Ligand‐Controlled Cross‐Dimerization and ‐Trimerization of Alkynes under Nickel Catalysis

Abstract

Abstract:The cross-dimerization of diphenylacetylene with trimethylsilylacetylene via C À H bond cleavage in the presence of a catalytic amount oftogether with a pyridine-based ligand efficiently proceeds to give the corresponding enyne compound with good yield. In contrast, their 1:2 cross-trimerization leading to a dienyne derivative takes place selectively using a triarylphosphine ligand. The regioselective cross-dimer-

Pyridine‐Enhanced Head‐to‐Tail Dimerization of Terminal Alkynes by a Rhodium–N‐Heterocyclic‐Carbene Catalyst

“…Competitively to the formation of head-to-head (E/Z) and head-to-tail (gem) enynes, other products such as butatrienes, [6] diynes, [7] dieneynes, [8] cyclotrimers, [9] oligomers, [10] or polymers [11] can also be obtained, depending on the catalyst and reaction conditions (Scheme 1). A broad range of catalysts based on Pd, [12] Ru, [13] Rh, [14] Ni, [15] Ir, [16] Fe, [17] Au, [18] Co, [19] Os, [20] Ti, [21] Zr, [22] Re, [23] Y, [24] Sc, [25] Hf, [26] Cr, [27] lanthanides, [28] actinides, [29] or main group elements [30] can promote alkyne dimerization, albeit with different grades of success with regard to selectivity. In particular, preferential preparation of head-to-tail enynes has been disclosed for aroA C H T U N G T R E N N U N G mat-A C H T U N G T R E N N U N G ic [13i, 14e, 16b, 24a] or aliphatic [12d, 14a,c,g,k, 22a, 24c, 26, 29b] alkynes, although examples of selective initiators regardless of the substituent on the alkyne are limited to the catalysts of Nakamura (Ti), [21a] Trost (Pd), [12b] Eisen (Al), [30b] and Zhang (Au) [18] and their respective co-workers.…”

Palladium‐Catalyzed Regio‐ and Stereoselective Cross‐Addition of Terminal Alkynes to Ynol Ethers and Synthesis of 1,4‐Enyn‐3‐ones

“…The thus synthesized enynyl ethers with allylic hydroxy tethers, which survived the reaction, were shown to be ready precursors for valuable 1-en-4-yn-3ones. Given the importance of conjugated enynes and enol ethers as subunits in bioactive molecules as well as valuable synthetic intermediates, [1] a number of efforts have been made toward their synthesis. Alkyne-alkyne cross-addition was developed as a practical and atom economical means to obtain conjugated enynes, [2][3][4][5] with couplings ranging from dimerization to polymerization. After the pioneering studies by Trost et al on the homodimerization of terminal alkynes (Scheme 1 A) [2] and the selective coupling of terminal alkynes with electron-deficient conjugated alkynes (Scheme 1 B), [3] many groups investigated the selective synthesis of enynes through this pathway.…”

“…Although the reaction can be predicted (from the previous studies) [2][3][4] to occur through a concerted syn addition of the hydroalkynyl palladium species to the alkyne using the intrinsic polarization for the regioselectivity, we were interested in finding any possible involvement of the hydroxy functionality in the assembly of the metal-substrate complex. Furthermore, we wanted to expand the scope of the reaction to nonpropargyl substrates to further improve the generality of the approach.…”

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

“…[11] Although several Ru, [7b] Rh, [6d] and Ir [3b] metal complexes selectively catalyzed the head-to-head cross-dimerization and afforded both Z and E isomers, reports on the formation of head-to-tail products are scarce. [5f, 8a] In contrast, the selective cross-trimerization of three alkynes by combining silylacetylene and internal alkynes leading to 1,3-dien-5-ynes has been performed with Ni [4,12] and Pd [5c] catalysts. However, the formation of selective (2-alkynyl)-gem-1,3-dienes with a combination of three terminal alkynes has not been reported till date.…”

“…In this context, different chemo-, regio-, and stereoselectivities have been achieved with Ir, [3] Ni, [4] Pd, [5] Rh, [6] Ru, [7] Ti, [8] and Co [9] catalysts, among others.…”