Iridium alkoxides

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)... 

Although the precise mechanism has not yet been clarified, a possible mechanism is shown in Scheme 5.2. First, the iridium alkoxide 3 is produced from 1 and an alcohol, this step being stimulated by a base (K2CO3). A ]3-hydride elimination of 3 then yields a carbonyl product and the iridium hydride 4. The insertion of acetone into the iridium-hydride bond in 4, giving metal isopropoxide 5, is followed by exchange of the alkoxy moiety to regenerate 3. 

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

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. 

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... 

The mechanism for the iridium-catalyzed hydrogen transfer reaction between alcohols and ketones has been investigated, and there are three main reaction pathways that have been proposed (Scheme 4). Pathway (a) involves a direct hydrogen transfer where hydride transfer takes place between the alkoxide and ketone, which is simultaneously coordinated to the iridium center. Computational studies have given support to this mechanism for some iridium catalysts [18]. 

The carbonyl intermediate then reacts readily with a primary amine to afford an imine and water. A subsequent addition of the iridium hydride to the C=N double bond of the imine, followed by amide-alkoxide exchange, would then occur to release the product. 

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. 

P,S -Unsaturated alcohols undergo an oxidative esterification with aliphatic aldehydes in the presence of an iridium(I) catalyst and potassium carbonate.330 Precoordination of the ene-alkoxide with iridium is proposed, followed by reaction with aldehyde. Although the ester yield is high, a mixture of unsaturated and saturated esters is typically obtained, except for secondary alcohols. 

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... 

HYDROGENATION CATALYSTS Aretie-chnomium tricarbonyls. (1,5-Cycloocta-diene)(pyridine)(tricyclohexyl-phosphine)iridium(I) hexafluorophosphate. Di-fi.-chlorobis( 1,5-hexadlene)dirhodium. Lindlar catalyst. Palladuim(II) acetate-So-dium hydride-/-Amyl alkoxide. Rhodium catalysts. 

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 Oppenauer Oxidation. When a ketone in the presence of an aluminum alkoxide is used as the oxidizing agent (it is reduced to a secondary alcohol), the reaction is known as the Oppenauer oxidation. This is the reverse of the Meerwein-Ponndorf-Verley reaction (19-36) and the mechanism is also the reverse. The ketones most commonly used are acetone, butanone, and cyclohexanone. The most common base is aluminum ferf-butoxide. The chief advantage of the method is its high selectivity. Although the method is most often used for the preparation of ketones, it has also been used for aldehydes. An iridium catalyst has been developed for the Oppenauer oxidation, and also a water-soluble iridium catalyst An uncatalyzed reaction under supercritical conditions was reported. 

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... 

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