Mechanism of isomerization

Chandler D 1978 Statistical mechanics of isomerization dynamics in liquids and the transition state approximation J. Chem. Phys. 68 2959... 

The reversible reactions are initiated by an equilibrium between neutral and ionized forms of the monosaccharides (see Fig. 6). The oxyanion at the anomeric carbon weakens the ring C-O bond and allows mutarotation and isomerization via an acyclic enediol intermediate. This reaction is responsible for the sometimes reported occurrence of D-mannose in alkaline mixtures of sucrose and invert sugar, the three reducing sugars are in equilibrium via the enediol intermediate. The mechanism of isomerization, known as the Lobry de Bruyn-... 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

Metallacyclobutanes have been proposed as intermediates in a number of catalytic reactions, and model studies with isolated transition metallacyclobutanes have played a large part in demonstrating the plausibility of the proposed mechanisms. Since the mechanisms of heterogeneously catalysed reactions are especially difficult to determine by direct study, model studies are particularly valuable. This article describes results which may be relevant to the mechanisms of isomerization of alkanes over metallic platinum by the bond shift process and of the oligomerization or polymerization of alkenes. 

The conjugated diene (including the trans-trans, trans-cis, and cis-cis isomers) can further add ethylene to form Cg olefins or even higher olefins (/). The mechanism of isomerization is proposed to be analogous to butene isomerization reactions (4, 8), i.e., 1-butene to 2-butene, which involves hydrogen shifts via the metal hydride mechanism. A plot of the rate of formation of 2,4-hexadiene vs. butadiene conversion is shown in Fig. 2. 

Chemistry Serpone, N. and Bickley, D. G., Kinetics and Mechanisms of Isomerization and 29 167... 

The stmcture of pyochelin (for a detailed bibliography, see (57)), a secondary siderophore of Pseudomonas aeruginosa and of Burkholderia cepacia was established (75) as 2-(2-o-hydroxyphenyl-2-thiazolin -yl)-3-methylthiazolidine-4-carboxylic acid. It consists of a mixture of two easily interconvertible stereoisomers (pyochelin I and II) differing in the configuration of C-2". They can be separated by chromatography, but in methanolic solution (not in DMSO) the equilibrium (ca. 3 1) is restored quickly. For a discussion of the mechanism of isomerization, see (57, 577). 

N-labelling has been used to aid the investigation of the mechanisms of isomerization no oxygen exchange between the NO and N02 ligands was observed. " ... 

All the olefins involved in these studies are simple olefins to which energy transfer from ketones should be endothermic. Although the details of the mechanism of isomerization are still a matter of some debate,5,71 it is generally agreed that isomerization takes place by addition of the sensitizer to the olefin to form a new intermediate which may be of a biradical nature. Thus if energy transfer is not favored, there is another mechanism by which ketone sensitizers can induce olefin isomerization, and the observed quantum yields and photostationary states may differ sharply from those predicted by the energy transfer mechanism. 

The acetylene may be unsubstituted (R = H) or substituted, in which case the reaction proceeds even more readily. The resulting compound is known as 1.2-dicarbtw/aY -dodecaborane(l2), or the ortho carborane, and is isoelectronic and isostructur.il with [BI2HI2]2-. It is stable to both heat and air, but it isomerizes at high temperatures to the 1,7 ( meta or neo isomer) and the 1,12 ( para isomer) (Fig. 16.51). The mechanism of isomerization, thought to be intramolecular, has been discussed for many years.133... 

Indications of the mechanism of isomerization of saturated hydrocarbons were obtained by Ciapetta (C3), who observed that olefins were isomerized over nickel-silica-alumina catalyst at appreciably lower temperatures than were the corresponding saturated hydrocarbons, suggesting that olefins were intermediates in the reaction. Ciapetta also suggested that the rearrangement of the carbon skeleton took place via a carbonium... 

The topic of electron transfer induced geometric isomerization is treated in a separate section, because we wish to emphasize the existence of two fundamentally different mechanisms of isomerization. Although both mechanisms are initiated by an electron transfer step, the key intermediates involved in these isomerizations are fundamentally different. The interaction of acceptor sensitizers with donor olefins leads to the isomerization of alkene radical cations, whereas the reaction of donor sensitizers with acceptor olefins leads, eventually, to the population of alkene triplet states. 

The third mechanism of isomerization, photoinduced rearrangements of radical cations, has been pursued in a variety of systems. Matrix isolated radical cations have been noted to undergo some rigorous reorganizations as well as subtle ones. For example, the ring opening of cyclohexadiene to hexatriene radical cation and the interconversion of its different rotamers have been achieved by irradiation with UV or visible light [173-174]. 

Fig. 2. Mechanisms of bond-shift isomerization on metal catalysts, (a) Anderson-Avery mechanism (63). (b) Muller-Gault mechanism for isomerization of neopentane on Pt (64). (c) McKervey-Rooney-Samman mechanism (69). (d) Muller-Gault mechanism of isomerization of isobutane on Pd (64). (e) Clarke-Rooney mechanism (46). (f) Garin-Gault mechanism (65).

As discussed in Section IV, Barron et al. (55, 61) found the cyclic mechanism of isomerization to be predominant, perhaps the sole route, on a highly dispersed platinum-alumina (0.2% w/w Pt). The cyclic mechanism was shown to be important also over platinum films and supported platinum of moderate dispersion (>100 A). Here, although the product distributions were very different from that found over the dispersed catalyst, the initial product distributions at 300°C were practically identical in the isomerization and in methylcyclopentane hydrogenolysis. At lower temperatures they were somewhat different as they also were at all temperatures on platinum films. It was suggested that, especially on platinum films, a bond-shift isomerization could accompany the cyclic... 

The mechanism of isomerization of amine glycosides has been discussed by Howard, Kenner, Lythgoe and Todd,107 while the literature regarding N-glycosides in general has been briefly reviewed by Honeyman and Tatchell.143... 

Scheme 24. Reactions of MjCfCO), (M = Fe, Ru, Os) with phosphines and diphosphines and the mechanism of isomerization of the Ru5C(CO),3(L-L) species (L L = dppm, dppe, dppp, and dppb).

Ono and Ware"" have measured the absorption, emission, and excitation spectra, the fluorescence decay times, and the quantum yields of a series of substituted diphenylmethylenes in rigid matrices at low temperatures. Acean-thrylene shows S2- So emission in hexane with a yield of 0.017 and lifetime of 4.3 ns. The low-temperature fluorescence spectra of bis-2-naphthyl-alkanes and their derivatives have been studied. Excimer formation is an activated process. The fluorescence and absorption spectra of 1,1-diphenyl-ethylenes have been analysed in some detail by Gustav and Bolke. " The S — Si transitions in trans isomers of phenylnaphthylethylenes have been assigned by picosecond absorption spectroscopy. Effects of solvent viscosity and the role of conformers in the mechanism of isomerization are elucidated. The production of non-equilibrium conformer concentrations in glassy solutions of diarylethylenes at 77 K due to restrictions imposed by the solid matrix has also been reported. Free jet excitation and emission spectra of diphenyl-butadiene show clearly the lowest excited Ag state and give a lifetime of 52.8 ns for 0-0 excitation.Electric field-induced charges in the optical... 

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