Amination of ketenes: Kinetic and mechanistic studies

The rate constants for reaction of PhMe2SiCH=C=O (6) with amines to form amides in CH3CN are best fitted with a mixed second- and third-order dependence on [amine], in stark contrast to previous studies of Ph2C=C=O and other reactive ketenes in which only a first-order dependence on [amine] was observed in H2O or in CH3CN. Derived third-order rate constants for 6 depend on the amine basicity, with a 1.7 x 10(7) greater reactivity for n-BuNH2 compared to CF3CH2NH2. These kinetic results are consistent with recently reported theoretical studies for reaction of CH2=C=O with NH3. For 6 the relative reactivity k(n-BuNH2)/k(H2O) is estimated to be 10(13) in CH3CN. The crowded ketene t-Bu2C=C=O (10) is enormously deactivated toward amination and reacts in neat n-BuNH2 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-BuNH2]. 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 CF3CH2NH3+ catalyses the addition of CF3CH2NH2 to 6 in a process independent of [CF3CH2NH2]. The reactivity of 6 with n-BuNH2 is 370 times greater in CH3CN compared to isooctane, a result attributed to the polar nature of the transition state and possible catalysis of the addition by CH3CN.