HYDROGEN SHIFTS IN CYCLOHEXYLCARBENES - SPATIAL DEPENDENCE OF ACTIVATING POWER AND OF PRIMARY DEUTERIUM-ISOTOPE EFFECTS

The conformationally biased ketones 4-t-butyl-cis-2-methylcyclohexanon e (Ic) and 4-t-butyl-cis-2-trans-6-dimethylcyclohexanone (7a; and its 2,6-dideuterio derivative 7c) were converted into p-toluenesulfonylhyd razone Li salts. Thermolysis or photolysis generated putative singlet carbenes, which underwent competitive axial vs equatorial H shift (or D Shift in the case of 7c) to give alkenes. Product analysis showed th at a bystander Me-eq substituent promotes a geminal H shift several ti mes more efficiently than does a bystander Me-ax. This geometry-depend ent activating power parallels behavior noted earlier for OMe and Ph b ystander groups; but as Me groups are rotationally symmetric and posse ss no lone pair or pi electrons this phenomenon cannot be attributed s olely to rotameric considerations or to effects involving mobile elect ron clouds. For the trans-dimethylcarbenes 10a and 10c the primary deu terium isotope effect (k(H)/k(D)) for axial migration (I-ax) was deter mined to be ca. 1.5 times larger than that for equatorial migration (I -eq). This finding invalidates the common assumption that I-ax = I-eq and suggests that published data on deuterium isotope effects and on H -ax/H-eq migration selectivities need to be adjusted. (C) 1997 Elsevie r Science Ltd.