Abstract
This review will outline the recent advances in chemo-, regio-, and stereoselective (cross-) dimerization of terminal alkynes to generate 1,3-enynes using different types of iron and cobalt catalysts with altering oxidation states of the active species. In general, the used ligands have a crucial effect on the stereoselectivity of the reaction; e.g., bidentate phosphine ligands in cobalt catalysts can generate the E-configured head-to-head dimerization product, while tridentate phosphine ligands can generate either the Z-configured head-to-head dimerization product or the branched head-to-tail isomer. Furthermore, the hydroalkynylation of silyl-substituted acetylenes as donors to internal alkynes as acceptors will be discussed using cobalt and nickel catalysts.
Highlights
Specialty section: This article was submitted to Organic Chemistry, a section of the journal Frontiers in Chemistry
While cobalt catalysts seem to be superior in comparison to iron catalysts for the E-selective dimerization of terminal alkynes, iron catalysts gave better results in the Z-selective transformations
Head-to-tail homodimerization has been well studied for piano-stool type iron(II) catalysts, which are able to cross-dimerize alkynes if a propargylic alcohol or propargylic amine is used as the donor-alkyne
Summary
1,3-Enynes are powerful building blocks in organic synthesis, due to their unique reactivity of the triple bond and/or double bond which can be addressed selectively (Sasaki et al, 2011; Zhou and Moberg, 2013) or react together as one functional group, for example, in cycloaddition reactions (Gevorgyan and Yamamoto, 1999; Pünner and Hilt, 2012; Röse et al, 2015). Some reports with main group (Dash and Eisen, 2000; Hasenbeck et al, 2019) and lanthanide catalysts (Nishiura et al, 2003; Ge et al, 2007; Platel and Schafer, 2012), early transition metal- (Cembellín et al, 2020), but mainly noble-metal-catalyzed dimerization protocols have been published (e.g., Ru (Barsu et al, 2020), Rh (Katagiri et al, 2008), Ir (Ohmura et al, 2000), Pd (Chen et al, 2013)) (Trost and Masters, 2016) Because of their higher toxicity and lower availability, earth-abundant transition metal catalysts, especially iron (Plietker, 2008) and cobalt (Hapke and Hilt, 2020) catalysts, have gained increasing attention in the last years with quite remarkable results (Liang et al, 2020b), which shall be summarized in this review. Due to the high number of possible isomers in the dimerization/hydroalkynylation process, the used catalyst must 1) control the chemo-, regio-, and stereoselectivity and 2) suppress further transformation of the 1,3-enynes with additional alkynes toward oligomerization products
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