Isomerization twist mechanisms

Intriguingly, the conical intersection model also suggests that E,Z-isomerization of acyclic dienes might be accompanied by conformational interconversion about the central bond, reminiscent of the so-called Hula-Twist mechanism for the efficient ,Z-photo-isomerization of the visual pigment rhodopsin in its rigid, natural protein environment101. A study of the photochemistry of deuterium-labelled 2,3-dimethyl-l,3-butadiene (23-d2) in low temperature matrices (vide infra) found no evidence for such a mechanism in aliphatic diene E,Z -photoisomerizations102. On the other hand, Fuss and coworkers have recently reported results consistent with the operation of this mechanism in the E,Z-photoisomerization of previtamin D3 (vide infra)103. 

The E,Z-photoisomerization of previtamin D to tachysterol has also received recent attention. Jacobs and coworkers examined the process in various solvents at 92 K and found evidence for the formation of a triene intermediate which converts thermally (Ea ca 6.5 kcal mol 1) to the more stable tEc rotamer of tachysterol (tEc-T equation 58)230. The rate of this conversion is viscosity dependent. They identified this intermediate as the cEc rotamer, produced by selective excitation of the cZc rotamer of previtamin D. In a re-examination of the low temperature ,Z-photoisomerization of previtamin D as a function of excitation wavelength, Fuss and coworkers have suggested an alternative mechanism, in which tEc-1 is produced directly from cZc-P and cEc-T directly from tZc-P (equation 59)103. This mechanism involves isomerization about both the central double bond and one of its associated single bonds—the hula-twist mechanism of Liu and Browne101 — and involves a smaller volume change than the conventional mechanism for ,Z-isomerization. The vitamin D system has also been the subject of recent theoretical study by Bemardi, Robb and Olivucci and their co workers232. 

These parameters often parallel one another since they are related to similar characteristic of the system (ehange in number of particles involved in the reaction etc.). The catalyzed hydrolysis of CrjO by a number of bases is interpreted in terms of a bimolecular mechanism, and both AS and AK values are negative. In contrast the aquation of Co(NH2CH3)5L (L = neutral ligands) is attended by positive AS and AK values. The steric acceleration noted for these complexes (when compared with the rates for the ammonia analogs) is attributed to an mechanism.There is a remarkably linear AK vs AS plot for racemization and geometric isomerization of octahedral complexes when dissociative or associative mechanisms prevail, but not when twist mechanisms are operative (Fig. 2.15). For other examples of parallel AS and AF values, see Refs. 103 and 181. In general AK is usually the more easily understandable, calculable and accurate parameter and AK is... 

Linkage isomerization cannot occur by a twist mechanism. 

Mechanisms leading to geometrical isomerization in complexes of this type resemble those already discussed with M(AA,)3 and M(Li)(L2), Secs. 7.6.2 and 7.6.3. As well as twist mechanisms, dissociation of either the unidentate ligand or one-ended dissociation of the... 

However, isomerization process has been found to be rapid and leads to an equilibrium mixture of both the tram and the cis isomers (Fig. 21. VI, VII)359,3645. The isomerization process proceeds via the twist mechanism and does not appear to involve an antimony-oxygen bond repture3645. 

An electrochemical study has shown reversible reduction to Mn" and oxidation to Mnlv species and a H NMR study shows that isomerization, of the compounds of unsymmetrical ligands such as MePhdtc, is slow on the NMR time scale at - 60°C and occurs via a trigonal twist mechanism rather than a dissociative one. 

To illustrate the approach, let us consider some of the data and deductions for the system Co[CH3COCHCOCH(CH3)2]3, measured in C6H5C1. It was found that both the isomerization and the racemization are intramolecular processes, which occur at approximately the same rate and with activation energies that are identical within experimental error. It thus appears likely that the two processes have the same transition state. This excludes a twist mechanism as the principal pathway for racemization. Moreover, it was found that isomerization occurs mainly with inversion of configuration. This imposes a considerable restriction on the acceptable pathways. Detailed consideration of the stereochemical consequences of the various dissociative pathways, and combinations thereof, leads to the conclusion that for this system the major pathway is through a tbp intermediate with the dangling ligand in an axial position as in Fig. l-12(c). 

Other isomerization mechanisms involving compounds containing chelating ligands are different types of twists. A number of twist mechanisms have been described, with different movements of the rings those most commonly considered are shown in Figure 12-10. 

Figure 7.11. Schematic state correlation diagram for the cis-trans isomerization of azobenzene for two reaction paths that correspond to a twist mechanism and an inversion mechanism, respectively (adapted from Rau, 1984).

By analogy with thermal processes (92), photoinduced isomerization s may occur either by intramolecular or intermolecular mechanisms. Intramolecular isomerizations are further classified either as "twisting" mechanisms, which involve no metal-ligand bond breaking, or as bond-rupture mechanisms. The latter term applies to reactions of chelate compounds in which one of the ligands is for a short time partially dissociated. Intermolecular mechanisms involve species other than those that make up the reactant complex. 

A particularly well studied system which undergoes photochemical cis -> trans isomerization is Pt(gly)2- The work on these compounds through 1969 has already been discussed in detail elsewhere (7), and thus only the major conclusions thereof will be mentioned here. A definitive study by F. Scandola et al. (93) showed that the photochemical cis - trans isomerization of this species occurs with an intramolecular "twisting" mechanism (Fig. 7). The thermally relaxed reactive excited state was proposed to have a pseudo-tetrahedral geometry as shown in Fig. 7. 

Mechanisms for the isomerization of square-planar complexes are either intra- or intermolecular. The simplest intramolecular process is a twist mechanism where the initial square-planar complex rearranges to its isomer via an intermediate or transition state having a T configuration ... 

Thermal isomerization, also occurs, but only with prolonged heating and only in the presence of excess glycine. Quantum yields of photoisomerization, however, are independent of the free glycine. In addition, when isomerizations are carried out in the presence of radioactively labeled free glycine, the thermal trans product contains labeled glycine, whereas the photochemical product contains none. Therefore, the two reactions must proceed by different mechanisms, the thermal reaction by an intermolecular pathway and the photochemical isomerization hy an intramolecular pathway involving a twist mechanism. 

---