Journal of the American Chemical Society, Vol.121, No.47, 10924-10927, 1999
Cobaloxime pi-cation steric and stereoelectronic effects: The amazing effect of a single methyl group adjacent to the site of reaction
Typically, beta-hydroxylalkylcobaloximes are very sensitive to acid, readily forming transient cobaloxime pi-cations which can either be trapped with nucleophiles or undergo irreversible alkene loss. Surprisingly, 2,3-dihydroxy-3,7-dimethyl-6-octenyl cobaloxime (1) was found to be stable to acid treatment, including that by the strong acid camphorsulfonic acid. A comparative study of the acid stability of 2,3-dihydroxypropyl cobaloxime (8), 2,3-dihydroxylbutyl cobaloxime (9), and 2,3-dihydroxy-3-methylbutyl cobaloxime (10) was done, showing that 8 and 9 exhibit normal acid sensitivity whereas 10 is stable to acid. The inclusion of an additional methyl group in going from 9 to 10 completely turns off an otherwise very facile reaction, presumably due to severe steric effects of the py(dmgH)(2)Co moiety of the cobaloxime which forces the acyclic chain of 10 to adopt a conformation which is stereoelectronically nonproductive for cobaloxime pi-cation formation.