Semipinacol rearrangements mechanisms

A new semipinacol rearrangement mediated by Sn(IV) was proposed by Bates and to explain the formation of 579 from 578 (equation 256). As stated by the authors, the mechanism of formation of 579 most likely involves an intermediate hydroxylamine 580 (equation 257). Nucleophilic addition of the hydroxylamine to the ketonic carbonyl leads to 581, which may undergo a tin-mediated pinacol-type rearrangement with preferred migration of the phenyl substituent to produce amide 582. 

Fig. 14.18. Mechanism-based diastereoselectivity in the semipinacol rearrangement of an epoxide. This rearrangement is stereoselective, since there is only one H atom in the position next to the sextet center and the H atom undergoes the [l,2]-migration on the same face of the five-mem bered ring.

The mechanism of this benzilic acid rearrangement starts with attack of hydroxide on one of the carbonyl groups. The tetrahedral intermediate can collapse in a reaction reminiscent of a semipinacol rearrangement, carbonyl group is formed here... 

The rearrangement of acetals of 2-haloalkyl aryl ketones is a well-documented process yielding esters of 2-arylalkanoic acids by 1,2-aryl shift (equation 7). The mechanism of this rearrangement is reminiscent of other semipinacol rearrangements. Loss of the halogen (usually assisted by Lewis acid), yields a carbocation (4), which then undergoes a 1,2-aryl shift with carbonyl group formation. 

I Interactive mechanism for semipinacol rearrangements of diazonium salts... 

Scheme 1.9 General features and possible mechanisms of the semipinacol rearrangement.

The proposed mechanism for this transformation is closely related to that depicted in Scheme 40.47 above, except that the intermediate iminium ion derived from the primary amine catalyst 32 is epoxidated stereoselectively [60] and, after hydrolysis of the iminium, an acid-induced diastereoselective semipinacol rearrangement affords the spirocychc hydroxy-diketone product (Scheme 40.53). 

Subsequently, organocatalytic asymmetric halogenation/semipinacol rearrangement sequences, both leading to chiral quaternary haloketones, have been reported by Tu and coworkers [61] and by Hermecke and coworkers [62]. The mechanism of these reactions probably involves an acid-catalyzed diastereoselective semipina-col rearrangement of a chiral halonium ion intermediate obtained by chiral tertiary amine-promoted asymmetric halogenation of an enone [63]. 

---