Amination of Ketenes: Kinetic and Mechanistic Studies

The rate constants for reaction of PhMe(2)SiCH=C=O (6) with amines to form amides in CH(3)CN are best fitted with a mixed second- and third-order dependence on [amine], in stark contrast to previous studies of Ph(2)C=C=O and other reactive ketenes in which only a first-order dependence on [amine] was observed in H(2)O or in CH(3)CN. Derived third-order rate constants for 6 depend on the amine basicity, with a 1.7 x 10(7) greater reactivity for n-BuNH(2) compared to CF(3)CH(2)NH(2). These kinetic results are consistent with recently reported theoretical studies for reaction of CH(2)=C=O with NH(3). For 6 the relative reactivity k(n-BuNH(2))/k(H(2)O) is estimated to be 10(13) in CH(3)CN. The crowded ketene t-Bu(2)C=C=O (10) is enormously deactivated toward amination and reacts in neat n-BuNH(2) with rates 10(12) and 2 x 10(5) times slower than those for t-BuCH=C=O and t-BuC(i-Pr)=C=O (11), respectively. The observed rate constants for 11 also show a higher than first-order dependence on [n-BuNH(2)]. The absence of higher order kinetic terms in [amine] for more reactive ketenes is attributed to irreversibility of addition of an initial amine to the ketene, while with more stable ketenes the initial step is reversible and later steps involving additional amine molecules are kinetically significant. The general acid CF(3)CH(2)NH(3)(+) catalyses the addition of CF(3)CH(2)NH(2) to 6 in a process independent of [CF(3)CH(2)NH(2)]. The reactivity of 6 with n-BuNH(2) is 370 times greater in CH(3)CN compared to isooctane, a result attributed to the polar nature of the transition state and possible catalysis of the addition by CH(3)CN.