NHC rhodium complexes

The performance of this ligand class in different catalytic reactions depends not only on the individual reaction, but also on the metal used. The rhodium NHC complex in Figure 4.23 catalyses the hydrosilylation of acetophenone with 98% conversion and... 

Gribble MW Jr, Ellman JA, Bergman RG (2008) Synthesis of a benzodiazepine-derived rhodium NHC complex by C-H bond activation. OrganometaUics 27 2152-2155... 

A diastereoselective tandem intramolecular hetero-5 + 2-cycloaddition/Claisen rearrangement of vinylic oxirane-alkyne substrates using a rhodium NHC complex to provide regiospecific and diastereoselective access to [3.1.0] bicyclic products has been reported (Scheme 153). ... 

The activity of rhodium (and ruthenium) NHC complexes in hydrosilylation of ketones was already reported in 1977 by Hill and Nile (300). Recently, several new rhodium NHC complexes allowing enantioselective hydrosilylation of ketones and j8-ketoesters were developed (for review, see (220)). 

A series of rhodium NHC complexes of the type RhCl(NHC)(cod) were obtained by the direct reaction of [Rh( -Cl)(cod)2l2 with the NHC by insertion of the rhodium atom into the C=C bond of bis(imidazolidin-2-ylidene) olefins, or by transmetallation from the corresponding silveifi) complexes. The restricted rotation about the Rh-C bond was studied and the steric size of the N-alkyl wingtips of the carbene and the size of the auxiliary ligands were shown to be influenced significantly. The uiCO) of the carbonyl derivatives obtained by bubbling CO over solutions of the cod precursors provided an efficient tool to estimate the electron-donor power of the carbene ligands. The preparation of unsymmetrically saturated NHCs by the direct reaction of the carbene with [Rh( -Cl)(cod)2]2 323 also provided examples of coordination to Rh. ... 

PubUl-UUdemolins C, Poyatos M, Bo C, Fernandez E. Rhodium-NHC complexes mediate diboration versus dehydrogenative borylation of cyclic olefins a theoretical explanation. Dalton Trans. 2013 14 746-752. 

The Rh-catalyzed intramolecular cycloaddition has proven reliable for the construction of polycyclic carbocycles and heterocycles. The cascade processes involving this reaction have also been achieved. A recent example is the cascade intramolecular hetero-[5 + 2] cycloaddition/Claisen rearrangement reaction of vinylic oxirane-aUcyne substrates 73, which uses the rhodium NHC complex as catalyst and provides atom-economical, regiospecific, and diastereoselective access to [3.1.0] bicyclic products 74 (Scheme 5.50) [48]. 

Rhodium-NHC complexes have been employed in a diverse range of chemical transformations of which only selected recent examples will be highlighted in this section. ... 

Rhodium-NHC complexes [Rh( <-Cl)(IPr)( -olefin)]2 and RhCl(IPr)(py)(t/"-olefin) (IPr= l,3-bis(2,6-diisopropylphenyl)imidazol-2-carbene, py = pyridine, olefin = cyclooctene) have been designed as highly active catalysts for hydrothiolation of alkynes RC=CH with R SH. The dinuclear catalyst was found to promote the formation of the linear product RCH=CHSR, whereas the mononuclear catalyst favoured the branched isomer R(R S)C=CH2- A complex interplay between electronic and steric effects exerted by the carbene (IPr), pyridine, and hydride ligands accounts for the observed regioselectivity. DFT calculations suggested that migratory insertion of the alkyne into the rhodium-thiolate bond is the rate-determining step. ... 

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. 

The ability of enzymes to achieve the selective esterification of one enantiomer of an alcohol over the other has been exploited by coupling this process with the in situ metal-catalysed racemisation of the unreactive enantiomer. Marr and co-workers have used the rhodium and iridium NHC complexes 44 and 45 to racemise the unreacted enantiomer of substrate 7 [17]. In combination with a lipase enzyme (Novozyme 435), excellent enantioselectivities were obtained in the acetylation of alcohol 7 to give the ester product 43 (Scheme 11.11). A related dynamic kinetic resolution has been reported by Corberdn and Peris [18]. hi their chemistry, the aldehyde 46 is readily racemised and the iridium NHC catalyst 35 catalyses the reversible reduction of aldehyde 46 to give an alcohol which is acylated by an enzyme to give the ester 47 in reasonable enantiomeric excess. 

For rhodium and iridium compounds alkoxo ligands take over the role of the basic anion. Using /z-alkoxo complexes of ( -cod)rhodium(I) and iridium(I)— formed in situ by adding the /r-chloro bridged analogues to a solution of sodium alkoxide in the corresponding alcohol and azolium salts—leads to the desired NHC complexes even at room temperature [Eq. (10)]. Using imidazolium ethoxyl-ates with [(r " -cod)RhCl]2 provides an alternative way to the same complexes. By this method, it is also possible to prepare benzimidazolin-2-ylidene complexes of rhodium(I). Furthermore, an extension to triazolium and tetrazolium salts was shown to be possible. ... 

