Diradicals configurations

For the next higher polymer, 1, 4-butadiene, rotation is considerably restricted in the (tc, ir ) state because of migration of the double bond in the centre in the diradical configuration but the ground state rotamers are in rapid equilibrium. 

This ring-opening and reclosure process does not apply for the [a, it] diradical configuration, because at least one of the radical lobes is coplanar with the [Pg.58]

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... 

Scheme 8 Electron configuration and polarization in the triplet diradicals...

The orbital phase theory is applicable to the singlet diradicals [20]. The electron configuration of the singlet states of the cross- (TMM) and linear (BD) conjugate diradicals is shown in Scheme 9, where the mechanism of the delocalization of a and P spins between the radical centers through the double bond are separately illustrated by the arrows. The cyclic [-a-Tr-b-T -] interaction is readily seen to occur for the spin delocalizations. The p orbital a) in one radical center and the n orbital are occupied by a spins, and therefore, electron-donating orbitals. The p orbital (b) in the other radical center and the ii orbital are not occupied by a spins. 

Scheme 9 Electron configuration and delocalization, cyclic orbital interaction, and orbital phase properties in the singlet diradicals...

The early stages in the oxidation of disilene have been treated theoretically for the parent molecule H2Si=SiH2.95 The first intermediate along the reaction coordinate is the open-chain trans diradical 64 (Scheme 16), which is in equilibrium with a gauche form, 65. From the latter, closure to the 1,2-dioxetane 66 would probably be rapid. The open-chain form can react with a second molecule of disilene to give the diradical 67, which could collapse into two molecules of the disilaoxirane 68. If similar steps are followed in the oxidation of 3, they must be quite rapid, since the relative configuration at the silicon atoms is maintained in both products, 59a and 61a.93... 

Scheme 6.25 shows the formulas of the compounds 88 [68], 89 [70], 90 [78], 91 and 92 [70], which emerge from the dimerization of 74, 76, 77 and 79, respectively. The structure is of the same type as that of the dimer 38 of 6 (Scheme 6.10). The configuration has not been determined, but it is assumed to be trans as in the case of 38. Hence the cyclization of the tetramethyleneethane diradicals of type 37 (Scheme 6.10), the immediate precursors of the isolated dimers, should proceed as a least-motion process at the unsubstituted radical centers. Only the trimethylsilyl group causes a predominantly alternative course, since the dimers 91 and 92 were obtained in a ratio of 1 2.

In the case of the [4+ 2]-cycloadditions, the diradical analogous to 172 should contain an allyl radical subunit in the side-chain having the Z-configuration. There the closure of the six-membered ring occurs also employing the central carbon atom of the pentadienyl radical system. A quantum-chemical study reproduced the preference of the step 172 —y 163 over that from 172 to 173 [47]. This may have its origin in the higher spin density at C3 of the cyclohexadienyl radical as compared with Cl and C5 [108]. 

The [2 + 2]-cycloaddition of allene proceeds via a stepwise diradical mechanism rather than a concerted one-step mechanism. The allenes come together in a crossed configuration. The bond formation between the central sp carbon atoms is accompanied by a simultaneous conrotatory twisting leading to a perpendicular 2,2 -bisallyl diradical 3. Rotation about the central bond of 3 gives the planar diradical and a disrotatory closure leads to the formation of dimer 2. The stereochemistry of some of the following examples is explained by this mechanism. 

Since then, the photocycloaddition reaction has been extensively studied and has become a powerful tool for the construction of complex polycyclic molecules. High stereoselectivities are observed in some cases. The configuration of the diradical intermediate determines the stereochemistry of the adduct [33], Typical examples... 

Further supporting evidence for the occurrence of diradicals was obtained by Reich and Cram when they heated [2.2]paracyclophane with either dimethyl maleate or fumarate esters at 200 °C for 40 h in the absence of air. The cis- and trans-2,3-dicarboxymethyl[4.2]paracyclophanes 162 and 163 were formed in about equal amounts, irrespective of the configuration of the olefin employed. Other similar reactions would also suggest a radical mechanism for this reaction furthermore, a concerted addition of the olefinic double bond to 2, or to the postulated intermediate diradical 157, can be ruled out because of lack of stereospecificity of insertion. 

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