Hydroboration Of Imines Research Articles

Synergistic Photoredox Catalysis and Organocatalysis for Inverse Hydroboration of Imines.

The first catalytic inverse hydroboration of imines with N-heterocyclic carbene (NHC) boranes has been realized by means of cooperative organocatalysis and photocatalysis. This catalytic combination provides a promising platform for promoting NHC-boryl radical chemistry under sustainable and radical-initiator-free conditions. The highly important functional-group compatibility and possible application in late-stage hydroborations represent an important step forward to an enhanced α-amino organoboron library.

Asymmetric Imine Hydroboration Catalyzed by Chiral Diazaphospholenes

AbstractThe first use of diazaphospholenes as chiral catalysts has been demonstrated with enantioselective imine hydroboration. A chiral diazaphospholene prepared in a simple three‐step synthesis from commercial materials has been shown to achieve the highest enantioselectivity for the hydroboration of alkyl imines with pinacolborane reported to date. Enantiomer ratios of up to 88:12 were obtained with low (2 mol %) catalyst loadings. Twenty examples of asymmetric reduction employing this main‐group catalysis protocol, including the synthesis of the pharmaceuticals ent‐rasagiline and fendiline, are shown.

Asymmetric Imine Hydroboration Catalyzed by Chiral Diazaphospholenes.

The first use of diazaphospholenes as chiral catalysts has been demonstrated with enantioselective imine hydroboration. A chiral diazaphospholene prepared in a simple three-step synthesis from commercial materials has been shown to achieve the highest enantioselectivity for the hydroboration of alkyl imines with pinacolborane reported to date. Enantiomer ratios of up to 88:12 were obtained with low (2 mol %) catalyst loadings. Twenty examples of asymmetric reduction employing this main-group catalysis protocol, including the synthesis of the pharmaceuticals ent-rasagiline and fendiline, are shown.

BArF3-Catalyzed Imine Hydroboration with Pinacolborane Not Requiring the Assistance of an Additional Lewis Base

The rarely used boron Lewis acid tris[3,5-bis(trifluoromethyl)phenyl]borane (BArF3) is found to be an excellent catalyst for metal-free hydroboration of imines. In the presence of 1.0 mol % of BArF3, several ketimines and aldimines undergo hydroboration with pinacolborane (HBpin) at room temperature without the aid of an external Lewis base. BArF3 is more reactive than other Lewis acidic boranes, including the often-used tris(pentafluorophenyl)borane (B(C6F5)3). The steric hindrance imparted by the six fluorine atoms ortho to the boron center in B(C6F5)3 accounts for this. Mechanistic control experiments indicate conventional Lewis acid catalysis involving imine activation and hydride transfer from HBpin.

P-N Cooperative Borane Activation and Catalytic Hydroboration by a Distorted Phosphorous Triamide Platform.

Studies of the stoichiometric and catalytic reactivity of a geometrically constrained phosphorous triamide 1 with pinacolborane (HBpin) are reported. The addition of HBpin to phosphorous triamide 1 results in cleavage of the B-H bond of pinacolborane through addition across the electrophilic phosphorus and nucleophilic N-methylanilide sites in a cooperative fashion. The kinetics of this process of were investigated by NMR spectroscopy, with the determined overall second-order empirical rate law given by ν = -k[1][HBpin], where k = 4.76 × 10-5 M-1 s-1 at 25 °C. The B-H bond activation process produces P-hydrido-1,3,2-diazaphospholene intermediate 2, which exhibits hydridic reactivity capable of reacting with imines to give phosphorous triamide intermediates, as confirmed by independent synthesis. These phosphorous triamide intermediates are typically short lived, evolving with elimination of the N-borylamine product of imine hydroboration with regeneration of the deformed phosphorous triamide 1. The kinetics of this latter process are shown to be first-order, indicative of a unimolecular mechanism. Consequently, catalytic hydroboration of a variety of imine substrates can be realized with 1 as the catalyst and HBpin as the terminal reagent. A mechanistic proposal implicating a P-N cooperative mechanism for catalysis that incorporates the various independently verified stoichiometric steps is presented, and a comparison to related phosphorus-based systems is offered.

