Diradical compounds

Scheme 10.27 Pyrrole-based PNP-pincer ligand precursor, its dinickel(l) compound 60, and a related dinickel(l) diradical compound 61.

Pig. 2 Chemical structure and names of representative diradical compounds presenting large second hyperpolarizabilities. l,2-bisfphenalen-l-ylidene)ethene (1), 1,2-bisfphenalen-l-ylidene)-ethane (2), 5-indaceno[l,2,3-c / 5,6,7-c d ]diphenalene (IDPL, 3), 1,4-bis-(imidazole-2-ylidene)-cyclohexa-2,5-diene (BI2Y, 4). 

P-Peroxylactones undergo thermal decarboxylation to carbonyl compounds by the initial formation of a 1,5-diradical (238). a-Peroxylactones undergo similar decarboxylation, emitting light since the ketone is generated in the triplet excited state (85,239,240) ... 

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... 

For conjugated carbonyl compounds, such as a,) -enones, the orbital diagram would be similar, except for the recognition that the HOMO of the ground state is ij/2 of the enone system, rather than the oxygen lone-pair orbital. The excited states can sometimes be usefully represented as dipolar or diradical intermediates ... 

Pyridine, and its monomethyl and 3,5-dimethyl derivatives " combine exothermically with dimethyl acetylenedicarboxylate in ether yielding some ether soluble materials, including trimethyl pyrrocoline-1,2,3-tricarboxylate (Section III,F,3) and deep red ether-insoluble gums. A number of crystalline compounds have been isolated from these gums by fractional crystallizations and will now be considered in detail. In the case of pyridine, Diels et al. ° isolated a red labile 1 2 molar adduct, which they formulated as (75), which isomerized rapidly on standing to a yellow stable adduct (76). These formulations are no longer accepted. Diels and Alder also suggested that the acetylenic ester first dimerized to the diradical (74) which then combined with the pyridine. 

Pyrolyses of Nl- or N3-substituted derivatives of compounds 4 and 5 have continued to find use as routes to azacarbazoles, although the yields are often indifferent and there are no recent examples. The photochemical reactions are dealt with in Section IV.G. Pyrolysis media are paraffin (P) or PPA, and examples of products are compounds 247 (P, cytostatic) (83MI2), 248 (P) (84MI1), and 249 (from a 1-substituted derivative) (86MI2). Indications of diradical intermediates are provided by the thermolysis of compound 250 (P) (83MI2) where one product is a dimer. 

Upon heating the enediyne la rearranges reversibly to the 1,4-benzenediyl diradical 2a, which in its turn can rearrange to the enediyne lb or—in the presence of a hydrogen donor (e.g. cyclohexa-1,4-diene)—react to the aromatic compound 3a. 

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 ... 

In addition to cyclopentenes, other types of compounds may be formed upon heating of vinylcyclopropanes. For example pentadienes 6a/b may be formed by a competitive route from a diradical intermediate. 

Hydrogen shifts are often observed in thermal isomerizations of vinylaziridines. Heating of compounds 221 at 180 °C produced mixture of 3-pyrrolines 222 and hydrazones 223 (Scheme 2.54) [87]. The formation of 223 can be explained in terms either of a concerted hydrogen shift as depicted in 224 or of diradical intermediates 225, both of which would be followed by thermal isomerization of the (Z)-carbon-carbon double bonds to provide the ( ) isomers 223. 

Dimer and trimer byproducts have been isolated from MMA polymerizations and these are suggestive of 1,4-diradical intermediates.323 28 Lingnau and Mcycrhoff523 found that rates of spontaneous polymerization of MMA were substantially higher in the presence of transfer agents (RH). They were able to isolate the compound (98) that might come from trapping of the biradical intermediate (Scheme 3.65). 

As will be shown in Section V, T -S mixing has so far been identified in reactions involving diradicals and in certain reaction of organo-metallic compounds. 

Both singlet and triplet n,n states undergo the reaction." The intermediate diradical can also cyclize to a cyclobutanol, which is often a side product. Carboxylic esters, anhydrides, and other carbonyl compounds can also give this... 

There is much evidence that the mechanism" of the 1-pyrazoline reactions generally involves diradicals, though the mode of formation and detailed structure (e.g singlet vs. triplet) of these radicals may vary with the substrate and reaction conditions. The reactions of the 3 f-pyrazoles have been postulated to proceed through a diazo compound that loses N2 to give a vinylic carbene." ... 

The anticancer activity of complex natural products having a cyclodecenediyne system [for a review see <96MI93>] has prompted the synthesis of 54 (X = CH2 and OCH2) <96CC749> and 55 (R = a-OH and p-OH) <95AG(E)2393> on the basis that such compounds are expected to develop anticancer activity as the P-lactam ring opens. This is because cycloaromatization can only occur in the monocyclic enediyne and the diradical intermediate in the cyclization is thought to be the cytotoxic species. 

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... 

Many of the problems and misconceptions occurring for dithiolene compounds are related to the fact that the ligands are redox-active and can be oxidized to monoanionic radicals. Typical examples for this phenomenon are the mono and diradical complexes [Fe ( "bdt )( "bdt)(PMe3)] (9) and [Fe ( "bdf)2(PMe3)]-" (10) for which bdt and bdt are tcrt-butyl-dithiolene and its one-electron oxidized form. Originally, these and other bdt derivatives had been described as... 

Finally, in some cases diradical negative ions can even be generated directly npon ionization of appropriate precnrsors. For example, nitrene and carbene anions can be formed by El of organic azides, diazo-compounds, and diazirines, whereas Branman and co-workers have reported the formation of oxyallyl anions by El of flnorinated acetyl componnds (Eq. 5.12). ... 

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