Diradicals, as intermediates

Elliott et al. [155] proposed a concerted cycloaddition also for the reaction of 417 with 1,1-dimethylallene to yield 426. However, even the 1,2-bismethylenecyclobu-tane entity of 426 provides support for a diradical intermediate, since it is characteristic of the dimers of many allenes (see, for example, Schemes 6.10 and 6.25). It is generally accepted that these [2+ 2]-cycloadditions proceed via diradicals as intermediates. 

Thermal and photochemical decompositions of cyclic diazenes usually proceed via diradicals as intermediates. Good synthetic routes to polycyclic diazenes are available, and consequently many interesting members of this class of compounds, containing three-membered rings, have been prepared and their decompositions studied under a variety of conditions. The thermal conversion of exo-6,7-diazatricyclo[3.2.1.0 ]oct-6-ene to cyclohexa-1,4-diene and nitrogen (Table 18, entry 1) was so much more rapid than the analogous decompositions of model cyclic diazenes that it probably occurred by a synchronous reaction, i.e. a diradical intermediate... 

The assumption of a singlet diradical as intermediate in 1,3-cycloadditions would account for the feeble polar character of several such reactions, as well as for other of their features. However, the generality of this two-step mechanism is questionable , mainly on stereochemical grounds. It seems favoured in particular cases . 

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 decomposition of diphenylenediazomethane produces the di-phenylenemethylene diradical as an intermediate. This intermediate will only be a diradical in case the two electrons follow the usual rule... 

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). 

The occurrence of an additional two-membered bridge as intermediate in a [2.2]paracyclophane system has been postulated by Forrester and Ramasseul 47b>. During the synthesis of the bishydroxylamine 82, a precursor of the diradical 19 these authors were able to isolate as byproduct the pseudo-para hydroxy compound 83 which is converted into the violet quinone 84 through oxidation by atmospheric oxygen or reac-... 

Similarly, ab initio calculations on the thermal reaction of propene forming methyl-cyclopentane suggested a three-step biradical reaction with 1,4-biradical and 1,5-biradical as intermediates. Quantum-chemical calculations have been carried out for the cyclization of the neocarzinostatin chromophore cyclonona-l,2,3,5-tetraen-7-yne to 1,5-didehydroindene biradical. The degree of stereoselectivity of the Diels-Alder reaction of 2-methylfuran and maleic acid in water has been found to reduce significantly in the presence of heavy atoms. Taking into account the relatively low concentration (3.5-7 m) of heavy-atoms, and the rapid fall off of the heavy-atom effect with distance, these results show that a large portion of the Diels-Alder reaction occurs via diradical intermediates. " ... 

Other 2-substituted cyclopropylideneacetates of type 3-X also entered this cycloaddition (Scheme 15) [19]. The endolexo selectivity is low but usually still higher than that of simple acrylic acid esters. The relative Diels-Alder reactivities of dienophiles 1-Me and 3-X as determined by competition experiments (Scheme 15) suggest a mechanism involving either diradicals or zwitterions as intermediates [19]. Surprisingly, the 2-fluoro derivative 3-F is less reactive than the parent compound 3-H. The 2-chloro and 2-bromo derivatives 1-Me and 3-Br have similar reactivities and cycloadd to furan (57) about 16 times faster than methyl acrylate. 

The postulation of trimethylene and tetramethylene diradicals as reactive intermediates involved in many thermal isomerization and fragmentation reactions has a long history,but not until 1994 had they ever been detected in real time. The validity of the diradical hypothesis was tested through femtosecond studies, and the tests provided dramatic evidence confirming that these short-lives species are indeed real, directly experimentally accessible chemical entities. 

This femtosecond study confirmed the involvement of the oxytetramethylene diradical as a reactive intermediate, and found that the trimethylene formed from it had the same hfetime as the trimethylene generated through the photodecarbonyl-ation of cyclobutanone. For tetrahydropyran, the oxypentamethylene drradical (86 amu) is formed readUy and the 85 amu transient, from the p-cleavage of a C H bond, is the dominant fragmentation product. 

A fragmentation reaction which appears to proceed via the generation of 1,4-diradicals is the decomposition of 1,1-tetramethylenediazenes. Unlike the more stable 1,2-diazenes (tetrahy-dropyridazines, see Section 4.2.1.), the 1,1-isomers are not usually isolated or characterized by physical methods but are proposed as intermediates in the thermal decomposition of iV-phenyl-sulfonamidopyrrolidines 1, giving 1,4-diradicals which recombine to yield cyclobutanes 3 and 4. 39 These intermediates are also formed in the photochemical decomposition at low temperature of 1,1-tetramethylenediazenes, prepared in situ from 1-aminopyrrolidines and /er/-butyl hypochlorite.141... 

Density functional theory computational studies have been used to determine die importance of secondary orbital interactions for the stability of transition-state structures for die 4 + 2-cycloaddition of furan with cyclopropene.175 Kinetic studies of die 2 + 4-cycloaddition of 2-cyclopropylidene acetates with furan and dimethylful-vene suggest a mechanism involving diradicals or zwitterions as intermediates.176 Cyclopropene, produced by die reaction of allyl chloride with sodium bis(bimediyl-silyl)amide, reacts with 1,3-diphenylisobenzofuran to produce both endo- and exo-Diels-Alder cycloadducts isolated for the first tune.177... 

In one of the earliest reports on ortho photocycloaddition, in which the reaction of benzonitrile with 2-methylbut-2-ene is described, a diradical (triplet) intermediate was proposed [73], The structure of the product corresponds to the most stable of the four possible diradical intermediates. When benzophenone was added as a sensitizer in an attempt to increase the yield of the photoadduct, only 0.05% of ortho adduct was isolated along with 54% of an oxetane formed by the addition of benzophenone to 2-methylbut-2-ene. In the absence of benzophenone, the ortho adduct was isolated in 63% yield. It is, however, thermally as well as photochemically unstable and reverts to starting materials, supposedly also via a biradical. The authors propose that benzophenone catalyzes bond cleavage of the adduct more efficiently than ortho addition and this would account for the low yield of photoadduct in the presence of benzophenone. From these experiments, no conclusion about the identity of the reactive excited state can be drawn. 

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. 

From organic chemistry it is known that cycloaddition reactions leading to cyclobutanes are required to be stepwise reactions, according to the Woodward-Hoffmann rules [131]. A bond is formed between the two olefins, leading to a tetramethylene intermediate (T). In a subsequent step, the second bond is formed, yielding the cycloadduct. Depending on the reactants, either zwitterionic or diradical tetramethylenes can be proposed as intermediates [132, 133]. 

Tetramethylene Diradicals as Key Intermediates in Photoreactions of Donor/Acceptor Olefins... 

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