Diastereomers solid-state effects

Another way to obtain enantio-enriched tandem addition products from racemic coordination diastereomers is to use solid-state induced control of TpRe(CO)(L) ( -aromatic) stereoisomers where the aromatic substrate is prochiral. This method is effective for L = Melm, Bulm, py or PMes and f/ -aromatic = anisole, 3-methylanisole... 

Derivatives in which the vicinal difluoromethylene motif is extended to include 3,4, 5 and 6 fluorine atoms have been examined to determine conformational preferences [24], The most sophisticated study focused on the hexafluo-romethylene motif, carefully assembled in the diastereomers 3 and 4, that extended earlier studies on tetra- and pentafluoro alkanes [24, 27]. The conformation of these compounds, both in the solid state and in solution, indicates that in order to avoid unfavorable 1,3-dipolar interactions between F atoms, 3 adopts a helical topography that simultaneously allows the vicinal fluorine atoms to adopt the preferred gauche arrangement. The same effect favors an extended anti conformation for 4 that presents no unfavorable 1,3-dipolar interactions and facilitates 3 out 5 possible gauche arrangements of the vicinal fluorine atoms [24, 27],... 

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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