Ruthenium-based olefin metathesis catalysts are used in laboratory-scale organic synthesis across chemistry, largely thanks to their ease of handling and functional group tolerance. In spite of this robustness, these catalysts readily decompose, via little-understood pathways, to species that promote double-bond migration (isomerization) in both the 1-alkene reagents and the internal-alkene products. We have studied, using density functional theory (DFT), the reactivity of the Hoveyda-Grubbs second-generation catalyst 2 with allylbenzene, and discovered a facile new decomposition pathway. In this pathway, the alkylidene ligand is lost, via ring expansion of the metallacyclobutane intermediate, leading to the spin-triplet 12-electron complex (SIMes)RuCl (R21, SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene). DFT calculations predict R21 to be a very active alkene isomerization initiator, either operating as a catalyst itself, via a η-allyl mechanism, or, after spin inversion to give R21 and formation of a cyclometalated Ru-hydride complex, via a hydride mechanism. The calculations also suggest that the alkylidene-free ruthenium complexes may regenerate alkylidene via dinuclear ruthenium activation of alkene. The predicted capacity to initiate isomerization is confirmed in catalytic tests using p-cymene-stabilized R21 (5), which promotes isomerization in particular under conditions favoring dissociation of p-cymene and disfavoring formation of aggregates of 5. The same qualitative trends in the relative metathesis and isomerization selectivities are observed in identical tests of 2, indicating that 5 and 2 share the same catalytic cycles for both metathesis and isomerization, consistent with the calculated reaction network covering metathesis, alkylidene loss, isomerization, and alkylidene regeneration.

Whereas a number of highly Z-selective ruthenium-based olefin metathesis catalysts bearing N-heterocyclic carbene ligands have been reported in recent years, Zselectivity has so far been difficult to achieve for phosphinebased catalysts. Guided by predictive density functional theory (DFT) calculations, we have developed phosphine-based ruthenium olefin metathesis catalysts giving 70−95% of the Zisomer product in homocoupling of terminal alkenes such as allylbenzene, 1-octene, allyl acetate, and 2-allyloxyethanol. Starting from a moderately selective catalyst, [P(Cy) 3 ](-S-2,4,6-Ph-C 6 H 2 )ClRu(CH-o-O i PrC 6 H 4 ) (4, Cy = cyclohexyl, i Pr = isopropyl), obtained by substituting a chloride of the Hoveyda−Grubbs first-generation catalyst with 2,4,6-triphenylbenzenethiolate, we moved on to replace Cl and PCy 3 by chelating, anionic phosphine ligands. Such ligands increase selectivity by limiting rotation around the P−Ru bond and by specifically directing the steric bulk of the phosphine substituents toward the selectivity-inducing thiolate ligand. In particular, DFT calculations predicted that o-(dialkylphosphino)phenolate ligands should improve selectivity and activity compared to 4. The most promising of these compounds (8b), based on the o-(ditert-butylphosphino)phenolate ligand, directs the two P-bonded tert-butyl substituents toward the 2,4,6-triphenylbenzenethiolate and has little steric hindrance trans to the thiolate. This compound metathesizes terminal olefins such as allylbenzene and 1-octene with Z-selectivities above 80% and allylacetate above 90%. Although these phosphine-based ruthenium monothiolate catalysts in general achieve somewhat lower activities and Z-selectivities than their second-generation counterparts, they also offer examples giving less substrate and product isomerization and thus higher yields. ■ INTRODUCTIONOlefin metathesis is an important carbon−carbon coupling method of the organic chemist's toolbox. It is, for instance, used extensively in the synthesis of natural products 1−3 and polymers 4,5 and in oleochemistry 6 and has found applications even in peptide and protein modifications. 7−10 Metathesis of terminal olefins to give disubstituted olefins typically results in mixtures of the Z-and E-isomers, with the thermodynamically more stable E-isomer usually being the major product. Separating these isomers is costly and often challenging, which may hamper the assessment of their activity and the application of olefin metathesis in medicinal chemistry. 11−13 In recent years, several catalysts with enhanced Z-selectivity have been reported. The first highly Z-selective catalysts, based on molybdenum and tungsten, were developed by Schrock and Hoveyda. 14−16 More recently, ruthenium-based counterparts, containing an N-heterocyclic carbene (NHC) chelated to the metal center via a Ru−C bond, were discovered by Grubbs and co-workers. 17−19 Highly Z-selective ruthenium-based catalysts were also developed by Hoveyda and co-workers, by replacing the chloride ligands of the Hoveyda−Gru...

