NHC-Ru complexes

However, similar NHC architectures employing aromatic side chains have shown more encouraging results. In 2000, Nolan and co-workers reported the synthesis and characterisation of the NHC-Ru complex 20 bearing a sterically more demanding N,N -bis-[2,6-(di-/xo-propyl)phenyl]imidazol-2-ylidene (IPr) ligand [27, 28] (Fig. 3.5). Standard RCM substrate 1 was used to test the catalytic performance of 20. The ring closure was found to be complete after 15 min by using 5 mol% 20 as catalyst at room temperature. Under identical conditions, 15... 

In 2001, Furstner reported the preparation and characterisation of the NHC-Ru complex 22 containing iV,iV -bis[2,6-(diisopropyl)phenyl]imidazolidin-2-ylidene (SIPr) [29] (Fig. 3.6), which is the congener of complex 20. Subsequently, Mol and co-workers revealed that complex 22 was a highly active metathesis initiator [30]. More recent comparative studies showed that catalyst 22 could catalyse the RCM of 1 faster than any other NHC-Ru catalyst, while it was not stable enough to obtain complete conversion in the RCM of 3 and was inefficient for the construction of the tetrasubstituted double bond of cyclic olefin 6 [31]. 

Some attempts have also been made to vary the para- or meta-substituents on the phenyl ring of the SlMes ligand, however none of the resulting NHC-Ru complexes showed better RCM activity than the original Grubbs 1114 when tested with standard substrates 1 and 7 [37 0]. 

A NHC-Cu complex 9 has also been used in the cyclopropanation of 5 and cyclooctene 8 using EDA 6 (Scheme 5.3) [5]. Complex 9 was isolated prior to use and, as in the case of NHC-Ru complex, the cyclopropanation reaction did not display high diastereoselectivity. However, products 7 and 10 were obtained in good to excellent yields depending on the ratio between the alkenes and EDA. Improved yields were obtained when alkenes were used in six- or ten-fold excess. 

For the last 2 decades ruthenium carbene complexes (Grubbs catalyst first generation 109 or second generation 110, Fig. 5.1) have been largely employed and studied in metathesis type reactions (see Chapter 3) [31]. However, in recent years, the benefits of NHC-Ru complexes as catalysts (or pre-catalysts) have expanded to the area of non-metathetical transformations such as cycloisomerisation. 

A chiral NHC-Ru complex 158 was used in the Diels-Alder reaction between methacrolein 156 and cyclopentadiene 157 (Scheme 5.41) [47]. The adduct 159 was obtained in an excellent yield under mild conditions, albeit with low enantioselectivity. 

More recently, two types of Ru complexes were obtained by the reaction of mesityl-substituted electron-rich olefins with [RuCl2(p-cymene)]2 [27]. Cleavage of the chlorine bridges occurs first to yield the expected (NHC)(p-cymene)Ru(II) complex 9. Under harsher reaction conditions (140 °C in p-xylene) further arene displacement takes place to yield the chelated ( 6-mesityl-NHC - Ru complex 10 (Scheme 6). The olefin 8 was easily obtained by deprotonation of the corresponding dihydroimidazolium salt. 

Dragutan V, Dragutan I, Delaude L et al. (2007) NHC-Ru complexes Friendly catalytic tools for manifold chemical transformations. Coord Chem Rev 251 765-794... 

NHC—Ru complexes-friendly catalyhc tools for manifold chemical transformations 07CCR765. 

Chiral NHC-Ru complexes have been successfully involved in the asymmetric hydrogenation of quinoxalines. Complete conversions have been achieved but ees remained under 90%. 

Herrmann et published an array of NHC-Ru complexes (13-16) resulting from the diphosphane Ru-benzylidene complex 12 and IMes carbenes, in toluene or tetrahydrofuran, at room temperature. Products with one or two SIMes ligands, depending on the molar ratio of the complex 12 and the SIMes carbene (in practice molar ratios of 1 1.2 or 1 2.2 are used), were obtained in high yield (80-90%). 

Some unsymmetrically substituted eomplexes, e.g. 26a (n = 1, 2, 4) possess the unique ability to metathesize their own ligands to form ehelated NHC-Ru complexes in which the NHC and die tegulaf earbene unit, Ru = CHR, are tethered by a variable eyelie structure. In one example, heating a solution of complex 26a (n = 2) in refluxing toluene afforded the metallacyclic complex 26 in 75% isolated yield (Scheme 16). ... 

