Transfer LUMO-lowering activation

It is important to note for the following discussion that in electron-transfer processes the reductant s highest occupied molecular orbital (HOMO) should combine with the oxidant s lowest unoccupied molecular orbital (LUMO) of the same symmetry to ensure proper overlap of reductant and oxidant orbitals to initiate electron transfer. That is, electron transfer will occur readily from n to n orbitals on different species or from a to a but not n to a in a linear arrangement of atoms [e.g., A-B-C in Appendix I (following references at the end of this chapter)]. In the case of outer-sphere electron-transfer processes, n- to 7r-electron transfers are favored over a to a because (1) such transfers do not require major changes in bond lengths in the precursor complex (lower activation energy) and (2) the n orbitals are more diffuse or better exposed than a orbitals. This process is well documented for transition metals. For inner-sphere electron-transfer processes, both n- to n- and a- to n-electron transfers are most favored (Purcell and Kotz, 1980). 

What is the role of water under these conditions It has been suggested that water suppresses the formation of proHne-oxazoUdinone, which has been considered to be a parasitic species [11]. Then, the role of water is to prevent deactivation rather than to promote activity. Studies, carried out on the proUne-catalyzed reaction between acetone and 2-chlorobenzaldehyde allow one to hypothesize a conflicting role of water. Water increases the total catalyst concentration due to suppression of unproductive species and decreases the relative concentration of productive intermediates by shifting the iminium ion back to proline [12]. Addition of water suppresses formation of both on- and ofF-cyde iminium ions 1 and 2 by Le Chatelier s principle (Scheme 24.2a). The net effect of added water on the globally observed rate will depend on the relative concentrations of iminium ions 1 and 2, which may be different for different aldehydes and can be a function of substrate concentrations and rate and equilibrium constants. Seebach and Eschen-moser have raised doubts about the fact that oxazoUdinones are unproductive and parasitic species in proline-catalyzed aldol reactions [13]. The small excess of water will potentially facilitate proton-transfer in the transition state (Scheme 24.2b), which both lowers the LUMO of the incoming electrophile as well as directs the enantioselectivity of the newly formed stereocenters. 

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