Iridium complexes carbene

Organometallic Compounds. The predominant oxidation states of indium in organometalUcs are +1 and +3. Iridium forms mononuclear and polynuclear carbonyl complexes including [IrCl(P(C3H3)3)2(CO)2] [14871-41-1], [Ir2014(00)2] [12703-90-1], [Ir4(CO)22] [18827-81 -1], and the conducting, polymeric [IrCl(CO)3] [32594-40-4]. Isonitnle and carbene complexes are also known. 

The ionic iridium(III) carbene complex (47) is prepared from the reaction of IrHCl(03SCF3)-(CO)(PPh3)2] with [RC=NMe](03SCF3).61 Addition of Na(barf) (barf = B(3,5-C6H3 (CF3)2)4) to [Ircp (PMe3)(CH3)(0S02CF3)] in CH2C12 yields the structurally determined species... 

C-C bond formation can be favored over /3-hydride elimination by changing the nature of the catalyst. Hence, cyclizations can be mediated by iridium carbene complexes resulting from a formal intramolecular cross-coupling of the alkene with an. s -C-H bond (Equation (40)). 

There have been many reports of the use of iridium-catalyzed transfer hydrogenation of carbonyl compounds, and this section focuses on more recent examples where the control of enantioselectivity is not considered. In particular, recent interest has been in the use of iridium A -heterocyclic carbene complexes as active catalysts for transfer hydrogenation. However, alternative iridium complexes are effective catalysts [1, 2] and the air-stable complex 1 has been shown to be exceptionally active for the transfer hydrogenation of ketones [3]. For example, acetophenone 2 was converted into the corresponding alcohol 3 using only 0.001 mol% of this... 

The same catalyst has also been used for the reduction of aldehydes to primary alcohols [7]. Several other iridium W-heterocyclic carbene complexes have been shown to be successful as catalysts for the transfer hydrogenation of ketones [8-12], including the interesting complex 6, where the cyclopentadienyl ring is tethered to the 77-heterocyclic carbene. Complex 6 was employed at low catalyst loading for the reduction of a range of ketones including the conversion of cyclohexanone 11 into cyclohexanol 12 [13]. 

The catalyst is also effective for the reduction of styrenes, ketones, and aldehydes. Cyclohexenone 16 was reduced to cyclohexanone 11 by transfer hydrogenation, and using a higher catalyst loading, styrene 17 was reduced to ethylbenzene 18. The elaboration of [Ir(cod)Cl]2 into the triazole-derived iridium carbene complex 19 provided a catalyst, which was used to reduce aUcene 20 by transfer hydrogenation [25]. 

Marr and coworkers have used the combination of the iridium W-heterocyclic carbene complex 100 (0.1 mol%) with the lipase Novozyme 435 to convert racemic alcohols 101 into enantiomerically enriched esters 102 with excellent yield and enantioselectivities (Scheme 25) [85]. Peris and coworkers have used complex 25... 

Iridium N-Heterocyclic Carbene Complexes and Their Application as Homogeneous Catalysts... 

A large number of reports have concerned transfer hydrogenation using isopropanol as donor, with imines, carbonyls-and occasionally alkenes-as substrate (Scheme 3.17). In some early studies conducted by Nolan and coworkers [36], NHC analogues of Crabtree catalysts, [Ir(cod)(py)(L)]PF,5 (L= Imes, Ipr, Icy) all proved to be active. The series of chelating iridium(III) carbene complexes shown in Scheme 3.5 (upper structure) proved to be accessible via a simple synthesis and catalytically active for hydrogen transfer from alcohols to ketones and imines. Unexpectedly, iridium was more active than the corresponding Rh complexes, but... 

Perhaps most dramatically of all, for the first time, bis(carbene)-substituted iridium complexes, such as [Ir(cod)(NHC)2] (NHC = 1,3-dimethyl- or 1,3-dicyclohexylimidazolin-2-ylidene] were successfully used by Herrmann and coworkers as C—H-activation catalysts in the synthesis of arylboronic acids starting from pinacolborane and arene derivatives [46]. 

Iridium 2-pyridinylmethyl imidazolylidene C,N-chelates were obtained by transmetallation of the silver carbene complexes and tested for catalytic activity in the TH of benzophenone and nitroarenes by isopropanol [55]. The neutral monodentate complexes [(L-KC)Ir(COD)Clj [61a,b L = l-methyl-3-(6-mesityl-2-pyridinylmethyl)-2-imidazolylidene, l-mesityl-3-(6-mesityl-2-pyridinylmethyl)-2-... 

AhHCl BulPCHzCHiCRCHiCH BuDlR = H(flux-ional) = CH3(not fluxional), olefinic and carbene complexes, and some new hydrides of iridium. 

Iridium-carbon multiple bonds allenylidene complexes, 7, 355 carbene complexes, 7, 344 carbyne complexes, 7, 361 higher cumulenylidene complexes, 7, 358 vinylidene complexes, 7, 352 Iridium-carbon single-bonded complexes alkenyl complexes, 7, 319 alkyl and aryl complexes, 7, 303 in C-C bond-forming catalysis, 7, 335 characteristics, 7, 303... 

Further restrictions to the scope of the present article concern certain molecules which can in one or more of their canonical forms be represented as carbenes, e.g. carbon monoxide such stable molecules, which do not normally show carbenoid reactivity, will not be considered. Nor will there be any discussion of so-called transition metal-carbene complexes (see, for example, Fischer and Maasbol, 1964 Mills and Redhouse, 1968 Fischer and Riedel, 1968). Carbenes in these complexes appear to be analogous to carbon monoxide in transition-metal carbonyls. Carbenoid reactivity has been observed only in the case of certain iridium (Mango and Dvoretzky, 1966) and iron complexes (Jolly and Pettit, 1966), but detailed examination of the nature of the actual reactive intermediate, that is to say, whether the complexes react as such or first decompose to give free carbenes, has not yet been reported. A chromium-carbene complex has been suggested as a transient intermediate in the reduction of gfem-dihalides by chromium(II) sulphate because of structural effects on the reaction rate and because of the structure of the reaction products, particularly in the presence of unsaturated compounds (Castro and Kray, 1966). The subject of carbene-metal complexes reappears in Section IIIB. 

Although carbene complexes of platinum group metals are fairly rare, iridium forms several. One is made by the following sequence 113... 

Not surprisingly, these rhodium and iridium carbene complexes were tested for their catalytic behaviour in the transfer hydrogenation of benzophenone and acetophenone (M +3), the hydrosilylation of alkynes (M +1) and also the catalytic cyclisation of acetylenic carboxylic acids (M +1). Hydrogenation works better for iridium than rhodium and for aromatic than for aliphatic ketones [40,43,44]. The iridium(I) complex is the first iridium catalyst showing activity for the cyclisation of acetylenic carboxylic acids [40]. The results for the hydrosilylation reactions were very moderate. 

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