Regioselectivity of Hydroboration of Unsymmetrically Substituted Alkynes with HBBr2-SMe2
JOC 45, 389-395 (1980)
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.
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.
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.