Olefin metathesis selectivity

Supported metal oxide catalysts are widely employed in industrial applications alkane dehydrogenation, olefin polymerization, olefin metathesis, selective oxidation/ammoxida-tion/reduction of organic molecules (alkyl aromatics and propylene), and inorganic emissions (N2O, NO , H2S, SO2, and VOC) [1,3,7,11-13]. The initial industrial applications of supported metal oxide catalysts were limited to hydrocarbon dehydrogenation/hydro-genation and olefin polymerization/metathesis reactions. In more recent years, the number of applications of supported metal oxide catalysts for oxidation reactions has grown significantly due to their excellent oxidation characteristics in the manufacture of certain... 

Abstract For many years after its discovery, olefin metathesis was hardly used as a synthetic tool. This situation changed when well-defined and stable carbene complexes of molybdenum and ruthenium were discovered as efficient precatalysts in the early 1990s. In particular, the high activity and selectivity in ring-closure reactions stimulated further research in this area and led to numerous applications in organic synthesis. Today, olefin metathesis is one of the... 

Although olefin metathesis had soon after its discovery attracted considerable interest in industrial chemistry, polymer chemistry and, due to the fact that transition metal carbene species are involved, organometallic chemistry, the reaction was hardly used in organic synthesis for many years. This situation changed when the first structurally defined and stable carbene complexes with high activity in olefin metathesis reactions were described in the late 1980s and early 1990s. A selection of precatalysts discovered in this period and representative applications are summarized in Table 1. 

Olefin metathesis of vinylboronates [102] and allylboronates [103, 104] has been investigated over the past few years because organoboranes are versatile intermediates for organic synthesis. Cross metathesis of vinylboronate 108 and 2-butene 109, for example, yields the boronate 110, which can be converted to the corresponding vinyl bromide 111 with high Z selectivity. Vinyl iodides can be obtained analogously. It should be noted that vinyl bromides and vinyl... 

Bent ansa-metallocenes of early transition metals (especially Ti, Zr, Hf) have attracted considerable interest due to their catalytic activity in the polymerization of a-olefins. Ruthenium-catalyzed olefin metathesis has been used to connect two Cp substituents coordinated to the same metal [120c, 121a] by RCM or to connect two bent metallocenes by cross metathesis [121b]. A remarkable influence of the catalyst on E/Z selectivity was described for the latter case while first-generation catalyst 9 yields a 1 1 mixture of E- and Z-dimer 127, -127 is the only product formed with 56d (Eq. 19). 

In the case of olefin metathesis, the selectivity in initiation products can be understood in terms of minimization of the steric interactions in the metal-lacyclobutane intermediates (vide supra), which are governed by the relative position of the substituents the metallacyclobutane with substituents in pos-... 

Olefin metathesis enables the catalytic formation of C=C double bonds under mild conditions.1 After the development of well-defined catalysts,1 2 selective cross-couplings between functionalized terminal alkenes (CM) have been noted.2 A general problem... 

The Ghadiri work set the stage for later experiments employing olefin metathesis in a library selection. The Nicolaou group reported the first of... 

Figure 1.13 Self-selection of molecular boxes via olefin metathesis.

Intermolecular olefin metathesis starts to compete with traditional C=C-bond forming reactions such as the Wittig reaction and its modifications, as illustrated by the increasing use of electron-deficient conjugated alkenes for the ( )-selective construction of enals and enoates. 

TABLE 2. Summary of Olefin Metathesis Catalysts Prepared by Reacting a Selected Schiff Bases with RuCI2( p-cymeneb and then Postreacting with Alkyl or Aromatic Lithium Salt"... 

Title Alkylidene Complexes of Ruthenium Containing A-Heterocyclic Carbene Ligands Use as Highly Active, Selective Catalysts for Olefin Metathesis... 

All these catalytic results, however, were usually achieved at very low (2-3%) conversions. The only exception is a paper reporting up to 80% selectivity at 20% conversion over a M0CI5—R4Sn-on-silica olefin metathesis catalyst (700°C, 1 atm, CH4 air = l).42 In general, higher temperature and lower—about ambient— pressure compared to homogeneous oxidation, and high excess of methane are required for the selective formation of formaldehyde in catalytic oxidations.43 The selectivity, however, decreases dramatically at conversions above 1%, which is attributed to the decomposition and secondary oxidation of formaldehyde.43,44 It is a common observation that about 30% selectivity can be achieved at about 1% conversion. 

Such higher order prerequisites could be fulfilled by ensemble operation of several sites. For example, a dimeric cluster of cuprous ions on silica gel is very active for the oxidation of CO with NzO at room temperature, but isolated cuprous ions are entirely inactive for this reaction 60). More interesting selectivity may be found in the reaction of olefins with methylene complexes the reaction of olefins with mononuclear methylene undergoes an olefin metathesis reaction, but the reaction of ethylene with bridging methylene in /i-CH2Co2(CO)2(Cp)2 61), /<-CH2Fe2(CO)8 (62), and /<-CH2-/i-ClTi(Cp)2Al(Me)2 (65) (Cp = cyclopentadiene) leads to propene formation (homologation reaction). 

Scheme 21. Air-stable chiral Ru-based catalyst for olefin metathesis can be used for highly effective and selective AROM/CM reactions...

For select reviews on catalytic olefin metathesis, see Grubbs RH, Chang S (1998) Tetrahedron 54 4413 Furstner A (2000) Angew Chem Int Ed 39 3012... 

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