Propene cross-metathesis

Note also that (1) d° Ta alkyhdene complexes are alkane metathesis catalyst precursors (2) the cross-metathesis products in the metathesis of propane on Ta are similar to those obtained in the metathesis of propene on Re they differ only by 2 protons and (3) their ratio is similar to that observed for the initiation products in the metathesis of propane on [(=SiO)Ta(= CHfBu)(CH2fBu)2]. Therefore, the key step in alkane metathesis could probably involve the same key step as in olefin metathesis (Scheme 27) [ 101 ]. 

Note also that, in contrast to classical heterogeneous catalysts, the initiation step of [=SiORe(=CtBu)(=CHtBu)(CH2tBu)] is well defined and corresponds to the cross-metathesis of the alkene with the neopentyhdene ligand. In fact, in the metathesis of propene, 0.7 equiv of a 3 1 mixture of 3,3-dimethyl-l-butene and 4,4-dimethyl-2-pentene is formed (Figure 3.27) the nearly quantitative formation of cross-metathesis products is consistent with a real single-site catalyst. Moreover,... 

C, no solvent). The grafted catalyst proved to be highly active, with equilibrium reached in less than Ih. A TOP of 0.25molmoT s was attained, which corresponds to one of the best rates observed for a Re metathesis catalyst. Also observed during the metathesis reaction was the evolution of approximately 1 equivalent of a 1 3 mixture of 3,3-dimethylbutene and 4,4-dimethyl-2-pentene, which arises from the cross-metathesis of the neopentyl ligand of the grafted complex and propene. 

Vinylsilanes undergo productive cross-metathesis (CM) and silylative coupling (SC) with allyl-substituted (N, B)functionalized alkenes to yield l-silyl-3,Ar, -substituted propenes with preference (for V-derivatives) and exclusive formation (for boronates) of the f-isomer. 

The aesthetically pleasing silica-supported alkyl, alkylidene, alkylidyne Re complex =SiORe[=C u][=CH Bu][CH2 Bu] (Scheme 3, Structures 15-17), prepared from the molecular precursor, has also been found to be highly reactive for the metathesis of propene [13]. Moreover, the evolution of roughly one equivalent of a 1 3 mixture of 3,3-dimethylbutene and 4,4-dimethyl-2-pentene is consistent with a cross-metathesis of the neopentylidene ligand of 1 and propene. The turnover obtained also exceeds those obtained with classical heterogeneous catalysts such as W03/Si02 used industrially for decades (Lummus process). 

Dimersol E is used to upgrade C2 + C3 fuel gas. Co-oligomerization of ethylene and propene leads to a gasoline stream very similar to the Dimersol G product. Mixed butenes are also obtained with Dimersol E (from ethylene dimerization). They can be used in paraffinic alkylation or to make propene through a subsequent cross-metathesis reaction with ethylene. 

Metathesis is a versatile reaction that forms the basis for several important industrial processes, such as the Phillips triolefin process, which produces propene by cross-metathesis of 2-butene with ethene, and the Shell higher olefins process (SHOP), which involves a combination process that converts ethene to detergent-range olefins. Several interesting polymeric materials are commercially produced via the ROMP of different types of unsaturated cyclic monomers, including nor-bornene, cyclooctene, and dicyclopentadiene [1]. 

The reaction of ds-pent-2-ene with 4-methylpent-l-ene has been investigated in some detail under conditions that allowed most of the ethene and propene to escape reaction (7). As may be seen from Fig. 9.2, the reaction is very rapid when catalyzed by Bu4N[MoCl(CO)5]/MeAlCl2 and reaches a steady state after 8 min. The cross-metathesis products, 6-methylhept-3-ene and 5-methylhex-2-ene, are formed in somewhat larger amount than the self-metathesis products, hex-3-ene and 2,7-dimethyloct-4-ene. The initial and final trans/cis ratios, obtained by plotting Doyle s data according to the method of Fig. 6.2, are (final values shown in brackets) but-2-ene 0.9 (2.5), 5-methylhex-2-ene 1.6 (3.3), 6-methylhept-3-ene 2.5... 

Various industrial companies have developed a metathesis process for the production of 2-methylbut-2-ene (isoamylene), which can be used to make isoprene via oxidative dehydrogenation. It is produced via cross-metathesis of isobutene and but-2-ene reaction (8). Cross-metathesis can also be carried out with propene instead of but-2-ene, or a mixture of the two. 

When cross-metathesis was first discovered, propene enjoyed only limited use and the reaction was viewed as a potential source of ethylene. Once methods were developed for the preparation of stereoregular polypropylene, however, propene became more valuable and cross-metathesis of ethylene and 2-butene now serves as a source of propene. 

