Rhodium complexes carbene

Scheme 15 Formation of 4-alkenyl(phenyl)-substituted 5-dialkylamino-2-ethoxycyclopen-tadienes 75 via transmetallated alkyne-inserted rhodium-carbene complexes 74 [73]. For further details see Table 2...

Herrmann et al. reported for the first time in 1996 the use of chiral NHC complexes in asymmetric hydrosilylation [12]. An achiral version of this reaction with diaminocarbene rhodium complexes was previously reported by Lappert et al. in 1984 [40]. The Rh(I) complexes 53a-b were obtained in 71-79% yield by reaction of the free chiral carbene with 0.5 equiv of [Rh(cod)Cl]2 in THF (Scheme 30). The carbene was not isolated but generated in solution by deprotonation of the corresponding imidazolium salt by sodium hydride in liquid ammonia and THF at - 33 °C. The rhodium complexes 53 are stable in air both as a solid and in solution, and their thermal stability is also remarkable. The hydrosilylation of acetophenone in the presence of 1% mol of catalyst 53b gave almost quantitative conversions and optical inductions up to 32%. These complexes are active in hydrosilylation without an induction period even at low temperatures (- 34 °C). The optical induction is clearly temperature-dependent it decreases at higher temperatures. No significant solvent dependence could be observed. In spite of moderate ee values, this first report on asymmetric hydrosilylation demonstrated the advantage of such rhodium carbene complexes in terms of stability. No dissociation of the ligand was observed in the course of the reaction. 

Inhibition of diazoester decomposition by a large excess of olefin speaks in favor of intermediarily liberated W(CO)5 as direct metal precursor of425. Stereoselectivities in the cyclopropanation reaction are very similar to those observed in the Rh2(OAc)4 catalyzed version, which underlines once more the close relationship of tungsten and rhodium carbene complexes. 

Recently, various rhodium carbene complexes were investigated as catalysts for hydrosilation of olefins, acetylenes, and dienes to see whether carbene ligands modify catalytic activity. All reactions were... 

An understanding of the mechanism [10] for rhodium-mediated intramolecular C-H insertion begins with the recognition that these a-diazo carbonyl derivatives can also be seen as stabilized ylides, such as 15 (Scheme 16.4). The catalytic rhodium(II) car-boxylate 16 is Lewis acidic, with vacant coordination sites at the apical positions, as shown. The first step in the mechanism, carbene transfer from the diazo ester to the rhodium, begins with complexation of the electron density at the diazo carbon with an open rhodium coordination site, to give 17. Back-donation of electron density from the proximal rhodium to the carbene carbon, with concomitant loss of N2, then gives the intermediate rhodium carbene complex 18. 

The mechanism by which this intermediate rhodium carbene complex 18 reacts can be more easily understood if it is written as the inverted ylide 19, as this species would clearly be electrophilic at carbon. We hypothesized that for bond formation to proceed, a transition state 20 in which the C-Rh bond is aligned with the target C-H bond... 

Figure 5.18. Preparation of cycloalkenes from rhodium carbene complexes [165,168],...

The product of reaction (30) is thought to coordinate a further molecule of ethyldiazoacetate trans to the iodoalkyl group which looses dinitrogen, yielding a hexacoordinate rhodium carbene complex according to Eq. (31) which transfers its carbene moiety to an attacking alkene molecule. 

Pyridone is O-alkylated more readily than normal amides, because the resulting products are aromatic. With soft electrophiles, however, clean N-alkylations can be performed (Scheme 1.7). The Mitsunobu reaction, on the other hand, leads either to mixtures of N- and O-alkylated products or to O-alkylation exclusively, probably because of the hard, carbocation-like character of the intermediate alkoxyphosphonium cations. Electrophilic rhodium carbene complexes also preferentially alkylate the oxygen atom of 2-pyridone or other lactams [20] (Scheme 1.7). 

Catalytic Functionalization of N-Heterocycles via their Rhodium-Carbene Complexes... 

A C(C02Me)2 unit can add to C=C double bonds by means of another carbenoid, namely a rhodium-carbene complex A (B is aresonace form, Figure 3.17). Again, these additions are... 

