Direct Competition of Intramolecular C±H Insertions into Benzylic C±H and 4-Substituted Benzylic C±H by Rhodium(II)-mediated Carbenoids

Intramolecular C±H insertion by Rh-mediated carbenoids has attracted considerable interest in recent years. In addition to its synthetic applications, there have been extensive investigations into the mechanism of this reaction. We have recently studied the electronic e€ects of this reaction through the measurement of the relative reactivities of Rh-mediated carbenoid insertion into a series of substituted benzylic C±H versus benzylic C±H by an indirect method, in which the insertion into an intramolecular secondary aliphatic C±H served as an internal standard. The relative reactivities thus obtained were found to give a good Hammett linear correlation with substituent constants. Since steric and conformational factors can also in̄uence the C±H insertion selectivity, it is of interest to see if direct intramolecular competition gives the same results. We report in this paper the direct intramolecular competition between 4-nitro and 4-methoxy substituted benzylic C±H and its non-substituted counterpart (Scheme 2).