A special type of reaction is observed with the platinum(IV) complex [PtI(Me)3] which cleaves the Af,N,Af, A -tetraphenyltetraaminoethylene under reduction to form the dimeric cyclometallated mono(NHC) complex of platinum(II) iodide [Eq. (31)]. Cyclometallation with the same ligand is also observed for ruthe-nium. Additional cyclometallations with various substituents of NHCs have been reported for ruthenium(II), rhodium(III), iridium(I), palladium(II), " and platinum(II). In the case of iridium, alkyl groups can be activated twice. In rare cases like for nickel(II) /x-bridging NHCs have been obtained. ... 

Historically, NHC complexes were investigated for the first time as catalysts and discussed as catalytic intermediates in the dismutation of electron rich tetraamino-ethylenes [Eq, (49)] Mixtures of two differently substituted olefins were reacted in the presence of rhodium(I) complexes and the products obtained showed mixed substitution patterns. Starting from Wilkinson s catalyst [(Ph3P)3RhCl], NHC complexes are formed as intermediates which could be isolated and used as even more active catalysts. In this first example, however, the NHC actively participates in... 

Rhodium(I) and ruthenium(II) complexes containing NHCs have been applied in hydrosilylation reactions with alkenes, alkynes, and ketones. Rhodium(I) complexes with imidazolidin-2-ylidene ligands such as [RhCl( j -cod)(NHC)], [RhCl(PPh3)2(NHC)], and [RhCl(CO)(PPh3)(NHC)] have been reported to lead to highly selective anti-Markovnikov addition of silanes to terminal olefins [Eq. 

Using the catalyst system described above in combination with a rhodium phosphine catalyst Lebel reported the de novo synthesis of alkenes from alcohols [100]. They developed a one-pot process, avoiding the isolation and purification of the potentially instable aldehyde intermediate. They combined the oxidation of alcohols developed by Sigman [89] with their rhodium-catalyzed methylenation of carbonyl derivatives. The cascade process is compatible with primary and secondary aliphatic as well as benzyUc alcohols in good yields. They even added another reaction catalyzed by a NHC complex, the metathesis reaction, which has not been addressed in this review as there are many good reviews, which exclusively and in great depth describe all aspects of the reaction. 

The first chiral NHCs of this type were developed by Herrmann and En-ders in 1996. Herrmann s group [6] synthesized a symmetric imidazolium salt 1 (as carbene precursor), starting from an enantiopure chiral amine which was readily converted to the heterocycle using a multi-component reaction previously developed by Arduengo [7]. After coordination to a rhodium(I) complex precursor (Scheme 1), this ligand was tested in the hydrosilylation of acetophenone. 

Figure 3.27 Berry pseudorotation in five coordinate rhodium(l) complexes with linker-free oxa-zoline functionalised NHC ligands.

Seeing the dicationic complex in Figure 3.42 that is stabilised by two solvent molecules (weakly coordinating acetonitrile) immediately raises the expectation that two imino functionalised NHC ligands should in principle be able to coordinate in a chelate fashion to the same metal atom. This is realised in a trigonal bipyramidal rhodium(I) complex, where the two carbene moieties are in the axial position [130] (see Figure 3.43). 

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. ... 

Figure 3.100 Synthesis of rhodium(l) and rhodium(lll) complexes from doubly phosphino functionalised NHC ligands.

Switching from palladium to rhodium, we encounter some very interesting chemistry. Zeng et al. [302] reacted the tiidentate PCP phosphino functionalised imidazolium salt with silver(I) oxide and subsequently transferred the carbene to rhodium(I) (see Figure 3.100). Careful selection of the rhodium precursor complex and reaction conditions enables tetrahedral, square bipyramidal and octahedral rhodium(I) and rhodium(III) complexes to be formed. As the authors explained, the activation of the C-Cl bond in methylene chloride in an oxidative addition reaction on rhodium(I) resulting in a rhodium(in) complex requires an electron rich rhodium(I) complex. The presence of a NHC ligand is advantageous in this respect. 

Labande et al. tested the rhodium(I) complexes of diphenylphosphinoferrocenyl functionalised NHC ligands (Cp,Cp and Cp,Cp substitution) in the hydrosilylation of ketones finding these complexes of only moderate activity [186]. As no attempt was made for the chiral resolution of the catalysts prior to use in catalysis, the prochiral acetophenone could not be tested in asymmetric catalysis. 

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). 

In the event, only the rhodium(I) complex features sulfur coordination, whereas the iridium(I) complex prefers a second carbene Ugand over the sulfur mediated chelate effect. The two complexes were tested for their activity in the hydrogenation of dimethyl itaco-nate. The iridium complex was inactive and the rhodium complex showed 44% conversion with a disappointingly low chiral resolution of 18% ee (R). The corresponding phosphine functionalised NHC rhodium(I) complex reacted under milder conditions, but without improvement of chiral resolution, 13% ee (S). 

Some other intermolecular C-H activations involving the NHC ligand have been observed during the synthesis of particular NHC-containing pincer -type complexes also called CCC-NHC complexes. In addition to zirconium- and rhodium-based complexes (210) and (211)/ several examples involving palladium of general structure (271) have been synthesized. Whereas Faller... 

Direct C-H activation of 2-imidazolines in the addition to alkenes has been observed under rhodium catalysis as shown for the formation of 586 (Scheme 139) <20040L1685>. The proposed intermediate was thought to be similar to that involved in metal-N-heterocyclic carbene (NHC) complexes <2002AGE1290>. 

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