Carbene Complexes of Transition Metals

CARBENE COMPLEXES OBTAINED FROM ACYL, AND CARBAMOYL METALLATES 

Alkoxy carbene complexes are useful starting compounds for other organometallic complexes,1 5 particularly methylidyne (carbyne) complexes.16 By modification of the coordinated (noncarbene) ligands or of the carbene ligand, other carbene complexes can be synthesized. The use of carbene complexes in organic syntheses has been reviewed recently.17 18 

---

During the last decade N-heterocyclic carbene complexes of transition metals have been developed for catalytic applications for many different or-... 

Carbene complexes of transition metals [2,21,225-236] are typical representatives of compounds with a double metal-carbon bond. They are seen as derivatives of a two-covalent carbon in their singlet state [226,232,236]. As a rule, the carbene ligand is an effective a-donor and a comparatively weak n-acceptor. Formation of a cr-bond M — C takes place via transference of a nonbonding electronic pair with a nucleophilic a-orbital of the carbenic carbon to the metal atom. Simultaneously, it is also possible to form a 7t-bond as a result of the interaction of symmetrically appropriate metallic d-AO with a vacant electrophilic /7-orbital of the carbene [236,237], This situation is a key factor that determines the polarization of most of the carbene complexes according to type 145 (Fig. 2.6). 

Metallacycles, formation from carbene complexes of transition metals 91UK169. 

Carbene complexes of transition metals containing 1,3-butadienyl fragment in syntheses of heterocycles 00EJO17. 

Schrock-type carbene complexes of transition metals other than titanium are also utilized for carbonyl olefination, although their synthetic utility has not yet been fully investigated. In some cases, their reactions differ from those of titanium-carbene complexes in terms of stereo- and chemoselectivity and are complementary to olefination with titanium reagents. This section describes the use of carbene complexes of various transition metals in carbonyl olefination. 

A decade after Fischer s synthesis of [(CO)5W=C(CH3)(OCH3)] the first example of another class of transition metal carbene complexes was introduced by Schrock, which subsequently have been named after him. His synthesis of [((CH3)3CCH2)3Ta=CHC(CH3)3] [11] was described above and unlike the Fischer-type carbenes it did not have a stabilizing substituent at the carbene ligand, which leads to a completely different behaviour of these complexes compared to the Fischer-type complexes. While the reactions of Fischer-type carbenes can be described as electrophilic, Schrock-type carbene complexes (or transition metal alkylidenes) show nucleophilicity. Also the oxidation state of the metal is generally different, as Schrock-type carbene complexes usually consist of a transition metal in a high oxidation state. 

As heavier analogs of carbenes141) stannylenes can be used as ligands in transition-metal chemistry. The stability of carbene complexes is often explained by a synergetic c,7t-effect cr-donation from the lone electron pair of the carbon atom to the metal is compensated by a a-backdonation from filled orbitals of the metal to the empty p-orbital of the carbon atom. This concept cannot be transferred to stannylene complexes. Stannylenes are poor p-a-acceptors no base-stabilized stannylene (SnX2 B, B = electron donor) has ever been found to lose its base when coordinated with a transition metal (M - SnXj B). Up to now, stannylene complexes of transition metals were only synthesized starting from stable monomoleeular stannylenes. Divalent tin compounds are nevertheless efficient cr-donors as may be deduced from the displacement reactions (17)-(20) which open convenient routes to stannylene complexes. 

The development of the chemistry of carbene complexes of the Group 8a metals, Ru, Os, and Ir, parallels chemistry realized initially with transition metals from Groups 6 and 7. The pioneering studies of E. O. Fischer and co-workers have led to the characterization of many hundreds of carbene complexes in which the heteroatoms N, O, and S are bonded to the carbene carbon atoms. The first carbene ligands coordinated to Ru, Os, and Ir centers also contained substituents based on these heteroatoms, and in this section the preparation and properties of N-, O-, S-, and Se-substituted carbene complexes of these metals are detailed. 

