Camphor reductive coupling

Dissolving metal reduction of camphor produces a mixture of bor-neol, isobomeol, and pinacol coupling products (Scheme 39). The ratios of the stereoisomers are affected profoundly by whether the starting ketone is enantiomerically pure or racemic, implying the chirality recognition at the stage of ketyl radical (65). 

Wynberg studied stereochemistry of the McMurry reductive dimerization of camphor in detail (64). In Scheme 37, A and B are homochiral dimerization products derived by the low-valence Ti-promoted reduction, while C and D are achiral heterochiral dimers. The reaction of racemic camphor prefers homochiral dimerization (total 64.9%) over the diastereomeric heterochiral coupling (total 35.1 %). Similarly, as illustrated in Scheme 38, oxidative dimerization of the chiral phenol A can afford the chiral dimers B and C (and the enantiomers) or the meso dimer D. In fact, a significant difference is seen in diastereoselectivity between the enaritiomerically pure and racemic phenol as starting materials. The enantiomerically pure S substrate produces (S,S)-B exclusively, while the dimerization of the racemic substrate is not stereoselective. In the latter case, some indirect enantiomer effect assists the production of C, which is absent in the former reaction. Thus, it appears that, even though the reagents and reaction conditions are identical, the chirality of the substrate profoundly affects the stability of the transition state. 

S-shaped current potential curves emerge when a surface phase transition of an organic adsorbate is coupled with a faradaic reaction of some electroactive species. As a representative of such a system, the periodate reduction on Au(lll) single crystal electrodes in the presence of camphor was studied [160], Camphor adsorbed on Au(lll) electrodes exhibits two first-order phase transitions upon variation of the electrode potential [161]. In a cyclic voltammogram, the phase transition manifests itself in a pair of needle-like peaks (Fig. 28 (A)). Between the peak pairs, a condensed, well-ordered camphor film exists. At more negative potentials, the camphor coverage is low, while the state of the adsorbate at positive potentials beyond the second phase transition is not yet known. The small hystereses between the respective anodic and cathodic peaks are caused by the finite nucleation rate of the respective thermodynamically stable phase. 

It was recommended that alkali-metal-NH3 reduction of ketones should be conducted in the presence of NH4+ this follows from the finding that reduction of [3,3-2H2]camphor and quenching in the absence of the ion gave epimeric alcohols with one or two atoms of tracer and complex products formed by disproportionation, abstraction of H from the medium, and pinacolic coupling. In the presence of NH4C1, exclusively [3,3-2H2]-alcohols were formed with Li, Na, or K. It was concluded that a mechanism proposed by House (1972) predominated when NH4+ was the proton source.120... 

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