Chirality bond breaking

In the alkylation with most commonly used stabilized nucleophiles, one quite obvious problem associated with this process is the physical separation between the chiral ligand and the reaction site. Both bond-breaking and -forming occur on the Jt-allyl face opposite the metal and its attendant ligand. Thus, efficient transfer of the chirality over the allyl barrier is key to a successful strategy for asymmetric allylic alkylation. 

The term stereoselective is often confused with the term stereospecific, and the literature abounds with views as to the most satisfactory definition. To offer some clarification, it is perhaps timely to recall a frequently used term, introduced a decade or so ago, namely the stereoelectronic requirements of a reaction. All concerted reactions (i.e. those taking place in a synchronised process of bond breaking and bond forming) are considered to have precise spatial requirements with regard to the orientation of the reactant and reagent. Common examples are SN2 displacement reactions (e.g. Section 5.10.4, p. 659), E2 anti) elimination reactions of alkyl halides (e.g. Section 5.2.1, p.488), syn (pyrolytic) elimination reactions (Section 5.2.1, p.489), trans and cis additions to alkenes (e.g. Section 5.4.5, p. 547), and many rearrangement reactions. In the case of chiral or geometric reactants, the stereoisomeric nature of the product is entirely dependent on the unique stereoelectronic requirement of the reaction such reactions are stereospecific. 

Several doublets for methylene protons with AS = 0.73-0.89 ppm confirm the all-jyn-like orientation of adjacent aromatic groups in compounds 53 and 54. On the basis of Cotton effects that are clearly observed in the CD spectra of 53 and 55 (and with much lower intensity in the case of 54), a chiral arrangement of the aromatic groups is proposed, which is induced by the presence of the chiral groups in these molecules.107 The stabilization of these chiral conformations by hydrogen bonds was proved by a much lower intensity of the Cotton effects in hydrogen-bond breaking solvents such as methanol. 

Now, let us see how the axiom about bond breaking is applied in relating tne configuration of one chiral compound to that of another. 

Rule 2 Bond breaking in a retrosynthetic sense avoids disturbing chiral centres asmuch as possible to give the longest carbon chain that contains the maximum overlap of chirality and functionality with the precursors. 

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