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

Abstract

Abstract:The reaction of complex [Ir(H) 2 (NCCH 3 ) 3 (P-i-Pr 3 )]BF 4 (1) with excess of 1-alkynes such as t-BuC CH and PhC CH gave the butadiene compounds Ir[h 4 -(R) 2 C 4 H 4 ](NCCH 3 ) 2 (P-iPr 3 )}BF 4 (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 ¥ BF 4 , 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 ¥ BF 4 as solvent.

C−H Activation and C−C Coupling Reactions in 2-Vinylpyridine Cationic Complexes of Iridium

“…In particular, thermally induced reactions of C−C reductive elimination have not been observed even under harsh experimental conditions. In this respect, the behavior of these bis-alkenyl and alkenyl-alkyl compounds is similar to that reported for the related bis-phosphine cation [Ir(CHCH 2 ) 2 (NCCH 3 ) 2 (PPh 3 ) 2 ] + , although is different from that of the monophosphine analogues [Ir(CHCHR) 2 (NCCH 3 ) 2 -(P i Pr 3 )] + , postulated as intermediates in the formation of Ir(I) butadiene complexes . However, complexes 7 − 11 have been found to readily react with a new equivalent of 2-vinylpyridine.…”

LabileN-Heterocyclic Carbene Complexes of Iridium

“…Terminal alkynes such as phenylacetylene can also work as hydrogen acceptors against dihydrides 5 and 6 , although, given that they are good oxidative addition reactants too, their reactions afford products very different from those in eq 3. Unfortunately, the intricate evolution of the more reactive complex 5 in the presence of phenylacetylene has not yet been understood, although its complexity is by itself suggestive of similarities with the triisopropylphosphine analogue of 5 , the reactivity of which against 1-alkynes has been found to be particularly rich . On the contrary, the reaction of 6 with an excess of phenylacetylene has been observed to selectively produce styrene and the hydride alkynyl complex [IrH(CCPh)(NCMe) 2 (IMes)(P i Pr 3 )]PF 6 ( 14 ).…”

Nitrile-Functionalized Pyridinium Ionic Liquids: Synthesis, Characterization, and Their Application in Carbon−Carbon Coupling Reactions

“…Palladium-catalyzed carbon−carbon bond formation, such as in the Suzuki and Stille reactions, usually takes place in high yield under relatively mild conditions and tolerates a wide variety of functional groups on either coupling partner . The development of biphasic (or multiphasic) systems for these types of reactions has received considerable attention, and both Suzuki , and Stille reactions have previously been studied in ionic liquids.…”