The Biradical Mechanism

The symmetry analysis is, however, still incomplete The correspondence diagram in Fig. 6.2 was drawn for concerted closure of both bonds this pathway was shown to be formally allowed under a b2g perturbation, a prediction - perhaps more properly a retrodiction - borne out by the facile head-to-tail dimerization of silaethylene. Where there is no effective substitutional desym-metrization, the allowing perturbation is necessarily a big displacement that foils the concerted process. To be sure, the nuclei move in the right direction to form a transoid biradical, but it has yet to be confirmed that the stepwise process is consistent with the requirements of orbital symmetry conservation. This is done in Fig. 6.4 with the aid of a correspondence diagram specifically set up in to analyse the first step, formation of a tranS Stahle bradical. 

The MOs on the left side of Fig. 6.2 are relabelled according to their irreps. The better p-orbital overlap across the diagonal puts below tt, but 

1) The four zero-order orbitals chosen are a bonding-antibonding pair of a orbitals localized in the new C2-C3 bond lined up along z, and the two combinations of Pz orbitals on Ci and C4. Of the latter two, through-space interaction is assumed to stabilize tt slightly relative to ttI. 

2) The orbitals of like symmetry interact, the lower being stabilized at the expense of the upper. The resulting through-bond interaction [24] reverses their order, so that the electronic configuration of the trans-stable radical becomes [a 6 ] like that of the reactant pair of ethylenes. 

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The two most widely accepted mechanisms for the spontaneous generation of radicals from S are the biradical mechanism (top half of Scheme 3.61) first proposed by Flory314 and the Mayo315 or MAH (molecule assisted homolysis) mechanism (lower pari of Scheme 3.61). 

Two extreme mechanisms have been proposed for the unimolecular dioxetane decomposition the concerted mechanism , whereby cleavage of the peroxide and the ring C—C bond occurs simultaneously, and the biradical mechanism whereby the initial cleavage of the 0—0 bond leads to the formation of a 1,4-dioxy biradical whose subsequent C—C bond cleavage leads to the formation of the two carbonyl fragments (Scheme 8). Although the biradical mechanism adequately explains the activation parameters obtained for most of the dioxetanes smdied, it appears not to be the appropriate mechanistic model for the rationalization of singlet and triplet quanmm yields. Therefore, an intermediate mechanism has been proposed, whereby the C—C and 0—0 bonds cleave in a concerted, but not simultaneous, manner (Scheme 8) . 

Although the activation parameters obtained from the thermal decomposition of a great number of diverse dioxetane derivatives have been interpreted on the basis of the biradical mechanism, no general interpretation of the excitation efficiencies has been given on fhe basis of fhis mechanism Furfhermore, most theoretical... 

From the variation in product quantum yields, Heicklen and Knight82 were able to deduce the steps for the biradical mechanism ... 

A stepwise reaction involving a biradical intermediate accounts for the formation of both 45 and 46. In the biradical mechanism the first step is formation of just one C-C bond between the reactants, and this could occur in two different ways to give 47 or 48ab ... 

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. 

The energy diagram shown in Fig. 2 gives a possible interpretation of the geometrical and structural isomerization in terms of the biradical mechanism. This... 

Fig. 2. Energy diagram for the structural and geometrical isomerizations of cyclopropane, on the basis of the biradical mechanism.

The formation of the diene in the isomerization of vinylcyclopropane is analogous to the formation of propene from cyclopropane. The reaction has a higher energy of activation than the ring-closure reaction. Fig. 3 gives an interpretation of these results in terms of the biradical mechanism. 

In spite of the very considerable amount of work on this system the mechanism is by no means firmly established. The biradical mechanism is consistent with all of the experimental results, but it is difficult to exclude the alternative mechanism. 

The biradical formed in Process a will be relatively more stable than that formed in b, since the free electrons are both located on secondary carbon atoms Path a should therefore have a lower activation energy than Path b, in agreement with experiment cf. Table 6 for the cis compound = 60.4, 2 = 63.0 kcal.mole for the irons compound 4 = 61.6, 5 = 63.4 kcal.mole" ). Both cis- and trans-butene-2 are formed from each of the dimethylcyclobutanes, but not in equilibrium amounts. This can be reconciled with the biradical mechanism if the lifetime of the radical is comparable with the time of rotation of the groups in the biradical. The results are equally satisfactorily explained if the intermediate has only partial biradical character. 

Scheme (5), like (3), predicts the formation of the enol form, thus, all the evidence put forward in support of mechanism (3) may also be regarded as evidence in favour of mechanism (5). According to Wagner and Hammond the biradical mechanism is supported by the relative reactivities of the excited states of the ketones substituted to various extents. 

