Chemoselective Reductions with Sodium Borohydride. Aldehydes vs. Ketones

References

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• The use of NaBH4 for selective reduction of aldehydes in the presence of ketones has been reported (ref. 1c) in 2 examples. The method uses 1 eq. of hydride in ethanol at room temperature. This method gives poor selectivity (70%) with 4-methylcyclohexanone/hexanal
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• Selectivity as high as 90% has been reported in few cases (ref 1i,1k,1s,1z,1bb)
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• The use of undistilled acetaldehyde results in much poorer selectivity presumably due to the presence of acetic acid. The presence of acetic acid dramatically increases the rate of reduction (for an example see ref. 3)
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• The amount of alcohol in the solvent is directly related to the rate of reduction. Increased amounts gave poorer selectivity while reduced amounts gave poorer conversion (during 1h)
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• This suggests the possibility of selective reduction. Indeed, hexanal is reduced in the presence of benzaldehyde in 50% iPrOH in CH2Cl2 at -78° C for 1 h (selectivity-85%)
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• Excellent selectivity has been achieved in this case using highly hindered reagents1k, 1s
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