Tungsten diynes

The series of hydrido phosphine-substituted clusters, [HWRu3(p4-BH)(CO)n x(PPh3)xCp] (x = 1, 2) have been prepared. The chemistry of the tungsten diyne [W(C = CC = CH)(CO)3Cp] with a number of ruthenium cluster complexes has been elaborated, and is summarised in Scheme 6. Reaction with [Ru3(CO)io(NCMe)2] afforded 151 which readily converted to the hydrido-alkynyl cluster 152 on refluxing in benzene. Reaction with the phosphine-substituted [Ru3( u-dppm)(CO)io] afforded three isomers of 153, a product analogous to 152. Cluster core expansion of 152 by reaction with [Co2(CO)s] or [Ru3(CO)i2]/[Fc2(CO)9] gave the linked clusters 154 and 155, respectively. 

An obvious drawback in RCM-based synthesis of unsaturated macrocyclic natural compounds is the lack of control over the newly formed double bond. The products formed are usually obtained as mixture of ( /Z)-isomers with the (E)-isomer dominating in most cases. The best solution for this problem might be a sequence of RCAM followed by (E)- or (Z)-selective partial reduction. Until now, alkyne metathesis has remained in the shadow of alkene-based metathesis reactions. One of the reasons maybe the lack of commercially available catalysts for this type of reaction. When alkyne metathesis as a new synthetic tool was reviewed in early 1999 [184], there existed only a single report disclosed by Fiirstner s laboratory [185] on the RCAM-based conversion of functionalized diynes to triple-bonded 12- to 28-membered macrocycles with the concomitant expulsion of 2-butyne (cf Fig. 3a). These reactions were catalyzed by Schrock s tungsten-carbyne complex G. Since then, Furstner and coworkers have achieved a series of natural product syntheses, which seem to establish RCAM followed by partial reduction to (Z)- or (E)-cycloalkenes as a useful macrocyclization alternative to RCM. As work up to early 2000, including the development of alternative alkyne metathesis catalysts, is competently covered in Fiirstner s excellent review [2a], we will concentrate here only on the most recent natural product syntheses, which were all achieved by Fiirstner s team. 

The reaction of alkenes with alkenes or alkynes does not always produce an aromatic ring. An important variation of this reaction reacts dienes, diynes, or en-ynes with transition metals to form organometallic coordination complexes. In the presence of carbon monoxide, cyclopentenone derivatives are formed in what is known as the Pauson-Khand reaction The reaction involves (1) formation of a hexacarbonyldicobalt-alkyne complex and (2) decomposition of the complex in the presence of an alkene. A typical example Rhodium and tungsten ... 

Although the structure of the complex arising from I52/CH2CI2 is not clear, this catalyst is excellent in terms of ease of preparation. The catalyst is very active for formation of cycloalkynes with ring sizes different from those of diynes (Table 6.5). In contrast to tungsten alkylidyne complex 150, catalyst 152/ CH2CI2 is sensitive toward an acidic proton such as amide proton and exhibited remarkable tolerance towards many polar functional groups (Table 6.5). 

TABLE 6.5. Diyne Metathesis Using Tungsten and Molybdenum Catalysts... 

Table 6 Diyne metathesis using tungsten and moiybdenum compiexes...

PPEs by Acyclic Diyne Metathesis (ADIMET) Utilizing Schrock s Tungsten Carbyne Complex 11... 

Electrocycylic ring closure combined with Diels-Alder addition has been employed to derive all four rings of the tetracyclic ring system in the steroids by means of a one pot synthesis (ref.198). Thus, from the triflate of hex-4-ynol and the readily available hexa-1,5-diyne a triyne intermediate was obtained which with chromium or tungsten hexacarbonyl formed a triyne carbene complex. This with a 1,3-dialkoxybuta-1,3-diene in acetonitrile with carbon monoxide 0 atmosphere) initially at ambient temperature and then at 110°C over 24 hours afforded a 62% yield of the two products shown [(1) R = TBS (2) R = H, (1) (2) 5 2]. 

Acyclic diyne metathesis (ADIMET) is the method of choice for the preparation of PPEs. After the first successful attempts with a tungsten carbyne complex [148],... 

The commercially available complex 2 shows higher activity and is metathesis active under mild conditions, typically ambient temperature up to 90 °C [3c, hj. RCAM reactions with 2 are often run at 80 °C in toluene or a related aromatic solvent However, rigorously inert (anhydrous and oxygen-free) conditions are required. The core structure is amenable to tuning by ligand modifications, reflected in a recent report which demonstrated that the effective RCAM of diyne 6 can be performed at room temperature to afford cyclic alkyne 8 in excellent yield when imidazolin-2-iminato tungsten alkylidyne catalyst 7 was employed (Scheme 7.5) [12], The same catalyst has also been employed in the preparation of cyclophanes [13]. 

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