Cationic complexes four-centred

Figure 3.34 Supposed four-centred activated cationic complexes for primary insertion of a propylene molecule...

The course of these additions of lithium hydride resembles that found for the addition of borane (Nagase et al., 1980 Graham et al., 1981). With ethylene, the initial step is exothermic formation of a Jt-complex without barrier, then rate-determining transformation to the borane via a four-centre transition structure. In both the borane and lithium hydride additions, there is relatively little development of the new C—H bond with distances of 1.692 and 1.736 A respectively in the transition structures. When a carbanionic product is not formed, for example in the reaction of lithium hydride with cyclopropenyl cation yielding cyclopropene and lithium cation (Tapia et al., 1985), reaction again occurs via a hydride-bridged complex, but the C- H- -Li array remains nearly linear throughout the reaction. 

The phthalocyanine macrocyclic system, formally derived from four isoindoles, is the basis for many blue dyestuffs. Metal derivatives have a cation complexed at the centre, much like the iron atom in heme (see... 

The phthalocyanine macrocyclic system, formally derived from four isoindoles, is the basis for many blue dyestuffs. Metal derivatives have a cation complexed at the centre, much as the iron atom in haem. Phthalocyanine can be produced by the reductive cyclisation of 2-cyanobenzamide or, in a route which makes its relationship to isoindole more obvious, by the combination of four molecules of 1,3-diiminoisoindoline with the elimination of ammonia. ... 

Secondly, a calix[4]arene with four polyether substituents prepared by the Cragg group demonstrated no K+ transport but significant Na+ transport [55], Here, as with the Gokel group s more complex molecule described above, the calixarene was in the 1,3-alt conformation so that at best the cation could only bind to two -OCH2CH2O- chelation sites at any time. This would allow two water molecules to bind the cation giving a sixfold coordination centre ideal for Na+. Consequently 1,3-alt calix[4]arenes with simple polyether substituents appear to be selective for Na+ over K+. 

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