Rearrangement biradicaloid

It is easy to see how simple MO arguments lead one to expect that two non-bonding orbitals will result if a single bond is stretched sufficiently (Fig. 6), as a double bond is twisted by 90 degrees (Fig. 5), or as atoms are rearranged to make a carbene. Return through such biradicaloid... 

Using a valence bond scheme parametrized with an effective Hamiltonian technique, it was shown that the mechanistic preference for a synchronous pathway with an aromatic transition state versus a non-synchronous mechanism via biradicaloid intermediate can be controlled by two factors (1) the stability of the long bond in the Dewar valence bond structure, and (2) the softness of the Coulomb interaction between the end methylene groups in the 1,5-diene chain. This means that the mechanism of rearrangement (equation 153) can strongly depend on substituents218. 

The boron-centred biradicaloid (R2PBR )2 (5.17, R = Pr, R = Bu) is an isoelectronic analogue of the (RPCR)2 biradicaloids, 5.14 and 5.15. Both of these four-membered rings contain 22 valence electrons for R=H. The P2B2 system 5.17 is synthesised by the reaction of a 1,2-dichlorodiborane with two equivalents of LiP Pr2. The P2B2 ring is presumably formed by rearrangement of the initially formed acyclic P-B-B-P skeleton (Scheme 5.2). 

Figure 6. Logarithmic plot values of LUm/fc versus pressure for the Cope rearrangement of bullvalene (torr at the experimental temperature of 356 K). Experimental values are signified by solid circles. Pressures are the total sample pressure at 356 K. Errors in frUni/fc are reported to 2o. The solid (upper) line represents the values calculated from RRKM theory using the biradicaloid transition state model. The lower line represents calculated rate constants using the aromatic transition-state model. The collision diameter was 3.6 A in both cases.

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