Iridium complexes, cations

C4H5N2S is the anion of l-methyl-2-mercaptoimidazole, the thiol sulfur may participate in coordination with three copper atoms (265) [80JCS(CC)867]. In the iridium complex cation 266 with 2-mercaptopyri-... 

By contrast, reaction of XeFi with the iridium carbonyl complex cation [Ir(CO)3(PEt3)2] in CH2CI2 results in addition across one of the Ir-CO bonds to give the first example of a metal fluoroacyl complex ... 

Schemes 6-13 Reaction of the cationic iridium complex 67 with carboxylic acids...

In 1998, Ruiz et al. reported the synthesis of new chiral dithioether ligands based on a pyrrolidine backbone from (+ )-L-tartaric acid. Their corresponding cationic iridium complexes were further evaluated as catalysts for the asymmetric hydrogenation of prochiral dehydroamino acid derivatives and itaconic acid, providing enantioselectivities of up to 68% ee, as shown in Scheme 8.18. 

Bergman s group showed that low-temperature selective C-H activations of -alkanes could be achieved using cationic-solvated iridium complexes (Equation (2)).10 10a... 

Double incorporation reactions of iV-benzylmaleimide with styrene analogs are catalyzed by cationic iridium complexes (Equation (102)). 

A two-component bimetallic catalytic system has been developed for the allylic etherification of aliphatic alcohols, where an Ir(i) catalyst acts on allylic carbonates to generate electrophiles, while the aliphatic alcohols are independently activated by Zn(n) coordination to function as nucleophiles (Equation (48)).194 A cationic iridium complex, [Ir(COD)2]BF4,195 and an Ru(n)-bipyridine complex196 have also been reported to effectively catalyze the O-allylation of aliphatic alcohols, although allyl acetate and MeOH, respectively, are employed in excess in these examples. 

Eisenberg reported product inhibition in the hydrogenation of acetone with a cationic iridium complex [42]. Apparently, the bond between iridium and isopropanol is too strong on account of the cationic nature of the complex (Scheme 44.7). 

The first example of dehydrogenation of alkanes by a transition metal complex was also achieved with iridium. The cationic iridium(III) complex, [IrH2-(acetone)2(PPh3)2]" BF4, with TBE (3,3-dimethyl-l-butene or t-butylethene) as... 

Several arylations involving reactive alkenes such as norbomene or allenes have been reported. Togni and coworkers have shown that norbomene is selectively added to the ortho positions of phenols to produce a mixture of 30 and 31 in 69% and 13% yield, respectively, after 72 hours at 100°C (22) [108, 109]. 1,1-dimethylallene also reacts with aromatic carboxamides (33) to produce prenylation products (34) in the presence of cationic iridium complexes (23) [110]. In both cases, initial ortho C-H bond activation in arenes directed by coordinating groups followed by olefin insertion has been proposed. 

Insertion of aUcynes into aromatic C-H bonds has been achieved by iridium complexes. Shibata and coworkers found that the cationic complex [Ir(COD)2]BF4 catalyzes the hydroarylation of internal alkynes with aryl ketones in the presence of BINAP (24) [111]. The reaction selectively produces ort/to-substituted alkenated-aryl products. Styrene and norbomene were also found to undergo hydroarylation under similar condition. [Cp IrCl2]2 catalyzes aromatization of benzoic acid with two equivalents of internal alkyne to form naphthalene derivatives via decarboxylation in the presence of Ag2C03 as an oxidant (25) [112]. 

Recently, Shibita et al. reported catalysis of alkyne insertion into an arylamide sp C-H bond to give allylamides (42) by a cationic iridium complex [118]. An interesting aspect of this work is the unusually selective cleavage of an sp C-H bond over sp aromatic C-H bonds so that the alkenyl arylamide (43) is only a very minor product (30). The carbonyl group is required for the reaction as no coupling... 

The cationic iridium complex [Ir(cod)(PPh3)2]OTf, when activated by H2, catalyzes the aldol reaction of aldehydes 141 or acetal with silyl enol ethers 142 to afford 143 (Equation 10.37) [63]. The same Ir complex catalyzes the coupling of a, 5-enones with silyl enol ethers to give 1,5-dicarbonyl compounds [64]. Furthermore, the alkylation of propargylic esters 144 with silyl enol ethers 145 catalyzed by [Ir(cod)[P(OPh)3]2]OTf gives alkylated products 146 in high yields (Equation 10.38) [65]. An iridium-catalyzed enantioselective reductive aldol reaction has also been reported [66]. 

As mentioned above, iridium complexes are also active in the formation of amines via the hydrosilylation/protodesUylation of imines. In the presence of 2 equiv. of HSiEts, the cationic complex [lr bis(pyrazol-l-yl)methane (CO)2][BPh4] (C4) catalyzes the reduction of various imines, including N-alkyl and N-aryl imines and both aldimines and ketimmes. Excellent conversions directly to the amine products were achieved rapidly at room temperature in a methanol solution (Scheme 14.7) [53]. 

With the knowledge that 14 can activate aldehydes in 1, the role of 1 in the reaction was explored further. Specifically, the relative rates of C—H bond activation and guest ejection, and the possibility of ion association with 1, were investigated. The hydrophobic nature of 14 could allow for ion association on the exterior of 1, which would be both cn t h al pi cal I y favorable due to the cation-it interaction, and entropically favorable due to the partial desolvation of 14. To explore these questions, 14 was irreversibly trapped in solution by a large phosphine, which coordinates to the iridium complex and thereby inhibits encapsulation. Two different trapping phosphines were used. The first, triphenylphosphine tris-sulfonate sodium salt (TPPTS), is a trianionic water-soluble phosphine and should not be able to approach the highly anionic 1, thereby only trapping the iridium complex that has diffused away from 1. The second phosphine, l,3,5-triaza-7-phosphaadamantane (PTA), is a water-soluble neutral phosphine that should be able to intercept an ion-associated iridium complex. 

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