The Diradical Mechanism

Thermodynamic calculations [29], based on the scheme shown, with the reasonable assumptions that step 1 is rate limiting and that because of the extreme exothermicity, k2 l i 

The results were in agreement with the trend in activation energy observed experimentally for a series of dioxetans with an increasing number of alkyl groups. The unsubstituted parent dioxetan is not sufficiently stable (as predicted) to allow its isolation, but the others gave good experimental values. This analysis does not constitute a proof of the diradical formation, but is compatible with it. 

A clear distinction between the concerted and diradical routes would appear possible by placing substituents on the methylene groups so that the cleavage of the C-C bond would be enhanced by developing conjugation. 

Various dioxetans of this sort, particularly with Ri = H, R2 = R3 = R4 = Ph, have been synthesised [15]. The activation energy is exactly within the range for simple alkyl substitution, and there is no effect of solvent polarity on the decomposition. The significance of this last point will be apparent after the discussion of the concerted and electron transfer mechanisms. This useful approach has been extended [30] to a study of substituent effects in the series based on (1) and (2). Although 3,3-bis (p-anisyl)-l,2-dioxetan has a lower Ea (87.8 12.6kJM- ) than the 

Others, this is about what would be expected for a substituent effect on the oxygen-centred diradical. Certainly the reaction constant would be very different for a rate-determining step involving C-C bond breakage. Isotope effects -substitution by deuterium - have also been used to support the diradical mechanism [31]. No isotope effect was noted for (3), the conclusion being that rehybridisation at carbon does not occur in the transition state. An inverse 

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The diradical mechanism b is most prominent in the reactions involving fluorinated alkenes. These reactions are generally not stereospecificand are insensitive to solvent effects. Further evidence that a diion is not involved is that head-to-head eoupling is found when an unsymmetrical molecule is dimerized. Thus dimerization of F2C=CFC1 gives 106, not 107. If one pair of electrons moved before the other, the positive end of one molecule would be expeeted to attack the negative end of the other. 

Thermal cleavage of cyclobutanesto give two alkene molecules (cyclorever-sion, the reverse of 2 -I- 2 cycloaddition) operates by the diradical mechanism, and the [ 2s -I- o2a] pathway has not been found " (the subscripts a indicate that cr bonds are involved in this reaction). 

A special problem is the high yield of triplet carbonyl compounds being formed — neither the concerted nor the diradical mechanism are fully explaining this fact. Further data on the identities and yields of excited products from different dioxetanes are needed. 

The above reactions reinforce the diradical mechanism proposed for the BF3 reaction. Hexafluorobenzene and the various fluorinated ethylenes 73,74 however, react quite differently. The products in these reactions formally correspond to C-F bond insertion by an SiF2 monomer. 

Investigations on the thermal isomerization of various disubstituted [2.2]paracyclophanes at 200 °C by Cram and Reich 111 112> showed that only the diradical mechanism (Route A) was consistent with experimental findings. Thermal isomerization starting from the pure pseudo-geminal... 

One method which is diagnostic of the diradical intermediate is the extent of retention of cis- and trans-substitution in the product from a cis- or tram-olefin. In a concerted process both bonds are formed concurrently, thereby prohibiting isomerization, whereas in the diradical mechanism the bond can rotate. 

It has been argued that the mechanism here is not the diradical mechanism, but the [,2, + 2, mechanism Roberts Tetrahedron 1985, 41, 5529. 

For evidence favoring the diradical mechanism, see Willcott Cargle J. Am. Chem. Soc. 1967,89, 723 Doering Schmidt Tetrahedron 1971,27, 2005 Roth Schmidt Tetrahedron Lett. 1971,3639 Simpson Richey Tetrahedron Lett. 

The diradical mechanism as applied to this system is as hypothetical as in the cases of cyclopentanone and cyclohexanone. The only data on the use of radical scavengers is a report on the photolysis of cycloheptanone in the presence of 3.2 mm. of oxygen. In this case, both of the C hydrocarbons and 6-heptenal was observed to be formed. It is quite likely that the reactions 30-33 are concerted processes. 

While compound (3) obtained in the cocondensation reaction may still be formed through the contribution from the silirane mechanism, just as it is formed in the gas phase, the formation of (4) and (5) cannot be rationalized with the same reaction mechanism. The fact that compound (4) was found to be the major product in the cocondensation reaction strongly indicates that the reaction proceeds via the diradical mechanism shown in Scheme 12. Compounds (3) and (4) are both thermally stable. It is interesting to note that compound (4) can be converted into compound (3) in n-hexane solution by UV irradiation. 

At this point, we feel that the difference between the silirane mechanism and the diradical mechanism has been overemphasized in the past. In fact, the silirane intermediate may be considered as a special case in the (SiF2) homologue with n = 1. 

