Coupled reactions 174-8 intermediate isomerization

This bi-exponential behavior confirms the presence of reversible isomerization steps coupled with irreversible degradation steps and accounts for the role of the di-cis isomers as reaction intermediates, according to the general reaction scheme presented in Figure 12.1. The dependence of the rate constant of each elementary step on temperature allowed the calculation of the respective activation... 

In addition to benzenoid diazo components, diazotised heterocyclic amines in which the amino group is attached to a nitrogen- or sulphur-containing ring figure prominently in the preparation of disperse dyes [87,88], since these can produce marked bathochromic shifts. The most commonly used of these are the 6-substituted 2-aminobenzothiazoles, prepared by the reaction of a suitable arylamine with bromine and potassium thiocyanate (Scheme 4.31). Intermediates of this type, such as the 6-nitro derivative (4.79), are the source of red dyes, as in Cl Disperse Red 145 (4.80). It has been found that dichloroacetic acid is an effective solvent for the diazotisation of 2-amino-6-nitrobenzothiazole [89]. Subsequent coupling reactions can be carried out in the same solvent system. Monoazo disperse dyes have also been synthesised from other isomeric nitro derivatives of 2-aminobenzothiazole [90]. Various dichloronitro derivatives of this amine can be used to generate reddish blue dyes for polyester [91]. 

The c /.s-bis(phosphine)-substituted complexes 68 were found in our laboratory to isomerize to the trans derivatives under irradiation with visible light [Eq. (57)] (97). The isomerization reaction was postulated to involve a stereochemically nonrigid pentacoordinate intermediate which is generated by photochemical carbyne-carbonyl coupling. (The evidence for the ligand coupling step as well as other photoinduced coupling reactions are discussed in Section IV,F.)... 

Because the coupled isomerization-cyclization of geranyl pyrophosphate proceeds without detectable free intermediates, no ready means has been available to examine the isomerization to linalyl pyrophosphate in isolation. In an attempt to dissect the normally cryptic isomerization step from the tightly coupled reaction sequence, a non-cyclizable analog of geranyl... 

The cis-trans isomerization of diorganonickel complex via dissociative or associative intermediates was investigated in detail from both experimental and theoretical aspects [98]. All these results suggest that the stable trans diorganonickel intermediate of the cross-coupling reaction may be converted into the reactive cis isomer during the reaction. Further details are not clear at present in the Ni-catalyzed system. 

Methylmagnesium compounds (MgMgX X = Me, I) also catalyze the trans-cis isomerization of PdMe2L2 (Scheme 9.13) [38], This reaction was found to serve as a key elementary process in cross-coupling reaction between Phi and MeMgl catalyzed hy tra 5-PdPh(I)L2 (L = PEt2Ph) (Scheme 9.14) [16], The tra 5-PdPh(I)L2 intermediate, which is formed hy oxidative addition of... 

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