Regioselectivity on Generation of Boron Enolates

Subsequent aldol reactions of boron enolates vith aldehydes proceed vithout loss of regiochemical integrity, but the reactions vith ketones proceed slo vly and the regiochemical integrity of an aldol product does not reflect that of an enolate. 

A small amount of trifluoromethanesulfonic acid (1.0 g) was added to tri-butylborane (15.16 g, 83.3 mmol) at room temperature under argon. The mixture was stirred and warmed to 50 °C until evolution of butane began (there is an induction period). After cooling of the mixture to 25 °C the remaining trifluoromethanesulfonic acid (11.51 g, total 83.3 mmol) was added dropwise at such a rate as to maintain a temperature between 25 and 50 °C. 

The mixture was then stirred for further 3 h at 25 °C. Distillation under reduced pressure gave pure dibutylboryl triflate 8 (19.15 g, 84% bp 60 °C/ 

Trifluoromethanesulfonic acid (18.75 g, 125 mmol) was added to 9-BBN (15.33 g, 127 mmol) in hexane (100 mL) under argon. After overnight stirring the reaction mixture was concentrated and distilled in vacuo to afford 9-BBNOTf 19 (38 °C/0.03 mmHg, 28.84 g, 85%). 

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