NHC-Rh complexes

Asymmetric hydrosilylation using a chiral NHC-Rh complex was first reported by Herrmann et al. in 1996 (Scheme 3.19) [7]. After the synthesis of the novel chiral NHC-Rh complex 34, which was stable in air, they used this precatalyst to catalyze the asymmetric hydrosilylation of acetophenone. Good catalytic conversions were obtained, but with low enantioselectivities (up to 32% ee) and very long reaction times (up to 12 days). 

Lai et al. disclosed the asymmetric hydroformylation of styrene using chiral NHC-Rh complex 47 as the catalyst (Scheme 3.25) [45], However, the enantio-selectivity and regioselectivity were rather low. Under optimized reaction conditions, the branched aldehyde 48 was obtained in 12.5% ee with the branched/ linear ratio of 42/58 in the presence of 3 equivalents of PPhs. 

In 2006, the same group reported the synthesis of novel hexadentate imidazolium salts (Scheme 7.11) from hexakis(bromomethyl)benzene and 1-substituted imidazole and decided to evaluate their efficiency in the arylation of aldehydes with arylboronic acids [18]. The cattJyst generated in situ from the hexadentate imidazolium sailts and [Rh(COD)Cl]2 represents an easy-to-handle and high-yielding procedure for the addition of arylboronic acids to aldehydes. Luo and coworkers [19] then reported a new catalytic system on novel chiral imidtizolium salts and a stable NHC-Rh complex derived from L-proline (see Scheme 7.11). Excellent yields were achieved, but, unfortunately, their attempt to undergo an efficient asymmetric reaction failed, as a maximum enantioselectivity of 21% ee was achieved. 

In a study of NHC-Rh complexes aimed at developing an NHC-based alternative to the Monsanto acetic acid catalyst, Haynes and co-workers prepared [(IMe)2Rh(CO)(I)] and examined its reaction with Mel. Interestingly, even though several observed intermediates had alkyl and acyl groups cis to an NHC, reductive elimination was not observed. However, when subjected to a milder version of the industrial reaction conditions, namely 10% MeOH and 2% Mel in chlorobenzene at 120 °C under 10 atm of CO, sequential loss of the two NHCs was observed, generating the NHC-free catalyst [Rh(CO)2l2], previously employed by Monsanto. This catalyst proved to be considerably more reactive than the NHC-containing species. ... 

The first efficient catalytic applications of N-heterocyclic carbene ligands with late transition metals were reported for rhodium. Since the mid 1990s, NHC-rhodium chemistry has been extensively studied and may be comparable to palladium or nickel in terms of the number of catalytic applications. Outside the context of catalysis, NHC-Rh complexes have featured prominently in studies probing the electronic properties of NHCs, usually quantified by comparison of the pco frequencies in the IR spectra of [LRh(CO)2Cl] complexes. ... 

Herrmann et al. reported the hydrogenation activity of various NHC-Rh complexes [(ICy)Rh(COD)Cl] 14, [(IMe)2Rh(COD)]Cl 15, and [(IBn)RhBr(CO)2] 16. " ICy-containing 14 produced the best results, whereas the carbonyl complex 16 was inert in the hydrogenation of cyclohexene. In these reactions, PPhs was used as additive to stabilized low coordinated catalytic species. 

The application of NHC ligands in this reaction has naturally been extended to asymmetric processes. In general, the chiral induction of NHCs was low, probably due to the rapid internal rotation of the chiral substituents around the C-N axis. Also, metalation procedures involving deprotonation of a chiral azolium precursor with strong bases may cause loss of chirality in the desired complex. In contrast, the silver transmetalation route generally yields the optically active target complex. Since the first reports by Herrmann et al., and shortly after by Enders et al. in 1996-1997 on the synthesis and applications of chiral NHC-Rh complexes in the hydrosilylation of carbonyl compounds, increased efforts were made in this area. ... 

Hydrosilylation reactions catalyzed by achiral NHC-Rh complexes were first reported by Lappert and Maskell in 1984, who found 96 and 97 effective for ketones and alkynes (Figure 13.16). In recent studies by Ozdemir and coworkers, different NHC-Rh complexes were applied to the hydrosilylation of acetophenone derivatives with triethylsilane and bimetallic complex 98 was particularly efficient. Also, Rh complexes 99, bearing a hydrophilic tetra-ethylene glycol and/or hydrophobic long-chain alkyl-functionalized NHC, were active catalyst for the hydrosilylation of ketones with Ph2SiH2. ... 

In 2007, Tsuji and co-workers showed NHC-Rh complexes 107 with dendrimer substituents were more active than Rh analogues in the hydrosilylation of 2-cyclohexenone (Figure 13.18). Furthermore, 107 gave the 1,4-adduct as major product, whereas Rh complexes 100 led preferentially to the 1,2-adduct instead. Alternatively, related complex 108 was fairly elfective for the hydrosilylation of ketones with diphenylsilane to give the corresponding alcohols in moderate to good yields (67-93... 

Hydroboration of terminal alkynes catalyzed by NHC-Rh complexes 125 was found to be influenced by the electronic nature of the substituents on the NHC backbone, 7i-withdrawing groups affording even lower yields than... 

Triazolium-based carbenes also afforded mono- and bis-NHC Rh complexes." ... 

Scheme 4.22 Rh-catatyzed C-H activation and cyclization of substituted heterocycles via (a) the formation of a [(NHC)Rh] complex or (b) a C-H activation. Relative energies given in kcal mol". ...

In a very recent study, Oro et al. prepared MCM-41 supported NHC-Rh complexes 170 and tested them in the solvent-free hydrosilylation of acetophenone with HSiMe(OSiMe3)2 (Figure 13.20). Even if an induction period was observed, these catalysts eventually exhibited good catalytic activity. 

In 2007, Tsuji and co-workers showed NHC-Rh complexes 171 with dendrimer substituents were more active than Rh analogues in the... 

Recent work towards NHC-Rh complexes as anticancer agents has been conducted by Ott, who tested a series of NHC-Rh complexes (61-63) against the MCF-7 and HT-29 cancer cell lines. The complexes demonstrated a high degree of cytotoxicity, with cyclooctadiene-bearing complexes of types... 

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