Iridium alkoxide complexes

The hydrido(ethoxo) complex carrying an electron-donating q -CsMes (= Cp ) ligand, [Cp IrH(OEt)(PPh3)] (4), was prepared by a metathesis reaction between [Cp Ir Cl2(PR3)] (3) and NaOEt followed by P-H elimination from the intermediate diethox-ide complex (Eq. 6.4) [7]. Several other iridium alkoxide analogs [Cp IrH(OR)... 

Iridium alkoxides, synthesis, 7, 383 Iridium alkyl complexes reactivity, 7, 284-... 

Shopov DY, Rudshteyn B, Campos J, Batista VS, Crabtree RH, Brudvig GW. Stable iridium(IV) complexes of an oxidation-resistant pyridine-alkoxide ligand highly divergent redox properties depending on the isomeric form adopted. J Am Chem Soc. 2015 137 7243-7250. 

Hydroaryloxylation of terminal alkenes RCH=CH2 with phenols ArOH can be catalysed by the pincer-iridium complex (133) at 150 °C to afford the corresponding Markovnikov ethers RCH(OAr)-CH3 as an attractive alternative to the Williamson synthesis. The reaction is believed to proceed via alkene insertion into an iridium-alkoxide bond, followed by the rate-determining C-H reductive elimination. ... 

Since the iridium(III) complex [(Cp )IrCl2]2 (Cp = pentamethylcyclo-pentadienyl) is an active catalyst for the p-alkylation of secondary alcohols with primary alcohols, a series of iridium(III) complexes 26-28 bearing a Cp unit tethered to an imidazolyhdene was synthesized (Equation (8.15)). These complexes displayed similar activities in the p-alkylation of secondary alcohols with primary alcohols as electrophiles (Equation (8.15)), and surpassed the performance of their parent compound [(Cp )IrQ2]2. Control of the reaction time was found to be crucial to avoid the undesirable dehydrogenation of the product (see Section 8.4.2 for further details). The sequence of catalytic reaction steps was thought to involve the oxidation of both alcohols and the formation of an iridium hydride species. Base-promoted cross-aldolization and elimination to form the ot-enone and hydrogenation of the C=C and C=0 bonds to regenerate an iridium-alkoxide species would complete the cycle. 

Hydrogen elimination from iridium alkyl or alkoxide derivatives is also a general pathway to generate Ir-H species. The mechanism of the reversible /3-hydrogen elimination from square-planar Ir(i) alkoxide complexes has been studied in detail by Hartwig," Milstein," " " and their co-workers. 

The acetylacetonates are stable in air and readily soluble in organic solvents. From this standpoint, they have the advantage over the alkyls and other alkoxides, which, with the exception of the iron alkoxides, are not as easily soluble. They can be readily synthesized in the laboratory. Many are used extensively as catalysts and are readily available. They are also used in CVD in the deposition of metals such as iridium, scandium and rhenium and of compounds, such as the yttrium-barium-copper oxide complexes, used as superconductors. 1 1 PI Commercially available acetyl-acetonates are shown in Table 4.2. 

A similar type of immobilization was obtained by reacting the phosphonylated 2,2 -bipyridine ligand depicted in Figure 42.10 with excess titanium alkoxide. Rhodium and iridium complexes of this immobilized ligand showed activity for... 

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. ... 

The mechanism operating in rhodium-catalyzed and iridium-catalyzed hydrogen transfer reactions involves metal hydrides as key intermediates. Complexes such as [ M(p.-C1)(L2) 2], [M(cod)L2](Bp4) (M = Rh, Ir L2 = dppp, bipy), and [RhCl(PPh3)3] are most likely to follow the well-established mechanism [44] via a metal alkoxide intermediate and elimination to generate the active hydride species, as shown in Scheme 2. 