In addition to copper and rhodium catalysts commonly used in the generation of metal carbene complexes, other transition metals have also been explored in the diazo decomposition and subsequent ylide generation.Che and co-workers have recently studied ruthenium porphyrin-catalyzed diazo decomposition and demonstrated a three-component coupling reaction of a-diazo ester with a series of iV-benzylidene imines and alkenes to form functionalized pyrrolidines in excellent diastereoselectivities (Scheme 20). ... 

The role of carbenes and metal carbene complexes in transition metal-catalyzed processes is suspected of being quite extensive (61). For example, the role of carbenes in the olefin metathesis reaction as described in the previous section is probably important (55, 60). It is quite possible that the o-v rearrangement is important in these reactions also, but this has not been investigated in detail. 

H. Fischer, The Synthesis of Carbene Complexes, in Transition Metal Carbene Complexes (Eds. K. H. Dotz, H. Fischer, P. Hofmann, F. R. Kreissl, U. Schubert, and K. Weiss, Verlag Chemie, Weinheim, 1983, pp. 4-8). 

The additional adamantyl substituent on the phenol moiety was introduced by an acid catalysed reaction with 1-adamantol prior to the reduction step. The significance of this ligand is that it stabilises a Pd-alkyl group cis to the NHC ligand in a palladium(II) carbene complex. These transition metal carbene complexes with a cis alkyl ligand are still rare... 

The synthesis and reactivity of titanoxo units as fragments of transition metal Fischer carbene complexes have been reviewed.1349 Other reviews have appeareed covering structurally characterized organometallic hydroxo complexes of transition metals including mono- and bis-Gp titanium derivatives.809... 

Stoichiometric use of transition-metal-carbene complexes in organic synthesis has been thoroughly reviewed.Various examples using carbene complexes containing cyclopropane subunits have been reported.Here, the cyclopropyl moiety is either attached directly to the carbene carbon or is placed in a more remote position. This section only discusses isolated carbene complexes. Related transition-metal-catalyzed conversions of diazo compounds containing cyclopropane subunits, which are interpreted to proceed via carbene intermediates (e.g. ref 130), are not discussed here. 

For reviews of carbene complexes and transition metal mediated cycloaddition reactions of alkynes in organic synthesis see a) Dotz KH (1984) Angew Chem 96 573 Angew Chem Int Ed Engl 23 587 b) Schore NE (1988) Chem Rev 88 1081 c) Casalnuova JA, Schore NE (1995) Organomettalic cycloaddition reactions of acetylenes. In Stang PJ, Diederich F (eds) Modern acetylene chemistry, VCH, Weinheim, p 139 d) de Meijere A (1996) Pure Appl Chem 68 61... 

As well as phosphorus ligands, heterocyclic carbenes ligands 10 have proven to be interesting donor ligands for stabilization of transition metal complexes (especially palladium) in ionic liquids. The imidazolium cation is usually presumed to be a simple inert component of the solvent system. However, the proton on the carbon atom at position 2 in the imidazolium is acidic and this carbon atom can be depro-tonated by, for example, basic ligands of the metal complex, to form carbenes (Scheme 5.3-2). 

Electrophilic and nucleophilic phosphinidene complexes have been related to the corresponding carbene complexes of which the Fischer-type is usually considered as a singlet-singlet combination and the Schrock-type as a triplet-triplet combination. However, both the strongly preferred triplet state of R-P and the M=P bond analysis suggest this schematic interpretation to be less appropriate for transition metal complexed phosphinidenes. 

A60. J. P. Candlin, K. A. Taylor, and D. T. Thompson, "Reactions of Transition-Metal Complexes. Elsevier, Amsterdam, 1968. A review of types of reactions of metal complexes (e.g., substitution, combination, redox) reactions with various reagents (e.g., hydrocarbons, halides, carbon monoxide, and isonitrile) and preparation of new stabilised organic systems (e.g., metallocenes, carbenes). Intended for research workers, consequently written at a fairly high level, with emphasis on organometallics. A61. H. J. Keller, NMR-Untersuchungen an Komplexverbindungen. Springer, Berlin, 1970. Expansion of review article 37.1. 

---