Carbon ligands neutral

No reaction of soft carbon nucleophiles takes place with propargylic acet-ates[37], but soft carbon nucleophiles, such as / -keto esters and malonates, react with propargylic carbonates under neutral conditions using dppe as a ligand. The carbon nucleophile attacks the central carbon of the cr-allenylpal-ladium complex 81 to form the rr-allylpalladium complex 82, which reacts further with the carbon nucleophile to give the alkene 83. Thus two molecules of the a-monosubstituted /3-keto ester 84, which has one active proton, are... 

Transition Metal Complexes of Neutral Tti -Carbon Ligands... 

Dihalocarbene ligands, like other neutral 2-e donor carbon ligands, are expected to participate in migratory-insertion reactions when bound adjacent to a rx-bound alkyl or hydride ligand. An example is provided by the following reaction (119) ... 

Neutral r -Carbon Ligands Beyond Carbon Monoxide 3... 

Carbon monoxide, the paradigm of neutral q -carbon ligands, was early recognized to be well described by three limiting forms C=0, and " C-O , to... 

Class A. Neutral -Carbon Ligands Ylides and Carbodiphosphoranes... 

Scheme 3 Typology of neutral r) -carbon ligands. The coordinating carbon atom of the free ligand obeys the octet rule in one of its main resonance form (form a)...

Class B. Neutral -Carbon Ligands NHC and Non-NHC TT-Conjugated Carbenes... 

Class C. Neutral si -Carbon Ligands Amino- and Nonamino-Cumulenylidenes... 

Both categories of sp-hybridized neutral carbon ligands, namely the all-carbon-substituted and heteroatom-conjugated allenylidene and cumulenylidene ligands, are presented in great detail by V. Cadiemo and S. E. Garcia-Garrido in the sixth and seventh chapters of this volume, respectively. 

After the discovery by Fischer and Maasbol of the first stable carbene complexes in 1964, i.e., [(CO)5W =C(OMe)R ] [21], generation of related metaUacumulene derivatives [M]=C(=C) =CR2 (n > 0) was obviously envisaged. Thus, it is presently well-established that stabilization of these neutral unsaturated carbenes by coordination to a transition metal center is possible by the use of the lone pair of electrons on the carbenic carbon atom, via formation of a metal-carbon a-bond (electron back-donation from the metal fragment to the carbon ligand may strengthen this bond). This has allowed the development of a rich chemistry of current intense interest due to the potential applications of the resulting metallacumulenic species in organic synthesis, as well as in the construction of molecular wires and other nanoelectronic devices [22]. 

In this chapter, the most efficient synthetic routes, the main stmctural features as well as reactivity patterns of odd-chain metallacumulene complexes bearing 7i-donor substituents, i.e., [M]=C(=C) =CR R ( = 1, 3, 5 R /R = NR2, OR, SR, SeR), are reviewed. In addition, the coordination chemistry of phosphonioace-tylides (R3P C=C ) and tricarbon monoxide (C3O) will also be discussed since these heteroatom-containing 77 -carbon ligands lead to closely related bonding situations, with participation of both neutral cumulenic and zwitterionic alkynyl-type mesomeric forms (Fig. 3). 

Part III sp -Hybridized Neutral iti -Carbon Ligands All-Carbon-Substituted Allenylidene and Related... 

Type III reaction proceeds by an attack of a nucleophile at the central sp carbon of the allenylpalladium. Soft carbon nucleophiles such as / -keto esters and malonates react with propargylic carbonates under neutral conditions using DPPE as a ligand [35], The 2,3-disubstituted propenes 151 and 152 are obtained by the reaction of 2-propynyl carbonate (145) with two moles of malonate under neutral conditions in... 

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