Metathesis of terminal olefins

A classical catalyst for metathesis reactions 1 reminiscent of a polymerization Ziegler catalyst it is essentially a combination of a transition metal halide (WCU. MoCl ) and an alkyl metal derivative (AIR). SnK. etc). It is noteworthy that a reduction step occurs during the constitution of the active center, since an elTicieni metathesis of terminal olefins has been achieved under electro-catalytic conditions [40]. (CI4 being the active carbenic entities in... 

Apart Ifom the molybdenum carbene complexes already listed in Tables 2.1 and 2.2 Mo-based catalysts are of three main types (i) other Mo complexes, activated by a suitable cocatalyst (ii) M0CI5, also activated by a cocatalyst and (iii) supported oxides, generated in various ways. For the metathesis of terminal olefins higher than propene. Mo-based catalysts are generally more effective than the corresponding W-based catalysts. 

W-based catalysts for the metathesis of terminal olefins are comparatively few in number. However, this is partly an illusion because systems such as WClg/EtAlCla/ EtOH, although not effective in the sense of yielding ethene and an internal olefin, cause rapid non-productive metathesis in which the products can only be distinguished from the reactants by isotopic labelling see Ch. 5. 

Table 6.2 Initial percentage trans content of the products RCH=CHR of the metathesis of terminal olefins RCH=CH2 at 25°C...

Degenerate metathesis of terminal olefins by Cp2Ti(CH2AlMe2Cl) [45]... 

Molybdenum catalyst are usually efficient for the metathesis of terminal olefins, whereas tungsten complexes are not. However tungsten complexes are active for the metathesis of internal olefins. One possible explanation of this observation is that metallacarbenes combine with terming alkenes selectively according to ... 

Snapper and coworkers [38] developed a tandem process where olefination was used in conjunction with olefin metathesis. In this process, Ru complex 51 or 52 was an efficient catalyst for cross metathesis of terminal olefins 47 and 48. 

The reversible nature of cross metathesis is of synthetic importance because, by the use of a sufficiently active metathesis catalyst, it generally ensures the preferential formation of the most thermodynamically stable product. This results in the transformation of terminal olefins into internal ones, and we have seen that undesired self-metathesis products can be recycled by exposing them to a second CM process. 

It also explains the /Z selectivity of products at low conversions (kinetic ratio. Scheme 19). In the case of propene, a terminal olefin, E 2-butene is usually favoured (E/Z - 2.5 Scheme 19), while Z 3-heptene is transformed into 3-hexene and 4-octene with EjZ ratios of 0.75 and 0.6, respectively, which shows that in this case Z-olefins are favoured (Scheme 20). At full conversion, the thermodynamic equilibriums are reached to give the -olefins as the major isomers in both cases. For terminal olefins, the E olefin is the kinetic product because the favoured pathway involved intermediates in which the [ 1,2]-interactions are minimized, that is when both substituents (methyls) are least interacting. In the metathesis of Z-olefins, the metallacyclobutanes are trisubstituted, and Z-olefins are the kinetic products because they invoke reaction intermediates in which [1,2] and especially [1,3] interactions are minimized. 

One needs to emphasize that the extent of these hydride shifts, in comparison to the overall reaction, is infinitesimal. Otherwise, gross hydrogen exchanges would have obscured the effectiveness of reported labeling experiments, particularly those which establish the site of scission (59) and the occurrence of regenerative metathesis in terminal olefins (60, 61). 

Boelhouwer s discovery (23) prompted a flurry of activity in this area. Baker applied the Boelhouwer catalyst to the metathesis of w-olefinic esters (88). At an ester/W molar ratio of 20/1 (68°C), symmetrical olefinic diesters were formed in 34-36% yields with concomitant elimination of ethylene. In addition, Baker identified products recovered in 3-8% yield corresponding to addition of HC1 across the terminal double bond. 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

Recendy, we found that A -allyl-o-vii rlaniline 44 gave 1,2-dihydroquinoline 45 by normal RCM and developed silyl enol ether-ene metathesis for the novel synthesis of 4-siloxy-1,2-dihydroquinoline and demonstrated a convenient entry to quinolines and 1,2,3,4-tetrahydroquinoline [13], We also have found a novel selective isomerization of terminal olefin to give the corresponding enamide 46 using rathenium carbene catalyst [Ru] and silyl enol ether [14], which represented a new synthetic route to a series of substituted indoles 47 [12], We also succeeded an unambiguous characterization of mthenium hydride complex [RuH] with ACheterocyclic carbene... 

Recent examples of the first route have been described by Kobayashi and coworkers, who reported the synthesis and characterization of polymeric Pcs obtained through the olefin metathesis polymerization of terminal olefin groups in the side chains of unsymmetrical Pc monomers [159], X-ray analysis of the solid material indicates that the Pcs are ordered in stacks. 

Metathesis reactions are essentially reversible, which under normal circumstances ensures the preferential formation of the thermodynamic product. The employment of terminal olefins (particularly in RCM and CM processes) results in the liberation of ethylene gas from the equilibrium, a tactic often used to drive metathesis reactions. Over the last decade, the advent of active and functional-group-insensitive catalysts have broadened the scope of the reaction to the extent that currently only minimal (if any) protection of Lewis-basic functionality is required. 

Substituted vinylphosphonates (195) and allylphosphonates (196) with E-olefin stereochemistry have been prepared for the first time via intermolecular olefin cross-metathesis (CM) using ruthenium alkylidene complex (197) in good yield. A variety of terminal olefins, styrenes and geminally substituted olefins has been successfully employed in these reactions (Scheme 49). ... 

The conversion can be done by metathesis in the form of the ethenolysis reaction. In this, inner olefins are treated with ethylene to form two moles of terminal olefins. For example, by the reaction of... 

The i-oleftns obtained have a purity of more than 95 molar per cent of terminal olefins and are devoid of branched structures or diolefinic or cyclic impurities. These z-olefms, which all have an even number of carbon atoms, are produced with yields that vary according to a statistical distribution, with a maximum of C, 0-C, 2-Cj, for example, if the final product is intended for the manufacture of linear alkylbenzene. Due to the low yield of a given cut, it is understandable that the upgrading of all the other fractions is economically necessary. Since the upgrading of the light and heavy fractions is often a problem. Shell in the United States has developed the SHOP (Shell Higher Olefin Process), in which these effluents are converted to a cut for detergents by isomerization and metathesis. 

Many stable tungstacyclobutane complexes are known, but few will initiate the metathesis of internal olefins or ROMP of cyclic olefins. Yet many will undergo exchange reactions with ethene or terminal olefins by a mechanism which must involve dissociation to a tungsten carbene complex. A great deal can therefore be learnt about the olefin metathesis mechanism from a study of such reactions. The following is a short summary. 

The behaviour of pent-l-ene is typical of that of terminal olefins. Its metathesis to form oct-4-ene and ethene, reaction (9), is most readily achieved with high... 

Examples of processes involving a-bond metathesis are recently increasing [119]. Hydroboration of terminal olefins is catalyzed by lanthanoid hydride or... 

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