Timothy M. Champagne

Highly Selective Ruthenium Metathesis Catalysts for Ethenolysis

N-Aryl,N-alkyl N-heterocyclic carbene (NHC) ruthenium metathesis catalysts are highly selective toward the ethenolysis of methyl oleate, giving selectivity as high as 95% for the kinetic ethenolysis products over the thermodynamic self-metathesis products. The examples described herein represent some of the most selective NHC-based ruthenium catalysts for ethenolysis reactions to date. Furthermore, many of these catalysts show unusual preference and stability toward propagation as a methylidene species and provide good yields and turnover numbers at relatively low catalyst loading (<500 ppm). A catalyst comparison showed that ruthenium complexes bearing sterically hindered NHC substituents afforded greater selectivity and stability and exhibited longer catalyst lifetime during reactions. Comparative analysis of the catalyst preference for kinetic versus thermodynamic product formation was achieved via evaluation of their steady-state conversion in the cross-metathesis reaction of terminal olefins. These results coincided with the observed ethenolysis selectivities, in which the more selective catalysts reach a steady state characterized by lower conversion to cross-metathesis products compared to less selective catalysts, which show higher conversion to cross-metathesis products.

Low Catalyst Loadings in Olefin Metathesis: Synthesis of Nitrogen Heterocycles by Ring Closing Metathesis

A series of ruthenium catalysts have been screened under ring-closing metathesis (RCM) conditions to produce five-, six-, and seven-membered carbamate-protected cyclic amines. Many of these catalysts demonstrated excellent RCM activity and yields with as low as 500 ppm catalyst loadings. RCM of the five-membered carbamate series could be run neat, the six-membered carbamate series could be run at 1.0 M, and the seven-membered carbamate series worked best at 0.2-0.05 M.

C–H bond activation through steric crowding of normally inert ligands in the sterically crowded gadolinium and yttrium (C5Me5)3M complexes

Synthesis of the sterically crowded Tris(pentamethylcyclopentadienyl) lanthanide complexes, (C5Me5)3Ln, has demonstrated that organometallic complexes with unconventionally long metal ligand bond lengths can be isolated that provide options to develop new types of ligand reactivity based on steric crowding. Previously, the (C5Me5)3M complexes were known only with the larger lanthanides, La–Sm. The synthesis of even more crowded complexes of the smaller metals Gd and Y is reported here. These complexes allow an evaluation of the size/reactivity correlations previously limited to the larger metals and demonstrate a previously undescribed type of C5Me5-based reaction, namely C–H bond activation. (C5Me5)3Gd, was prepared from GdCl3 through (C5Me5)2GdCl2K(THF)2, (C5Me5)2Gd(C3H5), and [(C5Me5)2Gd][BPh4] and structurally characterized by x-ray crystallography. Although Gd3+ is redox-inactive, (C5Me5)3Gd functions as a reducing agent in reactions with 1,3,5,7-cyclooctatetraene (COT) and triphenylphosphine selenide to make (C5Me5)Gd(C8H8), [(C5Me5)2Gd]2Se2, and [(C5Me5)2Gd]2Se depending on the stoichiometry used. When the analogous synthetic method was attempted with yttrium in arene solvents, the previously characterized (C5Me5)2YR complexes (R=C6H5, CH2C6H5) were isolated instead, i.e., C–H bond activation of solvent occurred. To avoid this problem, (C5Me5)3Y was synthesized in high yield from [(C5Me5)2YH]2 and tetramethylfulvene in aliphatic solvents. Isolated (C5Me5)3Y was found to metalate benzene and toluene with concomitant formation of C5Me5H, a reaction contrary to the normal pKa values of these hydrocarbons. In this case, the normally inert (C5Me5)1− ligand engages in C–H bond activation due to the extreme steric crowding.

