Dissociation phosphine

Phosphine dissociation is not rate determining, resulting in a high initiation rate. The rate determining step is the coordination of ihe olefin that is competing with recoordination... 

In a related work, Morokuma and co-workers reported their own MP2 calculations on the associative reaction pathway, in which the phosphine dissociation is not considered [25], In the model system, the HB(OH)2 and HB02(CH2)3 boranes were used, while the catalyst was RhCl(PH3)2Cl and the olefin C2H4. 

DR. GEOFFROY We obviously don t know why the photochemistry changes. We know that it is reasonable that phosphine dissociation does not occur when we have a chelating diphosphine ligand. But we don t understand why we see hydrogen loss in this system when we don t see it in the other cases. 

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. 

Most recently, Chen et al. studied the solubility of trans-Co2(CO)e[ p-CF3C6H4)3]2 in SCCO2 (p = 0.45 gcm ) in the presence of 0.74 MPa carbon monoxide in view of utilizing this complex as a pre-catalyst for hy-droformylation of ethylene and propylene [123]. The presence of additional P(p-CF3C6H4)3 enabled measurements above 373 K without phosphine dissociation against carbon monoxide. Solubilities were measured between 0.2 mmol cm (353 K) and 2.1 mmol cm (403 K). 

As a consequence of the higher coordination energy, the dicarbene complexes 53 disfavor a dissociative pathway similar to that of 52. A mixed NHC/phosphine complex of type 54, however, reveals a phosphine dissociation energy in the same order of magnitude as 52. Therefore, 54 is able to populate the dissociative pathway just as readily as 52. In contrast to 52, however, a phosphine-free species 58 is to be considered as the key intermediate in the catalytic cycle. 

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

When subjected to shock-waves, phosphine dissociates into hydrogen and red phosphorus. The radiation thus emitted is only visible in reflection. In contrast, the shock-wave induced dissociation of phosphine diluted with argon is accompanied by emission of visible light. In this case the reaction products are hydrogen and white phosphorus... 

In basic medium the catalytic species was postulated to be a Ru-dihydride complex. In this case, the regioselectivity was determined by the proton-transfer step (65). The complete catalytic cycle in basic medium is depicted in Scheme 14. First the phosphine dissociation generating a vacant site for the substrate coordination takes place. Next step is the insertion of the substrate into the Ru-H bond (inner-sphere mechanism) followed by water coordination in order to occupy the vacant site. This step has the highest relative energy barrier for the overall process. To generate the final product this intermediate must be somehow protonated however, in basic medium there are no easily available protons in solution. Thus, bulk water molecules are the only proton source. The transfer of a proton from a water molecule to the C=C bond requires at least 36.6 kcal mol-1, which is much more than the highest barrier found for C=0 hydrogenation... 

The ultimate products from these mixed species depend on the nature of the solvent and the tertiary phosphine (197, 199, 200) and are formed by reductive elimination of two organic fragments from the gold(III) center. A mechanism involving phosphine dissociation has been proposed (200) (Scheme 2). 

It has been shown (Section V,A and B) that phosphine dissociation is involved in the decomposition of organogold(III) complexes. Addition of... 

There is an alternative pathway to II, in which the phosphine dissociates before the alkene group coordinates pathway III. On the basis of electron accountancy alone, this should be viewed as unfavourable as it involves two 14-electron intermediates (26 and 27). However, it should be noted that the mechanism-derived rate equation for reaction via pathways I/III rather than I/II would be equally consistent with the empirical rate equation. 

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