The decomposition of diazo ketones in the presence of Rh(II) and Cu(I) catalysts affords products of C-H bond insertion in high yields. The effect of structural variation on intramolecular and transannular C-H insertions of diazo ketones has been investigated. The enantioselectivity of the insertion was examined with 15 chiral catalysts of different structural types. It was low in all cases. The poor enantioselectivity of the insertion of diazo ketones in comparison to that obtained in insertions of diazo esters and diazo amides is attributed to two factors: The oxocarbenes derived from diazo esters and diazo amides are stabilized by resonance of the carbonyl group with the heteroatom. Furthermore, the conformational constraints which must be overcome in order to reach the transition state for intramolecular insertion are lower in the case of carbenes derived from diazo ketones than in those from diazo esters and amides owing to the higher rotational barriers of amides and esters in comparison with that of ketones. This results in an earlier, and therefore less selective transition state for insertion of diazo ketones.