Cyclobutanes may be converted to alkenes thermally, the reverse of the [2 + 2] cycloaddition reaction. These retroaddition or cycloreversion reactions have important synthetic applications and offer further insights into the chemical behavior of the 1,4-diradical intermediates involved they may proceed to product alkenes or collapse to starting material with loss of stereochemistry. Both observations are readily accommodated by the diradical mechanism. Generation of 1,4-tetramethylene diradicals in other ways, such as from cyclic diazo precursors, results in formation of both alkenes and cyclobutanes, with stereochemical details consistent with kinetically competitive bond rotations before the diradical gives cyclobutanes or alkenes. From the tetraalkyl-substituted systems (5) and (6), cyclobutane products are formed with very high retention stereospecificity,while the diradicals generated from the azo precursors (7) and (8) lead to alkene and cyclobutane products with some loss of stereochemical definition. ... 

The argument in favour of the latter was that the diradical should be lower in energy, because its formation requires disruption of only one carbon-carbon rc-bond instead of two in case of carbene formation. Both ESR work and optical spectroscopy have meanwhile confirmed the diradical mechanism for growth of oligomeric chains up to length of 5 repeat units. Upon further addition of monomers, the acetylenic structure becomes energetically more stable causing a cross-over to the carbene mechanism. For further discussion of this topic the reader is referred to the article by H. Sixl in this volume. 

Both mechanisms have in common a spin-multiplicity change however, the fundamental difference between them is that in the diradical mechanism (Eq. 66), the intersystem-crossing step is reversible, while in the concerted mechanism (Eq. 67) it is irreversible. Thus, the classical mechanistic dilemma of distinguishing between normal spin-conserved diradical and concerted reactions, particularly [2+2]-cycloaddition, is still further complicated by the fact that in the dioxetane retrocyclization distinct spin-multiplicity changes are involved. The theoretical and experimental work on this challenging problem will be briefly discussed. 

Most of the experimental evidence also points to the diradical mechanism as the preferred decomposition mode. Thus, the very earliest experimental evidence in support of the diradical mechanism rests on the fact that alkyl and phenyl substitution does not significantly alter the activation parameters for dioxetane decompositon. It was argued that if C-C bond cleavage occurs simultaneously with 0-0 bond cleavage, the incipient carbonyl group in the activated complex (23) should be stabilized in the relative order phenyl > alkyl > hydrogen. Thus, the activation energies should obey the relative order ii a(Ph)< fl(R)< a(H), that is, lowest for phenyl-substituted dioxetanes. Since this expectation was not borne out by the experimental data, the diradical (24) was proposed as an intermediate. 

As additional support for the diradical mechanism, it was shown that the 3,4-diethoxy-l,2-dioxetane (8) and the p-dioxene-l,2-dioxetane had identical activation energies, implying that the C-C bond is not significantly stretched in the activated complex. That these notions on substituent effects in dioxetane decomposition are grossly oversimplified has come clearly into focus in recent years (Table 4). The fact that little yet is understood about the correspondence between activation parameters and dioxetane structure has already been amply expounded in Section V.l.B. Nevertheless, a few additional comments seem appropriate on this subject in... 

Probably the strongest support in favor of the diradical mechanism is the lack of a deuterium isotope effect in the thermal decomposition of franx-3,4-diphenyl-1,2-dioxetane. In the concerted mechanism, the ring carbon of the dioxetane changes its hybridization state from sp to sp in the activated complex (23) and an inverse secondary isotope effect k lkp) would be expected. Consequently, a diradical mechanism was argued to accommodate these results. Similarly, in the thermal decarboxylation of the dimethyl a-peroxylactone, a negligible (A ///A ) = 1.06 0.04) secondary isotope effect was observed. Presumably, in the a-peroxylactone decomposition, a diradical mechanism similar to that of dioxetanes (Eq. 66) upholds. 

The photochemical cycloaddition method provides good yields of spirothietanes, as illustrated in the preparation of 50. The (—)3-menthyl ester of methacrylic acid gives thietane 51 in 17% optical purity via the diradical mechanism Si Ti), but in only 6% optical purity via the S2 state. ... 

Thermal cleavage of cyclobutanes ° to give two alkene molecules cyclorever-the reverse of [2 + 2]-cycloaddition) operates by the diradical mechanism. 

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. 

PhCH=N(0)Ph, and styrene to yield 2,3,5-triphenylisoxazoline 226 are also consistent with Huisgen s concerted, cyclic mechanism and inconsistent with the diradical mechanism (structures 227 and 228). 

Yamaguchi, K., Yabushita, S., Fueno, T., Houk, K. N. Mechanism of photooxygenation reactions. Computational evidence against the diradical mechanism of singlet oxygen ene reactions. J. Arrr. Chem. Soc. 1981, 103, 5043-5046. 

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