Direct observation of reaction intermediates for a well defined heterogeneous alkene metathesis catalyst

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 105; no. 34; pp. 12123 - 12127
• Main Authors: Blanc, Frédéric, Berthoud, Romain, Copéret, Christophe, Lesage, Anne, Emsley, Lyndon, Singh, Rojendra, Kreickmann, Thorsten, Schrock, Richard R
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 26.08.2008; National Acad Sciences
• Subjects: Alkenes; Carbon; Catalysis; Catalysts; Chemicals; Ligands; Metathesis; NMR; Nuclear magnetic resonance; Physical Sciences; Protons; Reaction intermediates; Reactivity; Silica; Spectroscopy
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0802147105

Grafting of [W([identical with]NAr)(=CHtBu)(2,5-Me₂NC₄H₂)₂] on a silica partially dehydroxylated at 700°C (SiO₂₋ ₍₇₀₀₎) generates the corresponding monosiloxy complex [([identical with]SiO)W([identical with]NAr)(=CHtBu)(2,5-Me₂NC₄H₂)] as the major species ([almost equal to]90%) along with [([identical with]SiO)W([identical with]NAr)(CH₂tBu)(2,5-Me₂NC₄H₂)₂], according to mass balance analysis, IR, and NMR studies. This heterogeneous catalyst displays good activity and stability in the metathesis of propene. Very importantly, solid state NMR spectroscopy allows observation of the propagating alkylidene as well as stable metallacyclobutane intermediates. These species have the same reactivity as the initial surface complex [([identical with]SiO)W([identical with]NAr)(=CHtBu)(2,5-Me₂NC₄H₂)], which shows that they are the key intermediates of alkene metathesis.