Electronic and Steric Effects in the Addition of Electrophilic 1,3-Dicarbonylalkyl Radicals to Styrenes

Article abstract of DOI:10.1021/jo00015a037

The addition reactions of 1,3-dicarbonylalkyl radicals to ring-substituted styrenes have been kinetically investigated in MeOH and/or MeCN.It has been observed that the rate effect of ring substituents is nearly identical in the reactions of MeCOCHCOMe<*> (1), MeOCOCHCOMe<*> (2) and MeOCOCHCOOMe<*> (4), the ρ value, in MeOH being -0.96, -1.01 and -1.06, respectively.Since the three radicals are relatively strong oxidants and have similar reduction potentials, an important contribution of the transfer structure RCOCHCOR(1-)*CH2CHAr<*>(1+) to the addition transition state is suggested.It has also be found that in the reactions of 1 and 4 with α-alkyl-substituted styrenes the rate of additions is strongly influenced by the nature of the alkyl group, decreasing in the order: Me > Et > i-Pr > t-Bu.The observed effects are much larger than those reported for the corresponding reactions of the nucleophilic cyclohexyl radical.It is suggested that the α-alkyl substituents exert an effect of steric inhibition of resonance thereby ring delocalization of the charge and/or unpaired electron in the transition state is significantly reduced.Delocalization may be more important in the reactions of 1 and 4 than in those of the cyclohexyl radical since it is possible that the former utilizes a transition state occurring later along the reaction coordinate and/or characterized by a larger extent of charge transfer.