A past project in my research group involved the synthesis of chiral amino-alcoh

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A past project in my research group involved the synthesis of chiral amino-alcohols. One step in the sequence of reactions that we used to produce these compounds was the following reduction of a ketone to a chiral alcohol, in which we hoped to stereo-selectively obtain one stereoisomer of the alcohol over the other. When we used NaBH4 as the reducing agent, we obtained a 2.5:1 mixture of the two diastereomers. Explain why a racemic mixture (i.e. a 1:1 mixture) is not formed even though we are using an achiral reducing agent. When we used S-Alpine Borane (a chiral reducing agent similar to the CBS reagent) to form the alcohol, we obtained a 49:1 ratio of products. Explain the increased stereoselectivity as compared to that obtained with NaBR4. Predict the relative selectivity you would expect to see if we were to use LiA1H4 as the reducing agent (choices: less than 2.5:1, greater than 49:1, or something in between). Give a brief justification for your answer!

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