Let us first consider the question of the mechanism from the point of view of the existing hypotheses. On the basis of the concerted mechanism we have no a priori reason to favour either the triplet or the excited singlet state as the likely source of the primary reactions, however, if the biradical mechanism is valid for primary steps II and III, then the triplet state is expected to be the precursor of these reactions. 

Confirmation of the biradical mechanism was attempted by means of experiments involving addition of oxygen, which is known to be an efficient radical scavenger. However, it was found - that the formation of both the cycloalkanes and the olefins was practically not influenced by oxygen, while the rate of formation of the unsaturated aldehydes was increased instead of being decreased by O2. The effect of oxygen is essentially similar to that of inert gases. 

On photolysing cyclobutanone and cyclopentanone in the presence of excess ethylene. Flowers and Frey reported the formation of long-chain aliphatic and cyclic alkanes, considering it as a confirmation of the biradical mechanism. However on repeating these experiments,other workers did not observe the formation of the products reported by Flow.r and Frey, not even in the presence of 1 atm of C2H4. 

Srinivasan favoured the concerted mechanism for reactions I-IV. His reasoning was based, above all, on the lack of any effect caused by O2. In support of his view he claimed that inert gases exert a different, in some cases even opposite, influence on the formation of the various products, which, thus, cannot all originate from the same biradical. However, as appears from the above discussion and from that which follows, the inert gas effects are, in fact, compatible with the biradical mechanism as well. 

Summarizing, it can be seen that the experimental results do not point conclusively to either mechanism. It seems, however, that the biradical mechanism, which... 

From the above results it is evident that a lower energy content of the decomposing molecule favours the formation of the unsaturated aldehyde at the expense of decarbonylation. The explanation of this fact has been attempted on the basis of both the concerted and the biradical mechanisms. 

The dependence of the mode of decomposition on the energy possessed, was also interpreted on the basis of the biradical mechanism. It has been suggested that whatever change in the experimental conditions (an increase in pressure or wave-... 

Methyl cyclobutane carboxylate decomposes cleanly and unimolecularly to ethylene and methyl acrylate in the gas phase between 653 and 693 °K in static systems. The reaction can be well represented, both qualitatively and quantitatively by the biradical mechanism, viz. 

The four-membered ring oxide decompositions (oxetane and 3,3-dimethyl oxetane ) are well-behaved unimolecular reactions. Products are shown in Table 22. Reaction products and experimental Arrhenius parameters are consistent with the biradical mechanism below. Calculated parameters are those of O Neal and Benson . 

In a later study. Swanson and coworkers [81] studied the cure of acetylene-terminated poly(imide)s selectively labelled at various positions with nuclei. Curing of the sample, labelled at the imide carbonyl group, confirmed the completion of the imidization reaction on heating. The product of addition onto the carboxyl group was not observed. Four new peaks were identified in the spectrum of the cured sample labelled at the Ci-acetylene group, while a similar result was obtained for the sample labelled at the C2-acetylene position. Analysis of these results rules out the participation of coupling reactions and the biradical mechanism, which would produce triple-bond structures, but confirms the presence of the product of cyclotrimerization and Friedel-Crafts reactions. The latter mechanism is confirmed from the presence of small peaks due to aliphatic carbons in the spectra of the materials labelled at the acetylene groups. 

The biradical mechanism is commonly proposed to explain the reaction regioselec-tivity.629,630,632 633 Two alternative ring-opening processes for the 1,4-biradical 81, obtained from the photolysis of 82, may lead to two 1,3-biradicals, 83 and 84 (Scheme 6.32). The latter, which is thermodynamically more stable because the unpaired electron is delocalized on the benzhydryl moiety, should be a precursor to the diphenylvinylpropene 85. Indeed, the irradiation of 82 afforded 85 as a sole product.655... 

A group of papers dealing with the intramolecular photodimerization reaction of a,a>-bis(9-anthryl)alkanes (185 n = 2—10) fails to produce agreement about the detailed mechanism of the reaction. Measurements of quantum yields for fluorescence, photoreaction, and intramolecular deactivation as a function of temperature are said to provide no support for a biradical intermediate, but rather to support a concerted mechanism. In reply, the proposers of the biradical mechanism reinterpret these data and find them consistent with their mechanism. A third research group reports results in fluid solution at room tempera-ture " their concern is more with the question of excimer involvement in the mechanism, and they report that in many of the systems unidentified photoproducts are formed via excimers that do not lead to the normal 9,9 -linked photodimer. The internal photodimer from (185 n = 3) has been studied in a matrix at 10 and photodissociation is shown to lead to two different modifications of (185 n = 3) with different reactivities. A geometrical constraint on the intramolecular photoreactions of 9,9 -linked bisanthracenes is demonstrated by the failure of the di-substituted ethylenes (186) to give internal dimers. 

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