Thermal interconversion

The Cornforth rearrangement involves the thermal interconversion of 4-carbonyl substituted oxazoles, with exchange between the C-C-O side-chain and the C-C-O fragment of the oxazole ring. These reactions generally involve compounds where a heteroatom (-OR, -SR, -Cl) is attached to the 5-position (R2) of the starting oxazole. 

The 1,3-allylic rearrangement has also been observed with several cyclic sulfoxides. For example, the interesting thermal interconversion of the bicyclic stereoisomeric pair 141... 

W. E. von Doering, C. Sotiriou-Leventis, and W. R. Roth, Thermal interconversions among 15-cis, 13-cis, and... 

For the thermal interconversion (the equilibrium tends to lie over towards the diene), the HOMO for the diene (17 cf. p. 344) will be... 

The fusion of the benzene ring at the 3-4 bond of the furoxan system dramatically decreases the energy barrier of thermal interconversion, with the consequence that, in benzofuroxan derivatives 10a and 10c, the equilibrium is active at room temperature [6, 7, 10] (Scheme 6.2). 

In its heyday, DTA analysis was very useful for the study of compound polymorphism and in the characterization of solvate species of drug compounds. It was used to deduce the ability of polymorphs to undergo thermal interconversion, providing information that could be used to deduce whether the system in question was monotropic or enantiotropic in nature. For instance, the enthalpies of fusion and transition were measured for different polymorphs of sulfathiazole and methylprednisolone [24]. The DTA thermograms shown in Fig. 4.5 demonstrate that Form-I is metastable with respect to Form-II, even... 

However, from elegant labelling studies by Wentrup and coworkers on the gas-phase thermal interconversion of phenylnitrene and 2-pyridylcarbene, it became evident that an alternative and equally viable intermediate in these nitrene ring expansions is the azacycloheptatrienylidene (216) or its tautomer, the cumulated system (217) (81AHC(28)23l). 

Early kinetic studies on the structural isomerization of cyclopropane to propene provided estimates of activation parameters73 75 and prompted speculation that the reaction might well involve a trimethylene diradical intermediate. This possibility seemed reinforced when the thermal interconversion of the els and trans isomers of l,2-d2-cyclo-propane at 414 to 474 °C (equation 1) was reported in 195876. This structurally degenerate isomerization was found to be substantially faster than conversion to deuterium-labeled propenes—about 24 times faster at the high pressure limit76 77. 

The forward and reverse directions of an elementary reaction represent a particular case of a reversible process. The principle of microscopic reversibility states that, for such a reaction, there is a single common pathway, i.e. a single transition structure for forward and reverse directions [ 14]. For the thermal interconversion of cis- and trans-stilbenes, the single transition structure comprises two PhCH moieties, singly bonded with a dihedral angle of 90°. (In contrast, the forward and reverse paths in the more complicated photochemical interconversion of cis- and trans-stilbenes are different, and the principle of microscopic reversibility does not apply.)... 

Scott, L. T. Roelofs, N. H. 1987 Benzene ring contractions at high temperatures. Evidence from the thermal interconversions of aceanthrylene, acephenanthrylene and fluoranthrene. J. Am. chem. Soc. 109, 5461-5465. 

Scheme 5 Feringa s unidirectional rotor [32], The 360° unidirectional rotation is achieved in a four-step process involving alternative photo-isomerization of the double bond and thermal interconversion of helicity...

Photochemical 1,2-silyl migration from l-tris(trialkylsilyl)cyclotrisilene 72 occurred to give the corresponding bicyclo[1.1.0]tetrasilane 70 (equation 54)132. During the photochemical and thermal interconversion among the SLiRg isomers 69, 70 and 72, the corresponding 1,3-tetrasiladiene derivative was never detected. 

The analysis in Section 22.3 indicates that the thermal interconversion of a diene with a cyclobutene should occur by conrotation. The reaction is allowed in both directions, as long as a conrotatory motion is followed. However, usually only the conversion of the cyclobutene to the diene is observed because the cyclobutene is destabilized by angle strain and is present only in trace amounts at equilibrium. An example of the opening of a cyclobutene to form a diene is provided by the following equation ... 

Addition of N-phenylmaleimide to compound (78 R1 = R2 = Ph) gives a mixture of the exo and endo adducts (80 R1 = R2 = Ph). When pure samples of these are heated in boiling xylene, thermal interconversion of the adducts is observed indicating that under these conditions retro-cycloaddition occurs (69JA6891,79CL1029). The unsubstituted derivative (78 R1 = R2 = H) and N-phenylmaleimide yielded only the endo adduct (80 R1 = R2 = H) (79TL4493). 

In Scheme 1 the respective rate constants of thermal interconversion of R and S(k), R and S (k ), and of deactivation of R and S (B) are equal. If this were not so, spontaneous or nonpolarized-light induced deracemization would occur, which never has been observed. The interconversion rates of Pr and Ps(k3 and k 3 ) may be different, if the products are diasteromers. If the ground state enantiomerization is slow compared to the other reactions then the kinetics of independent parallel reactions apply if not, those of dependent parallel reactions. 

The structural isomerization of cyclopropane to propene does not involve radical chain processes if a trimethylene diradical intermediate is involved, it must be of such a short lifetime that it may not be easily trapped through gas-phase intermolecular reactions Yet the trimethylene diradical hypothesis does account for the thermal interconversion of cis and trans 1,2-d2-cyclopropanes in a most plausible manner. Homolytic cleavage of one carbon-carbon bond would form a 1,3-diradical intermediate rotations of terminal methylene groups in this trimethylene diradical followed by reformation of the cyclopropane ring would rationalize the isomerization According to this model, the net outcome of a stereomutation process would be dictated by the relative magnitudes of rate... 

A. Kitagawa, T. A frustrated Cope rearrangement thermal interconversion of... 

Cordes, M. H. J. Berson, J. A. Thermal interconversion of a pair of diastereomeric cyclopropanones. An upper limit for a cyclopropanone-oxyallyl energy separation, 7. Am. Chem. Soc. 1992,114,11010-11011. 

Valuable information about the photocycle is derived from low temperature spectroscopy, because thermal interconversions of the photointermediates have characteristic transition temperatures, and at certain chosen temperatures single intermediates or mixtures of only two intermediates will accumulate. Thus, the batho-intermediate (626 nm absorption peak at this temperature) is stable [222,245-247] at and below 150 °K, but is converted into the L form (543 nm absorption band) above this temperature [248,249]. The L intermediate is converted into the M form (418 nm absorption peak) upon warming to 220 °K, particularly at higher salt concentrations [48]. 

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