Diradicals, also

A number of diradicals (also called biradicals) are known. When the unpaired electrons of a diradical are widely separated (e.g., as in CH2CH2CH2CH2 ), the species behaves spectrally like two doublets. When they are close enough for interaction or can interact through an unsaturated system (as in trimethylene-methane, " they... 

Another almost identical example is the pyrolysis of 6,6-dimethylbicyclo[3.1, l]heptan-2-one (nopinone, 18), whose corresponding diradical also enjoys stabilization by an adjacent carbonyl group. Thus, at 570 °C, 18 is converted substantially to the secondary ene products cis-2-methyl-3-isopropenylcyclopentanone (19) and /ra .s-2-methyl-3-isopropenylcyclopentanone (20), along with a small amount of the hydrogen transfer product 4-isopropenylcyclohexanone (21).100... 

Acyl, alkyl diradicals also apparently close to oxacarbenes. This statement derives from the most rational mechanism for the observed photoproducts of the 3-oxacyclohexanone below 42>. 

Compound 35 contains a thermolabile C-C bond which, on thermally induced fragmentation, yields a high proportion of macroradicals. The PDMS diradical also acts as a counter radical and can undergo chain extension at both ends in the presence of vinylic monomers (acrylonitrile, maleic anhydride, diethylfumarate) or styrenic monomers, leading to diblock copolymers in 95% yield according to the following scheme [211, 212] ... 

The concept of intramolecular alkylation of AT-substituted amino acid derivatives via 1,5-diradicals also turned out to be an excellent system for studying the different stereochemical course of spinisomers as discussed in Section 6.2.2. Thus, the a-ketoester 9, which contains an alanine moiety, was prepared. In contrast to aryl ketones, a-ketoesters are not completely converted into the triplet state after photochemical excitation. Upon addition of either a triplet quencher (naphthalene) or a triplet sensitizer (benzophenone), each of the two spin states may be forced (Scheme 3, Table 1). The chiral center at the d-position with respect to the keto carbonyl group raises the question whether a memory effect of chirality may be observed during the cyclization. The results summarized in Table 1 amply demonstrate the specific properties of spinisomeric biradicals. In the presence of naphthalene, which probably acts not only as a triplet quencher but also as a singlet sensitizer, the chiral information of the reactant 9 is almost entirely conserved in the helical diradical 10 because of its very short lifetime. In contrast, the addition of benzophenone results in almost complete racemization, and also the cis/trans selectivity is... 

Figure 4c also describes the spontaneous polymerisation ofpara- s.yX en.e diradicals on the surface of soHd particles dispersed in a gas phase that contains this reactive monomer (16) (see XylylenePOLYMERS). The poly -xylylene) polymer produced forms a continuous capsule sheU that is highly impermeable to transport of many penetrants including water. This is an expensive encapsulation process, but it has produced capsules with impressive barrier properties. This process is a Type B encapsulation process, but is included here for the sake of completeness. 

Intramolecular hydrogen-atom abstraction is also an important process for acyclic a,/ -unsaturated ketones. The intermediate diradical then cyclizes to give the enol of a cyclobutyl ketone. Among the by-products of such photolyses are cyclobutanols resulting from alternative modes of cyclization of the diradical intermediate ... 

The structure of the excited state of 1,3-dienes is also significant with respect to Z E isomerization. If the excited state is an allylmethylene diradical, only one of the two double bonds would be isomerized in any single excitation event ... 

Nonfluonnated allenes also readily react with fluoroalkenes to give diverse fluonnated alkylidenecyclobutanes [727, 12S, 129, 130] (equations 55 and 56), except for tetramelhylallene, which rearranges to 2,4-dimethyl 1,3-pentadiene under the reaction conditions prior to cycloaddition (equation 57) Systematic studies of l,l-dichloro-2,2-difluoroethylene additions to alkyl-substituted allenes establish a two-step, diradical process for alkylidenecyclobutane formation [131, 132, 133]... 

