Metathesis by Molybdenum and Tungsten Catalysts

Carbon–carbon double bonds are an integral part of the chemical industry and are widely found in natural products, from the small and simple (ethylene) to the large and complex. The ability to manipulate carbon–carbon double bonds to make other carbon–carbon double bonds in a catalytic and stereospecific fashion has revolutionized the way organic molecules and polymers are made today. This article outlines the development of modern molybdenum and tungsten alkylidene catalysts that can be designed at a molecular level to achieve a given result. Carbon–carbon triple bonds also can be manipulated in a similar manner with the appropriate alkylidyne catalyst. Although the 'alkene metathesis' and 'alkyne metathesis' reactions are now fifty to sixty years old, many problems remain that will require an even more detailed understanding of these most intricate, superficially simple reactions

Preparation of Macrocyclic

The first examples of catalyst-controlled stereoselective macrocyclic ring-closing metathesis reactions that generate Z-enoates as well as (E,Z)- or (Z,E)-dienoates are disclosed. Reactions promoted by 3.0–10 mol % of a Mo-based monoaryloxide pyrrolide complex proceed to completion within 2–6 h at room temperature. The desired macrocycles are formed in 79:21 to >98:2 Z/E selectivity; stereoisomerically pure products can be obtained in 43–75% yield after chromatography. Utility is demonstrated by application to a concise formal synthesis of the natural product (+)-aspicilin.United States. National Institutes of Health (GM-59426

Direct synthesis of Z-alkenyl halides through catalytic cross-metathesis

Olefin metathesis has had a large impact on modern organic chemistry, but important shortcomings remain: for example, the lack of efficient processes that can be used to generate acyclic alkenyl halides. Halo-substituted ruthenium carbene complexes decompose rapidly or deliver low activity and/or minimal stereoselectivity, and our understanding of the corresponding high-oxidation-state systems is limited. Here we show that previously unknown halo-substituted molybdenum alkylidene species are exceptionally reactive and are able to participate in high-yielding olefin metathesis reactions that afford acyclic 1,2-disubstituted Z-alkenyl halides. Transformations are promoted by small amounts of a catalyst that is generated in situ and used with unpurified, commercially available and easy-to-handle liquid 1,2-dihaloethene reagents, and proceed to high conversion at ambient temperature within four hours. We obtain many alkenyl chlorides, bromides and fluorides in up to 91 per cent yield and complete Z selectivity. This method can be used to synthesize biologically active compounds readily and to perform site- and stereoselective fluorination of complex organic molecules.National Institute of General Medical Sciences (U.S.) (GM-59426 and GM-57212

Synthesis of Molybdenum and Tungsten Alkylidene Complexes That Contain Sterically Demanding Arenethiolate Ligands

Imido alkylidene complexes of Mo and W and oxo alkylidene complexes of W that contain thiophenoxide ligands of the type S-2,3,5,6-Ph[subscript 4]C[subscript 6]H (STPP) and S-2,6-(mesityl)[subscript 2]C[subscript 6]H[subscript 3] (SHMT = S-hexamethylterphenyl) have been prepared in order to compare their metathesis activity with that of the analogous phenoxide complexes. All thiolate complexes were significantly slower (up to ∼10× slower) for the metathesis homocoupling of 1-octene or polymerization of 2,3-dicarbomethoxynorbornene, and none of them was Z-selective. The slower rates could be attributed to the greater σ-donating ability of a thiophenoxide versus the analogous phenoxide and consequently a higher electron density at the metal in the thiophenoxide complexes.National Institutes of Health (U.S.) (Grant GM-59426

Alkyne metathesis by molybdenum and tungsten alkylidyne complexes

Alkyne metathesis by molybdenum and tungsten alkylidyne complexes is now ~45 years old. Progress in the practical aspects of alkyne metathesis reactions with well-defined complexes, as well as applications, in the last decade, guarantees that it is destined to become a useful method for the synthesis of organic molecules

Reducing Them Down To Charge Them Up: Low Temperature Catalyst Activation

The largest industrial application of olefin metathesis today is the synthesis of propylene from ethylene and butenes(1) employing WO₃ on SiO₂, a relatively long-lived and regenerable catalyst that operates at 350–400 °C. It is widely proposed that high temperatures are required because the percentage of metal sites actually involved in the metathesis reaction is extremely low, or the reaction that generates alkylidenes is not a high yield reaction, or both. A recent paper by Copéret, Mashima, and co-workers(2) tackles head-on the question concerning how in WO₃/SiO₂ catalysts the alkylidene is formed from an olefin alone. Hundreds of papers have attempted to answer this question, although one has to admit that there may not be a single answer for all supported oxide catalysts or all olefins

