Mechanisms of cis-trans Isomerization

One of the possible mechanisms of cis-trans isomerization of olefins is excitation to the triplet or diradical state.98-96 The two paths, one by way of singlet and triplet states and the other solely by way of singlet states, are diagrammed in Fig. 1. The two lines with minima at 0° and... 

Oxalate (ox, 204 ) complexes of Cr° have been known since the very beginning of coordination chemistry. Thus, the resolution of chiral [Cr(ox)3] with strychninium counterion by Werner in 1912 prodnced the first optically active anionic coordination compound. There also exists a series of bis(oxalato) complexes of the type [Cr(ox)2X2] + , where X can be any of a variety of nentral donors (e g. H2O, NH3, etc.) or anionic ligands (e g. SCN, N3, etc.). These compounds have been used to study the mechanism of cis/trans isomerization and racemization of optically active octahedral coordination compounds. 

These experiments demonstrate that two mechanisms of cis-trans isomerization concurrently work in A -pyrazolines the C4 epimerization and the C5 epimerization, the first one being the most efficient. The different rates of exchange of protons on C4 correspond to the protonation of the intermediate A -pyrazoline by the less hindered face. Toth <89JCS(P2)319,93T863> has devoted a great effort to the conformational analysis and stereochemistry of A -pyrazolines. 

The Visual Transduction Process. Picosecond absorption spectroscopy which utilizes OMCDs also has provided important mechanistic information that previously was not available by means of other techniques. Detailed pathways of a number of reactions which are important from a physical, chemical, and/or biological viewpoint have been elucidated by means of this technique. A recent picosecond spectroscopic study by Spalink et. al. (30) has demonstrated that an experimental criterion, which has been used to support the hypothesis that cis-trans isomerization (31) is the primary event in the visual transduction process, is not true. This criterion is based on the commonly occurring statement that both the naturally occurring 11-cis-rhodopsin and the synthetic 9-cis-rhodopsin lead to the same primary photochemical product, bathorhodopsin. Of course, the existence of a common intermediate generated from either 11-cis- or 9-cis-rhodopsin would support the commonly proposed mechanism of cis-trans isomerization as the primary event in the visual transduction process. However, the data obtained by Spalink et. al. (30) indicate that a common intermediate is not generated from both rhodopsins. 

General Mechanisms of Cis-Trans Isomerization A Rapid Survey... 

Mechanisms of Cis-Trans Isomerization around the Carbon-Carbon Double Bonds via the Triplet State 3.2.2... 

Nonempirical calculations of reaction complexes formed during anionic polymerization ofbutadiene, in the presence of stilbenes, have been made [87]. The mechanism of cis-trans isomerization in the terminal unit of the living polymer consisted in concerted rotation about the Cp bond, and the migration of Li between and Cy... 

Figure 7.4. Syn-syn, anti-anti exchange (a) the angular twist mechanism of cis-trans isomerization and (b) Berry s pseudorotation mechanism of the exchange of equatorial and axial ligands in the trigonal bipyramid.

Rather less success has attended continuing efforts to label the mechanism of cis-trans isomerization in [MCl2(PR3)2] and related complexes. The best that can be said in view of current data is that a unique mechanism seems unlikely. 

The mechanisms of cis-trans isomerization of Pt(II) and Pd(II) complexes have been the subject of a great deal of study and controversy for many years. The arguments continue, but some clarification of the issues has been brought about by the publication of a welcome review by Anderson and Cross. The types of mechanism hitherto proposed have also been reviewed briefly by Favez et as an introduction to their study of the cis-trans isomerization of... 

The mechanisms of cis-trans isomerization of Pd(II) and Pt(II) complexes (except organometallics) have been considered in Chapter 4. The uncatalyzed cis-trans isomerization of [PtXR(PEt3)2] (X = halide, R = aryl) occurs by an initial release of halide ion. The dependence of rate on the solvent (acetonitrile or alcohols) has been attributed to hydrogen bonding which assists the dissociation of halide to give T-shaped three-coordinate intermediates, rather than to nucleophilic displacement of halide by solvent.However, others have shown that the first step has a value of -12 cm mol (X = Br, R = mesityl. 

---