Complexo de Metal de Transição de Carbino

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    Recent advances in the development ofalkyne metathesis catalysts

    Xian Wu and Matthias Tamm*

    Review Open AccessAddress:Institut fr Anorganische und Analytische Chemie, TechnischeUniversitt Braunschweig, Hagenring 30, 38106 Braunschweig,Germany

    Email:Xian Wu - wuxn03@hotmail.com;Matthias Tamm* - m.tamm@tu-bs.de

    * Corresponding author

    Keywords:alkynes; homogeneous catalysis; metathesis; molybdenum; tungsten

    Beilstein J. Org. Chem. 2011, 7, 8293.doi:10.3762/bjoc.7.12

    Received: 13 September 2010Accepted: 08 December 2010Published: 18 January 2011

    Guest Editor: K. Grela

    2011 Wu and Tamm; licensee Beilstein-Institut.License and terms: see end of document.

    AbstractThe number of well-defined molybdenum and tungsten alkylidyne complexes that are able to catalyze alkyne metathesis reactionsefficiently has been significantly expanded in recent years.The latest developments in this field featuring highly active imidazolin-2-iminato- and silanolatealkylidyne complexes are outlined in this review.

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    ReviewIntroductionCC bond formation is one of the most important types of reac-tion in organic synthesis. Transformations employing organo-metallic compounds as catalysts have achieved a significant rolebecause of their advantages such as simplicity (fewer reactionsteps) and efficiency (higher yields) in comparison with tradi-tional synthetic strategies. Nowadays, a plethora of methods isknown, which can be used for the formation of CC single anddouble bonds, whereas simple ways to create CC triple bondsare less common, despite the importance and ubiquity of CCtriple bonds in research areas such as natural product synthesisand advanced material science [1].

    Alkyne metathesis, which deals with the breaking and makingof CC triple bonds, has only relatively recently become part ofthe tool box of organic and polymer chemists for the prepar-

    ation of their target molecules [2-11]. Catalyzed by organotran-sition metal complexes, this reaction type creates new CCtriple bonds very simply via the Katz mechanism (Scheme 1)[12], based on which a series of different reaction types such asalkyne cross metathesis (ACM), ring-closing alkyne metathesis(RCAM), ring-opening alkyne metathesis polymerization(ROAMP) and acyclic diyne metathesis polymerization(ADIMET) are known (Scheme 2).

    In contrast to olefin metathesis, the number of catalysts foralkyne metathesis is far more limited. The first catalyst foralkyne metathesis was a heterogeneous system based on WO3/silica, which was first reported by Pennella, Banks and Baileyin 1968 [13], while the first homogeneous system, whichconsisted of [Mo(CO)6] and resorcinol [14], was discovered by

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    Scheme 1: Alkyne metathesis based on the Katz mechanism.

    Scheme 2: Reaction patterns of alkyne metathesis.

    Mortreux and Blanchard in 1974. Since then, great efforts havebeen made to develop highly efficient alkyne metathesis cata-lysts and this has led to three major systems which have domin-ated this area, i.e., the Mortreux system, the Schrock system andthe CumminsFrstnerMoore system. Only recently, twonovel systems, which exhibit highly promising catalytic perfor-mance in alkyne metathesis, were successfully introduced: 1. Amodified Schrock system containing imidazolin-2-iminatoligands that was developed by our group; 2. silanolate-supported complexes such as molybdenum nitride and alkyli-dyne complexes with Ph3SiO ligands developed by Frstnerand tungsten alkylidyne complexes with (t-BuO)3SiO ligandsintroduced by us. Since there are already several reviews avail-able that cover research progress up to 2006 [2-11], this articlewill focus on the two novel catalyst systems, which were estab-lished over the last four years (200710), commencing with abrief introduction to the established systems that have alreadybeen widely used by synthetic chemists.

    Traditional catalyst systemsMortreux systemFirst reported in 1974, the Mortreux system consists of twocomponents: [Mo(CO)6] and phenol or derivatives thereof [14-19]. During the last decades, this system was intensively studied

    and its performance was significantly improved. However,some drawbacks including the requirement of high reactiontemperatures and low functional group tolerance greatly limit itsapplicability. Moreover, the catalytic mechanism and the activespecies involved remain unknown, preventing a further rationalcatalyst design. Nevertheless, because of the commercial avail-ability and high stability of the pre-catalysts as well as thesimplicity of operation, this classical system is still widely usedby chemists [20-28].

    Schrock systemSchrock-type catalysts are high oxidation state molybdenum ortungsten alkylidyne complexes which form metallacyclobutadi-enes (the key intermediate in the Katz mechanism) upon treat-ment with internal alkynes. Among these, the tungstenneopentylidyne complex [Me3CCW(OCMe3)3] is the mostwidely used species and is reliably synthesized in several stepsfrom commercially available WCl6. Accordingly, numerousapplications of this catalyst have been reported, which usuallyrequires elevated reaction temperatures and relatively high cata-lyst loadings [29-35].

