2.1.5.3 Zinc Borohydride [Zn(BH4)2]
The zinc borohydride is prepared from zinc chloride and NaBH4 in ether. It is most useful for the reduction of α,β-unsaturated aldehydes and ketones to give saturated alcohols (Scheme 7).
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Scheme 7 |
2.1.5.4 Lithium or Potassium Trialkylborohydride
The presence of three alkyl groups in boron atom makes the hydride strongly nucleophile and hence it is a strong reducing agent compared to other borohydrides, and it can reduce the aldehydes and ketones even at -78°C temperature.The commonly used trilakylborohydrides are lithium triethylborohydride, Li(Et3BH), and lithium and potassium tri-sec-butylborohydride (L-and K-electrides). The bulky L- and K-selectrides can reduce carbonyl compound from less hindered side resulting selective products (Scheme 8).
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Scheme 8 |
J. L. Hubbard, Encyclopedia of Reagents for Organic Synthesis, John Wiley and Sons, Inc., L. A. Pauette, Ed., New York, 1995, 5, 3172. |
Examples:
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W. Oppolzer, C. Chapuis,G. Bernardinelli, Tetrahedron Lett. 1984, 25, 5885. |
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E. Piers, M. A. Romero, Tetrahedron 1993, 49, 5791. |
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J. Nokami, M. Ohkura, Y. Dan-Oh, Y. Sakamoto, Tetrahedron Lett. 1991, 32, 2400. |