Additives in Acyclic Systems

The previous discussion focused on the functional group changes that are possible when hydride reducing agents are used and, when appropriate, their regiochemical preferences. A major aspect of hydride reduction is the stereoehemistry of the reduction, as introduced in Table 4.5. A simple example is 2-pentanone, which is reduced to 2-pentanol. The carbonyl is a prochiral center (sec. 1.4.E) since addition of hydride from the two 

Chiral boranes can be used as reducing agents for carbonyl compounds. 30 Brown prepared a variety of chiral boranes in connection with extensive studies of hydroboration reactions (sec. 5.4.B). Reaction of diborane and a-pinene, for example, gave (-)-( / ,25,3/ ,5/ )-diisopinocampheylborane (206), which reduced carbonyl derivatives to alcohols with high asymmetric induction.33 Trialkylboranes are also capable of 

With all reagents generated by addition of chiral materials to L1A1H4, their exact structure must be determined before facial and orientation preferences can be determined for a given substrate. As illustrated by Alpine borane , the mechanism of hydride transfer must also be known. When these parameters are known, careful examination of models (Dreiding models or computer generated modeling) should determine the actual steric interactions. A casual inspection is usually insufficient for accurate predictions however. 

Morrison and Mosher collected data for asymmetric induction in the reduction of several acyclic ketones and this monograph is an excellent source when one must choose an asymmetric reducing agent.236 xhe use 

Ipc2B-Cl 224. Binal-H A. LiAlH4-MEP-NEAa 223. LiAlH4-aminobutanol 

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