Iridium direct synthesis

A diverse group of secondary and tertiary amines are readily synthesized from the reaction of primary and secondary amines with allylic carbonates in the presence of preformed iridium metalacycles, but the direct synthesis of primary amines via iridium-catalyzed allylic amination requires the use of ammonia as a nucleophile. The asymmetric allylation of ammonia had not been reported until very recently, and it is not a common reagent in other metal-catalyzed reactions. Nonetheless, Hartwig and coworkers developed the reactions of ammonia with allylic carbonates in the presence of la generated in situ [89]. Reactions conducted in the initial work led exclusively to the products from diallylation (Scheme 16). Further advances in... 

Aqueous organometalHc catalysis allows the use of NH3-solutions in water for the direct synthesis of amines from olefins in a combined hydroformylation/reductive amination procedure (Scheme 4.19). The hydroformylation step was catalyzed by the proven Rh/TPPTS or Rh/BINAS (44) catalysts, while the iridium complexes formed from the same phosphine ligands and [ IrCl(COD) 2] were found suitable for the hydrogenation of the intermediate imines. With sufficiently high NH3/olefin ratios (8/1) high selectivity towards the formation of primary amines (up to 90 %) could be achieved, while in an excess of olefin the corresponding... 

Machado AS, Oleskta- A, Castillon S, Lukacs G (1985) Hydroxy group directed hydrogenation with rhodium and iridium catalysts. Synthesis of protected chiral carbocyclic analog of daunosamine. J Chem Soc Chem Commim 530-532... 

Can the iridium chemistry tolerate substrates with primary alcohols Need to generate an aryltrifluoroborate salt Yes, a version of the iridium-catalyzed C—H borylation is tolerant to the presence of primary alcohols [75] A convenient precursor for the direct synthesis is not readily available. Try generating an arylboronic acid or arylboronate, and treat it with a solution of KHP [37, 38]. For arylboronates, a 4 1 THF/water mixture was successful [38]... 

These recent mechanistic studies have provided the foundation for the most recent work that has expanded the scope of iridium-catalyzed allylic substitution. The synthesis and characterization of the ethylene-bound complex lb resulted directly... 

The use of ethylene adduct lb is particularly important when the species added to activate catalyst la is incompatible with one of the reaction components. Iridium-catalyzed monoallylation of ammonia requires high concentrations of ammonia, but these conditions are not compatible with the additive [Ir(COD)Cl]2 because this complex reacts with ammonia [102]. Thus, a reaction between ammonia and ethyl ciimamyl carbonate catalyzed by ethylene adduct lb produces the monoallylation product in higher yield than the same reaction catalyzed by la and [Ir(COD)Cl]2 (Scheme 27). Ammonia reacts with a range of allylic carbonates in the presence of lb to form branched primary allylic amines in good yield and high enantioselectivity (Scheme 28). Quenching these reactions with acyl chlorides or anhydrides leads to a one-pot synthesis of branched allylic amides that are not yet directly accessible by metal-catalyzed allylation of amides. 

The direct silylation of arenes through C—H bond activation provides an attractive route for the synthesis of useful aromatic compounds [64]. Vaska s complex was the first of the iridium catalysts to be reported for activation of the C—H bond in benzene by Si—H of pentamethyldisiloxane to yield phenylsubstituted siloxane [65]. However, a very attractive method for the aromatic C—H silylation with disilanes has been recently reported by the groups of Ishiyama and Miyaura [66-68]. 

In support of the conclusion based on silver, series of 0.2, 0.5, 1.0, 2.0, and 5.0 % w/w of platinum, iridium, and Pt-Ir bimetallic catalysts were prepared on alumina by the HTAD process. XRD analysis of these materials showed no reflections for the metals or their oxides. These data suggest that compositions of this type may be generally useful for the preparation of metal supported oxidation catalysts where dispersion and dispersion maintenance is important. That the metal component is accessible for catalysis was demonstrated by the observation that they were all facile dehydrogenation catalysts for methylcyclohexane, without hydrogenolysis. It is speculated that the aerosol technique may permit the direct, general synthesis of bimetallic, alloy catalysts not otherwise possible to synthesize. This is due to the fact that the precursors are ideal solutions and the synthesis time is around 3 seconds in the heated zone. 

