Houk-List transition state model

Horeau principle 1286,1294 Horner-Wadsworth-Emmons (HWE) sequence 1309 host-guest complexes 687 Houk-List transition state model 475, 477, 478... 

Thus, a unified model for proline-catalyzed asymmetric a-functionalization of carbonyl compoimds by electrophiles uncovered in the period 1971-2006 is provided by the Houk-List transition state and its analogs, which embody three important and... 

FIGURE 2.1. Working transition state models for the electrophilic attack to the enamine intermediate, (a) List-Houk model, (b) Steric model, (c) Seebach-Eschenmoser model. 

The enantioselectivity of a closely related reaction, the proline-catalyzed a-ami-nation of aldehydes with diazodicarboxylic esters, independently disclosed by List and by Jprgensen in 2002 [30], can also be accounted for by a List-Houk transition state model (Figure 2.10). 

The proline-catalyzed intermolecular Michael reaction of unactivated ketones with nitroalkenes [33] can also be fitted in a mechanistic scenario involving the List-Houk transition state model (Figpre 2.11). 

Figure 17.7 (a) Syn-face addition in Houk-List models (b) onti-face addition in Seebach-Eschenmoser transition state models. 

Agami s model was subsequently challenged by List, Lerner, and Barbas III in 2000 [8a], when they proposed a one-proline enamine mechanism for the proline-catalyzed intermolecular aldol reaction between ketones and aldehydes. Shortly afterwards, on the basis of DFT calculations, Houk and co-workers proposed a very similar mechanism for the Hajos-Parrish intramolecular aldol [19]. Using the B3LYP/6-31H-G(2df,p) level of DFT theory, Houk and co-workers [20] have seen that the energy difference between the two possible chair Zimmermann-Traxler-like transition states, which differ in the orientation of the enamine with... 

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