Direct Nucleophilic SN1-Type Reactions of Alcohols

In 2005, the ACS Green Chemistry Institute (GCI) and the global pharmaceutical corporations developed the ACS GCI Pharmaceutical Roundtable to encourage the development of green chemistry and green engineering in the pharmaceutical industry. The Roundtable has established a list of key research areas including the direct nucleophilic reactions of alcohols. The substitution of activated alcohols is a frequently used approach for the preparation of active pharmaceutical ingredients. Alcohols are transformed into the reactive halides or sulfonate esters, thereby allowing their reaction with nucleophiles. Although the direct nucleophilic substitution of an alcohol should be an attractive process, as one of the byproducts from the reaction yields water, hydroxide is a poor leaving group that hinders the reaction. Recently, the direct substitution of allylic, benzylic, and tertiary alcohols has been achieved through an S(N)1 reaction with catalytic amounts of Bronsted or Lewis acids. In this review, the approaches leading to a greener process are examined in detail, and the advances achieved to date in this important transformation are presented.