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

Abstract

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.