Rhodium heterocyclic carbenes

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

Steric effects were also found to be important for determining the reactivity of rhodium complexes containing N-heterocyclic carbene (NF1C) ligands [47] (Scheme 10), which have been the subject of intense in-... 

Tejel C, Ciriano MA (2007) Catalysis and Organometallic Chemistry of Rhodium and Iridium in the Oxidation of Organic Substrates. 22 97-124 Tekavec TN, Louie J (2006) Transition Metal-Catalyzed Reactions Using N-Heterocyclic Carbene Ligands (Besides Pd- and Ru-Catalyzed Reactions). 21 159-192 Tesevic V, see Gladysz JA (2008) 23 67-89... 

In asymmetric ketone hydrosilylation, axially chiral bidentate N-heterocyclic carbene ligands derived from BINAM proved to be moderately effective when bound to iridium, but less so with rhodium (Scheme 3.19) [44]. 

New N-heterocyclic carbene rhodium and iridium complexes derived from 2,2 -diaminobiphenyl were successfully synthesized and their structures unambiguously characterized by X-ray diffraction (XRD) analysis. These are cata-lytically active for the hydrosilylation of ketones with diphenylsilane, although an NHC—rhodium complex was found to be the best among those investigated [45]. 

Iridium(l) precursors [lr(cod)(L)] with bidentate N-heterocyclic carbene ligands L3 appeared slightly less active in the hydrosilylation of acetophenone with diphe-nylsilane than did the similar rhodium complexes, giving respectively yields of 85% of I and 15% of II for the Pr substituent, and 83% of I and 17% of II for the benzyl moiety, after 2 h reaction at room temperature [47]. However, when carbene ligands of type L3 were used a significant increase in the ee-value of the sec-phenethyl alcohol R isomer of up to 60% was observed. 

In order to vary the electronic situation at the carbene carbon atom a number of carbo- and heterocycle-annulated imidazolin-2-ylidenes like the benzobis(imida-zolin-2-ylidenes) [58-60] and the singly or doubly pyrido-annulated A -heterocyclic carbenes [61-63] have been prepared and studied. Additional carbenes derived from a five-membered heterocycle like triazolin-5-ylidenes 10 [36], which reveals properties similar to the imidazolin-2-ylidenes 5 and thiazolin-2-ylidene 11 [37] exhibiting characteristic properties comparable to the saturated imidazolidin-2ylidenes 7 have also been prepared. Bertrand reported the 1,2,4-triazolium dication 12 [64]. Although all attempts to isolate the free dicarbene species from this dication have failed so far, silver complexes [65] as well as homo- and heterobimetallic iridium and rhodium complexes of the triazolin-3,5-diylidene have been prepared [66]. The 1,2,4-triazolium salts and the thiazolium salts have been used successfully as precatalysts for inter- [67] and intramolecular benzoin condensations [68]. 

A rhodium(I)-.A-heterocyclic carbene complex has brought about a chemoselective decarbonylation, converting a cyclobutanone to the corresponding cyclopropane, while leaving an aldehydic substituent untouched.336... 

Fischer-type carbenes, 8, 218 iV-heterocyclic carbenes, 8, 219 non-heteroatom carbenes, 8, 217 overview, 8, 197—268 photochemistry, 1, 252 with Re(I), 5, 900 with rhodium, 7, 184 rotation, dynamic NMR studies, 1, 420 in Ru and Os half-sandwich preparations, 6, 575 silver-catalyzed insertions, 9, 568 with silver(I), 2, 203 as spectator ligands, 1, 66... 

An N-heterocyclic carbene rhodium complex derived from L-proline has been reported (Scheme 10). The rhodium complex 9 is active catalyst for the addition reaction of arylboronic acids to aldehydes, albeit with low ee s (21% at best) [29]. 

Gade and co-workers reported the synthesis of an oxazolinyl-carbene which is obtained by direct linkage of the two heterocycles. The new ligand system was obtained by reacting the 2-bromooxazoline 65 [122] with an imidazolium precursor in THF (Scheme 46) [123]. N-heterocyclic carbene rhodium complexes could be obtained by reaction of the imidazolium salt 66 with [ Rh(/i.-OfBu)(nbd) 2] generated in situ [124]. 

Metal-catalyzed [4- -2] and [5- -2] cycloadditions devoped by Wender and Ttost are powerful transformations for the construction of polycyclic ring skeletons. Their intramolecular versions incorporating an ether group in the tether lead to oxygen heterocycles in good yields. A rhodium N-heterocyclic carbene catalyst has been shown to be particularly effective in such [4-1-2] and [5-1-2] cycloaddition reactions to form [r]-annulated tetrahydrofurans (Scheme 66). In all cases reported, excellent yields have been obtained in less than lOmin reaction time at 15-20 °C <2006JOC91>. 

The case studies below will provide selected examples of calorimetric studies carried out on metal-ligand exchange reactions of phosphorous-based ligands (such as phosphines, phosphite, A -pyrrolyl-phosphines) and N-heterocyclic carbene ligands (NHCs) as well as addition reactions relevant to catalytic transformations, involving metals such as ruthenium, rhodium,platinum,and molybdenum. ... 

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>. 

The bond between carbon and rhodium is extremely stable, thus allowing mono- or biphasic hydroformylations without any excess of ligands. Due to this stability, for the first time ever an anchoring to a polymer support seems possible without leaching. These N-heterocyclic carbenes appear to be excellent ligands to stabilize catalytically active metals even under harsh temperature conditions, e. g. Heck C-C-coupling reactions at 130 °C [152 b]. 

Rhodium Complexes with iV-Heterocyclic Carbene Ligands... 

Rhodium complexes such as 16 with A-heterocyclic carbenes can be prepared in one step proceeding from commercially available precursors (e. g. [( /" -1,5-COD)RhCl]2 (eq. (20)) or Wilkinson s complex [RhCl(PPh3)3] and the free carbene, which is generated from the storable imidazolium salt by deprotonation [47 9]. For more details, see Section 3.1.10. 

Dupont and van Leeuwen studied hydroformylation reactions in [bmim]PF6 [63] and observed the formation of catalytically active rhodium species in operando (Fig. 13). The same class of reaction has been studied again by Dupont, this time in the IL [bmim]Tf2N [64]. Here, they reported H/D exchange in the 2-position of the imidazolium ring of the IL, especially in the presence of a base - a finding that has been reported earlier already ([3,9], and references therein). The involvement of A-heterocyclic carbene species was discussed but not proven. 

It was also reported that a chiral heterocyclic carbene rhodium complex catalyzed the reduction of acetophenone with diphenylsilane giving 32% ee and 90% yield [47]. 

RHODIUM AND IRIDIUM A-HETEROCYCLIC CARBENE COMPLEXES FROM IMIDAZOLIUM CARBOXYLATES... 

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