Hydrogen shifts in cyclohexylcarbenes. Spatial dependence of activating power and of primary deuterium isotope effects.

The conformationally biased ketones 4-t-butyl-cis-2-methylcyclohexanone (Ic) and 4-t-butyl-cis-2-trans-6-dimethylcyclohexanone (7a; and its 2,6-dideuterio derivative 7c) were converted into p-toluenesulfonylhydrazone 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 that a bystander Me-eq substituent promotes a geminal H shift several times more efficiently than does a bystander Me-ax. This geometry-dependent activating power parallels behavior noted earlier for OMe and Ph bystander groups; but as Me groups are rotationally symmetric and possess no lone pair or pi electrons this phenomenon cannot be attributed solely to rotameric considerations or to effects involving mobile electron clouds. For the trans-dimethylcarbenes 10a and 10c the primary deuterium isotope effect (k(H)/k(D)) for axial migration (I-ax) was determined 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 Elsevier Science Ltd.