Stereochemical implications contributions

This reaction has been extensively studied " and I discuss it here merely to point out certain pitfalls. The conversion of a Ni(0) bis (butadiene) cranplex XIX to a Ni(II) bis- r-allylic complex XX (the structures are drawn withmit stereochemical implication) by reductive coupling appears at first sight to be an allowed process each ligand contributes four electrons and the metal contributes two, making ten in all. Thus Eq. (1) is satisfied, provided the metal itself introduces no discontinuity ... 

The Editors note with regret the recent passing of Louis Malaprade, University of Nancy, discoverer of the stoichiometric oxidation of glycols by periodate, a reaction that has had such profound implications in the structural investigation of carbohydrates and of Karl Freudenberg, Heidelberg, last surviving student of Emil Fischer s, pioneer of important stereochemical concepts, and a scientist whose extensive contributions to synthesis included the classic, widely used acetone derivatives (isopropylidene acetals) of the monosaccharides. 

With the preceding reviews of the enzymology of monoterpene cyclization and of model studies relevant to the cyclization process, it is possible to formulate a unified stereochemical scheme for the enzymatic cyclization of geranyl pyrophosphate (Figure 4). The proposal which follows is consistent with the implications of parallel advances in related fields, most notably the contributions of Cane (8,16,24,25,52), Arigoni (67) and Coates (68,69) on the stereochemistry of sesquiterpene and diterpene cyclizations, and of Poulter and Rilling (29,70) on the stepwise, ionic mechanism of prenyl transferase, a reaction type of which several monoterpene, sesquiterpene and diterpene cyclizations are, in a sense, the intramolecular equivalents. 

The suggestion is then made that the stereoconlrolling step in asymmetric Mizoroki-Heck reactions is oxidative addition (via dynamic kinetic resolution) rather than alkene association or migratory insertion. The implication is that only substrates capable of a dynamic kinetic resolution may cyclize with high enantioselectivity. This would limit the substrate scope of the asymmetric intramolecular Mizoroki-Heck reaction. While the dynamic kinetic resolution during the oxidative addition may be a component of the overall stereoselectivity, it does not rule out contributions from later events in the mechanistic pathway and does not explain the effect of additives on selectivity. What has been shown is that the axial chirality of the o-iodoanilides (as with any enantioenriched isomer of a chiral precursor) influences the stereochemical outcome of their reactions. 

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