Sterically hindered group bonding chemistry

Figure 8.65 Correlation of carbon acidity (pKa in DMSO) with hydrogen-bonded C---0 distances (D). Points marked are not sterically hindered, whereas in those marked A, the approach of the C—H and O groups is difficult. For (CN)2CHR, the one unclathrated structure is marked A , while clathrates are marked B . (Reproduced with permission from The Royal Society of Chemistry).

From elementary organic chemistry, we know that the positions and hence reactivities of the electrons in unsaturated molecules are influenced by the nature, number, and spatial arrangement of the substituents on the double bond. As a result of these influences, the double bond reacts well with a free radical for compounds of the types CHj = CHY and CHj = CXY. These compounds constitute the so-called vinyl monomers where X and Y may be halogen, ally l, ester, phenyl, or other groups. It must, however, be noted that not all vinyl monomers produce high polymers. In symmetrically disubstituted double bonds (e.g., 1,2 disubstituted ethylenes) and sterically hindered compounds of the type CHj = CXY, polymerization, if it occurs at all, proceeds slowly. 

The position of the hydroxyl groups has significant influence on the reactivity, depending on the type of chemistry involved. Whereas the exo-hydroxyl group is less sterically hindered, the endo-hydroxyl group is more nucleophilic due to intramolecular hydrogen bonding. This difference in reactivity is an intrinsic feature in isosorbide chemistry. 

Interest in the synthesis of sterically hindered bonds between boron and the heavier Group 15 elements continues to parallel similar interest in the chemistry of Al, Ga and In, a fertile area in recent years. The most recent report on the reaction of R2BCI and LiPR 2 gave monomeric phoshinoboranes only when R = Cy, Bu, R = Bu. in other instances, the de-hydro-... 

---