ABSTRACT
In the SN2 substitution, the reaction invokes a second-order process, with inversion of the configuration of the chiral center in a secondary leaving group (Fig. 2). In allylic and propargylic compounds the SN2' reaction in addition to undergoing nucleophilic substitution takes place with a rearrangement of a double bond (Fig. 3). This mechanism is a second-order allylic rearrangement, and occurs at the y-position when a-substi tution is sterically more demanding. This process involves the simultaneous movement of three pairs of electrons. However, some authors have brought evidences for and against this proposal [2] . The stereochemistry of the SN2' substitution is rather complex, but in an acylic system the reaction is stereospecific and the nucleophile enters syn to the leaving group. A frontier orbital analysis has been proposed to rationalize this process [3,4] . Nevertheless, anti-reactions can take place, depending on the nature of the nucleophile and leaving group.
