Secondary α-hydrogen kinetic isotope effects in nucleophilic additions to ferrocenyl-stabilised carbocations

Abstract

The secondary α-hydrogen kinetic isotope effects upon the solvolyses of Fc₂C⁺H (1a) and p-MeCO₆-H₄C⁺HFc (1b)(Fc = ferrocenyl) in H₂O, H₂O–MeCN, and H₂O–MeOH (k_f^H/k_f^D=ca. 0·9), and upon the acid heterolyses of the corresponding alcohols (k_r^H/k_r^D=ca. 1·1), suggest that there is appreciable C ⋯ O bond formation in the transition state for carbocation solvolysis; for additions of HO^–, N₃^–, and BH₄^– to (1a), k_f^H/k_t^D=ca. 1·0, indicating ‘early’ carbocation-like transtition states for these reactions, whereas intermediate values (k_f^H/k_f^D=ca. 0·96) are found for additions of primary amines.