Asymmetric Hydroboration and Hydrosilylation: Chiral Metal Catalysts for Boron and Silicon Installation
Asymmetric hydroboration and hydrosilylation install boron and silicon, respectively, across a C=C or C=O bond with enantiocontrol. The resulting chiral organoboranes and organosilanes are versatile intermediates: organoboranes are oxidized to chiral alcohols or used in Suzuki coupling, while organosilanes undergo Tamao–Fleming oxidation or cross-coupling. These reactions are powerful because they convert abundant feedstock olefins and ketones into high-value chiral building blocks.
Asymmetric Hydroboration
Rhodium-Catalyzed
Rh(I) complexes with chiral diphosphines or phosphoramidites catalyze hydroboration of styrenes and vinylarenes with catecholborane (HBcat) or pinacolborane (HBpin). The regioselectivity (branched vs linear) and enantioselectivity are both ligand-controlled. Chiral phosphoramidites (MonoPhos family) are particularly effective.
Copper-Catalyzed
Cu(I)-phosphoramidite complexes, generated in situ from CuCl and ligand with an organoboron reductant, catalyze asymmetric hydroboration of β-substituted styrenes, 1,1-disubstituted alkenes, and internal alkenes — substrate classes where Rh underperforms. The Cu system is also effective for hydroboration of ketones to give chiral boronic esters.
Asymmetric Hydrosilylation
Rhodium and Titanium-Catalyzed Ketone Hydrosilylation
Rh(I)-BINAP and Ti-TADDOLate complexes catalyze hydrosilylation of aryl ketones with diphenylsilane or trichlorosilane, giving chiral silyl ethers that hydrolyze to chiral alcohols. The Ti-TADDOLate system is notable for its low cost and excellent functional group tolerance.
Copper-Catalyzed Hydrosilylation
Cu(I)-phosphoramidite complexes catalyze enantioselective hydrosilylation of ketones and imines under mild conditions with inexpensive silanes (PMHS, TMDS). The Cu system is among the most practical for large-scale application.
Recommended Products
- (S)-MonoPhos — phosphoramidite for Rh- and Cu-catalyzed hydroboration
- (R)-BINOL / (S)-BINOL — backbone for phosphoramidite library
- (S)-TADDOL / (R)-TADDOL — diol for Ti-catalyzed hydrosilylation
- (S)-BINAP / (R)-BINAP — diphosphine for Rh-catalyzed hydrosilylation
- (S)-t-Bu-BOX / (R)-t-Bu-BOX — bisoxazoline for Cu-catalyzed hydrosilylation
Why These Reactions Matter
- Versatile products — organoboranes and organosilanes are intermediates for many downstream transformations.
- Mild conditions — often room temperature, no high-pressure H₂.
- Earth-abundant metals — Cu and Ti alternatives to Rh.
- Complementary to hydrogenation — different substrate scope and functional group tolerance.
Frontier Developments
Recent advances include: (1) enantioselective hydroboration of unactivated internal alkenes (long a challenge), (2) Cu-catalyzed hydroboration with cheaper diboron reagents (B₂pin₂), (3) photocatalytic asymmetric hydroboration under visible light, and (4) asymmetric hydroboration of imines to chiral aminoboranes. The maturation of Cu-catalyzed variants is pushing hydroboration and hydrosilylation into mainstream process chemistry.