Conformational analysis hyperconjugation

The application of /hc couplings in conformational analysis of six-membered heterocycles has been shortly overviewed by Juaristi et al. and by Alabugin et a/. The couplings have been also shortly addressed by Ribeiro and Rittner in their work on the role of hyperconjugation in the conformational analysis of methylcyclohexane and methylheterocyclohexanes. 

Cramer CJ (1996) Hyperconjugation as it affects conformational analysis. J Mol Struct (Theochem) 370 135-146... 

NBO analysis has been used to dissect the contributions of the three principal contributors to ethane s structural preferences and separate steric and hyperconjugative interactions. It was found that removal of vicinal hyperconjugation interactions yields the eclipsed structure as the preferred conformation, whereas Pauli exchange (steric) and electrostatic (Coulombic) repulsions have no decisive influence on the preference for a staggered conformation. The hyperconjugative preference for the staggered conformation is attributed to the antiperiplanar stereoelectronic requirement sununarized in Figure 6.5. 

Key words Carbohydrate - Conformational analysis -Stereoelectronic effects - Hyperconjugation... 

AMI Cambridge Structural Database Carbohydrate Force Fields Conformational Analysis 1 Conformational Analysis 2 Conformational Analysis 3 Continuum Solvation Crystal Structure Calculations 1 Crystal Structure Calculations 2 Cyclodextrins Force Fields MM3 Hydrogen Bonding 1 Hydrogen Bonding 2 Hydrogen Bonds Semiempirical Methods Hyperconjugation MNDO Natural Bond Orbital Methods PM3 Solvation Carbohydrates. 

The beta silicon effect , referring to the strong stabilization of carbocations -substituted with silicon, is another well-known manifestation of hyperconjugation. The C-Si empty orbital on a cationic center, very significant delocalization is observed. Finally, an important feature in the conformational analysis of alkenes is the tendency for allylic sp centers to prefer to eclipse the C=C double bond (Scheme I) (see Conformational Analysis I Conformational Analysis 2 and Conformational Analysis 3). This preference, typically on the order of 2 kcal moI , is ascribed to improved hypercon-jugative interactions of the non-eclipsing C-H bonds with the jr orbital of the alkene for this conformation. 

The nature of the anomeric effect in the bicyclic /ra r-fused octahydro-l-methyl-l//-2,l-benzothiazine 2,2-dioxide 37 has been examined by single crystal X-ray structure analysis (Figure 5) <1998CJC164>. The crystal structure of 37 shows that the A -Me group assumes an axial position in the solid state. The authors suggest that this conformation is also the most stable in solution and propose that this hyperconjugation effect is >2.0 kcal mol . ... 

We have recently argued [39] that facial selectivity in the Diels-Alder reactions of 5-substituted cyclopenta-1,3-dienes is a reflection of hyperconjugative effects a frontier orbital analysis is shown in Fig. 6-10 [40], The molecule adopts a conformation where the better electron-donating group (C-Me rather than C-X) hinders approach of the dienophile from that face. Overlap of the C-C (T-bond (a better donor than C-X) increases the energy of the diene HOMO. Furthermore overlap of the C-C (T -orbital with the diene LUMO yP reduces the energy of the... 

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