Further studies of imido alkylidene complexes of tungsten, well-characterized olefin metathesis catalysts with controllable activity

Article abstract of DOI:10.1021/om00158a025

An alternative synthesis of W(CH-t-Bu)(NAr)(dme)Cl₂ (Ar = 2,6-C₆-H₃-i-Pr₂) consists of the five steps WCl₆ → W(O)Cl₄ → W(NAr)Cl₄ → W(NAr)(O-t-Bu)₂Cl₂(THF) → W(NAr)(O-t-Bu)₂(CH₂-t-Bu)₂ → W-(CH-t-Bu)(NAr)(dme)Cl₂, in which tert-butoxide "protecting groups" are replaced by chlorides in the last step upon addition of PCl₅. The easiest synthesis to a catalyst precursor consists of the three steps WO₂Cl₂ → W(NAr)₂Cl₂(dme) → W(NAr)₂(CH₂R)₂ → W(CHR)(NAr)(OTf)₂(dme) (R = t-Bu, CMe₂Ph; OTf = OSO₂CF₃), in which an imido ligand protecting group is ultimately replaced by two triflate ligands upon addition of triflic acid in the last step. An X-ray study of W(CH-t-Bu)(NAr)(O-t-Bu)₂ shows it to be a pseudotetrahedral complex in which the tert-butyl group points toward the imido ligand (syn conformation; space group P1, a = 14.050 (5) ?, b = 18.885 (5) ?, c = 11.123 (5) ?, α = 92.22 (3)°, β = 108.30 (3)°, γ = 79.25 (2)°, V = 2752 (2) ?³, Z = 4, M_r 572.46, ρ = 1.381 g cm^-3, μ = 43.03 cm^-1; R = 0.039, R_w = 0.043). Complexes of the type W(CH-t-Bu)(NAr′)(OR)₂ (NAr′ = N-2,6-C₆H₃Me₂; OR = O-t-Bu, OCMe₂(CF₃), OCMe(CF₃)₂, OC(CF₃)₂(CF₂CF₂CF₃)) were prepared by methods analogous to those used originally to prepare NAr complexes. Reactions between NAr′ complexes and olefins in general yield less stable organometallic products than when the NAr ligand is present. In one case (addition of internal olefins to W(CH-t-Bu)(NAr′)[OCMe(CF₃)₂]₂) a product was isolated that was consistent with the formation {W(NAr′)-[OCMe(CF₃)₂]₂}₂. Some of the W(CH-t-Bu)(NAr)X₂ variations that were prepared include X = OAr, OCEt₃, OCMe₂Ph, SAr, and CH₂-t-Bu. Other variations include W(CHEt)(NAr)X₂ complexes (X = OCEt₃, NPh₂), W(CHSiMe₃)(NAr)X₂ complexes (X = OAr, OCMe₂(CF₃), OCMe(CF₃)₂), and W[CHSi(OMe)₃](NAr)X₂ complexes (X = OAr, OCMe₂(CF₃), OCMe(CF₃)₂). Syn and anti rotamers of W(CHSiMe₃)(NAr)(OAr)₂ were observed and found to interconvert on the NMR time scale (ΔG?₂₉₈ = 15.0 (1) kcal mol^-1). None of the variations have any obvious advantage over known alkoxide/NAr complexes for metathesis of ordinary or strained cyclic olefins. An attempt to prepare a derivative containing the OC(CF₃)₂(tolyl) ligand yielded W[OC(C₆H₃Me)(CF₃)₂](NAr)[OC(CF ₃)₂(tolyl)](CH₂-t-Bu), formed by addition of an ortho CH bond to the W=C bond (space group P2₁/c, a = 16.821 (2) ?, b = 11.951 (1) ?, c = 19.455 (4) ?, β = 93.852 (8)°, V = 3920.5 ?³, Z = 4, M_r 943.6, ρ(calcd) = 1.606 g cm^-3, μ = 31.09 cm^-1; R = 0.038, R_w = 0.043).