Isomerization activity, double-bond metathesis

Rhenium oxide-alumina catalysts are reduced at ambient temperatures and sub-atmospheric pressure by propene and higher alkenes, generating metathesis activity. Ethylene at these conditions did not show any reduction capabilities. Reduction with CO or NH3 at 300-500° C did not result in metathesis activity. At room temperature CO did not adsorb on reduced catalysts however, NO adsorbs and is a poison for the olefin metathesis reaction. Water generated in reducing catalysts with alkenes is mainly associatively adsorbed and, at ambient temperatures, exchanges hydrogen atoms with propene and butene. Activity for double-bond isomerization is partly accounted for by associatively adsorbed water, which generates acidity. ... 

Other catalysts containing WO have been shown to have activity in double bond shift and in metathesis (ref. 6,7,8). The conditions for skeletal isomerization developed in the present work do not favour the metathesis reaction and it makes no apparent contribution to the product. However double bond shift does accompany skeletal isomerization. 

Pent-l-ene metathesis is observed with W(CO)6/ZrCl4//iv and W(CO)6/TiCl4/Av, but is accompanied by double-bond isomerization and cross-metathesis of pent-2-ene with pent-l-ene. In the series of [WL(CO)5]-type complexes (L = CI, CO, py, PPh3), the most active for pent-2-ene metathesis is (Et4N)[WCl(CO)5] (Szymanska-Buzar 1987). 

The elucidation of the role of double-bond isomerization activity in metathesis process is an example of the helpfulness of the four-center mechanism. As the scheme predicted, in certain applications the elimination, of double-bond isomerization activity (acidic isomerization sites were destroyed by various mild caustic treatments) prevented secondary metathesis reaction resulting in very high selectivity to specific products ( 5). In contrast, in other applications (e.g., linear olefin and neohexene processes) to obtain a high level of productive metathesis, the mechanistic scheme indicated a need for enhanced isomerization activity this was accomplished by addition of a very selective double-bond isomerization catalyst to the scheme ( ),... 

Our recent study on the activity for metathesis revealed a high reactivity of 1,9-decadiene and cyclooctene in their reactions with triethoxy- and trisiloxy-substituted vinylsilanes [20]. When the mixture containing vinylsilane and 1,9-decadiene was heated in the presence of Grubbs catalyst, the formation of mono- and bis(silyl)dienes, accompanied by a polymeric product, was detected. The replacement of 1,9-diene with cyclooctene in the reaction mixture has resulted in the same products. The reactions can be illustrated by Eqs. 6 and 7. l,10-bis(silyl)-substituted dienes were isolated with 50-70 % yields and characterized spectroscopically. In the case of disubstituted products separated by distillation, no double bond migration was observed. The only process observed in that case was E/Z isomerization, so only Z,Z-disubstituted products could be isolated. 

---