Observation axial

The origin of stereofacial selectivity in electrophilic additions to methylene-cyclohexanes (2) and 5-methylene-l,3-dioxane (3) has been elucidated experimentally (Table 2) and theoretically. Ab initio calculations suggest that two electronic factors contribute to the experimentally observed axial stereoselectivity for polarizable electrophiles (in epoxidation and diimide reduction) the spatial anisotropy of the HOMO (common to both molecules) and the anisotropy in the electrostatic potential field (in the case of methylenedioxane). The anisotropy of the HOMO arises from the important topological difference between the contributions made to the HOMO by the periplanar p C-H a-bonds and opposing p C—O or C—C cr-bonds. In contrast, catalytic reduction proceeds with equatorial face selectivity for both the cyclohexane and the dioxane systems and appears to be governed largely by steric effects. ... 

Molecular models of polynucleotide helices consistent with the observed axial rise per nucleotide (7z) and its turn angle ( ), which are accurately measurable from the fibre... 

Alternatively, electrostatic repulsive forces between the dipoles due to the ring oxygen lone pairs and the exocyclic oxygen or halogen may account in part for the observed axial preferences."... 

LUMO density on one side of the macrocycle were suggested to be the primary factors leading to the experimentally observed axial lability of Fe(III) correlates. 

The axial transition states with LiH were computed for the aza-, oxa-, and thia-derivatives and subjected to NBO analysis to explore n —> a interaction as advocated by Cieplak in support of the observed axial preference. Such an interaction was found to be absent. The corresponding equatorial transition states were found to be devoid of the n energy differences in the axial and equatorial transition states are collected in Table 13. These energy profiles point to poorest axial selectivity for the thia-species and highest axial selectivity for the oxa-species. The axial selectivity of the aza-species is predicted to be intermediate of the other two. However, this contrasts the experimental findings in as much as the aza-species exhibited the highest axial selectivity. 

Four years later, the anomeric effect was more simply attributed, by Lemieux and Chii (24, 185), to interactions between the dipoles of the C(6)—0(1) and C(2)—OR bonds in 96 (Figure 13b). This approach enabled Anderson and Sepp (125) to find a qualitative explanation of the observed axial preference in 2-halotetrahydropyrans 97 (Scheme 33) for it allowed one... 

Rapid equilibration at room temperature between chair conformations leads to one peak. As one lowers the temperature, the interconversion is slowed down until, at temperatures below -66.7°C, peaks due to the axial and equatorial hydrogens are observed. Axial and equatorial hydrogens have different chemical shifts under these conditions. 

Very recently, Benn and collaborators explained the experimentally observed axial conformational preferences of 2-cyano derivatives of oxane, thiane, and selenane in terms of an attractive through-space n interaction between the nonbonded lone electron pair of the endocyclic heteroatom and the n orbital of the axial cyano group. This proposal was supported with computational (NBO) analysis (Scheme 22) (10CJC831). 

Using field-emission microscopy (FEM) to observe axial rotation and preferential adsorption of dimeric C2 units during growth Marchand, M. Joumet, C. Guillot, D. Benoit, J. -M. Yakobson, B. 

On a completely wetting chemical channel ( eq = 0), one can observe axially homogeneous rivulets only if the contact angle 6 of the rivulet (which is pinned at the channel edge by the chemical step if 9 is smaller than the equilibrium contact angle on the surrounding substrate) is smaller than a certain critical angle. Macroscopically... 

Prins cyclizations, which proceed by intramolecular addition of alkenes to oxocarbenium ions, provide a simple, efficient method for the stereoselective synthesis of carbocycles and cyclic ethers [77]. Halosilanes and (la) have been used for Prins cyclizations not only as Lewis acids but also as heteroatom nucleophiles. For instance, in the presence of MesSil or MesSiBr, and lutidine, mixed acetals (26) are efficiently cyclized to 4-halotetrahydropyrans (27) with high diastereoselectivity [78]. The halide is introduced into the axial site of the C(4) position. The proposed mechanism for the MesSiBr-promoted reaction involves the initial formation of a-bromoethers (28) from (26). Solvolysis of (28) provides the intimate ion pair (29). Cyclization to the chair transition structure (30) and proximal addition of the bromide produces the observed axial adduct (27). The role of lutidine is to suppress a less selective HBr-promoted cyclization (Scheme 9.23). Acetals bearing an alkyne or allene moiety also undergo the halosilane-promoted cyclization to form haloalkenes [79, 80]. 

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