Fundamental Studies of Tungsten Alkylidene Imido Monoalkoxidepyrrolide Complexes

Abstract

Two diastereomers of the monoalkoxidepyrrolide (MAP) species, W(NAr)(CH₂)(Me₂Pyr)(OR*) (1; Ar = 2,6-diisopropylphenyl, Me₂Pyr = 2,5-dimethylpyrrolide, OR* = (R)-3,3′-dibromo-2′-(tert-butyldimethylsilyloxy)-5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-binaphthyl-2-olate), were generated through addition of R*OH to W(NAr)(CH₂)(Me₂Pyr)₂. The unsubstituted tungstacyclobutane species, W(NAr)(C₃H₆)(Me₂Pyr)(OR*) (2), was isolated by treating the mixture of diastereomers of 1 with ethylene. An X-ray study revealed 2 to have a trigonal bipyramidal structure in which the imido and phenoxide ligands are in axial positions. A variety of NMR experiments were carried out on 1 and 2. The major findings are the following: (i) the methylidene ligands in the two diastereomers of 1 rotate readily about the W═C bond (k = 2−7 s⁻¹ at 22 °C); (ii) NMR studies are consistent with 2 breaking up to give an intermediate alkylidene/ethylene complex, (R)- and (S)-W(CH₂)(C₂H₄); and (iii) the ethylene in the (R)-W(CH₂)(C₂H₄) intermediate can rotate about the W−ethylene bond axis at approximately the same rate as 2 re-forms or ethylene is lost to give 1. Compound 1 reacts with trimethylphosphine to yield (R)-1(PMe₃). Two intermediate PMe₃ adducts were observed and found to convert to (R)-1(PMe₃) in an intramolecular fashion with an average rate constant at 5 °C of ∼1.4 × 10⁻⁴ s⁻¹. Both neophylidene (4) and methylidene (5) MAP species containing 2,3,5,6-tetraphenylphenoxide ligand also were prepared. Compound 5 can be heated to 80 °C, where methylidene rotation about the W═C bond is facile and observable in a variable-temperature ¹H NMR spectrum. A ¹H−¹H EXSY spectrum of 5 in benzene-d₆ at 20 °C showed that the methylidene protons are exchanging with k = 90 s⁻¹. A structure of 5(THF) showed it to be a square pyramid with the methylidene ligand in the apical position and THF coordinated trans to the imido ligand. Exposure of 5 to ethylene generated the tungstacyclobutane complex, W(NAr)(C₃H₆)(Me₂Pyr)(OR) (6), whose structure is analogous to that of 2. Treatment of 5 with PMe₃ yielded yellow 5(PMe₃), an X-ray study of which revealed it to be a square pyramid with the methylidene ligand in the apical position and the phosphine trans to the pyrrolide. These studies suggest that metallacyclobutane intermediates in metathesis reactions with MAP species are likely to contain axial imido and phenoxide ligands, that metallacycles are formed when an olefin approaches the metal in a MAP species trans to the pyrrolide, and that the configuration at the metal inverts as a consequence of each forward metathesis step.