Steric effects computational studies

The work of Melander and Carter (1964) on 2,2 -dibromo-4,4 -di-carboxybiphenyl-6,6 -d2 (1) has been referred to above in the introductory and theoretical sections, where it was pointed out that the availability of two detailed theoretical computations of the inversion barrier (Westheimer and Mayer, 1946, Westheimer, 1947 Hewlett, 1960) made this system especially attractive for the study of steric isotope efifects. Furthermore, in the preferred initial-state conformation the two bromines are probably in van der Waals contact (cf. Hampsoii and Weissberger, 1936 Bastiansen, 1950), and thus initial-state steric effects are unaffected by deuterium substitution in the 6 and 6 positions. The barrier calculations provided two different theoretical values for the non-bonded H Br distance in the transition state which, together with the corresponding H Br potential function, could be inserted in equation (10) to yield values for A AH. For... 

Two separate computational investigations of the Rh/dppms catalyst and related systems have appeared in the literature. One study [40] concluded that steric effects were important in promoting migratory CO insertion in [Rh(CO)(dppms)l2Me], while the other [41] proposed that an electronic effect, arising from the sulfur donor atom of dppms, was responsible. It is likely that a combination of steric and electronic effects result in the observed reactivity. 

The diastereoselectivity of protonation of enolate anions has been studied by H/D exchange.156 /i-Substituted ethyl butanoates were chosen as substrates, with conditions that rigorously excluded ion-pairing and aggregation effects. Stereoelectronic effects were found typically to produce higher stereoselection than purely steric effects, hi the specific case of H/D exchange in 3-ethoxybutanoate in ethanol-4 protonation of the enolate of 3-fluorobutanoate was chosen as a computational model.157 Similar... 

Figure 4 shows the main geometric features of the transition structures associated with the [n4c] and [n6d] steps in the reaction between ketene and prop-2-en-l-imine. Experimental and computational studies [42, 43] showed that the periselectivity of this reaction is very sensitive to substituent effects. Thus, in general disubstituted ketenes and/or imines possessing bulky substituents at the (3-position favor the formation of [2+2] cycloadducts because of severe steric... 

Boehme, C., Wipff, G. 2001. The energetic and structural effects of steric crowding in phosphate and dithiophosphinate complexes of M3+ lanthanide cations. A computational study. Chem. Eur. J. 1 1398-1407. 

The enthalpies of trimers and tetramers were also calculated as part of this study. At the trimer level - important because this is the initial product of the reaction [dimer + zinc alkyl + aldehyde] - the most stable structures are macrocyclic. At the tetramer level there are two species of comparable enthalpy. One is the well-described cubic tetramer, and the other a barrel-like species that is conceptually related to two N - Zn lined dimers. The specific value derived from these calculations is only apparent when compared with the later computational work on the real diisopropylzinc-derived species. This highlights the importance of steric effects involving the more bulky isopropyl groups as a defining feature of asymmetric autocatalysis [80]. 

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