Iridium vinyls

A more complex series of iridium vinyls have been derived from DMAD, directly from Tp Ir( -C2H4)2 (194), Tp Ir(f/ -C4H4-2,3-Me2) (234) or Tp Ir(C6H5)2(N2)... 

The syntheses and spectroscopic and electrochemical characterization of the rhodium and iridium porphyrin complexes (Por)IVI(R) and (Por)M(R)(L) have been summarized in three review articles.The classical syntheses involve Rh(Por)X with RLi or RMgBr, and [Rh(Por) with RX. In addition, reactions of the rhodium and iridium dimers have led to a wide variety of rhodium a-bonded complexes. For example, Rh(OEP)]2 reacts with benzyl bromide to give benzyl rhodium complexes, and with monosubstituted alkenes and alkynes to give a-alkyl and fT-vinyl products, respectively. More recent synthetic methods are summarized below. Although the development of iridium porphyrin chemistry has lagged behind that of rhodium, there have been few surprises and reactions of [IrfPorih and lr(Por)H parallel those of the rhodium congeners quite closely.Selected structural data for rr-bonded rhodium and iridium porphyrin complexes are collected in Table VI, and several examples are shown in Fig. 7. ... 

The stereoselective isomerization of unsymmetrical diallyl ethers to allyl ( )-vinyl ethers was also carried out in the presence of a cationic iridium(l) catalyst. The catalyst prepared in situ by treating [Ir(cod)(PPh2Me)2]PF6 with hydrogen was found to be an excellent catalyst for the selective isomerization of a less substituted allyl group to an ( )-vinyl ether (Scheme 44).72... 

The main species in solution has been identified to be the hydrido-alkynyl complex [IrH(C2Ph)(cod)(//2-iPrPCH2CH2OMe)]+BF4 (23). This is, however, only a sink that results from direct reaction of 22 with the 1-alkyne, draining the active catalyst from the system. The catalysis proceeds via the dihydrido-diene intermediate [IrH2(cod)(//2- PrPCH2CH2OMe)]+ BF4 (24), which reacts reversibly with the alkyne to yield the hydrido-iridium-styryl complex 25, followed by a rate-determining reaction of this hydrido-vinyl species with hydrogen to re-... 

Boronic esters have been used in a wide range of transformations. These useful reagents have been transformed into numerous functional groups and are essential reagents for several C-C bond-forming reactions. Transition metal-catalyzed hydroboration of olefins often leads to mixtures of branched and linear products. Several groups have reported asymmetric reductions of vinyl boronic esters [50-52] with chiral rhodium P,P complexes however, the first iridium-catalyzed reduction was reported by Paptchikhine et al (Scheme 10) [53]. 

Keywords Iridium, Enantioselective, Hydrogenation, Transfer hydrogenation, Allylation, Vinylation... 

Periana and Goddard et al. also found that the iridium(III) vinyl bis-acetylace-tonate complex (Vinyl-Ir-Py, 26) catalyzes dimerization of olefins (e.g., 21) [106, 107]. Analogously to the hydroarylation of Scheme 8, the mechanism is proposed to... 

Addition to linear 1,1-disubstituted allylic acetates is slower than addition to monosubstituted allylic esters. Additions to allylic trifluoroacetates or phosphates are faster than additions to allylic carbonates or acetates, and reactions of branched allylic esters are faster than additions to linear allylic esters. Aryl-, vinyl, alkynyl, and alkyl-substituted allylic esters readily undergo allylic substitution. Amines and stabilized enolates both react with these electrophiles in the presence of the catalyst generated from an iridium precursor and triphenylphosphite. 

Electron-rich iridium(l) complexes can perform C—H activation reactions under mild conditions [13]. In this line, acetone-dis solutions of the [(ri -l,3,5-C 5H3Me3)) lr(Ti -C2H4)(P Pr3)]BF4 complex, at room temperature, show deuterium incorporation to the ethane ligand, most likely due to the participation of hydrido vinyl iridium(lll) species, formed by the C—H activation of ethane, according to Scheme 2.25 [21]. 

The iridium-catalyzed hydroboration of heteroatom-containing substrates (vinyl sulfides, sulfoxides, sulfones and sulfonates) was examined by Westcottand Baker [43]. These authors observed the selechve formation of the linear boronate ester... 

