Beta-H transfer from the metallacyclobutane: a key step in the deactivation and byproduct formation for the well-defined silica-supported rhenium alkylidene alkene metathesis catalyst.
The surface complex [([triple bond]SiO)Re([triple bond]CtBu)(=CHtBu)(CH2tBu)] (1) is a highly efficient propene metathesis catalyst with high initial activities and a good productivity. However, it undergoes a fast deactivation process with time on stream, which is first order in active sites and ethene. Noteworthy, 1-butene and pentenes, unexpected products in the metathesis of propene, are formed as primary products, in large amount relative to Re (>>1 equiv/Re), showing that their formation is not associated with the formation of inactive species. DFT calculations on molecular model systems show that byproduct formation and deactivation start by a beta-H transfer trans to the weak sigma-donor ligand (siloxy) at the metallacyclobutane intermediate having a square-based pyramid geometry. This key step has an energy barrier slightly higher than that calculated for olefin metathesis. After beta-H transfer, the most accessible pathway is the insertion of ethene in the Re-H bond. The resulting pentacoordinated trisperhydrocarbyl complex rearranges via either (1) alpha-H abstraction yielding the unexpected 1-butene byproduct and the regeneration of the catalyst or (2) beta-H abstraction leading to degrafting. These deactivation and byproduct formation pathways are in full agreement with the experimental data.
Leduc
Anne-Marie
AM
Institut de Chimie, Université de Lyon, C2P2 CNRS-Université Lyon 1-ESCPE Lyon, F308-43 F-69616 Villeurbanne Cedex, France.
Salameh
Alain
A
Soulivong
Daravong
D
Chabanas
Mathieu
M
Basset
Jean-Marie
JM
Copéret
Christophe
C
Solans-Monfort
Xavier
X
Clot
Eric
E
Eisenstein
Odile
O
Böhm
Volker P W
VP
Röper
Michael
M
D000475:Alkenes / Q000737:chemistry*; D000480:Alkynes / Q000737:chemistry*; D002384:Catalysis; D003503:Cyclobutanes / Q000737:chemistry*; D003517:Cyclopentanes / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry; D007536:Isomerism; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D012211:Rhenium / Q000737:chemistry*; D012822:Silicon Dioxide / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013816:Thermodynamics
D000475:Alkenes; D000480:Alkynes; D003503:Cyclobutanes; D003517:Cyclopentanes; D009942:Organometallic Compounds; D012211:Rhenium; D012822:Silicon Dioxide; D006859:Hydrogen; C013658:propylene