Sp-centers

It is noteworthy that some catalysts convert thioethers to quaternary salts where the reactive electrophilic center is no longer one of the two C centers but the C sp center of the thiazolium salt (284. 285). Thus... 

The effect of introducing -hybridized atoms into open-chain molecules was discussed earlier, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are noted when sp centers are incorporated into six-membered rings. Whereas the fiee-energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecyclohexane and to 4.9 kcal/mol in cyclohexanone. ... 

Hybridization of substrate at the electrophilic center undergoing reaction—small at sp centers" large a-effects have been reported for reactions at sp and sp hybridized centers ... 

Molecules with tetrahedral symmetry can be realized with adamantane cores or tetrasubstituted sp centers (Fig. 3.15). Braun et al. [92] synthesized some adamantane glasses with glass transition temperatures of 116°C for 1,3,5,7-tetrakis (biphenyl-4-carbonyloxy)adamantane (45a) and 118°C for 1,3,5,7-tetrakis (4-iodobenzoyloxy)adamantane (45b), respectively. It is interesting to note that if the direction of the ester linkage is reversed (esters of adamantanetetracar-boxylic acid), Tg is lowered by 30 K. 

In the following we have broadly divided the available C(l) nucleophiles according to structure and reactivity. C(l)-Nucleophiles based on metallation at an sp -center are categorized in terms of the presence and, as appropriate, the nature of a hetero atom (O or N) substituent at C(2). C(l)-Lithiated glycals correspond to nucleophilic sp -centers, and variations on this theme (concerning the nature of the metal component) are covered in depth. 

A 1,2-oxastannetanide, which is an example of an anionic pentacoordinate tin compound with a four-membered ring oxygen and one organic substituent in apical position, was prepared at room temperature by deprotonation of the corresponding / -hydroxystannane using KH in THF and 18-crown-6 (equation 59)76. The presence of a stannyl anion was evident by the appearance of a double quartet with Sp centers at —75.09 ppm... 

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