 ° ) (5). I t appears t h a t i n t h i s case the semidione i s a r e a s o n a b l e s p i n l a b e l f o r the o l e f i n . At 4 - t - b u t y l group f r e e z e s t h e c y c l o h e x a n e semi­ dione i n t o c o n f o r m a t i o n 2 up t o a t l e a s t 90°. On the o t h e r hand f o r a 4-methyl s u b s t i t u e n t b o t h 2 and J a r e

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3

6

p o p u l a t e d w i t h 2 b e i n g p r e f e r r e d b y an e n t h a l a p y d i f ­ f e r e n c e o f 1.4 Kcal/mole ([3]/(2] = 0.13 a t 40°) (4). Cycloheptane-lj2-semidTone i s a r i g i d s p e c i e s (τ>10~ sec.) up t o a t l e a s t 90°. From an a n a l y s i s o f the e s r h y p e r f i n e i n t e r a c t i o n s i t c a n be c o n c l u d e d t h a t the s i n g l e p o p u l a t e d c o n f o r m a t i o n i s the s t a g g e r e d con­ formation 4 ( 6 ) . 7

H

a (G)

C

C

equatorial

0.28

2.05

0.54

axial

6.60

0.28