gem‐Selective Cross‐Dimerization and Cross‐Trimerization of Alkynes with Silylacetylenes Promoted by a Rhodium–Pyridine–N‐Heterocyclic Carbene Catalyst

Abstract

The gem‐selective cross‐dimerization and ‐trimerization of silylacetylenes with alkynes through CH activation using a rhodium(I)–pyridine–N‐heterocyclic carbene catalyst have been developed. This reaction is applied to various aliphatic or aromatic terminal alkynes, internal alkynes, and gem‐1,3‐disubsituted enynes to afford the corresponding enynes and dienynes with high regio‐ and stereoselectivities and in good isolated yields (up to 91 %).

New Aspects of Recently Developed Rhodium(N‐Heterocyclic Carbene)‐Catalyzed Organic Transformations

“…A similar strategy was applied for the regioselective dimerization of alkynes using a Rh‐NHC catalyst. While [Rh(IPr)(coe)Cl] 2 alone gave several side products, such as the trisubstituted benzene obtained from the [2+2+2] cycloaddition, head‐to‐head dimerization products, and trimerization products, when pyridine was added it suppressed the generation of side products but not the generation of the desired head‐to‐tail dimerization product (Scheme ) ,. The reaction was initiated by the generation of rhodium hydride species, which are obtained by oxidative addition to a terminal alkyne.…”

“…A Rh(I) catalyst can also activate unreactive C−H bonds through oxidative addition. Jun and co‐workers reported a series of reactions involving rhodium hydride species ,. These C−H functionalization reactions were often highly efficient synthetic routes to complex synthetic targets.…”

Cobalt‐Catalyzed E‐Selective Cross‐Dimerization of Terminal Alkynes: A Mechanism Involving Cobalt(0/II) Redox Cycles

“…Thec obalt precatalyst 1a is reduced by EtMgBr to give the cobalt(0) species A.N ext, oxidative addition of at erminal silylacetylene occurs to generate the trimethylsilylalkynylcobalt(II)h ydride complex C via intermediate B,aterminal C À H s-coordinated species of silylacetylene to the cobalt(0) center found in DFT calculations.No other terminal alkynes are incorporated for the oxidative addition processes because of the low bond dissociation energy of the terminal C À Hbond of silylacetylenes as well as the preferential coordination of the terminal C(sp) À H bond. [14,15] Then ext step is either ah ydrometallation or alkynylmetallation with as econd terminal alkyne (2)t o produce either D or E,r espectively.T he exceptionally high selectivity for the formation of E isomers strongly suggests the hydrometallation pathway because of the steric repulsion of the bulky ligand and the substituent of the terminal alkyne. Finally,r eductive elimination from either D or E produces 4 with regeneration of A.T he high cross-selectivity without forming any homodimers in our catalytic system is explained by the steric repulsion between the incoming silylacetylenes and 2,4,6-triisopropylphenyl substituents on L 1 in C. [16] In fact, the cobalt complex 1b,h aving al ess bulky 1,10-phenanthroline ligand, showed better catalytic activity and cross-selectivity for the bulkier silylacetylene 6 with 2a (91 %y ield for 10)along with ahomodimer of 6 (26 %yield with respect to 6) compared with the cross-dimerization in entry 2, Table 1.…”

Elected to the Academy of Europe: F. Lloret and P. Samorì / EuCheMS Lecture: C. Moberg and G. Férey / EuCheMS Award for Service: L. A. Oro / International Society of Electrochemistry Prizes: A. M. Bond, J. Rusling, M. Osawa, Y.‐G. Guo, F. La Mantia, and Y. Wang

“…His main research interests are in organometallic and homogeneous catalysis using late transition metals. He has reported in ChemCatChem on CH activation using a rhodium(I)–pyridine– N ‐heterocyclic carbene catalyst,5a and in Angewandte Chemie on CN coupling mediated by an iridium complex 5b. Oro is the Co‐Chair of the Editorial Board of ChemCatChem , and was on the International Advisory Board of Angewandte Chemie from 2006–2013…”