Carbene complexes zirconium

Entry Carbene complex 54 Carbonyl compound Product Yield 

54a proceeds with preferential formation of the (Z)-styrene derivative. In contrast, the use of imines under the similar reaction conditions reverses the /Z selectivity. The stereoselectivity is improved as the substituent on the nitrogen atom is made more bulky, although the yield of the olefination is depressed. In contrast to the olefination of lactones with 54a, the corresponding imidates react with the zirconium-carbene complex 54a to give enol ethers with Z selectivity. 

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A further way of generating carbon-metal double bonds is based on the [2 -I- 2] cycloreversion of metallacyclobutanes. This method has proven particularly useful for the generation of synthetically valuable titanium and zirconium carbene complexes (Section 3.1.7). 

Cycloreversion of four-membered metallacycles is the most common method for the preparation of high-valent titanium [26,27,31,407,599-606] and zirconium [599,601] carbene complexes. These are usually very reactive, nucleophilic carbene complexes, with a strong tendency to undergo C-H insertion reactions or [2 -F 2] cycloadditions to alkenes or carbonyl compounds (see Section 3.2.3). Figure 3.31 shows examples of the generation of titanium and zirconium carbene complexes by [2 + 2] cycloreversion. 

WITTIG-TYPE REACTIONS Chromium car-bene complexes. Di-n-butyltelluronium carboethoxymethylide Zirconium carbene complexes. 

Investigations into the coordination chemistry of ylides and iminophosphor-anes continue apace, an interesting development being the use of iminophos-phorane fragments as the pendant arms on new pincer ligands which help to stabilise unusual samarium and zirconium carbene complexes. 

Tab. 4.16. Carbonyl olefination with zirconium carbene complexes 54. ...

Using the zirconium carbene complexes 54 as olefination agents, stereochemical control can be achieved by replacing a carbonyl group of the substrates with an imino group (=NR) (Scheme 4.50) [119). The olefination of benzaldehyde with... 

Inter- and intramolecular (cyclometallation) reactions of this type have been ob-.served, for instance, with titanium [408,505,683-685], hafnium [411], tantalum [426,686,687], tungsten [418,542], and ruthenium complexes [688], Not only carbene complexes but also imido complexes L M=NR of, e.g., zirconium [689,690], vanadium [691], tantalum [692], or tungsten [693] undergo C-H insertion with unactivated alkanes and arenes. Some illustrative examples are sketched in Figure 3.37. No applications in organic synthesis have yet been found for these mechanistically interesting processes. 

Similarly, neither zirconium, tantalum, molybdenum, nor tungsten carbene complexes have been applied extensively by organic chemists for carbonyl olefination [609,727-729], probably because of the difficulty of their preparation and the high price of some of these compounds. These reagents can, however, have appealing chemo- and stereo-selectivity (Table 3.11). 

Figure 4.35 Synthesis and reactivity of amino functionaiised carbene complexes of zirconium(IV).

Edworthy, I.S., Blake, A.J., Wilson, C. etal. (2007) Synthesis and NHClability of d° lithium, yttrium, titanium, and zirconium amido bis(N-heterocyclic carbene) complexes. Organometallics, 26, 3684. 

Downing, S.P., Guadano, S.C., Pugh, D. et al. (2007) Indenyl- and fluorenyl-functionahzed N-heterocyclic carbene complexes of titanium, zirconium, vanadium, chromium, and yttrium. Organometallics, 26, 3762. 

The analogous zirconium dichloride complex bearing a phosphoranimino-carbene ligand 188 was obtained from the salt metathesis reaction between the dilithiated salt of the ligand and ZrCl4(THF)2161 (Scheme 37). The Zr=C... 

Carbonyl Olefination Using Zirconium, Tantalum, Niobium, Molybdenum, and Tungsten Carbene Complexes... 

Barger, P.T., Santarsiero, B.D., Armentraut, J. and Bercaw, J.E. (1984) Carbene complexes of zirconium. Synthesis, structure, and reactivity with carbon monoxide to afford coordinated ketene. Journal of the American Chemical Society, 106, 5178-5186. 

Babu RPK, Mcdonald R, Decker SA, Klobukowski M, Cavell RG (1999) New zirconium hydrocarbyl bis (phosphoranimino) pincer carbene complexes. Organometallics 18 4226 229. doi 10.1021/om9901473... 

Reuclin R, Griinstein D, Le Goff X-F, Le Floch P, Mezailles N (2010) Phosphorus stabilized carbene complexes bisphosphonate dianion synthesis, reactivity and DFT studies of 0 C 0 zirconium(IV) complexes. Dalton Trans 492-499. doi 10.1039/b915468j... 

Figure 4 shows the remarkable structural similarity between the bimetallic carbene (1 2) and alkoxy complexes formed from diverse paths 1,2 addition of Zr-H to a carbonyl bound to tungsten (eq. 1) and 1,1 addition of Re-H to a zirconium-bound acetyl (eq. 2). 

Cyclic amino-carbenes, in molybdenum carbonyls, 5, 457 Cyclic bis(phosphine) dichlorides, with iron carbonyls, 6, 48 Cyclic carbenes, as gold atom ligands, 2, 289 Cyclic carbometallation, zirconium complexes, 10, 276 Cyclic carbozirconation characteristics, 10, 276 intermolecular reactions, 10, 278 intramolecular reactions, 10, 278 Cyclic dinuclear ylides, and gold , 2, 276 Cyclic 1,2-diols, intramolecular coupling to, 11, 51 Cyclic enones, diastereoselective cuprate additions, 9, 515 Cyclic esters, ring-opening polymerization, via lanthanide catalysis, 4, 145 Cyclic ethers... 

The pincer like diamino functionalised carbene ligand stabilises the group 4 metal sufficiently to perform a few very interesting insertion reactions with the M-Me bond. The reactions were performed with the hafnium rather than the zirconium complexes [113]. Small molecules used for these insertion reactions include isonitriles (aryl and alkyl) and CO (see Figure 4.36). Reaction of the hafnium carbene dimethyl complex with xylyl-isonitrile results in addition of this excellent donor ligand and subsequent insertion into the Hf-Me... 

Complexes of type (90) represent a new class of zirconium hydrocarbyl derivatives in which a strong metal carbene interaction in a pincer hgand structure is formed. The crystal structure of such complexes shows a short metal carbon link common to two fused four-membered rings which are almost coplanar. 

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