Olefinic donors, fragmentation reactions

DFT calculations confirmed the similarities with the alkyne/vinylidene transformation but have revealed that additional parameters were essential to achieve the isomerization [8, 20-23]. The hydride ligand on the 14-electron fragment RuHC1L2 opens up a pathway for the transformation similar to that obtained for the acetylene to vinylidene isomerization. However, thermodynamics is not in favor of the carbene isomer for unsubstituted olefins and the tautomerization is observed only when a re electron donor group is present on the alkene. Finally the nature of the X ligand on the RuHXL2+q (X = Cl, q=0 X = CO, q=l) 14-electron complex alters the relative energy of the various intermediates and enables to stop the reaction on route to carbene. 

M(tj-CsH5)2 fragment with 7r-acceptor and -donor ligands and have emphasized the orbital interactions with conformational consequences, e.g., for carbene and olefin ligands. They have also discussed insertion reactions of olefins and carbon monoxide coordinated to this metal fragment 134). 

The addition of transition metal fragments ML (L = two-electron donor ligand) across formally unsaturated metal-metal or metal-carbon bonds is a well-developed synthetic route to heteronuclear clusters (1,2,11,12,27) and has received theoretical justification from Hoffmann s isolobal principle (46). The addition of a PtL2 fragment across an M=M double bond may be considered as analogous to the reaction of a carbene with an olefin, resulting in a cyclopropane. The use of isolobal analogies in the directed synthesis of heteronuclear clusters has been reviewed (11,12,27). 

This experiment demonstrates that tlie eyanoisopropyl radieal is eapable of addition to the double bond of trimer 4 but the radieal 7 so formed is ineapable of sustaining propagation and fragments to olefin 8 (a major produet) and radieal 9. Radieal 9 proceeds to stable products by combination or disproportionation with itself or other radicals (4). The mechanism in Scheme 2, therefore, is likely to be the process responsible for the chain transfer observed in the copolymerization experiments summarized in Table 1. An additional but minor reaction pathway for radical 7 was observed to be termination by hydrogen abstraction. Other radicals in the mixture (e.g. 9 or eyanoisopropyl) would be the most likely hydrogen donors (i.e. disproportionation). 

---