Metal-carbene complexes thermal decomposition

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. 

Since transition-metal-carbene complexes with amino substitution at the carbene carbon atom are thermally very stable, their application as aminocarbene transfer reagents necessitates the use of high decomposition temperatures which, in many cases, is disadvantageous for the isolation of cyclopropylamines. Thus, for example, the cyclopropylamine 13 was not isolated from the reaction of an aminocarbene-chromium complex 12 with acrylonitrile in cyclooctane at 140°C instead, the respective decomposition product 15 was obtained in low yield.It is well known that cyclopropylamines readily undergo thermal ring opening so this result is not surprising. 

Most commonly, photolytic, thermal, or transition metal catalyzed decomposition of diazoalkanes is used to create carbene molecules. A variation on catalyzed decomposition of diazoalkanes is the Bamford-Stevens reaction, which gives carbenes in aprotic solvents and carbenium ions in protic solvents. Another method is induced elimination of halogen from gem-dihalides or HX from a CHXj moiety, employing organoHthium reagents (or another strong base). It is not certain that in these reactions actual free carbenes are formed. In some cases there is evidence that completely free carbene is never present instead, it is likely that a metal-carbene complex forms. Nevertheless, these metallocarbenes (or carbenoids) give the expected products. 

Where (3 ehmination cannot occur, a elimination sometimes takes over. This leads to the formation of metal carbene complexes with M=C double bonds. For example, the first step in the thermal decomposition of Ti(CH2CMe3)4 is known to be a elimination to Ti(=CHCMe3)-(CH2CMe3)2. Similarly, attempts to prepare Ta(CH2CMe3)s led to formation of the carbene complex, t-BuCH=Ta(CH2CMe3)3. Carbenes and a elimination are discussed in Sections 11.1 and 7.5. 

Several stable Group 6 metal-ketene complexes are known [14], and photo-driven insertion of CO into a tungsten-carbyne-carbon triple bond has been demonstrated [15]. In addition, thermal decomposition of the nonheteroatom-stabilized carbene complexes (CO)5M=CPh2 (M=Cr, W) produces diphenylke-tene [16]. Thus, the intermediacy of transient metal-ketene complexes in the photodriven reactions of Group 6 Fischer carbenes seems at least possible. 

The q1-coordinated carbene complexes 421 (R = Ph)411 and 422412) are rather stable thermally. As metal-free product of thermal decomposition [421 (R = Ph) 110 °C, 422 PPh3, 105 °C], one finds the formal carbene dimer, tetraphenylethylene, in both cases. Carbene transfer from 422 onto 1,1-diphenylethylene does not occur, however. Among all isolated carbene complexes, 422 may be considered the only connecting link between stoichiometric diazoalkane reactions and catalytic decomposition [except for the somewhat different results with rhodium(III) porphyrins, see above] 422 is obtained from diazodiphenylmethane and [Rh(CO)2Cl]2, which is also known to be an efficient catalyst for cyclopropanation and S-ylide formation with diazoesters 66). 

An interesting carbene-alkyne intermediate was postulated for the conversion of carbene complexes to chrysene derivatives, which again illustrates the potential of cr,Tr-activation of bridging ligands by metal centers [Eq. (32)] (225). The thermal decomposition of the carbene complex Cr-... 

There are several methods to release the carbene ligand from the metal, but in no case the intermediacy of free carbenes has been established. The thermal decomposition of carbene complexes generally results in a dimerization of the carbene ligand to give aUcenes. The replacement of the carbene ligand by carbon monoxide, tertiary phosphines or amines is a less drastic way to cleave the metal-carbene bond and, in principle, allows for a recovery of the low-valent metal species. 

The metal-donor bonds are predominantly ionic and become more labile for calcium, strontium, and barium compared to beryllium and magnesium. The solubility and stability of the complexes decrease from calcium to barium. The 1 1 adducts of NHCs with BH3 or BF3 (28 and 29) are thermally stable and can be sublimed without decomposition. This is in sharp contrast to the properties of conventional carbenes, which rely on a pronounced metal-to-ligand back donation and are, thus, not suited to forming adducts with electron-poor fragments such as... 

Addition of certain copper salts to solutions of diazo compounds also leads to evolution of nitrogen and formation of products of the same general types as those formed in thermal and photochemical decompositions of diazoalkanes. The weight of the evidence, however, indicates that free carbene intermediates are not involved in such reactions. Instead, complexes of the carbene unit with the metal ion catalyst seem to be the actual reactants. Such a complex would be an example of a carbenoid species. Although the product suggests the involvement of a carbene-like reactivity, other evidence rules out a completely free carbene of the type generated by photochemical expulsion of a molecule of nitrogen. 

These methodologies do present some drawbacks, as thermolysis is typically necessary to liberate the free carbene. If the free carbene is particularly unstable and subsequent reaction with the metal precursor is slow, or if a thermally sensitive organometallic complex is to be formed, decomposition may compromise the yield of the reaction. 

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