Unusual 1-Alkyne Dimerization/Hydrogenation Sequences Catalyzed by [Ir(H)2(NCCH3)3(P-i-Pr3)]BF4: Evidence for Homogeneous-Like Mechanism in Imidazolium Salts

• Published in: Advanced synthesis & catalysis Vol. 345; no. 1-2; pp. 280 - 288
• Main Authors: Navarro, Janeth, Sági, Mária, Sola, Eduardo, Lahoz, Fernando J., Dobrinovitch, Isabel T., Kathó, Ágnes, Joó, Ferenc, Oro, Luis A.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 01-01-2003; WILEY‐VCH Verlag
• Subjects: alkynes; biphasic catalysis; homogeneous catalysis; hydrogenation; ionic liquids; iridium
• ISSN: 1615-4150; 1615-4169
• DOI: 10.1002/adsc.200390023

The reaction of complex [Ir(H)2(NCCH3)3(P‐i‐Pr3)]BF4 (1) with excess of 1‐alkynes such as t‐BuC≡CH and PhC≡CH gave the butadiene compounds Ir[η4‐(R)2C4H4](NCCH3)2(P‐i‐Pr3)}BF4 (R=t‐Bu, 5; R=Ph, 6). Compound 5 was obtained as a single isomer containing a 1,3‐disubstituted butadiene ligand, whereas 6 was formed as a 7:1 mixture of isomers containing 1,3‐ and 1,4‐disubstituted butadienes, respectively. Spectroscopic observations showed alkenyl hydride reaction intermediates, consistent with a double insertion/C‐C coupling sequence. Complexes 5 and 6 were found to react with dihydrogen to give 1 and alkenes resulting from the partial hydrogenation of the butadiene moieties. This dimerization/hydrogenation sequence has been found to be the major reaction of t‐BuC≡CH under conditions of homogeneous hydrogenation whereas that of PhC≡CH produced styrene and ethylbenzene as major products. Similar selectivity was observed for these hydrogenations using organic/ionic liquid biphasic conditions with toluene/BMIM⋅BF4, suggesting reaction mechanisms similar to those operating under homogeneous conditions. This conclusion is also supported by the spectroscopic observation of alkenyl hydride intermediates during the formation of 6 in BMIM⋅BF4 as solvent.