Highly sterically hindered carbon acids: the intrinsic reactivity of 5,5′,5″-trimethyl- and 3,3′,3″,5,5′,5″-hexamethyl-2,2′,2″,4,4′,4″-hexanitrotriphenylmethanes

Abstract

Rate constants (k_p^B,k_^–p^BH) for the reversible deprotonation of 5,5′,5″-trimethyl- and 3,3′,3″,5,5′,5″-hexamethyl-2,2′,2″,4,4′,4″-hexanitrotriphenylmethanes (2 and 3) by primary aliphatic amines, piperidine and morpholine as well as by phenoxide anions and hydroxide anion have been measured in H₂O–Me₂SO (20 : 80) at 25 °C. Comparison of the results obtained with those for 2,2′,2″,4,4′,4″-hexanitrotriphenylmethane (1a) shows that the introduction of methyl groups in positions adjacent to the nitro groups decreases markedly the thermodynamic acidity of the exocyclic CH group: ΔpK_a1a²= 1.68; ΔpK_a1a³= 6.48. It is suggested that these decreases are very likely the reflection of a twisting of the nitro groups out of their attached aromatic planes and therefore of a reduced resonance stabilization of the conjugated carbanions C-2 and C-3. Other important steric effects are operating in the ionization of 2 and 3. These arise from the accumulation of ortho-nitro groups in the triphenylmethane system which makes the approach of the base reagents from the exocyclic carbon of 2 and 3 very difficult. The finding of extremely low intrinsic reactivities for 2 and 3 and the observation of a much greater catalytic efficiency of primary amines than of secondary amines in assisting the proton transfers are the two most striking manifestations of these F-strain effects.