Iridium complexes chelating ligands

In 2004, Bolm et al. reported the use of chiral iridium complexes with chelating phosphinyl-imidazolylidene ligands in asymmetric hydrogenation of functionalized and simple alkenes with up to 89% ee [17]. These complexes were synthesized from the planar chiral [2.2]paracyclophane-based imida-zolium salts 74a-c with an imidazolylidenyl and a diphenylphosphino substituent in pseudo ortho positions of the [2.2]paracyclophane (Scheme 48). Treatment of 74a-c with t-BuOLi or t-BuOK in THF and subsequent reaction of the in situ formed carbenes with [Ir(cod)Cl]2 followed by anion exchange with NaBARF afforded complexes (Rp)-75a-c in 54-91% yield. The chela-... 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

Except for one recent example, all iridium-catalyzed allylic substitution reactions have been performed under an inert atmosphere with dry solvent and reagents. The iridium metalacycle is sensitive to protonation, which opens the metalacycle and results in the formation of a less-active complex containing a K -phosphoramidite ligand. A paper by Helmchen et al. addressed this issue [107]. Nearly all iridium catalysts used for allylic substitution consist of an iridium fragment chelated by COD. In the presence of a catalyst containing dibenzo[a,c]cyclooctatetraene (dbcot) in place of COD, allylic substimtion reactions occur in air with results that are comparable to those of reactions performed under an inert atmosphere (Scheme 35). 

In contrast, 1,5-cyclo-octadiene remains coordinated during the catalytic cycle of hydrogenation of phenylacetylene to styrene, catalyzed by the related iridium complex [Ir(C0D)( Pr2PCH2CH20Me)]BF4. This complex, which contains an ether-phosphine-chelated ligand, catalyzes the selective hydrogenation reaction via a dihydrido-cyclo-octadiene intermediate. The reaction is first order in each of catalyst, phenylacetylene and hydrogen [11] the proposed catalytic cycle is shown in Scheme 2.3. 

The beneficial effects of chelating ligands were also demonstrated by Hartwig, Ishiyama and Miyaura [62]. This group isolated the iridium(I) complex [lr(Bpin)3(COE)(DTBPY) modified with simple 2,2 -bipyridine ligands (such as 4,4 -di-tert-butyl-2,2 -bipyridine DTBPY), which seemed to be responsible for the first catalytic C—H borylation at room temperature (Scheme 7.30). An extension... 

Cyclometallated iridium complexes, for OLEDs, 12, 145 Cyclometallated palladium(II) complexes from amines and pyridines, 8, 280 with C,C-chelating ligands, 8, 291 enantioselective synthesis, 8, 296 ferrocene-based palladacycles, 8, 292 four-membered palladacycles, 8, 297 imine- and oxime-based complexes, 8, 285 with N-N and N=N bonds, 8, 288 palladacycle catalysis, 8, 297... 

Okazaki, M., Kawano, Y., Tobita, H., Inomata, S., and Ogino, H. (1995) Light-and heat-induced isomerization of chloro (hydrido)iridium(III) complex containing a (2-phosphinoethyl)silyl chelate ligand. Chemistry Letters, 1005—1006. 

Alkyl and aryl isothiocyantes, RCNS, parallel CS2 in their capability to insert into M—H bonds and yield products having the JV-alkyl- or JV-aryl-formamide chelate ligand RN=CH=S. The iridium(III) complexes [Ir(X)2(RN=CH=S)(PPh3)2] (207 X = Cl, Br R = Me, Et, Ph) can be prepared from trans-[Ir(H)(X)2(PPh3)2] and the appropriate alkyl or aryl isothiocyanate. Two isomeric forms of (207) were deemed plausible, with (207a) the preferred stereochemistry for the alkyl products and (207b) for the aryl products.453... 

Iridium complexes such as IrCl(COD)(L-L), [Ir(COD)(L-L)]+ and [Ir(COD)-(py)(PR3)2]+ are active catalysts for the hydrogenation of a variety of substrates, particularly tetrasubstituted alkenes, e.g. [Ir(COD)(py)(PCy3)] known as Crabtree s catalyst, hydrogenates tetramethylethylene. On the other hand, [Ir(COD)(py)2]+ is inactive, apparently since it does not add H2. The hydrido cation [Ir(H)2(COD)(i72-Pr,2PCH2CH2OMe)]+, with a chelating P—O ligand, selectively hydrogenates phenylacetylene to styrene, via Ir—CH=CHPh intermediates.2... 

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