Magnesium bis(hydridotriphenylborate), isolated as a solvent-separated ion pair [Mg(thf)][HBPh], effectively catalyzed the hydroboration of several unsaturated substrates including CO.

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Light alkali metal hydridotriphenylborates M[HBPh3] (M = Li, Na, K), characterized as tris{2-(dimethylamino)ethyl}amine (L) complexes [(L)M][HBPh3], act as efficient catalysts for the chemoselective hydroboration of a wide range of aldehydes and ketones using pinacolborane HBpin. The lithium derivative showed a remarkably high TOF of ≥17 s(-1). These compounds also catalyze the hydroborative reduction of CO2 to give formoxyborane HCO2Bpin without any over-reduction.

The first example of a catalytic hydroboration of amides for their deoxygenation to amines is reported. This transformation employs an earth-abundant magnesium-based catalyst. Tertiary and secondary amides are reduced to amines at room temperature in the presence of pinacolborane (HBpin) and catalytic amounts of ToMMgMe (ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate). Catalyst initiation and speciation is complex in this system, as revealed by the effects of concentration and order of addition of the substrate and HBpin in the catalytic experiments. ToMMgH2Bpin, formed from ToMMgMe and HBpin, is ruled out as a possible catalytically relevant species by its reaction with N,N-dimethylbenzamide, which gives Me2NBpin and PhBpin through C-N and C-C bond cleavage pathways, respectively. In that reaction, the catalytic product benzyldimethylamine is formed in only low yield. Alternatively, the reaction of ToMMgMe and N,Ndimethylbenzamide slowly gives decomposition of ToMMgMe over 24 h, and this interaction is also ruled out as a catalytically relevant step. Together, these data suggest that catalytic activation of ToMMgMe requires both HBpin and amide, and ToMMgH2Bpin is not a catalytic intermediate. With information on catalyst activation in hand, tertiary amides are selectively reduced to amines in good yield when catalytic amounts of ToMMgMe are added to a mixture of amide and excess HBpin. In addition, secondary amides are reduced in the presence of 10 mol % ToMMgMe and 4 equiv of HBpin. Functional groups such as cyano, nitro, and azo remain intact under the mild reaction conditions. In addition, kinetic experiments and competition experiments indicate that B-H addition to amide C═O is fast, even faster than addition to ester C═O, and requires participation of the catalyst, whereas the turnover-limiting step of the catalyst is deoxygenation. ABSTRACT:The first example of a catalytic hydroboration of amides for their deoxygenation to amines is reported. This transformation employs an earth-abundant magnesium-based catalyst. Tertiary and secondary amides are reduced to amines at room temperature in the presence of pinacolborane (HBpin) and catalytic amounts of To M MgMe (To M = tris(4,4-dimethyl-2-oxazolinyl)phenylborate). Catalyst initiation and speciation is complex in this system, as revealed by the effects of concentration and order of addition of the substrate and HBpin in the catalytic experiments. To M MgH 2 Bpin, formed from To M MgMe and HBpin, is ruled out as a possible catalytically relevant species by its reaction with N,N-dimethylbenzamide, which gives Me 2 NBpin and PhBpin through C−N and C−C bond cleavage pathways, respectively. In that reaction, the catalytic product benzyldimethylamine is formed in only low yield. Alternatively, the reaction of To M MgMe and N,N-dimethylbenzamide slowly gives decomposition of To M

Solid magnesium hydride [MgH ] has been pursued as a potential hydrogen-storage material. Organic chemists were rather interested in soluble magnesium hydride reagents from mid-20th century. It was only in the last two decades that molecular magnesium hydride chemistry received a major boost from organometallic chemists with a series of structurally well-characterized examples that continues to build a whole new class of compounds. More than 40 such species have been isolated, ranging from mononuclear terminal hydrides to large hydride clusters with more than 10 magnesium atoms. They provide not only insights into the structure and bonding of Mg-H motifs, but also serve as models for hydrogen-storage materials. Some of them are also recognized to participate in catalytic transformations, such as hydroelementation. Herein, an overview of these molecular magnesium hydrides is given, focusing on their synthesis and structural characterization.