Tungsten propargyl complex

An analogy can be made between 0 to N migration and the rearrangement of allylic and propargylic alcohols promoted by d° vanadium and tungsten oxo complexes ... 

Fig. 2. 33. Generation of anionic propargyl tungsten complexes and conversion to pyrroles [280].

The reaction of alkynyllithium compounds with alkoxycarbene tungsten complexes leads to anionic propargyl tungsten complexes (Figure 2.33 see also Figure 3.9). These intermediates are stable at low temperatures and react upon Lewis acid catalysis with aldehydes or A -sulfonyl imines to yield five-membered heterocycles [280]. Oxidative methoxycarbonylation [375] of the intermediate vinyl tungsten complex, followed by elimination of methanol leads to pyrroles or furanes (Figure 2.33 Entry 6, Table 2.22). 

The application of tungsten-<7r-allyl complexes provided yet another elegant and efficient access to paraconic acids (Scheme 29) [68]. The propargylic al-... 

Cycloaddition reactions of a,/3-unsaturated chromium and tungsten complexes have been studied to a great extent and have been reviewed.3 -6 Our report on cycloaddition of (l-alkynyl)carbene complexes is restricted to a short abstract and an update including more recent results. A most remarkable feature of [4+2] cycloadditions of 1,3 dienes to C=C bonds of (l-alkynyl)carbene complexes, e.g., li, is that such reactions proceed under very mild conditions, compared to those for reactions of propargylic esters, e.g., 41. Thus, formation of a Diels-Alder adduct, e.g., a norbornadiene derivative 42, can be achieved at 25°C via carbene complexes instead of at 190°C via the direct route (Scheme 15).68 Ligand disengagement from compound 40 can be achieved in various ways, e.g., by formation of an ester 43 through oxidation of the Cr=C bond, or by formation of an allyl silane 4369 or a stannane.70 71... 

Synthesis of heterocyclic and carbocyclic compounds via alkynyl, allyl, and propargyl organometallic cyclopentadienyl iron, molybdenium and tungsten complexes 00CRV3127. 

Reverse processes, that is reactions of propargyl or allenylidene ligands coordinated to mononuclear centers with water, methanol, amines, and other ligands are known. Tungsten l and platinum complexes give a series of reactions (some of which are mediated by surfaces) comparable with those discussed for clusters. 

LactonizattonT Lactone formation from propargylic alcohols and homologs in which the other propargylic position is substituted with a tungsten residue is readily achieved by treatment with triflic acid (0.25 equiv) in CH Clj at -40°. The tt-allyl complexes are demetallated on further treatment with CFjCOOH. 

Related -propargylic tungsten complexes 4.259 have been found to be sufficiently nucleophilic, with Lewis acid assistance, to attack aldehydes in an intramolecular fashion (Scheme 4.90). The initial cy-clisation products, believed to be Trf-allene complexes 4.260, cyclize to t -complexes 4.261, which can be oxidatively carbonylated to esters 4.262. 

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