Iridium vinyl hydride complex

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. 

The same cycle is followed during the reactions of linear alkanes to form linear alk-enes. Although the thermod)mamics for dehydrogenation of cyclooctene are more favorable than those for the dehydrogenation of linear alkanes, primary C-H bonds typically undergo oxidative addition faster than secondary C-H bonds, as discussed in Chapter 6. Thus, linear alkanes react faster than cyclic alkanes. However, the accumulation of a-olefin inhibits the catalytic process. An T) -olefin complex formed from the a-olefin becomes the resting state of the catalytic cycle for reactions catalyzed by the POCOP system, instead of the vinyl hydride complex that is the resting state of the PCP system. The accumulation of the olefin complex that lies off the cycle leads to a lower concentration of the iridium complexes within the cycle and slower reactions as the concentration of a-olefin product increases. 

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... 

In another example, the cyclometalated iridium complex [Ir(ppy)2(4-vinylpyridine)Cl] has been attached via hydrosilation see Hydrosilation) to hydride-terminated poly(dimethylsiloxane) to produce a luminescent material. Evaluation of this material as a luminescent oxygen sensor revealed significantly improved sensitivity over dispersions of the original vinyl pyridine complex in poly(dimethylsiloxane). The luminescent material was blended with polystyrene to give a new sensor that exhibited increased sensitivity and maintained short response times to rapid changes in air pressure. 

Thermolysis of [(triphos)Ir(H)2Et] with styrene afforded [(triphos)IrH(ti2-CH2=CHI%)] which on photolysis underwent an insertion of iridium into a C-H bond to give a 1 1 mixture of the ( -) and (Z-) styryl complexes [(triphos)Ir(H)2(CH=CHPh)]. The ethene complexes lCpIr(L)(C2H4)] (L = CO, PPh3) were reported" to undergo two competing photochemical reactions in solution - isomerisation to a vinyl hydride and dissociation of ethene widi insertion of iridium into solvent C-H bonds. The mechanisms of these reactions were studied in detail. 

When the iridium hydride is reacted with a hydrogen acceptor, simple oxidative addition adducts can be seen for aromatic and vinylic C-H containing substrates. With nitrobenzene, although a thermodynamic preference is seen for an orthometallated chelate product, the kinetic preference is for meta- and para-Gr-W activation, which is then followed by rearrangement to the o/n4n-activated product, which in turn coordinates the nitro group. Hence, chelate assistance is found to have no kinetic benefit for C-H activation in this complex (Equation (22)). 

Stepwise double alkyne to vinylidene tautomerization is the key step responsible for the formation of the 77 -butadienyl iridium(in) complex [Ir K -0,C -O=G(Me)CH=CPh (77 -PhCH=CHC=CHPh)(PPh3)2]SbF6 579 346 proposed mechanism, which is illustrated in Scheme 82, involves an alkyne to vinylidene rearrangement (I —> II) followed by a hydride insertion (II — III), a second alkyne to vinylidene rearrangement (III — V), and a migratory insertion of the vinyl to the vinylidene (V — VI) resulting in the G-C bond formation. 

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