Brown s chloroborane

The stereoselectivities of the carbonyl group reductions with Alpine-Borane (Figure 10.24) or with Brown s chloroborane (Figure 10.25) are explained as shown in the for-... 

Fig. 10.25. Asymmetric carbonyl group reduction with diisopinocampheylchloroborane [Brown s chloroborane, (IPC)2BCL]. Concerning the reduction product depicted in the top row, the designation 5 of the configuration relates to the aryl-substituted and R to the Rttrt-substituted propargylic alcohol.

Fig. 8.20. Asymmetric carbonyl group reduction with diisopinocampheylchloro-borane [Brown s chloroborane, (IPC)2BC1].

There is a second effect. In the transition state in which the stronger Lewis acid complexes the carbonyl oxygen, the carbonyl group is a better electrophile. Therefore, it becomes a better hydride acceptor for Brown s chloroborane than in the hydride transfer from Alpine-Borane. Reductions with Alpine-Borane can actually be so slow that decomposition of this reagent into a-pincnc and 9-BBN takes place as a competing side reaction. The presence of this 9-BBN is problematic because it reduces the carbonyl compound competitively and of course without enantiocontrol. 

Monodiloroborane in ether added dropwise at 0° to a soln. of 3-hexyne in the same solvent, and stirred 2 hrs. under Ng -> bis-(c/5 -3-hexenyl)chloroborane (Y 88% after distillation), without prior distillation dissolved in tetrahydrofuran, and aq. NaOH added at 0° followed by dropwise addition of a soln. of iodine in tetrahydrofuran until a slight color of iodine persists -> c/5 ,tranM,5-diethyl-3,5-octadiene (Y 83%). F. e. and reactions s. H. C. Brown and N. Ravindran, J. Org. Chem. 38, 1617 (1973). 

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