Carbene complex Subject

This review focuses on the cross-metathesis reactions of functionalised alkenes catalysed by well-defined metal carbene complexes. The cross- and self-metath-esis reactions of unfunctionalised alkenes are of limited use to the synthetic organic chemist and therefore outside the scope of this review. Similarly, ill-defined multicomponent catalyst systems, which generally have very limited functional group tolerance, will only be included as a brief introduction to the subject area. 

An alternative mechanistic scenario for the initial steps of this reaction (associative route) was the subject of a study by TDS [27, 39], It considers the possibility that the cycloaddilion with alkynes takes place initially by direct reaction with the coordinatively saturated chromium carbene complex 9. 

Acceptor-substituted carbene complexes are electrophilic intermediates which react readily with lone pairs, giving the corresponding ylides. These can be valuable intermediates, capable of undergoing a broad range of synthetically useful transformations. This subject has been treated in several reviews [38,995,1077-1079,1086]. 

The subject of this book has been organized in three main sections preparation and applications of heteroatom-substituted carbene complexes (Fischer-type carbenes), non-heteroatom-substituted carbene complexes, and acceptor-substituted carbene complexes. In each section the different types of reaction have been ordered either according to the mechanism or according to the type of product. In addition to a selection of illustrative examples, several experimental procedures have been included. These were chosen taking into account safety, availability of starting materials, relevance of the products, and general interest. 

Since the pioneering studies of Fischer, carbene complexes have become the subjects of very extensive investigations in several laboratories, and they have become known for a large number of the transition metals.100 A fairly general statement about their chemistry is that these compounds usually are neither prepared directly from free carbenes nor are they to be considered as sources of free carbenes, i.e. free carbenes are generally not in equilibrium with the carbene complexes. Instead, they are prepared by indi-... 

The reactions of Fischer carbene complexes with an anionic nucleophile may be represented by Equation (49).179 181 Typical carbene complexes that have been the subject of kinetic studies are 76-M and... 

Further restrictions to the scope of the present article concern certain molecules which can in one or more of their canonical forms be represented as carbenes, e.g. carbon monoxide such stable molecules, which do not normally show carbenoid reactivity, will not be considered. Nor will there be any discussion of so-called transition metal-carbene complexes (see, for example, Fischer and Maasbol, 1964 Mills and Redhouse, 1968 Fischer and Riedel, 1968). Carbenes in these complexes appear to be analogous to carbon monoxide in transition-metal carbonyls. Carbenoid reactivity has been observed only in the case of certain iridium (Mango and Dvoretzky, 1966) and iron complexes (Jolly and Pettit, 1966), but detailed examination of the nature of the actual reactive intermediate, that is to say, whether the complexes react as such or first decompose to give free carbenes, has not yet been reported. A chromium-carbene complex has been suggested as a transient intermediate in the reduction of gfem-dihalides by chromium(II) sulphate because of structural effects on the reaction rate and because of the structure of the reaction products, particularly in the presence of unsaturated compounds (Castro and Kray, 1966). The subject of carbene-metal complexes reappears in Section IIIB. 

Heinrich Dotz, from the Kekule-Institut (a predestined name ) of the University of Bonn, is another famous chemist who has given his name to a reaction. Coming from E. O. Fischer s school, he advantageously exploited his serendipitous discovery of the very rich reactivity of Fischer-type carbene complexes in synthesizing polycydic arene derivatives. This chromium-templated carbene benzannulation approach to densely functionalized arenes (Dotz reaction) is the subject of the chapter (No. 8) that he has co-authored with J. Stendel Jr. 

Pyrazolyl- [85], dihydropyridyl- [86], and pyrrolizinylcarbene complexes [87] have also been subjected to the benzannulation to give the respective oxygenated benzo-N-heterocycles. Finally, a,/ ,y,dienyl carbene complexes containing a heterocyde at the internal double bond have been utilized to prepare 2,3-dihydro-l,2-benzisoxazoles and indazoles by intramolecular benzannulation [60h]. 