Transition metal free catalytic hydroboration of aldehydes and aldimines by amidinato silane.

The transition metal free catalytic hydroboration of aldehydes and ketones is very limited and has not been reported with a well-defined silicon(iv) compound. Therefore, we chose to evaluate the previously reported silicon(iv) hydride [PhC(NtBu)2SiHCl2], (1) as a single component catalyst and found that it catalyzes the reductive hydroboration of a range of aldehydes with pinacolborane (HBpin) under ambient conditions. In addition, compound 1 can catalyze imine hydroboration. DFT calculation was carried out to understand the mechanism.

Magnesium Catalysis of Imine Hydroboration

The β-diketiminato magnesium alkyl complex [LMgnBu] (L=CH{CMe(NDipp)}(2), Dipp=diisopropylphenyl) is shown to be a highly effective precatalyst for the hydroboration of alkyl and aryl substituted aldimines and ketimines with pinacol borane (HBpin). Catalysis is proposed to occur through a sequence of Mg-N/B-H metathesis and rate-determining Mg-H/N=C insertion steps, a proposal strongly supported by stoichiometric studies and kinetic analysis. The reactions are observed to proceed through the intermediacy of well-defined magnesium amides, two examples of which have been isolated and structurally characterized. Mechanistic investigations suggest that the catalytic rate-determining process occurs at an isolated magnesium center and requires the presence of two molecules of the imine substrate for effective turnover. This latter observation is rationalized as a requirement for the secondary substrate molecule to displace HBpin from the coordination sphere of the catalytic magnesium center.

Rhodium-catalyzed hydroborations of allylamine and allylimines1

The in situ rhodium-catalyzed addition of catecholborane (HBcat, cat = 1,2-O2C6H4) and pinacolborane (HBpin, pin = 1,2-O2C2Me4) to allylamine, allylimine, 2- and 4-vinylpyridines, and a thienyl imine has been examined using multinuclear NMR spectroscopy. Although reactions of allylamine (H2NCH2CH=CH2) and HBcat gave complex product distributions arising from competing dehydrogenative borylation pathways, addition of HBpin to allylamine using a rhodium catalyst afforded only products arising from hydroboration (RN(Bpin)CH2CH2CH2Bpin, where R = H, Bpin) and hydrogenation (RN(Bpin)CH2CH2CH3). Hydroboration of allylimines (RHC=NCH2CH=CH2, R = Ar) with HBcat occurs initially at the more reactive imine functionality to give unsaturated borylamines (RCH2N(Bcat)CH2CH=CH2). Further reaction with HBcat gives varying amounts of hydroboration products RCH2N(Bcat)CH2CH2CH2Bcat and RCH2N(Bcat)CH2CH(Bcat)CH3 as well as the diboration product RCH2N(Bcat)CH2CH2CH(Bcat)2, depending on the choice of catalyst. Reactions with related unsaturated pyridine derivatives are complicated by extensive degradation, which can be avoided by coordination of the pyridine nitrogen to a Lewis acid. The first examples of metal-catalyzed hydroboration of imines using HBpin are also reported.Key words: catalysis, hydroboration, boronate esters, dehydrogenative borylation, allylimines.

Coinage metal-catalyzed hydroboration of imines

The preparation of several new coinage metal complexes containing bulky, chelating bis(phosphine) ligands is described. The molecular structures of [AuCl( μ-DiPPE)] n ( 4), [Cu(μ-Cl)(DHP)] 2 ( 6a), and [Au(DHP)(PEt 3)]Cl ( 8) have been determined (DiPPE = 1,2-bis(diisopropylphosphino)ethane, DHP = deerhead phosphine, o-phenylenebis(diisopropylphosphine). These new complexes are selective catalysts for the hydroboration of imines and thiazolinesus ing catecholborane. The molecular structure of ▪ ( 11) is also reported.