Ground state conformational control

Wagner, P.J., Hasegawa, T., Zhou, B., and Ward, D.L. (1991) Diverse photochemistry of sterically congested a-arylacetophenones ground-state conformational control of reactivity. Journal of the American Chemical Society, 113, 9640-9654. 

The importance of conformational considerations in bichromophoric systems has been recognized by several workers [10,20,54-59]. The influence of ground-state conformational control, which has been invoked in explanation of intramolecular exciplex photophysics in a-aryl-0)-N-alkylakanes [60,61] and excimer formation in 1,1 -di-2-naphthyldiethyl-ether [20], has been suggested to be of importance too in macromolecular photophysics [20,59]. 

The decay processes in P2VN are considered to be super-position of excimer forming processes from different conformational sites. Further, the efficiency of excimer formation at different conformational sites differs substantially. The dual decay process in P2VN, if indicative of the existence of two excimer sites, is supposed to result from ground state conformational control of excimer formation. 

Application of the Curtin-Hammett Principle would suggest that the different ground state conformers have minimal influence on the product composition. It is the difference in activation energies for the two different isomers that controls the reaction, and the diastereomeric transition states would be attained from either ground state conformation. 

Further experimental and theoretical studies on the rotational barriers of the metal fragment in (cycloheptatriene)Cr(CO)3 complexes195 suggest that (cycloheptatriene)-Cr(CO)3 complexes in general are in equilibrium with their norcaradiene valence isomers and their ground state conformation is controlled by the same electronic factors which effect the cycloheptatriene-norcaradiene equilibrium195. 

A conclusion can therefore be reached that electronic effects are not important in the rate of hydrolysis of acetals, but this is valid only if these compounds hydrolyze in their ground state conformation. The reality could well be completely different. For instance, an acetal could prefer to hydrolyze with stereoelectronic control via a higher energy conformation. Thus, the hydrolysis of 127 and 128 could occur with stereoelectronic control at a more or less competitive rate. Compound 127 would hydrolyze via its ground state conformation whereas 128 would hydrolyze via the boat conformation 129. This situation is possible because the enthalpy of activation for the cleavage of the 4-nitrophenoxy group in 127 and 128 is close to 25 kcal/mol, a much greater value than the barrier for the formation of the boat conformation 129 which is of the order of 10 kcal/mol (94). Therefore, 129 could lie on the reaction coordinates. 

The first examples of control of stereochemistry in free-radical addition to alkene radical traps utilized 2,5-dimethylpyrrolidine carboxamides as the chiral auxiliary [8j, In this approach, the symmetry of the pyrrolidine reduces the conformational options for the a, -unsaturated carboxamide. Thus, the preferred Z conformation for carboxamides of 2,5-dimethylpyrrolidine is shown in Fig. 1 (structure 1). Single-crystal X-ray analysis of several dimethyl pyrrolidine carboxamides, including 1 and 2, have been carried out, and these structures support the notion that the preferred ground state conformation for these carboxamides is as shown [9], The X-ray structures for I and 2 are shown in f ig. 1. 

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