In 2008-2009, Grela and co-workers disclosed the synthesis of NHC Ru complexes bearing chelating sulfoxide ligands (25) that showed no catalytic activity in RCM or enyne metathesis reactions at room temperature, but became active upon heating to 40 110 °C. Concomitantly, Lemcoff et al. prepared a closely related series of sulfur-chelated latent ruthenium alkene... 

In 2006, Delaude et al. showed that imidazol(in)ium-2-carboxylates were eonvenient NHC preeursors to produee NHC-Ru complexes in situ for the photoinduced ROMP of norbornene and cyclooctene in the presence of [RuCl2(p-cymene)]2. Subsequent work allowed the isolation of compounds 30 in pure form starting from the ruthenium dimer and three different NHC-C02 adducts. Alternatively, Fischmeister and Dixneuf used the free IMes carbene to synthesise the known [(IMes)RuCl2(p-cymene)] complex, which was found to be an efficient pre-catalyst for the CM of various styrene derivatives and the RCM of a tetrasubstituted cycloolefin when activated with styrene. ... 

Besides alkene or alkyne metathesis, a broad range of other non-metathetical reactions promoted by NHC Ru complexes was reported in the literature. " Some of them were discovered serendipitously, as they constituted side reactions in metathesis catalysis. Other were deliberately investigated and optimised. Despite the usefiilness of several of these processes, their significance has remained undervalued due to the huge appeal of olefin metathesis and related reactions. 

The RCM of 1,6-dienes into cyclopentenes is sometimes accompanied by a cycloisomerisation reaction to give cyclic products with an exo-methylene substituent, especially when ruthenium-arene complexes are used as catalyst precursors (see for instance compounds 32 in Section 7.3.1.6). In many cases, this side reaction was undesired and could be effectively suppressed by the addition of various co-catalysts. It is nevertheless possible to alter the reaction path to obtain the cycloisomerisation products with very high selectivities. Early examples of NHC-Ru complexes suitable for this task included the unstable cationic alle-nylidene complexes 36 prepared in situ from more robust chelated precursors 35 (Equation (7.6)). Alternatively, a combination of IMes-HCl/Cs2C03/[RuCl2(p-cymene)]2 also provided an efficient catalytic system. A mechanism involving oxidative coupling of the 1,6-diene to a ruthenium(II) centre followed by p-elimination to generate a hydrido ruthenium(IV) intermediate and reductive elimination was proposed for the transformation (Scheme 1.9)2 ... 

Although ROMP or ADMET reactions hold a prominent position among polymerisation processes initiated by NHC-Ru complexes, other catalytic paths leading to macromolecular products were also investigated. The activity of compounds 30 and other similar monometallic [(NHQRuCl2(p-cymene)] complexes was tested in the atom transfer radical polymerisation (ATRP) of vinyl monomers by Delaude and Demonceau, along with 32. These complexes led to the controlled polymerisation of methyl methacrylate at 85 °C (Equation (7.8)). Attempts to polymerise n-butyl acrylate and styrene turned out to be more challenging, because of difficulties to control the acrylate polymerisation and of competition with the self-metathesis of styrene. 

Scheme 7.13 NHC Ru complexes for stoichiometric or catalytic C F bond activation.

A series of mixed PCys/NHC Ru complexes, such as [(IMes)-RuCl2(=CHPh)(PCy3)] 86, favored the formation of the Z-isomer, while the a-isomer was only formed in minor amounts. ... 

Ruthenium s supremacy in the carbene chemistry of Group 8 elements is a direct consequence of the tremendous interest raised by NHC-Ru complexes as catalysts for olefin metathesis. Indeed, the synergy of a late transition metal tolerant of a wide variety of functional groups, together with a class of ligands whose physical and chemical properties are easily modulated to tailor the activity, selectivity, stability, water-solubility, recoverability, or latency of the resulting catalytic species, translated into an unprecedented success story of modern synthetic chemistry. Yet, the ability of ruthenium complexes to promote carbon-carbon bond formation goes well beyond... 

A predictable move in the development of NHC-Ru complexes bearing indenylidene ligands was the preparation of the third-generation complex 38 by the groups of Slugovc and Verpoort in 2008. The first team established its X-ray crystallographic structure and demonstrated its aptitude to efficiendy promote the controlled living ROMP of norbomene monomers, while the second one probed its catalytic activity in diverse RCM, CM, and ROMP reactions. ... 

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