Olefinic donors, fragmentation

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). 

Vicinally donor-acceptor-substituted olefins usually are rather unreactive species. Nevertheless cyclopropanation of 2,2-dimethyl-3(2 H)-furanone 70 could be executed with dimethyl oxosulfonium methylide as a methylene source. The bicyclic compound 71 is formed in modest yield accompanied by the spiro epoxide as a second product in almost equal amounts. Carbinols 72 derived from 71 by alkyl lithium addition can be nitrosated and photolyzed to suffer a Barton fragmentation. The resulting y-oxoaldehydes are directly cyclized to afford the 2-substituted cyclopentenones 73 in good yield 41. ... 

Typically, the orientation of the olefin is controlled by the orientation of the metal HOMO that would serve as the electron donor to the Tr -orbital of the olefin. For example, the HOMO of the tP° ML fragment lies in the plane of the metal and the metal-bound atoms of the two ligands, and as shown in Figure 2.24 the olefinic carbons lie in this plane in the most stable geometry. The HOMO of a sawhorse C, geometry d fragment lies in the plane... 

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