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

Abstract

The reaction of [PhC[triple bond]MBr(3)(dme)] (dme=1,2-dimethoxyethane) with the hexafluoro-tert-butoxides LiX or KX [X=OC(CF(3))(2)Me] afforded the benzylidyne complexes [PhC[triple bond]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-butylimidazolin-2-imine (Im(tBu)NH), to form the imidazolin-2-iminato complexes [PhC[triple bond]MX(2)(Im(tBu)N)] (3a: M=W, 3b: M=Mo). The propylidyne complex [EtC[triple bond]MoX(2)(NIm(tBu))] (4) was obtained by treatment of 3b with an excess of 3-hexyne. Complexes 3a and 3b 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 3a 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 2a, 2b, 3a, 3b, 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 3a and 3b by choosing the alkyne metathesis of 2-butyne as the model reaction; the studies revealed a lower activation barrier for the tungsten system.

Phosphoraneiminato tungsten alkylidyne complexes as highly efficient alkyne metathesis catalysts

“…Running the experiments under the same conditions for 1 h afforded the product 5 in virtually quantitative yields (>99%) as a colourless oil after filtration through alumina, exhaustive extraction with diethyl ether and evaporation of the solvent. 1 H and 13 C NMR spectroscopic characterisation indicated in both cases the presence of (spectroscopically) pure 5 [11], see Supporting Information.…”

“…Gas chromatographic (GC) measurements were carried out on an SHIMADZU GC-2010 instrument. [Me 3 CC^W {OCMe(CF 3 ) 2 } 3 (DME)] (DME ¼ 1,2-dimethoxyethane) [18], R 3 PNSiMe 3 , R 3 PNH, R 3 PNLi (R ¼ Cy, iPr) [19e21] and the substrates 4 [11] and 6 [22] were prepared according to published procedures. The 1 H and 13 C NMR spectra of the organic metathesis products 5 and 7 are shown as Supporting Information.…”

“…1). These complexes were shown to serve as efficient catalysts for alkyne cross-metathesis (ACM), ring-closing alkyne metathesis (RCAM) and ring-opening alkyne metathesis polymerization (ROAMP) even at room temperature and with low catalyst loadings (1e2 mol%) [10,11,15].…”

“…In particular, significant efforts were made to establish alkylidyne complexes as homogeneous catalysts for applications in alkyne metathesis and to develop catalyst systems that live up to expectations with regard to their activity, substrate compatibility, and required reaction temperature [8]. More recently [9], a number of new tungsten-and molybdenum-based catalysts were introduced that rely on the use of ancillary imidazolin-2-iminato [10,11], silanolate [12,13], or chelating polyphenolate ligands [14]; our own main contribution to this field comprises the preparation of imidazolin-2-iminato neopentylidyne and benzylidyne complexes [RC^M(NIm tBu …”

Towards Highly Active and Robust Alkyne Metathesis Catalysts: Recent Developments in Catalyst Design

“…Among the variants of the Schrock-type alkylidyne complexes, the imidazolin-2-iminatotungstenneopentylidyne complexes such as 1 and 2 (Scheme 2 a) recently reported by Tamm and co-workers display high catalytic activity in a variety of metathesis reactions at ambient temperature and with low catalyst loadings. [10] The approach of introducing imidazolin-2-imide ligands is based on the strong electrondonating capacity of the imidazolium ring towards the Mo or W center. Theoretical calculations showed that the combination of electron-withdrawing alkoxides, such as hexafluorotert-butoxide, with strongly electron-donating imidazolin-2-imide ligands provides a proper balance between the stability and the electrophilicity of the resulting tungsten alkylidyne complexes of type 1.…”