The addition of a B–H bond to an unsaturated bond (polarized or unpolarized) is a powerful and atom-economic tool for the synthesis of organoboranes. In recent years, s-block organometallics have appeared as alternative catalysts to transition-metal complexes, which traditionally catalyze the hydroboration of unsaturated bonds. Because of the recent and rapid development in the field of hydroboration of unsaturated bonds catalyzed by alkali (Li, Na, K) and alkaline earth (Mg, Ca, Sr, Ba) metals, we provide a detailed and updated comprehensive review that covers the synthesis, reactivity, and application of s-block metal catalysts in the hydroboration of polarized as well as unsaturated carbon–carbon bonds. Moreover, we describe the main reaction mechanisms, providing valuable insight into the reactivity of the s-block metal catalysts. Finally, we compare these s-block metal complexes with other redox-neutral catalytic systems based on p-block metals including aluminum complexes and f-block metal complexes of lanthanides and early actinides. In this review, we aim to provide a comprehensive, authoritative, and critical assessment of the state of the art within this highly interesting research area.

https://pubs.acs.org/doi/pdf/10.1021/acs.chemrev.1c00641

s-Block Metal Catalysts for the Hydroboration of Unsaturated Bonds

Magnesium hydridotriphenylborate [Mg(thf)₆][HBPh₃]₂: a versatile hydroboration catalyst

. Hydroboration of alkynes is of special interest to researchers since it is the most straightforward process for the synthesis of highly important vinyl borane synthetic intermediate compounds. It is significant in terms of both regioselectivity and stereoselectivity, since several positional isomers are formed during the process of hydroboration. Given the importance of this class of compounds, an extensive study has been carried out regarding catalytic condition development and detailed mechanistic studies for selectively obtaining a single isomer. This elaborate review comprehensively summarizes both the regioselective and stereoselective catalytic hydroboration of alkynes. It is aimed at giving readers accurate knowledge of the catalytic systems available for the hydroboration of certain alkynes. We anticipate that this inclusive review will encourage researchers to explore more new types of catalytic systems to achieve the remarkable process of hydroboration of alkynes.

Overview of Regioselective and Stereoselective Catalytic Hydroboration of Alkynes

Catalytic reduction of nitriles is considered as an attractive and atom-economical route to a diversity of synthetically valuable primary amines Compared to other methods, dihydroboration approach has been developed relatively recently but has already attracted the attention of many research groups due to reasonably mild reaction conditions, selectivity control and the access to N,N-diborylamines, which emerged as powerful reagents for C-N bond forming reactions Early developments in catalytic dihydroboration of nitriles implied precious metal catalysts along with harsh conditions and prolonged reaction times, whereas recent advances mostly rely on base and main group metal catalytic systems with significantly improved profiles This minireview aims to provide an overview of advances and challenges of dihydroboration of nitriles with d-, f- and main group metal catalysts Mechanistic features of different catalytic systems, functional group tolerance and scope of the methods are also presented The synthetic utility of N,N-diborylamies, beyond simple protodeborylation, is discussed in the aspect of N-arylation, imine and amide synthesis

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

Three new dimeric bis-guanidinate zinc(II) alkyl, halide, and hydride complexes [LZnEt]2 (1), [LZnI]2 (2) and [LZnH]2 (3) were prepared. Compound 3 was successfully employed for the hydrosilylation and hydroboration of a vast number of ketones. The catalytic performance of 3 in the hydroboration of acetophenone exhibits a turnover frequency, reaching up to 5800 h-1, outperforming that of reported zinc hydride catalysts. Notably, both intra- and intermolecular chemoselective hydrosilylation and hydroboration reactions have been investigated.

Zinc Hydride-Catalyzed Hydrofuntionalization of Ketones

The reaction of LH [L = {(ArNH)(ArN)-C=N-C=(NAr)(NHAr)}; Ar =2,6-Et2-C6H3] with a commercially available alane amine adduct (H3Al·NMe2Et) in toluene resulted in the formation of a conjugated bis-guanidinate (CBG)-supported aluminum dihydride complex, i.e., LAlH2 (1), in good yield. The new complex has been thoroughly characterized by multinuclear magnetic resonance, IR, mass, and elemental analyses, including single-crystal structural studies. Further, we have demonstrated the aluminum-catalyzed hydroboration of a variety of nitriles and alkynes. Moreover, aluminum-catalyzed hydroboration is expanded to more challenging substrates such as alkene, pyridine, imine, carbodiimide, and isocyanides. More importantly, we have shown that the aluminum dihydride catalyzed both intra- and intermolecular chemoselective hydroboration of nitriles and alkynes over other reducible functionalities for the first time.

Aluminum-Catalyzed Selective Hydroboration of Nitriles and Alkynes: A Multifunctional Catalyst.

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Aluminum-Catalyzed Selective Hydroboration of Nitriles and Alkynes: A Multifunctional Catalyst.