A copper (I or II)/diethylphosphite catalytic system for base-free additive dimerization of alkynes
Graphical abstract
Introduction
In our earlier study of the copper-catalyzed addition of SH1 and P(O)H2 bonds to alkynes we have serendipitously observed that the Cu(II) catalyzed reaction of phenylacetylene with (RO)2P(O)H (R = Et, Ph), gives in high yield (up to 90%) the corresponding conjugated enyne, resulting from the additive dimerization of alkyne, instead of the expected HP(O) addition product (Scheme 1).
Since conjugated enynes are very important building blocks for the synthesis of biologically active molecules3 and materials for molecular electronics,4 we decided to study this reaction in more detail. It is known that stereodefined enynes (E or Z) can be obtained by cross-coupling reactions of alkenyl halides with terminal alkynes under Cu(I)5 or Pd6 catalysis, by the reaction of vinylsilanes with bromoalkynes,7 or using a Heck reaction.8
However, the simplest and most atom-economical route to conjugated enynes is certainly the metal-catalyzed additive dimerization of alkynes. This reaction was studied for various (early or late) transition metal catalysts and lanthanides.9, 9(a), 9(b) Surprisingly, the general copper catalyzed version of this transformation is still unknown, with a notable exception of acetylene itself; its dimerization on CuCl-based Nieuwland catalyst is used in the industrial production of vinylacetylene and is mentioned in many textbooks,10 while acetylene dimerization with copper acetylenide is known as the Strauss reaction.11 The most common problem in the additive dimerization of unsymmetrical alkynes is the formation of head-to-head (E and Z) and head-to-tail isomer mixtures (Scheme 2).
Dimerization can be also accompanied by the formation of Glaser-type12 1,3-diynes (R−≡−≡−R), and, under early transition metal catalysis, of triene byproducts (R− = · = · = −R). In addition to all these difficulties, related to regio- and stereoselectivity, there is a problem of finding a catalyst equally effective for aromatic and aliphatic acetylenes. In his pioneering work B. Trost developed the palladium catalyzed conditions, which allowed to obtain head-to-tail isomers selectively.13 However, it was soon shown that changing the palladium catalyst, the type of alkyne, using a different base and other additives, allows to obtain either head-to-head14, 15 or head-to-tail isomers.15(a), 15(b) For example, head-to-head isomer is formed in the reaction of terminal alkyne with activated internal alkyne in water under joint CuBrPdCl2(PPh3)2 catalysis.16 In this paper we extended our reaction2 on variety terminal alkynes using Cu (I) and Cu (II) salts and oxides in the presence of catalytic amount of (EtO)2P(O)H.
Section snippets
Results and discussion
Initial optimization of reaction conditions was performed using phenylacetylene dimerization as a model reaction.
Under aerobic conditions this reaction17 in DMF-THF (3:1) gave 1,4-diphenylbutadiyne as the only product formed. Depending on the Cu(II) salt catalyst and additive (Table 1) its yield varied from low (entries 2, 4, 5, 8) to moderate (entries 1, 6, 7, 9) or even nearly quantitative (entry 3).
To avoid this unwanted dimerization all subsequent reactions were carried out under a dry
Conclusion
We have elaborated a simple and efficient method of regioselective Cu-catalyzed head-to-head additive dimerization of aromatic and aliphatic alkynes utilizing various sources of copper and, as an essential additive, a catalytic amount of dialkylphosphite. The methodology is general for a variety of terminal alkynes possessing various functional groups such as aryl, alkyl, hydroxy and amino groups.
General
All reactions were performed under argon atmosphere. Solvents were dried and purified before use by standard procedure, diethylphosphite, copper compounds and all alkynes were purchased from commercial sources. 1H and 13C NMR spectra were recorded with TMS (0.00 ppm) or CDCl3 (1H NMR at 7.26 ppm and 13C NMR at 77.0 ppm) as internal standards, 85% H3PO4 was used as the external reference of 31P NMR spectra.
General procedure for the additive dimerization of terminal alkynes
To a mixture of 1 mmol alkyne, 5 mol% of copper compound in 2.0 mL of solvents
Acknowledgments
The authors thank Dr. P.K. Sazonov (M.V. Lomonosov MSU) for help in preparing the manuscript.
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