Roush Asymmetric Allylation
The Roush asymmetric allylation allows the enantioselective synthesis of homoallylic alcohols from the reaction of allylboronates with aldehydes.
General features:
1. The preparation of allylboronates may be achieved by esterification of allylboronic acid or by transesterification of triisopropylallylboronate. 2. The reaction is typically carried out in toluene, in the presence of 4Å molecular sieves at low temperatures: -78 °C. 3. Selecting the proper boronate gives access to one enantiomer of the homoallylic alcohol product or the other. 4. (E)- crotylboronate derivatives lead to the formation of the anti-diastereomer, while (Z)- crotylboronates give the syn product. Also, (E)-crotylboronates usually exhibit higher enantioselectivities than (Z)- crotylboronates.
Roush Asymmetric Allylation
Reaction Mechanism
Experimental Procedure
To neat D-(–)-bis(2,4-dimethyl-3-pentyl) tartrate (4.22 mmol) was added (2-bromallyl)diisopropoxyborane (2.81 mmol) at 22 °C, and the mixture was stirred for 1 h before it was warmed to 60 °C and stirred under vacuum for 1 h. The resultant mixture was cooled to 22 °C, and toluene (5.6 mL), pentane (1.4 mL), and 4 Å molecular sieves (600 mg) were sequentially added. The mixture was stirred for 5 min and then cooled to −95 °C before a solution of the aldehyde (0.703 mmol) in toluene/pentane (2.0 mL, 3:1) was added. The resultant mixture was stirred for 2 h before it was quenched with water and EtOAc. The mixture was extracted with EtOAc. The combined organic phases were washed with brine, dried over anhydrous Na2SO4, filtered, and concentrated under vacuum. The residue was subjected to flash column chromatography to give the homoallylic alcohol (88% yield).
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