Iridium chemoselectivity

Iridium hydride complexes effectively catalyze addition of nitriles or 1,3-dicarbonyl compounds (pronucleophiles) to the C=N triple bonds of nitriles to afford enamines.42S,42Sa Highly chemoselective activation of both the a-C-H bonds and the C=N triple bonds of nitriles has been observed (Equation (72)). To activate simple alkane dinitriles, IrHs(P1Pr3)2 has proved to be more effective (Equation (73)). The reaction likely proceeds through oxidative addition of the a-C-H bonds of pronucleophiles to iridium followed by selective insertion of the CN triple bonds to the Ir-C bond. 

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-monotosylated ethylenediamine [Ts(en)] and Ir-CF3Ts(en) are highly active and chemoselective catalysts for the aqueous-phase transfer hydrogenation of aldehydes using sodium formate as the hydrogen donor.376... 

An important feature of the present reaction is the chemoselective addition of activated nitriles to the CN triple bonds of nitriles in the presence of carbonyl groups, because of the strong coordination ability of nitriles toward metals. The iridium-catalyzed addition of ethyl cyanoacetate to 4-acetylbenzonitrile (30) gives ethyl (Z)-3-(4-acetylphenyl)-3-amino-2-cyano-2-propenoate (31, 59%) chemoselec-tively, while the same reaction promoted by a conventional base such as AcONH4 and NaOH gives ethyl 2-cyano-3-(4-cyanophenyl)-2-butenoate (32) ( Z= 55 45) [20]. 

Owing to strategic reasons, Lonza exited the dextrometorphan business before the process could be transferred. Subsequently a further study of the hydrogenation of 10, using iridium complexes with chiral amidophosphine-phosphonite ligands, was published by another group [14]. An enantioselectivity of 86% ee was reached, but the chemoselectivity was low. 

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... 

Heteroaromatic substrates have been productive targets for both TH and ATH reactions. Quinoline and isoquinolium salts can be reduced by Ru(II) and Rh(III) catalysts by ATH [104, 105]. Chemoselective reductions of quinolines were achieved by TH using IPA with an iridium catalyst (Fig. 31) [106]. 

Bhat S, Sridharan V (2012) Iridium catalysed chemoselective alkylation of 2 -aminoacetophenone with primary benzyl type alcohols under microwave conditions. Chem Commiin 48(39) 4701 703... 

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