Computational study of the general base catalysed aminolysis of 2-benzoxazolinone

Zwitterionic Ring-Opened Oxyphosphonium Species from the Ph3P-I2 Mediated Reactions of Benzo[d]oxazol-2(3H)-ones with Secondary Amines

Instead of the expected substituted 2-aminobenzo[d]oxazoles, relatively stable ring-opened oxyphosphonium betaines were isolated for the first time from the Ph₃P-I₂-mediated reactions of benzo[d]oxazol-2(3H)-ones with acyclic secondary amines. The structure of one of these compounds was unambiguously confirmed by single-crystal X-ray analysis. Thermolysis of the betaines gave rise to 2-dialkylaminobenzoxazoles with concomitant loss of triphenylphosphine oxide, suggesting their possible role as intermediates in an alternative reaction path.

Concerted amidation of activated esters: reaction path and origins of selectivity in the kinetic resolution of cyclic amines via N-heterocyclic carbenes and hydroxamic acid cocatalyzed acyl transfer

The N-heterocyclic carbene and hydroxamic acid cocatalyzed kinetic resolution of cyclic amines generates enantioenriched amines and amides with selectivity factors up to 127. In this report, a quantum mechanical study of the reaction mechanism indicates that the selectivity-determining aminolysis step occurs via a novel concerted pathway in which the hydroxamic acid plays a key role in directing proton transfer from the incoming amine. This modality was found to be general in amide bond formation from a number of activated esters including those generated from HOBt and HOAt, reagents that are broadly used in peptide coupling. For the kinetic resolution, the proposed model accurately predicts the faster reacting enantiomer. A breakdown of the steric and electronic control elements shows that a gearing effect in the transition state is responsible for the observed selectivity.

Mechanism of the aminolysis of Fischer alkoxy and thiocarbene complexes: a DFT study

B3LYP calculations have been carried out to study the reaction mechanism of the aminolysis of Fischer carbene complexes of the type (CO)(5)Cr=C(XMe)R (X = O and S; R = Me and Ph). We have explored different possible reaction mechanisms either through neutral or zwitterionic intermediates as well as a general base catalysis assisted by an ammonia molecule. Our results show that the most favorable pathway for the aminolysis of Fischer carbene complexes is through a stepwise reaction via a zwitterionic intermediate generated by the initial nucleophilic attack. We have found that the ammonia-catalyzed mechanism entails a significantly lower barrier for the rate-determining step than the uncatalyzed one. At lower pressure gas-phase conditions, the rate-determining step corresponds to the concerted proton transfer and MeXH elimination. Thiocarbene complexes show a higher energy barrier for this rate-determining step due to the lower basicity of the MeS(-) substituent. At higher pressure or in solution, the rate-determining step corresponds to the initial nucleophilic attack. Our results indicate that the transition state of the nucleophilic attack is more advanced and has a higher barrier for alkoxycarbene than thiocarbene complexes due to the stronger pi-donor character of the alkoxy group that reduces the electrophilicity of the attacked carbene atom making the nucleophilic attack more difficult.

Mechanism of the Aminolysis of Methyl Benzoate: A Computational Study†

Density functional and ab initio methods were applied in examining the possible mechanistic pathways for the reaction of methyl benzoate with ammonia. Transition state structures and energies were determined for concerted and neutral stepwise mechanisms. The theoretical results show that the two possible pathways have similar activation energies. The general base catalysis of the process was also examined. The predictions reveal that the catalytic process results in considerable energy savings and the most favorable pathway of the reaction is through a general-base-catalyzed neutral stepwise mechanism. The structure and transition vectors of the transition states indicate that the catalytic role of ammonia is realized by facilitating the proton-transfer processes. Comparison of the energetics of the aminolysis for methyl benzoate and methyl formate shows the more favorable process to be that for the aliphatic ester. The differing reactivity of the two esters is explained in terms of the electrostatic potential values at the atoms of the ester functionality.