Equatorial addition pattern

In this brief overview of C70-addition patterns, it may also be mentioned that photochemical addition of l,l,2,2-tetramesityl-l,2-disilirane afforded a C2-symmetric adduct for which the unusual addition across the equatorial,... 

Upon further functionalization, the regioselectivity of the additions becomes remarkably enhanced. A third di(ethoxycarbonyl)methano addend is preferentially introduced (40%) into the equatorial (e) position. Starting from e,e,e-tris-adduct, a remarkable sequence of stepwise e additions, with regard to the previous addends, affords hexakis-adduct 8 [12]. TTiis compound features a pseudo-octahedral addition pattern and can also be directly obtained in 14% yield by reacting C o with 8 equiv of diethyl 2-bromomalo-nate/l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) [12],... 

In another approach (see NMR Data Correlation with Chemical Structure),three-dimensional (3D) substructure descriptors are used as a basis for the prediction. In this case the distribution function is simplified and a subsequent analysis is not required. This extension of the HOSE code is based on the ability to count the total number of steric interactions over a three-, four-, and five-bond sphere and additional parameters to describe the axial and equatorial substitution pattern for each atom. In the simple case of cis/trans ksomerism the number of interactions is calculated by means of the display coordinates. More complex situations are described by up/down bonds in this case the calculation of the number of steric interactions for each atom is performed by comparison of the query structure with carefully selected references from a library of ring skeletons. Figure 6 exemplifies the method. The discussion of the stereochemical effects is focused on the two diastereotopic methyl groups at position 4. The experimental shift values are 22.0 and 33.6 ppm. The prediction without stereodescriptors results in 27.4 ppm for both atoms (Figure 6, upper part). If the stereochemical effects are considered, the predicted shifts (21.5 and 33.5 ppm) deviate insignificantly from the experimental values (Figure 6, bottom). 

The stereoselectivity of organometallic additions with carbonyl compounds fits into the general pattern for nucleophilic attack discussed in Chapter 3. With 4-r-butylcyclohex-anone, there is a preference for equatorial approach but the selectivity is low. Enhanced steric factors promote stereoselective addition. 

The structural anisotropy in crystalline or structurally ordered BPDA-PFMB films was studied in this laboratory with wide-angle X-ray diffraction (WAXD) methods. In brief, WAXD experiments were designed to examine both the reflection and transmission modes of thin-fihn samples. In addition, uniaxially oriented polyimide fiber WAXD patterns were obtained to aid in the identification of the film structure. The film WAXD pattern obtained from the reflection mode corresponded well to the fiber pattern scanned along the equatorial direction (Figure 16.3), " which indicates that the reflection mode pattern represents the (hkQ) diffractions. On the other hand, as shown in Figure 16.4, the (001) diffractions were predominant in the film WAXD pattern obtained via the transmission mode. This pattern corresponded to the fiber pattern scanned along the meridian direction. These experimental observations clearly indicate that the c-axes of the crystals are preferentially oriented parallel to the film surface however, within the film, they are randomly oriented. 4.2 5 j( should be pointed out that the WAXD experiments are only sensitive to crystalline or ordered structures in polyimide films. They do not provide any information on the amorphous regions. 

Additional evidence of that hypothesis is given In Tables 4 and 5. The catalysts prepared with carbonyl clusters in n-hexane medium must avoid the MgO hydrolysis. The selectivity patterns for such catalysts show notable differences in comparison with the aqueous Impregnated type catalysts. The carvotanacetone formation is largely diminished and the stereospecificity to axial-equatorial carvomenthol is totaly inhibited. However in Rhodium silica supported catalysts the selectivity to carvotanacetone practically does not change. The effects in stereospecifity towards the carvomenthol product may be due to a small silica hydrolysis effect. 

---