Iridium, alkyl halide complex

Abstraction of one of the metal-bound hydrides from complex 5a provides the cationic iridium(lll) complex 28, which is an efficient precatalyst for alkyl halide reduction in the presence of EtsSiH (Equation 12.11) [31]. 

Oxidative Addition of Alkyl Halides to Palladium(0). The stereochemistry of the oxidative addition (31) of alkyl halides to the transition metals of group VIII can provide information as to which of the many possible mechanisms are operative. The addition of alkyl halides to d8-iridium complexes has been reported to proceed with retention (32), inversion (33), and racemization (34, 35) via a free radical mechanism at the asymmetric carbon center. The kinetics of this reaction are consistent with nucleophilic displacement by iridium on carbon (36). Oxi-... 

The products of oxidative addition of acyl chlorides and alkyl halides to various tertiary phosphine complexes of rhodium(I) and iridium(I) are discussed. Features of interest include (1) an equilibrium between a five-coordinate acetylrhodium(III) cation and its six-coordinate methyl(carbonyl) isomer which is established at an intermediate rate on the NMR time scale at room temperature, and (2) a solvent-dependent secondary- to normal-alkyl-group isomerization in octahedral al-kyliridium(III) complexes. The chemistry of monomeric, tertiary phosphine-stabilized hydroxoplatinum(II) complexes is reviewed, with emphasis on their conversion into hydrido -alkyl or -aryl complexes. Evidence for an electronic cis-PtP bond-weakening influence is presented. 

There are quite a number of routes available for the production of iridium(ni) alkyl compounds. In addition to the halide displacement and olefin insertion pathways noted above for iridium(l) compounds, oxidative addition of C-H bonds to iridium(l) to form iridium(in) hydrido alkyl complexes is also a possibihty. This subject will be covered in detail in Section 9 and will not be discussed here. However, there are other oxidative addition routes that lead to the formation of iridium(lll) alkyls. First, oxidative addition of O2 or HCl to some alkyl and aryl iridium(l) complexes can produce iridium(lll) alkyl or aryl compounds. In some cases, HgCl2 can add, but this appears to lead to tractable products only for the very stable pentafluorophenyl complex. Of course, oxidative addition see Oxidative Addition) of alkyl halides such as H3CI will also yield alkyl iridium(lll) compounds. Addition of Mel to Vaska s compound yields a stable iridium(III) complex, but addition of Etl does not produce a stable compound, presumably due to subsequent /J-hydride elimination see fi-Hydride Elimination). A number of mechanistic studies have been done on the oxidative addition of alkyl halides to iridium(l), especially Vaska s complex see Vaska s Complex). 

In an important report, Janowicz and Bergman have described the photochemical incorporation of an alkyl group from an alkane into an iridium complex. The process does not appear to involve alkyl radicals, but is thought to occur via two successive concerted steps. This report may point the way to new catalytic processes for the functionalization of alkanes. The photoaddition of alkyl halides to alkenes catalysed by Cu complexes represents a novel process for formation of C-C bonds, and may have useful synthetic applications (Mitani et ai). The possibility of analogous intramolecular reactions comes readily to mind. 

Evidence for a radical pathway includes the observation that the reaction is accelerated by radical initiators (such as oxygen or peroxides) and the presence of UV light. Moreover, the order of reactivity for the R group is IIP > II0 > 1°, which is inconsistent with a direct displacement mechanism, but is in accord with the stability of alkyl radicals. Radical inhibitors (such as steri-cally hindered phenols) retard the rate of reaction with sterically-hindered alkyl halides, but not when R = methyl, allyl, and benzyl. When stereoisomerically pure alkyl halides are used, OA results in the formation of a 1 1 mixture of stereoisomeric alkyl iridium complexes, consistent with the formation of an intermediate radical R-. 

In-depth study of Pd(IV) alkyl halide reductive elimination showed competitive reactions in operation (Scheme 11). Several intriguing, albeit unsupported, mechanisms were considered for the elimination of ethane from (45). In contrast to the palladium complex, the iridium species, (46), did not undergo reductive elimination when R, R = Neither did elimination occur when 1 was... 

A more prevalent approach has been the nucleophilic alkylation of a metal halide, as in the metathesis of 344 with methyllithium to afford Tp Rh(Me) (rj -C Rs) (348). This approach was also employed to prepare the iridium complex Tp Ir(Me)Br(PMe3) (372) from the respective dibromide. Similarly, a range of alkyl halides of general formula Tp Rh(R)X(CNCH2 Bu) (374-385,... 

In 1970 two conflicting reports on the stereochemistry of the addition of chiral alkyl halides to square-planar iridium(I) complexes appeared. In one report it was claimed that the reaction of /ra/w-[IrCI(CO)(PPh2Me)2] with optically active CHjCHBrCOjEt occurred with retention of configuration as shown in Scheme 5 (Pearson and Muir, 1970). This result is consistent with a six-coordinate intermediate , as is the lack of incorporation of any free halide into the product (Section 8). However, the conclusions should again be treated with care since the study employs the cleavage of the iridium-carbon bond by halogen, and without knowing the stereochemistry of this reaction little can be said about the stereochemistry of the displacement. 

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). 

