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Stereochemistry of the fluorination of an alcohol with phenyltetrafluorophosphorane was investigated in the case of steroidal alcohols. 5α-Cholestan-3β-ol and 3α-ol gave 3α- and 3β-fluoro-5α-cholestane, respectively, which suggests that the fluorination proceeded through SN2 mechanism ; while, cholesterol and 3β-acetoxy-5-bromo-5α-cholestane-6β-ol gave fluoro compounds with retention of configuration, which shows that a neighboring group may have participated in stabilizing the carbonium intermediate in special cases. Diphenyltrifluorophosphorane gave similar results.
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Kobayashi et al. (1975) studied this question.
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