Catalytic IrCl

Today, iridium compounds find so many varied applications in contemporary homogeneous catalysis it is difficult to recall that, until the late 1970s, rhodium was one of only two metals considered likely to serve as useful catalysts, at that time typically for hydrogenation or hydroformylation. Indeed, catalyst/solvent combinations such as [IrCl(PPh3)3]/MeOH, which were modeled directly on what was previously successful for rhodium, failed for iridium. Although iridium was still considered potentially to be useful, this was only for the demonstration of stoichiometric reactions related to proposed catalytic cycles. Iridium tends to form stronger metal-ligand bonds (e.g., Cp(CO)Rh-CO, 46 kcal mol-1 Cp(CO)Ir-CO, 57 kcal mol ), and consequently compounds which act as reactive intermediates for rhodium can sometimes be isolated in the case of iridium. 

Iridium made its first major mark in 1965, in the arena of organometallic chemistry with the discovery of Vaska s complex, [IrCl(CO)(PPh3)2] (1) [1]. Only weakly catalytic itself, Vaska s complex is nevertheless highly relevant to cataly-... 

Catalysts other than homogeneous (molecular) compounds such as nanoparticles have been used in ionic liquids. For example, iridium nanoparticles prepared from the reduction of [IrCl(cod)2] (cod = cyclooctadiene) with H2 in [bmim][PF6] catalyses the hydrogenation of a number of alkenes under bipha-sic conditions [27], The catalytic activity of these nanoparticles is significantly more effective than many molecular transition metal catalysts operating under similar conditions. 

Oligomerization and polymerization of terminal alkynes may provide materials with interesting conductivity and (nonlinear) optical properties. Phenylacetylene and 4-ethynyltoluene were polymerized in water/methanol homogeneous solutions and in water/chloroform biphasic systems using [RhCl(CO)(TPPTS)2] and [IrCl(CO)(TPPTS)2] as catalysts [37], The complexes themselves were rather inefficient, however, the catalytic activity could be substantially increased by addition of MesNO in order to remove the carbonyl ligand from the coordination sphere of the metals. The polymers obtained had an average molecular mass of = 3150-16300. The rhodium catalyst worked at room temperature providing polymers with cis-transoid structure, while [IrCl(CO)(TPPTS)2] required 80 °C and led to the formation of frani -polymers. 

Zhou and Hartwig recently discovered the beneficial effect of added potassium hexamethyldisilazanide (KHMDS) base for the asymmetric addition of aniUnes to norbornenes, thereby widening the synthetic scope of the original CMM system (see Table 6.2) [17]. [IrCl(COE)2]2 and two equivalents of variants of the Segphos and Biphep Ugands first presumably form complexes 8, 11, and 12 in situ (see Chart 1) and then in combination with co-catalytic KHMDS generate the catalytically active species (see Table 6.2 and Section 6.4 for a discussion of the mechanism). 

IrCl(cod)]2, in the presence of PPh3 and KOH, catalyzed the a-alkylation of ketones with alcohols [41]. As an example, the reaction of 2-octanone 87 with 1-butanol 88 was catalyzed by the iridium complex to give 6-dodecanone 89 in 80% yield (Equa-hon 10.19). The alkylation proceeded with complete regioselectivity at the less-hindered side of 2-octanone, and the reaction was promoted by a catalytic quantity of KOH (10mol%) in the absence of both a hydrogen acceptor and a solvent. 

Another type of N-alkylation was achieved by the [IrCl(cod)]2-catalyzed reductive alkylation of secondary amine with aldehyde and silane (Equation 10.29) [53], For example, the treatment of dibutylamine 117 with butyraldehyde 118 and EtsSiH 119 (a 1 1 1 molar ratio amine, aldehyde and silane) or polymethyUiydrosiloxane (PMHS) in 1,4-dioxane at 75 °C under the influence of a catalytic amount of [lrCl(cod)]2, gave tributylamine 120. 

Vaska s complex ([IrCl(CO)(PPh3)2]) also catalyzed the carbonylative coupling of diynes, which provided bicyclic cyclopentadienones (Scheme 11.23) [35]. Due to the instability of the products, the substrates are limited to symmetrical diynes with aromatic groups on their termini nonetheless, this reaction still serves as the catalytic and practical procedure for the synthesis of cyclopentadienones, which are anti-aromatic with a 47t system and serve as active synthetic intermediates. 

