Boron-Catalyzed N-Alkylation of Amines using Carboxylic Acids

A boron-based catalyst was found to catalyze the straightforward alkylation of amines with readily available carboxylic acids in the presence of silane as the reducing agent. Various types of primary and secondary amines can be smoothly alkylated with good selectivity and good functional-group compatibility. This metal-free amine alkylation was successfully applied to the synthesis of three commercial medicinal compounds, Butenafine, Cinacalcet. and Piribedil, in a one-pot manner without using any metal catalysts.

Facile Protocol for Catalytic Frustrated Lewis Pair Hydrogenation and Reductive Deoxygenation of Ketones and Aldehydes

A series of ketones and aldehydes are reduced in toluene under H2 in the presence of 5 mol % B(C6F5)3 and either cyclodextrin or molecular sieves affording a facile metal-free protocol for reduction to alcohols. Similar treatment of aryl ketones resulted in metal-free deoxygenation yielding aromatic hydrocarbons.

Frustrated Lewis pair chemistry: development and perspectives

Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases in solution that are deterred from strong adduct formation by steric and/or electronic factors. This opens pathways to novel cooperative reactions with added substrates. Small-molecule binding and activation by FLPs has led to the discovery of a variety of new reactions through unprecedented pathways. Hydrogen activation and subsequent manipulation in metal-free catalytic hydrogenations is a frequently observed feature of many FLPs. The current state of this young but rapidly expanding field is outlined in this Review and the future directions for its broadening sphere of impact are considered.

Autoinduced catalysis and inverse equilibrium isotope effect in the frustrated Lewis pair catalyzed hydrogenation of imines

The frustrated Lewis pair (FLP)-catalyzed hydrogenation and deuteration of N-benzylidene-tert-butylamine (2) was kinetically investigated by using the three boranes B(C6F5)3 (1), B(2,4,6-F3-C6H2)3 (4), and B(2,6-F2-C6H3)3 (5) and the free activation energies for the H2 activation by FLP were determined. Reactions catalyzed by the weaker Lewis acids 4 and 5 displayed autoinductive catalysis arising from a higher free activation energy (2 kcal mol(-1)) for the H2 activation by the imine compared to the amine. Surprisingly, the imine reduction using D2 proceeded with higher rates. This phenomenon is unprecedented for FLP and resulted from a primary inverse equilibrium isotope effect.

B(C6F5)3-mediated transformations and hydrogenation of carbodiimides

The reactivity of a series of carbodiimides RNCNR (R=tBu (1 a), iPr (1 b), SiMe3 (1 c), and Dipp (2,6-di-iso-propylphenyl (1 d)) with B(C6 F5 )3 was investigated. After initial adduct formation, several distinct reaction pathways were identified. These pathways involve either isomerization of the carbodiimide to cyanamide derivatives or insertion of a carbodiimide into a BC bond of B(C6 F5 )3 to yield four-membered heterocycles. In the presence of dihydrogen, stepwise hydrogenation of the heteroallene moiety in the carbodiimides was achieved, which yielded the respective amidine-B(C6 F5 )3 adducts and amidinium borate salts upon reaction with one or two equivalents of H2 , respectively.

Metal-free hydrogenation catalyzed by an air-stable borane: use of solvent as a frustrated Lewis base

In recent years 'frustrated Lewis pairs' (FLPs) have been shown to be effective metal-free catalysts for the hydrogenation of many unsaturated substrates. Even so, limited functional-group tolerance restricts the range of solvents in which FLP-mediated reactions can be performed, with all FLP-mediated hydrogenations reported to date carried out in non-donor hydrocarbon or chlorinated solvents. Herein we report that the bulky Lewis acids B(C6Cl5)x(C6F5)(3-x) (x=0-3) are capable of heterolytic H2 activation in the strong-donor solvent THF, in the absence of any additional Lewis base. This allows metal-free catalytic hydrogenations to be performed in donor solvent media under mild conditions; these systems are particularly effective for the hydrogenation of weakly basic substrates, including the first examples of metal-free catalytic hydrogenation of furan heterocycles. The air-stability of the most effective borane, B(C6Cl5)(C6F5)2, makes this a practically simple reaction method.

Gallium-assisted transfer hydrogenation of alkenes

We report a rare case of alkene transfer hydrogenation using a main-group compound instead of a transition-metal complex as catalyst. We disclosed that 1,4-cyclohexadiene can be used as H2 surrogate towards olefin reduction in the presence of [IPrGaCl2 ][SbF6 ]. Hydrogenative cyclizations have also been carried out because this cationic gallium complex is also a potent hydroarylation catalyst.

Frustrated Lewis pair chemistry of chiral (+)-camphor-based aminoboranes

Dimethylamino-(+)-camphorenamine reacted with an equimolar amount of Piers' borane, HB(C6F5)2, to give the corresponding iminium-hydroborate zwitterionic salt. Being in equilibrium with the parent enamine-HB(C6F5)2 N-B pair, this salt was able to split hydrogen heterolytically, hydrogenating the iminium group in the molecule. Detailed studies revealed that the hydrogen splitting in this reaction proceeded through an intermolecular pathway leading to a bornylamine-HB(C6F5)2 adduct. When the starting enamine is present in excess over HB(C6F5)2, the produced bornylamine-HB(C6F5)2 adduct breaks up, eliminating free bornylamine and forming the initial camphorenamine-HB(C6F5)2 pair. This results in hydrogenation of the camphorenamine framework in a catalytic fashion.

Metal-free HB(C6F5)2-catalyzed hydrogenation of unfunctionalized olefins and mechanism study of borane-mediated σ-bond metathesis

Out with the metal: Metal-free hydrogenation of unfunctionalized olefins can be achieved by employing HB(C6F5)2 as the catalyst. The key step in the catalytic reaction is believed to involve a novel borane-mediated σ-bond metathesis, which has been investigated both experimentally and theoretically.