Transition Metal Complexes of Vinylketenes

Publisher Summary This chapter discusses the various transition-metal complexes of vinylketenes—namely, η 4 -(vinylketene) chromium(0)complexes, η 4 -(viny1ketene)molybdenum and tungsten complexes, η 4 -(vinylketene)manganese(I) and η 4 -(vinylketene)vanadium(I), η 4 -(vinylketene)cobalt(I) complexes, and η 4 -(vinylketene)iron(0) complexes. The stabilizing effect of metal complexation has great import when considering the use of catalytic amounts of a transition metal reagent to accelerate organic transformations that pass through unstable vinylketene species. The annulation reactions take place with certain classes of vinylketenes in both their free and complexed states. The extension of this methodology to incorporate a catalytic transition metal, or the application of this concept to alternative organic transformations, yields some important results. Many of the syntheses depend on the carbonylation of a vinylcarbene complex for the generation of the vinylketene species. The ease of this carbonylation process is controlled by the identity of the metal. The electronic characteristics of the metal have a great effect on the strength of the metal–carbon double bond and could be a regulating factor in the carbeneketene transformation. Methods for the generation and use of enantiomerically pure vinylketene complexes are well developed. Stereocontrol is exerted over subsequent reactions to yield optically active organic or organometallic products. Investigations are to be carried out into the reaction mechanisms of several fundamental processes.