Biradicals formation

Occasionally, equilibria between a quinoid and a diradicaloid form of tetraazafulvaleiies of type 77 have been discussed (66AG303 72NKK100 79JOC1241). Based on ESR measurements, only traces of radicals (0.1% at 200°C) could be observed and therefore 77 (Ar = Ph) exists at room temperature predominately in the quinoid structure. Other authors stated that the thermochromism of 77 mainly results from a change in intermolecular interaction, not from biradical formation (84MI1030). 

Drivers for Performing Photochemical Biradical Formation in Micro Reactors... 

Beneficial Micro Reactor Properties for Photochemical Biradical Formation... 

Photochemical Biradical Formation Investigated in Micro Reactors Organic synthesis 87 [OS 87] Benzopinacol formation by two-radical combination... 

The final type of reaction that will be discussed is the highly interesting cross photocycloaddition of cyclic a, (3- unsaturated ketones with olefins. Examples were given in Eqs. 28—31. A general mechanism 94), to which there may be exceptions to be discussed later, would involve a triplet state of the enone and the reactions steps given in Eqs. 32, 33, and 35, complex formation, biradical formation, and product formation. An earlier idea that two different excited triplet states were reacting has been discounted. 100,141,142) The inefficiency of the reaction is attributed to an alternate decay of complex 77,78,ioo,i42)( an(j the excited state has a n-n configuration. 100,142)... 

A nucleophilic attack on the acetylenic ketone functionality of golfomycin A (184) was proposed as a potential pathway to form the benzannulated enyne-allene 185 (Scheme 20.38) [71]. Subsequent biradical formation has been postulated as a possible mechanism to account for its DNA-cleaving properties and antitumor activity. 

That is, the CH2 biradical formation process is far too endothermic to be of any significance compared to reactions (62) and (63). On the other hand, 2-C3H7 decomposes primarily by the following CH3CHCH2-H bond scission ... 

Irradiation of cycloalkenones in the presence of excess olefin results in cycloaddition to the olefin.420,421 The major products are those which would be predicted to arise from stepwise addition with intermediate biradical formation if the 2 position adds first to the double bond. 

Wagner and Sakamoto [95] have studied the triplet decay of para-alkeny-loxyacetophenones with a substituent (X = Cl, CN, OMe, Me) at the position ortho to the alkenyloxy side chain. They propose that the cycloaddition reaction involves charge-transfer interaction between donor double-bond and acceptor triplet benzene first, followed by biradical formation (Scheme 18). 

Application of the XH CIDNP technique has allowed us to confirm the hypothesis of the biradical formation and to propose a detailed scheme of the photolysis of 1768. 

Biradical formation via photolysis of an intermediate mono-halo radical has also been suggested for l,3-bis-(chloromethyl)benzene (218) with spectral evidence for both the biradical and mono-halo radical obtained at room temperature [131] (Scheme 12). However, it was not possible to determine whether biradical... 

Koga, N. Morokuma, K. Comparison of biradical formation between enediyne and enyn-allene. Ab initio CASSCF andMRSDCI stndy, J. Am. Chem. Soc. 1991,113, 1907-1911. 

Nagata, R. Yamanaka, H. Okazaki, E. Saito, I. Biradical formation from acyclic conjugated eneyne-allene system related to neocarzinostatin and esperamicin-calichemicin, Tetrahedron Lett. 1989, 30,4995-4998. 

Even in the meta-phenylenevinylenes 22 the neighboring units can be charged to the biradical form. This is in line with the observation of spin localization in the monoradical, where the unpaired electron is distributed in one stilbene unit only. Upon further charging of e.g. 22b the biradical formation is obtained [44, 434]. 

The photolytic cleavage of alkyl aryl sulfoxides has been shown to occur via initial C—S bond homolysis, in accordance with the common mechanistic assumption. Secondary and tertiary alkyl groups show high chemoselectivity for alkyl C—S cleavage. Uniquely, alkene products have been isolated, formed by disproportionation of the initial alkyl radical, with the formation of benzaldehyde and racemization of primary alkyl compounds. An investigation into the photochemical conversion of N-propylsulfobenzoic imides into amides in various solvents revealed a solvent dependence of the observed mechanism. In ethanol, sulfur dioxide extension forms a biradical which abstracts a hydrogen atom from the solvent, whereas in aromatic solvents biradical formation by a single electron transfer is implicated. The photolysis and thermolysis of l,9-bis(alkylthio)dibenzothiophenes and /7-aminophenyl disulfide have been studied. 

The intermediacy of an exciplex in Patemo-Buchi reactions has been supported [118a-g] by (i) a normal KIE, unexpected for direct biradical formation (ii) stereoselectivety only of state reactions (iii) rate... 

The search for these exciplexes provided evidence against their existence and in favor of direct formation of spin protected triplet biradicals [8], which would explain points (i) and (ii) above. Whether the initial step in the luminescence quenching by olefins is exciplex formation or 1,4-biradical formation, is still an open question [118h]. Using TET it is possible to conciliate direct biradical formation with observations (i)-(vii). 

Myers, A.G. Kuo, E.Y. Finney, N.S. Thermal generation of alpha,3-dehydrotoluene from (Z)-l,2,4-heptatricn-6-ync. J. Am. Chem. Soc. 1989, 111, 8057-8058 Nagata, R. Yamanaka, H. Okazaki, E. Saito, I. Biradical formation from acyclic conjugated eneyne-allene system related to neocarzinostatin and esperamicin-calichemicin. Tetrahedron Lett. 1989, 30, 4995 998. 

For pairs of neutral radicals, the singlet exit channel is usually the only viable geminate pathway. The situation is very different for radical ion pairs, which frequently imdergo geminate reactions such as electron return or biradical formation in the triplet state, and the importance of the triplet exit channel in electron transfer reactions has been emphasized in several recent reviews. ° ... 

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