Stereospecific Ring-Opening Metathesis Polymerization (ROMP) of Norbornene and Tetracyclododecene by Mo and W Initiators

We report the synthesis of >98% cis,isotactic and cis,syndiotactic polynorbornene (poly(NBE)) and poly(endo,anti-tetracyclododecene) (poly(TCD)). Cis,isotactic poly(NBE) and poly(TCD) were prepared employing Mo-based biphenolate imido alkylidene initiators, Mo(NR)(CHCMe[subscript 2]Ph)(Biphen) (Biphen = e.g., 3,3′-(t-Bu)[subscript 2]-5,5′-6,6′-(CH[subscript 3])[subscript 4]-1,1′-biphenyl-2,2′-diolate), while cis,syndiotactic poly(NBE) and poly(TCD) were prepared employing W-based imido or oxo monoaryloxide pyrrolide (MAP) initiators, W(X)(CHR′)(Pyrrolide)(OTer) (X = NR or O; OTer = a 2,6-terphenoxide). Addition of 1-hexene or coordinating solvents such as THF do not decrease the stereospecificity of the polymerization. Cis,iso and cis,syndio dyads can be distinguished through examination of [superscript 1]H and [superscript 13]C NMR spectra of the two polymers in a mixture. The polymers were hydrogenated to give isotactic and syndiotactic H-poly(NBE) and H-poly(TCD).United States. Department of Energy (grant DE-FG0286ER13564

Formation of High-Oxidation-State Metal–Carbon Double Bonds

This tutorial explores the major pathways of forming metal–carbon double bonds in high-oxidation-state alkylidene complexes that began with the alkylidene chemistry of tantalum complexes in the 1970s and continued with the organometallic chemistry of Mo, W, and Re and the development of homogeneous catalysts for the metathesis of olefins. It also explores recent findings in surface organometallic chemistry and discusses the link between molecularly defined and heterogeneous catalysts. Recent results suggest that heterogeneous olefin metathesis catalysts that are activated toward metathesis upon exposure to olefins produce a d[superscript 0] alkylidene through formation of a metallacyclopentane ring at d[superscript 2] metal sites followed by “a ring contraction” to a metallacyclobutane, a reaction that was first observed in tantalum chemistry.National Science Foundation (U.S.) (Grant CHE-1463707

Synthesis of Tungsten Oxo Alkylidene Complexes

Reaction of W(O)[subscript 2](CH[subscript 2]-t-Bu)[subscript 2](bipy) with a mixture of ZnCl[subscript 2](dioxane), PMe[subscript 2]Ph, and trimethylsilyl chloride in toluene at 100 °C produced the known tungsten oxo alkylidene complex W(O)(CH-t-Bu)Cl[subscript 2](PMe[subscript 2]Ph)[subscript 2] (1a) in 45% isolated yield. The neophylidene analogue W(O)(CHCMe[subscript 2]Ph)Cl[subscript 2](PMe[subscript 2]Ph)[subscript 2] was prepared similarly in 39% yield. The reaction between 1a and LiOR (LiOR = LiOHIPT, LiOHMT) in benzene at 22 °C led to formation of the off-white W(O)(CH-t-Bu)Cl(OR)(PMe[subscript 2]Ph) complexes 4a (OR = OHMT = 2,6-dimesitylphenoxide) and 4b (OR = OHIPT = 2,6-(2,4,6-triisopropylphenyl)[subscript 2]phenoxide). Compound 4a serves as a starting material for the synthesis of W(O)(CH-t-Bu)(OHMT)(2,6-diphenylpyrrolide) (6), W(O)(CH-t-Bu)[N(C[subscript 6]F[subscript 5])[subscript 2]](OHMT)(PMe[subscript 2]Ph) (7), W(O)(CH-t-Bu)[OSi(t-Bu)[subscript 3]](OHMT) (8), and W(O)(CH-t-Bu)(OHMT)[subscript 2] (10). The reaction between 8 and ethylene was found to yield the square-pyramidal metallacyclobutane complex W(O)(C[subscript 3]H[subscript 6])[OSi(t-Bu)[subscript 3]](OHMT) (9), while the reaction between 10 and ethylene was found to yield the square-pyramidal metallacyclobutane complex W(O)(C[subscript 3]H[subscript 6])(OHMT)[subscript 2] (11). Compound 11 loses ethylene to yield isolable W(O)(CH[subscript 2])(OHMT)[subscript 2] (12). X-ray structures were determined for 6, 7, 9, and 12.National Science Foundation (U.S.) (CHE-1111133)National Science Foundation (U.S.) (CHE-9808061

Synthesis of Molybdenum and Tungsten Alkylidene Complexes that Contain a tert-Butylimido Ligand

National Science Foundation (U.S.) (CHE-1111133)National Science Foundation (U.S.) (CHE-0946721