Sterically Driven Olefin Metathesis: The Impact of Alkylidene Substitution on Catalyst Activity

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• 作者：Justin A. M. Lummiss ; Frédéric A. Perras ; Robert McDonald ; David L. Bryce ; Deryn E. Fogg
• 刊名：Organometallics
• 出版年：2016
• 出版时间：March 14, 2016
• 年：2016
• 卷：35
• 期：5
• 页码：691-698
• 全文大小：460K
• ISSN：1520-6041

文摘

The dramatic reactivity difference between the Grubbs metathesis catalysts and their resting-state methylidene derivatives was probed in an integrated crystallographic, solid-state NMR and localized molecular orbital analysis study. A principal focus was the second-generation Grubbs system RuCl₂(H₂IMes)(PCy₃)(═CHR) (GII, R = Ph; GIIm, R = H); supporting studies were carried out with the first-generation species RuCl₂(PCy₃)₂(═CHR) (GI, GIm). The compiled rate constants for PCy₃ dissociation demonstrate the limited lability of the methylidene complexes (e.g., ca. 275-fold lower for GIIm than GII and nearly 2000 times lower for the IMes analogue GIIm′). This is important because it impedes catalyst re-entry from the resting state into the active cycle. The ³¹P chemical shift (CS) tensors for the PCy₃ ligand exhibited the expected changes (i.e., those characteristic of an increased Ru–P orbital interaction) in GIIm relative to GII, as did GIm vs GI. Greater insight was offered by the ¹³C CS tensors. Whereas calculations on truncated models predict significant differences in ¹³C CS tensor values for GII compared with GIIm, the experimental values are equivalent, implying a compensating effect that weakens the Ru═C interaction in the benzylidene complex. Published X-ray crystallographic parameters for GII and GI reveal that one chloride ligand is displaced below the basal plane by steric interactions with the benzylidene phenyl group, an effect absent in GIIm and GIm. During PCy₃ loss from the [Ru]═CHPh systems, established processes of alkylidene rotation transform Ph–Cl repulsion into Ph–PCy₃ repulsion. Displacing the PCy₃ ligand below the plane does not relieve this conflict, instead incurring steric interactions with the H₂IMes ligand. Enhanced PCy₃ lability in the benzylidene complexes, relative to their methylidene analogues, is hence proposed to originate in the steric pressure exerted by the Ph substituent.