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

Article abstract of DOI:10.1039/P29920001259

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 deg 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 theexocyclic CH group: ΔpK²_a1a = 1.68; ΔpK³_a1a = 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.

Full text of DOI:10.1039/P29920001259

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