Besides formaldehyde, Michael acceptors such as acrylonitrile and ethyl acrylate also serve as substrate to undergo the addition in the presence of various metal complexes [10-14]. Acrylonitrile affords P(CH2CH2CN)3 tcep (Scheme 3). The order of catalytic activity is reported to be Pt[P(CH2CH2CN)3]3>Pd[P(CH2CH2CN)3]3P IrCl[P(CH2CH2CN)3]3>Ni[P(C-H2CH2CN)3]3. The solvent effect on the rate is not significant. In acetonitrile, however, a small amount of a telomer is formed. 

Murai and Chatani speculated that the two acetylene carbons should be converted into two carbene equivalents to give XVIII during the reaction." To trap this intermediate, the reaction of 6,11-dien-l-yne 69c, which has an olefin moiety in a tether, is carried out in the presence of [RuCl2(CO)3]2 in toluene at 80 °C for 4 h to give tetracyclic compound 71 in 84% yield. It is interesting to note that other transition metal complexes, such as PtCl2, [Rh(OOCCF3)2]2, [IrCl(CO)3] , arid ReCl(CO)s also show catalytic activity for this very complex transformation (Scheme 27). 

Following the success with cobalt and rhodium, Shibata reported Ir(i)-based enantioselective catalytic reaction. Right after their observation that the efficiency of [IrCl(COD)]2-catalyzed PKR substantially increased by addition of a phosphane co-ligand, they moved directly to use chiral phosphanes and examined the enantioselectivity. " TON and TOE of the reaction were low and the number of examples was limited. Typically, the reaction required a fair amount of Ir(i) catalyst [IrCl(COD)]2 (0.1-0.15 equiv.) and (reaction time. However, this has remained as the best in terms of enantioselectivity to date. Moreover, this catalytic system provided the first asymmetric intermolecular reaction as well. 

At temperatures above ca. 200°C, the decarbonylation reaction can be driven catalytically (1,4,14, 20). Scheme I illustrates the proposed catalytic reaction scheme (15,16). This catalytic reaction is slow (activity for benzaldehyde decarbonylation at 178°C is 10 turnovers hr-1) presumably because the oxidative addition of RCOX to RhCl(CO)(PPh3)2 is difficult (7, 21, 22). Consistent with this, the rate is significantly greater when IrCl(CO)(PPh3)2 is used as the catalyst (benzaldehyde, 178°C, activity is 66 turnovers hr-1) (23). Oxidative addition to iridium complexes is well known to be more facile than with their rhodium analogues. 

Vaska s complex trans-IrCl(CO)(PPh ) has served as an important model for mechanistic investigation of catalytically relevant reactions such as the oxidative addition and reductive elimination of small molecules(15). The latter processes have also been the subject of some photochemical investigation. For example, the reductive elimination of H2 depicted in Equation 5, which is a relatively slow thermally activated process (k = 3.8 x 10- s l in 25° benzene solution (15)), has been shown to occur readily when the dihydride complex was subjected to continuous photolysis with 366 nm light(16). However, Vaska s compound itself was reported to be... 

The displacement of cyclooctene or C2H4 from an iridium(I) centre by a variety of chiral phosphines (L) leads to the formation of [ (L)IrCl 2] which, in conjunction with a source of F (phosphazenium fluoride), has been used for catalytic hydroamination of olefins. This combination leads to a 6.5 fold increase in the activity of the system and a total reversal in the enantioselectivity compared to that of the chloride analogue. There is no direct evidence of formation of a metal fluoride complex, but it is proposed that it may well form in situ and that this might explain these interesting results [75]. 

The chiral switch of the metolachlor was achieved in 1997. It was put on the market with a content of approximately 90% of the Sc active diastereomers. The key step of the large-scale enantioselective synthesis is the catalytic hydrogenation of the MEA imine shown in Figure 17. A mixture of [IrCl(l,5-cyclooctadiene)]2, the chiral diphosphine (i ,5)-xylyphos, iodide (as tetrabutylammonium or sodium salts) and acetic (30%) or sulfuric (at low... 

IrCl(N,)(PPh 0.> 19, 20), Co(PPh3)3N,> 91, 92) and CoH-(PPh3)3N2 (68). Although several of these complexes have been prepared from molecular nitrogen, the effectiveness of such complexes in catalytic nitrogen fixation remains to be demonstrated. 

Many catalytic reactions are not sensitive to the presence of a sulfur atom on the substrate. Two examples can be quoted the Nozaki-Hiyama-Kishi reaction where a chlorosilane-mediated Cr-Mn-catalyzed C-C coupling occurs between a halogenoalkene and an aldehyde [63], and the [IrCl(CO)3]-catalyzed intramolecular allyl transfer in functionalized 1,3-thiozanes [64]. 

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