NHC-iridium complexes

Fig. 2.13 Diboration catalysts based on palladium and iridium NHC complexes...

Iridium NHC complexes have also been used to catalyse the coupling of alcohols 7 and 8. Complexes 12 [4] and 13 [5], containing chelating NHC ligands, are also effective for catalysing the same 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. 

A tetracyclic bis(imidazolidine), containing two fused diazepine rings, was prepared and served as a building block for formation of iridium- and palladium-N-heterocyclic carbene (NHC) complexes (130M6445). The tetracyclic system 95 was formed via condensation of 1,4-diaminobutane 92, glyoxal 93, and formaldehyde 94.The reaction of 95 with [IrCl(COD)]2 in the presence of excess 1,5-cyclooctadiene led directly to the iridium-NHC complex 96.The palladium-NHC complex 97 was accessed via oxidation of bis(imidazolidine) 95 with N-bromosuccinimide and reaction with Pd(OAc)2 in the presence of KL... 

Scheme 2.3 Synthesis of rhodium- and iridium-NHC complexes via an intermediate with basic ligands.

The most common isomerisation reactions catalysed by transition metals are those involving the isomerisation of alkenes. Taller, Crabtree and co-workers have reported that the iridium bfy-NHC complex 55 is effective for the isomerisation of... 

Following on the triazole mohf a series of new iridium(I) l,2,4-triazole-3-ylidene NHC complexes [lr(cod)(NHC)L]BF4 (56a-c, L=PPh3, pyridine Scheme 4.23) were synthesized and tested and good results found for TH on C=0, C=N and C=C double bonds in 2-propanol with K2CO3 [52]. 

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. ... 

Several systematic experimental and computational studies have compared the sigma-donating abilities of NHCs and tertiary phosphines for a variety of transition-metal complexes [8-17]. As illustrative examples, analyses of the nickel-carbonyl complex 1 and iridium carbonyl complex 2 (Fig. 1) re-... 

Figure 3.96 Rhodium(l) and iridium(l) complexes of a phosphino functionalised NHC ligand.

Figure 3.97 Synthesis of an iridium (I) complex with abnormally coordinated phosphino functionalised NHC ligand.

In both cases, no catalytic reactions were performed with the iridium(I) and palladium(II) complexes synthesised with these ligands. However, Peris and coworkers used the synthesis of their iridium carbene complex to show that the formation of the M-NHC bond is an oxidative addition that can be assisted by a weak base. This base does not deprotonate the imidazolium salt, but traps the protons formed in the process [152],... 

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). 

Peracetylated glucose, brominated in the 1-position, was reacted with V-methylimi-dazole to yield the corresponding carbohydrate functionalised imidazolium salt [114], Reaction with silver(l) oxide and subsequently with [Cp lrCyj yields the silver(I) and iridium(lll) NHC complexes, respectively (see Figure 6.47). The structural parameters of the Cp lr(NHC) complex are similar to those of analogous Cp lr(NHC) complexes featuring nonfunctionalised carbenes [115,116]. 

Recently, Peris and coworkers reported the synthesis of highly stable orthometallated Cp -Ir NHC complexes and a catalytic application in the deuteration of organic molecules. Complexes (227) were synthesized from imidazolium salt (225) and [IrCp Cl2]2 as metal precursor either by a one-step procedure with sodium iodide to minimize the mixture I versus Cl or by transmetallation from the corresponding silver carbene affording (226) followed by the C-H activation of the phenyl ring by the iridium... 

Major achievements in cyclometallation processes involving NHC ligands have been achieved by Nolan and coworkers. It was observed that the solvent could play a crucial role in the formation of rhodium- and iridium-based complexes. As shown in Scheme 39, the reaction performed in pentane between [M(COE)2Cl]2 (M = Rh or Ir) and four equivalents of FBu led only to COE substitution by the NHC ligand affording rhodium-based product (243) (the iridium complexing proving difficult to isolate). On the other hand, the same reaction carried out in hexanes gave, via C H activation, the hydride complexes (244) and (245). Finally, in benzene, a unique double cyclometallation process occurred to yield the coordinatively unsaturated 16-electron... 

Interestingly, treatment of enantiomerically pure (S,5 )-l,3-di(methyl-benzyl)imidazolium chloride with the same source of iridium in the presence of NaOAc results in the formation of a [lrCp Cl(NHC)] complex as a unique diastereoisomer, in which the ligand chelates via the carbene and ort/jo-position of one phenyl group (Fig. 38). ... 

Research in the chemistry of rhodium and iridium Af-heterocyclic carbene (NHC) complexes has extraordinarily evolved since 2000. A quick search for rhodiimi-NHC and iridiimi-NHC complexes in the SCl-expanded database, with a 2005-2013 timespan, results in more than 360 hits for rhodium, and more than 340 for iridiiun, which gives a good idea on the interest that rhodium and iridium NHC-based chemistry have achieved in the last few years. It is important to note that a nimiber of reviews and book chapters specifically concerning the chemistry of NHC-based compounds of rhodium and iridiiun have recently appeared [1]. This chapter will deal with all new aspects of the NHC-M (M = Rh, Ir) chemistry not reviewed before, and therefore is mainly restricted to the last 4-5 years. The chapter is classified into two main sections, the first of which deals with relevant structural and electronic features of Rh-NHC and Ir-NHC complexes, and the second with the catalytic applications of these compounds. While not pretending to be completely comprehensive, we have tried to describe the most relevant examples assigned to each section. Some other relevant applications of these complexes have not been considered, such as the emerging biochemical applications, mostly referred to Rh-NHC complexes [2], and the luminescent properties of some Ir-NHC complexes, mostly used for the fabrication of electro-optical devices [3]. 

Catalytic Applications of Rhodium and Iridium NHC-Based Complexes... 

The numerous and ever-increasing applications of rhodium and iridium NHC-based complexes have been recently reviewed [lb,lc, 47,104]. We present here an updated account on the catalytic applications of rhodium and iridium NHC-based complexes, paying special attention to reduction and oxidation processes. 

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