Diradical species

A new type of rearrangement has been reported for certain l-(o-nitrophenyl)pyrazoles (169), giving cis- and trans-benzotriazole 1-oxides (170 Scheme 12) (73TL891). The reaction was rationalized in terms of an intermediate azo compound (171 formed in turn either from the diradical species (172) or from the intramolecular 1,4-adduct (173). Subsequently... 

The reactive structural element for the Myers cyclization is an enyne allene, the heptatrienyne 6, which reacts to form a diradical species 7 ... 

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 ... 

When buta-1,3-diene 3 is irradiated in the presence of a photosensitizer (e.g. benzophenone), the isomeric divinylcyclobutanes 6 and 7 are formed via the intermediate diradical species 4 and 5 respectively in addition the [4 + 2] cycloaddition product 4-vinylcyclohexene (see Diels-Alder reaction) is obtained as a side product ... 

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 overall reaction includes allylic transposition of a double bond, migration of the allylic hydrogen and formation of a bond between ene and enophile. Experimental findings suggest a concerted mechanism. Alternatively a diradical species 4 might be formed as intermediate however such a species should also give rise to formation of a cyclobutane derivative 5 as a side-product. If such a by-product is not observed, one might exclude the diradical pathway ... 

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 fragmentation/cyclization ratio is determined by the relative orientation of the respective molecular orbitals, and thus by the conformation of diradical species 2. The quantum yield with respect to formation of the above products is generally low the photochemically initiated 1,5-hydrogen shift from the y-carbon to the carbonyl oxygen is a reversible process, and may as well proceed back to the starting material. This has been shown to be the case with optically active ketones 7, containing a chiral y-carbon center an optically active ketone 7 racemizes upon irradiation to a mixture of 7 and 9 ... 

The irradiation is usually carried out with light of the near UV region, in order to activate only ihc n n transition of the carbonyl function," thus generating excited carbonyl species. Depending on the substrate, it can be a singlet or triplet excited state. With aromatic carbonyl compounds, the reactive species are usually in a Ti-state, while with aliphatic carbonyl compounds the reactive species are in a Si-state. An excited carbonyl species reacts with a ground state alkene molecule to form an exciplex, from which in turn diradical species can be formed—e.g. 4 and 5 in the following example ... 

Diradical species 4 is more stable than diradical 5, and the oxetane 6 is thus formed preferentially oxetane 7 is obtained as minor product only. Evidence for diradical intermediates came from trapping experiments, as well as spectroscopic investigations. ... 

For the mechanistic course of that reaction two pathways are discussed " a concerted [l,3]-sigmatropic rearrangement, and a pathway via an intermediate diradical species. Experimental findings suggest that both pathways are possible. The actual pathway followed strongly depends on substrate structure the diradical pathway appears to be the more important. 

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 ... 

Several mechanistic explanations98—including both concerted symmetry-allowed nonlinear chelatropic paths96, and nonconcerted stepwise mechanisms (such as that in which a diradical species is involved99)—have been advanced to accommodate the stereospecific experimental results2,173,73,99. 

The only conclusion that can be drawn is that the lifetime of the benzophenone triplet in benzene is shorter than in other inert solvents due to the formation of a diradical species with the solvent which happens to absorb in the same region as the benzophenone triplet itself and the ketyl radical. Thus the advent of nanosecond flash spectrometry has allowed the solution of an interesting and difficult problem. 

In principle, the triplet state of ketones could react in either of two modes to produce the product ketone and olefin. These are via a concerted pathway or by a distinct diradical species ... 

In cyclohexane the same two ketones (12) and (13) are obtained from the photolysis of (11) but in aqueous dioxane two phenols are isolated as well as the bicyclic ketone (12). Swenton(10) suggested that the gas-phase reaction involves diradical species, whereas in polar solvents zwitterionic intermediates are favored ... 

Ogura and Matsuura<30) argued that which mechanism (the slither or pivot) takes place depends on whether the rearrangement takes place from the ground state zwitterionic species or from the excited state diradical species. They claim that the following transformation occurs via the diradical pivot mechanism ... 

The energy available from the anthracene triplet (42 kcal/mole) is sufficient to produce either of these states. The singlet excited molecule subsequently attacks a ground state anthracene to produce the observed endoperoxide. The 1Aff state is believed to be responsible for the addition to anthracene to form the endoperoxide since it closely resembles a diradical species, while the 1Ss+ state more closely resembles a dipolar ion. 

In all the latter cases the easier dimerization reaction is connected with the particular stability of the intermediate diradical species. This is also the reason for the recently found facile dimerization of the 1-donor substituted allylidene-cyclopropane 136a (Scheme 66) [127]. Allylidenecyclopropane 136a cyclodimer-izes to the expected cyclobutane 467 in very mild thermal conditions, due to the stabilization of the intermediate 466. At higher temperature (120 °C) both 136a and 467 give a more complex mixture of products, with the cyclooctadiene dimer 468 being the prevailing one (Scheme 66) [127],... 

Electron-deficient olefins such as acrylonitrile can participate in the cross [2 + 21-cycloaddition with allenes. 3-Methylenecydobutanecarbonitrile (17) was obtained in 60% yield by the reaction of allene with a large excess of acrylonitrile under autogenous pressure at 200 °C [16]. Initial bond formation takes place between the central carbon of allene and the terminal carbon of acrylonitrile to give a diradical species, which cydizes to form the cydoadduct [17]. 

The Characterization and Reactivity of Photochemically Generated Phenylene Bis(diradical) Species as Revealed by Matrix Isolation Spectroscopy and Computational Chemistry... 

---