Excited state enantiomeric excess

Thus, we can write an expression for the time dependence of the excited state enantiomeric excess, T n(t)... 

Photochemical reactions—asymmetric as well as nonasymmetric ones—take their outset from electronically excited states of the reactant. In this chapter, direct cpl excitation is covered, sensitized reactions are discussed in Chap. 4. Asymmetric photochemistry produces new chirality in a reaction system. The cpl-induced reactions are often called absolute asymmetric, as there is no net chirality in the reactants. This discriminates asymmetric photochemistry from the photochemistry of chiral molecules, which can be induced by nonpolarized light. The newly created chirality in cpl-induced reactions becomes apparent in an excess of the amount of one reactant or product enantiomer over the other. This enantiomeric excess (ee) in a mixture of R and S enantiomers is defined as... 

To be very enantioselective, this reaction has to meet several important requirements. First, photoenols have to be produced as either pure Z or E stereoisomers to allow enantioselective photodeconjugation. Even so, protonation of the Z or stereoisomers from the same, rear side, for example, would produce opposite enantiomers and a low enantiomeric excess (ee) would result (Scheme 3). Fortunately, photoenolization of aliphatic enones is only possible from the Z isomer excited in its singlet state, and the excited molecule has to adopt an s-cis conformation to place the excited carbonyl and the y-H close enough to allow y-H abstraction. Consequently, the enol is formed in a unique configuration. All these observations have led several groups to propose a concerted process involving a 1,5 antarafacial sigmatropic shift for the formation of photodienols [16]. 

The area of stereoselective electron transfer has attracted considerable attention in recent years. The nature of the products obtained depends upon the enantiomer which is undergoing reaction. For example, excited state quenching of A-[Ru(bpy)3] by racemic [Co(acac)j] leads to an enantiomeric excess of A-[Co(acac)j]. 

As for the more usual chemical means of resolutions described earher in this book, there are ways around the statistical Hmitation of a kinetic resolution. It is theoretically possible for a process to involve racemization of the exited states of the chiral compound to give a photostationary state - a steady enantiomeric excess. These processes, sometimes caRed photoderacemizations or photoresolutions axe rare, partly because of the need to ensure that the excited state does not undergo some other reaction but instead reverts cleanly to the ground state. This combination... 

The key to partnership between theory and experiments in organocatalysis rests with the ability of the former to precisely identify the transition states responsible for stereoselectivity. In the following section, some prototypical organocatalytic reactions are presented wherein the computational methods were impressively successful. There are more exciting applications wherein a priori predictions were attempted ahead of experimental verification. As a prelude to in silica catalyst design, a comparison between the computational predictions of the stereochemical outcome and the experimentally observed enantiomeric excess values for a representative set of proline-catalyzed reactions (Scheme 17.13) is compiled in Table 17.1. 

Mariano and co-workers have employed chiral eniminium salts as surrogates for enones to control the stereoselectivity of the enone-olefin [2-1-2]-photocycloaddition. Eniminium salts have only n,n excited states and intersystem crossing to triplet excited states is therefore slow. As a consequence, the photocyclization should occur from the singlet state via a concerted mechanistic pathway. The chiral iminium salt 3, accessible by condensation of the corresponding ketone with an enantiomerically pure tra s-2,5-disubstituted pyrrolidine, cychzed in good yield. After removal of the chiral auxihary, the tricyclic ketone 4 was obtained in enantiomeric excesses of up to 82% ee (Scheme 3). 

---