Bisallyl biradical

Dimethylenecyclobutane (DMC) undergoes a degenerate carbon scrambling reaction at temperatures above 250°C at low pressures in the gas phase with log k = 14.45 - 46 800. Further, the isotopomers are formed kinetically as a 2 1 mixture of 1,3- and 3,3-shift products suggesting that an orthogonal or rapidly rotating 2,2 -bisallyl biradical is involved (Scheme 1.51) ... 

Finally, the thermolysis of 2,3-dimethylenebicyclo[2.2.0]hexane has been examined in an effort to determine the behavior of a 2,2 -bisallyl biradical forced to be nearly planar. The hydrocarbon dimerizes in a first-order reaction in modestly concentrated solution, and the presumed biradical can be trapped with 2,4-hexadiene with loss of stereochemistry of the diene.However, in the vapor phase at 110°C, first-order isomerization to a 2 1 mixture bicyclo[4.2.0]octa-1,5-diene (BOD), and 3,4-dimethylene-l,5-hexadiene (DMHD), occurs. ... 

Indeed, these motions represent the lowest energy pathway for ring closure of a singlet bisallyl biradical according to 6-3IG MCSCF calculations by Gilbert. 

The bisallyl biradical, however, has been demonstrated to undergo rapid rotation around the central bond. Thus, pyrolysis of optically active T gives optically inactive 1,3-shift product at only 9% conversion further, starting Tracemized to the extent of... 

The question of whether the bisallyl biradical is involved in the allene dimerization has been answered in the affirmative. Dimerization of 1,1-dimethylallene at 150°C and thermolysis of 4,5-dimethylene-3,3,6,6-tetramethyl-3,4,5,6-tetrahydropyridazine at the same temperature gave the three isomeric allenes in the same ratio. Further, direct photolysis of the azo compound just above room temperature gave the same product distribution when the thermal data was extrapolated to that temperature. 

The high temperatures required for this reaction would appear to require cleavage to bisallyl biradical intermediates. But the fact that this reaction was no faster than the equivalent 3,3-shift of 4-vinylcyclohexene was interpreted as requiring... 

For other substituent effects, see Oth et al Upon heating at 305°C for 24 h homotropilidiene gives bicyclo[3.3.0]octa-2,6-diene, a reaction which would appear to occur via the bisallyl biradical above (Scheme 9.33). 

The origin of the 3.3.0 diene would appear to be a 1,3-shift in the bisallylic biradical which forms the 3.2.1 diene (Scheme 9.55). However, this biradical species must be formed with rotation around the CHD group to give the stereochemical scrambling observed. 

The similarity in the product ratios from the two 3.2.0 dideuterio isomers would appear to minimize the role of isotope effects here. It is possible that the 1,3- and 3,3-shift pathways are separate, but ring opening of cyclobutanes by the coupled motions depicted in Scheme 9.59 would give conformationally distinct, incompletely equilibrated, bisallyl biradicals which could rationalize the preferences. 

To characterize the stereochemistry of the conversions, the x6>-7-methyl-6-methylenebicyclo[3.2.0]hept-2-ene derivative was pyrolyzed and found to give 88% inversion at Cl in the 1,3-shift product (mostly) /i(i(9-6-methyl-5-methylenebicy-clo[2.2.1]hept-2-ene. However, the endo-1-mtihyX 3.2.0 isomer also gave 60% retention in its 1,3-shift to give the endo-6-mQ hy 2.2.1 product. It would appear that the bisallyl biradical responsible for the major product is formed preferentially from... 

However, pyrolysis of the 7-methoxy derivatives reveals a strong preference for a 3,3-shift from the syn isomer and a preference for 1,3-shift from the anti isomer so that either very distinct conformations of a bisallyl biradical are being generated or there is some concert in the pyrolysis of the syn material (Scheme 10.51). ... 

Bullvalene (BV) gives c/5 -9,10-dihydronaphthalene (C9,10) at high temperatures with = 45 kcal/mol and = 2 e.u. A suggested pathway involved tricyclo[5.3.0.0" ]deca-2,5,9-triene or lumibullvalene (LBV), the vinyl cyclopropane rearrangement product of BV. The stereochemistry required for this rearrangement represents a forbidden pathway. However, the Eact is sufficiently higher than the BDE of the cyclopropane bond in BV that a bisallylic biradical derived from cyclopropane bond fission in BV can easily be formed, and it can close to LBV (Scheme 11.23). 

AUene Cycloadditions. —The consensus of opinion is that allene dimerization to 1,2-dimethylenecyclobutanes is a two-step process which proceeds via an intermediate bisallyl biradical (315). Preference for formation of those stereoisomers having larger groups (R) on the inside positions of the double bonds of the product (316) can be explained on this model by assuming that they occupy the less hindered inward positions in (315). Dimerization of 1,3-diadamantylallene was studied since models suggested that a product such as (316 R = adamantyl) would be impossible to form. In fact dimerization... 

Maier, G. and Senger, S., 2-Vinylmethylenecyclopropane/3-methylenecyclopentene rearrangement matrix infrared spectroscopic identification of 4-methylene-2-pentene-l,5-diyl (1,2-bisallyl biradical),/. Am. Chem. Soc., 119, 5857, 1997. 

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... 

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