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The umpolung of alkynyl 1,2-diketones via N-heterocyclic carbene (NHC) catalysis was achieved for the first time, allowing rapid access to a large variety of synthetically and pharmaceutically important α-pyrones under very mild conditions. A completely new NHC-catalyzed umpolung pattern involving an O-acylated allenolate as the key intermediate was proposed. Moreover, an unprecedented reaction pathway, featured by a series of group migrations and new bond formation, was postulated to demonstrate the formation of the products.

Growing attention in developing new N-heterocyclic carbene (NHC)-mediated reactions involving homoenolate intermediates has prompted our interest in exploring the mechanistic details of the related reactions. In this work, we carried out a detailed theoretical study for the NHC-catalyzed annulation reaction of cinnamaldehyde (A) and benzodi(enone) (B) in the presence of 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU). By performing density functional theory calculations, we show clearly the detailed reaction mechanism and rationalize the experimental observation. The reaction of A and B falls into two stages: the formation of homoenolate intermediate and the annulation of homoenolate with B. In the homoenolate formation stage, three possible paths are characterized. The pathway involving the DBU-assisted 1,2-proton transfer with a stepwise mechanism is kinetically more favorable, and the DBU-assisted C1 proton departure is the rate-determining step of the total reaction. The annulation of homoenolate with B involves four elementary steps. The conformational difference of homoenolate (cis and trans) leads to two slightly different reaction processes. In the total reaction, the process involving cis-conformation of A is kinetically more feasible. This can be clearly understood through the frontier molecular orbital analysis and the electronic inductive effect. The calculated results are expected to offer valuable information for further design and development of NHC-mediated reactions. Chirality 25:521-528, 2013. © 2013 Wiley Periodicals, Inc.

The efficient, highly enantioselective preparation of a variety of tetralone derivatives with two contiguous stereocenters and diverse functional groups is described.

The asymmetric desymmetrization of 1,4-dienes via chiral N-heterocyclic carbene catalyzed Stetter-type umpolung reaction was demonstrated. A variety of differently substituted dienes were tolerated very well, affording cyclic ketones with two consecutive stereogenic centers (including one quaternary carbon) in moderate to high yields and with high to excellent enantioselectivities. This work expanded both reaction types of catalytic diene desymmetrizations and substrate diversity in NHC catalyzed desymmetric transformations.

A general method for the asymmetric desymmetrization of 1,3-diketone substrates via chiral N-heterocyclic carbene catalyzed intramolecular benzoin reactions was developed. Five- and six-membered cyclic ketones bearing two contiguous fully substituted stereocenters were generated with excellent diastereoselectivities and moderate to excellent enantioselectivities.

The stereoselectivity in the 6-exo cyclization of α-carbamoyl radicals was investigated experimentally and theoretically. The BEt3/O2-initiated iodine-atom-transfer radical cyclization reactions of substituted N-(but-3-en-1-yl)-N-(tert-butyl)-2-iodoalkanamides were carried out, which led to the predominant formations of 3,4-cis, 4,5-trans, or 4,6-trans substituted δ-lactams. Density functional calculations at the B3LYP/6-31G* level revealed that the 6-exo radical cyclization proceeds via boat-conformational transition states. Furthermore, a mechanistic insight into the stereoselectivity was provided and the calculation results were in excellent agreement with the experimental observations.

The synthesis of the cis-homoenolate equivalents can be realized from cis-enols in the presence of an NHC-catalyst.

The asymmetric Stetter reaction of β-alkyl (I), β-aryl (IV) as well as β,β-disubstituted enals (VI) with 2-alkylidene-1,3-dicarbonyl compounds (II), (VII) or (IX) proceeds smoothly in the presence of a chiral triazolium-based NHC organocatalyst.

Highly enantioselective (formal) hetero-Diels–Alder reactions between chalcones and formylcyclopropanes are disclosed. The challenging N-heterocyclic carbene (NHC)-bounded enolate intermediates from formylcyclopropanes were captured for new C–C bond forming reactions. The reaction products were obtained with high diastereo- and enantioselectivities and could be easily transformed to optically pure multisubstituted cyclohexane derivatives.

Substrates with electron-withdrawing groups other than -CO-Ar at the α-position of the chalcone are tested (-CO-O-Et, -CN, -Ph) but fail to react.

A new asymmetric annulation of bis-enone derivatives of type (I) is achieved in the presence of a NHC-catalyst.

The mild, efficient, and general aerobic aromatization of Hantzsch 1,4-dihydropyridines was realized at room temperature with 5 mol % of 9-phenyl-10-methylacridinium perchlorate as the photocatalyst, which could be easily recovered and reused.