Preparation of iniidazolin-2-iminato molybdenum and tungsten benzylidyne complexes: A new pathway to highly active alkyne metathesis catalysts

摘要

The reaction of [PhC=MBr_3(dme)] (dme = 1,2-dimethoxyethane) with the hexafluoro-fert-butoxides LiJ. or KX [X = OC(CF_3)_2Me] afforded the benzylidyne complexes [PhC=MX_3(dme)] (2a: M = W, 2b: M = Mo), which further reacted with the lithium reagent Li(Im_(tBu)N), generated with MeLi from 1,3-di-tert-butylimidazolin2-imine (Im ~(tBu)NH), to form the imidazolin-2-iminato complexes [PhC=MX _2(Im~(tBu)N)] (3a: M = W, 3b: M = Mo). The propylidyne complex [EtC=MoX_2(NIm~(rBu))] (4) was obtained by treatment of 3b 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-pentyny loxymethyl)ben- zene (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.