Copper-catalysed homo-coupling

Although the copper-catalysed homo-coupling reaction of aryldiazonium salts is still unemployed, the Cohen s work is a good basis for further achievements of this perspective approach. 

Table 8. Palladium-catalysed homo-coupling reaction of arylboronic acids to symmetrical biaryls in the presence of copper(II) nitrate as an oxidant...

Aryltrimethyl- or tri- -butylstaimanes as common Stille reagents are also successfully homo-coupled under palladium catalysed reactions in the presence of ethyl 2,3-dibromophenylpropionate (443) [26], or more conveniently by oxidation with copper(I) salts, e.g. CuCl [27], or copper(II) salts, e.g. Cu(N03)2 3H20 [28,29], which proceed smoothly at room temperature in tetrahydrofuran or DMF to afford symmetrical biaryls in excellent 3uelds. The Cu(N03)2-mediated homo-coupling works well also with diaryldimethyl(or -butyl)stannanes [30]. Moreover, the reaction can be accomplished with a catalytic amount of copper(II) chloride or manganese(II) bromide (10 mol%) in the presence of iodine as stoichiometric oxidant [31]. For example, compound 443, acting as an oxidant, converts the phenyltri-n-butylstannane (184) to biphenyl (8) in 86% yield [26], Scheme 11. 

The homo-coupling reaction with only catalytic amount of copper(II) chloride or manganese(II) bromide has been affected in polar solvents such as DMF or NMP with iodine as an ultimate oxidant [31]. Namely, 2-thienyltri- -butylstannane (173) was converted to 2,2 -bithienyl (98) with CuCl2 in 72% yield, whereas the MnBr2-catalysed reaction resulted with a 83% yield [31], Scheme 14. 

A common by-product in the Sonogashira reaction is the homo-coupled diyne (Scheme 2.127). This is usually due to the presence of O2. This is a long-known copper-catalysed reaetion called Glaser coupling, which is a form of homo-coupling (Section 2.10). Palladium may also catalyse this reaction. 

The monoiodo derivative 74 was then converted to the ethynyl substituted phthalocyanine 76 by reaction with trimethylsilylacetylene ([Pd(PPh3)2Cl2], Cul, EtjN, THF, RT, 24 h) followed by desyUlation of 75 (TBAF, THF, 0 C-rt, 2 h, 65 %). Then 1,3-butadiyn bridged dimer 77 was obtained by copper catalysed Glaser homo-coupling reaction (CuCl, pyridine, rt, 3 days, 75 %) [62],... 

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