    CumminsFrstnerMoore systemCummins introduced triamido molybdenum(III) complexes ofthe type [Mo{NR(Ar)}3] in the mid 1990s, which are able tocleave the NN triple bond in the dinitrogen molecule [36-38].Based on this discovery, Frstner developed a catalyst systemthat is formed upon treatment of [Mo{N(t-Bu)Ar}3]with dichloromethane to give the methylidyne complex[HCMo{N(t-Bu)Ar}3] and the chloro complex [ClMo{N(t-Bu)Ar}3] [39]. Although the detailed reaction mechanism hasnot been fully uncovered, the latter complex is, somewhat coun-terintuitively, considered to be the active species. Similarly,Moore was able to isolate molybdenum alkylidyne complexessuch as [EtCMo{N(t-Bu)Ar}3], which are able to catalyzealkyne metathesis reactions efficiently, albeit only after treat-ment with phenol derivatives or by capture on silica [40-46].The reaction with phenolic compounds presumably leads topartial or complete cleavage of the MoN bonds to producecatalytically active phenolate complexes. In agreement with thisassumption, Cummins was able to report the synthesis of well-defined molybdenum benzylidyne complexes from the molyb-daziridine [Mo(H)(2-Me2CNAr){N(i-Pr}Ar)] and could

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    Scheme 4: Ligand synthesis and catalyst design.

    demonstrate that these systems are efficient initiators for alkynemetathesis even at ambient temperature and low catalyst load-ings [47]. Scheme 3 shows some typical examples of the threetraditional catalyst systems.

    Scheme 3: Typical examples from traditional catalyst systems.

    Novel catalyst systemsImidazolin-2-iminato tungsten and molybdenumalkylidyne complexesImidazolin-2-iminato ligands, which are isolobal to phospho-raneimides (R3PN) and cyclopentadienides (C5R5) [48-52],can be described by the resonance structures shown inScheme 4, indicating that the ability of the imidazolium ring tostabilize a positive charge affords highly basic ligands with a

    strong electron-donating capacity towards early transitionmetals or metals in a higher oxidation state [53-55]. In recentyears, our group has significantly expanded the use of these2,4-electron donor ligands in organometallic chemistry andhomogeneous catalysis [56-67]. Their synthesis starts fromN-heterocyclic carbenes 1 which react with trimethylsilyl azideto afford 2-trimethylsilyliminoimidazolines 2. After treatmentwith methanol, the corresponding imidazolin-2-imines 3 can beconveniently isolated [60]. Deprotonation by alkyl lithiumreagents leads to imidazolin-2-iminato lithium compounds 4,which serve as ligand transfer reagents during the catalystpreparation (Scheme 4).

    The idea to use imidazolin-2-iminato ligands for the modifica-tion of Schrock-type alkylidyne complexes is based on theconsideration that they can be regarded as monoanionicanalogues of dinegative imido ligands, which are present insome of the most active olefin metathesis catalysts, i. e.,SchrockHoveyda-type tungsten and molybdenum imido-alkylidene complexes [10]. We presumed that substitution ofthe imido ligands by imidazolin-2-iminato ligands and concur-rent conversion of the metalcarbon double bond into a triplebond would afford metal alkylidyne species with a well-preserved structural and electronic integrity, and therefore withpotentially undiminished catalytic activity (Scheme 4). Thus,the resulting new complexes should then be highly activealkyne metathesis catalysts.

    In order to verify this design strategy, high oxidation state tung-sten and molybdenum alkylidyne complexes bearing imida-zolin-2-iminato ligands (5 and 6) were synthesized by twodifferent routes. The low-oxidation-state route (on the right-hand side in Scheme 5) starting from metal hexacarbonyl hasadvantages such as higher atom economy, easier operation andsuitability for both tungsten and molybdenum [68-70] in com-

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    Scheme 5: Catalysts synthesis using high- and low-oxidation-stateroutes (for 6a, X = Li or K; for 6b, X = K).

    parison with the high-oxidation-state route (on the left- handside in Scheme 5) starting from tungsten hexachloride [71-73].The use of partially fluorinated alkoxides such as hexafluoro-tert-butoxide, OCCH3(CF3)2, proved to be essential for creatingactive catalysts [74], indicating that successful catalyst designin this system relies on establishing a push-pull situation in asimilar fashion present in SchrockHoveyda olefin metathesiscatalysts (Scheme 4) [10] and also in an isolobal rhenium(VII)imido-alkylidyne complex [Re(NAr)(Ct-Bu)(ORF)] (Ar = 2,6-diisopropylphenyl, RF = C