Yoshihito Kayaki

N‐Heterocyclic Carbenes as Efficient Organocatalysts for CO2 Fixation Reactions

Getting a fix: N-heterocyclic carbenes (NHCs) and NHC-CO(2) adducts serve as potent organocatalysts for carbonate synthesis by the addition of a CO(2) unit to propargylic alcohols or epoxides under mild and solvent-free reaction conditions (see scheme). The enhanced Lewis basicity of imidazol-2-ylidenes bearing electron-donating alkyl groups on the nitrogen atoms leads to utilizing CO(2) as a nucleophilic fragment in the chemical fixation processes.

Oxo-Tethered Ruthenium(II) Complex as a Bifunctional Catalyst for Asymmetric Transfer Hydrogenation and H2 Hydrogenation

Newly developed oxo-tethered Ru amido complexes (R,R)-1 and their HCl adducts (R,R)-2 exhibited excellent catalytic performance for both asymmetric transfer hydrogenation and the hydrogenation of ketonic substrates under neutral conditions without any cocatalysts to give chiral secondary alcohols with high levels of enantioselectivity.

Aerobic Oxidation of Alcohols with Bifunctional Transition-Metal Catalysts Bearing C–N Chelate Ligands

The aerobic oxidation of alcohols with a family of bifunctional Ir, Rh, and Ru complexes bearing C-N chelating ligands derived from primary benzylic amines was investigated. The isolable amido-Ir complexes [Cp*Ir{kappa(2)(N,C)-(NHCR(2)-2-C(6)H(4))}] (R=C(6)H(5), CH(3); Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) effected the oxidation of secondary alcohols smoothly under atmospheric pressure of air at 30 degrees C in THF to give the corresponding ketones in good yields. The hydrido(amine)-Ir complexes [Cp*IrH{kappa(2)(N,C)-(NH(2)CR(2)-2-C(6)H(4))}] and the combined catalyst system involving the chloro(amine)-Ir complex [Cp*IrCl{kappa(2)(N,C)-(NH(2)CR(2)-2-C(6)H(4))}] and KOC(CH(3))(3) were also found to be effective catalysts, whereas the tertiary amine complex [Cp*IrCl{kappa(2)(N,C)-(N(CH(3))(2)CH(2)-2-C(6)H(4))}], which does not have a metal/NH moiety, did not show catalytic activity. The employment of primary alcohols in the aerobic reaction with the Cp*IrCl complex and KOC(CH(3))(3) resulted in the formation of esters through oxidative dimerization.

Carboxylative cyclization of propargylamines with supercritical carbon dioxide

Propargylic amines were found to react smoothly with carbon dioxide under supercritical conditions to give (Z)-5-alkylidene-1,3-oxazolidin-2-ones exclusively, even in the absence of any metal or base catalyst.

Cycloaddition of tertiary aziridines and carbon dioxide using a recyclable organocatalyst, 1,3-di-tert-butylimidazolium-2-carboxylate: straightforward access to 3-substituted 2-oxazolidones

Imidazolium-2-carboxylates derived from N-heterocyclic carbenes (NHCs) and CO2 serve as efficient catalysts for CO2-carboxylation of tertiary aziridines bearing various substituents such as halogens, ether, olefin, ester, acetal, and nitro groups on the aziridine ring in 2-propanol, leading to 3-substituted-2-oxazolidones in good to excellent yields and with high selectivity. The NHC–CO2 adducts facilitate nucleophilic attack of the CO2 moiety on the aziridines, in which the substituents are intact during the carboxylation. The catalyst is successfully recycled up to five times with no apparent loss in activity.

Utilization of N,N-dialkylcarbamic acid derived from secondary amines and supercritical carbon dioxide: stereoselective synthesis of Z alkenyl carbamates with a CO2-soluble ruthenium-P(OC2H5)3 catalyst.

Reversible transformation of diethylamine (1) and supercritical carbon dioxide (scCO(2)) into N,N-diethylcarbamic acid (2) was confirmed by direct acquisition of (1)H and (13)C{(1)H} NMR spectra. The equilibrium between 1+CO(2) and 2 is strongly influenced by conditions of the supercritical state. Low temperature favors formation of carbamic acid, whereas high temperature causes decarboxylation. On the basis of the spectroscopic results of carbamic acid formation under scCO(2) conditions, the ruthenium-catalyzed formation of alkenyl carbamates from terminal alkynes, 1, and carbon dioxide was investigated to demonstrate the useful transformation of elusive carbamic acids. Selectivity toward the CO(2)-fixation products over enynes obtained by dimerization of the alkyne substrates was improved by the use of scCO(2) as a reaction medium. In particular, a CO(2)-soluble ruthenium complex, trans-[RuCl(2){P(OC(2)H(5))(3)}(4)], was found to be effective in affording Z alkenyl carbamates with high stereoselectivity.

Enhanced Hydrogen Generation from Formic Acid by Half‐Sandwich Iridium(III) Complexes with Metal/NH Bifunctionality: A Pronounced Switch from Transfer Hydrogenation

By switching the catalytic function from transfer hydrogenation based on the metal/NH bifunctionality, facile dehydrogenation of formic acid was achieved by amido- and hydrido(amine)-Ir complexes derived from N-triflyl-1,2-diphenylethylenediamine (TfDPEN) at ambient temperature even in the absence of base additives. Further acceleration was observed by the addition of water, leading to a maximum turnover frequency above 6000u2005h(-1). A proton-relay mechanism guided by the protic amine ligand and water is postulated for effective protonation of metal hydrides.

Asymmetric Mukaiyama aldol reaction of a ketene silyl acetal of thioester catalyzed by a binaphthol–titanium complex in supercritical fluoroform

Abstract The Mukaiyama aldol reaction catalyzed by a binaphthol-derived chiral titanium(IV) complex proceeds smoothly in an unorthodox reaction medium, supercritical fluid (SCF) such as fluoroform (scCHF 3 ). The chemical yield and enantioselectivity of the reaction in SCFs are found to be tuned by changing the supercritical fluids, scCHF 3 versus carbon dioxide (scCO 2 ), and adjusting the matched polarities by varying the pressure of CHF 3 .

Highly Selective Carboxylative Cyclization of Allenylmethylamines with Carbon Dioxide Using N-Heterocyclic Carbene-Silver(I) Catalysts

Silver(I) carboxylate complexes promote the carboxylative cyclization of allenylmethylamines to afford 5-alkenyl-1,3-oxazolidin-2-ones in 2-propanol. The use of an N-heterocyclic carbene ligand (IPr) under pressurized CO2 is effective in suppressing the intramolecular hydroamination that leads to 2,5-dihydropyrroles. The mechanism involving a nucleophilic attack of the carbamate of the allene moiety and a subsequent protonation was realized on the basis of experimental and theoretical results involving a model intermediate, the alkenylgold(I) complex, which was synthesized from Au(OH)(IPr) and 1-methylamino-2,3-butadiene.

Supercritical fluids as reaction media for molecular catalysis

Chemical transformations in supercritical fluids (SCFs) have enormous potential advantages. The possibilities of rate enhancement and adjustable selectivities will motivate the research of molecular catalysis in SCFs. Recent progress in the organometallic catalysis under supercritical conditions is reviewed with emphasis on the benefits of utilization of supercritical carbon dioxide (scCO2) both as a reaction medium and reactant.