Man Sing Cheung

Spectroscopic and Structural Characterization of the CyNHC Adduct of B2pin2 in Solution and in the Solid State

The Lewis base adduct of B(2)pin(2) and the NHC (1,3-bis(cyclohexyl)imidazol-2-ylidene), which was proposed to act as a source of nucleophilic boryl groups in the β-borylation of α,β-unsaturated ketones, has been isolated, and its solid state structure and solution behavior was studied. In solution, the binding is weak, and NMR spectroscopy reveals a rapid exchange of the NHC between the two boron centers. DFT calculations reveal that the exchange involves dissociation and reassociation of NHC rather than an intramolecular process.

Copper-mediated reduction of CO2 with pinB-SiMe2Ph via CO2 insertion into a copper-silicon bond

Reaction of [(IPr)Cu-OtBu] (1) with pinB-SiMe(2)Ph (2) leads to the Cu-silyl complex [(IPr)Cu-SiMe(2)Ph] (3). Insertion of CO(2) into the Cu-Si bond of 3 is followed by transformation of the resulting silanecarboxy complex [(IPr)Cu-O(2)CSiMe(2)Ph] (4) to the silanolate complex [(IPr)Cu-OSiMe(2)Ph] (5) via extrusion of CO. As 5 reacts readily with 2 to regenerate 3, a catalytic CO(2) reduction to CO is feasible. The individual steps were studied by in situ(13)C NMR spectroscopy of a series of stoichiometric reactions. Complexes 3, 4, and 5 were isolated and fully characterized, including single-crystal X-ray diffraction studies. Interestingly, the catalytic reduction of CO(2) using silylborane 2 as a stoichiometric reducing agent leads not only to CO and pinB-O-SiMe(2)Ph but also to PhMe(2)Si-CO(2)-SiMe(2)Ph as an additional reduction product.

Synthesis, Structure, and Reactivity of Anionic sp(2) -sp(3) Diboron Compounds: Readily Accessible Boryl Nucleophiles.

Lewis base adducts of tetra-alkoxy diboron compounds, in particular bis(pinacolato)diboron (B2 pin2 ), have been proposed as the active source of nucleophilic boryl species in metal-free borylation reactions. We report the isolation and detailed structural characterization (by solid-state and solution NMR spectroscopy and X-ray crystallography) of a series of anionic adducts of B2 pin2 with hard Lewis bases, such as alkoxides and fluoride. The study was extended to alternative Lewis bases, such as acetate, and other diboron reagents. The B(sp(2) )-B(sp(3) ) adducts exhibit two distinct boron environments in the solid-state and solution NMR spectra, except for [(4-tBuC6 H4 O)B2 pin2 ](-) , which shows rapid site exchange in solution. DFT calculations were performed to analyze the stability of the adducts with respect to dissociation. Stoichiometric reaction of the isolated adducts with two representative series of organic electrophiles-namely, aryl halides and diazonium salts-demonstrate the relative reactivities of the anionic diboron compounds as nucleophilic boryl anion sources.

Iridium-catalyzed C–H borylation of quinolines and unsymmetrical 1,2-disubstituted benzenes: insights into steric and electronic effects on selectivity

Borylation of quinolines provides an attractive method for the late-stage functionalization of this important heterocycle. The regiochemistry of this reaction is dominated by steric factors but, by undertaking reactions at room temperature, an underlying electronic selectivity becomes apparent, as exemplified by the comparative reactions of 7-halo-2-methylquinoline and 2,7-dimethylquinoline which afford variable amounts of the 5- and 4-borylated products. Similar electronic selectivities are observed for nonsymmetrical 1,2-disubstituted benzenes. The site of borylation can be simply estimated by analysis of the 1H NMR spectrum of the starting material with preferential borylation occurring at the site of the most deshielded sterically accessible hydrogen or carbon atom. Such effects can be linked with C–H acidity. Whilst DFT calculations of the pKa for the C–H bond show good correlation with the observed selectivity, small differences suggest that related alternative, but much more computationally demanding values, such as the M–C bond strength, may be better quantitative predictors of selectivity.

Metal-free borylation of electron-rich aryl (pseudo)halides under continuous-flow photolytic conditions

A metal-free borylation reaction of electron-rich aryl chlorides, fluorides, mesylates and phosphates under continuous-flow photolytic conditions is reported. The flow setup was designed to facilitate this process efficiently in comparison with the batch mode. Owing to its unique chemical selectivity, mild reaction conditions, good functional group tolerance and substrate scope, this reaction adds a complementary protocol to the current synthetic methods for boronic acid derivatives. The proposed reaction mechanism involves a photolytically generated triplet aryl cation, and DFT calculations suggest that the borylation product is formed in an anion-mediated single step process passing a minimum energy crossing point.

Computational Insight into Nickel-Catalyzed Carbon–Carbon versus Carbon–Boron Coupling Reactions of Primary, Secondary, and Tertiary Alkyl Bromides

The nickel-catalyzed alkyl-alkyl cross-coupling (C-C bond formation) and borylation (C-B bond formation) of unactivated alkyl halides reported in the literature show completely opposite reactivity orders in the reactions of primary, secondary, and tertiary alkyl bromides. The proposed Ni(I) /Ni(III) catalytic cycles for these two types of bond-formation reactions were studied computationally by means of DFT calculations at the B3LYP level. These calculations indicate that the rate-determining step for alkyl-alkyl cross-coupling is the reductive elimination step, whereas for borylation the rate is determined mainly by the atom-transfer step. In borylation reactions, the boryl ligand involved has an empty p orbital, which strongly facilitates the reductive elimination step. The inability of unactivated tertiary alkyl halides to undergo alkyl-alkyl cross-coupling is mainly due to the moderately high reductive elimination barrier.