Geminal group

This dependence of p/fa on inductive effects was also found in nitrobenzene218 and aqueous methanol.219 Anomalies were found, however, particularly where the predominant con former had an axial OH group.215,217,218 These were explained in terms of interaction between inductive effects of geminal groups, or of steric effects,215,218,219 termed by the authors a stereopolar effect. These studies led to the conclusion that the inductive effect of substituents in such systems is registered at... 

When two groups, for example two hydroxyl groups, are adjacent to each other, they are known as vicinal groups, whilst two groups attached to the same atom are referred to as geminal groups. 

Hence, in equation 5 the expression a(R ) a(R ) (//(R ) — a (R )) for the geminal groups R and R, for instance, may be viewed as that part of the rotation contribution of an asymmetric carbon atom which can be transferred from acyclic compounds V to the cyclic compounds I. 

The asymmetry in the electron density distribution of COR, hence, is induced by lifting the local C symmetry of COR by the geminal phenyl rings and H atoms, respectively. These geminal groupings lie above or below the plane of COR. As in XIX R lies in the relevant polar plane, it should not affect the chirality of the electron density distribution of COR significantly. Therefore, the rotation contribution from the ligand COR should be (almost) independent from the substituent at site 3. As a first approximation the rotation contribution from COR therefore can be viewed as a constant. In this way the molar rotations of 1,1-diphenylcyclopropanes XIX are described by equation 13. ... 

If the CXOH (X = O, H2) substituent has no geminal group (71,73), the rotations can be predicted correctly. With a methyl group geminal to CXOH the TTOR approach fails. In 72 and 74 the CXOH group is strongly shielded. 

Complications in the treatment of optical rotations have also been observed for 1,1-diphenylcyclopropanes of the type XX. Here, however, probably intramolecular hydrogen bonds between geminal groups are responsible for the breakdown of the TTOR approach. 

It turns out that there are three possible relationships for such geminal groups They can be homotopic, enantiotopic, or diastereotopic. 

Table 9.20). The silanol composition on the surface of three types of silicas remains to be elucidated, but it would be anticipated that the HDS silicas would have higher concentrations of geminal groups, whereas the conventional silica would have a greater amount of isolated silanols (Rodgers et al., 2004 Blume et al., 2004). 

As noted earlier, three types of functional silanol groups exist on the silica surface isolated hydroxyl groups, geminal groups (two -OH groups on one Si atom), and vicinal groups (see Figure 9.6). The silanization reaction is illustrated in Eq. (9.5). 

Cyclolinear polymers can be made by condensation reactions with diols involving geminal (12.244) or non-geminal groups (12.245) [93,94]. 

Oxidative cyclocarbonylation-alkoxycarbonylation is usually observed when the reaction of alkenols and alkynols is carried out in the presence of external nucleophiles such as alcohols. /3-Lactones and /3-lactams bearing an exocyclic (Z)-(alkoxycar-bonyl)methylene moiety can be prepared according to a very simple oxidative technique starting from propynyl alcohols or amines. Geminal groups have been found to exert a powerful effect on cyclization. 

The concerted loss of two geminal groups also fall under this cateogy (scheme 7.3). This reaction is also symmetry-allowed. 

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