Iridium Phosphines

To date, the only soluble catalysts reported to have significant activity in mediating exchange between atmospheric (isotopic) hydrogen and substrates in solution are organometallic complexes of iridium. They can be considered as consisting of three classes iridium phosphine complexes, iridium dionate complexes, and iridium cyclopentadiene complexes. 

The class first to be investigated, and to date the most extensively studied, are iridium phosphines. The first study of labeling by this method utilized the complex [Ir(H)2(acetone)2(PPh3)2]BF4. Test substrates became labeled to high isotopic abundance 

With this simple mechanistic model, it became possible to anticipate whether and where any new compound might be labeled by this method. The isotopes introduced by this method are generally introduced into positions where they are chemically stable and in most cases metabolically stable, making the method valuable for biological and pharmaceutical research applications. Few other isotope exchange methods possess this degree of predictability. 

The first reported application of the new method with tritium was on the steroid 104. 

An important feature of the iridium phosphine class of complexes is that the triphenyl-phosphine hgands can be replaced with a wide variety of phosphine and related donor 

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Schemes 6-4 Oxidative addition of water and methanol to cationic iridium phosphine complex 37...

Schemes 6-5 Oxidative addition of water to neutral iridium phosphine complexes...

Schemes 6-8 Oxidative addition of aicohois to neutrai iridium phosphine compiexes...

Schemes 6-10 Oxidative addition of alcohol, phenol, and water to cationic iridium phosphine complex 67...

The iridium phosphine complex [IrC PEt,),] 39b can also activate O-H bonds of carboxylic acids. The stoichiometric reaction with a,(o-alkynoic acids RC=C(CH2)2 CO2H (R = Me, Ph) gave cis-hydrido(carboxylato)iridium(III) complexes 92 (Eq. 6.26), and the molecular structure of 92a was determined crystallographically [59]. 

The fourth chapter gives a comprehensive review about catalyzed hydroamina-tions of carbon carbon multiple bond systems from the beginning of this century to the state-of-the-art today. As was mentioned above, the direct - and whenever possible stereoselective - addition of amines to unsaturated hydrocarbons is one of the shortest routes to produce (chiral) amines. Provided that a catalyst of sufficient activity and stabihty can be found, this heterofunctionalization reaction could compete with classical substitution chemistry and is of high industrial interest. As the authors J. J. Bmnet and D. Neibecker show in their contribution, almost any transition metal salt has been subjected to this reaction and numerous reaction conditions were tested. However, although considerable progress has been made and enantios-electivites of 95% could be reached, all catalytic systems known to date suffer from low activity (TOP < 500 h ) or/and low stability. The most effective systems are represented by some iridium phosphine or cyclopentadienyl samarium complexes. 

In the hydrogenation of alkenes, rhodium-, ruthenium- and iridium-phosphine catalysts are typically used [2-4]. Rhodium-phosphine complexes, such as Wilkinson s catalyst, are effective for obtaining alkanes under atmospheric pres-... 

In a subsequent detailed study, other iridium phosphine systems prepared in situ were used. Depending on the steric properties of the phosphine employed and... 

Spogliarich et al. published the details of a study [15] on the electronic effect implicated in reactions catalyzed by iridium/phosphine systems, and found there to be a slight dependence on the charge distribution around the carbonyl group of the substrates, while electron-withdrawing groups enhance the reduction rate... 

In contrast to the Pt(0) and Pt(II) complexes and the corresponding Rh(I) and Rh(III) complexes, the iridium complexes have rarely been employed as hydrosilylation catalysts [1-4]. Iridium-phosphine complexes with d metal configura-tion-forexample, [Ir(CO)Cl(PPh3)2] (Vaska s complex) and [Ir(CO)H(PPh3)3]-were first tested some 40 years ago in the hydrosilylation of olefins. Although they underwent oxidative addition with hydrosilanes (simultaneously to Rh(I) com-... 

Iridium-phosphine complexes were found to be efficient carbonylative alkyne-alkene coupling catalysts [62]. Although frequently applied in other transformations, the dimeric complex [ Ir( x-Cl)(cod) 2] appeared to be a very active catalyst in the coupling of silylated diynes with CO [63], giving bicyclic products with a carbonyl moiety (Scheme 14.12). 

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

WINK AND FORD Chemistry of Rhodium and Iridium Phosphine Complexes 201... 

