Carbenes, alkynes

Keywords Alkenes Alkynes Carbene complexes Ruthenium Tandem reactions... 

Key words DFT, isomerization, alkene, alkyne, carbene, vinylidene. 

P. Hofmann, and M. Hammerle, Mechanism of Dotz Reaction Alkyne-Carbene Linkage to Chromacyclobutenes , Angew. Chem. Int. Ed. Engl. 28, 908-910 (1989). 

A catalytic tandem cyclopropanation-ring-closing metathesis of dienyne 80 led to derivative 81 in good yield (Scheme 30 <2004JA9524>). For internal alkynes, carbene-mediated ring-closing enyne metathesis was observed. Less favorable alkyne binding leads to preferential reactions of the metal carbene with the 1-alkene moiety. 

The observed photochemistry of this class of compounds has been exploited for both synthetic and mechanistic applications. In our study of (CO).sW=C(OMe)Ph, we observed that -40°C irradiation of the complex in the presence of alkynes led to the formation of alkyne-carbene complexes [Eq. (141)] 165). The PhC=CPh adduct complex was isolated... 

The first examples of a [2 -i- 2] cycloaddition at an unsaturated substituent on a carbene ligand have been recently reported." In the example shown in Scheme 2 it can be seen that the cycloadditions can occur very rapidly between enol ethers and alkynic carbene complexes. Alkynic carbene complexes can serve as surrogates for alkynic esters since the cycloadducts can be very efficiently oxidized to their corresponding esters. While the [2 + 2] cycloaddition of methyl tetrolate and dihydropyran is unknown, a comparison can be made between the alkynic carbene complex (12b), which reacts at room temperature, and the doubly activated alkynic ester (15), which will only react at 180 C with the same substrate. 

IV. SYNTHESIS AND PROPERTIES OF ALKENE, ALKYNE, CARBENE AND RELATED COMPLEXES OF GOLD... 

Scheme 58. Novel synthesis of centrosymmetric chrysenes (127) by dimerization of intramole-cularly stabilized alkyne-carbene chromium chelate complexes [160a, d]...

Finally, Katz and subsequently Rooney (386-388) showed that (CO)sW [C(OMe)Ph] catalyzes the polymerization of alkynes via carbene-alkyne and purported metallacyclobutene intermediates (Scheme 34). Later computational work by Hofmann (388b) called into question the formation of the latter species, suggesting that the reaction proceeds directly via the vinyl-carbene. Formation of the alkyne-carbene intermediate by low-temperature photolysis of (CO)5W[C—(OMe)Ph] in the presence of alkynes leads to polymerization upon warming. The use of a pre-formed carbene complex is not required since active polymerization catalysts can be formed by photolysis of W(CO)5 in the presence of terminal alkynes in hydrocarbon solutions. A key step in catalyst generation is rearrangement of a coordinated alkyne to a vinylidene ligand (389). 

K. H. Dotz, T. Schafer, F. Kroll, and K. Harms, Angew. Chem., 104, 1257 (1992). Alkin(carben)-Komplexe Stabilisierung einer Zwischenstufe der Carbenanellierung. Angew. Chem., Int. Ed. Engl, 31, 1236 (1992). Alkyne(carbene) Complexes Stabilization of an Intermediate of Carbene Annelation. 

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