Asymmetric Rhodium-Directed anti-Markovnikov Regioselective Boracyclopentannulation

Wittig/B─H insertion reaction: A unique access to trisubstituted Z-alkenes

The Wittig reaction, which is one of the most effective methods for synthesizing alkenes from carbonyl compounds, generally gives thermodynamically stable E-alkenes, and synthesis of trisubstituted Z-alkenes from ketones presents notable challenges. Here, we report what we refer to as Wittig/B─H insertion reactions, which innovatively combine a Wittig reaction with carbene insertion into a B─H bond and constitute a promising method for the synthesis of thermodynamically unstable trisubstituted Z-boryl alkenes. Combined with the easy transformations of boryl group, this methodology provides efficient access to a variety of previously unavailable trisubstituted Z-alkenes and thus provides a platform for discovery of pharmaceuticals. The unique Z-selectivity of the reaction is determined by the maximum overlap of the orbitals between the B─H bond of the borane adduct and the alkylidene carbene intermediate in the transition state.

Regioselective Radical Hydroboration of gem-Difluoroalkenes: Synthesis of α-Borylated Organofluorines

A regioselective radical hydroboration of gem-difluoroalkenes was developed for the synthesis of α-difluoroalkylborons. The reaction features excellent regioselectivity, broad substrate scope, and good functional group capability. DFT calculations implicated the remarkable α-selectivity was driven from the kinetically and thermodynamically more favorable α-addition step. The resulting α-difluoroalkylborons could be readily converted into NHC-borane-tethered monofluoroalkenes, which demonstrated unique reactivity and applicability in the synthesis of monofluoroalkene derivatives through transformations of the boron unit.

Highly diastereoselective preparation of chiral NHC-boranes stereogenic at the boron atom

Stereogenic main group elements are clearly generating interest in the enantioselective catalysis field. Surprisingly, while chiral organoboron reagents are very useful in stereoselective transformations, few scaffolds stereogenic at boron and configurationally stable have been reported to date. Herein, we describe an original library of chiral NHC-boranes, stereogenic at the boron atom, that has been prepared in only a few steps and in good yields (up to 93%). Key steps involve a chlorination/arylation sequence in the presence of simple Grignard reagents from bicyclic NHC-boranes. The high and unprecedented diastereoselectivity observed during the second step (up to 99 : 1 dr) has been rationalized through a plausible SRN1 mechanism thanks to EPR observations and DFT calculations.

Investigation of the rhodium-catalyzed hydroboration of NHC-boranes: the role of alkene coordination and the origin of enantioselectivity

The mechanism of the intramolecular enantioselective rhodium(i)-catalyzed hydroboration of NHC-boranes is investigated by experiments and calculations, using Density Functional Theory and Random Phase Approximation methods.

Catalytic B-H Bond Insertion Reactions Using Alkynes as Carbene Precursors

Herein, we report transition-metal-catalyzed B-H bond insertion reactions between borane adducts and alkynes to afford organoboron compounds in excellent yields under mild reaction conditions. This successful use of alkynes as carbene precursors in these reactions constitutes a new route to organoboron compounds. The starting materials are safe and readily available, and the reaction exhibits 100% atom-economy. Moreover, an asymmetric version catalyzed by chiral dirhodium complexes produced chiral boranes with excellent enantioselectivity (up to 96% ee). This is the first report of highly enantioselective heteroatom-hydrogen bond insertion reactions of metal carbenes generated in situ from alkynes. The chiral products of the reaction could be easily transformed to widely used borates and diaryl methanol compounds without loss of optical purity, which demonstrates its potential utility in organic synthesis. A kinetics study indicated that the Cu-catalyzed B-H bond insertion reaction is first order with respect to the catalyst and the alkyne and zero order with respect to the borane adduct, and no kinetic isotopic effect was observed in the reaction of the adduct. These results, along with density functional theory calculations, suggest that the formation of the Cu carbene is the rate-limiting step and that the B-H bond insertion is a fast, concerted process.

Synthesis and Suzuki Reactions of N-Heterocyclic Carbene Difluoro(aryl)-boranes

Readily available NHC-arylboranes (NHC-BH2Ar) are converted in high yield to stable NHC-difluoro(aryl)boranes (NHC-BF2Ar) by treatment with 2 equiv of 1-chloromethyl-4-fluoro-1,4-diazonia-bicyclo[2.2.2]octane bis(tetrafluoroborate) (Selectfluor). In turn, the NHC-difluoro(aryl)boranes participate directly in Suzuki reactions under conditions previously used for anionic trifluoroborate salts. Accordingly, NHC-difluoroboranes are a new class of stable precursors for Suzuki reactions.

Exploring the mechanism of the hydroboration of alkenes by amine–boranes catalysed by [Rh(xantphos)]+

The [Rh(xantphos)]⁺ fragment acts as an effective catalyst for the hydroboration of the alkene TBE (tert-butyl ethene) using the amine–borane H₃B·NMe₃ at low (0.5 mol%) catalyst loadings to give the linear product.

Copper-catalyzed B-H bond insertion reaction: a highly efficient and enantioselective C-B bond-forming reaction with amine-borane and phosphine-borane adducts

A copper-catalyzed B-H bond insertion reaction with amine- and phosphine-borane adducts was realized with high yield and enantioselectivity under mild reaction conditions. The B-H bond insertion reaction provides a new C-B bond-forming methodology and an efficient approach to chiral organoboron compounds.

Insertion of reactive rhodium carbenes into boron-hydrogen bonds of stable N-heterocyclic carbene boranes

Readily available rhodium(II) salts catalyze reactions between NHC-boranes (NHC-BH3) and diazocarbonyl compounds (N2CRCOR'). Stable α-NHC-boryl carbonyl compounds (NHC-BH2-CHRCOR') are isolated in good yields. The reaction is a reliable way to make boron-carbon bonds with good tolerance for variation in both the NHC-borane and diazocarbonyl components. It presumably occurs by insertion of a transient rhodium carbene into a boron-hydrogen bond of the NHC-borane. Competitive experiments show that a typical NHC-borane is highly reactive toward rhodium carbenes.