Tungsten-alkynyl complexes

Etherification using a metal vinylidene has also been combined with G-G bond formation through the reaction of an alkynyl tungsten complex with benzaldehyde (Scheme 14). The addition of an internal alcohol to the incipient /3,/Udialkylvinylidene that is generated leads to dehydration and the formation of a Fischer-type alkylidene complex. Further reactions of this carbene with a range of nucleophiles have provided access to various furan derivatives.374,375... 

Surface-catalyzed isomerization of mononuclear alkynyl tungsten complexes to allyl derivatives has also been reported it occurs on silica gel. ... 

Another example of a [4S+1C] cycloaddition process is found in the reaction of alkenylcarbene complexes and lithium enolates derived from alkynyl methyl ketones. In Sect. 2.6.4.9 it was described how, in general, lithium enolates react with alkenylcarbene complexes to produce [3C+2S] cycloadducts. However, when the reaction is performed using lithium enolates derived from alkynyl methyl ketones and the temperature is raised to 65 °C, a new formal [4s+lcj cy-clopentenone derivative is formed [79] (Scheme 38). The mechanism proposed for this transformation supposes the formation of the [3C+2S] cycloadducts as depicted in Scheme 32 (see Sect. 2.6.4.9). This intermediate evolves through a retro-aldol-type reaction followed by an intramolecular Michael addition of the allyllithium to the ynone moiety to give the final cyclopentenone derivatives after hydrolysis. The role of the pentacarbonyltungsten fragment seems to be crucial for the outcome of this reaction, as experiments carried out with isolated intermediates in the absence of tungsten complexes do not afford the [4S+1C] cycloadducts (Scheme 38). 

Experimental Procedure 2.2.5. Gyclopentannulation with a Tungsten Alkynyl-carbene Complex l-(l-Ethoxy-3-phenyl-5,9b-dihydro-4H-cycldpenta a]naph-thalen-9 yl)pyrrolidine [353]... 

Experimental Procedure 2.2.5. Cyclopentannulation with a Tungsten Alkynyl-carbene Complex I-(I-Ethoxy-3-phenyI-5,9b-dihydro-4f/-cyclopenta[a]naph-thaIen-9-yl)pyrroIidine... 

Cycloadditions of alkynyl carbenes. Alkynyl carbene complexes of chromium and tungsten undergo [2 + 2]cycloaddition at room temperature with a wide range of enol ethers. 

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... 

Addition of l,3-thiazolium-4-olates to (l-alkynyl)carbene tungsten complexes lb,n affords thiophene and/or pyridone complexes, 64 and 65, by extrusion of sulfur and isocyanate, respectively, from the [3+2] adduct formed initially (Scheme 21).97 [3+2] cycloaddition reactions of azomethine... 

The reaction of an azadiene RIN = CH—CH = CHR2 (R = Ph, Me R2 = rc-Pr, Bn, i-Pr, n-Pr) with a (l-alkynyl)carbene chromium complex la (R = Ph, SiMe3, f-Bu) quite unexpectedly affords [4+3] cycloaddition products 133 in 57-93% yield, from which azepinones 132 are obtained on contact with wet silica (Scheme 52).141 Reaction of azadienes R N=CH — CH = CHR2 (R = Ph, Me R2 = n-Pr, Bn, i-Pr, n-Pr) with a (l-alkynyl)car-bene tungsten complex lb (R = Ph) was shown to produce 1,4-dihydropyri-dine complexes (of the type shown in Scheme 50) as the major products.142... 

Molecular wires, 208 211 in which two metal centers are connected by an unsupported C3 chain, as in compound 191, have been generated by addition of BF3 to binuclear bridged (l-alkynyl)carbene complex lw (Scheme 80).27 29,212 Similar compounds were derived from tungsten, rhenium, and iron moieties. A binuclear complex 192 containing a C5 bridge has been also obtained 27... 

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

Tungsten alkynyl Fischer carbene complexes are excellent dienophile partners in the classical Diels-Alder reaction with 1-azadienes (see Sect. 2.9.2.1). On the contrary, the chromium-derived complexes exhibit a different behaviour and they react through a [4S+3C] heterocyclisation reaction to furnish azepine derivatives [116] (Scheme 68). The reaction is initiated by a 1,2-addition of the nitrogen lone pair to the carbene carbon followed by a [l,2]-Cr(CO)5 shift-pro-... 

---