The regioselectivity of hydroboration of 1-phenyl-1-propyne with HBBr₂-SMe₂ (dibromoborane/dimethylsulfide) was determined by oxidizing the intermediate alkenylborane to the corresponding carbonyl compounds with basic hydrogen peroxide.

By ¹H NMR Analysis:

To a 25°C solution containing 0.62mL (0.575g, 4.95mmol) of CH₂Cl₂ in 2.0mL of 1-phenyl-1-propyne and 0.26mL (0.329g, 3.87mmol) of CH₂Cl₂ in 2.0mL of CDCl₃ was added 0.66mL (5.0mmol) of HBBR₂-SMe₂. After being stirred at RT for 3 hours, the reaction was cooled to 0°C and quenched by adding 5.0mL of 3N NaOH. After warming to RT and equilibrating, the mixture was oxidized by the dropwise addition of 2.5mL of 30% H₂O₂. The reaction was stirred at 25°C for 6 hours to ensure complete oxidation. The aqueous layer was saturated with K₂CO₃ and the organic layer separated. After being dried over anhydrous Na₂SO₄, the clear organic layer was analyzed by ¹H NMR. The benzylic protons of phenylacetone (3.67) were integrated against the methyl triplet of propiophenone vs. the CH₂Cl₂. After normalization of the number of protons contributing to each resonance, there was found 1.74mmol (35%) of phenylacetone and 3.01mmol (61%) of propiophenone. Hence, 64% of the boron had been placed alpha to the phenyl group of 1-phenyl-1propyne with 36% beta to the phenyl.

By GC Analysis:

To a 25°C solution of 0.62mL of 1-phenyl-1-propyne (0.581g, 5.00mmol) and 0.80mL of n-undecane (0.605g, 3.87mmol) in 2.0mL of CH₂Cl₂ was added 0.64mL (5.0mmol) of HBBr₂SMe₂ dissolved in 3.3mL of CH₂Cl₂. After 3 hours at 25°C, the reaction as cooled to 0°C, quenched with 5mL of 3N NaOH, and oxidized by adding 2.5mL of 30% H₂O₂. The aqueous layer was saturated with K₂CO₃ and the organic layer separated. Ethanol was added (2.0mL) followed by an excess of NaBH₄. After the mixture was stirred at 25°C for 5 hours, the excess NaBH₄ was quenched with dilute HCl and the aqueous layer saturated with NaCl. [red]After this mixture was dried over K₂CO₃ and analyzed by GC on a 10% carbowax 20M (on Chromosorb W; 6ftx1/4in.) column, there was obtained 2.88mmol (58%) of 1-phenyl-1-propanol and 1.63mmol (33%) of 1-phenyl-2-propanol. This corresponds to a regioselectivity identical with that obtained above by ¹H NMR analysis.