Effect of steric hindrance on the rates and kinetic isotope effects of the reactions of 1-nitro-1-(4-nitrophenyl)alkanes with TBD and MTBD bases in THF

The rates of the reactions of 1-nitro-1-(4-nitrophenyl)alkanes and their deuteriated analogues with two bicyclic guanidines of comparable basicity, 1,5,7-triazabicyclo[4.4.0] dec-5-ene (TBD) and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD), in tetrahydrofuran have been measured. The results are discussed in terms of the effects of steric hindrance in the C-acid and the base on the rates and deuterium kinetic isotope effects (KIEs), The reactions of TBD are 118-287 times faster than reactions of MTBD with the same nitroalkanes, The stabilization of the transition state of the TBD reactions by the N-H...O hydrogen bond is plausible. With the most sterically crowded C-acid, the steric hindrance in the base gives a reduced deuterium KIE, Deuterium KIEs for the reactions of MTBD with various C-acids decreases with the steric hindrance in the C-acid but the reverse is true for TBD reactions, Results of this work disagree with the notion that steric hindrance leads to enhanced kinetic isotope effects.