Stereochemical model

These initial reports demonstrated that a catalytic asymmetric variant of the Simmons-Smith reaction could be developed. Although good yields and selectivities were obtained, the lack of a clear understanding of the origin of activation, the limited structural information on the active species and the absence of a stereochemical model made rational improvements difficult at best. The next... 

Scheme 2.25. Proposed catalytic cycle and stereochemical model.

Gadella, T. W. J. and Jovin, T. M. (1995). Oligomerization of epidermal growth-factor receptors on A431 cells studied by time-resolved fluorescence imaging microscopy—a stereochemical model for tyrosine kinase receptor activation. J. Cell Biol. 129, 1543-58. 

One of the most intriguing features of olefin metathesis, and an aspect which continues to receive increasing attention, is the stereochemistry of the reaction. Rationalizations of stereochemical peculiarities have undergone a metamorphosis which parallels advances of mechanistic theories from the pairwise scrambling schemes to the now-popular car-bene-to-metallocycle scheme. As yet there exists no unified stereochemical model which can adequately account for all the observed results. 

Lotz and coworkers have used poly (y-benzyl-D-L-glutamate) as a stereochemical model to study the conformation of gramicidin A89. 

Structural evidence culled from the Cambridge Crystallographic Database have been incorporated where possible. The original stereochemical models have been reproduced, even those that have been improved by subsequent evidence. In many instances, a detailed description of the course of these processes is still lacking. Nowhere is this lack of information more striking than in the Cu(I) catalyzed cyclo-propanation of alkenes. 

Scheme 2. Stereochemical model proposed by Aratani for the cyclopropanation of alkenes using 16. [Adapted from (14).]...

The stereoselectivities in this reaction are governed by steric interactions in the formation of metallacyclobutane 60 (35). Of two possible intermediates (Fig. 5), 61 suffers from steric interactions between the ligand and the ester functionality. Avoidance of these interactions and minimization of 1,2-interaction in the metallacyclobutane leads to the formation of the observed major enantiomer and dias-tereomer (trans). The model suggests that increased diastereoselectivity should be observed with increasing steric bulk of the diazoester, a relationship that has already been established as discussed (cf. Eqs. 24 and 26). It is interesting to note that this model loosely corresponds to the stereochemical model proposed by Aratani for the Sumitomo cyclopropanation with one important difference the Aratani model is based on a tetrahedral metal while the Evans-Woerpel model is predicated on square-planar copper. Applying the Aratani model to the Evans ligand would predict formation of the opposite enantiomer as the major product (35). 

Scheme 11. Results and stereochemical model proposed by Singh for the allylic oxidation of alkenes using 157-Cu complexes [Adaptedfrom (111).]...

Figure 14. Proposed mechanism and stereochemical model for Nakajima s phenolic coupling using proline-derived copper complexes. [Adapted from (129).]...

Figure 18. Stereochemical model for conjugate additions catalyzed by arenethiolato—copper complexes. [Adapted from (145, 146).]...

Scheme 16. Results and stereochemical model for the 271d catalyzed Diels—Alder reaction of 286. [Adapted from (205).]...

The stereochemistry of the reactions catalyzed by 269c may be rationalized by invoking the model advanced by Evans in relation to the Diels-Alder studies, Fig. 26. However, an adequate stereochemical model that rationalizes selectivities observed with catalyst 269d/271d and accounts for the turnover in selectivities between the two systems remains elusive. The effect of the vinyl ether substituent on... 

Figure 26. Proposed stereochemical model for the hetero-Diels-Alder reaction of ethyl vinyl ether and acylphosphonate catalyzed by 55c-Cu(II) complex.

Figure 31. Proposed stereochemical model for 271c mediated ene reaction of various alkenes with ethyl glyoxylate.

Scheme 27. Stereochemical model for the copper-catalyzed enol animation reaction. [Adapted from...

Figure 32. Stereochemical model for the 1,3-dipolar cycloaddition of electron-rich alkenes to nitrone (401) catalyzed by 269c. [Adapted from (247).]...

Belokon et al. (261) subsequently found that salen-Cu(II) complexes are effective catalysts for the asymmetric alkylation of amino acid derivatives. Excellent se-lectivities are observed with 1 mol% of 88b-Cu in toluene at ambient temperature, Eq. 225. Although no stereochemical model is advanced to account for the selec-tivities, these workers suggest the catalyst may be acting as a chiral phase-transfer agent. 

This stereochemical model explains the stereochemistry of the hydrogenation of MAA over TA-Ni system. It also predicts that the TA-Ni catalyst can be effective for the enantioselective hydrogenation of some prochiral ketones with excellent e.e. values (70%). 

First the structures of cytochrome cytochrome c peroxidase [21] are both known at high resolution. Although the precise three dimensional structure of the protein-protein complex is unknown (and, we shall argue, unknowable), molecular modeling has produced detailed stereochemical models for the c ccp complex which are subject to experimental testing and subsequent improvement, as detailed below. 

Scheme 18 Plausible stereochemical model for antl-diastereoselective iridium-catalyzed allyla-tion of alcohols employing alkynes as allyl donors...

Fig. 3 Proposed stereochemical model accounting for the observed sense of absolute stereoinduction (chiral ligand = (5)-Cl,MeO-BIPHEP)...

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