Mechanistic work on the olefin metathesis reaction by well-characterized ruthenium carbene complexes led to the rational design and synthesis of a modified, unsymmetrical "first-generation" catalyst, which, in contrast to either first-or second-generation systems with symmetrical ligands, converts a mixture of two cycloolefin monomers to a largely alternating copolymer. The mechanistic concept of a homogeneous catalyst that switches between more than one state at each turnover is general. The structures of the complexes, determined by mass spectrometry, NMR, and X-ray crystallography, reveal some unexpected features, which explain sequence errors in the copolymer. Experimental SectionGeneral Remarks. Unless otherwise stated, all manipulations were carried out under an argon atmosphere on a vacuum line using standard Schlenk techniques. The solvents were dried by distillation from the following drying agents prior to use and were transferred under N 2 : diethyl ether (Na/K), n-hexane (Na/K), THF (K), CH 2 Cl 2 (CaH 2 ), ethanol (Mg), methanol (Mg). Flash chromatography employed Fluka silica gel 60, type 60752 (230-400 mesh). TLC was done with Merck silica gel 60 F 254 plates and visualized by UV 254 light. Low-resolution ESI-MS measurements were done on a Finnigan MAT LCQ MS ion trap mass spectrometer, which were then used to set up high-resolution mass spectrometric measurements on a Finnigan MAT TSQ Quantum instrument with tetradodecylammonium bromide as an external standard for absolute mass calibration. NMR measurements are reported for a Varian Mercury XL 300 ( 1 H, 300 MHz; 13 C, 75 MHz; 31 P, 121 MHz) spectrometer. Chemical shifts (δ values) are reported in ppm with respect to Me 4 Si (δ ) 0 ppm), used as an internal standard for 13 C and 1 H NMR, and an 85% aqueous H 3 PO 4 solution, used as an external standard for 31 P NMR. Coupling constants (J) are given in Hz. 13 C NMR and 31 P NMR spectra were proton broad-banddecoupled. The multiplicities of peaks are denoted by the following abbreviations: s, singlet; d, doublet; t, triplet; m, multiplet; br, broad. Elemental analysis was performed by the Microanalytical Laboratory of the Laboratorium fu ¨r Organische Chemie, ETH-Zu ¨rich. Gel permeation chromatography (GPC) was carried out with a Polymer Laboratories PL-GPC 220 fitted with a refractive index and viscometry detector using 1,3,5-trichlorobenzene at 140 °C and a polystyrene reference.Ligand Syntheses. tert-Butylphenylchlorophosphine. A 0.95 M t-BuMgCl solution in Et 2 O was prepared by addition of 60 g (652 mmol) of t-BuCl to a suspension of 23.8 g (978 mmol) of magnesium turnings in 400 mL of Et 2 O. The suspension was stirred for 2 h at room temperature, filtered, and titrated. To a solution of 33 g (188 mmol) of phenyldichlorophosphine in 100 mL of Et 2 O at -50 °C was added 200 mL (188 mmol) of the 0.95 M t-BuMgCl solution over 1 h with vigorous stirring. The formed gray-white suspension was allowed to reach room temperature over an additional 2 h of stirring. Filtration, evaporation of the solv...

We report here the mechanism-based design of a ring-opening metathesis polymerization (ROMP) [1] catalyst which preferentially assembles a mixture of cyclooctene and norbornene into an alternating copolymer. While there have been reports of regular copolymerizations, highly selective examples are uncommon outside of free-radical polymerization, with the 1:1 alternating copolymers of ethylene and carbon monoxide, [2] or of epoxides and carbon dioxide, [3] being perhaps the most prominent examples. Given that the exemplary functional characteristics of biopolymers, for example, structural or catalytic functions in polypeptides, information storage and transmission in polynucleotides, and molecular recognition in polysaccharides, derive ultimately from sequence-selective copolymerization of simple monomer units, one can presume that synthetic polyolefins, as high-performance materials, would be substantially enhanced if even simple sequenceselective copolymerizations could be achieved routinely. Treating regular copolymerization more generally as a problem in metal-catalyzed organic synthesis, one sees that the issue is not stereoselectivity, or even regioselectivity. We seek to introduce into polymerization catalysts the element of chemoselectivity, which is the most basic kind of selectivity in organic synthesis. We encode the sequence information in the catalyst itself, and in this present study succeed in the most primitive, two-component, alternating sequence of ring-opening metathesis copolymerization.Phosphine ligand 1 was prepared by sequential alkylation and arylation of phenyldichlorophosphine with tBuMgCl and ortho-methoxyphenyllithium, isolation by careful distillation, cleavage of the methyl ether by BBr 3 , and then deprotonation with NaH. Reaction of one equivalent of 1 and [(Cy 3 P) 2 RuCl 2 ( = CHPh)] (2, Scheme 1) resulted in phosphine exchange, followed by elimination of NaCl. A similar approach has been reported by Hoveyda and co-workers for the preparation of a tethered, second-generation metathesis catalysts. [4] The resulting carbene complex 3 was purified by column chromatography under rigorous exclusion of oxygen, and checked by 1 H and 31 P NMR spectroscopy, as well as ESI-MS, to confirm that a single compound was prepared. Importantly, no other carbene species was present (analytical details and further information on the synthesis of 1 and 3 can be found in the Supporting Information). Polymerization of norbornene, cyclooctene, and mixtures thereof, together with 0.05 mol % catalyst, were conducted under dry N 2 at room temperature in either dichloromethane or cyclooctene as solvent. In experiments with cyclooctene as solvent, the norbornene is consumed quantitatively after 17 h. If a less-coordinating solvent, such as CH 2 Cl 2 , is used, then norbornene is consumed much more rapidly, with phenomenological rates comparable to those observed with catalyst 2 under the same conditions. The resulting polymer was isolated by removal of solvent at 0.01 mbar pressure until no further weight los...

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Wir berichten hier von der Entwicklung eines Katalysators für die ringöffnende Metathesepolymerisation (ROMP), [1] der in einer Mischung von Cycloocten und Norbornen die Monomere vorzugsweise abwechselnd in die Polymerkette einbaut. Zwar gibt es bereits einige Berichte über gleichmä-ßige Copolymerisationen, aber außer in der Radikalcopolymerisation sind hoch selektive Systeme nur selten gefunden worden. Die Palladium-katalysierte Copolymerisation von Ethen und Kohlenmonoxid [2] und die Ringöffnungspolyme-risation von Epoxiden mit Kohlendioxid [3]