Casey 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]) ... 

Fischer carbene Fischer carbene Casey carbene... 

It was proved that metal carbynes are sources of metal carbenes [e.g. scheme (9) in Chapter 6] promoting the polymerisation of acetylenic monomers. Therefore, related metal carbynes and carbenes appeared to catalyse the polymerisation of alkynes in the same way as regards the identity of the products, in particular as regards stereochemistry. For the terminal and internal alkynes, the Fischer carbyne acts much like the Casey and Fischer metal carbenes. The Fischer carbyne also promotes acetylene polymerisation, and it does this where the Fischer carbene fails and the Casey carbene is much less effective [22,143]. 

Metal Carbene Catalysts. The first use of isolated single-component car-bene catalysts showed that the Fischer (4) and Casey carbenes (5) polymerize phenylacetylene, ferf-butylacetylene, and cyclooctyne in low yields (130). For example, the bulk polymerization of tert-butylacetylene with (4) gives a high molecular weight (Mn = 260,000) polymer in 28% yield. Polymer-supported Fischer carbene (4) is also active for the polymerization of phenylacetylene under photoirradiation (145). As a catalyst, the Casey carbene (5) is less stable but more active than the Fischer carbene (130). The Rudler carbene (6) readily releases the in-tramolecularly ligated double bond upon the approach of an acetylenic monomer. Thus, it is more active than the Fischer and Casey carbenes (146-148). These carbene complexes are, however, unable to control the polymerization. 

Fischer-type and Casey carbenes are 18-electron complexes and are too stable to work as catalysts. Photoirradiation and/or addition of Lewis acids promotes the generation of active species. Schrocktype electron-deficient Mo and W alkylidene... 

WeU-defined carbene catalysts show excellent activity for polymerization, and are isolable and thus informative to suppose the polymerization mechanism as metathesis type. The first example of isolated single-component carbene catalysts is Kscher and Casey carbenes (1 and 2, respectively. Figure 1), which polymerize PA, tert-butylacetylene, and cyclooctyne in... 

Casey CP (1981) Metal-carbene complexes. In Jones M Jr, Moss RA (eds) Reactive intermediates. Wiley, New York, p 135... 

Following Casey s experiment (5) depicted in Eq. (4), Katz and coworkers (26, 27) demonstrated that the stable W(0) carbene complex (CO)5W=CPh2 may also be employed as a metathesis catalyst. 

In comparison with other catalysts, (diphenylcarbene)tungstenpenta-carbonyl is surprisingly sluggish (26). This observation is significant as it relates to diverse views regarding the carbene-to metallocyclobutane interconversion. Whereas Casey emphasizes a need to accommodate the incoming olefin within the coordination sphere of the metal prior to rearrangement to a metallocycle [Eq. (5)], Katz (28) has described the process essentially as a dipolar attack of a polarized carbene-metal (R2C+—M-) on the olefin [Eq. (6)]. The latter does not specify a need for w-complexation of the olefin as a precondition to metathesis. 

Hence, Casey s scheme calls for an empty coordination site on the metal in addition to the complexed carbene. Recently, Casey demonstrated that carbonyls of (CO)5W=CPh2 are vulnerable to exchange processes under relatively mild reaction conditions (29) ... 

In marked contrast to the results of Gassman and Schrock, major differences were noted by Casey and co-workers in a series of studies utilizing phenylcarbene-substituted W(0) complexes in reactions with olefins. The H NMR spectra of new phenylcarbene tungsten and iron (69) complexes indicate a substantial positive charge residing on the carbene carbon, and as expected, these complexes readily form ylides on reaction with phosphines ... 

Some notable reversals in the order of olefin reactivities were observed in contrast to the tantalum carbenes. Further, in comparing the reactivities of (CO)5W=CPh2 and (CO)5W=CHPh, Casey (70) noted additional striking contrasts The monophenyl carbene complex reacted rapidly at... 

The reactions reported by Casey (14) for a third carbene complex, (CO)5W=C(CH3)Ph, are not pertinent to the present topic because extremely rapid /8-H transfer to tungsten at -78°C initiates a sequence of reactions unique to this particular complex. This type of H transfer is atypical of conventional metatheses. 

Dotz reaction is proposed. According to our calculations the addition of the alkyne molecule to the carbene complex takes place before CO loss in the initial steps of the reaction. Further, our study shows that a novel proposal involving a chromahexatriene intermediate entails lower energy barriers and more stable intermediates than the previous reaction mechanisms postulated by Dotz and Casey. The novel findings query revision of the classically assumed paths and put forward that additional experimental and theoretical studies are necessary to definitely unravel the reaction mechanism of this intringuing reaction. 

Casey and coworicers have shown that ketone etiolates add efficiently to a,3-unsaturated vinyl carbene complexes (164), irrespective of 3,3-disubstitution on the complex or high substitution on the enol-ate 133 thus, contiguous 3 and y quaternary centers are easily assembled. When coupled with the ease of release of the carbene ligand from the complexes by either oxidation to the ester functionality1331 or elimination to the corresponding enol ether,133 the vinyl carbene complexes are synthetic equivalents for a,3-unsaturated esters or a,3-unsaturated aldehydes, respectively (Scheme 63). 

It is over eight years since the last comprehensive review appeared on metal isocyanide chemistry. In the interim, reviews have appeared on specific aspects of isocyanide chemistry. Lippard reviewed seven and eight coordination in molybdenum isocyanide compounds (7), Yamamoto reviewed metal(O)- isocyanide complexes (2), and in related reviews on car-bene complexes by Cotton and Lukehart (5), Lappert et al. (4), and Casey (5), mention was made of carbenes synthesized from metal isocyanides. 

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... 

Ofele s, Casey s and Chatt s Routes to Metal Carbenes... 

Referring to Lappert s work, it is also worth mentioning that in the same year that Casey described the synthesis of the tungsten carbene W(CPh2)(CO)5 (23)... 

C. P. Casey, and R. L. Anderson, Thermolysis of (2-Oxacyclopentylidene)-pentacarbo-nylchromium(O) Evidence Against Free Carbenes in Thermal Decomposition of Metal-Carbene Complexes, J. Chem Soc., Chem. Comm. 1975, 895-896. 

C. P. Casey, and T. J. Burkhardt, Reactions of (Diphenylcarbene)pentacarbonyl-tung-sten(0) with Alkenes. Role of Metal-Carbene Complexes in Cyclopropanation and Olefin Metathesis Reactions, J. Am. Chem. Soc. 96, 7808-7809 (1974). 

T1)C nature of the carbene-meta) bond has received much attention and the poIa i atiun of its Mouble bond has been a matter of discussion, Gassinan has proposed a nucleophilic (negative carbon) carbcnc as an intermediate, whereas Casey s observations give some support to a positively charged carbene carbon atom in some cases. 

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