Pyridyl based systems

R=-0CH2 CH2 OMe (a) side view and (b) top view one of the ethoxy groups is disordered 

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Fig. 6. The P-NMR Chemical shift scale for pyridyl phosphane based systems...

In an excellent review by Roesky et al. in 1994 [70a] a vast number of examples for coordination complexes of cyclic phosphazanes and phosphazenes and other related systems have already been compiled. In the following section, an attempt is made to cover the latest features of group 13 systems along with some earlier examples with phosphorus-nitrogen based systems other than pyridyl phosphanes. 

The first example of a counterthermodynamic one-way E - Z photoisomerization based upon intramolecular hydrogen bonding was reported for the 7V-methyl-3-(2-pyridyl)-propenamide systems. 

Persaud, L., Bard, A. J., Campion, A., Fox, M. A., Mallouk, T. E., Webber, S. E., White, J. M. (1987). Photochemical hydrogen evolution via singlet-state electron-transfer quenching of zinc tetra(N-methyl-4-pyridyl)porphyrin cations in a zeolite L based system, J. Am. Chem. Soc., 109 7309. 

Costas and coworkers have shown recently that manganese complexes based on BPMEN-type ligands (where BPMEN is bis((pyridin-2-yl)methyl)ethylene-l,2-diamine, Figure 11.11) show enhanced activity in the epoxidation of alkenes with H2O2 in the presence of excess acetic add, albeit with lower activity and with a narrower substrate scope than for the pyridyl-tmtacn based systems reported in the same study (see above. Table 11.7) [102]. 

There are other initiator systems of lesser commercial importance. Cumene hydroperoxide is reported to cure acrylic adhesives in the presence of alkyl or pyridyl thioureas [105]. These initiators have been combined with a phosphated acrylate to promote adhesion to metal [106]. Thiourea-based initiators can be applied as a one-part on galvanized metal, where the metal surface provides the second part of the redox initiator [107]. 

Further development in the chemistry of oxazolidinone antibacterials was based mainly on the assumption that the 4-pyridyl moiety of one of Dupont s lead compounds, E-3709, might be amenable to replacement by suitably saturated heterocyclic bioisosteres [48]. This assumption was based on an example in which successful replacement of the piperazine ring system in the quinolone antibacterials, such as ciprofloxacin, with a pyridine fragment, such as seen in Win-57273, results in improvement of both the antibacterial and the pharmacokinetic profiles of the compounds. Similarly, as in the case of ciprofloxacin and Win-57273, it was predicted that the presence of a small but highly electron-withdrawing fluorine atom would be tolerated at the meta position(s) of the central phenyl ring, and would confer enhanced antibacterial activity and/or other desirable properties to the targeted oxazolidinones, as shown in Fig. 3. 

Recently, bis(imino)pyridyl Fe(II)-based catalysts have been reported to afford isospecific chain-end controlled propene polymerization occurring through secondary monomer insertion.138 139 Even more recently, catalytic systems based on the octahedral bis(salicylaldiminato)Ti complex have been reported to afford syndiospecific chain-end controlled propene polymerization,140 which possibly occurs through secondary monomer insertion.141... 

A mechanistic study by Haynes et al. demonstrated that the same basic reaction cycle operates for rhodium-catalysed methanol carbonylation in both homogeneous and supported systems [59]. The catalytically active complex [Rh(CO)2l2] was supported on an ion exchange resin based on poly(4-vinylpyridine-co-styrene-co-divinylbenzene) in which the pendant pyridyl groups had been quaternised by reaction with Mel. Heterogenisation of the Rh(I) complex was achieved by reaction of the quaternised polymer with the dimer, [Rh(CO)2l]2 (Scheme 11). Infrared spectroscopy revealed i (CO) bands for the supported [Rh(CO)2l2] anions at frequencies very similar to those observed in solution spectra. The structure of the supported complex was confirmed by EXAFS measurements, which revealed a square planar geometry comparable to that found in solution and the solid state. The first X-ray crystal structures of salts of [Rh(CO)2l2]" were also reported in this study. 

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