Racemization electronic effects

AS -8 to +4 e.u. Even though the racemization rate constants differ slightly, their distinct dependence on the steric and to a lesser extent on the electronic effects of the substituents bonded to the sulfinyl sulfur atom was noted. It deserves adding that the activation volume for racemization of methyl p-tolyl sulfoxide 41, A F 0 ml/mol, is also consistent with the pyramidal inversion mechanism (249). 

It would usually be assumed that abstraction/recombination reactions such as those illustrated in this section would proceed with racemization at the reacting centers. It has been reported22, however, that photocyclization of amide 1 proceeds with complete retention of absolute configuration. Racemization at the site of abstraction requires orbital rehybridization, passing through a planar intermediate. In this case rehybridization appears to be markedly slowed. This may be an electronic effect due to the heteroatom substituent on the intermediate radical, or simply a steric effect. The structure of product 2 was established by X-ray crystallographic analysis. 

The complexes [Cr(S2CNMe2)3], [Cr S2P(OEt)2 3] and [Cr(S2COEt)3] have Pfeiffer CD activity but this develops only when there is a large ratio of optically active environment compound to the racemate.1021 A normal coordinate analysis of the IR spectra of [Cr(S2CNR2)3] (where R = Me, Et, Pr or Bun) has shown that the alkyl substituent affects the thioureide band at ca. 1550 cm"1 through kinematic as well as electronic effects.1022... 

It is not easy to determine the differences in structural selectivity among all possible derivatives. A rationalization based on an electronic effect [18] and the length of the acyl substituent on cellulose were attempted [42]. CTA-I (micro-crystalline cellulose triacetate) is very specific and can be used for resolution of racemic compounds both having aromatic rings and carbonyl groups. On the other hand, a CTA-II (cellulose triacetate) CSP has a different selectivity. Both... 

In general, catalysis by 84a and 85c resulted in good to excellent enantioselec-tivities in the reduction of lcetimines derived from methyl aryl ketones and aromatic amines (80, R1, R3 = aryl, R2 = Me), where the electronic effects of substituents in both aromatic groups did not show any significant influence [79, 80]. On the other hand, imines obtained from aliphatic amines (80, R3 = alkyl) gave virtually racemic products with 85a [80b]. In the reduction of non-aromatic imines, such as 80c, only catalyst 84a maintained high enantioselectivity (Table... 

Realistically, we did not have the option of incorporating a second thio substituent to enhance the acidity of the substrate. Rather, we expected the twin effects of the thioester s electronic influence and the resonance stabilization of the Michael/ retro-Michael tandem to enable the formation of the enolate under mildly basic conditions. The electronic effect of the thioester was supported by a calculation, indicating that the pKa of the a-proton of the thioester would lower (by 1.5 p/C, units) in comparison with 9 [30]. It was not clear if this would be enough to permit racemization under reaction conditions that would support sufficient enzyme activity but relatively little work would be required to test the hypothesis. 

Anthonsen, and P. Sandford (Eds.), Chitin and Chitosan, Elsevier, London, 1990, pp. 415 29. K. Bock, C. Pedersen, I. Defaye, and A. Gadelle, Steric and electronic effects in the formation of dihexulose dianhydrides. Reaction of racemic sorbose in anhydrous hydrogen fluoride and a facile synthesis of D-sorbose, Carbohydr. Res., 216 (1991) 141-148. 

The experimental data state that [6]helicene 2 having six alkoxy substituents racemizes considerably slower than the parent [6]helicene 1. Hence, we can conclude that the electron-donating ability of oxygen substituents should not be responsible for the increase in the racemization rates observed for compounds 3 and 4 regarding to 1. We can discard the electronic effect of the substituents as the origin of the acceleration in the racemization process. 

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