Fischer carbene complex thermal reaction

The ability of Fischer carbene complexes to transfer their carbene ligand to an electron-deficient olefin was discovered by Fischer and Dotz in 1970 [5]. Further studies have demonstrated the generality of this thermal process, which occurs between (alkyl)-, (aryl)-, and (alkenyl)(alkoxy)carbene complexes and different electron-withdrawing substituted alkenes [6] (Scheme 1). For certain substrates, a common side reaction in these processes is the insertion of the carbene ligand into an olefinic C-H bond [6, 7]. In addition, it has been ob-... 

One of the earliest reported thermal reactions of Fischer carbene complexes was the reaction with olefins to give cyclopropanes [127]. More recently it has been shown that photolysis accelerates inter molecular cydopropanation of electron-poor alkenes [128]. Photolysis of Group 6 imine carbenes with alkenes... 

Thermal cyclization reaction of Fischer carbene complexes with a, 3-unsaturated ketones and aldehydes led to 2,3-dihydrofurans, which were converted to the corresponding furans under various conditions, including treatment with silica gel, use of non-aqueous acid (HBF4 or CF3CO2H) or simply by heating <07AGE4136>. 

Besides the thermal reactions of Fischer car-benes, photochemical transformations of such complexes also deserve attention. Since the discovery of McGuire and Hegedus [4] in the early 1980s that irradiation of chromium carbene complexes (4) with visible light in the presence of N-substituted benzaldimines (5) leads to (racemic) )ff-lactams of type 6, photolytic reactions of Fischer carbene complexes have been intensively investigated by L.S. Hegedus and his group in Fort Collins, Colorado [5]. 

Fischer carbene complexes rmdergo elimination reactions under thermal conditions to form enol ethers, enol acetates, or enamines, depending on the heteroatom substituent on the carbene carbon and, depending on the conditions, aldehydes. Fligh (Z) to (E) ratios of isomers are obtained in most cases. Although thermal reactions have been reported, this formal 1,2-hydrogen shift is in most cases facilitated by tertiary... 

The reactions of both alkoxy and amino complexes are highly stereoselective and give the unnatural epimer at the C-6 position in the penicillin analogs, but methods are known for inversion at this center, llie mechanism of these reactions is thought to involve the intermediacy of a metal-4cetene complex whose formation is photo-induced.Early indications that this was the case came when it was found these reactions fail to give cyclic products of any kind under thermal conditions. More recently, vi-nylketene complexes have been trapped from these reactions.With the recent realization that metal-ketene intermediates are likely to be involved in these reactions further development of the photo-induced reactions of Fischer carbene complexes can be anticipated. 

Photolysis of hydroxy Fischer carbene complexes (96) (Scheme 20) in the presence of alcohols under several atmospheres of carbon monoxide gives low to moderate yields of a-hydoxy esters (97). It is proposed that the reactions proceed via ketenes formed from the liberated or complexed carbenes and CO. In some cases, acetals formed via thermal decomposition of the carbenes are the major products. Photolysis of iron porphyrin carbene complexes results in cleavage of the iron-carbon double bond, producing a four coordinate iron(II) porphyrin and the free carbene. The carbenes can be trapped in high yield with a variety of alkenes. 

It should be noted that a key element in most of these reactions is that they are initiated by the coordination of the substrate to the metal.Since Fischer carbene complexes are coordinatively saturated, the coordination of the substrate needs to be preceded by the loss of a ligand (see, e.g., equation 8, S = substrate). This ligand loss can be initiated thermally as well as photochemically. This loss of CO is usually the rate-limiting step which is a major reason why kinetic experiments give little information about the complex steps that follow the initial ligand loss. 

In order to explore the effect of the heteroatom in Fischer-carbene type ligands on the reactivity and thermal stability of ruthenium complexes, Grubbs et al. prepared and characterized a series of well-defined bis-phosphine 4-8, NHC imidazole (IMes) 9-12, and NHC imidazolidine (HglMes) 13 complexes (Figure 12.3) [17]. The exceptionally high stability of 8 at 55 C (20 days before half the complex was decomposed) could be explained by the chelation ofthe amide carbonyl to the ruthenium center. When tested in ROMP reactions, all Fischer carbene complexes demonstrated rapid and quantitative polymerization of norbornene (NBE) derivatives at room temperature, although the polymerization... 

Thermal Metalla-Diels-Alder Reaction Formation of five-membered carbocycles via a thermal metalla-Diels-Alder reaction is reported in a formal [3+2] cycloaddition of a, 3-unsaturated Fischer carbene complexes 63... 

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. 

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

Several reaction sequences have been reported in which Fischer-type carbene complexes are converted in situ into non-heteroatom-substituted carbene complexes, which then cyclopropanate simple olefins [306,307] (Figure 2.22). This can, for instance, be achieved by treating the carbene complexes with dihydropyridines, forming (isolable) pyridinium ylides. These decompose thermally to yield pyridine and highly electrophilic, non-heteroatom-substituted carbene complexes (Figure 2.22) [46]. 

H. Fischer, A. Mosch, and W. Kleine, Unusual Selectivity in the Thermal Rearrangement-Elimination Reaction of a Carbene Complex in Solution and in the Solid State, Angew. Chem. Int. Ed. Engl. 17, 842-843 (1978). 

Heteroatom-stabilized carbene complexes of type 1, first discovered by E.O. Fischer in 1964 [1], nowadays belong to the best investigated classes of transition metal compounds. Such complexes are coordinatively saturated, intensely colored solids = 350-400 nm), which exhibit a sufficient stability for normal preparative use. Especially chromium carbene complexes (2) enjoy increasing importance in organic synthesis, and it must be added that thermal reactions such as benzannulations (i.e. the Ddtz reaction), cyclopropanations and additions to a,j8-unsatu-rated complexes clearly predominate [2J. 

In his initial paper in 1975, D6tz reported that the thermal cycloaddition of pentacar-bonyl(methoxyphenylcarbene)chromium with diphenylacetylene in di-n-butyl ether yielded a chromium-complexed 4-methoxy-l-naphthol [2]. Soon thereafter, he related that the same reactants in w-heptane produced not only naphthol product, but also indene, furan, and cyclobutenone products [4]. As it turned out, these results foreshadowed the extraordinary richness of organic structural types that may be derived from cycloadditions of alkynes with Fischer carbenes, as well as very recent contributions to reaction chemoselectivity through control of reaction conditions. Indeed, in the years since, the field has seen the introduction of a number of newly discovered cycloaddition types and, maybe more importantly, has... 

Reactions of Fischer Carbenes Thermal decomposition of carbene complexes usually leads to one or both of two types of alkenes one type is formed by the 1,2-shift of a hydride, and the other by dimerization... 

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