Cleavage thermal

Finally the chemical aromatization of Ring A which occurs in nature in the biosynthesis of estrogens must be mentioned. It can be done by thermal cleavage of the C-19 methyl group in 1,4-dien-3-ones (H.H. Inhoffen, 1940 C. Djerassi, 1950) and was later achieved at lower temperatures with lithium — biphenyl in THF (H.L. Dryden, Jr., 1964). 

In the commercial Gorham process (2), the extremely reactive PX is conveniendy generated by the thermal cleavage of its stable dimer, Vo-di- -xyljIene (DPX), a [2.2]paracyclophane [1633-22-3] (3). In many instances, substituents attached to the paracyclophane framework are carried through the process unchanged, ultimately becorning substituents of the polymer in the coating. 

Thermal cleavage of boron tris(trifluoromethane)sulfonate and boron tris(pen-tafluoroethane)sulfonate was studied. Careful identificadon of the products has led to the proposal of a mechanism and an account for this process [121] (equation 93). 

Electrochemical addition of perfluorobutyl iodide to 2-methyl-3-butyn-2-ol followed by basic dehydroiodmation and thermal cleavage gives perfluorobutyl-acetylene in an overall yield of 83% [34] (equation 26)... 

The remaining major method for the reduction of the C=N+ functionality is the reaction with formic acid. The first report was that of Luke , who found (95) that thermal cleavage of l,l-dimethyl-2-methylenepyrrolidinium formate was accompanied by reduction. Lukes then explored the generality... 

In general, the Ge-Ge bond is readily cleaved by Br2 either at ambient or elevated temperatures but the compounds are stable to thermal cleavage at moderate temperatures. Ge2Re compounds can even be distilled unchanged in air (like Si2R but unlike the more reactive Sn2R ) and are stable towards hydrolysis and ammonolysis. 

For a summary of methods of radical formation, see Giese, B. Radicals in Organic Synthesis Formation of Carbon-Carbon Bonds Pergamon Elmsford, NY, 1986, p. 267. For a review on formation of free radicals by thermal cleavage, see Brown, R.F.C. Pyrolytic Methods in Organic Chemistry Academic Press NY, 1980, p. 44. 

Thermal cleavage of cyclobutanesto give two alkene molecules (cyclorever-sion, the reverse of 2 -I- 2 cycloaddition) operates by the diradical mechanism, and the [ 2s -I- o2a] pathway has not been found " (the subscripts a indicate that cr bonds are involved in this reaction). 

In the thermal cleavage of cyclohexadienes, then, the positive lobes must lie on the same side of the plane, requiring disrotatory motion ... 

Carbocation-carbanion zwitterionic intermediates were proposed for the thermal cleavage of several cyclic compounds. In most of these reactions the ionically dissociating bond belongs to one of four strained ring systems, i.e. cyclopropane (13), cyclobutane (14), cyclobutene (15) or norbornadiene (16). The mechanism is distinguished from the formation of a diradical intermediate through homolysis in terms of solvent and substituent effects... 

As previously pointed out in the discussion of ring size effects on bond homolyses the largest steric acceleration by bulky substituents is expected for the thermal cleavage of C-C-bonds in tetra- or hexasubstituted ethanes 26. Im comparison to azoal-... 

THERMAL CLEAVAGE OF BENZYLIC BONDS IN AROMATIC SYSTEMS... 

Reich and Cram 111>112> have reported the results of a series of experiments, which all indicate thermal cleavage of [2.2]paracyclophane to the p,p-dimethylenebibenzyl diradical (157). After heating [2.2]para-cyclophane (2) at 250 °C in p-diisopropylbenzene, they isolated p,p-dimethylbibenzyl in 21% yield as the sole non-polymerizable product. 

Thermal cleavage of C—C bonds has been studied in cyclopropanes and cyclophanes, and particularly extensively in highly substituted alkanes. Riichardt and his school discovered a linear correlation between the experimental activation enthalpy for the homolysis of the weakest bond in overcrowded ethanes and the strain in the ground states of these molecules in accordance to MM2 calculations (284). 

Recent calculations [329] suggest that the first reaction step of the Dotz benzannulation reaction may not necessarily be the thermal cleavage of one carbonyl ligand. 

Electron-rich olefins are nucleophilic and therefore subject to thermal cleavage by various electrophilic transition metal complexes. As the formation of tetraaminoethylenes, i.e., enetetramines, is possible by different methods, various precursors to imidazolidin-2-ylidene complexes are readily available. " Dimerization of nonstable NHCs such as imidazolidin-2-ylidenes is one of the routes used to obtain these electron-rich olefins [Eq. (29)]. The existence of an equilibrium between free NHC monomers and the olefinic dimer was proven only recently for benzimidazolin-2-ylidenes. In addition to the previously mentioned methods it is possible to deprotonate imidazolidinium salts with Grignard reagents in order to prepare tetraaminoethylenes. " The isolation of stable imidazolidin-2-ylidenes was achieved by deprotonation of the imidazolidinium salt with potassium hydride in THF. ... 

Cp2Ti=CH2 (Cp = cyclopentadienyl) is liberated from a titanacyclo-butane primer by thermal cleavage in the presence of an iminoborane (product IX) (81). Alkynes may also undergo [2 + 2]-cyclodimerizations with unsaturated polar molecules. Rather polar alkynes seem to be favorable, e.g., ethoxyethyne, which can react with hexafluoroacetone to give the rather unstable product X (78). 

Triazolidones and their sulfur analogues are less thermally stable than triazoles as illustrated by the thermal cleavage of (26) (Equation (10)) <73BCJ2215>. 

A wealth of experimental data on the thermal dioxetane decomposition and the excited-state generation in the thermolysis process has been comprehensively surveyed in previous reviews". During the last decade, computational elucidation of the thermal cleavage received major attention and in the present subsection we consider the relevant smd-ies. Computations on the dioxetane thermolysis were conducted by both ab initio and semiempiricaP methods at different levels of sophistication. 

There often is a correlation between the strain relief in a reaction and the rate of thermolysis, but other factors may also be of importance. The Cg propellanes 16 and 17 have quite different reactivities. Whereas 16 undergoes thermal cleavage at 360 17 undergoes cleavage at 25 °C. ° Aside from the difference in... 

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