Recent contributions from the Baylis-Hillman reaction to organic chemistry

The aza-Morita-Baylis-Hillman reaction: a mechanistic and kinetic study

The aza-Morita-Baylis-Hillman (aza-MBH) reaction has been studied in a variety of solvents, a selection of imine substrates and with various combinations of PPh3 and para-nitrophenol as the catalyst system. The measured kinetic data indicates that the effects of solvent and protic co-catalyst are strongly interdependent. These results are most easily reconciled with a mechanistic model involving the reversible protonation of zwitterionic intermediates in the catalytic cycle, which is also supported by (31)P NMR spectroscopy and quantum chemical studies.

Ultrasound mediation for efficient synthesis of monoarylidene derivatives of homo- and heterocyclic ketones

Ultrasonic irradiation was efficiently used for high yield synthesis of monoarylidene derivatives of cyclic systems directly from the reaction of ketone with various aldehydes under solvent-free conditions. Reactions took place rapidly in the presence of catalytic amounts of pyrrolidine, while no significant formation of the undesired bis by-products was observed. Moreover, the procedure was applicable to both homo- and heterocyclic ketones.

Computational design of enone-binding proteins with catalytic activity for the Morita-Baylis-Hillman reaction

The Morita-Baylis-Hillman reaction forms a carbon-carbon bond between the α-carbon of a conjugated carbonyl compound and a carbon electrophile. The reaction mechanism involves Michael addition of a nucleophile catalyst at the carbonyl β-carbon, followed by bond formation with the electrophile and catalyst disassociation to release the product. We used Rosetta to design 48 proteins containing active sites predicted to carry out this mechanism, of which two show catalytic activity by mass spectrometry (MS). Substrate labeling measured by MS and site-directed mutagenesis experiments show that the designed active-site residues are responsible for activity, although rate acceleration over background is modest. To characterize the designed proteins, we developed a fluorescence-based screen for intermediate formation in cell lysates, carried out microsecond molecular dynamics simulations, and solved X-ray crystal structures. These data indicate a partially formed active site and suggest several clear avenues for designing more active catalysts.

Metal-free α-amination of secondary amines: computational and experimental evidence for azaquinone methide and azomethine ylide intermediates

We have performed a combined computational and experimental study to elucidate the mechanism of a metal-free α-amination of secondary amines. Calculations predicted azaquinone methides and azomethine ylides as the reactive intermediates and showed that iminium ions are unlikely to participate in these transformations. These results were confirmed by experimental deuterium-labeling studies and the successful trapping of the postulated azomethine ylide and azaquinone methide intermediates. In addition, computed barrier heights for the rate-limiting step correlate qualitatively with experimental findings.

A novel synthesis of 1-aryl-3-piperidone derivatives

A novel method to construct the 1-aryl-3-piperidone scaffold is described here. Starting from 3,5-dichloroaniline, a seven-step synthesis, without the use of protecting groups, generates the desired 3-piperidone ring in an overall yield of 30% through a key Morita-Baylis-Hillman reaction and ring-closing metathesis, providing easy access to diverse and useful heterocycles.