Preparation of Imidazolin‐2‐iminato Molybdenum and Tungsten Benzylidyne Complexes: A New Pathway to Highly Active Alkyne Metathesis Catalysts

The reaction of [PhCMBr 3 (dme)] (dme=1,2‐dimethoxyethane) with the hexafluoro‐ tert‐ butoxides LiX or KX [X=OC(CF 3 ) 2 Me] afforded the benzylidyne complexes [PhCMX 3 (dme)] ( 2 a : M=W, 2 b : M=Mo), which further reacted with the lithium reagent Li(Im t Bu N), generated with MeLi from 1,3‐di‐ tert ‐butylimidazolin‐2‐imine (Im t Bu NH), to form the imidazolin‐2‐iminato complexes [PhCMX 2 (Im t Bu N)] ( 3 a : M=W, 3 b : M=Mo). The propylidyne complex [EtCMoX 2 (NIm t Bu )] ( 4 ) was obtained by treatment of 3 b with an excess of 3‐hexyne. Complexes 3 a and 3 b are able to efficiently catalyse alkyne cross metathesis of various 3‐pentynyl benzyl ethers 5 and benzoic esters 7 at room temperature, to afford 2‐butyne and the corresponding diethers 6 and diesters 8 . The tungsten complex 3 a proved to be a superior catalyst for ring‐closing alkyne metathesis, and the [10]cyclophanes 10 and 12 were synthesised in high yield from 1,3‐bis(3‐pentynyloxymethyl)benzene ( 9 ) and bis(3‐pentynyl) phthalate ( 11 ), respectively. The molecular structures of compounds 2 a , 2 b , 3 a , 3 b , 4 , and 12 were determined by single‐crystal X‐ray diffraction. DFT calculations have been carried out for catalyst systems based on the imidazolin‐2‐iminato tungsten and molybdenum complexes 3 a and 3 b by choosing the alkyne metathesis of 2‐butyne as the model reaction; the studies revealed a lower activation barrier for the tungsten system.