The dimerization of functional alkenes such as acrylates and acrylonitrile represents an attractive route to obtain bifunctional compounds such as dicarboxylates and diamine, respectively. The head-to-tail dimerizahon of acrylates and vinyl ketones was catalyzed by an iridium hydride complex generated in situ from [IrCl(cod)]2 and alcohols in the presence of P(OMe)3 and Na2C03 [26]. The reaction of butyl acrylate 51 in the presence of [IrCl(cod)]2 in 1-butanol led to a head-to-tail dimer, 2-methyl-2-pentenedioic acid dibutyl ester (53%), along with butyl propionate (35%) which is formed by hydrogen transfer from 1-butanol. In order to avoid... 

The iridium-catalyzed transformation between carboxyhc acid and vinyl acetate [28] or allyl acetate [30] was also promoted to afford vinyl or allyl carboxylates in good yields. 

Phosphinite pincer iridium systems have also been shown to have a lower tendency to oxidatively add TEE to give (vinyl)(hydride) complexes similar to 3 [18]. While this has been identified as one of the major catalyst deactivation processes in phosphine pincer iridium catalysis, apparently with complexes such as 5, only olefin coordination can occur. However, this is a considerably weaker bonding and is less detrimental to catalyst activity. Eased on steric arguments, product olefin coordination (e.g. COE) is favored over TEE coordination, and therefore at a high TON and high product concentrations the phosphinite catalysts 5 are markedly less active than the phosphine analogues 1. 

Attempts to turn this acetylene dimerization reachon into a catalyhc polymeriza-hon process have failed thus far. In the presence of excess phenylacetylene, the iridium(I) complex 23 activates another Caikynyi—H bond and hansforms, after a hydrogen shift, to the stable (vinyl)(alkynyl) iridium(III) system 27 (Equahon 12.10). 

In contrast to reactions with vinyl epoxides and palladium catalysts, the reactions with rhodium retain the stereochemistry of the alkene fragment during the reaction [20]. This is illustrated by the reactions of trans-37a/h and cis-37a/b, which give only one product possessing the same olefin geometry as the starting epoxides (Eqs. 4 and 5). The retention of olefin stereochemisty has also been documented in allylic functionalizations with iridium catalysts, indicating that similar modes of action may be present [21, 22]. 

The need for a base additive in this reaction implies the intermediacy of acetylide complexes (Scheme 9.10). As in the Rh(III)-catalyzed reaction, vinylidene acetylide S4 undergoes a-insertion to give the vinyl-iridium intermediate 55. A [l,3]-propargyl/ allenyl metallatropic shift can give rise to the cumulene intermediate 56. The individual steps of Miyaura s proposed mechanism have been established in stoichiometric experiments. In the case of ( )-selective head-to-head dimerization, vinylidene intermediates are not invoked. The authors argue that electron-rich phosphine ligands affect stereoselectivity by favoring alkyne C—H oxidative addition, a step often involved in vinylidene formation. 

Abstract The purpose of this chapter is to present a survey of the organometallic chemistry and catalysis of rhodium and iridium related to the oxidation of organic substrates that has been developed over the last 5 years, placing special emphasis on reactions or processes involving environmentally friendly oxidants. Iridium-based catalysts appear to be promising candidates for the oxidation of alcohols to aldehydes/ketones as products or as intermediates for heterocyclic compounds or domino reactions. Rhodium complexes seem to be more appropriate for the oxygenation of alkenes. In addition to catalytic allylic and benzylic oxidation of alkenes, recent advances in vinylic oxygenations have been focused on stoichiometric reactions. This review offers an overview of these reactions... 

Palladium, platinum and iridium complexes of thiirene 1,1-dioxides (10) show upfield shifts of about 5 p.p.m. for vinyl protons and 1 p.p.m. for methyl protons, the shifts being attributed to back-donation of electrons by the metal and its associated ligands <73JOM(57)403>. 

The iridium-catalysed hydrosilylation of alkynes in the presence of 4,4, 5,5,-tetra-methylbiphosphinine (tmbp) has been explored and shown to proceed effectively to afford /S-( >vinylsilanes with high selectivity in moderate to high yields, whereas a similar hydrosilylation in the absence of tmbp produced /i-(Z)-vinyl si lanes. This stereoselectivity reversal is believed to be a function of the electron-withdrawing properties of tmbp coordinated to iridium.111... 

Iridium catalysts have been employed only infrequently in asymmetric hydroformylation with a catalytic system prepared in situ from [Ir(CO)3(PPh3)2]BPh4 and DIOP, a low optical yield (0.5 %) was obtained in the synthesis of 2-acetoxy-propanal from vinyl acetate32). 

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