Catalytic reactions involving olefins

Our study on the synthesis, structure and catalytic properties of rhodium and iridium dimeric and monomeric siloxide complexes has indicated that these complexes can be very useful as catalysts and precursors of catalysts of various reactions involving olefins, in particular hydrosilylation [9], silylative couphng [10], silyl carbonylation [11] and hydroformylation [12]. Especially, rhodium siloxide complexes appeared to be much more effective than the respective chloro complexes in the hydrosilylation of various olefins such as 1-hexene [9a], (poly)vinylsiloxanes [9b] and allyl alkyl ethers [9c]. 

This is a new entry for alkylation of benzene, though the applicability of this reaction is narrow. These authors proposed that a catalytic cycle involves olefin/acetonitrile ligand exchange followed by olefin insertion into the Ru-Ar bond. The C-H bond activation in another arene allows elimination of alkylbenzenes. 

Strictly related to catalytic reactions involving CO and H20 are reactions in which CO and alcohols, ROH, or CO and amines, R2NH, are used as building blocks. The catalytic addition of carbon monoxide and an alcohol to an olefin yields carboxylic esters (hydroesterification). Thus, the synthesis of methyl propionate from ethylene, CO, and methanol using a catalytic system composed of Ru3(CO)u and [PPh4]I (190°C, 20 bar C2H4, 45 bar CO, 2.5 hr, yield 74%, CT 1000) has been reported (323) ... 

The present paper reports on the catalytic properties of selected aluminophosphate molecular sieves in model hydrocarbon reactions. The molecular sieves were selected to represent large and medium pore sizes with a variety of framework elements including transition metals, in addition to aluminum and phosphorus. Model reactions were chosen to explore catalytic performance in paraffin, olefin and aromatic rearrangement reactions to probe molecular sieve character, shape selectivity and catalytic activity, particularly for reactions involving olefins or olefin reaction intermediates. 

In catalytic reactions involving Pd(II) salts, carboxypalladation yields an alkylpal-ladium species that can often undergo (3-H elimination instead of protonolysis. Subsequently, Stoltz and coworkers demonstrated that Wacker-type processes can also afford lactones under oxidative conditions (Scheme 2.35). The proposed mechanism involves Pd(II) coordination to the alkene, followed by oxypalladation and (3-H elimination. After elimination of HX to form Pd(0), aerobic oxidation is required to regenerate a Pd(II) species. The net result is olefin transposition to an adjacent position [80]. 

Olefin exchange reactions are particularly related to catalytic reactions involving metal 7r-complexes. One of the first steps in the hydrogenation, isomerization, and polymerization of olefins involves the exchange of a ligand or solvent molecule to form the corresponding metal 7c-complex intermediate. 

Furthermore, in homogeneous catalytic reactions involving transition metal complexes, solvent molecules may coordinate to unsaturated intermediates and transition states, playing an important role in determining the reaction pathway. For example, it is believed that a solvent molecule coordinates to three- and five-coordinated unsaturated intermediates in the catalytic cycle of olefin hydro-genafion by RhCl(PPh3)3 (92). Such effects have recently been quantified by fhe Ab Inifo Molecular Orbifal Sfudy of the Full Cycle of Olefin Hydroformylafion Cafalyzed by RhH(CO)2(PH3)2 (93). Coordination of an olefin represenfafive of a solvent molecule into various intermediates was found to have a dramatic... 

The Jacobsen-Katsuki epoxidation reaction is an efficient and highly selective method for the preparation of a wide variety of structurally and electronically diverse chiral epoxides from olefins. The reaction involves the use of a catalytic amount of a chiral Mn(III)salen complex 1 (salen refers to ligands composed of the N,N -ethylenebis(salicylideneaminato) core), a stoichiometric amount of a terminal oxidant, and the substrate olefin 2 in the appropriate solvent (Scheme 1.4.1). The reaction protocol is straightforward and does not require any special handling techniques. 

The main aim of this review is to survey the reactions by which the Co—C bond is made, broken, or modified,.and which may be used for preparative purposes or be involved in catalytic reactions. Sufficient evidence is now available to show that there exists a general pattern of reactions by which the Co—C bond can be made or broken and in which the transition state may correspond to Co(III) and a carbanion (R ), Co(II) and a radical (R-), Co(I) and a carbonium ion (R ), or a cobalt hydride (Co—H) and an olefin. Reactions are also known in which the organo ligand (R) may be reversibly or irreversibly modified (to R ) without cleavage of the Co—C bond, or in which insertion occurs into the Co—C bond (to give Co—X—R). These reactions can be shown schematically as follows ... 

Dimerization, oligomerization, and similar reactions of olefins have been reported to be catalyzed by systems involving the majority of the Group VIII metals (3). The reasons for the particular interest in nickel-containing catalysts are their exceptionally high catalytic activity (catalytic reactions have been performed at temperatures as low as - 100°C), the diversity of catalytic reactions of obvious synthetic value, as well as the possibility to direct the course and control the selectivity of a catalytic reaction by tailoring the catalyst which are perhaps without parallel among transition metal complex catalysts. 

The following conclusions can be drawn (a) ir-Allylnickel compounds are probably not involved in the catalytic dimerization of cyclooctene, because the highest reaction rate occurs when only traces of these compounds can be detected further, the concentration of the new 7r-allyl-nickel compound (19) becomes significant only after the catalytic reaction has ceased, (b) The complex formed between the original 7r-allylnickel compound (11) and the Lewis acid is transformed immediately upon addition of cyclooctene to the catalytically active nickel complex or complexes. In contrast to 7r-allylnickel compounds, this species decomposes to give metallic nickel on treatment of the catalyst solution with ammonia, (c) The transformation of the catalytically active nickel complex to the more stable 7r-allylnickel complex occurs parallel with the catalytic dimerization reaction. This process is obviously of importance in stabilizing the catalyst system in the absence of reactive olefins. In... 

The use of organic halide to reactivate a decayed catalyst has been known for other catalytic processes involving transition metal catalysts, especially in olefin polymerization reactions (18-21). 

It should be noted here again that the catalytic reaction does not involve a change of valence of the metal. In general, catalytic olefin addition reactions that involve a hydride transfer do not require change of valence in the metal catalyst. On the other hand, carbon-carbon bond formation by coupling reactions which involve electron shifts, such as in Wilke s Ni°-catalyzed butadiene oligomerization reaction [Eq. (1)], requires a valence change on the metal. 

In 1993, we reported that various unsaturated heterocycles can be alkylated with Et-, wPr- and nBuMgCl in the presence of optically pure (EBTHI)ZrCl2 (3a) or (EBTHI)Zr-binol (3b) to afford the derived unsaturated products in >90% ee (cf. 5 6, Scheme 2) [4a]. Many of the simpler five- and six-membered starting materials are available commercially or can be prepared by established procedures. In contrast, catalytic enantioselective reactions involving unsaturated medium ring hetero cycles were not a trivial undertaking the synthesis of these olefinic substrates, by the extant methods, was prohibitively cumbersome. 

When alkenes are allowed to react with certain catalysts (mostly tungsten and molybdenum complexes), they are converted to other alkenes in a reaction in which the substituents on the alkenes formally interchange. This interconversion is called metathesis 126>. For some time its mechanism was believed to involve a cyclobutane intermediate (Eq. (16)). Although this has since been proven wrong and found that the catalytic metathesis rather proceeds via metal carbene complexes and metallo-cyclobutanes as discrete intermediates, reactions of olefins forming cyclobutanes,... 

---