Diradical pathways

In a definitive study of butadiene s reaction with l,l-dichloro-2,2-difluoio-ethylene, Bartlett concluded that [2+4] adducts of acyclic dienes with fluorinated ethylenes are formed through a mixture of concerted and nonconcerted, diradical pathways [67] The degree of observed [2+4] cycloaddition of fluorinated ethylenes IS related to the relative amounts of transoid and cisoid conformers of the diene, with very considerable (i.e., 30%) Diels-Alder adduct being observed in competition with [2+2] reaction, for example, in the reaction of 1,1 -dichloro-2,2-difluoro-ethylene with cyclopentadiene [9, 68]... 

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 ... 

For the mechanistic course of that reaction two pathways are discussed " a concerted [l,3]-sigmatropic rearrangement, and a pathway via an intermediate diradical species. Experimental findings suggest that both pathways are possible. The actual pathway followed strongly depends on substrate structure the diradical pathway appears to be the more important. 

The mechanisms of these ring expansions are not certain. Both concerted" and diradical" pathways have been proposed, and it is possible that both pathways operate, in different systems. 

The mechanism can be described by the diradical pathway given (the C-3 substituents act to stabilize the radical), though the species shown are not necessarily intermediates, but may be transition states. It has been shown, for the case of certain substituted substrates, that configuration is retained at C-1 and C-5 and inverted at... 

Dimerization of lff-azepines is an extensively studied phenomenon and involves a temperature dependent cycloaddition process. At low (0°C for 1 R = Me) or moderate (130 °C for 1 R = C02R or CN) temperatures a kinetically controlled, thermally allowed [6 + 4] dimerization to the exo -adduct (73) takes place, accompanied by a small amount (<10%) of symmetrical dimer (74). The latter are thermodynamically favored and become the major products (83%) when the Iff-azepines are heated briefly at 200 °C. The symmetrical dimers probably arise by a non-concerted diradical pathway since their formation from the parent azepines by a concerted [6+6]tt cycloaddition, or from dimer (73) by a 1,3-sigmatropic C-2, C-10 shift are forbidden on orbital symmetry grounds. Dimerization is subject to steric restraint and is inhibited by 2-, 4- and 7-substituents. In such cases thermolysis of the lif-azepine brings about aromatization to the correspondingly substituted JV-arylurethane (69JA3616). 

MINDO/3 calculations of cyclobutane formation reveal a large singlet-triplet splitting of 10 kcal - mol-1 in the cisoid butyl 1,4-diradical, which might be due to a radical-radical interaction.79 The results of a more recent SINDOl calculation are also in agreement with a nonconcerted cleavage of cyclobutane via a diradical pathway.80 Additional quantitative evidence for the 1,4-diradicals has been obtained by thermochemical studies.81... 

Finally, the relative rates of cleavage and rotation of 1,4-diradicals have been directly studied by thermal decomposition of the tetrahydropyridazines 9 and 10 (R = Me or D) at 415 "C.85-87 Judging from the rate constants and product distributions obtained for various processes, it is likely that the fate of the diradicals 11 and 12 is identical to those generated by thermolysis of cyclobutanes.85 - 87 Obviously, a choice in favor of the nonconcerted diradical pathway can, therefore, be made on the basis of the aforementioned theoretical as well as experimental endeavors. 

The concerted nature of these reactions is also supported by the pyrolytic cleavage of cis-2,3-dimethylcyclobutanone and rra i-2,3-dimethylcyclobutanone at a temperature of 325 °C under a pressure of 10 Torr, from which approximately 99% retention of configuration in the resulting but-2-enes is observed.92,93 Moreover, the results obtained from the thermolysis of cis- and m -2,4-dimethylcyclobutanone also lend support to the concertedness of this reaction.92,93 However, as substantiated by Arrhenius parameters, a diradical pathway is the most likely competing reaction in the pyrolysis of 2-chlorocyclobutanone.94 On the other hand, the kinetics of the gas-phase pyrolysis of bicyclo[3.2.0]hept-2-en-6-one (14),95 bicyclo[3.2.0]heptan-6-one (IS)96 and 2,2-dimethyl-3-ethoxycyclobutanone (16)97 are commensurate with a concerted cycloreversion mechanism involving four-center auasi-zwitterionic transition structures. 