A systematic library of 24 nickel(II) complexes with bidentate diphosphane ligands was synthesized, and the solid-state structures of five of them were determined with X-ray crystallography. The compounds C1-C3 are common P2Ni(II)X2-type complexes, while C4 contains a unique [P2Ni(II)(NH3)(OAc)](+) square-planar structure with a P2NO donor set and C5 constitutes a rare [(P2Ni(II))2(μ-OH)2](2+) dinuclear compound. The catalytic activity of all complexes was tested in the hydrogenation and/or isomerization of 1-octene in a CH2Cl2/CH3OH reaction medium. Catalyst precursors bearing ligands with o-alkoxy aryl rings selectively hydrogentate 1-octene to n-octane, while catalytic systems comprising ligands without the o-alkoxy functionality selectively isomerize the substrate to a mixture of internal alkenes, mostly cis- and trans-2-octene. The conversion is enhanced by equipping the ligand aryl rings with electron-donating alkoxy groups, by increasing the steric bulk of the backbone and/or the aryl rings, by employing relatively noncoordinating anions, and by adding a base as the cocatalyst. Using the compound [Ni(L3X)I2] as the catalyst precursor and upon application of standard hydrogenation conditions, full conversion of the substrate was achieved in 1 h to isomerization products only (TON = 1940). When a catalytic amount of the base is added, a similar result is obtained even in the absence of H2. A maximum TON of 4500 in 1 h with 96% selectivity for n-octane was achieved by employing [Ni(oMeO-L3X)(NH3)(OAc)]PF6 as the catalyst precursor.

Ru-alkylidenes bearing sterically demanding arylthiolate ligands (SAr) constitute one of only two classes of catalyst that are Z-selective in metathesis of 1alkenes. Of particular interest are complexes bearing pyridine as a stabilizing donor ligand, [RuCl(SAr)(CHR)(NHC)(py)] (R = phenyl or 2-thienyl, NHC = N-heterocyclic carbene, py = pyridine), which initiate catalysis rapidly and give appreciable yields combined with moderate to high Z-selectivity within minutes at room temperature. Here, we extend this chemistry by synthesizing and testing the first two such complexes (5a and 5b) bearing 3-phenylindenylidene, a ligand known to promote stability in other ruthenium-based olefin metathesis catalysts. The steric pressure resulting from the three bulky ligands (the NHC, the arylthiolate, and the indenylidene) forces the thiolate ligand to position itself trans to the NHC ligand, a configuration different from that of the corresponding alkylidenes. Surprisingly, although this configuration is incompatible with Z-selectivity and slows down pyridine dissociation, the two new complexes initiate readily at room temperature. Although their thermal stability is lower than that of typical indenylidene-bearing catalysts, 5a and 5b are fairly stable in catalysis (TONs up to 2200) and offer up to ca. 80% of the Z-isomer in prototypical metathesis homocoupling reactions. Density functional theory (DFT) calculations confirm the energetic cost of dissociating pyridine from 5a (= M1-Py) to generate 14-electron complex M1. Whereas the latter isomer does not give a metathesis-potent allylbenzene π-complex, it may isomerize to M1-trans and M2, which both form π-complexes in which the olefin is correctly oriented for cycloaddition. The olefin orientation in these complexes is also indicative of Z-selectivity.

Assembly processes can drive the selection of self-assembling molecules in dynamic combinatorial libraries, yielding self-synthesizing materials. We now show how such selection in a dynamic combinatorial library made from an amphiphilic building block which, by itself, assembles into micelles, can yield membranous aggregates ranging from vesicles to sponge phases. These aggregates are made from a mixture of unconventional surfactant molecules, showing the power of dynamic combinatorial selection approaches for the discovery of new, not readily predictable, self-assembly motifs.