Cobalt-Catalyzed gem-Cross-Dimerization of Terminal Alkynes

Abstract

Transition-metal-catalyzed dimerization of two different terminal alkynes provides an atom-economic synthesis of valuable conjugated 1,3-enynes. Despite many catalyst systems developed, the state-of-the-art solutions are still limited to special alkynes. A practical catalyst, which could be used to cross-dimerize general aryl alkynes and aliphatic alkynes, is still highly desired. Herein we present the earth-abundant Co­(II)-catalyzed highly gem-selective cross-dimerization of aryl alkynes and aliphatic alkynes or gas acetylene under mild reaction conditions. Conjugated 1,3-enynes with various useful functional groups can be prepared in high yields. The application of Co­(OAc)2 and t-butyl group substituted triphos ligand is essential to distinguish the alkynes at different steps and realize the gem-selectivity.

Late 3d Metal-Catalyzed (Cross-) Dimerization of Terminal and Internal Alkynes

“…In contrast to iron catalysis, the cobalt-catalyzed hydroalkynylation of internal alkynes had been demonstrated by Li using CoCl 2 with a pyridyl-imine or a bidentate phosphine SCHEME 19 | Cobalt(II)-catalyzed gem-specific (cross-) dimerization of terminal alkynes (Chen and Li, 2020). SCHEME 20 | Postulated mechanism of the cobalt(II)-catalyzed head-to-tail dimerization of terminal alkynes (Chen and Li, 2020 ligand, which was in situ reduced to the active cobalt(I) species with zinc as the reducing agent in N-methylpyrrolidone (NMP) at 50°C.…”

“…In contrast to iron catalysis, the cobalt-catalyzed hydroalkynylation of internal alkynes had been demonstrated by Li using CoCl 2 with a pyridyl-imine or a bidentate phosphine SCHEME 19 | Cobalt(II)-catalyzed gem-specific (cross-) dimerization of terminal alkynes (Chen and Li, 2020). SCHEME 20 | Postulated mechanism of the cobalt(II)-catalyzed head-to-tail dimerization of terminal alkynes (Chen and Li, 2020 ligand, which was in situ reduced to the active cobalt(I) species with zinc as the reducing agent in N-methylpyrrolidone (NMP) at 50°C. With (2,6-diisoproyphenyl)-1-(pyridine-2-yl)methanimine as ligand, different diaryl-substituted alkynes 5 were hydroalkynylated by silyl-substituted alkynes 1 in good to excellent yields, using a small excess of 1.30 equivalents of terminal alkyne 1 (Scheme 21) (Sakurada et al, 2013).…”

“…While cobalt-catalyzed cross-dimerization of terminal alkynes normally needs a donor-alkyne with a high steric demand, Li described an Co(II)-TriPhos tBu -catalyzed cross-coupling of an aryl-substituted alkyne 1a and an alkyl-substituted acceptor-alkyne 1b , which led to the head-to-tail dimerization product 4ab ( Scheme 19 ) ( Chen and Li, 2020 ).…”

“…Besides cross-dimerization of two different alkynes, the authors also tested the homodimerization of aryl- and aliphatic-substituted alkynes, which resulted in the gem -1,3-enyne as the highly preferred product. Based on mechanistic investigations the following mechanism was postulated ( Scheme 20 ) ( Chen and Li, 2020 ).…”

“… Postulated mechanism of the cobalt(II)-catalyzed head-to-tail dimerization of terminal alkynes ( Chen and Li, 2020 ). …”