Iridium unsaturated aldehydes

Reduction of unsaturated aldehydes seems more influenced by the catalyst than is that of unsaturated ketones, probably because of the less hindered nature of the aldehydic function. A variety of special catalysts, such as unsupported (96), or supported (SJ) platinum-iron-zinc, plalinum-nickel-iron (47), platinum-cobalt (90), nickel-cobalt-iron (42-44), osmium (<55), rhenium heptoxide (74), or iridium-on-carbon (49), have been developed for selective hydrogenation of the carbonyl group in unsaturated aldehydes. None of these catalysts appears to reduce an a,/3-unsaturated ketonic carbonyl selectively. 

Ir/tppts catalysts exhibit almost the same selectivity as Ru/tppts in the hydrogenation of a,p-unsaturated aldehydes albeit with approximately 70 times lower rates.485 In sharp contrast to the ruthenium and iridium based tppts catalysts, RhJ tppts complexes catalyse the chemoselective hydrogenation of a,fl-unsaturated aldehydes to the corresponding saturated aldehydes (Figure 14, III).54-485... 

Iridium,204,205 together with osmium, has been not widely used in catalytic hydrogenation. Recently, however, iridium or iridium-based catalysts have been shown to be effective in various hydrogenations, such as in selective hydrogenation of a,P-unsaturated aldehydes to allylic alcohols (Section 5.2), of aromatic nitro compounds to the corresponding hydroxylamines (Section 9.3.6), of halonitrobenzenes to haloanilines without loss of halogen (Section 9.3.2), and in the stereoselective hydrogenation of carbon to carbon double bonds (see, e.g., eqs. 3.25-3.27 and Table... 

Osmium and iridium catalysts have been shown to be highly selective for the formation of unsaturated alcohols by hydrogenation of a,P-unsaturated aldehydes without any additive. Good yields of allyl alcohol (73%), crotyl alcohol (90%), and cinnamyl alcohol (95%) (eq. 5.26) were obtained by the hydrogenation of acrolein, crotonaldehyde, and cinnamaldehyde, respectively, over 5% Os-C catalyst both with and without solvent.63... 

In the 1990s, BP re-examined the iridium-catalyzed methanol carbonylation chemistry first discovered by Paulik and Roth and later defined in more detail by Forster [20]. The thrust of this research was to identify an improved methanol carbonylation process using Ir as an alternative to Rh. This re-examination by BP led to the development of a low-water iridium-catalyzed process called Cativa [20]. Several advantages were identified in this process over the Rh-catalyzed high-water Monsanto technology. In particular, the Ir catalyst provides high carbonylation rates at low water concentrations with excellent catalyst stability (less prone to precipitation). The catalyst system does not require high levels of iodide salts to stabilize the catalyst. Fewer by-products are formed, such as propionic acid and acetaldehyde condensation products which can lead to low levels of unsaturated aldehydes and heavy alkyl iodides. Also, CO efficiency is improved. 

Silica-supported homogeneous catalysts, especially phosphino-iridium compounds, appear more promising in the hydrogenation of a,y9-unsaturated aldehydes, provided that their productivity can be improved and catalyst deactivation is avoided so that recycling of these materials could be meaningful [35]. 

ML Khidekel, EN Bakhanova, AS Astakhova, KhA Brikenshtein, VI Savchenko, IS Monakhova, VG Dorokhov. Preparation of unsaturated alcohols by the hydrogenation of a,P-unsaturated aldehydes in the presence of an iridium catalyst. Izvestiya Akademii Nauk SSSR, Seria Khimicheskaya 499, 1970. 

Recently, it has been shown by Xiao et al. that cyclometalated iridium(III) complexes can be switched on by controlling the solution pH to function as catalysts for the TH of carbonyl compounds in water using formate as the hydrogen source. Substituted benzophenones as well as alkyl aryl and dialkyl ketones were reduced in high yields (79-99 %) using 0.05 mol% of catalyst 202 (Fig. 61) [197]. Catalysts 203 and 204 were examined in the chemoselective TH of various a-substimted ketones, keto esters, and a,p-unsaturated aldehydes in water [198]. It was clearly shown that the pH of the reaction solution plays the crucial role, and pH... 

