Biradicals transition state model

At present rate parameters for cis-trans isomerization reactions can be estimated by using the empirical model involving biradical transition states (Benson, 1976). That is, the transition state can be viewed as the —C —C — biradical, which rapidly rotates. Experimental rate parameters for a variety of cis-trans isomerization reactions are presented in Table XL As seen from this table, the A factors for these reactions are consistent with a tight transition-state model. Although not directly evident from Table XI, activation energies... 

Figure 8.1 Transition state model for 1,5 biradical cydization.

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

In general, the stereoelectronic influence of substituents in [2 + 2]-photocycload-dition reactions is minor, and the preferred ground-state conformation often accounts for the formation of the major diastereoisomer. Inspection of molecular models and force field calculations provide a good picture of possible transition states leading via 1,4-biradicals to cyclobutane products. The total synthesis of (+)-guanacastepene represents another recent example for the use of stereoselective intramolecular cyclopentenone-olefin photocycloadditions in natural products synthesis [32]. 

From a theoretical point of view, in multi-bond reactions it is essential to use a wavefunction where the possibility of biradical and aromatic transition states can be treated with a balanced level of accuracy. Thus the MC-SCF results of Morokuma et al [21] on the Cope rearrangement of the "model" reaction of 1,5 hexadiene are very convincing and provide reliable evidence that the lowest energy pathway for the "model" reaction is the synchronous one with the biradical intermediate lying 22 Kcal mole higher in energy than the synchronous transition state. 

Ultimately, these experimental studies cannot completely rule out biradical intermediates in the Cope rearrangement, especially for the heavily substituted systems. In the end, only theory can make a definitive statement about the structure of a transition state. The interplay between theory and experiment in the Cope rearrangement and other pericyclic processes has at times been contentious. In addition, differing theoretical models have often made diametrically opposed predictions (see the Going Deeper highlight on page 900). This further fueled the debate over the true nature of pericyclic transition states. 

Figure 16.15 presents a model for the photochemical cis-trans isomerization of an olefin. First consider thermal cis-trans isomerization. We start at the trans form, rotate 90° to a transition state structure with a great deal of biradical character, and then continue on to the cis form. Now consider the it,it state of a simple olefin. The -ir bond is now "broken", and there is no reason for it to stay planar. In fact, steric effects and electron repulsion effects favor the twisted form, and that is the minimum on both Si and Ti. We have a situation in which a geometry that is a maximum on So is a minimum in the excited state, and this is ideal for funnel formation. For direct irradiation of olefins, the Si and So states are close enough that fairly efficient hopping from Si to So can occur. If we add a sensitizer to the mix, Ti of the olefin is formed, and this can actually be lower in energy than So at the twisted geometry. Now a relatively stable, triplet biradical intermediate may lie on the cis-trans isomerization pathway. 

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