Associative Substitution with Isomerization

The cis-trans isomerization of PtCl2(Bu P)2 and similar Pd complexes, where the isomerization is immeasurably slow in the absence of an excess of phosphine, is very fast when free phosphine is present. The isomerization doubtless proceeds by pseudorotation of the 5-coordinate state. In this case an ionic mechanism is unlikely, since polar solvents actually slow the reaction. Similar palladium complexes establish cis/trans equilibrium mixtures rapidly. Halide ligand substitution reactions usually follow an associative mechanism with tbp intermediates. Photochemical isomerizations, on the other hand, appear to proceed through tetrahedral intermediates. 

Gas chromatographic analysis of the reaction mixture yielded three product peaks associated with isomeric alkyl substituted hydroxyl-amines of the general formula... 

More recently, this aromatic hydroxylation reaction has been examined in greater detail by rR spectroscopy and with a nitro-substituted XYL-H derivative. The relative rate of the hydroxylation reaction of [Cu2(N02-XYL-H)] is diminished compared to formation of the peroxo intermediate [Cu2 (N02-XYL-H)(02)], making the latter accessible to spectroscopic analysis 109). The oxygenated product [Cu2 (N02-XYL-H)(02)] " has UV-VIS absorption bands at 358nm (s = 20,000 cmand 530 nm (e = 1,200 M cm ), similar to other side-on peroxo dicopper(II) complexes, but it also has an additional intense peak at 435 nm (s = 5,000 M cm ). Despite the tendency for certain side-on peroxo compounds to be in equilibrium with isomeric bis(p-oxo) compounds (see above), which also have LMCT transitions near 400-420 nm, this new peak is not due to the presence of a bis(p-oxo) species. With resonance excitation, no symmetric Cu2 (0)2 core stretching mode was observed near 600 cm H7). Instead, a rR profile of the 435 nm absorption band shows that it is associated with the 277 cm peak associated assigned as the side-on peroxo Cu... Cu stretch. As determined by comparison with a known bis(p-oxo) standard, the amount of residual Cu2 (0)2 possibly in equilibrium was determined to be 0.13 % (0.0013) relative to the amount of Cug-(side-on-02) present. 

The enthalpy changes associated with proton transfer in the various 4, -substituted benzophenone contact radical ion pairs as a function of solvent have been estimated by employing a variety of thermochemical data [20]. The effect of substituents upon the stability of the radical IP were derived from the study of Arnold and co-workers [55] of the reduction potentials for a variety of 4,4 -substituted benzophenones. The effect of substituents upon the stability of the ketyl radical were estimated from the kinetic data obtained by Creary for the thermal rearrangement of 2-aryl-3,3-dimethylmethylenecyclopropanes, where the mechanism for the isomerization assumes a biradical intermediate [56]. The solvent dependence for the energetics of proton transfer were based upon the studies of Gould et al. [38]. The details of the analysis can be found in the original literature [20] and only the results are herein given in Table 2.2. 

Tertiary amides, such as those associated with prolyl amide bonds frequently influence turn architectures. The importance of the cis Xaa-Pro bond on activity was recognized and proposed to be the source of differentiation in biological activity [86] therefore, isomerization of the prolyl amide bond is central to regulation of protein folding, immunosuppression, and mitosis. These functions are not surprisingly associated with several disease states and thus substitution of the acyl-proline amide bond with the fluoroolefin isostere has received considerable attention. 

The infrared (IR) spectra of 1,10-phenanthroline, its hydrate and perchlorate in the region 600-2000 cm-1 have been obtained, and the principal features of the spectra interpreted.66 Further studies on the IR spectra of 1,10-phenanthroline,67-69 substituted 1,10-phenanthrolines,70,71 and 1,7-phenanthroline67 have also been reported. The IR spectrum of 4,7-phenanthroline in the region 650-900 cm-1 has been analyzed, and the C—H out-of-plane deformation frequencies were compared with those of phenanthrene and benzo[/]quinoline.72 The IR spectra of salts of 1,10-phenanthroline have been taken, and the NH vibrations determined.28,73 Infrared spectroscopy has been used to detect water associated with 1,10-phenanthroline and some of its derivatives on extraction into nitromethane from aqueous solution.74 The Raman spectrum of 1,10-phenanthroline has been compared with its IR spectrum.75 Recently, the Raman and IR spectra of all ten isomeric phenanthrolines were measured in solution and solid states, and the spectra were fully discussed.76... 

Analysis of the 13C chemical shifts of C-2 and C-6 in a series of 2-substituted 6-methoxy-3,6-dihydropyran-3-ones (82) allows the assignment of configuration to the isomeric pairs. C-2 is shielded by ca. 5 p.p.m. and C-6 by ca. 2.5 p.p.m. in the trans isomer compared with the corresponding signals of the cis compound (79MI22200). The upheld shift is associated with the 1,3-diaxial disposition of the substituent and the proton on the y-carbon atom. This y-effect has also been noted in the dihydropyran ring system (74MI22200). 

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