Transition absolute configuration

The absolute configuration of transition metal complexes. R. D. Gillard and P. R. Mitchell, Struct. Bonding (Berlin), 1970,7, 46-86 (165). 

Rearrangement of sulfoxides 38a, b exhibited the interplay of several conformational factors. Both diastereomers afford predominant axial (trans) alcohol, but with opposite absolute configuration. The (R, R)-diastereomer strongly prefers the exo-transition state, whereas the (R, S)-isomer prefers the endo conformation. Hoffmann interprets these results in terms of an approximately 3-fold preference for the exo-transition state but a 6-fold preference for formation of an axial bond, these effects reinforcing each other in one isomer but opposing each other in the second. 

In view of the absolute configuration and their optical yields (88-96 %), it follows that the precursor of the (S)-4 should be 2a, which are formed highly diastereoselectively. It is likely that the predominant formation of 2a conforms to the mechanism of the bromolactonization, the S-trans transition state (ref. 3). 

Gillard RD, Mitchell PR (1970) The Absolute Configuration of Transition Metal Complexes. 7 46-86... 

In 1960, Montanari and Balenovic and their coworkers described independently the first asymmetric oxidation of sulfides with optically active peracids. However, the sulphoxides were formed in this asymmetric reaction (equation 130) with low optical purities, generally not higher than 10%. The extensive studies of Montanari and his group on peracid oxidation indicated that the chirality of the predominantly formed sulphoxide enantiomer depends on the absolute configuration of the peracid used. According to Montanari the stereoselectivity of the sulphide oxidation is determined by the balance between one transition state (a) and a more hindered transition state (b) in which the groups and at sulphur face the moderately and least hindered regions of the peracid,... 

The configuration of the 4R,5R-dihydrodiol was established by application of the exciton chirality method (6). To minimize undesired interactions between the electric transition dipoles of the two j>-N,N-dimethylaminobenzoate chromophores and the dihydrodiol chromo-phore, a 4,5-dihydrodiol enantiomer was first reduced to 1,2,3,3a,4,5,7,8,9,10-decahydro and 4,5,7,8,9,10,11,12-octahydro derivatives (6). We found that it is not necessary to reduce the chrysene chromophore of a BaP 4,5-dihydrodiol enantiomer (Figure 2). Similarly, the absolute configurations of the K-region dihydrodiol enantiomers of BA (7), 7-bromo-BA (8), 7-fluoro-BA (9), 7-methyl-BA (10). and 7,12-dime thy 1-BA (DMBA) (7 ) can also be determined by the exciton chirality method without further reduction. 

Compounds having the same optical configuration show similar Cotton effects. If the absolute configuration is known (for example, from x-ray diffraction) for one optically active compound, a similar Cotton effect exhibited by another compound indicates that it has the same optical configuration as the known. In other words, if two compounds give electronic transitions that show Cotton effects that... 

While the collision theory of reactions is intuitive, and the calculation of encounter rates is relatively straightforward, the calculation of the cross-sections, especially the steric requirements, from such a dynamic model is difficult. A very different and less detailed approach was begun in the 1930s that sidesteps some of the difficulties. Variously known as absolute rate theory, activated complex theory, and transition state theory (TST), this class of model ignores the rates at which molecules encounter each other, and instead lets thermodynamic/statistical considerations predict how many combinations of reactants are in the transition-state configuration under reaction conditions. 

Figure 5-15 shows a possible transition state for the enantioselective cyclopropanation of cinnamyl alcohol in the presence of dioxaborolane 206. This model predictes the absolute configuration of the products. 

The assumption of a kinetically controlled course of the reaction, however, readily explains the observed results, even though the transition structures have not, as yet, been calculated. Because epoxide opening is exothermic, 39 can be regarded as a simple model for the transition structure according to the Hammond postulate. It is clear from the structure of 39 that the left-hand ethoxy substituent of the epoxide is in close proximity to the ligand of the catalyst, whereas the other substituent hardly encounters any steric interaction. Epoxide opening will release the former interaction. After reduction of the radical, this results in formation of the product with the absolute configuration observed experimentally. 

These reactions, performed many times, show, in addition to the reversal of the absolute configuration of the product with the change in the configuration at C-8 and C-9 of the alkaloids, a small but reproducible difference in the e.e. of the product. It is evident that the diastereomeric nature of quinine vs. quinidine and cinchonidine vs. cinchonine expresses itself via small but important energy differences in the best fits of the transition states. Noteworthy in this respect is the fine work of Kobayashi (20), who observed larger differences (in the e.e. s of products) when the diastereomeric cinchona alkaloids were used as catalysts after having been copolymerized with acrylonitrile (presumably via the vinyl side chain of the alkaloids). 

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