These compounds are nucleophihc at the noncoordinated sulfur, and undergo alkylation there with iodomethane or benzyl bromide to give initially (128) (which may be isolated as the PFe salt) and ultimately (129) (equation 28). They also react with electron-deficient alkynes, possibly by a dipolar mechanism, to afford cyclized iron carbene complex (130). Finally, compounds (129) are subject to attack by some nucleophiles hydride attack occurs initially at the metal, but ultimately gives ) -dithioester complexes, such as (131). ... 

One of the first reactions of carbene complexes to be investigated after their discovery was the cyclo-propanation of aikenes. This reaction has been extensively studied and will only be briefly surveyed here. Two excellent reviews of this subject have appeared, - and the cyclopropanation of aikenes with... 

As already mentioned for rhodium carbene complexes, proof of the existence of electrophilic metal carbenoids relies on indirect evidence, and insight into the nature of intermediates is obtained mostly through reactivity-selectivity relationships and/or comparison with stable Fischer-type metal carbene complexes. A particularly puzzling point is the relevance of metallacyclobutanes as intermediates in cyclopropane formation. The subject is still a matter of debate in the literature. Even if some metallacyclobutanes have been shown to yield cyclopropanes by reductive elimination [15], the intermediacy of metallacyclobutanes in carbene transfer reactions is in most cases borne out neither by direct observation nor by clear-cut mechanistic studies and such a reaction pathway is probably not a general one. Formation of a metallacyclobu-tane requires coordination both of the olefin and of the carbene to the metal center. In many cases, all available evidence points to direct reaction of the metal carbenes with alkenes without prior olefin coordination. Further, it has been proposed that, at least in the context of rhodium carbenoid insertions into C-H bonds, partial release of free carbenes from metal carbene complexes occurs [16]. Of course this does not exclude the possibility that metallacyclobutanes play a pivotal role in some catalyst systems, especially in copper-and palladium-catalyzed reactions. 

The thrust of this chapter deals with reactions of Fischer carbene complexes that have been the subject of kinetic and/or thermodynamic studies. The number of these is relatively limited. They include the reactions of equations (1) and (4), proton transfers that generate carbanions such as 5 (see equation 2), reactions at the metal center such as the loss or exchange of ligands as well as rearrangement reactions. 

Reactions of the type of equation (51) have also been the subject of a kinetic investigation. Rate constants in the forward (fei) and reverse direction ( -1) were determined for a number of combinations of R, R, M in acetone at 25°C. The results are summarized in Table 8. A major conclusion drawn from these results is that steric effects are important. In the absence of steric effects one would expect Ki to show the following patterns (i) It should be larger for P(n-Bu)3 than for P(n-OBu)3 because the former is more basic. (2) It should be larger for R = Ph than R = Me because the former is more electron withdrawing. For the methyl carbene complex (R = Me) Ail is indeed larger with P(n-Bu)3 than with P(n-OBu)3 but for the bulkier phenylcarbene complex (R = Ph) the Ki values for the two nucleophiles are comparable or even smaller with P(n-Bu)3 in the case of the chromium complex. This reversal reflects the larger size of P(n-Bu)3 relative to P(n-OBu)3." ... 

Figure 9 shows a typical rate-pH profile the points below pH 8.5 represent a water reaction, those above 8.5 an OH catalyzed reaction while the leveling off at pH > 12 is due to the ionization of the carbene complex. The hydrolysis is subject to general base catalysis the points on the pH-rate profile between pH 4.8 and 11.3 were obtained by extrapolation of buffer plots to zero buffer concentration. 

Compounds of the type formed in reactions such as 23.88 are called Fischer-type carbenes, they possess a low oxidation state metal, a heteroatom (O in this example) and an electrophilic carbene centre (i.e. subject to attack by nucleophiles, e.g. reaction 23.89). Resonance pair 23.51 gives a bonding description for a Fischer-type carbene complex. 

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