Stepwise biradical pathway

According to the various theories of pericyclic reactions, thermal [2+2] cycloadditions must occur in an [s + a] fashion. When both partners are alkenes, this geometry is unfavorable, suffering adverse sferic interactions. As such, all [2+2] cycloadditions of olefins to produce cyclobutanes occur by a stepwise, biradical pathway. 

Calculations at the PM3 or B3LYP/6-31G(d) SCF-MO levels of theory have revealed that pairs of electron-withdrawing substituents (R,R ) cause trapezoid geometric distortion during 2s -t- thermal cycloaddition-elimination, such that a fully synchronous transition state (36) becomes favoured for conversion of (35) to (37) the alternative Woodward-Hoffman antarafacial mode or stepwise biradical pathways need not be invoked. ... 

The photochemical production of vinylcyclopropane derivatives from compounds having two 7t-moieties bonded to an sp3-hybridized carbon648 is termed the din-methane rearrangement, also known as the Zimmerman reaction.649 A very broad spectrum of di-7t-systems can lead to photoproducts that are usually not obtainable by alternative routes.632,633 The reaction may be classified formally as a [l,2]-shift but, according to the proposed stepwise biradical mechanism,650 651 1,3- and 1,4-biradical (BR) intermediates and also the second 7t-bond may be involved652 (Scheme 6.29). A concerted (pericyclic) pathway for the di-7t-methane reaction from the excited singlet state is, however, not excluded. Typically, the singlet state reaction occurs upon direct... 

The question of whether the Diels-Alder reaction is concerted or stepwise via a biradical is an old but important one. The stereochemical preservation of the dienophilie added suggests concert, but the biradical pathway has had proponents. A careful analysis of the enthalpies of the various species on the energy surface " " showed that the biradical intermediate, generated from vinylcyclobutane, while nearly as stable as the Diels-Alder transition state in the parent case, is kinetically inaccessible under the reaction conditions. An extension of that analysis to include the contribution of entropy changes at 473 K is given in Scheme 7.105. 

A great deal of work has been focused on whether the ene reaction proceeds through a concerted or a stepwise mechanism. The initially proposed synchronous pathway was challenged by a biradical , zwitterionic or a perepoxide intermediate. Kinetic isotope effects in the photooxygenation of tetrasubstituted, trisubstituted and cis-disubstituted alkenes supported the irreversible formation of an intermediate perepoxide,... 

Nevertheless, the agreement that is obtained - which is probably the best one should expect given the small size of the basis set used in the CCSD(T) calculations and the possible problems associated with biradical character in the methyl vinyl ether pathway - suggests that the theoretically predicted TS structures are accurate representations of the actual transition states. This establishes the concerted nature of three of the rearrangements and the stepwise nature of the fourth. 

Computational study at B3LYP/6-31G showed three possible pathways. The first one was the concerted process that directly gives the ene product, 25. The TS, 23, is 23.8 kcalmol-1 higher in energy than 22. Biradical intermediate, 24, for a stepwise process, which is formed with an inward Me rotation optimized. However, the TS leading to 24 could not be obtained, and all attempts to locate the TS converged on 23. Alternative stepwise process with outward Me rotation gave a TS which is 8.8 kcal mol 1 less stable than 23. 

The [2 + 2] cycloaddition represents the most general and direct pathway for the formation of a cyclobutane structure from two alkene moieties, as outlined in Scheme 2.126. This process may occur as a concerted reaction via a cyclic transition state (mechanism a), as a stepwise reaction involving the formation of an acyclic biradical (mechanism b), or through bipolar (mechanism c) intermediates. Depending upon the structure of the reactants, cycloaddition may occur by any of these mechanisms. 

