DFT exploration of mechanistic pathways of an aza-Morita–Baylis–Hillman reaction

Acyl Transfer-Driven Rauhut-Currier Dimerization of Morita-Baylis-Hillman Ketones

A serendipitous Rauhut-Currier dimerization of 1,1-disubstituted activated olefins derived from Morita-Baylis-Hillman adducts was observed in the presence of DABCO. The reaction is driven by the migration of an acyl group and produces multifunctionalized enol esters in yields greater than 90% in most cases, without necessitating column chromatographic purification. The acyl transfer is thought to proceed via a transition state typical of a Morita-Baylis-Hillman (MBH) reaction, supported by a brief mechanistic study involving computational calculations.

Mechanism and reactivity in the Morita-Baylis-Hillman reaction: the challenge of accurate computations

A systematic density functional theory exploration of various reactive steps together with benchmark coupled cluster results are used to propose an accurate model of the mechanism of the Morita-Baylis-Hillman (MBH) reaction in organic chemistry. This reaction has attracted considerable interest from the synthetic and mechanistic points of view in recent years, with both computational and experimental mechanistic studies. It has recently (R. E. Plata and D. A. Singleton, J. Am. Chem. Soc., 2015, 137, 3811-3826) been correctly pointed out that previous computational studies failed to reproduce known mechanistic features of the reaction. The same study argued that computation is simply not able at the present time to provide accurate models for such reactions. This second claim is shown by our present work to overstate the problem: by using current 'state of the art' methodology, our results are fully consistent with observed behavior within the expected error bars of 1-5 kcal mol^-1, far smaller than the errors reported in Plata and Singleton's study. On the basis of exhaustive calculations reported here, we suggest that our proposed approaches for modeling electronic structure, solvation, and entropy should be able to provide accurate predictions for many more reactions. We also suggest that reactions like the MBH reaction, where solvation and entropy effects are particularly large, are among the hardest for computational mechanistic studies.

The Carbon-Nitrogen Bonds in Ammonium Compounds Are Charge Shift Bonds

A comprehensive investigation, utilizing valence-bond- and molecular-orbital-based techniques, reveals that C-N bonds in protonated and methylated ammonium compounds are charge shift bonds. Moreover, the ammonium compounds are predominantly covalent at equilibrium distance, yet have two competing dissociation channels, which determine whether or not a state function crossing occurs during dissociation. The location of the crossing point can be predicted with relative ionization potentials, and may be tuned by altering the stabilization of the respective fragments. By closely examining the nature of the C-N bond as it is stretched, a correlation is observed between the extent of the charge shift nature and bond length. Identification of charge shift bonds in these ubiquitous organic species specifically affects understanding of their role in organocatalysis, in which the C-N bond is stretched.