A MECHANISTIC COMPARISON BETWEEN [2-DIMETHYL-2,TRANS-4,6-OCTATRIENE -A VERY REMOTE SECONDARY H(2] AND [4+2] CYCLOADDITIONS OF TETRACYANOETHYLENE TO 2,7)D ISOTOPE EFFECT/

Cycloaddition of tetracyanoethylene to :1'-d(3)-7-methyl-2,trans-4,6-o ctatriene1,1,1-d(3) (DMOT-d(6)) shows a small inverse steric beta-seco ndary isotope effect for the [4 + 2] path (k(H)/k(D)=0.95+/-0.05), whi ch is consonant with a concerted mechanism. On the other hand a substa ntial remote eta-secondary isotope effect (k(H)/k(D)=0.81+/-0.05) was measured for the [2 + 2] path. This isotope effect, is the most remote reported in the literature, and it is a result of hyperconjugation in the dipolar intermediate which is formed in the rate determining step for the [2 + 2] path. (C) 1998 Elsevier Science Ltd. All rights reser ved.