Homocoupling alkynes

In 2006, Chatani and coworkers published a rhodium vinylidene-mediated alkyne cyclodimerization incorporating allylamine (Table 9.13) [26]. Although RhCl(PPh3)3 alone was a competent catalyst for the stereospecific formation of ( )-3-alkylidene-3,4-dihydro-2H-pyrroles (69), the addition of an ammonium salt helped to suppress undesired alkyne homocoupling. Under optimized conditions, a variety of functionalized alkynes underwent cyclodimerization with allyl- or crotylamine in good yield. 

K. Kamata, S. Yamaguchi, M. Kotani, K. Yamaguchi, N. Mizuno, Efficient oxidative alkyne homocoupling catalyzed by a monomeric dicopper-substituted silicotungstate, Angew. Chem. Int. Ed. 47 (2008) 2407-2410. 

The alkynyl iodide 359 undergoes cross-coupling with a terminal alkyne to give the 1,3-diyne 360[264]. No homocoupling product is formed. This reaction offers a good synthetic method for unsymmetrical 1,3-diynes. 

The use of imidazolium salts for in situ catalyst formation was shown to be optimal for the coupling of TMS-protected alkynes even with sterically demanding aryl bromides and avoids the formation of homocoupling-derived products. For this reaction, Nolan reported that the activation of chlorobenzene by this catalytic system was possible in moderate yield [125] (Scheme 6.41). 

For the development of the oxidative homocoupling reaction, in 1955 Chodkiewicz and Cadiot explored a Cu(I)-catalyzed heterocoupling reaction of terminal alkynes with 1-bromoalkyne in the... 

Trost s group examined the possibility of carrying out cross-coupling reactions of alkynes and transformed this into a very powerful synthetic method. Either homocoupling or perhaps, more interesting, heterocoupling procedures were performed using catalytic amounts of a palladium salt (Equation (191)). 

A second example from the same group is the synthesis of an elaborate diethynyltriphenylene derivative (Scheme 7 Table 8,entries 12,13) [58].Zn/Pd-promoted homocoupling of a 4-iodo-l,2-dialkoxybenzene furnishes the desired tetraalkoxybiphenyl, an electron-rich aromatic system. Iron trichloride-catalyzed Friedel-Crafts arylation of the biphenyl derivative with dimethoxy-benzene furnishes an unsymmetrical triphenylene derivative. Deprotection, oxidation, and subsequent Diels-Alder reaction with cyclohexadiene is followed by catalytic hydrogenation and reoxidation. TMS-CC-Li attack on the quinone delivers the alkyne modules, treatment with SnCl2 aromatizes the six-mem-bered ring, while KOH in MeOH removes the TMS groups cleanly to give the elaborate monomer. 

Suzuki-Miyaura as well as moderate activity in the Stille reaction ([M] = SnRa) were observed. In contrast to bis(NHC) complexes, inactivity in the Sonogashira reaction was due to increased activity in the homocoupling of alkynes [Eq. (47)], an undesired side reaction. 

Regioselective syntheses of 1,3,5-unsymmetrically substituted benzenes (309) are catalyzed by Pd(dba)2/PPh3 mixed alkyne/diyne reactants give mixtures containing homocoupled and mixed products (24 21 from HC CPh + HC=CC= CC Hn). The probable mechanism involves oxidative addition to the Pd(0) center, insertion of the second diyne into the Pd—H bond, reductive coupling and subsequent jr-complexation of this product to Pd(0), followed by Diels-Alder cycloaddition of the third diyne and elimination of product. 

Ranu and Banerjee developed a [bmim][OH] TSIL for oxidative homocoupling of terminal alkynes to 1,4-disubstituted 1,3-diynes in atmospheric conditions using Cu(ii) without using either palladium catalyst, amines, oxidants or organic solvents. Significant advantages stated by the authors include fast kinetics, high yields and mild reaction conditions. 

Cu(OAc)2 in combination with a solid base such as K2C03 has been reported to be an extremely efficient system in promoting the homocoupling of 2-azetidinones-tethered alkynes, whereas the cross-coupling of bromoalkynyl-(3-lactams with... 

The di-copper-substituted y-Keggin silicotungstate [y-H2SiWio036Cu2( j,-l, 1-N3)2]4- could act as an effective homogeneous catalyst for the oxidative homocoupling of various kinds of alkynes, including aromatic, aliphatic and heteroatom-containing ones (6.8) [108] ... 

Several alkyne-linked bis-2H-indazoles have been synthesized by the double cycliza-tion of butadiyne-linked (167) phenyltriazenes or phenyldiazenes, in turn obtained by Cu(OAc)2-mediated homocoupling of ethynylphenyltriazenes (Scheme 46).144 The reactions proceed rapidly under neutral conditions with mild heating, affording the heterocycles in excellent yields. By using Pd as catalyst under oxidative conditions, bis-2//-indazoles (170) were obtained directly from ethyne-linked phenyltriazenes (169). DFT calculations revealed for these reactions low barriers to cyclization that... 

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