With rhodium carbene complexes

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. 

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... 

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). 

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. ... 

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-... 

The stereochemical results observed in cyclization reactions of molybdenum carbene complexes with l,3-nonadien-8-ynes parallel the findings obtained in rhodium(ll)-catalyzed cyclopropa-nation reactions. Thus, treatment of dienyne 20 with molybdenum carbene complex 19 in benzene at 60 results in the formation of hexahydroazulenes 21 and 22 in a 1 4.8 ratio881. As the Cope rearrangement should proceed through the boatlike transition state, the diastereomeric ratio reflects the ratio of enol ether isomers at the divinylcyclopropane stage. 

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... 

Certain dinuclear Rh(II) carbene complexes react with alkanes to generate products from insertion of the carbene imit into the alkane C-H bond with high diastereo- and enantioselectivity (Equation 6.59). ° These reactions occur by mechanisms distinct from those of the reactions of C-H bonds witti the tungsten alkylidene and alkylid5me complexes just described. The reactions of the dinuclear Rh(II) carbene complexes appear to occur by a mechanism that involves direct reaction of the carbene at the C-H bond without coordination of the alkane and addition across the M=C bond of the carbene. Such rhodium carbene complexes have not been isolated, but the absence of an open coordination site cis to the carbene ligand in the accepted carbene intermediate is thought to preclude initial reaction of tire substrate at the metal center to form a new metal-carbon bond. The catalytic chemistry that occurs via these carbene complexes is presented in more detail in Chapter 18 (catalytic C-H bond functionalization). 

In a manner similar to platinum ones, rhodium—carbene complexes have been recently tested in the hydrosilylation of alkenes and alkynes (3). Especially, rhodium complexes with N-heterocyclic carbene (NHC) ligands have attracted considerable attention. Their performance is comparable with that of phosphine complexes. The following exemplary ligands were used 1,3-imidazoylidene chelate bis(imidazolinum-carbene) phosphine-functionalized NHC alkylammonium-imidazolium chloride salts NHC pincer complexes, pyridine-functionalized N-heterocyclic carbenes (also with iridium), and bis(dichloroimidazolylidene) (also with iridium). 

Diazo compounds, with or without metal catalysis, are well-known sources of carbenes. For synthetic purposes a metal catalyst is used. The diazo compounds employed are usually a- to an electron-withdrawing group, such as an ester or a ketone, for stability. In the early days, copper powder was the catalyst of choice, but now salts of rhodium are favoured. The chemistry that results looks very like the chemistry of free carbenes, involving cyclopropanation of alkenes, cyclopropenation of alkynes, C-H insertion reactions and nucleophilic trapping. As with other reactions in this chapter, free carbenes are not involved. Rhodium-carbene complexes are responsible for the chemistry. This has enormous consequences for the synthetic applications of the carbenes - not only does the metal tame the ferocity of the carbene, but it also allows control of the chemo-, regio- and stereoselectivity of the reaction by the choice of ligands. 

Another methodology is based on silver transmetallation [35]. In the first step, a silver-carbene complex is formed from the imidazolium salt and silver(I) oxide under reflux in dichloromethane. Subsequently, this complex is treated with a rhodium salt at room temperature to form the rhodium- carbene complex in high yields. In a few cases, also enetetramines have been used as precursors, delivering electron-rich carbenes by cleavage of the central C=C bond [36]. 

Triazoles also serve as a precursor of metal carbene complexes. For instance, Murakami and Fokin independently reported a denitrogenative rearrangement reaction of triazolyl alcohol 69 (Scheme 7.27) [41]. The ring-chain tautomeriza-tion of 69 generates its diazo imine form, which reacts with Rh2(Oct)4 to produce a rhodium carbene complex. The following 1,2-alkyl migration and elimination of rhodium produce enaminone 70. 

A series of carbene rhodium complexes of the general composition [Rh(L)( = CRR )(ri -C5H5)] (R = R = aryl L = SbPp3 or PR3) has been prepared from the square-planar precursors [Rh(Cl)( = CRR )(L)2] and NaCsHs and their reactivity towards nucleophiles studied. The complex [BpMe2Rh(co)(pyridine)] [Bp = dihydridobis(3,5-dimethylpyrazolyl)borate] reacts with Mel to yield the novel carbene complex [HB(Me2Pz)2Rh(H)(I)(py-ridine) C(0)Me ], resulting from formal addition of Mel across the Rh-C bond concomitant with hydride transfer from B to Rh. " A series of thermally stable PCP-type rhodium carbene complexes have been prepared from dinitrogen... 

Optically active furyl cyclopropane 402 was prepared from acetylene dicarboxylate and alkenes (Scheme 1.187) [261]. The acetylene dicarboxylate underwent dimerization to form metallocyclopentadiene 403, which decomposes to give cyclopropanes containing rhodium carbene complex 404. A good level of chiral induction was achieved using Segphos ligand with Rh(cod)2Bp4. 

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