Fig. 3.17. Two reactions that demonstrate the stereospecificity of Rh-catalyzed cis-cyclo-propanations of electron-rich alkenes. — The zwitterionic resonance form A turns out to be a better presentation of the electrophilic character of rhodium-carbene complexes than the (formally) charge-free resonance form B or the zwit-ter-ionic resonance form (not shown here) with the opposite charge distribution ( adjacent to the C02Me groups, on Rh) rhodium-carbene complexes preferentially react with electron-rich alkenes.

In consideration of conceivable strategies for the more direct construction of these derivatives, nitriles can be regarded as simple starting materials with which the 3+2 cycloaddition of acylcarbenes would, in a formal sense, provide the desired oxazoles. Oxazoles, in fact, have previously been obtained by the reaction of diazocarbonyl compounds with nitriles through the use of boron trifluoride etherate as a Lewis acid promoter. Other methods for attaining oxazoles involve thermal, photochemical, or metal-catalyzed conditions.12 Several recent studies have indicated that many types of rhodium-catalyzed reactions of diazocarbonyl compounds proceed via formation of electrophilic rhodium carbene complexes as key intermediates rather than free carbenes or other types of reactive intermediates.13 If this postulate holds for the reactions described here, then the mechanism outlined in Scheme 2 may be proposed, in which the carbene complex 3 and the adduct 4 are formed as intermediates.14... 

A truly hemilabile amino functionalised NHC ligand was introduced by Jimdnez et al. who synthesised a series of rhodium(I) compounds using anunonium functionalised imidazolium salts as starting materials [150] (see Figure 3.52). Interestingly, initially an ionic rhodium(I) compound was obtained that did not contain a carbene-rhodium bond. The rhodium carbene complex could be obtained after further deprotonation and coordination of the amine sidearm to the metal occurred only after chloride abstraction with AgBF. ... 

These rhodium carbene complexes were successfully employed as catalysts in the hydrosilylation of terminal alkynes. It was found that selectivities improve when the remaining wingtip group on the imidazole ring is small. 

In a more conventional approach, Zarka et al. attached a rhodium carbene complex onto an amphiphilic block copolymer [252], The concept is simple and involves the utilisation of a hydroxyalkyl substituted NHC as a ligand for the rhodium(I) catalyst used in hydrofor-mylation of 1-octene. The catalyst is then loaded onto a water-soluble, amphiphilic block copolymer by reacting the alcohol group of the catalyst with a carboxylic acid group of the block copolymer (see Figure 4.81). 

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 absolute configuration of the cyclopentyl phenylacetate (R=H) produced by the S-isomer of the catalyst, was established as R by analogy, the other C-H insertion products were presumed to be R. A transition state model similar to that proposed for asymmetric cycloprop anation with the same catalyst was used to interpret the asymmetric induction observed in C-H insertion. The rhodium carbene complex is represented as in Fig. 12 with the catalyst presumed to be-... 

Rhodium carbene complexes also promote the reaction giving cis-products predominantly64 in a very similar ratio to that obtained by using RhCl(PPh3)3 as catalyst. 

Another tandem synthesis based on rhodium-carbene complexes, the cyclo-propanation-Cope rearrangement sequence 8-68, was extensively investigated by Davies and coworkers (review Davies, 1993). This sequence leads to cyclohepta-dienes (8.154), which are useful for the synthesis of important natural products containing densely functionalized seven-membered rings. The sequence 8-68 requires ready access to 3-diazoalk-l-enes (vinyldiazomethanes) as basis for the rhodium-... 

The authors proposed an electrophilic rhodium carbene complex to account for formation of 129. Thus reaction of 128 with Rh2(OAc)4 generates the carbene complex 130, which is trapped by benzonitrile to afford the nitrilium species 131. Cyclization of 131 through the enolate oxygen then yields 129 (Scheme 1.36). 

Rhodium(I)-catalyzed reaction of phenyl 2-propylcycloprop-2-en-yl ketone with terminal alkynes gives 2-alkyl-4-propyl-7-phenyloxepines (Scheme 13). The reaction involves the formation of a rhodium-carbene complex, which undergoes a [2 + 2] cycloaddition with a terminal ethyne the resultant rhodacycle rearranges by a 1,5-sigmatropic shift, followed by reductive elimination of rhodium <92JA588l>. 

Ahmed M, Buch C, Routaboul L, Jackstell R, Klein H, Spannenberg A, Beller M (2007) Hydroaminomethylation with novel rhodium-carbene complexes an efficient catalytic approach to pharmaceuticals. Chem Eur J 13 1594-1601... 

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