Reductive Reactivity of the Organolanthanide Hydrides, [(C5Me5)2LnH]x, Leads to ansa-Allyl Cyclopentadienyl (η5-C5Me4CH2−C5Me4CH2-η3)2- and Trianionic Cyclooctatetraenyl (C8H7)3- Ligands

The reductive reactivity of lanthanide hydride ligands in the [(C5Me5)2LnH]x complexes (Ln = Sm, La, Y) was examined to see if these hydride ligands would react like the actinide hydrides in [(C5Me5)2AnH2]2 (An = U, Th) and [(C5Me5)2UH]2. Each lanthanide hydride complex reduces PhSSPh to make [(C5Me5)2Ln(mu-SPh)]2 in approximately 90% yield. [(C5Me5)2SmH]2 reduces phenazine and anthracene to make [(C5Me5)2Sm]2(mu-eta(3):eta(3)-C12H8N2) and [(C5Me5)2Sm]2(mu-eta(3):eta(3)-C10H14), respectively, but the analogous [(C5Me5)2LaH]x and [(C5Me5)2YH]2 reactions are more complicated. All three lanthanide hydrides reduce C8H8 to make (C5Me5)Ln(C8H8) and (C5Me5)3Ln, a reaction that constitutes another synthetic route to (C5Me5)3Ln complexes. In the reaction of [(C5Me5)2YH]2 with C8H8, two unusual byproducts are obtained. In benzene, a (C5Me5)Y[(eta(5)-C5Me4CH2-C5Me4CH2-eta(3))] complex forms in which two (C5Me5)(1-) rings are linked to make a new type of ansa-allyl-cyclopentadienyl dianion that binds as a pentahapto-trihapto chelate. In cyclohexane, a (C5Me5)2Y(mu-eta(8):eta(1)-C8H7)Y(C5Me5) complex forms in which a (C8H8)(2-) ring is metalated to form a bridging (C8H7)(3-) trianion.

Structural studies of mono(pentamethylcyclopentadienyl)lanthanide complexes

The mono(pentamethylcyclopentadienyl) lanthanide complexes [(C5Me5)Yb(μ-I)(μ-η 5 : η 5-C5Me5)Yb(C5Me5)]n (1), {[(C5Me5)Sm]3(μ-Cl)4(μ 3-Cl)(μ 3-OH)(THF)}2 (2), {[(C5Me5)Sm]2 (μ-OH)(μ-Cl)4(μ 3-Cl)Mg(THF)2}2 (3), [(C5Me5)2Sm](μ-Cl)6(μ 3-Cl)2(μ 4-Cl)[(C5Me5)Sm]4 (4), {[(C5Me5)Nd]3(μ 3-Cl)4(μ 4-Cl)2(μ 3-O2CPh)2K2(η 6-C7H8)}2 (5), [(C5Me5)Nd(C8H8)]2(μ-dioxane) (6), [(C5Me5)Yb(MeOtBu)]2(μ-η 8 : η 8-C8H8) (7), [(C5Me5)Dy(μ-I)2]3 (8), and [(C5Me5) Tm(MeCN)6]I2 (9), have been identified by X-ray crystallography. 1 is unusual in that it has a μ-η 5 : η 5-C5Me5 ring that generates a local bent metallocene environment around ytterbium. Complexes 2–5 demonstrate the versatility of bridging chlorides in generating a variety of structures for mono(pentamethylcyclopentadienyl) lanthanide halides. Complex 6 shows how dioxane can generate a crystallographically-analyzable complex by bridging two mixed-ligand metallocene units that do not readily crystallize with THF. The structure of 7 shows how methyl tert-butyl ether (MTBE) ligates a lanthanide. Complex 8 is a trimeric cyclopentadienyl lanthanide halide unusual in that it has six bridging halides that roughly define a trigonal prism. Complex 9 constitutes an organometallic example of a lanthanide in which acetonitrile completely displaces iodide counterions.

Samarium versus aluminium Lewis acidity in a mixed alkyl carboxylate complex related to alkylaluminium activation in diene polymerization catalysis

[(C5Me5)2Sm(mu-O2CPh)]2 reacts with iBu3Al to form a mixed bridge samarium aluminium complex [(C5Me5)2Sm(mu-O2CPh)(mu-iBu)Al(iBu)2], that displays two different carboxylate orientations toward the metals in a single crystal.