There is a difficulty with the mechanism of Scheme 3 in that the fragmentation of a triplet diradical should conserve spin, yet neither triplet Me2Si nor triplet tetraphenyl-naphthalene have been detected. The diradical pathway for the photofragmentation of silanorbornadienes confirms an earlier proposal by Barton and coworkers52. There is always the possibility that diradical intermediates such as the singlet and triplet 13 S and 13 T could function as silylenoids. Thus, the assumption that products from pyrolysis of 7-silanorbornadienes are formed from free silylenes must be treated with caution. 

The nonconcerted diradical pathway was still considered viable in the thermal decomposition of 1,2-diazetines assuming that the loss of nitrogen from the intermediate was rapid relative to the C-C bond rotation. Olsen using mono- and dideuterated diazetines provided mechanistic evidence for this process <1982JA6836>. The kinetic isotope effect observed suggested that the transition state was unsymmetrical and should be proceeding via a diradical like pathway (Scheme 3). 

Fig. 7. The [1,3] sigmatropic shift of vinylcyclopropane and two transition structures involved in the reaction. Left transition structure for the concerted process right transition structure for the bond closure in the diradical pathway...

In order to explain the stereochemical results in terms of the diradical mechanism, one must assume that the diradical intermediates collapse to dienes before rotamer equilibration is achieved. This demand led to the conclusion that the concerted mechanism accommodates the results with less stringent demands. Thermochemical calculations provide additional support for the conclusion that there is only a small probability for a diradical pathway. 

A diradical pathway was suggested by Christl and coworkers for 95 (equation 124). This is supported by the lower activation energy for this compound as compared with its analog, 7, whose diradical intermediate is a less potent radical stabilizer (equation 125). 

Some of the adducts derived from the addition of a cyclopropene to a cyclopentadiene cycloadd intramolecularly upon heating to afford homoquadricyclane derivatives " . For example, cyclopropene (155) gives a ca. 1 2 ratio of the anticipated adducts 156 and 157 at — 78°C. Quantitative rearrangement of the exo to the endo isomer occurs on warming the mixture to ambient temperature, but thermolysis of the endo product 157 at 200°C surprisingly provides the homoquadricyclane 158 and a diradical pathway is likely. 

To distinguish between these two mechanisms, Baldwin and co-workers carried out the pyrolysis of l- C-2,2,3,3-i/4-cyclopropane (42). As shown in equation 6.12, cleavage of a CH2-CD2 bond in 42 by the diradical pathway would lead to propene in which the C label at Cl of the product bears two hydrogen ( H) atoms (43). If the carbene mechanism (equation 6.13) operates, the C label at Cl of the product (44) would be bonded to one H and one D. The C NMR spectrum of the product mixture showed the presence of both 43 and 44. The results were interpreted to mean that the p5n olysis of 42 occurred by both mechanisms, although relatively more of the product was formed by the diradical pathway. ... 

For a further discussion of sigmatropic shifts and a method for generating qualitative potential energy surfaces to rationalize them, see Epiotis, N. D. Shaik, S. /. Am. Chem. Soc. 1977,99,4936. Beno, B. R. Fennen, J. Houk, K. N. Lindner, H. J. Hafner, K.. Am. Chem. Soc. 1998,120,10490. The calculations also revealed, however, that diradical pathways were lower in energy than the concerted pathway. 

Since this report in 1976, numerous studies of stereoselective ring opening reactions of variously substituted and enantioenriched VCPs have been reported. The formation of different stereoisomeric cyclopentene products suggests that die mechanism of the rearrangement might involve both the concerted and diradical pathways to different extents, depending upon the nature of substituents and their precise orientation on the corresponding... 

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