Anionic Rings

This can be circumvented by choosing alkyl groups with no P H, eg, methyl, neopentyl, trimethylsilylmethyl, phenyl and other aryl groups, and benzyl. The linear transition state for -elimination can also be made stericaHy impossible. The most successful technique for stabilization combines both principles. The pentahaptocyclopentadienyl ring anion (Cp) has six TT-electrons available to share with titanium. Biscyclopentadienyltitanium dichloride... 

Lapachenole — see 2H-Naphtho[l,2-6]pyran a-Lapachone, dehydrosynthesis, 3, 751 Large heterocyelic rings anions, 7, 18 aromatieity, 7, 14, 16 basicities, 7, 24 bond angles, 7, 14-16 bond lengths, 7, 14-16 cations, 7, 18... 

The crystal structure determinations of trimetaphosphimates showed that N-H 0 hydrogen bonds between the ring anions play an important role. In most of these salts the (P02NH)lj rings adopt approximately a chair conformation. In a few cases a conformation is found that is best described as a linear combination of the boat and the twist conformation. Examples of these conformations as well as some structural aspects of trimetaphosphimates are given in Fig. 2. 

Hence, the anion-radical of diphenyl fulvene acquires a spin-charge distribution dictated by steric shielding at Ph2C node and six-n-electron delocalization in the C5H5 ring. Anion-radicals of sterically congested stilbenes represent examples that are quite different, but at the same time are similar in principle. Let us compare the E structures of stilbene and its congested derivative, namely, a,p-di(tert-butyl)stilbene in their neutral and anion-radical forms (Scheme 3.18). 

The pseudohalogen concept can be extended (i) to some special nonplanar anions such as CF3, (ii) to heavier elements (isovalence electronic exchange of O by S, Se, Te of N by P etc. in SCN , SeCN , TeCN , P(CN)2 ) " and (iii) to derivatives such as the five-membered ring anion [CS2N3] which can be obtained in the reaction of carbon disulfide and sodium azide as shown by Sommer as early as 1915. The introduction of heavier elements, however, results in weaker delocalization effects due to the formation of weaker tt-bonds. 

The six-membered-ring dianion A" is found to be 22.6 kcal/mol more stable than the four-membered-ring anion B". This result is confirmed by experiments, which show that substitution reactions occur at the six-membered-ring system. 

Later, he found equilibrium between the six- and the four-membered-ring anion, which depends very strongly on the temperature at which the reaction is conducted.41 Higher temperatures lead to a better Si->N contact across the ring and therefore isomerization is possible. Substitution is preferred at low temperatures. We have studied the anionic rearrangement in reactions with fluorosilanes and boranes and found three more factors that influence the equilibrium between the six- and four-membered rings illustrated in Scheme 18.42 43... 

A further equilibrium, which can be controlled by the reaction temperature, exists between the trissilyl-substituted asymmetrical four-membered, six-membered and the symmetrical four-membered-ring anions (Scheme 35).52... 

As the experiments show, the same principles that were found for the equilibrium of the six- and four-membered-ring anions can also be applied to the eight-, six- and four-membered systems. 

A six-membered ring anion prepared by electrolytic reduction at —120 °C will rearrange spontaneously to a five-membered ring, even at low temperature and in a short period (half-life 5.3 min at -120 °C). 

It is then possible to visualise how such anions could form more complex units and germ nuclei. An example is shown in Figure 2 in which 4-ring anions yield the cubic unit found in the zeolite A framework, or the double crankshaft chain found in felspars or in phillipsite-harmotome zeolites (J7). 

Figure 2. Formation from 4-ring anions of cubic units found in...

Fig. 5.75. The proposed large ring anions (a)Sie0 iand(b)Si80. ...

SiO, SiOj-, SijOj , and SijO ", etc. However, the ring anions also suggested are well confirmed in the corresponding solid silicates. 

---