Camphor dissolving metals

Electrochemical reduction of camphor-and norcamphoroxime at a Hg cathode proceeds with a high degree of stereoselectivity to give products of opposite stereochemistry to those formed in the dissolving metal (Na-alcohol) reduction of the oximes. The electrolyses are proposed to proceed by a kinetically controlled attack by the electrode on each oxime from the less hindered side (Fig. 62) [348]. In contrast, the corresponding N-phenyl imines yield products of the same stereochemistry as those isolated from a dissolving metal reduction. Cyclic voltammetry and polarographic data point to RH and intermediates in this case that are proto-nated from the least hindered side [349]. 

Electrochemical reductions of camphor oxime (R = Me) and norcamphor oxime (R = H) at a mercury cathode proceed with a high degree of diastereoselectivity (equation 3) . The products are in fact of opposite stereochemistry to those formed in dissolving metal (sodium-ethanol) reductions of the oximes. 

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). 

The dissolving metal reductions of bicyclo[2.2. l]heptanones have been studied extensively, and it has been established that both metal-alcohol and metal-NH -proton donor systems provide the cndr>-alcohol regardless of its stability relative to the exo isomer. " - In the case of camphor (1) which has been studied in the most detail, the ratio of endo-alcohol (bomeol 2) to CAo-alcohoI (isobomeol 3) is very close to 90 10 for all metal-NH3 conditions employed. The variables include temperature (-33 and -78 °C), cosolvent (ether and THF), metal (Li, Na, K, Rb) and proton donor (NH4CI and ethanol). - The same results are obtained with both (+)- and (+)-camphor. These results are, coincidentally, almost identical to the equilibrium ratio for alcohols (2) and (3). ... 

The reduction of bicycloheptanones, particularly camphor (1) in the absence of added proton donors has been studied extensively in connection with the mechanism of the dissolving metal reductions and are discussed in Section 1.4.2.2. As previously noted, reductions under these conditions are of considerably less utility in synthesis than those carried out in the presence of a relatively acidic proton donor (NH4CI or ethanol). 

Lemery defined precipitation as an expression chemists used to describe the fall of a body which had been suspended dissolved in a liquid from which it has been subsequently disunited. Although Fontenelle construed this as a physical definition based on the principles of hydrostatics, Lemery used it to differentiate true metallic precipitates, or the products of displacement reactions, from false ones. One could obtain false precipitates, or the matters that lost their initial metallic form and were reduced to a friable and indissoluble mass, in several ways. Calcination (red and violet mercury), incomplete dissolution in acids (antimony in spirit of salt or in regal water), and calcination after dissolution and evaporation (mercury in spirit of niter), all produced such precipitates. True metallic precipitates differed from false ones in that they were directly separated from their dissolution in liquid. As Lemery put it, false precipitates were abandoned by the liquid, while true precipitates abandoned the liquid themselves. True precipitates were made sometimes naturally through agitation, but mostly with recourse to the intermediates such as alkali salts or other metals. The choice of intermediates depended on the nature of the bodies to be precipitated. Lemery provided an exhaustive discussion for each case. In order to precipitate a resinous matter dissolved in spirit of wine, one could use common water which, by meshing intimately with the spirit, would precipitate the resinous matter. Camphor in spirit of wine could thus be... 

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