Ring geometry

After generating the entire ring system of the molecule, CORINA uses a reduced force field to optimise the ring geometries. Two simplifications lead to this so-... 

Accurate low-temperature ( —150°C) X-ra determinations have been performed on a set of polyalkyl-A-4-thia2oline-2-thiones in order to determine ring geometry and deformations related to steric overcrowding (83. 84). The geometry of the ring is given in Fig. VII-1. 

The phenyl cation is an extremely unstable cation, as is reflected by the high hydride affinity shown in Table 5.2. In this case, the ring geometry opposes rehybridization so the vacant orbital retains sp character. Because the empty orbital is in the nodal plane of the ring, it receives no stabilization firom the n electrons. 

Table 10 and Figure 17 carry all the necessary information. Since the only experimental geometry available is that for the sulphoxide, the ring geometry was kept constant in all three compounds, while OCS and OSO angles and S—O distances were taken from the analogous dimethyl derivatives. 

In general, M—Sb heterocycles either adopt four- or six-membered ring geometries of the type [R2MSbR x (x - 2, 3). The only structurally characterized Al—Bi heterocycle, [Me2AlBi(Tms)2]3 43, also forms a six-membered ring. A1—E heterocycles. The central structural parameters of Al—Sb heterocycles are summarized in Table 9. 

These examples illustrate the issues that must be considered in analyzing the stereoselectivity of enolate alkylation. The major factors are the conformation of the enolate, the stereoelectronic requirement for an approximately perpendicular trajectory, the steric preference for the least hindered path of approach, and minimization of torsional strain. In cyclic systems the ring geometry and positioning of substituents are often the dominant factors. For acyclic enolates, the conformation and the degree of steric discrimination govern the stereoselectivity. 

The mechanism and stereochemistry of the orthoester Claisen rearrangement is analogous to the Cope rearrangement. The reaction is stereospecific with respect to the double bond present in the initial allylic alcohol. In acyclic molecules, the stereochemistry of the product can usually be predicted on the basis of a chairlike TS.233 When steric effects or ring geometry preclude a chairlike structure, the reaction can proceed through a boatlike TS.234... 

Both the regio- and stereochemistry of Entry 6 are of interest. The regioselectivity is imposed by the rigid ring geometry, which favors enolization at the observed position. Inspection of a molecular model also shows that a-face of the enolate is more accessible. 

In this section, we present a unified picture of the different electronic effects that combine to determine methyl rotor potentials in the S0, Sp and D0 electronic states of different substituted toluenes. Our approach is based on analysis of ab initio wavefunctions using the natural bond orbitals (NBOs)33 of Weinhold and cowork-ers. We will attempt to decompose the methyl torsional potential into two dominant contributions. The first is repulsive steric interactions, which are important only when an ortho substituent is present. The second is attractive donor-acceptor interactions between CH bond pairs and empty antibonding orbitals vicinal to the CH bonds. In the NBO basis, these attractive interactions dominate the barrier in ethane (1025 cm-1) and in 2-methylpropene (1010 cm-1) see Figure 3. By analogy, donor-acceptor attractions are important in toluenes whenever there is a substantial difference in bond order between the two ring CC bonds adjacent to the C-CH3 bond. Viewed the other way around, we can use the measured methyl rotor potential as a sensitive probe of local ring geometry. 

In view of the clear correlation of local ring geometry with methyl rotor barrier height in the S0 and D0 states, the strong effects of S, <— S0 excitation on rotor potentials seem to indicate substantial distortion of the ring away from hexagonal symmetry in the S, state as well. There is little clear evidence of this from molecular spectroscopy. We have speculated that such a n-molecular orbital orientation effect in the S j state (similar to that in the cation) might explain the observed characteristic... 

M-E heterocycles of the type [R2MER2]X generally adopt either four- or six-membered ring geometries (x = 2, 3), as was shown already for a large number of M-N heterocycles. This is illustrated in Table XVII, summarizing the number of structurally characterized four- and six-membered heterocycles [R MER (E = N, P, As).94... 

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