A steric interaction

In a combined experimental and theoretical investigation it was found that the / -alkyl group in the dienophile gave a steric interaction in the transition-state structure which supported the asynchronous transition-state structure for the Lewis acid-catalyzed carbo- and hetero-Diels-Alder reactions. The calculated transition-state energies were of similar magnitude as obtained in other studies of these BF3-catalyzed carbo-Diels-Alder reactions. 

Diaxial interaction (Section 4.8) The strain energy caused by a steric interaction between axial groups three carbon atoms apart in chair cyclohexane. 

A model consistent with the stereochemical outcome of the reaction and with the role of the halide has been proposed (Fig. 10). This model suggests side-on olefin binding and reorganization of the halide ligands. In such geometry, a steric interaction between the unbound olefin and apical halide may justify the dramatic increase in enantioselectivity observed upon changing the halide from cr to r. 

The stereoselectivity is enhanced if there is an alkyl substituent at C(l). The factors operating in this case are similar to those described for 4-r-butylcyclohexanone. The tnms-decalone framework is conformationally rigid. Axial attack from the lower face leads directly to the chair conformation of the product. The 1-alkyl group enhances this stereoselectivity because a steric interaction with the solvated enolate oxygen distorts the enolate to favor the axial attack.57 The placement of an axial methyl group at C(10) in a 2(l)-decalone enolate introduces a 1,3-diaxial interaction with the approaching electrophile. The preferred alkylation product results from approach on the opposite side of the enolate. 

The role of Ti(OPri)4 in this process is shown in Figure 2-7. The aldehyde is illustrated in two conformations, the solid lines indicating the more favorable orientation. The conformation represented by the dashed line is disfavored by a steric interaction with a pseudo-axial aryl group. Assuming that the attack of a nucleophile comes from the direction of the viewer, this hypothesis accounts for the Sf-face selectivity in all known Ti-TADDOLate-mediated nucleophilic additions to aldehydes. 

Up to this point our discussion of y-gauche effects has demonstrated that the transmission mechanisms are not yet well understood and still open to speculation. The original concept of a steric interaction is highly controversial, and as long as there is no convincing explanation, the use of the deep-rooted term steric compression shift must be discouraged. 

The tertiary a-ester (26) and a-cyano (27) radicals react about an order of magnitude less rapidly with Bu3SnH than do tertiary alkyl radicals. On the basis of the results with secondary radicals 28-31, the kinetic effect is unlikely to be due to electronics. The radical clocks 26 and 27 also cyclize considerably less rapidly than a secondary radical counterpart (26 with R = H) or their tertiary alkyl radical analogue (i.e., 26 with R = X = CH3), and the slow cyclization rates for 26 and 27 were ascribed to an enforced planarity in ester- and cyano-substituted radicals that, in the case of tertiary species, results in a steric interaction in the transition states for cyclization.89 It is possible that a steric effect due to an enforced planar tertiary radical center also is involved in the kinetic effect on the tin hydride reaction rate constants. 

The enantioselectivity of these reagents is explained by comparison of transition structures 72 and 73 shown in Scheme 7. The disfavored transition structure 73 leading to the minor enantiomer displays a steric interaction between the methylene of the allylic unit and the methyl group of one of the pinane units. Unlike the tartrate boronates described above, the directing effect of the bis(isopinocampheyl) allylic boranes is extremely powerful, giving rise to high reagent control in double diastereoselective additions (see section on Double Diastereoselection ). 

The proposed mechanism of the enantiodifferentiation involves chelation of the ester carbonyl oxygen to the enolate as illustrated with A and B66. Transition state B is believed to be destabilized relative to A due to a steric interaction between the a-methyl group and the cyclopentadienyl ligand. The presence of hexamethylphosphoramide reduced the diastereomer-ic ratio to 86 14, supporting the intermediacy of chelated species. 

When the thickness of the draining film is less than twice the thickness of the adsorbed protein layer, (i.e. TLs), the approaching faces of the film experience a steric interaction because of the overlap of the adsorbed protein layers. 

For galactitol the same coordinations are possible but a steric interaction exists between the surface and the CHOH-CHgOH chain. The 2-0,3-0,4-0 and 3-0, 4-0, 5-0 coordinations are thus less favoured than for mannitol. 

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