Ring isomerization, mechanism

This reaction is particularly interesting because the starting compound has the trans, trans structure and the product must arise from a cis intermediate in order to form the ring. A possible isomerization mechanism involves a ketene intermediate. 

Fig. 13. C6 aging mechanism ring isomerization site deactivation.

By formation of seven- from five-membered rings The mechanism of thermal isomerization of the cyclobutene compound in equation (74) to thiepin (50) is directly analogous to that found in the synthesis of dihydrothiepins (221) and (222 equation 65). Compound (50) was the first monocyclic thiepin to have been identified (74T2093). This synthetic route has also been applied to thiepin (148) (74T2431). 

There are numerous other examples of stereochemical correspondence in propeller molecules. The propellane 19> 1e has only one energetically reasonable isomerization mechanism available a twisting motion about the C3 axis which corresponds to the three-ring flip as well as to the trigonal twist rearrangement (Fig. 5). Similarly, only one isomerization mechanism (stereochemically correspondent in the permutational sense to the zero-ring flip) is energetically reasonable for tri-o-thymotide (Id). 8,20)... 

Variable temperature xH-nmr studies of other hindered (in both ortho positions) triarylboranes have revealed 32> that these compounds are also mixtures of propeller-like stereoisomers on the nmr time scale at low temperatures, and that interconversion of these stereoisomers occurs with activation energies of ca. 14—16 kcal/mol (Table 3). These isomerizations are also interpretable in terms of two-ring flip mechanisms. 

Related to the mechanism of the above structure is the ring-chain-ring isomerization of 3-(2-pyridyl)-[l, 2,3 triazolo 1,5-zz pyrid-7-yl derivatives (139) into 6- 11, 2,3]triazolo 1,5-a ]pyrid-3-yl ] -2-pyridyl derivatives (141) via a diazo compound 140 [139],... 

Almost all rj2-arene complexes of [Os] are amenable to both intrafacial (i.e., ring-walk) and interfacial (i.e., face-flip) isomerization mechanisms, which allow the metal to coordinate to the most thermodynamically favorable position (Figure 2). Aromatic molecules bearing a redonor group (e.g., anisole, aniline, phenol) tend to place the metal across C5-C6 in order to... 

Treatment of acylated chloroenamines with sodium borohydride in methanol resulted in the formation of bicyclic lactams124 (Scheme 93). The final isomerization is the result of a proton catalyzed ring-opening-ring closure mechanism. 

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. 

It was suggested by Gegiou et al. that the photoisomerization of azobenzene probably proceeds via a mechanism different than the cis-trans isomerization of stilbene. Such an isomerization process in azobenzene could involve a pyramidal inversion of a nitrogen atom, in contrast to stilbene and its derivatives, where rotation about the central double bond is required. Based on the data, the AT->AC isomerization mechanism may be similar to that of the triphenylmethane dyes, whose ground state structure is known to resemble a three-dimensional, propeller-shaped D3 structure with the phenyl rings rotated 32 degrees from the central plane. 

The isotropic ESR spectra of 101 at temperatures accessible to the radical show signs neither of the existence of separate rotamers nor of line-broadening effects suggestive of their rapid interconversion. An INDO MO study has shown that the radical probably exists in two preferred propeller-like conformations (that of C3 symmetry is shown in 101 that of Cj symmetry is obtained by a rotation of any single thiophene ring in 101 through 180°). The calculations indicate the C3 conformer to be more stable by 0.75 kcal mol" and the barrier to interconversion of the two forms, by a preferred two-ring-flip mechanism, to be about 6 kcal mol . This low barrier to interconversion is consistent with the failure to observe effects of the isomerism in the ESR spectra at temperatures above those at which the radical dimerizes. 

The data presented here unlock the mystery of how fullerenes appear near C30 in laser desorption experiments when no graphitic building blocks are present at smaller sizes. The ring fullerene isomerization mechanism might also help explain the rather surprising propensity for C o formation in carbon arc rectors. ... 

In this section we present experimental evidence for a bifunctional alkane isomerization mechanism obtained by selective poisoning of the acidic sites of Pd-NiSMM with pyridine, which was pulse-injected into the liquid hydrocarbon feed stream. The possibility of additional poisoning of the metallic sites was checked by studying the hydrogenation of benzene and the isomerization and ring opening of methylcyclopentane (MCP). 

However, in studies conducted at about the same time on benzosilacyclobutenes, in which carbonyl compounds were inserted into the silacyclobutene ring (in yields which varied from 25-80%), the products were interpreted as arising from attack of an excited carbonyl species directly on the ring. The mechanism implies formation of a diradical intermediate, and explains the formation of minor isomeric products35 (equation 21). 

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