Ligand mediated iron catalyzed dimerization of terminal aryl alkynes: scope and limitations

Abstract

Regioselective dimerization of terminal aryl alkynes to produce conjugated enynes has been achieved using FeCl3 and KO(t)Bu in the presence of either DMEDA or dppe. The reaction proceeds smoothly in toluene at 145 °C for 2 h to give the corresponding head-to-head dimers in good to excellent yields (54 to 99%) with high E-selectivity (67 : 33 to 83 : 17 E/Z). Both strongly electron-donating and electron-withdrawing groups are compatible with this procedure. The bidentate phosphine (dppe) ligand exhibits better catalytic activity than the bidentate amine (DMEDA). The aliphatic acetylene fails to react under this catalytic system which suggests that potassium tertiary butoxide activates the conjugated system of aryl acetylene through cation-pi interaction and pi-pi interaction. A radical inhibitor (galvinoxyl or TEMPO) completely suppresses the reaction. Employing FeCl2 as a catalyst instead of FeCl3, only phenyl acetylene afforded the corresponding head to head dimer in good yield. Mechanistic pathways for both FeCl3 catalyzed dimerization of aryl alkynes and FeCl2 catalyzed dimerization of phenyl acetylene have been proposed.

Iron‐Catalyzed gem‐Specific Dimerization of Terminal Alkynes

“…Recently, Milstein and co‐workers reported a well‐defined iron(II)‐pincer catalyst that can achieve Z ‐selective alkyne dimerization of arylalkynes under mild conditions at 1–3 mol % catalyst loadings in the absence of any additive . Later, Mandal and co‐workers reported a cyclic (alkyl)amino carbene iron(0) catalyst that can catalyze the homodimerization of arylalkynes in the presence of a large excess of KO t Bu at high temperatures with moderate to high yields and poor to high E selectivity . Similar to Z and E 1,3‐enynes, geminal ( gem ) 1,3‐enynes are useful synthons for complex organic architectures ,,–.…”

Iron‐Catalyzed gem‐Specific Dimerization of Terminal Alkynes

“…[1a, 4, 5] In contrast, only af ew examples of iron-based catalysts have been reported. [6,7] Thef irst reported iron-catalyzed alkyne dimerization uses FeCl 3 as the catalyst in the presence of aligand and large excess of KO t Bu;this reaction requires an extremely high catalyst loading (30 mol %) and high temperatures,and gives E enyne products. [6a] Recently,Milstein and co-workers reported aw ell-defined iron(II)-pincer catalyst that can achieve Z-selective alkyne dimerization of arylal-kynes under mild conditions at 1-3 mol %catalyst loadings in the absence of any additive.…”

Z‐Selective (Cross‐)Dimerization of Terminal Alkynes Catalyzed by an Iron Complex

“…[2] Direct hydroalkynylation across the carbon-carbon triple bond or alkyne dimerization to form conjugated enynes is the most attractive method as it is fully atom economical. [5] We are aware of only am arginally catalytic (maximum 3turnovers) E-selective alkyne dimerization involving iron to give enyne products, [6] with FeCl 3 ,aligand, and KOtBu (300 mol %) at high temperatures. [3, 4] Since iron salts are readily available, inexpensive,and environmentally benign, the development of iron-based catalysts for organic transformations is currently av ery active area of research.…”

“…[3] However, this process has resulted in relatively few applications owing to issues of chemo-, regio-, and stereoselectivity.T he dimerization of terminal alkynes has been investigated with alarge number of transition-metal-based catalysts,m ostly based on precious metals. [5] We are aware of only am arginally catalytic (maximum 3turnovers) E-selective alkyne dimerization involving iron to give enyne products, [6] with FeCl 3 ,aligand, and KOtBu (300 mol %) at high temperatures. [5] We are aware of only am arginally catalytic (maximum 3turnovers) E-selective alkyne dimerization involving iron to give enyne products, [6] with FeCl 3 ,aligand, and KOtBu (300 mol %) at high temperatures.…”