Hydrogen transfer rearrangements

PET P-Hydrogen transfer, rearrangement and de-carboxylation Benzoic acid and vinyl terephthalate ... 

PET decomposes via P-hydrogen transfer, rearrangement and decarboxylation, with major products benzoic acid and vinyl terephthalate. 

In the reaction of two olefins, both olefins must be adsorbed on active sites that are close together. One of these olefins becomes a paraffin and the other becomes a cyclo-olefin as hydrogen is moved from one to the other. Cyclo-olefin is now hydrogen transferred with another olefin to yield a paraffin and a cyclodi-olefin. Cyclodi-olefin will then rearrange to form an aromatic. The chain ends because aromatics are extremely stable. Hydrogen transfer of olefins converts them to paraffins and aromatics (Equation 4-11). 

Duffield and coworkers65 studied the El- induced mass spectra of five arene- (215-219) and four alkane sulfonylthioureas (220-223) and observed two rearrangement processes, namely loss of S02 from 215-219 and the elimination of ArS02 and RS02 with the thione sulfur atom from 215-223. The other fragmentations involved simple bond cleavages with and without hydrogen transfer (equation 48). The loss of H2S was evident for all the compounds studied except 221 and 222. It was, however, found to be a thermal and not an ionization process. 

The rare [1,4] hydrogen transfer has been observed in radical cyclizations. With respect to [1,7] hydrogen shifts, the rules predict the thermal reaction to be antarafacial. Unlike the case of [1,3] shifts, the transition state is not too greatly strained, and such rearrangements have been reported, for example,... 

Finally, we note the informative work of Garcia-Garibay and co-workers, who have extensively studied QMT in hydrogen transfer reactions in the excited triplet states of ort/zo-alkylarylketones, for example, 6 —> 7, which are electronically similar to radical rearrangements. 

Two of the three general types of secondary reactions resulting from photochemical a-cleavage of carbonyls, namely molecular rearrangement and hydrogen transfer to yield aldehydes or ketenes, have been discussed. The third type of reaction observed, decarbonylation, will be discussed in this section. The discussion will begin with the decarbonylation of small ring carbonyls. By way of example of this type of reaction, diphenylcyclopropenone decarbonylates upon photolysis to yield diphenylacetylene(57) ... 

Sym-octahydrophenanthrene (HgPh) would be expected to follow the same rearrangement-dehydrogenation reactions as Tetralin, except with more isomer and product possibilities. The reactions shown in Figure 1 illustrate the many structures expected from sym-HgPh in the presence of free radical acceptors. Unlike Tetralin, hydrophenanthrenes have multiple structures which each, in turn, form various isomers. The amounts of these isomers are dependent upon the type of hydrogen-transfer reactions and the environment of the system. 

The above results show that the dihydro-aromatics do not directly contribute to rearrangements. Secondly the dihydroaromatics rapidly aromatize by hydrogen transfer or dispropositionation. This implies that the rearrangement... 

A different result was obtained in the cycloaddition to methylenecyclo-propanes 216-218 tearing alkoxycarbonyl substituents on the cyclopropyl ring. In this instance, 1,2,3-triazoles 220 isomeric with the triazolines 219 were formed in the reaction [57]. The formation of triazoles 220 is rationalised by the intermediate formation of triazolines 219, which are unstable under the reaction conditions and undergo a rearrangement to the aromatic triazoles via a hydrogen transfer that probably occurs with the assistance of the proximal ester carbonyl (Scheme 35). The formation of triazoles 220 also confirms the regio-chemistry of the cycloaddition for the methylene unsubstituted methylene-cyclopropanes, still leaving some doubt for the substituted ones 156 and 157. 

Gasanov, 1979, see also Gasanov et al., 1977). Here, the point of reference is the rate constant for trapping primary alkyl radicals, which seems to be established with reasonable precision. The experiments showed, as expected, that the most rapid rearrangement was with [26, n = 5], corresponding to 1,5-hydrogen transfer. 

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