At this juncture, Danishefsky decided to investigate the possibility of setting the C(ll) and C(17) methyl stereocentres of 23 by hydroxyl-directed hydrogenation.13 Some years earlier, Evans had shown that cationic rhodium- and iridium-phosphine complexes can mediate highly diastereoselective reductions of trisubstituted homoallylic alcohols. However, for excellent stereoselectivities to generally be observed, it... 

This ring thereby constitutes an unusual yu-ij. ij -vinyl group whereby the phosphino carbon is also metalated. Intramolecular oxidative addition of the ortho C-H bond of the phenyl substituents is a common feature in crowded iridium phosphine complexes [65, 309, 310]. 

Asymmetric hydrogenation of trisubstituted aryl alkenes and aryl alkene esters using iridium-phosphine thiazole complexes 220 have been reported <06JA2995>. The tetrahydrobenzo[rf]thiazole complex (220b) delivers higher enantioselectivity than the cyclopenta[(7]thiazole and cyclohepta[r/]thiazole counterparts (220a and 220c), and replacement of thiazole moiety with oxazole dramatically reduces the enantioselectivity. The... 

A successful study of non-phosphine iridium complexes Ir", Ir , and Ir e. g., IrX(cod)2 [60], IrH2(triso)(SiMePh2)2 [61, 62], Ir(triso)(coe)2 (coe = cyclooctene triso = tris(diphenyloxophosphoranyl)methanide), Ir(triso)(C2H4)2 [61], has demonstrated effective hydrosilylation of alkenes and alkynes. Iridium phosphine complexes, e. g., Ir(C=CPh)(CO)2PCy2 [63] and IrCl(CO)(PPh3)2 [64], are also found to be active for hydrosilylation of phenylacetylene and 1-hexyne. 

As shown in Eq. (6.1), for example, Vaska and DiLuzio reported in 1962 that hydrogen molecules react with iridium phosphine compounds at room temperature to produce a hydrogen complex [9,10]. 

In contrast to the studies with the iridium-phosphine complexes, the very reactive complex shown in [10] reacts with alkyl halides as shown in (30), but in the presence of a large excess of LiCI the reaction of Bu"Br yields the chloro-complex under conditions where the corresponding bromo-complex does not exchange. These observations, together with the isolation of the intermediate, /ranj-[Rh(Me)(Et2mgBF2)(NCMe)] BF 4 and the reactivity order with respect to the alkyl group (Me > Et > secondary alkyl > cyclohexyl), supports an Sn2 mechanism (Collman and MacLaury, 1974). 

Chiral aldehydes can be decarbonylated under full retention of the configuration, but occasionally partial racemization may take place [10]. In general, decarbonylation with stoichiometric rhodium-arylphosphine complexes can be achieved at ambient temperature, but usually the catalytic version requires more severe conditions. Goldman et al. [11] discovered that trialkylphosphines form significantly more active catalysts, as exemplified with the binuclear complex [Rh(PMe3)(CO)Cl]2. The complex operates even at room temperature. A similar effect was also noted with iridium-phosphine catalysts [12]. 

None of these three types of ESO2 has been identified in the iron model systems. However, the type of 41 has been isolated within rhodium phosphine complex (45) [109], and the type of 43 has been isolated within an iridium phosphine complex (46) [110, 111]. These results support the possibility of the iron analogues 41 and 43, but results obtained with different types of metal do not necessarily apply to iron. The high stability of the rhodium and iridium complexes leads to the very low reactivity for oxygenation. 

Sertchook, H. Avnir, D. Blum, J. Joo, F. Katho, A. Schumann, H. Weimann, R. Wernik, S. (1996) Sol-gel entrapped lipophilic and hydrophilic mthenium-phosphine, rhodium-phosphine, and iridium-phosphine complexes as recyclable isomerization catalysts, J. Mol. Catal. A. - Chem., 108,153-60. 

CPDN 394 is an important intermediate for the synthesis of corannulene 23 it can be prepared by [2-F2-F1] reaction of 1,8-diethynylnaphthalene 395 using Fe(CO)5 upon demetallation procedure (Scheme 6.97) [235]. Later, Shibata reported the iridium complex-catalyzed carbonylative alkyne-alkyne coupling, which provides 394 in high isolated yields without demetallation procedure. The iridium phosphine complexes, [IrCl(CO)2(PPh3)2l enables the catalytic coupling under carbon monoxide at atmospheric pressure or less (Scheme 6.97) [236]. 

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