The Hidden Role of Boranes and Borohydrides in Hydroboration Catalysis

Abstract

The continued development of hydroboration catalysts typifies the importance of this transformation as a testbed for catalytic activity and as a fundamental reaction for organic synthesis. Catalytic hydroboration studies routinely investigated the decomposition of HBcat but in the case of HBpin, decomposition is not commonly investigated due to its perceived stability.Organoboranes catalyze the hydroboration of alkenes and alkynes; these species can be formed

Manganese‐Catalyzed Hydroborations with Broad Scope

“…The same interpretation can be derived from the unaltered reaction rate when adding trimethylphosphine (PMe 3 )—a strong ligand for reduced Mn species. No “hidden” BH 3 catalysis was operating ( 11 B NMR) [38] . Based on the collected synthetic, kinetic, and spectroscopic data, we postulate a catalytic mechanism that is initiated with the formation of [MnH(hmds)] x complexes from reaction of Mn(hmds) 2 and HBpin (Scheme 5, bottom).…”

Recyclable Copper Nanoparticles‐Catalyzed Hydroboration of Alkenes and β‐Borylation of α,β‐Unsaturated Carbonyl Compounds with Bis(Pinacolato)Diboron

“…When the hydroboration of 1 a was performed with HBpin (pinacolborane) under the standard conditions, no desire product was observed, demonstrating that the reaction did not involve the formation of HBpin intermediate (see SI for details) [6j] . Further the stoichiometric reaction mixture was monitored by 11 B NMR spectroscopy, there were no resonance of any boron‐containing species other than the product and B 2 pin 2 , probably due to the paramagnetic nature of Cu(II).…”

“…Switching the catalyst to nano‐Fe 3 O 4 or nano‐Fe 3 O 4 ‐DOPA gave lower yield of 1 b (entries 6 and 7). The reaction of 1 a with B 2 pin 2 in DMF in the absence of a catalyst gave trace amount of desire product (entry 8) [6j] . Next, several bases were screened; LiO t Bu, NaO t Bu, KOMe, and NaOMe displayed poor reactivities (entries 9–12).…”

Imine Reduction with Me2S-BH3

“…This anion then displaces Me2S from Me2S-BH3 to generate the catalytically active species C, which acts as a hydride donor to another imine completing the cycle while also yielding reduced species D. As mentioned in the introduction, catalytic hydroboration of unsaturated C=X fragments (X=O, N, etc., but X =C) has been a controversial topic. However, there is a significant body of evidence suggesting that four-coordinate B-H containing compound(s) (usually anionic), generated by activation, decomposition, or redistribution of boron reagents, act as initial hydride donors and hence as initiators of catalytically active species [11,12]. Clark and co-workers suggested a mechanism (Scheme 2a) that involved "activation" of HBpin (or HBcat) by coordination of a nucleophile (the electrophilic step), followed by hydride transfer (the nucleophilic step) from boron to the substrate (e.g., aldehyde), to yield the corresponding anion (e.g., alkoxide) [46].…”

“…Several of the described hydroboration reactions were efficient as catalyst loadings as low as 0.001 mol%, resulting in excellent substrate conversions [7]. Nevertheless, the exact role of these presumed (pre)catalytic species has been divisive, as several reports provided convincing evidence for the existence of hidden boron catalysis (HBC), i.e., the main role of the species that were introduced in "catalytic" amounts was the formation, via activation, redistribution, or decomposition (Scheme 1) of HBcat/HBipn, of boron-based compounds (e.g., hydroborates and boranes) that then acted as the true catalysts [10][11][12]. hydroboration protocol(s) as certain BH 3 -containing species act as synthetic precursors to HBpin/HBcat [13,14].…”