Abstract
AbstractReduction of (+)‐[3,3−2H2]camphor ([3,3−2H2]1) with lithium, sodium or potassium in ammonia and a co‐solvent gave; 1) the enolate of [3,3−2H2]1 and the alcoholates of (−)‐[2,3,3−2H3]isoborneol ([2,3,3−2H3] and (+)‐[2,3,3−2H3]borneol ([2,3,3−2H3]3); 2) the alcoholates of [3,3−2H2]2 and [3,3−2H2]3; 3) the dialcoholates of the pinacols [3,3,3′,3′−2H4]4 and [3,3,3′,3′−2H4]5. It is proposed that these are formed from the ketyls [3,3−2H2]1‐ M+, by: 1) disproportionation; 2) H‐atom abstraction from the medium; 3) dimerization. Protonation upon work‐up afforded [endo−32H]1, [2,3,3−2H3]2, [2,3,3−2H3]3,[3,3−2H2]2, [3,3−2H2]3, [3,3,3′,3′−2H4]4 and [3,3,3′,3′−2H4]5. Pinacol [3,3,3′,3′−2H4]5 was the main and pinacol [3,3,3′,3′−2H4]4 a minor product in the reductions with lithium and both were minor products in the reductions with sodium; pinacols were not formed in the reductions with potassium. Parallel reductions of 1, unlabeled, analogously led to 2, 3, 4 and 5, and the ratios 2/3 differed from the ratios ([2,3,3−2H3]2+[3,3−2H2] 2/([2,3,3−2H3]+[3,3−2H2]3) under certain conditions. Different values for these ratios were found in the reductions with each metal, all of which corresponded to low overall diastereoselectivities. Reactions 1 and 3 persisted when the reductions were carried out in ammonia/water/co‐solvent mixtures and the enolate formed via reaction 1 was protonated and the resulting [endo‐3‐2H]1 recycled. Reaction 2 cannot be monitored under these conditions. Reactions 1 and 3, and by inference also reaction 2, were almost completely suppressed when analogous reductions were carried out in the presence of ammonium chloride, [3,3−2H2]2 and [3,3−2H2]3 being obtained almost exclusively, in a 6: 94 ratio. It is proposed that the mechanism outlined in House [1] was dominant when, and only when, ammonium ion was the proton source; it may have competed when water was the proton source.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have