The generally accepted mechanism for olefin cross-metathesis is outlined for the case of propene in Mechanism 14.4. The catalyst belongs to a class of organometallics known as a metallocarbene, carbene complex, or alkylidene complex. Its structure is characterized by a carbon-metal double bond. In olefin metathesis the metal is typically ruthenium (Ru), tungsten (W), or molybdenum (Mo). Transition-metal carbene complexes were first prepared by Ernst O. Fischer (Munich) who shared the 1973 Nobel Prize in Chemistry with Geoffrey Wilkinson. 

Various industrial companies have developed a process for the production of isoamylene (2-methyl but-2-ene), which is a precursor of isoprene (obtained by oxidative dehydrogenation). It can be produced by cross-metathesis of isobutene with but-2-ene or propene ... 

EXSY experiments were performed at -87 °C (186 K) to determine the rate of exchange between the cis and trans stereoisomers of metallacycles 26a-c and 27a-c (Table 8.2). These rate constants incorporate the rates of metallacycle cycloreversion, alkylidene rotation, and cycloaddition to interconvert the two species. Given that propene, 1-butene, and 1-hexene are all Type I olefins for cross-metathesis [42], it is not unexpected that these results correlate with each other. 

Bis(silyl)propenes have also been briefiy explored using Pd allyl chemistry,as well as cross-metathesis. ... 

Basset and his group have observed that propane and propene metathesis give similar Cn+i/Cn+2 ratios of cross-metathesis products on silica-supported tantalum-neopentylidene catalyst at 150°C. The olefin-metathesis activity of these Schrock-type supported complexes results from the presence of the silyloxy ligand (vide infra) - Organometallic complexes are bound to silica or alumina by reaction of soluble complexes and involve die formation of one or several bonds between the central metal and the oxygen atom of the oxide support. 

The only oxide that has been used for catalyzed olefin metathesis at 25°C is Re207/Al203 (in the middle of the 1960s by British Petroleum), but it suffered from a low number of active sites, side reactions caused by the acid support and deactivation of the catalyst. On die other hand, the silica-supported rhenium catalyst [(SiO)(Re(C-f-Bu)(=CH-f-Bu)(CH2-f-Bu)] catalyzes the metathesis of propene at 25°C with an initial rate of 0.25 mol/(mol Re x s). The formation of 3,3-dimethyl-butene and 4,4-dimethylpentene in a 3 1 ratio results from cross metathesis between propene and the neopentyl idene ligand, and die ratio of cross-metathesis products matches the relative stability of the metallacyclobutane intermediates. Cross metathesis of propene and isobutene and self-metathesis of methyl oleate can also... 

Mixed WOj/Al Oj/HY catalysts prepared by calcination of physically mixed WO3, Al Oj and HY zeolite showed unique behavior in the metathesis between ethene and 2-butene to produce propene [147]. Monomeric tetrahedrally coordinated surface tungstate species responsible for the metathesis activity were formed via the interaction with Bronsted acid sites of HY zeolite. Polytungstate clusters are supposed to be less active in the metathesis reaction. The best catalyst demonstrates the 2-butene conversion close to the thermodynamic equilibrium value ( 64%) at 453 K. The catalysts are bifunctional [148] they catalyze first isomerization of 1-butene to 2-butene and then cross-metathesis between 1-butene and 2-butene to produce propene and 2-pentene. 10%W03/Al203-70%HY exhibits the highest propene yield. 

For the Mo/H-Beta zeolites, the formation of the Al2(Mo04)3 phase and the decrease in the concentration of Brpnsted acid sites explains the low catalytic activity of Mo/H-Beta in metathesis of ethylene and 2-butylene to propylene [149]. A promoting effect of Mg was revealed in the Mo/H-Beta-Al203 catalyst for cross-metathesis of ethene and butene-2 to propene [150]. The stability is improved at the Mg content of l-2wt.% due to the elimination of weak acid sites and suppression of the side olefin oligomerization reaction. 

The Mechanism of the cross coupling reaction can be accommodated by an oxidative addition of 1-bromopropene to iron(l) followed by exchange with ethylmagnesium bromide and reductive elimination. Scheme 3 is intended to form a basis for discussion and further study of the catalytic mechanism. In order to maintain the stereospecificity, the oxidative addition of bromo-propene in step a should occur with retention. Similar stereochemistry has been observed in oxidative additions of platinum(O) and nickel(O) complexes.(32)(33) The metathesis of the iron(lll) intermediate in step b is ixp icted to be rapid in analogy with other alkylations.(34) The formation of a new carbon-carbon bond by the redilcTive elimination of a pair of carbon-centered ligands in step c has been demonstrated to occur... 

But-l-ene is allowed to cross-metathesize with propene, leading to pent-2-ene which is further isomerized into pent-l-ene. The sequence of these two reactions, metathesis and isomerization, is repeated as many times as necessary ... 

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