Although the metathesis of internal alkynes has been known for more than 40 years, [1] its importance as a synthetic tool has not undergone a growth similar to that of the related metathesis of alkenes. [2] Unquestionably, the success of the latter reaction is largely based on the availability of highly active and well-defined homogeneous catalysts with sufficient air stability and broad functional-group tolerance, [2b, 3] whereas comparatively few systems are known that live up to the expectations for an alkyne metathesis catalyst with regard to its activity, substrate compatibility, and required reaction temperature. [4,5] Among such catalysts, the well-defined, prototypical Schrocktype alkylidyne complex 1 has found the broadest use ( Figure 1), [6] although it usually requires elevated reaction temperatures of about 80 8C and relatively high catalyst loadings (10-20 mol %). [2h, 7] Drawing on the structure of 1, we recently introduced imidazolin-2-iminato alkylidyne complexes of type 2 (Figure 1), which can either be synthesized from WCl 6 (M = W; R = tBu) [8] or, more conveniently, from [M(CO) 6 ] (M= W, Mo; R = Ph). [9] The tungsten systems proved to be particularly efficient in promoting alkyne cross-metathesis (ACM) and ringclosing alkyne metathesis (RCAM) even at room temperature and low catalyst concentrations (1 mol %). [8] Moreover, these catalysts were shown to be compatible with a range of functional groups. [9] Recently, Fürstner and co-workers established a new class of molybdenum nitride and alkylidyne complexes containing triphenylsilanolate (Ph 3 SiO) ligands, [10] with the diethyl ether complex 3 outperforming most of the other Mo-based catalyst systems. [11] Notably, the performance was further improved by the addition of molecular sieve (MS 5 ) to adsorb 2-butyne, which is usually formed during the metathesis reaction. In addition, this catalyst was recently employed for the synthesis of several naturally occurring macrolactides, [12] confirming the great potential of alkyne metathesis as a tool in organic synthetic methodology. However, the corresponding tungsten complexes of the type [PhCW(OSiPh 3 ) 3 L] have not, to date, been synthesized in a similar fashion to that described for 3. [11] Independently of Fürstner's approach and inspired by the work of CopØret, [13] we aimed at the preparation of related tri-(tert-butoxy)silanolate-supported tungsten alkylidyne complexes, since the silanol HOSi(OtBu) 3 can be regarded as a model for Si-OH sites present on silica surfaces. [14] Accordingly, complexes of the type [RCW{OSi(OtBu) 3 } 3 ] could be regarded as homogeneous models for silica-supported alkyne metathesis catalysts, a few examples of which have been reported ( Figure 2). For instance, the well-defined heterogeneous rhenium catalyst 4 promotes both olefin and alkyne metathesis, [15] whereas the groups of Basset and Moore reported grafting reactions on amorphous silica for the alkylidyne complexes [tBuCW(CH 2 tBu)(ODipp) 2 ] (Dipp = 2,6-diisopropylphenyl) and [EtCMo{N(tBu)Ar} 3 ]...

Alkyne metathesis has recently witnessed a significant growth in the number of homogeneous catalysts that are able to efficiently promote the formation and cleavage of carboncarbon triple bonds. [1, 2] All well-defined catalyst systems reported to date can be regarded as variants of Schrock-type molybdenum(VI) or tungsten(VI) alkylidyne complexes [3] in which the metal-alkylidyne moiety, MCÀR, is supported by alkoxide, [4] arlyoxide, [5] amido, [6] imidazolin-2-iminato, [7] phosphoraneiminato, [8] or silanolate ligands. [9,10] With a few exceptions, [11] alkyne metathesis usually requires the presence of internal alkynes, such as RCCMe (R = alkyl or aryl), affording the symmetric alkynes RCCR and MeCCMe. To drive these equilibrium reactions to completion, the latter, 2butyne (b.p. = 27 8C), can be continuously removed from the reaction mixtures under vacuum-driven conditions or, as recently shown by Fürstner and co-workers, advantageously by addition of molecular sieves (5 M.S.) to adsorb 2butyne. [9,12] For our part, the molecular-sieve-promoted method was also clearly superior when alkyne homocoupling and ring-closing alkyne metathesis (RCAM) reactions were studied in the presence of catalytic amounts of the tri(tertbutoxy)silanolate-supported tungsten benzylidyne complex 1 (Scheme 1). [10]

The tungsten benzylidyne complex [PhC≡W{OSi(OtBu)3}3] (1) efficiently catalyses the metathetic conversion between symmetrical and unsymmetrical 1,3-diynes, which provides the opportunity to prepare the latter species directly from terminal alkynes by a combination of copper-catalysed homocoupling and catalytic alkyne cross-metathesis (ACM).

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The ring-opening metathesis polymerization of cyclooctyne was studied was studied in the presence of catalytic amounts of the tungsten alkylidyne complexes [RC À À À W(NIm tBu ){OCMe(CF 3 ) 2 } 2 ] (1a: R ¼ CMe 3 , 1b: R ¼ Ph). The resulting polymers show relatively narrow polydispersities with the PDI ranging from 1.2 to 2.4. Treatment of 1b with cyclooctyne in dilute toluene or hexane solution afforded only low molecular weight oligomers. The mass spectra of these oligomers indicate the existence of macrocycles of the formula [C(CH 2 ) 6 C] n (n ¼ 3-9). In contrast, reactions at high substrate concentration led to mixtures of cyclic oligomers and linear polymers, which is probably a result of ring-chain equilibria, established in agreement with the Jacobson-Stockmayer theory of macrocyclization. In contrast, treatment of neat cyclooctyne with a catalytic amount of the catalyst produced medium molecular weight polymers in good yields.