Ring-opening polymerization (ROP) of cyclopropane derivatives catalyzed by Lewis acids has been realized for the first time. In contrast to the conventional radical ROP of dialkyl 2-vinylcyclopropane-1,1-dicarboxylates that proceeds via 1,7-addition, the ROP catalyzed by SnCl4 under ambient conditions selectively affords 1,5-addition polymers with Mn up to 12 600 in good to high yields. The high selectivity of the 1,5-addition structure can be explained by the formation of six-membered transition states from propagating enols and monomers. The obtained polymers show higher glass transition temperatures and better solubilities than the 1,7-addition polymer. The ROP of dimethyl 2-phenylcyclopropane-1,1-dicarboxylate also occurred with SnCl4 as the catalyst in CH3NO2. Donor (vinyl or phenyl) and acceptor (two esters) substituents at the 1,2-positions of cyclopropanes are required to promote an efficient ROP, as the Lewis acids coordinate the two ester groups of the monomers to generate a 1,3-dipole intermediate stabilized by their substituents. This Lewis acid catalysis allows access to polymers that cannot be synthesized by previous radical, anionic, and Pd-catalyzed ROPs.

N-Heterocyclic carbenes (NHCs) with a variety of oxidants promote the Mitsunobu-type coupling reactions of alcohols with phenols, carboxylic acids, and phthalimide. Experiments using a chiral alcohol indicate that these reactions proceed via SN1 or SN2 pathways depending on the polarity of the used solvents. The NHCs are consumed as reducing reagents to form their oxides as readily separable byproducts.

The N-heterocyclic carbene (NHC)-catalyzed dimerizations of a variety of disubstituted Michael acceptors have been investigated. In addition to the tail-to-tail dimerization of methacrylates reported previously, the scope of vinylidene substrates expands to γ-methyl-α-methylene-γ-butyrolactone, dimethyl 2-methylenepentanedioate, dimethyl itaconate, methacrylamides, and 2-isopropenylbenzoxazole, none of which have been dimerized by metal-catalyzed counterparts. In contrast, vinylene substrates, such as crotononitrile, methyl crotonate, and 2-cyclohexen-1-one, do not undergo the dimerization probably due to the steric hindrance of the β-substituent. Instead, the Rauhut-Currier (RC) reaction of these was found to proceed. The stoichiometric reaction of crotononitrile with NHC indicates that this RC reaction also involves the deoxy-Breslow intermediate and the reversible proton transfer.

N-Heterocyclic carbenes (NHCs) promote the transfer hydrogenation of various activated CC, CN, and NN bonds with water as the proton source. The NHCs act as reducing reagents to be converted into their oxides. A detailed reaction mechanism is proposed on the basis of deuterium-labeling experiments.

N-heterocyclic carbenes (NHCs) catalyze the oxa-Michael addition polymerization of hydroxyl functionalized acrylate monomers. This polymerization smoothly proceeded at room temperature to produce poly(ester-ether)s, which include new polymers having alicyclic, alkene, and alkyne groups in the main chain. The post polymerization modification of the alkene-functionalized polymer using thiol–ene “click” chemistry is demonstrated. The structure of poly(2-hydroxyethyl acrylate) was analyzed in detail by NMR and ESI-MS analyses and methanolysis, thus allowing estimation of the frequency of the transesterification. The NHC catalyst was covalently linked to the C terminal of the polymer chain, indicating that the NHC acts as a Lewis base to generate the zwitterionic intermediate without directly activating the hydroxyl groups of the monomers.

N-Heterocyclic carbenes (NHCs), which were first isolated in the early 1990s, have received a great deal of scientific attention as ligands for transition metal complexes and organocatalysts for more than a decade. Organocatalysis by NHCs primarily involves the reaction of carbonyl compounds, particularly the umpolung of aldehydes, although we and others have been developing the reactions and the polymerizations of Michael acceptors. This review focuses on the NHC-catalyzed transformations of Michael acceptors that were developed by our research group, including (1) tail-to-tail dimerization of a wide variety of substrates, (2) cyclotetramerization of acrylates, (3) tandem oxa-Michael addition and head-to-tail dimerization of methacrolein and (4) oxa-Michael addition polymerization of hydroxyl-functionalized acrylates. For the former two reactions, the NHCs turn the β-carbon of the Michael acceptors into nucleophilic sites (umpolung), thereby generating the deoxy-Breslow intermediate and enabling bond formation between the β-carbon and electrophiles. The latter two reactions are based on the O–C bond formation between alcohols and Michael acceptors, in which the NHC catalysts act as a Lewis base. Thus, the use of NHC catalysts allowed new modes of reactivity of Michael acceptors other than the conventional addition polymerizations.