Photoinduced [2 + 2] cycloaddition (Section 4.9) of alkenes (alkynes) to form cyclobutane (cyclobutene) derivatives is one of the best studied reactions in photochemistry.680 682 According to the Woodward Hoffmann orbital symmetry rules,336 the cycloaddition of one singlet excited (Si) and one ground-state alkene is allowed by a suprafacial suprafacial concerted stereospecific pathway (Scheme 6.45) 695 699 700 Rare concerted [4 + 2] and [4 + 4] photocycloadditions of conjugated singlet excited dienes must occur in a suprafacial antarafacial and suprafacial suprafacial manner, respectively.690 Since the suprafacial antarafacial reactant approach is geometrically difficult to achieve, [4 + 2] reactions usually proceed stepwise (involving biradical intermediates). [2 + 2] or [4 + 4] photocycloadditions can occur in either a concerted or stepwise fashion. 

Intersystem crossing from the excited singlet state of simple alkenes is inefficient. Triplet state cycloadditions, therefore, are usually achievable via triplet sensitization rather than direct irradiation.701 702 Such a process often involves exciplexes, formed between electron-poor and electron-rich alkenes,703 or 1,4-biradical intermediates (Scheme 6.45).704 Thus the tendency to achieve loose geometries in such species then favours a nonconcerted (stepwise) pathway, in which rotation about the central C—C bond occurs, eventually leading to loss of reaction stereospecifity. In general, the cycloaddition... 

The principal component of the reaction coordinate is the approach of the two ethylene molecules towards one another with retention of the full symmetry assumed in the construction of the correspondence diagram as Fig. 6.2 Wcts set up in D2/1, this least motion approach has the irrep ag. The diagram then tells us that the reaction coordinate for concerted conversion of the two tt bonds into the two (T bonds of cyclobutane also has to include a 625 component. Several symmetry coordinates, and the subgroups of D2/1 to which they desymmetrize the reaction path, are shown in Fig. 6.3. If the correspondence diagram had called for an displacement, the relatively facile formation of cyclobutane in its stable puckered D2 conformation would have been expected. If a b u component were required to induce the neccesary correspondence, the favored pathway would generate a cisoid biradical, which would immediately collapse to cyclobutane. The nominally stepwise reaction would then be kinetically indistinguishable from one in which the formation of both bonds is synchronous. 

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. 

Careful application of the three simple postulates listed above can yield insight into the mechanism and stereochemistry of biradical reactions as complex as the thermal dimerization of cis, irons-1,5-cyclooctadiene [26] or the isomerization of allyl-substituted cyclopropanes via internal [2 + 2]-cycloaddition [27]. An attempt to do so here would take us too far afield, in view of the ease with which biradical intermediates interconvert. Instead let us move on to the considerably more stereoselective cycloaddition of reactant pairs with complementary polarity, that proceeds stepwise along a zwitterionic pathway. [23]... 

A comparison of the three dimerizations discussed in the latter part of this chapter illustrates nicely the interplay between symmetry and energy The presence of an additional tt bond in cyclobutadiene offers enough energetic advantage to concerted closure of the four-membered ring to syn-TCOD for it to take precedence over the more general - and no less allowed - stepwise pathway via a transoid biradical. Cyclopropene, with just the one 7r-bond, behaves like a normal alkene and finds the latter pathway more convenient. Silacyclopropene starts off along a similar pathway, but makes use of the relative weakness of the CSi bonds to react in an entirely different manner, but one that is still consistent with the requirements of orbital symmetry conservation. 

Also, as shown in this example and similar to the Diels-Alder reaction, the normal or traditional mode of reaction is for the lowest unoccupied molecular orbital (LUMO) of the electron-deficient enophile to react with the highest occupied molecular orbital (HOMO) of the electron-rich ene. However, there are some carbon and hydrogen containing systems that have been proposed to proceed via radical or stepwise pathways. For example, allenyl alkynes 7 and 8 were thermally cyclized using PhMe under relatively mild conditions to provide 10 and 11 in good yield. The authors suggested a radical mechanism for this transformation via the fulvene biradical intermediate 9. ... 

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