The reactivity of sulfur clearly depends sensitively on the molecular ctimplexity of the reacting species. Little systematic work has been done. Cyc/<7-Ss is obviously less reactive than the diradical catenas, and smaller oligomers in the liquid or vapour phase also complicate the picture. In the limit atomic sulfur, which can readily be generated photolytically, is an extremely reactive specie.s. As with atomic oxygen and the various... 

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. 

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 ... 

Despite the body of evidence in favor of the Mayo mechanism, the formation of diphenylcyclobutanes (90, 91) must still be accounted for. It is possible that they arise via the 1,4-diradical 94 and it is also conceivable that this diradical is an intermediate in the formation of the Diels-Alder adduct 95 (Scheme 3.64) and could provide a second (minor) source of initiation. Direct initiation by diradicals is suggested in the thermal polymerization of 2,3,4,5,6-pentafluorostyrene where transfer of a fluorine atom from Diels-Alder dimer to monomer seems highly unlikely (high C-F bond strength) and for derivatives which cannot form a Diels-Alder adduct. 

In eq. 8, the rate of polymerization is shown as being half order in initiator (T). This is only true for initiators that decompose to two radicals both of which begin chains. The form of this term depends on the particular initiator and the initiation mechanism. The equation takes a slightly different form in the case of thermal initiation (S), redox initiation, diradical initiation, etc. Side reactions also cause a departure from ideal behavior. 

In the case of 60, the ion has been prepared and has been shown to be a diradical in the ground state, as predicted by the discussion on page 58. Evidence that 60 is not only nonaromatic but also antiaromatic comes from studies on 64 and 66. When 64 is treated with silver perchlorate in propionic acid, the molecule is rapidly solvolyzed (a reaction in which the intermediate 65 is formed see Chapter 5). Under the same conditions, 66 undergoes no solvolysis at all that is, 60 does not form. If 60 were merely nonaromatic, it should be about as stable as 65 (which of course has no resonance stabilization at all). The fact that it is so much more reluctant to form indicates that 60 is much less stable than 65. It is noted that under certain conditions, 65 can be generated solvolytically. ... 

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... 

It must be emphasized once again that the rules apply only to cycloaddition reactions that take place by cyclic mechanisms, that is, where two s bonds are formed (or broken) at about the same time. The rule does not apply to cases where one bond is clearly formed (or broken) before the other. It must further be emphasized that the fact that the thermal Diels-Alder reaction (mechanism a) is allowed by the principle of conservation of orbital symmetry does not constitute proof that any given Diels-Alder reaction proceeds by this mechanism. The principle merely says the mechanism is allowed, not that it must go by this pathway. However, the principle does say that thermal 2 + 2 cycloadditions in which the molecules assume a face-to-face geometry cannot take place by a cyclic mechanism because their activation energies would be too high (however, see below). As we shall see (15-49), such reactions largely occur by two-step mechanisms. Similarly. 2 + 4 photochemical cycloadditions are also known, but the fact that they are not stereospecific indicates that they also take place by the two-step diradical mechanism (mechanism... 

It has been contended that another mechanism, involving single electron transfer, may be taking place in some cases Olah, G.A. Krishnamurthy, V.V. J. Am. Chem. Soc., 1982,104, 3987 Yamataka, H. Nagareda, K. Hanafusa, T. Nagase, S. Tetrahedron Lett., 1989, 30, 7187. A diradical mechanism has also been proposed for certain cases Ward Jr., W.J. McEwen, W.E. J. Org. Chem., 1990, 55, 493. 

Three-membered rings can also be cleaved to unsaturated products in at least two other ways. (1) On pyrolysis, cyclopropanes can undergo contraction to propenes. In the simplest case, cyclopropane gives propene when heated to 400-500°C, The mechanism is generally regarded as involving a diradical... 

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