Amination of ketene: A theoretical study

The reactions of NH₃, (NH₃)₂, and (H₃N·H₂O) with ketene have been studied by ab initio calculations. Attack by the dimer (NH₃)₂ or by (H₃N·H₂O) through six-membered cyclic transition states is found to be favored, with a preference of 12.8 kcal/mol for initial addition of (NH₃)₂ to the C=O bond of the ketene giving the enol amide, as compared to initial addition to the C=C bond to give the more stable amide directly. These results ate in contrast to previous theoretical studies for the reaction of monomeric NH₃ with ketene, in which direct addition to the C=C bond was reported to be favored, and for the reaction of ketene with (H₂O)₂, in which addition to the C=O bond is calculated to be only 1.9 kcal/mol more favorable than addition to the C=C bond. The barrier via 10(ts) for the NH₃- catalyzed rearrangement of the enol amide/NH₃ complex 6 to the amide is 16.5 kcal/mol above that of 6, consistent with the experimental observation of an intermediate in ketene amination, and the barrier for reversion of 6 to the reactant complex 5a is 4.0 kcal/mol lower than that for amide formation via 10(ts), suggesting the initial amination may be reversible. These results demonstrate that previous theoretical calculations of ketene amination used inadequate levels of theory. The structure of the zwitterion CH₂=C(O^- )NMe₃⁺ (14) from reaction of ketene with NMe₃ has also been calculated, and in the gas phase this reaction is calculated to be exothermic by only 0.6 kcal/mol.