Carbenes, carbon monoxide

Carbenes are both reactive intermediates and ligands in catalysis. They occur as intermediates in the alkene metathesis reaction (Chapter 16) and the cyclopropanation of alkenes. As intermediates they carry hydrogen and carbon substituents and belong therefore to the class of Schrock carbenes. As ligands they contain nitrogen substituents and are clearly Fischer carbenes. They have received a great deal of attention in the last decade as ligands in catalytic metal complexes [58], but the structural motive was already explored in the early seventies [59], 

Recent successes are partly based on the use of aryl or bulky aryl substituents at the nitrogen atoms of the imidazolium rings. As ligands they are strong o-donors (stronger than t-BujP) and hardly rr-acccptors, they are sometimes called singlet carbenes . 

Carbon monoxide is often both reactant and ligand. As a ligand it is a strong Jt-acceptor and a moderate o-donor. By comparison of IR data one might say that its %-value is around 55, i.e. similar to hexafluoroisopropyl phosphite. 

Extending the comparison with the steric properties of phosphorus ligands, clearly CO is one of the smallest ligands available The interesting feature of CO as a ligand of is that it can be studied by IR, and also in situ IR, in a frequency area that is transparent (1800-2200 cm4). A disadvantage is that in order to obtain concentrations in an organic liquid in the order of 0.1 M one needs pressures in the order of 10 bar. 

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Treatment of the vinylcarbene chromium complex 97 with /-BuC=P affords the dihydrophosphetylketene complex 98 (Scheme 26). This transformation is believed to proceed via r -phosphaalkyne carbene-, phosphaalkenylcar-bene-, and phosphaalkenylketene complexes as intermediates. An intramolecular [2+2] cycloaddition completes the reaction sequence.53 Different carbene/carbon monoxide/phosphaalkyne cycloaddition products (e.g., 1,3-oxaphospholes, phosphaphenanthrenes) are obtained depending on substitution at the carbene ligand (vide infra). 

Using a light source of higher wavelength (>300 nm) generates the carbene carbon monoxide species 4, C4O2, can also be generated in the vapor phase... 

The carbene-carbon monoxide coupling was suggested as a potential elementary C—C coupling step in catalytic carbon monoxide reduction systems based on the observation of the reaction product (r -C5(CH3)5)2ZrH 2(p-OCH=CHO) [31]. The zirconoxycarbene complex 3 was found to react with carbon monoxide at room temperature to afford a zirconium-coordinated ketene complex 4 in 30% isolated yield (reaction 8.18). The X-ray structures of both 3 and 4 were determined [32]. 

Diphenylketene was isolated in up to 70% yield from (CO)5Cr=CPli2 under 1 bar carbon monoxide at 50 °C. An intramolecular carbene-carbon monoxide coupling was assumed in the reaction based on the observation that no incorporation of CO... 

Despite the fragmentary results known to date, the dediazotation of diazoalkanes coupled with the carbene-carbon monoxide trapping reaction seems to be a promising synthetic path for the preparation of various carboxylic acid derivatives through the ketene intermediate product. Especially the highly effective and selective cobalt-catalyzed examples of the reactions are worth to explore in more detail. 

Aryl- and alkenylcarbene complexes are known to react with alkynes through a [3C+2S+1C0] cycloaddition reaction to produce benzannulated compounds. This reaction, known as the Dotz reaction , is widely reviewed in Chap. Chromium-Templated Benzannulation Reactions , p. 123 of this book. However, simple alkyl-substituted carbene complexes react with excess of an alkyne (or with diynes) to produce a different benzannulated product which incorporates in its structure two molecules of the alkyne, a carbon monoxide ligand and the carbene carbon [128]. As referred to before, this [2S+2SH-1C+1C0] cycloaddition reaction can be carried out with diyne derivatives, showing these reactions give better yields than the corresponding intermolecular version (Scheme 80). 

The first and rate-determining step involves carbon monoxide dissociation from the initial pentacarbonyl carbene complex A to yield the coordinatively unsaturated tetracarbonyl carbene complex B (Scheme 3). The decarbonyla-tion and consequently the benzannulation reaction may be induced thermally, photochemically [2], sonochemically [3], or even under microwave-assisted conditions [4]. A detailed kinetic study by Dotz et al. proved that the initial reaction step proceeds via a reversible dissociative mechanism [5]. More recently, density functional studies on the preactivation scenario by Sola et al. tried to propose alkyne addition as the first step [6],but it was shown that this... 

The thermal benzannulation of Group 6 carbene complexes with alkynes (the Dotz reaction) is highly developed and has been used extensively in synthesis [90,91]. It is thought to proceed through a chromium vinylketene intermediate generated by sequential insertion of the alkyne followed by carbon monoxide into the chromium-carbene-carbon double bond [92]. The realization that photodriven CO insertion into Z-dienylcarbene complexes should generate the same vinylketene intermediate led to the development of a photochemical variant of the Dotz reaction (Table 14). 

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. 

An interesting carbene, 1-oxobutatrienylidene [25], having cumulated double bonds, has been found by IR spectroscopy in the photolysis (A>230nm) products of matrix-isolated l,2,3,4-pentatetraene-l,5-dione [26] (Maier et al., 1988) (in its turn the unstable dione [26] was generated by thermo- or photo-destruction of compound [27]). The second product was carbon monoxide. The linear structure of the carbene [25] has been suggested on the basis of two intense IR bands at 2222 cm and 1923 cm indicating respectively ketene and allene fragments. 

A -Heterocyclic Carbene Complexes in Reactions Involving Carbon Monoxide... 

Abstract This chapter focuses on carbon monoxide as a reagent in M-NHC catalysed reactions. The most important and popular of these reactions is hydro-formylation. Unfortunately, uncertainty exists as to the identity of the active catalyst and whether the NHC is bound to the catalyst in a number of the reported reactions. Mixed bidentate NHC complexes and cobalt-based complexes provide for better stability of the catalyst. Catalysts used for hydroaminomethylation and carbonyla-tion reactions show promise to rival traditional phosphine-based catalysts. Reports of decarbonylation are scarce, but the potential strength of the M-NHC bond is conducive to the harsh conditions required. This report will highlight, where appropriate, the potential benefits of exchanging traditional phosphorous ligands with iV-heterocyclic carbenes as well as cases where the role of the NHC might need re-evaluation. A review by the author on this topic has recently appeared [1]. 

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