Asymmetric induction crystal structure-solid state

To this point, all the examples presented have been ones in which the origin of the asymmetric induction has been unimolecular in nature, that is, the molecules adopt homochiral conformations in the solid state that favor the formation of one enantiomer over the other, usually through the close intramolecular approach of reactive centers bimolecular crystal packing effects appear to play little or no role in governing the stereochemical outcome of such reactions. This raises the interesting question of whether the soUd-state ionic chiral auxiUary approach to asymmetric synthesis could be made to work for conformationally unbiased reactants, i.e., those possessing symmetrical, conformationally locked structures. Two such cases are presented and discussed below. 

Leibovitch, M., Olovsson, G., Scheffer,J.R., and Trotter, J. (1998) An investigation of the yang photocyclization reaction in the solid state asymmetric induction studies and crystal structure-reactivity relationships. Journal of the American Chemical Society, 120, 12755-12769. 

One reason that salts of general structure 7 were chosen for investigation is that molecular mechanics calculations showed the benzocyclohexadienone framework to be planar. This planarity translates into an equal opportunity of forming either enantiomer of photoproduct 8, or to put it another way, the photochemistry of these salts is conformationally unbiased with respect to enantioselec-tivity. As a result, any ee in the crystalline state would have to be due to an essentially pure topochemical effect. Only one of the salts investigated (7b) gave a respectable ee in the solid state, and unfortunately, it has not been possible to obtain an X-ray crystal structure of this material, thus precluding an analysis of the specific topochemical factors responsible for asymmetric induction in this case. 

Thus, in comparison to the situation in most conventional, ground state asymmetric induction reactions, where the chiral auxiliary is intimately involved in the enantiodifferentiating step through its stereoelectronic effects or coordinating ability, the role of the ionic chiral auxiliary in solid state cyclobutanol formation is a relatively passive one. For example, the ionic chiral auxiliary does not need to be located close to the site of reaction and all that is required is that its attachment to the reactant via salt formation does not give rise to diastereomers. In addition, there is no direct correlation between the size and structure of the ionic chiral auxiliary and the extent of ee, nor is it possible to predict which enantiomer of the photoproduct will be favored. This would be akin to making an a priori prediction of crystal and molecular structure, a feat that is currently beyond the scope of modem crj tal engineering. 

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