Recent Advances in High Oxidation State Mo and W Imido Alkylidene Chemistry

Abstract

Introduction 3211 2. Principles of Tungsten and Molybdenum Imido Alkylidene Chemistry 3212 3. Precursors to Bisalkoxides and Monoalkoxides 3214 4. Studies Concerning Monoalkoxide Pyrrolide (MAP) Species 3216 5. Catalyst Decomposition in Solution 3218 6. Regeneration of Alkylidenes 3220 7. Catalysts Supported on Silica 3221 8. New Reactions 3222 9. Concluding Remarks 3224 10. Acknowledgments 3224 11. References 3224

Ring Opening Metathesis Polymerization of Norbornene and Derivatives by the Triply Bonded Ditungsten Complex Na[W2(µ-Cl)3Cl4(THF)2]·(THF)3

“…As the reaction progresses, the intensity of the above peaks increases, along with parallel and increasing formation of PNBD. Corroborative evidence for the alkylidene ligand(s) by 13 C-NMR could not be obtained, presumably as a consequence of their minute concentration. The multiplicity of the signals could be attributed either to formation of several active species deriving from the 1/NBD interaction, to fasbo/esbo equilibria and/or concurrent metathetical reactions.…”

“…Among them, protagonists are those of ruthenium, molybdenum and tungsten. For the last two, the following major categories have been classified [1][2][3][4][5][6][9][10][11] ) exhibiting small to moderate activity; these become very reactive when activated by organometallic cocatalysts (e.g., SnMe 4 , AlEt 3 ); (ii) discreet alkylidenes, such as the well-defined Katz [(OC) 5 W=C(Ph)R] (R = OMe, Ph) [12] and Schrock ([M(NAr)(CHR)(OR') 2 ] (M = Mo, W; Ar = aryl; R = aryl or alkyl; R' = CMe 3 , CMe 2 CF 3 , CMe(CF 3 ) 2 , C(CF 3 ) 3 ) [13] catalysts and their numerous variations. Despite the abundance of mononuclear catalytic systems, there is only a limited number of bimetallic multiply metal-metal bonded complexes inducing the ROMP of NBE.…”

“…The stereochemistry of the polymers obtained (PNBE, PNBE-COOME, PVNBE and the soluble part of PNBD) was determined by 1 H and 13 C NMR [28][29][30][31][32]. tt).…”

“…The reactivity ratios r c = cc/ct = 7.4, r t = tt/tc = 1.3 and r c r t = 9.6 were calculated from the heights of the relevant signals in the 13 C NMR spectra. Comparison of 1 (as per reactivity, yield, M w , and cis-selectivity) with other representative catalytic systems revealed that it is much more reactive than the Masuda type W(VI) halides, resembling more to the bicomponent ones (e.g., WCl 6 /AlEtCl 2 , SnMe 4 etc.)…”

“…Similar was the fate of attempts to establish whether the active species are bimetallic or 1 serves as a precursor to mononuclear compounds (by ESI-MS). Nevertheless, valuable information has been obtained by monitoring in situ the reactions of 1 with the substrates by 1 H and 13 C NMR. Solvent of choice was tetrahydrofuran (THF) because of the rapidity of the polymerizations in halogenated solvents (CHCl 3 , CH 2 Cl 2 , PNBD formation even at low ratio (1/NBD: 1/2), gelation at higher ones) and the insufficient solubility of 1 in the above solvents at lower temperatures.…”