关键词: 铜催化, 苯乙烯, 1,2-苯基异噁唑, 不对称硼胺化, β-硼酯伯胺

Organoboron compounds are important intermediates in organic synthesis because of their high utilities for C—C and C—X bond formations. Transition metal-catalyzed borylative difunctionalization of alkenes, which can simultaneously introduce C—B, C—C or C—X bonds, could directly construct highly functionalized organoboron in one step. Among these reactions, copper catalyzed enantioselective aminoboration of styrenes is an efficient approach to generate enantioriched β-aminoboronate which is a class of useful chiral compounds. In this work, employing styrenes as substrates, 1,2-benzisoxazole as an electrophilic primary amine source, bis(pinacolato)diboron (B₂pin₂) as boron source and LiOCH₃ as base, an enantioselective Cu-catalyzed aminoboration of styrenes by using a chiral sulfoxide-phosphine (SOP) ligand was developed, and a board range of chiral β-aminoalkylboranes, which could be readily converted to a class of valuable β-hydroxylalkylamines, were accessed with high yields and ee values. A general procedure for this aminoboration of styrenes is described in the following: in a glove box, CuI (0.05 mmol), chiral sulfoxide phosphine ligand L1 (0.06 mmol), and 2 mL of anhydrous tetrahydrofuran were added into a flame-dried tube. The resulting mixture was stirred at room temperature for 30 min. Then bis(pinacolato)diboron (B₂pin₂) (0.75 mmol), LiOCH₃ (1.25 mmol), styrene 1 (0.5 mmol), 1,2-benzisoxazole (0.75 mmol) and another 2 mL of THF were added into the reaction system in sequence. The reaction tube was removed out from the glove box and stirred at 20 ℃ for 12 h. After the reaction was finished, the NMR yield was firstly determined with dimethyl terephthalate (9.7 mg, 0.05 mmol) as internal standard, then, the crude product was recovered and purified with a preparative TLC which was alkalized with triethylamine to give the desired β-aminoboronates in moderate to good yields (47%~84%) and enantioselectivities (81%~99%). To demonstrate the utility of this reaction, β-boronate primary amine could be easily obtained by removing the Schiff base group of β-aminoboronate 3 under the methanol solution of hydroxylamine hydrochloride, which could be further oxidized to give corresponding chiral β-amino alcohol in moderate yield (48%).

Key words: Cu-catalyzed, styrenes, 1,2-benzisoxazole, enantioselective aminoboration, β-boronate primary amine