Rhodium-Catalyzed Dimerization of Terminal Alkynes Assisted by MeI

Abstract

Dimerization of terminal arylalkynes at ambient temperature catalyzed by Rh(CO)(PPh 3 ) 2 -Cl (2) in the presence of MeI leads to formation of enyne with high conversion and high regio-and stereoselectivity. A rhodium intermediate captured from oxidative addition of MeI was used for dimerization of alkyne with selectivity controlled by the use of solvents. In aprotic solvent (such as acetone, CH 2 Cl 2 , or THF) dimerization of terminal alkynes HCtC(p-C 6 H 4 X) (1, X ) H, a; NO 2 , b; C(O)H, c; Me, d; CN, e; NMe 2 , f; CF 3 , g; F, h; Br, i; I, j) leads to the (E)-1,4-disubstituted enynes 6 (a-k) in high selectivity. However, when MeOH is used as a solvent, the dimerization of 1-arylalkynes containing an electronwithdrawing group affords selectively the (Z)-1,4-disubstituted enyne 8. Requirement of the presence of MeI for this conversion indicates that the process presumably involves initially a six-coordinated rhodium methylacetylide intermediate. Oxidative addition of ICH 2 CN to 2 yielded the catalytically inactive six-coordinated complex Rh(CO)(PPh 3 ) 2 (CtCPh)(I)(CH 2 -CN) (5a). The analogous complex 5b with a p-nitro group on the phenyl acetylide ligand is characterized by X-ray diffraction analysis.

Head‐to‐Head Homo‐Coupling of Arylethynes Catalysed by (Dicarbonyl)ruthenium Chloride Metallacycles: Selective Synthesis of (E)‐1,4‐Diarylbut‐1‐en‐3‐ynes

“…In all cases the products were found to result from a head-to-head coupling, and were identified as (E)-1,4-diarylbut-1-en-3-ynes. In the cases of 3b and c, assignement of the E stereochemistry was based on the typical positions of the signals arising from the two ethylenic protons that were found to be consistent with those reported for (E)-3a and (E)-3d-f [12,13] in the literature. The yields in isolated enynes lie in the range between 54 and ca.…”

Symmetrical Bis(acetylido)ruthenium(II) Complexes

“…[16] Although many metal complexes, including early and late transition metals and lanthanides, are known to catalyse the dimerisation reaction of terminal alkynes, [17] in most cases a mixture of regio-and stereoisomeric butenynes is obtained. To date there have been few reported examples of highly selective catalysis.…”

Regio- and Stereoselective Homodimerization of Monosubstituted Acetylenes in the Presence of the Second Generation Grubbs Catalyst

“…Many examples of effective run of the process in the presence of transition metal complexes have been described. Catalytic activity in the process has been reported to be shown by complexes of such metals as palladium [4,[7][8][9][10][11][12][13], rhodium [14][15][16][17][18][19], ruthenium [20][21][22][23][24][25][26][27][28][29], nickel [30], iridium [31][32][33], osmium [34,35], iron [36] and the f-block metals [37][38][39]. However, a highly selective synthesis of conjugated enynes by dimerization is still a challenging process.…”