Catalyst resting state

In most cases, the dppe complex decomposed rapidly, but the more robust dcpe derivative was observed to be the catalyst resting state by NMR. Smaller phosphines reacted more quickly but less selectively in these systems, and selectivity for the hydrophosphination products ranged from 29 to >95%. It is likely that the phosphine byproducts included telomers such as 1 (Scheme 54), but they were not characterized. [9]... 

Evans suggests that the catalyst resting state in this reaction is a 55c Cu alkene complex 58, Scheme 4 (35). Variable temperature NMR studies indicate that the catalyst complexes one equivalent of styrene which, in the presence of excess alkene, undergoes ready alkene exchange at ambient temperature but forms only a mono alkene-copper complex at -53°C. Addition of diazoester fails to provide an observable complex. These workers invoke the metallacyclobutane intermediate 60 via a formal [2 + 2] cycloaddition from copper carbenoid alkene complex 59. Formation of 60 is the stereochemistry-determining event in this reaction. The square-planar S Cu(III) intermediate 60 then undergoes a reductive elimination forming the cyclopropane product and Complex 55c-Cu, which binds another alkene molecule. 

In contrast to the rhodium process the most abundant iridium species, the catalyst resting state, in the BP process is not the lr(l) iodide, but the product of the oxidative addition of Mel to this complex. 

Oxidative addition to complex 1 is the slowest and rate-determining step in the reaction scheme and also it is a singular step, involving the conversion of the catalyst resting state to a more reactive 2. An obvious way to obtain a faster catalyst is the substitution of carbonyl ligands in 1 by electron-donating phosphines, as organometallic chemistry tells us this variation never fails. Indeed, several variants that are indeed fester are known [11], but none of them has found application. 

At high PPh3 concentrations, where the catalyst resting state is (PPh3)3Rh(CO)H, phosphine dissociation must occur to form the coordinatively unsaturated intermediates 3c and 3t. This dissociation is suppressed by increased PPh3 concentration, which serves to reduce the concentration of active Rh species in the catalytic cycle. 

For every reaction, it is also important to consider what is actually being recycled. This is always the catalyst rest state, which is not necessarily the catalyst precursor. Particularly in the case of transition metal-based systems, irreversible steps are often necessary for the catalyst precursor to enter the... 

Leaching can be analyzed with respect to both the catalyst (rest state) and catalyst degradation products. The above reactions involve the separation of an n-octane product solution from a 5a catalyst residue at - 30 °C, and a subsequent n-octane extraction at - 30 °C. The data in Fig. 2, together with the solvent quantities employed, predict catalyst leaching of < 0.33% per cycle (calculated from the solubility at - 20 °C). This rises to 1.0 and 3.6% if phase separations are conducted at 0 and 20 °C, respectively. 

Crabtree and coworkers proposed a catalytic cycle for the reaction outUned in Equation 6.10. The mechanism is based on labeling and kinetic studies, and is outlined in Scheme 6.4 [25]. Adduct 36 was observed in nuclear magnetic resonance (NMR) spectra and appears to be a catalyst resting state. It should be noted that there is no change in the oxidation state of Ir, and that the key step is thought... 

As described above (Section 3.3.1.1), in situ HP IR measurements under catalytic conditions identified the anion [MeIr(CO)2l3] as the catalyst resting state in the Cativa process. The rate controlling step in the catalytic cycle was proposed to be carbonylation of [MeIr(CO)2l3] (Eq. (7)). 

Proposed catalyst resting states under high olefin concentrations... 

Hydroboration of Rh(acac)(coe)2 (54) + L2 with excess HBcat affords acacBcat and the zwitterionic complex Rh(/)6-catBcat)L2 (55), which is believed to be the catalyst resting state in these reactions. Variation of the ligands has a profound effect on these... 

---