Metal-Free Regio- and Chemoselective Hydroboration of Pyridines Catalyzed by 1,3,2-Diazaphosphenium Triflate.

N-Heterocyclic phosphenium triflates (NHP-OTf) 1 serve as efficient catalysts for the regio- and chemoselective hydroboration of pyridines under ambient condition with good functional group tolerance. Mechanistic studies indicate that a boronium salt, [(Py)2·Bpin]OTf 4, is generated concomitant with NHP-H 5 via hydride abstraction from HBpin by 1 in the initial reaction step. Hydride reduction of the activated pyridine in [(Py)2·Bpin]OTf 4 by NHP-H 5 affords the 1,4-hydroboration product selectively. Thus, the phosphenium species act as a hydrogen transfer reagent in the catalytic cycle.

Bis(N-heterocyclic olefin) Derivative: An Efficient Precursor for Isophosphindolylium Species.

We have developed bis(N-heterocyclic olefin) derivatives 2 and demonstrated that 2 can be utilized as precursors for the synthesis of isophosphindolylium species 3. X-ray diffraction and density functional theory studies indicate the aromatic property of the PC4 five-membered ring in 3. Despite its cationic nature, the P center in 3b exhibits nucleophilic character and thus readily forms a bond with CuCl to afford a copper phosphenium complex 4, demonstrating the potential utility of 3 as a σ-donor ligand.

Hydrophosphination of CO2 and subsequent formate transfer in the 1,3,2-diazaphospholene-catalyzed N-formylation of amines.

Hydrophosphination of CO2 with 1,3,2-Diazaphospholene (NHP-H; 1) afforded phosphorus formate (NHP-OCOH; 2) through the formation of a bond between the electrophilic phosphorus atom in 1 and the oxygen atom from CO2 , along with hydride transfer to the carbon atom of CO2 . Transfer of the formate from 2 to Ph2 SiH2 produced Ph2 Si(OCHO)2 (3) in a reaction that could be carried out in a catalytic manner by using 5 mol % of 1. These elementary reactions were applied to the metal-free catalytic N-formylation of amine derivatives with CO2 in one pot under ambient conditions.

Metal-free σ-bond metathesis in 1,3,2-diazaphospholene-catalyzed hydroboration of carbonyl compounds.

The first metal-free catalytic hydroboration of carbonyl derivatives has been developed in which a catalytic amount of 1,3,2-diazaphospholene effectively promotes a hydroboration reaction of aliphatic and aromatic aldehydes and ketones. The reaction mechanism involves the cleavage of both the PO bond of the alkoxyphosphine intermediate and the BH bond of pinacolborane as well as the formation of PH and BO bonds. Thus, the reaction proceeds through a non-metal σ-bond metathesis. Kinetic and computational studies suggest that the σ-bond metathesis occurred in a stepwise but nearly concerted manner.

A concerted transfer hydrogenolysis: 1,3,2-diazaphospholene-catalyzed hydrogenation of N=N bond with ammonia-borane.

1,3,2-diazaphospholenes catalyze metal-free transfer hydrogenation of a N=N double bond using ammonia-borane under mild reaction conditions, thus allowing access to various hydrazine derivatives. Kinetic and computational studies revealed that the rate-determining step involves simultaneous breakage of the B-H and N-H bonds of ammonia-borane. The reaction is therefore viewed as a concerted type of hydrogenolysis.

1,2,4,3-Triazaborole-based neutral oxoborane stabilized by a Lewis acid.

The first example of 1,2,4,3-triazaborole-based oxoborane has been synthesized via hydrogen migration upon the coordination of AlCl3 to the corresponding borinic acid. X-ray diffraction analysis and computational study disclosed the partial B[double bond, length as m-dash]O double-bond property.