Retropinacol reaction

As such, the thermal process in equation (60) proceeds via the same reactive intermediates (arising from an adiabatic electron transfer) as that observed in the photochemical processes in equations (57) and (58). The proposed electron-transfer activation for the thermal retropinacol reaction is further confirmed by the efficient cleavage of benzpinacol with tris-phenanthroline iron(III), which is a prototypical outer-sphere one-electron oxidant195 (equation 61). 

An useful alternative to the already known retropinacol reactions is presented by Liu and co-workers [7], This works demonstrates that pinacols bearing (dimethylamino)phenyl substiments can be subjected to fast oxidative fragmentation via photoinduced electron transfer with chloroform as the electron acceptor in yields up to 80%. The extremely fast dechlorination of the chloroform radical anion inhibits back-electron transfer and thus leads to effective fragmentation of the pinacol radical cation (Scheme 8). 

The best illustration of the high reactivity of the acyloxycarbenium ions 163 is a reaction which is not quite typical for other carboxonium ions. This is the retropinacol rearrangement of a-branched aldehydes and ketones 169 to the 1,2-dioles 173 which involves the unusual 1,2-shift of the migrating group R3 to the carbon atom attached to the oxygen atom of the acyloxycarbocation 17092-94 (equation 55). 

It should be noted that the 2-oxazolines are connected directly with enamides, being their products of cyclization on the one hand ", as well as being the heterocyclic precursors of W-acylenamines on the other . However, it is no less important that the 2-oxazolines are cyclic derivatives (and precursors also) of the 1,2-aminoalcohols. In essence, the reactions 117 122 (equation 41) and 125- 128 (equation 43) are the transformations of ketones to 1,2-aminoalcohols, i.e. they are the reversal of one of the pinacol rearrangement variants, i.e. the Tiffeneau and MacKenzie reactions . These transformations which proceed via the iV-acyliminium intermediates 118, 126 are examples of a real retropinacol rearrangement according to structural, functional and redox features (see Section III.B.3). 

This reaction is related to the Wagner-Meerwein Rearrangement, Demjanov Rearrangement, Tiffeneau-Demjanov Ring Expansion and Retropinacol Rearrangement involving carbocation intermediates. 

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