1,5-Cyclooctadiene, complex with iridium

The bridging chloride ligands in these [Ir(olefin)2Cl]2 compounds are susceptible to metathesis reactions, yielding new dimeric compounds of the form [Ir(olefin)2B]2 where B represents a new bridging ligand. AUcoxides, thiolates, and carboxylates have all been employed successfully in the replacement of chloride. The complexes with B = Br, I have also been prepared, both by metathesis reactions and by direct reaction of cyclooctene or cyclooctadiene with IrBrs or Iris The olefin complexes also provide excellent starting materials for the syntheses of arene and cyclopentadienyl iridium complexes, a subject that will be discussed in the next section. 

Reduction of the more unsaturated 3-oxygenated menthanes is a well-known route to menthones thus treatment of pulegone (46) with cyclooctadiene and an iridium catalyst yielded 61% menthone (521) and 39% isomenthone (583). cw-Piperitol (584) was also converted to Isomenthone (583) by treatment with low-valence cobalt complexes coordinated with phosphines. ... 

In contrast to rhodium, the majority of work with iridium has focussed upon the chemistry of these materials, with little effort expended on attempts to elucidate structural systematics, though a small number of cyclooctadiene complexes (177-188, Table II) have been prepared from Ir2(jU-Cl)2(l,5-COD)2 and MTp (M = Na, K, or Tl) largely to this end. 

Dinuclear Pt-Rh (336a-c, 339) and Pt-Ir (337a-c, 338) complexes with doubly alkynyl bridging systems have been isolated from the reactions between platinum alkynyl complex, 40 and from cyclooctadiene complexes of rhodium and iridium (Scheme 91). ... 

A wide range of carbon, nitrogen, and oxygen nucleophiles react with allylic esters in the presence of iridium catalysts to form branched allylic substitution products. The bulk of the recent literature on iridium-catalyzed allylic substitution has focused on catalysts derived from [Ir(COD)Cl]2 and phosphoramidite ligands. These complexes catalyze the formation of enantiomerically enriched allylic amines, allylic ethers, and (3-branched y-8 unsaturated carbonyl compounds. The latest generation and most commonly used of these catalysts (Scheme 1) consists of a cyclometalated iridium-phosphoramidite core chelated by 1,5-cyclooctadiene. A fifth coordination site is occupied in catalyst precursors by an additional -phosphoramidite or ethylene. The phosphoramidite that is used to generate the metalacyclic core typically contains one BlNOLate and one bis-arylethylamino group on phosphorus. 

Because of the relatively poor coordinating properties displayed by the P-menthyl-substituted monodentate ligands toward some catalytically useful metals like rhodium and iridium, development of the coordination chemistry of chelating phosphetanes was required. Early studies established that the bidentate ligand P(6 ),C (6 )-43 binds well to rhodium centers. It gives the chelating complex 26 with [Rh(COD)2]PF6 and the bimetallic compound 68 when reacted with [Rh(COD)Cl]2 under an atmosphere of CO (COD = cyclooctadiene Scheme 5) <19950M4983>. 

An NMR study (597) of ligand exchange in the system (diene)MCl(L) (diene = norbornadiene or 1,5-cyclooctadiene M = Rh or Ir L = tertiary phosphine, arsine, or stibene) shows a first-order dependence of the rate upon both L and the olefin complex in the temperature range from —70° to —10°C. The exchange involves an 8 2 mechanism with the five-coordinate complex (diene)MCl(L)2 as intermediate. The intermediate iridium complexes (l,5-CgHi2)IrCl(L)2 can be isolated from ethanolic solution. The activation energy for the process ranges from 4 to 10 kcal/mole (597). 

Again like rhodium, iridium forms alkene complexes. Examples are the cyclooctene or 1,5-cyclooctadiene (COD) compounds, e.g., [IrCl(COD)2]2, formed by boiling (NH4)2irCl6 with the olefin in alcohols this product can be converted into lrCH3(COD)(PMe2Ph)2, which shows unusual flux-ional behavior.13 Ethylene forms the unusual 5-coordinate IrCl(C2H4)4.14... 

Dimethyl H-phosphonate oxidatively adds to Ir (I) and Rh (I) compounds with the formation of hydrido-fr(ni) and hydrido-Rh(lII) phosphonato complexes [439]. Treatment of chloro-bis(cyclooctadiene) iridium (I) [IrCl(C8Hj4)2]2 with two equivalents of triphenyl phosphine and subsequent reaction with dimethyl H-phosphonate affords two products of oxidative addition I and II, which have not been spectroscopically distinguished ... 

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