Unexpected Reactivity Patterns of Ruthenium Alkylidenes with N‑Phosphino-Functionalized N‑Heterocyclic Carbene Ligands (NHCPs)

N-phosphino-functionalized
N-heterocyclic carbene (NHCP) ligands
have been evaluated as potential supporting ligands in ruthenium-catalyzed
olefin metathesis. Initial density functional theory (DFT) calculations
suggested that these NHCP ligands may allow access to neutral 14 valence
electron (VE) speciesequivalents of the active 14 VE species
formed by phosphine dissociation from Grubbs II precatalystsvia
facile decoordination of the NHCP phosphino donor of the strained
four-membered [RuPNC] chelate systems. Their attempted synthesis from
NHCPs and Grubbs-type Ru carbenes revealed addition of an NHCP donor
atom (P or C) to the alkylidene fragment, forming a new C–P
or C–C bond in five-membered chelate structures. DFT investigations
showed that these reactions are controlled kinetically and must not
be neglected as important possible deactivation routes in olefin metathesis.