Fundamental Studies of Tungsten Alkylidene Imido Monoalkoxidepyrrolide Complexes (original) (raw)

Journal of The American Chemical Society, 2009

Abstract

Two diastereomers of the monoalkoxidepyrrolide (MAP) species, W(NAr)(CH(2))(Me(2)Pyr)(OR*) (1; Ar = 2,6-diisopropylphenyl, Me(2)Pyr = 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)(CH(2))(Me(2)Pyr)(2). The unsubstituted tungstacyclobutane species, W(NAr)(C(3)H(6))(Me(2)Pyr)(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 horizontal lineC bond (k = 2-7 s(-1) at 22 degrees C); (ii) NMR studies are consistent with 2 breaking up to give an intermediate alkylidene/ethylene complex, (R)- and (S)-W(CH(2))(C(2)H(4)); and (iii) the ethylene in the (R)-W(CH(2))(C(2)H(4)) 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(PMe(3)). Two intermediate PMe(3) adducts were observed and found to convert to (R)-1(PMe(3)) in an intramolecular fashion with an average rate constant at 5 degrees C of approximately 1.4 x 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 degrees C, where methylidene rotation about the W=C bond is facile and observable in a variable-temperature (1)H NMR spectrum. A (1)H-(1)H EXSY spectrum of 5 in benzene-d(6) at 20 degrees 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…

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