Decomposition pathways, catalysts

Olefin metathesis is one of the most important reaction in organic synthesis [44], Complexes of Ru are extremely useful for this transformation, especially so-called Grubbs catalysts. The introduction of NHCs in Ru metathesis catalysts a decade ago ( second generation Grubbs catalysts) resulted in enhanced activity and lifetime, hence overall improved catalytic performance [45, 46]. However, compared to the archetypal phosphine-based Ru metathesis catalyst 24 (Fig. 13.3), Ru-NHC complexes such as 25 display specific reactivity patterns and as a consequence, are prone to additional decomposition pathways as well as non NHC-specific pathways [47]. 

There has been little insight into potential decomposition pathways for the Ni(II) system due to sparse experimental evidence. Polymerization results with catalysts bearing different alkyl and fluorinated substituents have suggested that a C-H activation process analogous to that occuring with the Pd(II) catalysts is unlikely with Ni(TT) [28], Instead, side reactions between Ni and the aluminum coactivator, present as it is in such large excess, have been implicated. The formation of nickel dialkyl species and their subsequent reductive elimination to Ni(0) is one possible deactivation mechanism [68]. 

Evidence for a major mode of catalyst deactivation in this system came from the observation of phosphonium cations (HPR3) in the reaction mixture, which could form through the pro to nation of free PR3 by the acidic dihydride complex. It is not known which species decomposes to release free PR3, but the decomposition pathway is exacerbated by the subsequent reactivity in which protonation of phosphine removes a proton from the metal dihydride, effectively removing a second metal species from the cycle. 

These Mo catalysts with a C2-tether connecting the phosphine and cyclopenta-dienyl ligand provide an example of the use of mechanistic principles in the rational design of improved catalysts, in this case based on information about a decomposition pathway for the prior generation of catalysts. The new catalysts offer improved lifetimes, higher thermal stability, and low catalyst loading. The successful use of a triflate counterion and solvent-free conditions for the hydrogenation are additional features that move these catalysts closer to practical utility. 

Eshova et al. studied the hydrogenation of C02 to formic acid in several solvents in the presence of an equivalent of Et3N using Wilkinson s catalyst. These authors conducted an extensive NMR study into the various decomposition pathways of the catalyst [56]. Apparently, DMSO is capable of rapidly displacing one equivalent of PPh3 on the catalyst a second equivalent is slowly displaced. 

Nindakova and Shainyan studied the decomposition pathways of Rh-DIOP complexes. Even upon prolonged exposure to hydrogen, the catalyst decomposed in the absence of substrate with, according to these authors, the Rh-bis-... 

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

In the activation of AHM to M0S2, one of the intermediates that is observed is ammonium tetrathiomolybdate (ATTM). Previous studies by Lopez et al. (16) show that small amounts of ammonium tetrathiomolybdate [(NH4)2MoS4] are produced during the decomposition of AHM, which represents an intermediate of a minor decomposition pathway. One advantage of using ATTM as a dispersed-phase catalyst in... 

Decomposition of (3), which might be formed from (1) or (2), can result in the formation of styrene or may lead to chain end unsaturation and neighboring ring protonation. Hydride abstraction by (3) would result in a saturated chain end. The lack of significant styrene production from any of the PS-catalyst samples suggests that P-scission of (3) to form styrene is not a dominant decomposition pathway at low temperatures. Chain end unsaturation derived from (3) may result in formation of indenes, which were detected in substantial amounts only when HZSM-5 catalyst was present. The restricted volume... 

The ability of non-C2 symmetric ketones to promote a highly enantioselective dioxirane-mediated epoxidation was first effectively demonstrated by Shi in 1996 [114]. The fructose-derived ketone 44 was discovered to be particularly effective for the epoxidation of frans-olefins (Scheme 17 ). frans-Stilbene, for instance, was epoxidized in 95% ee using stoichiometric amounts of ketone 44, and even more impressive was the epoxidation of dialkyl-substituted substrates. This method was rendered catalytic (30 mol %) upon the discovery of a dramatic pH effect, whereby higher pH led to improved substrate conversion [115]. Higher pH was proposed to suppress decomposition pathways for ketone 44 while simultaneously increasing the nucleophilicity of Oxone. Shi s ketone system has recently been applied to the AE of enol esters and silyl enol ethers to provide access to enantio-enriched enol ester epoxides and a-hydroxy ketones [116]. Another recent improvement of Shi s fructose-derived epoxidation reaction is the development of inexpensive synthetic routes to access both enantiomers of this very promising ketone catalyst [117]. 

HCo(CO)4 is a volatile, very toxic liquid with an extremely unpleasant odor. It is rather unstable as a pure material, decomposing into Co2(CO)8 and H2 below room temperature. The equilibria between Co2(CO)8 and HCo(CO)4 under different H2 pressures have been studied. Dilute solutions are more stable, from which observation of a second-order decomposition pathway has been concluded. However, very pure, colorless samples of (3) are much more stable, and Co2(CO)8 acts as a catalyst for the decomposition of (3). Since this decomposition follows a rate law of half-order in C02 (CO)8, the radical Co (CO) 4 is invoked as the active species in the decomposition process according to equations (18-21). From electron diffraction measurements, the geometry of (3) is trigonal bipyramidal see Trigonal Bipyramidal), with hydrogen occupying an axial position. 

The selectivities of CO2 in COjc were not dependent on either the specific input energy or the type of VOCs, and showed constant values. This observation indicates that the channel to produce CO2 was determined by the decomposition pathway (i.e. the type of intermediates). Again, once CO is formed, further oxidation of CO to CQz in the PDC reactor is negligible under the tested SIE range. Since the decomposition of HCOOH preferably produced CO in the PDC reactor, HCOOH is believed to be an important precursor for CO2 formation. Except for HCOOH which shows 100% CO selectivity, 6 aromatic compounds showed almost the same CO2 selectivity at about 71-77%. These similar by-product distributions and selectivities indicate that the decomposition mechanism of aromatic compounds in the PDC reactor is quite similar. From a long-term test over 150 hours, the PDC system showed a stable performance without any catalyst deterioration for the decomposition of benzene and toluene [170]. 

Another important mechanistic issue is the thermal decomposition of ruthenium alkylidene catalysts. To understand the decomposition pathways available in these systems, the thermolysis of two ruthenium alkylidene complexes, the propylidene (PCy3)2(Cl)2Ru=CHEt (3) and the methyhdene (PCy3)2(Cl)2Ru=CH2 (4), was examined in detail [93]. These two compounds were chosen because a variety of alkylidenes [as modeled by the propylidene (3)] and the methyhdene (4) are key intermediates in a range of olefin metathesis reachons with terminal alkenes. The studies revealed that the thermal decomposihon of the propylidene... 

---