Iridium complexes reactivity

The coordinated quinone methide Jt-system of complex 24 can also undergo cycloaddition (Scheme 3.17). When 24 was reacted with /V-methylmaleimide, a [3+2] cycloaddition took place to give the tricyclic iridium complex 29. The closest example to this unprecedented reactivity pattern is a formal [3 + 2] cycloaddition of /)-quinone methides with alkenes catalyzed by Lewis acids, although in that reaction the QMs serve as electron-poor reagents. 36... 

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. 

For further details of this reaction, the reader is referred to Chapter 9. The catalytic allylation with nucleophiles via the formation of Ti-allyl metal intermediates has produced synthetically useful compounds, with the palladium-catalyzed reactions being known as Tsuji-Trost reactions [31]. The reactivity of Ti-allyl-iridium complexes has been widely studied [32] for example, in 1997, Takeuchi idenhfied a [lrCl(cod)]2 catalyst which, when combined with P(OPh)3, promoted the allylic alkylation of allylic esters 74 with sodium diethyl malonate 75 to give branched... 

These solid-gas reactions represent, at the moment, the single path to 3-metalla -l,2-dioxolane complexes of rhodium and iridium. Complexes of this type have been widely proposed in catalytic cycles. However, it is unlikely that they take part in oxygenations with rhodium because of their high reactivity (see below) and the special conditions for their preparation. 

An X-ray crystal structure of 55, redrawn as Fig. 10, supported the formulation of the complex as that of a peroxo system. Further, the structure demonstrated that no interactions between the [Of-] ligand and the borate moiety were possible because of the relative arrangement of the [Of-] and borate ligands about the iridium center. Such interactions were implicated in the oxygen-initiated decomposition of the iridium complex of 52, while the lack of reactivity of the iridium complexes of 53 and 54 was attributed to steric factors arising from the alkyl chains connecting the sulfur atoms. 

The main methods of reducing ketones to alcohols are (a) use of complex metal hydrides (b) use of alkali metals in alcohols or liquid ammonia or amines 221 (c) catalytic hydrogenation 14,217 (d) Meerwein-Ponndorf reduction.169,249 The reduction of organic compounds by complex metal hydrides, first reported in 1947,174 is a widely used technique. This chapter reviews first the main metal hydride reagents, their reactivities towards various functional groups and the conditions under which they are used to reduce ketones. The reduction of ketones by hydrides is then discussed under the headings of mechanism and stereochemistry, reduction of unsaturated ketones, and stereochemistry and selectivity of reduction of steroidal ketones. Finally reductions with the mixed hydride reagent of lithium aluminum hydride and aluminum chloride, with diborane and with iridium complexes, are briefly described. 

The activation of methane in solution by an organometallic complex presents some experimental difficulties because any solvent that is likely to be chosen will be more reactive than methane. In addition, insolubility of the complex in liquid methane may preclude reaction with the pure hydrocarbon. These problems were overcome in the case of the reaction of CH4 with the iridium complex of Eq. 15.106 by taking advantage of the fact that the desired hydndo methyl complex Is thermodynamically more stable than other hydrido alkyl complexes. The methyl complex was produced by first creating a hydrido cyclohexyl complex and then allowing it to react with methane, m... 

Iridium alkoxides, synthesis, 7, 383 Iridium alkyl complexes reactivity, 7, 284-... 

Only when this tricarbonyl species is formed does the rearrangement to the acetyl-iridium complex occur. The observation that the neutral tricarbonyl complex rearranges to form an acetyl species much more readily than the anionic dicarbonyl species undoubtedly can be related to the relative strengths of the individual Ir-CO bonds in the two complexes. Similarly, the marked contrast to this type of reactivity with that of the rhodium system, where the analogous CH3Rh(CO)2I3 has never been detected because of... 

In 1988, Tilley and coworkers first succeeded in the synthesis of a ruthenium-silene complex by the reaction of Cp Ru(PR3)Cl with a Grignard reagent, and the structure was confirmed by X-ray crystallography (Eq. 9) [10]. They later synthesized an iridium complex in a similar manner (Eq. 10) [11]. Berry synthesized a tungsten-silene complex by treatment of a tungsten-chloride complex with Mg (Eq. 11) [12]. Although reactions of transition metal-silene complexes with Mel, HX, and MeOH have been reported, little is known about their reactivities [11,12]. 

The catalytic cycle involves the same fundamental reaction steps as the rhodium system oxidative addition of Mel to Ir(I), followed by migratory CO insertion to form an Ir(III) acetyl complex, from which acetic acid is derived. However, there are significant differences in reactivity between analogous rhodium and iridium complexes which are important for the overall catalytic activity. In situ spectroscopy indicates that the dominant active iridium species present under catalytic conditions is the anionic Ir(III) methyl complex, [IrMe(CO)2l3] , by contrast to the rhodium system where the dominant complex is [Rh(CO)2l2] - PrMe(CO)2l3] and an inactive form of the catalyst, [Ir(CO)2l4] represent the resting states of the iridium catalyst in the anionic cycles for carbonylation and the WGSR respectively. At lower concentrations of water and iodide, [Ir(CO)3l] and [Ir(CO)3l3] are present due to the operation of related neutral cycles . 

An interesting variant in the family of cyclopentadienlyirid-ium complexes are those containing -mdenyl ligands. It had been shown by Mawby and coworkers that tndenyl metal complexes are far more reactive with respect to ligand snbstitntion chemistry than their cyclopentadienyl analognes." Basolo quantified this greatly accelerated reaction chemistry for a number of tndenyl metal systems and dubbed this acceleration the tndenyl effect . The tndenyl effect also appears to play an important role in the chemistry of indenyl iridium complexes. Indenyliridium complexes may be prepared in much the same way as the cyclopentadienyl and pentamethyl-cyclopentadienyl iridium complexes. However, once formed, they are far more reactive in terms of substitution chemistry. 

Results from studies on organoiridium compounds have made significant contributions in the area of catalysis. In some transformations, organometallic iridium compounds are the most active catalysts available. In others, where iridium may not yield the most active catalysts, iridium complexes nevertheless yield important information about the stmcture and reactivity of important catalytic intermediates. The following sections attempt to briefly cover some of the most important homogeneous catalytic process where iridium chemistry has had some impact. 

Iridium complexes appear to be less reactive towards carbonyl compounds, although they react with aldehydes " and hexafluoroacetone . This last reaction has been studied kinetically, and the authors find it to be first order in dioxygen complex and in hexafluoroacetone. They conclude the reaction takes place by a direct electrophilic attack on the coordinated dioxygen. This contrasts sharply with the results of Ugo et al. for (Ph3P)2Pt02 the slow substitution reactions of Ir(III) complexes and the absence of vacant coordination sites exclude the coordination of the substrate prior to the attack on coordinated dioxygen, and this may account for the much slower reactions of the iridium complexes. 

The complexes of rhodium and iridium are generally less reactive than those of palladium and platinum no reaction was observed between Rh and Ir dioxygen complexes and CO2, CS2, aldehydes and ketones or between an iridium complex and CO, CO2 and The iridium complexes are not totally inert however, and reactions with... 

The complex has enjoyed relatively little use in organic synthesis. For iridium-catalyzed homogeneous hydrogenation of alkenes, Crabtree s iridium complex ((1,5-Cycloocta-diene)(tricyclohexylphosphine)(pyridine)iridium(I) Hexafluoro-phosphate) is generally preferred, although this readily prepared Ir complex is active. It is more reactive than its rhodium counterpart in the catalytic isomerization of butenyl- to allylsilanes. ... 

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