Stability of the first-generation Grubbs metathesis catalyst in a continuous flow reactor

Ethylene pretreatment of the (PCy3)2Cl2RuCHPh catalyst (1) prior to cross-metathesis of ethylene and cis-2-butene to form propylene in the continuous flow reactor produced a direct effect on catalyst deactivation. Similar cis-2-butene pretreatment of the same catalyst exhibited far less change in the catalyst activity. These results support the assumption that the ruthenium methylidene intermediate generated from ethylene and 1 is unstable and promotes catalyst loss while ruthenium alkylidenes, e.g. derived from 2-butene, exhibit significantly enhanced stability and sustained catalyst integrity. Continuous removal of products in the continuous flow reactor was important for separating the catalyst decay and the catalyst deactivation caused by a terminal olefin, in this case propylene.The amount of produced propylene during the 1 lifespan was determined in a series of tests using identical catalyst concentrations ([Ru] = 60 ppm) in pentadecane while varying the olefin pretreatment times from 0 to 420 min. The catalyst turnover numbers in the cross-metathesis experiments proved inversely proportional to the duration of ethylene treatment prior to the reaction. The activity of 1 pre-exposed to ethylene closely matched with the activity of the catalyst that decayed in the reaction mixture containing ethylene and cis-2-butene for the same period of time. A significant contribution of the Ru-methylidene decay to the activity losses in metathesis reactions was demonstrated directly in the cross-metathesis reaction environment. The catalyst proved to be less sensitive to cis-2-butene pretreatment and showed turnover numbers for subsequent cross-metathesis essentially similar to the reference cross-metathesis test.

Graphical abstractPretreatment of the (PCy3)2Cl2RuCHPh catalyst with ethylene in the continuous flow reactor in pentadecane prior to charging cis-2-butene to initiate cross-metathesis with formation of propylene indicates that the active catalyst loss due to pre-exposure to ethylene occurs with the same rate as in the productive reaction environment with both ethylene and cis-2-butene initially present.Figure optionsDownload full-size imageDownload as PowerPoint slide