The first tail-to-tail dimerization of methacrylonitrile (MAN) has been realized by the cooperative use of N-heterocyclic carbene (NHC) and Brønsted acid catalysts, producing 2,5-dimethylhex-2-enedinitrile with the E/Z ratio of 24:76. Although the NHC alone was not effective for the catalysis, the addition of alcohols resulted in the significant increase of the dimer yield up to 82% in the presence of 5 mol % NHC. Detailed experimental studies including the ESI-MS analysis of the intermediates, stoichiometric (co)dimerizations, and deuterium-labeling experiments revealed the mechanistic aspects of the proton transfer, isomerization, umpolung, and rate-limiting steps, allowing us to observe several mechanistic differences between the dimerization of MAN and that of methyl methacrylate. The stoichiometric reactions in the presence and absence of an alcohol suggest that the alcohol additives play a role in promoting the intermolecular proton transfers from the deoxy-Breslow intermediate to the regenerated NHC in the second half of the catalytic cycle. In addition, the codimerizations of MAN with n-butyl methacrylate (n-BuMA) have been studied. While the dimerization of n-BuMA was sluggish in the presence of an alcohol, the catalytic activity for the codimerization was enhanced by the cooperative systems.

N-Heterocyclic carbenes (NHCs) were found to catalyze the unprecedented cyclotetramerization of acrylates, producing the trisubstituted cyclopentenones in moderate yields. The proton or deuterium adducts of the deoxy-Breslow intermediate derived from NHC and two molecules of methyl acrylate were obtained. A reaction mechanism involving the new umpolung/cyclization sequence is proposed.

We and others have previously reported the intermolecular umpolung reactions of Michael acceptors catalyzed by an N-heterocyclic carbene (NHC). The representative tail-to-tail dimerization of methyl methacrylate (MMA) has now been intensively investigated, leading to the following conclusions: (1) The catalysis involves the deoxy-Breslow intermediate, which is quite stable and remains active after the catalysis. (2) Addition of the intermediate to MMA and the final catalyst elimination are the rate-limiting steps. Addition of the NHC to MMA and the proton transfers are relatively very rapid. (3) The two alkenyl protons of the first MMA undergo an intermolecular transfer to C3 and C5 of the dimer. (4) The initial proton transfer is intermolecular. (5) Compared with the benzoin condensation, noticeable differences in the kinetics, reversibility, and stability of the intermediates are observed.

The head-to-tail dimerization of methacrolein via the conjugate addition of methanol is catalyzed by various organic bases, such as an amine, phosphine, and N-heterocyclic carbene, to give 2,4-dimethyl-2-methoxymethylpentane-1,5-dial in moderate yields. Based on the interpretation of the key intermediates by electrospray ionization mass spectrometry, we propose a reaction mechanism involving the initial conjugate addition of the organic bases to methacrolein to generate a zwitterionic base followed by the activation of methanol.

Poly(ethylene glycol) with a molecular weight of 400 (PEG400) as an environmentally benign and highly viscous polymeric solvent was found to effectively accelerate the free radical polymerization (FRP) of methyl methacrylate and to afford a high-molecular-weight polymer. When the polymerization in PEG400 was performed at a monomer concentration of 3.0 mol l−1 and an initiator (2,2′-azobis(isobutyronitrile)) concentration of 0.3 × 10−2 mol l−1, the monomer conversion was completed for 6 h to afford a polymer with a number-average molecular weight (Mn) of 838 000. After polymerization, the PEG400 solvent was readily recovered and reused. The solvent effects of highly viscous PEG400 were kinetically analyzed. The polymerization rate (Rp) was proportional to the monomer concentration and the square root of both the initiator concentration and the viscosity of the polymerization media (η). For the FRP in the mixed solvents of PEG400 and toluene, both Rp and Mn values increased linearly with the square root of the η value. The kinetic study has shown that the highly viscous PEG400 solvent largely suppresses diffusion-controlled bimolecular termination.

N-Heterocyclic carbenes (NHCs) with a variety of oxidants promote the Mitsunobu-type coupling reactions of alcohols with phenols, carboxylic acids, and phthalimide. Experiments using a chiral alcohol indicate that these reactions proceed via SN1 or SN2 pathways depending on the polarity of the used solvents. The NHCs are consumed as reducing reagents to form their oxides as readily separable byproducts.

N-Heterocyclic carbenes (NHCs) promote the transfer hydrogenation of various activated CC, CN, and NN bonds with water as the proton source. The NHCs act as reducing reagents to be converted into their oxides. A detailed reaction mechanism is proposed on the basis of deuterium-labeling experiments.