Alkylaluminum Complexes as Precatalysts in Hydroboration of Nitriles and Carbodiimides

Abstract

The metathesis reaction of aluminum(III) dihydride LAlH 2 (1) (L = (ArNCMe) 2 CH, Ar = 2,6-i-Pr 2 C 6 H 4 ) with n-BuLi and n-Bu 2 Mg, respectively, resulted in di-n-butyl aluminum complex LAl(n-Bu) 2 (2). Meanwhile 1 reacted with MeLi producing dimethyl aluminum LAlMe 2 (3). Complexes 2 and 3 were characterized by NMR and elemental analysis, and 2 was additionally studied by single-crystal X-ray diffraction. The reported synthesis allows for access of dialkylaluminum complexes from aluminum-(III) dihydride compound (1) with excellent yields. The chemoselective double hydroboration of nitriles or single hydroboration of carbodiimides was studied with pinacolborane (HBpin) using LAl(n-Bu) 2 (2) as the precatalyst to afford bis(boryl)amines (4a−4m) and N-borylamines (6a− 6d) under solvent-free and mild conditions (60 °C). The records show that the catalytic reactions proceed in quantitative yields and short time. Moreover, nearly quantitative yield of di(boryl)(silyl)amine ( 5) was obtained, when cyanobenzene was reacted with equal amounts of HBpin and phenylsilane in the presence of precatalsyt 2. A comparison of the previously reported hydroboration reactions with nitriles in the literature and our results in the presence of the precatalyst LAl(n-Bu) 2 show convincingly the better results of short reaction time, mild reaction conditions, and quantitative yields.

Catalytic Nitrile Hydroboration: A Route to N,N‐Diborylamines and Uses Thereof

“…Finally, efficient transition-metal-free dihydroboration of nitriles has been achieved with main group element catalysts based on Li, [40] Davit Hayrapetyan was born and raised in Armenia. Mg, [41,42] B, [43] Al [44][45][46][47][48] and Zn [49] systems. All these reports are discussed below with an emphasis on advantages and limitations of hydroboration methods, experimental conditions, substrate scope and functional group tolerance.…”

“…[56,57] The majority of main group metal systems developed for hydroboration of nitriles concern aluminum catalysts. [44][45][46][47][48] Thus, the first example of such transformations was reported in 2018 by Cowley and Thomas et al, who employed LiAlH 4 as a precatalyst for a series of hydroborations of alkenes, alkynes and ketones with HBPin. Within the scope of these catalytic trials, the authors also reported hydroboration of 4-trifluoromethylbenzonitrile proceeded with full conversion within 6 h at room temperature using 1 mol% of LiAlH 4 .…”

“…In 2019, the research groups of Ma and Yang [48] and Roesky [44] have almost simultaneously reported hydroboration of nitriles catalyzed by very similar ( EtPh NacNac)AlH 2 (23) and ( Dipp NacNac)Al(Bu) 2 (24) complexes (Scheme 22). Operating under solvent-free conditions, both systems were found to efficiently convert a series of aliphatic and aromatic nitriles to the corresponding N,N-diborylamines with 2 equivalents of HBPin and 3-5 mol% catalyst loadings.…”

“…The proposed mechanisms for both 23 and 24-catalyzed reactions are similar and involve insertion of the nitrile into the AlÀ H bond of the catalytically active species to form the aluminum imide, which reacts with HBPin to give first aluminum N-borylamide and then N,N-diborylamine products. [44,48] Recently, Nembenna and co-workers reported the related Al (III) hydride 25 supported by conjugated bis-guanidine ligand and showed its catalytic activity in hydroboration of aliphatic and aromatic nitriles with HBPin (Scheme 23). [46] Similar to the NacNac system 24, the reactions were performed at 60°C under neat conditions with 3 mol% catalyst loading.…”

Hydroboration of Aldehydes, Ketones, and Carbodiimides Promoted by Mono(imidazolin‐2‐iminato) Hafnium Complexes

“…Carbonyl hydroboration catalysts include alkaline earth metals,, transition metals, group 13 metal catalysts, lanthanide, actinides, and other unique and sophisticated systems . In addition to the hydroboration of C=O bonds, the selective monohydroboration of carbodiimides to produce amidinates is also of great interest, since these systems play an important role in coordination chemistry …”