Diradicals 1,4-aryl

D Auria synthesized bispyrrole[3,2-A3, 2 -f ]dithianes such as 453 as part of the investigation of the photochemical reactions of aryl and heteroarylalkenes 452 in the presence of nitroarenes <1996T14253>. A number of rearrangements lead to the diradical intermediate 455, which then dimerizes to 453 (Scheme 34). 

The feasibility of hydrogen abstraction at the peptidyl a-carbon hydrogen bond by 1,4-aryl diradicals has been determined by examining a model reaction, i.e. abstraction of deuterium from dideuterioglycine by aryl radicals. The results have biological implications for the reactivity of the enediyne anti-tumour antibiotics with proteins. The non-Arrhenius behaviour of hydrogen-abstraction reactions by radicals has been investigated. For a number of reactions studied the enthalpy of activation was found either to increase or to decrease as a function of temperature. 

Ketones with the general structure R — C(O) —C(R R2) — C(R3,R ) — CH(R5,R6) (R = aryl or alkyl) undergo intramolecular H-abstraction on excitation in competition with cleavage of one of the C(O) — C bonds.1 The resulting 1,4-diradical then either closes to a cyclobutanol or undergoes cleavage of the central C2 — C3 bond with formation of an enol and an alkene. 

A second reaction of this type is of interest because it reveals the nature of the diradical produced. Here we have l-benzoyl-2-aryl-3-phenylcyclopropanes. The aryl groups in the study were p-cyanophenyl, anisyl and phenyl50. It was observed that the 2,3-trans,trans isomer, in which the benzoyl group was trans to both aryl groups, on photolysis afforded the cis-trans isomers in which the substituted aryl group was now cis to benzoyl. One might... 

When electron-withdrawing groups are present on the 4-carbon, triazoline thermolysis is insensitive to solvent polarity a homolytic decomposition to a singlet diradical similar to that formed in photolysis is proposed to account for the selective formation of aziridine in these cases (Scheme 162).67,172,454 The lower thermolysis temperatures of N-aryltriazolines ( 25°C)6 7,322 as compared to those for the N-alkyl compounds ( 90° C)6 7 are consistent with the stabilization of the diradical by aryl groups. 

Orfanopoulos et al. studied the photochemical reaction of alkenes, aryalkenes, dienes dienones, and acyclic enones with [60]fullerene to obtain various substituted cyclobutylfullerenes [240,241,243,247], For example, the photocycloaddition of cis- and Irans-1 -(p-mcthoxyphenyl)-1 -propenc 68 to C6o gives only the trans [2 + 2] adducts (Scheme 27), thus the reaction is stereospecific for the most thermodynamically stable cycloadduct. A possible mechanism includes the formation of a common dipolar or biradical intermediate between 3C o and the arylalkene. Subsequent fast rotation of the aryl moiety around the former double bond leads exclusively to the trans-69 [2 + 2] adduct. Irradiation of this product, yielded 90% trans-68,10% cis-68 and cycloreversion products. Thus, a concerted mechanism can be excluded because the photocycloreversion is expected to give the trans-68 as the only product. These results can be explained by the formation of a common dipolar or diradical intermediate. Similarly, cycloreversion products from C6o and tetraalkoxyethylene... 

Inue et al. claimed the first report of the infrared detection of an aryl nitroso oxide, made from the reaction of 4-nitrophenylnitrene with molecular oxygen.70 The singlet nitroso diradical is formed from the triplet nitrene and oxygen at a temperature of 50 K in argon or xenon matrices. The reaction does not occur at 40 K. 

The lack of reactivity of the aryloxazolinones (65) in photocycloaddition to many of the olefins other than 1,1-dimethoxyethene and furan probably results from efficient decay of E2 or D. Exciplex E2 and diradical D are proposed as intermediates in these cases for several reasons. Exciplex formation is most likely dependent on olefin ionization potential, and the ionization potential of many of the un-reactive olefins are intermediate between the ionization potential of furan and 1,1-dimethoxyethene as determined from the maxima of tetracynoethylene olefin charge transfer bands60 66,67. Although ds-2-butene does not form a cycloadduct with 2-phenyl-2-oxazolin-4-one (65a), ds-2-butene is isomerized to rram-2-butene during the irradiation52. Cis-trans isomerization is expected from decay of a triplet diradical. Decay of the exciplex and diradical intermediates in competition with reaction presumably results from steric hindrance from the aryl substituent. The olefins which give cycloadducts, furan and 1,1-dimethoxyethene, are expected to produce low steric hindrance with the aryl substituent in an exciplex or diradical. 

Aryl cations are highly reactive intermediates with two possible electronic configurations, the singlet and triplet states. The former is a localized cation with a vacant a orbital at the dicoordinated carbon atom, whereas the latter has a diradical character with single occupancy of the g orbital and the charge being delocalized in the... 

A further example of the same type of rearrangement is afforded by the l 2-benzisothiazoline-3-thiones. If the benzisothiazolium salt (40) is treated with thioacetic acid, the product is the 3-arylimino-3/f-1,2-benzodithiole (41), provided R is an aryl group when R is alkyl, the product is the benzisothiazoline-3-thione (42).32 Structures of types 41 and 42 are, however, in equilibrium with each other at 150°, presumably via a dipolar intermediate [Eq. (3)].32 A diradical... 

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