The methylene biradical

This suggestion is supported by the fact that there is an induction period for CO formation, as well as by the coincidence of the end of the induction period with the maximum in the allene concentration. The methylene biradical formed in step (3) may react with the ketene molecule producing CH4, as well as through a chain... 

Calculations based on this second model give the observed value for the entropy of activation. In addition, this model may be used to account for the observed isotope effect (Benson and Nangia, 1963). If the tetra-methylene biradical is involved then it is to be expected that appropriately substituted cyclobutanes might undergo cis-trans isomerization reactions. This will be referred to again later. One final point should be mentioned in connection with biradical intermediates in both cyclopropane and cyclobutane reactions. This concerns the absence of any effect of radical inhibitors on these systems, when it might be expected that they would interact with the biradicals. In fact calculations show that, under the conditions of formation, the biradicals have extremely short lifetimes sec) and hence, unless radical inhibitors are... 

The methylene triplet adds to ethylene symmetrically through the triplet biradical directly (B). The central methylene group (formed from ethylene) is bent downwards by this process (Fig. 10). At this stage rotation or direct ring closure can occur, with loss of stereochemistry following bond formation to yield cyclopropane. The cyclo-addition of the triplet requires only a small activation energy of about 5 kcal/mole 52). 

Bohm, the methylene free radical, —CHoCCHL—, may be produced momentarily in polyethylene by eliminating molecular hydrogen during the irradiation. This process could replace or exist in addition to that represented by Equation 1. Such a biradical (or Lewis acid) would not be expected to be stable but could revert to the vinvlene group... 

Figure 51. Cartoon representations of hydroxy-1,4-biradicals from 2- and sym-alkanones. The methylene chains (solid lines) are assumed to be extended and the sizes of the atoms between the odd-electron centers and the hydroxy groups are depicted as circles and filled dots, respectively.

There is a striking similarity in the behavior attributed to the triplet biradicals believed to be formed in the reactions of both triplet methylene and triplet oxygen with the 2-butenes. It should also be pointed out that in the triplet addition reactions of methylene, cis and trans olefins do not give the same cis and trans product ratios, indicating that the rate of spin inversion necessary for closure in the biradical intermediate occurs at a rate comparable to the rate of rotation about C—C bonds. 

The activation energy is thus not far from that for methylcyclobutane, so that the presence of the double bond has little effect on the process. It might have been expected that the activation energy would have been smaller for the methylene compound, since an intermediate biradical would be allylically stabilized, viz. 

Magnetic field effects show a dependence of the modulated triplet yields for photogenerated biradicals of the type )n-2O" upon the methylene chain length SI. There is also an effect on the yield micellized triplet radical pairs2 2... 

The photochemical reactions of ethyl phenylglyoxalate (217) in benzene have been re-examined. The three new products (218), (219) and (220) have been isolated from the reaction mixture. The quantum yields for the formation of the products are dependent on concentration. Irradiation of the phenylglyoxalate derivatives (221) results in conversion into the lactones (222) from (221, R = H) and (223) and (224) from (221, R = Me). The reactions are proposed to involve an intramolecular electron transfer process forming a zwitterionic biradical. This leads to activation of the methylenes adjacent to the sulfur atom. A similar effect is observed with the nitrogen analogue (225) which affords (226) as the... 

Miyashi et alP have previously demonstrated that the isomerism and ring opening of the methylene cyclopropanes (95) is initiated by SET processes. The present study has examined in detail the steps within the system. SET photochemistry of (95) can be brought about by the use of DCA, TCNB or NMQ BF4 as sensitisers and the formation of the radical cation (96) was demonstrated as was the formation of the biradical (97) which is formed from (96) by a back electron-transfer step. 

Additions to Cyclopentenones and Related Systems. (2 + 2)-Cycloadditions are reported following the irradiation of mixtures of alkyl and aryl 2-thioxo-3/f-benzoxazole-3-carboxylates with alkenes. Cycloaddition also occurs to the CS double bond. The photochemical additions of arylalkenes to 3-phenylcyclo-pentenone and 3-phenyl cyclohexenone have been studied. The regio- and stereochemistry observed in the additions has been rationalized in terms of the stability of the intermediate biradicals. Photocycloaddition of allene to the cyclopentenone derivative (6) in methylene chloride solution at — 78°C affords... 

Photoaddition of benzophenone to the methylene ketone derivatives (88) yields the oxetan derivatives (89a) in 28 and 26% yield, respectively. The biradical (90) is presumed to be the intermediate in the formation of these compounds. The authors suggest that there are two competing pathways for reaction within this biradical, one leading to the oxetans (89), and another by rearrangement through... 

On the other hand, MINDO/3 calculations, which often favor biradical formation, suggest that the reaction occurs via a biradical.But subsequent calculations at various ab initio levels revealed a concerted asynchronous reaction pathway. In the latest effort using large basis sets and 10/10 MCSCF approaches, the pathway of Scheme 5.4 was found to be of lowest energy by 14 kcal/mol. The transition state has one bond stretched to 2.25 A while the cross-ring bond is stretched by only 0.05 A from starting material, and the methylene group is twisted by approximately... 

Irradiation of py-unsaturated ketones can give rise to methylenecyclobutanols, and the effect of structural modifications on the photochemistry has been investigated. Irradiation of the medium-ring ocp-unsaturated ketone (136) gives the methylene-cyclobutanol (137) with a cis ring fusion. Restricted rotation in the biradical intermediate (138) is invoked to account for the absence of any of the more usually found... 

Details of the coding system developed to handle the interconversions of 1,2-dimethylenecyclobutanes and the related biradicals have now appeared. Gajewski has presented stereochemical evidence for the interconversion of planar and orthogonal bisallyl radicals in the thermal rearrangement of trans-3,4-dimethyl-l,2-dimethylenecyclobutane (442). Pyrolysis of (442) at 230°C for 80 minutes gave a 9 % conversion into an 18 1 1 mixture of the isomers (443), (444), and (445). The recovered (442) was 19 % racemized, and the isomer (443) also appeared to be racemic. The pyrolysis products show the expected preference for conrotatory motions about the 1,2- and 3,4-bonds. The conrotatory out motion leads to the anti-anti-bismethallyl radical (446) which can only close to anri,anri-diethylidene-cyclobutane (445) or anti-l-ethylidene-2-methylene-3-methylcyclobutane (443). The small percentage of the syn-isomer (444) found in the products could arise from the... 

For a correct description of the latter, the effects of electron correlation must be exactly accounted for, as was noted earlier. This is necessary in the first place because of crossing of the orbitals when reaction moves along the disrotatory route giving rise to the 1,4-biradical structure of the transition state XXIIa [37]. Although a planar carbon skeleton is retained in XXIIa, its structure clearly indicates an asynchronous character of rotations of the methylene links on the MERP ... 

Interest in the decomposition of cyclopropanes and the role of the trimethylene biradical in the decay mechanism has spanned more than three decades and has been fueled by two experiments that give apparently irreconcilable results. Experiments of S,S-trans-cyclopropane-l,2-d2 at 695 K indicate that isomerization via double-terminal rotation (i.e., con- and disro-tation of the terminal methylene groups) is at least 6 times more prevalent than isomerization via single-terminal rotation. Similar experiments with chiral... 

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