Cyclization diradical mechanism

Along with a very wide synthetic application the Cope rearrangement continues to be a subject of intense debates. The key mechanistic question is whether the rearrangement of 1,5-hexadiene derivatives is concerted and passes via a six-electron aromatic transition state, or whether it involves the formation of a diradical intermediate, i.e. a cyclization-cleavage mechanism. In the former case, bond making and bond breaking occur synchronously (a survey of this question has been published210). 

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... 

Kinetic isotope effects have been reported that suggest that the thermal cyclization of enyne-allenes proceeds through a stepwise diradical mechanism (Scheme 121). This is even true if steric bulk at the alkyne terminus is large, contrary to theoretical predictions.182... 

An extension of the simplistic but practical diradical mechanism of equation (1) leads to the generalized di-TT-methane process given in equation (43). Thus, the initial 1,3-diradical, instead of cyclizing across C-3 and C-5 to the final vinylcyclopropane, bonds across C-2 and C-5 to generate a cyclopro-pyldicarbinyl diradical, which on opening of the C-2/C-4 bond affords the isomerized 1,4-diene. A num-... 

In 1970 Bauslaugh22 proposed that the regioselectivity of enone cycloadditions can be explained by a diradical mechanism without invoking exciplexes. In this approach, the regio-chemical outcome is regarded as a consequence of the competition between cyclization and reversion from 1,4-diradicals. It is assumed that the more stable biradicals must return at a faster rate to the ground state starting molecules and therefore lead to less product. 

The accumulated spectroscopic evidence clearly points to the participation of both 828 and 82a in the SET-photoinduced degenerate methylenecyclopropane rearrangement of d -lla. When combined with the chemical results, it is possible to propose a cation radical cleavage (CRCL)-diradical cyclization (DRCY) mechanism for this process (Scheme 24). Cation radicals < 2-773 and undergo... 

A possible mechanism for the observed transformation includes the sequence outlined in Scheme 2.327 (i) propargyl (A) - allene (B) tautomerization, (ii) 8jt-cyclization (C), (iii) N-0 cleavage (diradical D), (iv) diradical recombination (cyclopropanone derivative E), and (v) one or two step cyclizations of the azadienyl cyclopropanone into azepinone F. The occurrence of cyclopropanones (type E), as intermediates, is supported by the formation, in some cases, of isoindoles (type I) (789) as minor products (Scheme 2.327) (139, 850, 851). 

This finding confirms an opinion that, at least in some cases, diradicals such as 18 can be the actual intermediates in the non-concerted Cope rearrangement, so-called stepwise cyclization-then-cleavage mechanism. Berson and coworkers who previously excluded diyl intermediate in the acetylenic Cope rearrangement20 designed in their next work24... 

Thermal or photochemical extrusion of nitrogen from 1-arylbenzotriazoles (114) leads to the formation of carbazoles (Scheme 14). The mechanism is believed to involve cyclization of a diradical (115) or an iminocarbene (116) to the 4a/f-carbazole (117) followed by an aromatizing hydrogen... 

The cyclization of Z-hex-3-en-l,5-diyne (equation 81) forming the diradical intermediate 52 was discovered in 1972 and was initially of largely mechan-Then it was discovered that this reaction occurs in the... 

The pyrolysis of 1-acyl-1,2,4-triazoles 127 proceeds readily under flash vacuum conditions at 800 °C to provide a useful synthesis of 5-substituted oxazoles 13073. The mechanism involves initial 1,2-acyl migration to give the 3H-form 128 which loses Ni to give the diradical 129 and this then cyclizes. For aromatic R groups yields are... 

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