Diradicals 1,4-, cleavage

The kinetically controlled Cope rearrangement of 2,5-bis(4-methoxyphen-yl)hexa-l,5-dienes induced by photosensitized electron transfer to DCA was examined by Miyashi and co-workers [101-103]. Remarkable in this context was the temperature-dependent change of the photostationary ratio of this rearrangement, yielding the thermodynamically less stable compound at — 80°C in 96%. A radical cation-cyclization diradical cleavage mechanism (RCCY-DRCL) is... 

The subsequent dark reactions readily are understood as typical of diradicals. Cleavage of 1 at Ca-CB gives ethene and an enol, which rearranges to the ketone. Alternatively, 1 can cyclize to a cyclobutanol ... 

Ikeda, H., Minegishi, T., Abe, H., Konno, A., Goodman, J.L., and Miyashi, T., Photoinduced electron-transfer degenerate Cope rearrangement of 2,5-diaryl-l,5-hexadienes a cation radical cyclization-diradical cleavage process,/.Am Chem. Soc., 120, 87,1998... 

Photofragmentation of chloromethyldiazirine in the presence of several gases was assumed to involve C—N bond cleavage to form a diradical, which undergoes a second C—N bond cleavage to form the carbene (8OMI508OI). 

This result can be rationalized in terms of a cyclopropyl diradical structure by noting that the bond cleavage of the intermediate will occur to give the more stable of the two possible 1,3-diradicals. The cyclopropane ring in the final product will then incorporate this terminus ... 

The thermal reversal of the photochemical a-cleavage, i.e., the direct recombination of the resulting radical pair or diradical, can be recognized as such only when at least one of the a-atoms is chiral and is epimerized in the process. In fact, the frequently rather low quantum yields observed in the phototransformations of nonconjugated steroidal ketones may be largely due to the reversal of a-cleavage. 

With certain donor substituents at C-3 the experimental findings may be rationalized rather by a diradical mechanism, where formation of the new carbon-carbon single bond leads to a diradical species 6, which further reacts by bond cleavage to give the diene 2 ... 

A mechanism has been formulated that would involve formation of diradical species 3 and 4, which however might not be real intermediates. At least one substituent at C-3 is required in order to stabilize the radical 4, and thereby facilitate the cleavage of the C-2/C-3 bond ... 

The photolytic cleavage of cyclic ketones 14 leads to formation of a diradical species, that can undergo analogously the various reactions outlined above. The decarbonylation followed by intramolecular recombination yields a ring-contracted cycloalkane 15 ... 

The direction of ring opening by homolytic cleavage of a cyclopropane bond is controlled by the stability of the diradical species formed. Upon heating of the mono-deuterated vinylcyclopropane 3, a mixture of the two isomeric mono-deuterated cyclopentenes 4 and 5 is formed ... 

This reaction, called Norrish Type II cleavage, involves intramolecular abstraction of the y hydrogen followed by cleavage of the resulting diradical (a... 

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). 

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... 

Barltrop and Coyle<12) have presented strong evidence for the participation of a discrete diradical intermediate in type I cleavage. In this study a... 

The initial step in a photochemical extrusion reaction is again cleavage of a bond to give a pair of radicals or a diradical, but now a second bond is broken immediately after the first one, thus leading to the extrusion of a small molecule as CO, C02, N2 or S02. As in a recent review on photoextrusion of small molecules 248) only such reactions will be outlined wherein the small molecule that is to be expelled is initially part of a ring, and where the product formed is again a cyclic system. 

The cleavage of the intermediate 1,4-diradical can also become the major path as in the synthesis of methylenecyclopentane (4.8) 408). 

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