tert-Butylimino-tris(dimethylamino)phosphorane as a Proton Acceptor in the System of Reaction of 1-Nitro-1-(4-nitrophenyl)alkanes in THF

摘要

Unexpectedly the reaction of deprotonation of carbon acids with phosphazene bases occurred very slowly.The kinetic study of the proton transfer reactions between C-acids of the series of nitroalkanes with increasing bulk of R = H,Me,Et,i-Pr substituents as 4-nitrophenylnitromethane (1),l-nitro-l-(4-nitrophenyl)ethane (2),l-nitro-l-(4-nitrophenyl)propane (3),2-methyl-l-nitro-l-(4-nitrophenyl)propane (4) and the tert-butylimino-tris(dimethylarnino)phosphorane (5) named Pi-t-Bu phosphazene is elaborated.The reactions have been studied in THF under pseudo-first-order conditions.The product of the proton transfer reaction with P1-t-Bu in THF appeared to be associated into ion pairs.The equilibrium constants range from > 100000 to 11.8 decreasing along with growing bulk of alkyl substituent in the reacting C-acid.The second order rate constants (k2H) are rapidly declining: 9360,2.31,0.66,0.09 dm3 mol-1 s_1 for 1,2,3,and 4 respectively,and could not be accounted for the small values of the enthalpies of activation DELTA H_H~(not=) =6.1,18.0,20.7 and 11.1 kJ mol-1 .The reactions show negative and relatively large values of the entropies of activation DELTA S_H~(not=) =-149.7,-176.5,-178.7,-227.8 J mol-1 deg-1.The primary deuterium kinetic isotope effects are large showing tendency of reverse relation towards steric hindrance of the reacting C-acids,k_H/k_D = 15.8,13.6,13.2 for 1,2,and 3,respectively.The results have been discussed in terms of the influence of steric effects brought by the bulk of alkyl substituents in the C-acid and the base on formation of the transition state.Also the influence of traces of residual water present in the reaction system has been taken into consideration.