The large negative values of AS in the reactions of the iridium(I) complexes have been rationalised in terms of increased solvation of the transition state, attributable to its increased dipole. Such a dipole results not only from the interaction of the alkyl halides with the metal centre, but also from deformation of the iridium complex from planar to pseudo-octahedral geometry as would occur in a three-centre cw-addition (Harrod and Smith, 1970). In some cases the values of AS and the influence of solvent on AH and AS have been explained in terms of a polar, asymmetric, three-centre transition state, in which the interaction of the metal is predominantly with the carbon centre (Ugo et al., 1972). 

Other efficient three-component systems are based on the Dye/amine AH/alkyl halide R -X combinations. Examples of Dye are hydrocarbons, indanones, thiobarbituric acid, ruthenium or iridium complexes, etc. (see also below). They work according to the sequence [1.40], R and A being the initiating radicals. 

Osborn s group has continued its studies on whether alkyl halides add oxidatively to iridium(I) compounds by a radical chain or an S 2 mechanism by using more reactive metal complexes such as [Ir(PMe3)2(CO)Cl]. For simple alkyl (methyl excepted), vinyl, and aryl halides and a-halo-esters, evidence based on the effect of radical initiators and inhibitors, structure-reactivity relationships, the trapping of radicals by acrylonitrile, and the loss of stereospecificity at the reacting carbon atom all indicate a radical chain process, perhaps as in equations (14) and (15) ... 

Chiral amines have been conveniently prepared also by asymmetric reductive amination of ketones using iridium catalysts and intriguing results with up to 96% ee have been obtained by Zhang and co-workers employing a catalytic system based on Ir./-binaphane in the presence of Ti(OPr )4 and iodine (Scheme 61). Water-soluble aquo complexes [Cp lr(H20)3](0Tf)2 494, [CpP Ir(H20)2](0Tf)2 504, and [Cp Ir(bpy)(H20)](0Tf)2 505 have been used to catalyze the reductive amination of hydrosoluble aldehydes and ketones as well as the dehalogenation of alkyl halides. The activity is markedly pH dependent and inactivation of the catalyst takes place reversibly on increasing the solution basicity due to Ir(H20), deprotonation and formation of mono- or dinuclear hydroxo complexes which are catalytically inactive. The structure of one of these compounds, [Cp Ir(bpy)(OH)]OTf 506, which reversibly forms from 494 around pH 6.6, is presented in Figure 42. 

Studies on oxidative-addition reactions of alkyl halides to square-planar iridium(i) complexes and other low-valent metal centres have shown that the reactions may either be regarded as Ss2 processes in which the metal centre acts as a nucleophile or else involve a concerted, three-centre addition. However, it has now been found that the oxidative-addition reaction of many alkyl halides to /w j-[IrCl(CO)(PMe3)2] can also proceed via a free-radical pathway. The studies show that the rates of reaction are greatly enhanced if small quantities of oxygen or a radical initiator, e.g. benzoyl peroxide, are present and that reaction rates are retarded by traces of radical scavengers, e.g. duroquinone or hydroquinone. Studies with the halides (1) show that the reaction proceeds with loss of stereochemistry at carbon. It is also found that the reaction rate... 

Palladium.—In contrast to iridium(i) complexes there have been few studies on the stereochemistry of addition of alkyl halides to palladium(0) complexes. The addition of optically active ethyl a-bromopropionate to [Pd(CNBu )J gives rm j -[PdBr(CHMeC02Et)(CNBu )2] and results in loss of stereochemistry at the carbon centre. Since ethyl acrylate and propionate are formed in this reaction, an equilibrium involving the a-n rearrangement (Scheme 3) could be... 

While testing two different catalysts, Tanaka found that cationic rhodium in a binary system (cationic Rh(I)/H8-binap) is effective in chemo- and regioselective addition reactions of terminal alkynes with acetylenedicarboxylate to form 1,2,3,4-tetra-substituted benzenes with excellent yield of 99% [9, 44, 45]. It is also important to note that this reaction is tolerant to a large number of functional groups, including alkenes, alkyl halides, and esters. Although cationic iridium complex Ir(I) did not give a positive result in the cycloaddition reactions, the authors showed that the catalytic system with neutral Ir(I) can facilitate cycloaromatization of dimethyl acetylenedicarboxylate and terminal alkynes [45]. 

Structural types for organometallic rhodium and iridium porphyrins mostly comprise five- or six-coordinate complexes (Por)M(R) or (Por)M(R)(L), where R is a (T-bonded alkyl, aryl, or other organic fragment, and Lisa neutral donor. Most examples contain rhodium, and the chemistry of the corresponding iridium porphyrins is much more scarce. The classical methods of preparation of these complexes involves either reaction of Rh(III) halides Rh(Por)X with organolithium or Grignard reagents, or reaction of Rh(I) anions [Rh(Por)] with alkyl or aryl halides. In this sense the chemistry parallels that of iron and cobalt porphyrins. 

We conclude that in octahedral alkyliridium(III) complexes the presence of tertiary phosphines favors exclusively the n -alkyl over the corresponding secondary alkyl, irrespective of the size or basicity of the phosphine. This preference is probably largely electronic in origin, but steric factors cannot be ruled out. A key step that generates a vacant coordination site for both alkyl-group migration and isomerization in octahedral tertiary phosphine complexes of rhodium(III) and iridium(III) is dissociation of halide ion. 

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