In accessing chiral allyl vinyl ethers for Claisen rearrangement reactions, Nelson et al. employed the iridium-mediated isomerization strategy. Thus, the requisite enantioenriched diallyl ether substrate 28 was synthesized via a highly enantioselective diethylzinc-aldehyde addition protocol10 (Scheme 1.1k). The enantioselective addition of Et2Zn to cinnamaldehyde catalyzed by (—)-3-exo-morpholinoisobomeol (MIB 26)11 provided an intermediate zinc alkoxide (27). Treatment of 27 with acetic acid followed by 0-allylation in the presence of palladium acetate delivered the 28 in 73% yield and 93% ee. Isomerization of 28 with a catalytic amount of the iridium complex afforded the allyl vinyl ether... 

An interesting variant is the in situ preparation of transition metal alkoxides from the corresponding halogenides and subsequent reaction with an azolium salt to form the NHC transition metal complex [69]. This works particularly well with rhodium, iridium and ruthenium where [(ii -cod)MCl]j (M = Rh, Ir) and [Cp RuCl]2 are readily available [57,58,71]. 

The bridging chloride ligands in these [Ir(olefin)2Cl]2 compounds are susceptible to metathesis reactions, yielding new dimeric compounds of the form [Ir(olefin)2B]2 where B represents a new bridging ligand. Alkoxides, thiolates, and carboxylates have all been employed successfully in the replacement of chloride. The complexes with B = Br, I have also been prepared, both by metathesis reactions and by direct reaction of cyclooctene or cyclooctadiene with IrBrs or Irls. The olefin complexes also provide excellent starting materials for the syntheses of arene and cyclopentadienyl iridium complexes, a subject that will be discussed in the next section. 

A vast majority of the allylic substitution reactions have been reported with palladium catalysts. However, complexes of other metals also catalyze allylic substitution reactions. In particular, complexes of molybdenum,tungsten, ruthenium, rhodium, and iridium " have been shown to catalyze the reactions of a variety of carbon nucleo-pliiles. In addition, complexes of ruthenium, rhodium, and iridium catalyze the reactions of phenoxides, alkoxides, amines, and amine derivatives. " The regioselectivity of the allylic substitution process witli these metals can often complement the regioselectivity of the reactions catalyzed by palladium complexes. The regioselectivity... 

More recently, Hartwig and co-workers showed that iridium complexes of the phosphoramidite ligand shown in Equation 20.48 catalyzes these reactions with amines, ary-loxides, the combination of alkali metal alkoxide and Cul, the combination of alcohol and base, trifluoroacetamide, azoles, and ketone enolate equivalents with enantiomeric excess near or above 95% in most They have also shown that this irid-... 

Given the isoelectronic relationship between [CR] and [NO] and the ubiquity of this latter ligand in the coordination chemistry of later transition metals, the scarcity of mononuclear alkylidyne complexes of metals from groups 8-10 is surprising [1-4]. Isolated examples have been reported for iron [5], cobalt [6], ruthenium [4,7], osmium [4,8-9] and iridium [10]. Most of the examples known employ routes with extensive precedent in early transition metal systems, i.e., either electrophilic attack at the p-atom of a hetero carbonyl (CS [5], CTe [4], or C=CH2 [10]) or the Lewis-acid assisted abstraction of an alkoxide group from a carbene precursor [5] (Scheme 1). The one approach which is, too date, peculiar to group 8 metals involves reduction of a divalent dichlorocarbene complex by lithium aryls [4]. The limitation of this procedure to ruthenium and osmium is presumably not a feature of these metals but rather a result of the present lack of synthetic routes to suitable dihalocarbene precursor complexes of earlier metals. 

Sulphur dioxide inserts into palladium-oxygen bonds. When a solution of palladium chloride in an alcohol is treated with sulphur dioxide, a sulphinato-complex is produced, presumably by insertion into a palladium-alkoxide bond. The reaction of sulphur dioxide with molecular oxygen complexes of platinum and of iridium, producing sulphato-complexes, involves insertion of sulphur dioxide into one of the metal-oxygen links, as shown in Scheme 5. ... 

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