In recent years, the olefin metathesis reaction has attracted widespread attention as a versatile carbon−carbon bond-forming method. Many new applications have become possible because of major advances in catalyst design. State-of-the-art ruthenium catalysts are not only highly active but also compatible with most functional groups and easy to use. This Account traces the ideas and discoveries that were instrumental in the development of these catalysts, with particular emphasis on (PCy3)2Cl2RuCHPh and its derivatives. The discussion includes an analysis of trends in catalyst activity, a description of catalysts coordinated with N-heterocyclic carbene ligands, and an overview of ongoing work to improve the activity, stability, and selectivity of this family of L2X2RuCHR complexes.

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The Development of L2X2RuCHR Olefin Metathesis Catalysts: An Organometallic Success Story

Recent advances in olefin metathesis and its application in organic synthesis

The advent of well-defined catalysts for olefin metathesis which combine high activity, durability, and excellent tolerance towards polar functional groups has revolutionized the field. The past decade has seen the rapid embrace of these reagents as tools for advanced organic and polymer chemistry and the success of this development is witnessed by a plethora of elegant applications to the synthesis of natural and nonnatural products. This review article provides an overview of these developments and intends to familiarize the reader with some very recent advances which hold the promise to expand the scope of the reaction even further. Moreover, the positive impact of metathesis on the fundamental logic of retrosynthetic planning is demonstrated by means of typical examples. Finally, it will be shown that metathesis is by no means restricted to alkenes as substrates, and some comments on metathesis reactions following unconventional mechanistic pathways will also be presented.

Olefin Metathesis and Beyond

Living ring-opening metathesis polymerization

Inspired by the unique and efficient wound healing processes in biological systems, several approaches to develop synthetic polymers that can repair themselves with complete, or nearly complete, autonomy have recently been developed. This review aims to survey the rapidly expanding field of self-healing polymers by reviewing the major successful autonomic repairing mechanisms developed over the last decade. Additionally, we discuss several issues related to transferring these self-healing technologies from the laboratory to real applications, such as virgin polymer property changes as a result of the added healing functionality, healing in thin films v. bulk polymers, and healing in the presence of structural reinforcements.

Self-healing polymers and composites

Several ruthenium-based olefin metathesis catalysts of the formula (PR3)2X2RuCHCHCPh2 have been synthesized, and relative catalyst activities were determined by monitoring the ring-closing metathesis of the acyclic diene diethyl diallylmalonate. The following order of increasing activity was determined:  X = I < Br < Cl and PR3 = PPh3 ≪ PiPr2Ph < PCy2Ph < PiPr3 < PCy3. Additional studies were conducted with the catalyst (PCy3)2Cl2RuCH2 to probe the mechanism of olefin metathesis by this class of catalysts. The data support a scheme in which there are two competing pathways:  the dominant one in which a phosphine dissociates from the ruthenium center and a minor one in which both phosphines remain bound. Higher catalyst activites could be achieved by the addition of CuCl to the reaction.

Well-defined ruthenium olefin metathesis catalysts: Mechanism and activity

Synthesis and Investigation of Homo- and Heterobimetallic Ruthenium Olefin Metathesis Catalysts Exhibiting Increased Activities

Chemical modifications are powerful tools to probe nucleic acid structure and protein-nucleic acid interactions. Nucleic acids can be modified either randomly by a variety of chemical reagents or site-specifically with synthetic base analog substitutions. We report here the use of 8-bromodeoxyguanosine (8-Br-dG) as a modification that is directly sensitive to a dihedral angle. 8-Br-dG should prove useful as a chemical probe for the conformation about glycosidic bonds in unusual nucleic acid structures.

Chemical Probe for Glycosidic Conformation in Telomeric DNAs

Activation of ruthenium based catalyst compounds with acid to improve reaction rates and yields of olefin metathesis reactions, including ROMP, RCM, ADMET and cross-methasis reactions is disclosed. The ruthenium catalyst compounds are ruthenium carbene complexes of the general formula AxLyXzRu=CHR' where x = 0, 1 or 2, y = 0, 1 or 2, and z = 1 or 2 and where R' is hydrogen or a substituted or unsubstituted alkyl or aryl, L is any neutral electron donor, X is any anionic ligand, and A is a ligand having a covalent structure connecting a neutral electron donor and an anionic ligand. The use of acid with these catalysts allows for reactions with a wide range of olefins in a variety of solvents, including acid-initiated RIM processes and living ROMP reactions of water-soluble monomers in water.

Acid activation of ruthenium metathesis catalysts and living romp metathesis polymerization in water

Reactions of the bis(tricyclohexylphosphine) ruthenium alkylidene complexes (PCy3)2(Cl)2Ru=CHPh, (PCy3)2(Cl)2Ru=CHCH=CPh2, and (PCy3)2(Cl)2Ru=CHCH=CMe2 with excess pyridine (py) cleanly furnish the six-coordinate bis(pyridine) derivatives (PCy3)(py)2(Cl)2Ru=CHPh, (PCy3)(py)2(Cl)2Ru=CHCH=CPh2, and (PCy3)(py)2(Cl)2Ru=CHCH=CMe2, respectively. In solution, there is evidence for an equilibrium between (PCy3)(py)2(Cl)2-Ru=CHPh and a five-coordinate mono(pyridine) derivative, (PCy3)(py)(Cl)2Ru=CHPh. This mono(pyridine) complex can be isolated by heating (PCy3)(py)2(Cl)2Ru=CHPh in toluene under dynamic vacuum to remove dissociated pyridine as a toluene azeotrope. The diphenylvinylcarbene complex (PCy3)(py)2(Cl)2Ru=CHCH=CPh2 has been structurally characterized by X-ray diffraction, and it exhibits a vinylcarbene ligand tilted by ~30o with respect to the Cl(1)–Ru– Cl(2)–C(1) plane. In addition, the Ru–N bond located trans to the vinylcarbene is elongated by a substantial 0.136(2) A in comparison to the Ru–N bond located trans to the tricyclohexylphosphine. The dimethyl-vinylcarbene derivative (PCy3)(py)2(Cl)2Ru=CHCH=CMe2 can be isolated as well, but it decom-poses rapidly when redissolved. Surprisingly, reaction of (PCy3)2(Cl)2Ru=CHPh with 1-methyl-imidazole (1-MeIm) follows a different route and provides the cationic tris(imidazole) product [(PCy3)(1-MeIm)3(Cl)Ru=CHPh][Cl], which also has been structurally characterized. These new compounds are interesting examples of ruthenium alkylidene complexes coordinated with hetero-cyclic N-donor ligands, but they display mediocre catalytic activity for the ring-closing metathesis of diethyl diallylmalonate.

Eric L. Dias

Papers

Well-defined ruthenium olefin metathesis catalysts: Mechanism and activity

Synthesis and Investigation of Homo- and Heterobimetallic Ruthenium Olefin Metathesis Catalysts Exhibiting Increased Activities

Chemical Probe for Glycosidic Conformation in Telomeric DNAs

Acid activation of ruthenium metathesis catalysts and living romp metathesis polymerization in water

Ruthenium alkylidene complexes coordinated with tricyclohexylphosphine and heterocyclic N-donor ligands