Aluminum-Catalyzed Hydroboration of Alkenes

Abstract

The aluminum-catalyzed hydroboration of alkenes with HBpin is reported using simple commercially available aluminum hydride precatalysts [LiAlH 4 or sodium bis(2-methoxyethoxy)aluminum hydride (Red-Al)]. Good substrate scope and functional group tolerance is demonstrated for alkene hydroboration, and the protocol was also applied to the hydroboration of ketone, ester and nitrile functional groups, showing the potential for wider application. The aluminum-catalyzed hydroboration is proposed to proceed by alkene hydroalumination which generates an alkyl aluminum species that undergoes s-bond metathesis with HBpin to drive turnover of the catalytic cycle.

Organoaluminum Compounds as Catalysts for Monohydroboration of Carbodiimides

“…The organoboron compounds derived from hydroboration reactions are crucial organic intermediates in various chemical transformations and material synthesis . In recent years, the ability of organoaluminum complexes to work as efficient catalysts for the hydroboration of unsaturated bonds with C=E (E=O, C, N), and C≡E motifs (E=C, N) was discovered. Intense work was invested in the hydroboration of unsaturated bonds through the use of aluminum‐containing pre‐catalysts.…”

LiAlH4: From Stoichiometric Reduction to Imine Hydrogenation Catalysis

“…[23] Also the last s-bond metathesis step in the cycle seemed questionable.Acomprehensive calculational study, however, demonstrated that the metal hydride mechanism shown in Scheme 1isfeasible. [15,18,[31][32][33][34][35][36][37][38][39][40][41][42] Especially noteworthy are two very recent investigations on LiAlH 4 -catalyzed alkene and aldehyde/ketone hydroboration. Acrucial step is the hydrogenolysis of the metal À Cbond by H 2 ,for which the rate is known to decrease with decreasing bond ionicity:N a À C > Mg À C > Al À C. [25] since it is well-known that RNH 2 and R 2 NH react smoothly with LiAlH 4 to give aluminium amide products and H 2 , [26][27][28][29][30][31] hydrogenolysis of AlÀNbonds to give amines is anticipated to be even more cumbersome.S ubstitution of H 2 for polar borane or silane reductants was therefore al ogical step.…”

“…[21] Given the simplicity of these group 2m etal catalysts,w e wondered whether LiAlH 4 ,w hich in imine reduction is normally added stoichiometrically,c ould be catalytically active.A lready in the 1960s,L iAlH 4 was used as ac atalyst for alkyne reduction, [24] however, the very harsh conditions needed (35 mol %c at, 190 8 8C, 80 bar H 2 )d id not encourage follow-up research. [31,38] Aluminium-catalyzed hydrogenations using the simple reductant H 2 ,h owever, are limited to highly Lewis-acidic tricoordinate Al catalysts, [43] often using high temperatures and H 2 pressures. Indeed, hydroboration and hydrosilylation reactions were found to be more promising.…”

Comparing Neutral (Monometallic) and Anionic (Bimetallic) Aluminum Complexes in Hydroboration Catalysis: Influences of Lithium Cooperation and Ligand Set

“…[8] Further, the catalytic chemistry of LiAlH 4 has recently been explored by Cowley, Thomas and Bismuto in the challenging hydroboration of alkenes, however the role of the alkali metal was not elaborated. [9] Thus, the current state of the field dictates that a systematic analysis of the secondary metal cooperative effects and various ligand factors that Figure 1. Al complexes 1-6 assessed in this study: ates 1-3; neutral 4-6. contribute to efficient hydroboration, is required in order to establish empirical rules for a posteriori design of future catalysts.…”