Hydrogenation of styrenes

Catalytic hydrogenation of styrene oxide (86—99) is another process currendy used for the manufacture of PEA. The main requirements for this reaction are a low operating temperature to avoid side reactions and a good quaUty of styrene oxide [76-09-3] starting material. 

Hydrogenation of styrene oxide over palladium in methanol 66 gives exclusively 2-phenylethanol, but in buffered alkaline methanol the product is l-phenylelhanol. If alcoholysis of the epoxide by the product is troublesome, the problem can be eliminated by portion-wise addition of the epoxide to the reaction, so as always to maintain a high catalyst-to-substrate ratio. The technique is general for reactions in which the product can attack the starting material in competition with the hydrogenation. 

The much more stable MIL-lOO(Cr) lattice can also be impregnated with Pd(acac)2 via incipient wetness impregnation the loaded catalyst is active for the hydrogenation of styrene and the hydrogenation of acetylene and acetylene-ethene mixtures to ethane [58]. MIL-lOl(Cr) has been loaded with Pd using a complex multistep procedure involving an addition of ethylene diamine on the open Cr sites of the framework. The Pd-loaded MIL-lOl(Cr) is an active heterogeneous Heck catalyst for the reaction of acrylic acid with iodobenzene [73]. 

Independent studies of the reduction of C=C and C=C bonds indicate that the latter is kinetically favored. Thus, in the absence of phenylacetylene, the rate of hydrogenation of styrene to ethylbenzene is about one order of magnitude faster than those for C=C bond reduction, indicating that the origin of the selectivity cannot be kinetic. The styryl compound represents a thermodynamic sink that causes virtually all the osmium present in solution to be tied up in this form, and therefore the kinetically unfavorable pathway becomes essentially the only one available in the presence of alkyne.31... 

PEGs with average molecular weights above 1000 are waxy solids under ambient conditions, but they melt under C02 pressure to become liquids under typical conditions of scC02 catalysis [63], The approach was demonstrated for the rhodium catalysed hydrogenation of styrene as a test reaction using Wilkinson s complex [(PPhs RhCl] as the catalyst (Scheme 8.6) [61],... 

Scheme 4.8 Proposed pathway for the hydrogenation of styrene catalyzed by Pd(PNO)(OAc) complexes.

Finally, the only example of a polynuclear homogeneous catalyst is the dinuc-lear complex [Pt P sH ]4- [66], which catalyzed the hydrogenation of styrene, phenylacetylene, 1-octyne, and 1-hexyne (i-PrOH, 60°C, 20.7 atm H2 pressure, Pd substrate ratio 1 1800) to the corresponding alkanes within 10 h of reaction. 

The shift correlates in magnitude with the separation of each particular group distance-wise from the aromatic moiety of the substrate or product this points to the formation of an intermediate -complex, for which the rate of formation and the rate of decay can be determined. The 1H-PHIP-NMR spectrum, as well as the anticipated intermediate product-catalyst-re-complex observed during the hydrogenation of styrene, is outlined in Figure 12.19. 

Fig. 12.19 The H-PHIP-NMR spectrum and the anticipated intermediate product-catalyst-re-complex observed during the hydrogenation of styrene.

Fig. 12.21 Chiral and achiral Rh-catalysts employed for the hydrogenation of styrene.

Fig. 12.22 CH2- and CH3-resonances observed in the -PHIP-NMR spectrum of the intermediate attached to an achiral catalyst during the hydrogenation of styrene.

Fig. 12.26 Plots of the time-dependence of concentrations of the intermediates and products of the hydrogenation of styrene.

By using PHIP-NMR studies, various intermediates such as the previously elusive dihydrides of neutral and cationic hydrogenation catalysts, as well as hydrogenation product/catalyst complexes, have already been detected during the hydrogenation of styrene derivatives using cationic Rh catalysts. Information about the substituent effect on chemical shifts and kinetic constants has been obtained via time-resolved PASADENA NMR spectroscopy (DYPAS). 

An informative set of calculations was carried out by Brandt et al, coupled to experimental studies that demonstrated first-order dependence of the turnover rate on both catalyst and H2, and zero-order dependence on alkene (a-methyl-(E)-stilbene) concentration [71]. The incentive for this investigation was the absence of any characterized advanced intermediates on the catalytic pathway. As a result of the computation, a catalytic cycle (for ethene) was proposed in which H2 addition to iridium was followed by alkene coordination and migratory insertion. The critical difference in this study was the proposal that a second molecule of H2 is involved that facilitates formation of the Ir alkylhydride intermediate. In addition, the reductive elimination of R-H and re-addition of H2 are concerted. This postulate was subsequently challenged. For hydrogenation of styrene by the standard Pfaltz catalyst, ES-MS analysis of the intermediates formed at different stages in the catalytic cycle revealed only Ir(I) and Ir(III) species, supporting a cycle (at least under low-pressure conditions in the gas... 

Scheme 39.5 Hydrogenation of styrene as a test reaction for immobilization methods, using scC02 as the mobile phase.

Most recently, a catalyst system based on PEG-modified phosphine ligands was reported to allow for a highly effective C02-induced separation procedure. In this case, the scC02 was used only at the separation stage to precipitate the catalyst and extract the products. The hydrogenation of styrene to ethyl benzene was used as a benchmark reaction, and it was shown that the catalytic active species could be recovered and not only re-used for another hydrogenation but also be subjected as a cartridge to a series of different transformations [43]. 

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