Iridium turnover frequency

The first application of a heterocyclic carbenoid achiral ligand for hydrogenation of alkenes was reported in 2001 by Nolan and coworkers. Both ruthenium [36] and iridium [37] complexes proved to be active catalysts. Turnover frequency (TOF) values of up to 24000 b 1 (at 373 K) were measured for a ruthenium catalyst in the hydrogenation of 1-hexene. 

After extensive experimentation, a simple solution for avoiding catalyst deactivation was discovered, when testing an Ir-PHOX catalyst with tetrakis[3,5-bis (trifluoromethyl)phenyl]borate (BArp ) as counterion [5]. Iridium complexes with this bulky, apolar, and extremely weakly coordinating anion [18] did not suffer from deactivation, and full conversion could be routinely obtained with catalyst loadings as low as 0.02 mol% [19]. In addition, the BArp salts proved to be much less sensitive to moisture than the corresponding hexafluorophosphates. Tetrakis (pentafluorophenyl)borate and tetrakis(perfluoro-tert-butoxy)aluminate were equally effective with very high turnover frequency, whereas catalysts with hexafluorophosphate and tetrafluoroborate gave only low conversion while reactions with triflate were completely ineffective (Fig. 1). 

The iridium(III)-complex, [Ir(p-acac-0,0,C )(acac-0,0)(acac-C )]2, mediates the activation of unactivated aromatic C—H bond with unactivated alkenes to form anti-Markovnikov products [57]. The reaction of benzene 131 with propene 132 (0.78 MPa of propylene, 1.96 MPa of N2) leads to the formation of n-propylbenzene 133 in 61% selectivities (turnover number (TON) = 13 turnover frequency (TOE) = 0.0110 s ) (Equation 10.34). The reaction of benzene with ethane at 180 °C for 3h gave ethylbenzene (TON = 455 TOE = 0.0421s ). The anti-Markovnikov selectivity was also proven for the reaction with 1-hexane and isobutene, giving 1-phenyUiexane (69% selectivity) and isobutylbenzene (82% selectivity), respectively. 

The results of some initial experiments indicated that the catalytic activity of 1 is strongly temperature-dependent. For instance, at 100 °C, the rates are relatively low but there is an appreciable turnover [turnover frequency (TOE) = 20.5 h" j. Increasing the reaction temperature to 200 °C increases the TOE to 720 h hence, the thermal robustness of the iridium catalyst is pivotal for optimal catalyst performance. The tridentate pincer-type ligands provide a particularly stable platform for metal confinement through covalent Ir—C bonding combined with the terden-tate chelating coordination. 

Intermolecular asymmetric aminations are at an early stage of development, and consequently much lower turnover frequencies and catalytic yields have been observed at this stage. In the example shown, a key aspect is the activation of the iridium complex catalyst by fluoride ion [111] (Scheme 38). 

Catalytic activities of the zeolite-supported clusters (Table 4) are reported as turnover frequencies these are rates per total iridium atom for such small clusters. Rates were also reported for conventional (structurally nonuniform) supported catalysts consisting of aggregates of metallic iridium on supports, these rates, per unit of metal surface area, are markedly greater than those observed for the supported clusters [15]. Changing the support from zeolite NaY to MgO had little effect on the activities of the decarbonylated clusters. 

In the work with asymmetric hydrogenation in SC-CO2, Kainz et al. used modified iridium catalysts to hydrogenate imines. The presence of BARF as a counterion provided the best results in CO2. In comparison to dichloromethane, CO2 gave up to double the turnover frequency (TOF = TON/time) but enantioselectivities were lower by 10%. Again yield and selectivity were substrate specific. Lange et al. developed a chiral rhodium diphosphinite catalyst... 

These reactions have been investigated since the 1970s. Palladium, rhodium, nickel, ruthenium, and iridium complexes proved to be active catalysts, but at first with only low conversions. In the 1990s, these reactions were picked up again, first by Graf and Leitner [84] with Rh catalysts, then by Jessop and co-workers [85] and Baiker and co-workers [86] with Ru complexes. An enormous increase in activity was achieved in the synthesis of formic acid a turnover frequency (TOF) of 95 000 h 1 was obtained and for DMF a TOF of 370 000h 1. 

The water-soluble iridium(III) complex, [IrCp (H20)3]2+ (Cp = p5-C5Me5) was found a suitable catalyst precursor for reduction of aldehydes and ketones by hydrogen transfer from aqueous formate [254], Under the conditions of Scheme 3.34 turnover frequencies in the range of 0.3-4.3 h-1 were determined. Of the several water-soluble substrates the cyclic cyclopropanecarboxaldehyde reacted faster than the straight-chain butyraldehyde, and aldehydes were in general more reactive than the only simple ketone studied (2-butanone). While glyoxylic acid was reduced fast, pyruvic acid did not react at all. 

With substrate 8a, the use of SCCO2 leads to a significant improvement of the performance of the catalyst compared to the reaction in CH2CI2. Hence, significantly lower amounts of iridium complex are required for efficient catalysis in the supercritical medium and turnover frequencies (TOF, mole product per mole metal center per hour) above 2,000 h were achieved at catalysts loadings as low as 0.014 mol%. It is important to note that this effect is not related to a simple increase in reaction rate, but rather to a change in the overall kinetic behavior of the catalytic system. 

Table 15.2. A comparison of turnover frequencies for the homogeneous hydrogenation of different alkenes catalyzed by rhodium, ruthenium, and iridium catalysts.

A major breakthrough in transfer dehydrogenation of alkanes was achieved in 1996 by Jensen, Kaska, and coworkers [16, 17]. They reported that the iridium pincer complex ( " PCP)IrH2, la, was highly reactive and exceptionally thermally stable for transfer dehydrogenation of COA employing TBE as the acceptor [Eq. (3)]. For example, at 200°C the turnover frequency was reported to be 12/min with no noticeable catalyst decomposition over 7 days. 

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