Alternative synthetic approaches include enantioselective addition of the organometallic reagent to quinoline in the first step of the synthesis [16], the resolution of the racemic amines resulting from simple protonation of anions 1 (Scheme 2.1.5.1, Method C) by diastereomeric salts formation [17] or by enzymatic kinetic resolution [18], and the iridium-catalyzed enantioselective hydrogenation of 2-substituted quinolines [19]. All these methodologies would avoid the need for diastereomer separation later on, and give direct access to enantio-enriched QUINAPHOS derivatives bearing achiral or tropoisomeric diols. Current work in our laboratories is directed to the evaluation of these methods. 

Looking first at alcohol-directed reductions62, it is apparent that there have been many studies of the reduction of allylic and homoallylic alcohols using both the neutral and cationic reduction complexes based on rhodium, iridium etc. In the case of cyclic substrates where an alcohol is located on one side of a ring, the hydrogen is simply delivered cis to the alcohol function63. This is illustrated by key reduction steps in the synthesis of monensin (Scheme 5)64 and the marine natural product arenarol (Scheme 6)65. In each... 

Tilley and coworkers reported a more direct procedure for the synthesis of late transition metal silylenoid complexes by reaction with silanes [equation (7.1)].36,37 Exposing iridium complex 14 to dimesitylsilane afforded iridium silylenoid 15.36 In addition to dimesitylsilane, other silanes could be used, including diphenyl-, diethyl-, or dimethylsilane. Primary silanes such as mesitylsilane and 2,4,6-triisopropylphe-nylsilane were also tolerated as substrates.37... 

Examples for o-phenylene scaffolds for bis-carbene ligands come from the research groups of Peris [344,345] and Herrmann [346]. Synthesis of the bis-imidazolium salt is achieved by reaction of a,a -xylene dichloride and the N-substituted imidazole. The rhodium(l) and iridinm(I) complexes can then be made by addition of the imidazolium salt to a solution of [M(cod)Cl]2 (M = Rh, Ir) in ethanol or acetonitrile (with NEtj as auxiliary base) (see Figure 3.108). The rhodium complexes were used successfully in the hydrosi-lylation of styrene [344] whereas both the rhodium and iridium complexes were used for the direct borylation of arenes making functionalised arylboronic acid esters accessible by a simple one-pot reaction [346]. 

D.J. Williams, J.S. Kruger, A.F. McLeroy, A.P. Wilkinson, and J.C. Hanson, Iridium(III) Amine Complexes as High-stability Structure-directing Agents for the Synthesis of Metal Phosphates. Chem. Mater., 1999, 11, 2241-2249. 

Me]x-MCM-41 containing nanosized particles of platinum, palladium, rhodium, ruthenium and iridium were directly synthesised from surfactant stabilised spherical metal nanoparticles in the synthesis gel, and characterised with XRD, ICP-AES, TG/DSC, TEM, nitrogen physisorption and carbonmonoxide chemisorption, and Si MAS NMR. During the synthesis some agglomeration of the particles took place, but the metal particles were present inside the pore system of MCM-41. The matericils were active and selective catalysts in the hydrogenation of cyclic olefins such as cyclohexene, cyclooctene, cyclododecene and norbomene. 

In contrast to cobalt, simple alkene complexes of rhodium and iridium have been the subjects of prolific research, much of it ultimately directed toward the development of catalysts for hydrogenation and C—H bond activation processes. In view of the expansive literature on these materials, it would seem appropriate to consider their synthesis and structural facets separately from their reactivity. 

The first example of direct enantioselective addition of a C—H bond to a ketone was reported by Shibata and co-workers in 2009 using the cationic Ir/ (S)-Hg-BINAP as the catalyst in the synthesis of a chiral 4-acetyl-3-hydroxy-3-methyl-2-oxindole with 72% ee. Recently, Yamamoto and co-workers developed a cationic Ir/(R,R)-Me-BIPAM catalyzed asymmetric intramolecular direct hydroarylation of a-keto amides 178 affording the chiral 3-substi-tuted 3-hydroxy-2-oxindoles 179 in high yields with complete regioselectivity and high enantioselectivity (84-98% ee). In their proposed reaction mechanism, the aryl iridium complex formed via C—H bond activation is coordinated with the two carbonyl groups of the amide (Scheme 5.65). 

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