Substitution at saturated carbon. Part 25. The iododemetallation of benzyltrialkyltins in methanol

Abstract

The a c t i o n of I z / I -in methanol o n compounds PhCH2SnRj l e a d s o n l y t o c l e a v a g e o f t h e b e n z y l group (as PhCH2I) when R=Et, P r n o r Bun. WithPhCH2SnMe3, b o t h b e n z y l and methyl groups are removed. R a t e C o n s t a n t s a r e r e p o r t e d f o r t h e i o d o d e m e t a l l a t i o n o f a l l f o u r PhCH2SnR3 compounds, R=Me, E t , P r n and Bun.The ? r e s e n t r e s u l t s and p r e v i o u s l i t e r a t u r e d a t a show t h a t w i t h a c o n s t a n t SnR3 l e a v i n g group, t h e sequence o f a l k y l -t i n r e a c t i v i t y i s Ph>PhCH2>Me>Et>Prn, w i t h t h e b e n z y l -t i n bond b e i n g broken 3-4 t i m e s as r a p i d l y as t h e m e t h y l -t i n bond and a b o u t 4 0 t i m e s as r a p i d l y as t h e e t h y l -t i n bond.The e f f e c t o f added a n i o n s , C 2 , B r -, I-and CEO4-on t h e r a t e c o n s t a n t s (and p r o d u c t s ) a r e r e p o r t e d , and it i s shown t h a t b o t h k i n e t i c s t u d i e s and p r o d u c t a n a l y s e s l e a d to t h e c o n c l u s i o n t h a t f o r e i g n a n i o n s d o n o t p a r t i c i p a t e i n t h e r e a c t i o n mechanism.These a r e , however, s p e c i f i c k i n e t i c s a l t e f f e c t s i n t h e o r d e r NaBr<NaC2<NaCL04<NaI. INTRODUCTION L a r g e l y due t o t h e work o f G i e l e n and N a~i e l s k i , l -~ r a t e c o n s t a n t s are 3-4 now known f o r t h e c l e a v a g e of many o f t h e s i m p l e a l k y l g r o u p s from t i n by t h e e l e c t r o p h i l e s i o d i n e and bromine, e s p e c i a l l y i n p o l a r s o l v e n t s .The b e n z y l group, however, h a s n o t been so w e l l s t u d i e d .towards t h e e l e c t r o p h i l e HgCfi2 i n m e t h a n o l , t h e b e n z y l group i s n o t e s p e c ia l l y r e a c t i v e , t h e s e q u e n c e (1) b e i n g o b t a i n e d f o r t h e c l e a v a g e o f R-Sn bonds r e l a t i v e t o e t h y l t a k e n a s u n i t y . So f a r , i t h a s proved d i f f i c u l t t o o b t a i n an estimate o f t h e r e l a t i v e r e a c t i v i t y o f t h e b e n z y l -t i n group t o w a r d s t h e I t i s known5 t h a t R = P h ( 1 . 4 x 1 0~)~M e ( 4 3 0 )~P h C H~( l l )~E t ( l )~P r~( O . l 9 )~B u~( O . l 7 )halogens.i n D M s O ,~ and B r 2 / B r -i n DMSO,* o n l y t h e methyl group w a s s p l i t o f f from PhCH2SnMe3.was p r e f e r e n t i a l l y removed,' b u t i n t h i s n o n p o l a r s o l v e n t t h e k i n e t i c form i s t h i r d -o r d e r r a t h e r t h a n t h e second-order form o b s e r v e d i n p o l a r s o l v e n t s , and i n any case no comparison o f r a t e c o n s t a n t s f o r c l e a v a g e o f b e n z y l and a l k y l groups w a s made.' S i n c e w e had a v a i l a b l e 5 a series o f b e n z y l t i n compounds, PhCH2SnR3 where R = M e , E t , P r n and Bun, w e s e t o u t t o d e t e r m i n e t h e r e a c t iv i t y o f t h e b e n z y l -t i n group towards t h e e l e c t r o p h i l i c s y s t e m 12/1-i...