Phosphonium salts exhibit great utility in organic synthesis. However, tetraarylphosphonium salts (TAPS) have found limited use as catalysts. We demonstrate the TAPS-catalyzed carbon dioxide fixation at atmospheric pressure for the coupling reaction with epoxides. Five-membered cyclic carbonates were obtained, including enantio-enriched carbonates. Mechanistic studies revealed the origin of the behavior of TAPS to be the in situ formation of an active species by TAPS addition to epoxides via halohydrin intermediates.

Highly anti- and enantioselective synthesis of beta-amino aldehydes having an aliphatic substituent at the beta-position was accomplished by a combination of two catalytic reactions, that is, an initial Ni(II) complex-catalyzed isomerization of a double bond followed by a chiral phosphoric acid catalyzed aza-Petasis-Ferrier rearrangement, using hemiaminal allyl ethers as the initial substrate. The chiral phosphoric acid also functioned as an efficient resolving catalyst of racemic hemiaminal vinyl ethers.

Preparation of a range of oxazolidinones, including enantioenriched N-aryl-substituted oxazolidinones, in which tetraarylphosphonium salts (TAPS) catalyze the [3 + 2] coupling reaction of isocyanates and epoxides effectively, is described. The key finding is a Brønsted acid/halide ion bifunctional catalyst that can accelerate epoxide ring opening with high regioselectivity. Mechanistic studies disclosed that the ylide generated from TAPS, along with the formation of halohydrins, plays a crucial role in the reaction with isocyanates.

Carbonyl-stabilized phosphonium ylides exhibit great utility in modern organic synthesis, and they are known as an ambident nucleophile at the carbonyl oxygen atom and at the α-carbon atom. However, they have found limited use as catalysts. We focused on the inherent nucleophilicity of the oxygen atom to develop an ionic nucleophilic catalysis, and the phosphonium ylide-catalyzed primary alcohol selective acylation of mixed diols with acid anhydrides has been demonstrated. Mechanistic studies revealed the behavior of a catalyst, which would contribute to the field of ylide chemistry.

The transformation of ortho-alkynylaryl ketones through a cyclization/enantioselective-reduction sequence in the presence of a chiral silver phosphate catalyst afforded 1H-isochromene derivatives in high yield with fairly good to high enantioselectivity. An asymmetric synthesis of the 9-oxabicyclo[3.3.1]nona-2,6-diene framework, which has been found in some biologically active molecules, is presented as a demonstration of the synthetic utility of this method.

The first highly diastereo- and enantioselective additions of a halogen and phosphoramidic acid to unactivated alkenes have been developed, catalyzed by a chiral Brønsted acid. A unique feature of these additions is the opportunity for stereocontrol at two noncontiguous chiral centers, carbon and phosphorus, leading to cyclic P-chiral phosphoramidates. In addition to their inherent value, the phosphoramidates are precursors to enantioenriched epoxy allylamines.

The development of carbon dioxide fixation under mild conditions is a central theme in organic synthesis. Despite the tremendous progress in the field of organocatalysis in the past two decades, the coupling reactions of epoxides with carbon dioxide that proceed at atmospheric pressure at temperatures of less than 100 °C have remained challenging. In our aspirational studies of tetraarylphosphonium salts (TAPS) catalysis, we report here the bifunctional TAPS-catalyzed synthesis of five-membered cyclic carbonates by chemical fixation using 1 atm of carbon dioxide at 60 °C. Intriguing substituent effects of TAPS were observed, in which electron-donating groups enhanced their reactivity. In addition, the mechanism was thoroughly investigated by undertaking both experimental and theoretical studies, suggesting that the electronic properties of TAPS affect carbon dioxide insertion into halohydrin intermediates. The results provided fruitful information to understand the origin of the TAPS behavior, which would contribute to the design of novel catalysts for carbon dioxide capture.

The precise mechanism of the highly anti- and enantioselective aza-Petasis-Ferrier (APF) rearrangement of hemiaminal vinyl ethers catalyzed by a chiral phosphoric acid was investigated by undertaking experimental and theoretical studies. The APF rearrangement is characterized by the following unique mechanistic features: (i) efficient optical kinetic resolution of the starting racemic hemiaminal vinyl ether, (ii) enantioconvergent process from racemic hemiaminal vinyl ethers to optically active β-amino aldehyde products, and (iii) anomalous temperature effects on the enantioselectivity (enantioselectivity increases as reaction temperature increases). The following experiments were conducted to elucidate the unique mechanistic features as well as to uncover the overall scheme of the present rearrangement: (A) X-ray crystallographic analysis of the recovered hemiaminal vinyl ether to determine its absolute configuration, (B) rearrangements of enantiomerically pure hemiaminal vinyl ethers to validate the stereochemical relationship between the hemiaminal vinyl ethers and β-amino aldehydes, (C) theoretical studies on the transition states of the C-O bond cleavage and C-C bond formation steps to gain an insight into the optical kinetic resolution of the hemiaminal vinyl ether and the origin of the stereoselectivity, as well as to elucidate the overall scheme of the present rearrangement, and (D) crossover experiments of two hemiaminal vinyl ethers having different vinyl ether and aliphatic substituents to comprehend the mechanism of the anomalous temperature effect and the enantioconvergent process. The results of experiments and theoretical studies fully support the proposed mechanism of the present anti- and enantioselective APF rearrangement.