Elimination from diradicals

As an alternative to the diradical ring closure that occurs with the 3 -indazole ring contraction, the employment of a 1,3-elimination from an ortho, a-disubstituted aromatic has much appeal for cycloproparene formation, not least because of the simplicity of the process and the ready availability of the starting materials. It is not surprising, therefore, that such a report appeared as early as 1974. Radlick and Crawford found that 1-bromo-2-(methoxymethyl)benzene (33) underwent lithium-halogen exchanged and cyclization to 1 upon treatment with butyllithium. The yield was a modest but acceptable 30%. In similar vein, the cyclization methodology provided rocketene (34) albeit in 5% yield (equation 7a). ... 

An independent entry into the diradical manifold of the NBD Q valence isomerization was described by Turro and coworkers , who used the photochemical or thermal N2 elimination from the azoalkanes 3,4-diazatricyclo[4.2.1.0 ]nona-3,7-diene (48) and 8,9-diazatetracyclo[4.3.0.0 . 0 ]non-8-ene (49) to produce the... 

Berson and coworkers have reported that the addition of 9-phenylanthracene to MA is not affected by ferric-dipivaloylmethide. The latter is a known catalyst for singlet-triplet conversion. From this and other kinetic evidence on the sterochemistry of cyclopentadiene-acrylate adducts, Berson and coworkers argue that a diradical intermediate can be eliminated from consideration in the Diels-Alder reaction. Different products formed from photochemical versus thermal DA reaction may be considered as evidence of the absence of a triplet intermediate as well, although a singlet diradical cannot be ruled out. ... 

Unlike other members of this group, the DNA strand scission by C-1027 occurs without activation by a thiol or nucleophile [207, 208]. The formation of radical intermediates causing DNA cleavage by the antibiotic C-1027 was confirmed by spin trapping. The signals of the spin adducts with 2-methyl-2-nitrosopropane were recorded by ESR spectroscopy. The spin adducts were also detected by mass spectrometry (Scheme 3.1) [208]. It was shown that the antibiotic C-1027 is an equilibrium mixture of forms 3.407 and 3.410, the latter in very low concentrations. The process of hydrogen atom elimination from the DNA and the formation of aromatic structure 3. 411 were found to proceed in two steps first, slowly from atom C and then from atom C in a fast step [208, 209]. These data help us to understand the mechanisms of DNA strand scission by the antibiotic and thus contribute to the development of new anticancer drugs with specific delivery systems [208, 209]. Maduropeptin 3.408 is also able to form diradicals without activation [210]. 

The higher strain energy in thiirene dioxides (19) compared to thiirane dioxides (17) is obvious. Yet, the elimination of sulfur dioxide from the latter is significantly faster than one would expect for a thermally allowed concerted process. Consequently, either aromatic-type conjugative stabilization effects are operative in thiirene dioxides2,12 or the relative ease of S02 elimination reflects the relative thermodynamic stability of the (diradical )99 intermediates involved in the nonconcerted stepwise elimination process. 

Dimethylene-2,3-dihydrofuran derivatives, which are produced by fluoride-induced 1,4-conjugative elimination of trimethylsilyl acetate from the [(trimethylsilyl)methyl]-3-furan precursor 207, undergo subsequent [4-1-4] dimerization reactions to produce cycloocta[l,2-3 6,5-. ]difuran derivatives as a mixture of isomers (Equation 137) <1995JA841 >. A methyl substituent at the 3-methylene position was found to retard the rate of dimerization, an observation which is consistent with the proposed two-step mechanism involving the initial formation of a diradical intermediate in the rate-determining step (Table 16). 

Zewail and his co-workers addressed this question in their laboratory and studied the stability of the tetramethylene diradical generated from various precursors in real time. He showed with femtosecond spectroscopy that intermediate product was in fact formed and had a lifetime of 700 fs. Using cyclopentanone as the precursor they have shown that with two photons at 310 nm (pump) carbon monoxide is eliminated through an a-cleavage. 

Two proposed mechanisms for the polymerization of a,a -bis(dialkyl-sulfonio)-p-xylene dihalides (463) are shown in Fig. 70 [302]. Both mechanisms begin with the abstraction of an a-proton to produce ylid 464. The 1,6-elimination of dialkyl sulfide produces the p-xylylene pseudo-diradical 465. One mechanism involves the formation of polymer from this species via the dimerization of 465 to give the dication diradical 466. This species was proposed to grow rapidly to high molecular weight polymer (467) by head-to-tail additions to both ends [301]. An alternative mechanism involving polymer formation from 465 via a putative anionic mechanism has been proposed [302]. There were two main factors in the proposal of... 

Subsequent to these developments Mishra and Crawford , in a further attempt to elucidate the decomposition mechanism and nature of intermediates, have investigated the stereometric distribution of products in the thermolysis of (3R 5R)-(+)-rranj-3,5-dimethyl-l-pyrazoline. If the reaction proceeds as postulated by Crawford et al., the tra/w-l,2-dimethylcyclopropane product would result from the disrotatory closure of the planar biradical and should be racemic, while the above two alternatives, step-wise elimination of nitrogen or the involvement of pyramidal diradicals by inversion at both asymetric centres would be expected to yield optically active tranj-1,2-dimethylcyclopropane. 

The experimental results indicated an excess of double inversion to retention of absolute configuration of 23.6 %, which comprises about 6 % of the total decomposition, and it was concluded that 94 % of the products arises from the planar diradical and 6 % from either the pyramidal diradical or by step-wise elimination of nitrogen. 

The ratio of spirohexan-4-one/spirohexan-5-one was close to 1 1 which can be rationalized by assuming a diradical mechanism as proposed for the cycloaddition of dimethylketene to methylenecyclopropane. Dichloroketene also underwent nonregioselective addition to methylenecyclopropane (49% yield) however, 4,4-dichlorospiro[2.3]hexan-5-one was formed as the main product. When methylenecyclopropane was reacted with chloro(2,2,2-tri-chloroethyl)ketene, generated by elimination of chlorine from 2,2,4,4,4-pentachlorobutanoyl chloride with zinc and phosphoryl chloride, 4-chloro-4-(2,2,2-trichloroethyl)spiro[2.3]hexan-5-one (10) was isolated in 31% yield. The isolation of a single isomer instead of mixtures as described above is possibly due to the different reaction conditions which do not guarantee an uncatalyzed cycloaddition. 

Radicals are formed upon one-electron oxidation of tBuOOH, but also a v 00 diradical has been suggested as an intermediate. As can be seen from the formation of ketones and alcohols, it is impossible to completely eliminate the influence of oxygen, also because oxygen is formed in situ according to reaction 7. 

Aldehydes show an elimination reaction (loss of carbon monoxide, CO), that is not possible with ketones. Butanal, for example, photodissociates to propane and carbon monoxide. Cyclic ketones dissociate to a diradical (41 from cyclopentanone), which then reacts in any of several ways including elimination to ethene or 42 and coupling to cyclobutane. Formation of cyclobutane and ethene is accompanied by expulsion of CO prior... 

---