Osborn carbene

Transition metal carbene complexes can be divided into two classes electrophilic carbenes (Fischer carbene [69-71], Casey carbene [72,73]) and nucleophilic carbenes (Osborn carbene [74,75], Schrock carbene [76-79]) ... 

Osborn carbene Schrock carbene Schrock carbene... 

Osborn and Green s elegant results are instructive, but their relevance to metathesis must be qualified. Until actual catalytic activity with the respective complexes is demonstrated, it remains uncertain whether this chemistry indeed relates to olefin metathesis. With this qualification in mind, their work in concert is pioneering as it provides the initial experimental backing for a basic reaction wherein an olefin and a metal exclusively may produce the initiating carbene-metal complex by a simple sequence of 7r-complexation followed by a hydride shift, thus forming a 77-allyl-metal hydride entity which then rearranges into a metallocyclobutane via a nucleophilic attack of the hydride on the central atom of the 7r-allyl species ... 

These two greatly different properties of metal carbene complexes reflect a crucial difference in the metal-carbon bonding. Single carbene moieties (present in Fischer and Casey complexes) form a donor-acceptor metal carbene bond involving a a donor bond and a % re-donor bond. On the other hand, triplet carbene moieties (appearing in Osborn and Schrock complexes) form a covalent metal alkylidene double bond (a, n) via spin pairing of the sp2-hybridised carbon... 

J. Kress, M. Wesolek, and J. A. Osborn, Tungsten(IV) Carbenes for the Metathesis of Olefins. Direct Observation and Identification of the Chain Carrying Carbene Complexes in a Highly Active Catalyst System, J. Chem. Soc., Chem. Comm. 1982, 514-516. 

The initial observation of a metal carbene that reacted with an alkene to give a metallacyclobutane complex was reported by Osborn and coworkers for the reaction shown in equation (10). This reaction was observed by NMR spectroscopy at low temperature (—70°C). When this reaction mixture was allowed to warm to higher temperature, polynorbornene was produced in high yield. Shortly after this discovery, the titanocene complex (4) was shown to be an efficient catalyst for the synthesis of monodisperse polynorbornenes. These discoveries, along with the synthesis of a new family of tungsten (5a), molybdenum (5b), and rhenium (6) catalysts,shown in Figure 1, have opened a new era of ROMP chemistry in which the polymer synthesis is guided by the selection of a catalyst... 

With experimental support for the metal-carbene-mediated mechanism of olehn metathesis, a number of groups initiated studies with isolated metal-carbene and metallacyclobutane complexes. Early work by Chauvin and Katz on the polymerization of strained olefins using Fischer-type carbenes demonstrated the success of such an approach [56], The introduction of high oxidation state alkylidene complexes led to well-defined catalyst in which the propagating species could be observed and studied, such as the tungsten-based systems developed by Osborn, Schrock, and Basset [59,60], The best-studied and useful of these have been the Schrock arylimido alkylidene complexes, and we will return to these later in this chapter. 

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