Hydrosilylation in the presence of a carbon electrophile is often accompanied by C-C bond formation. For example, three-component coupling of hydrosilane, alkyne, and y unsaturated aldehyde is suggested to proceed via oxanickelacycle intermediate to give (Z)-enol silyl ether (Scheme 3-28). Hydrosilylation of alkenes under a carbon monoxide atmosphere allows carbonyl incorporation, giving silyl enol ethers by using a cobalt or iridium catalyst (Schemes 3-29 and 3-30). Under similar reaction conditions in the presence of a rhodium catalyst, alkynes are converted to y silyl-substituted acroleins (Scheme 3-31). ... 

One of the most active and selective catalysts in this kind of reaction is undoubtedly Ir/support, as recently demonstrated by Jacobs and coworkers [276], Therefore, by combining the carbonyl affinity of metallic iridium with the promotion effect of the H-fl zeolite, which is a strong Bronsted acid, one can reduce a large variety of unsaturated ketones and aldehydes to allylic alcohols with high conversions, selectivities, and diasteieoselectivities. 

Complexes 6 undergo the second migratory insertion in this scheme to form the acyl complexes 7. Complexes 7 can react either with CO to give the saturated acyl intermediates 8, which have been observed spectroscopically, or with H2 to give the aldehyde product and the unsaturated intermediates 3. The reaction with H2 involves presumably oxidative addition and reductive elimination, but for rhodium no trivalent intermediates have been observed. For iridium the trivalent intermediate acyl dihydrides have been observed [29], The Rh-acyl intermediates 8 have also been observed [26] and due to the influence of the more bulky acyl group, as compared to the hydride atom in 2e and 2a, isomer 8ae is the most abundant species. 

Unstabilized enolates react with allylic carbonates in the presence of metalacyclic iridium-phosphoramidite catalysts. Although ketones and aldehydes have not yet been used directly as pronucleophiles with this catalyst system, silyl enol ethers [80] and enamines [81] react with linear allylic carbonates to form, after workup, p-branched, y-8 unsaturated ketones (Scheme 13). Both methods form products in high yield, branched selectivity, and enantioselectivity for a range of cinnamyl and alkyl-substituted allylic carbonates. However, the silyl enol ethers derived from aliphatic ketones reacted in lower yields than enamines derived from the same ketones. 

A possible mechanism for the P-alkylation of secondary alcohols with primary alcohols catalyzed by a 1/base system is illustrated in Scheme 5.28. The first step of the reaction involves oxidation of the primary and secondary alcohols to aldehydes and ketones, accompanied by the transitory generation of a hydrido iridium species. A base-mediated cross-aldol condensation then occurs to give an a,P-unsaturated ketone. Finally, successive transfer hydrogenation of the C=C and C=0 double bonds of the a,P-unsaturated ketone by the hydrido iridium species occurs to give the product. 

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

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. 

Pincer-ligated iridium complexes have been used as homogeneous catalysts for the dehydrogenation of aliphatic polyalkenes to give partially unsaturated polymers. The catalyst appears to be selective for dehydrogenation in branches as compared with the backbone of the polymer.56 The mechanism shown in Scheme 1 has been suggested for an [IrCl(cod)]2-catalysed oxidative esterification reaction of aliphatic aldehydes and olefinic alcohols.57... 

Hydrogen transfer reactions are catalyzed by several iridium complexes, including the dimethyl sulfoxide (DMSO) complexes cis- and trans-[Ir(Cl)4(DMSO)2]", [Ir(Cl)3(DMSO)3] and [lr(H)-(Cl)2(DMSO)3], as well as the cyclooctadiene (cod) complexes [Ir(Cl)(cod)]2 and [Ir(3,4,7,8-Me4phen)(cod)], and tra 5-[Ir(Cl)(CO)(PPh3)2]. Vaska s complex catalyzes the conversion of p-methoxybenzoyl chloride to the corresponding aldehyde. The dimethyl sulfoxide iridium(III) complexes catalyze hydrogen transfer from propan-2-ol to unhindered cyclohexanones to yield cyclohexanols, while the cod complexes serve as catalysts in the transfer of hydrogen from propan-2-ol to alkenes, ketones and a,/3-unsaturated ketones. ... 

Morken and coworkers [39b] developed the first asymmetric reductive aldol reaction with silanes as reductants in combination with a chiral rhodium catalyst. a,P-Unsaturated esters were reacted with several aldehydes to provide the corresponding aldol products 79 in good yields and enantio- and diastereoselectivities (Scheme 8.23). Both aliphatic and aromatic aldehydes could be converted into aldol products 79 under these conditions. Furthermore, the group reported an iridium-catalyzed asymmetric version that tolerated various protected hydroxyaldehydes [39aj. On the basis of this precedence, a highly enantio- and diastereoselective... 

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