Dimerization of terminal alkynes

The dimerization of terminal alkynes, known as the Glaser coupling, the Eglinton coupling, and the Cadiot-Chodkiewicz coupling, is one... 

Scheme 5 Proposed mechanism for the catalysis of the dimerization of terminal alkynes by the mixed hafnacarborane complex. Reproduced by permission of the American Chemical Society from Organometallics 1997, 16, 4508.

Several examples are known of the transition metal-catalyzed synthesis of 1,2,3-buta-trienes, which possess one more cumulated C=C double bond than allenes. Most of the reported examples of the butatriene synthesis involve dimerization of terminal alkynes and conjugated enynes are typical side products of the reactions. 

Alkynes react with haloethenes [38] to yield but-l-en-3-ynes (55-80%), when the reaction is catalysed by Cu(I) and Pd(0) in the presence of a quaternary ammonium salt. The formation of pent-l-en-4-ynes, obtained from the Cu(I)-catalysed reaction of equimolar amounts of alk-l-ynes and allyl halides, has greater applicability and versatility when conducted in the presence of a phase-transfer catalyst [39, 40] although, under strongly basic conditions, 5-arylpent-l-en-4-ynes isomerize. Symmetrical 1,3-diynes are produced by the catalysed dimerization of terminal alkynes in the presence of Pd(0) and a catalytic amount of allyl bromide [41]. No reaction occurs in the absence of the allyl bromide, and an increased amount of the bromide also significantly reduces the yield of the diyne with concomitant formation of an endiyene. The reaction probably involves the initial allylation of the ethnyl carbanion and subsequent displacement of the allyl group by a second ethynyl carbanion on the Pd(0) complex. 

The regio- and stereoselective dimerization of terminal alkynes into disubstituted enynes is efficiently catalyzed by rare-earth metal alkyl and hydride complexes, as reported independently by Bercaw et al. and Teuben et al. in 1987 [211,212]. Takaki and coworkers have shown that complexes Ln[N(SiMe3)2]3 when combined with an amine additive (typically, ArNH2 compounds) afford an active species for the... 

Scheme 12.21 Catalytic dimerization of terminal alkynes to head-to-head (PJ and tail-to-head (Pj) products.

The indenylidene complexes IX and XXVIIIc were also reported to promote the addition of different carboxylic acids to terminal alkynes to give enol esters, the Markovnikov addition product being the major product with, in some cases, the competing catalytic dimerization of terminal alkynes [61]. 

Scheme 9.8 Solvent-dependent selectivity in Rh(lll)-catalyzed Dimerization of terminal alkynes.

Scheme 9.11 Proposed mechanism for hydrative dimerization of terminal alkynes.

Finally, it can be noted that some cross-dimerization of terminal alkynes with internal alkynes, where ruthenium vinylidene intermediates are postulated, have also been reported [74, 75]. 

Lanthanide metallocene compounds are also active catalysts for the dimerization of terminal alkynes, giving predominantly the linear head-to-head enyne dimer with a double bond of E configuration [17]. In recent years, however, novel organo-lanthanide [13] and organoactinide [18, 19] systems have shown their ability to produce, with high selectivity, Z and geminal enyne products, respectively. 

Typical Procedure for Ruthenium-Catalyzed Dimerization of Terminal Alkynes with Monocarboxylic Acids [23]... 

The first example involves the dimerization of terminal alkynes. It takes place via initial activation of the alkyne C-H bond, but several examples involve a vinylidene intermediate. In most cases, conjugated enynes are obtained by ruthenium-catalyzed tail-to-tail dimerization [84,85], as in the following example [85] (Eq. 63). 

The precatalyst Cp RuCl(COD) allowed the head-to-head oxidative dimerization of terminal alkynes and the concomitant 1,4-addition of carboxylic acid to stereoselectively afford 1-acyloxy-l,3-dienes in one step under mild conditions [89] (Eqs. 67,68). The first step of the reaction consists in the oxidative head-to-head alkyne coupling via the formation of a ruthenacycle intermediate that behaves as a mixed Fischer-Schrock-type biscarbene ruthenium complex, allowing protonation and nucleophilic addition of the carboxylate. 

The InCl3-NaBH4-MeCN system shows high regio- and stereoselectivity for the dimerization of terminal alkynes to give enynes 91-93 (Scheme 114). The reaction is considered to proceed via the radical addition and coupling mechanism (Scheme 115).386... 

Alkynes can be selectively dimerized, cyclotrimerized, or polymerized with a large variety of transition metal and lanthanide catalysts nickel also catalyzes the cyclote-tramerization of HC=CH to cyclooctatetraene. Very electrophilic complexes such as Cp 2LnR and Group 4 compounds,137 as well as 18-electron species such as Cp RuH3(L) and Ru(Tp)Cl(PPh3)2, catalyze the linear dimerization of terminal alkynes 138... 

The original Sonogashira protocol involves palladium-copper co-catalysis. Attempts have been made over the last few years to overcome some of the limitations in this method, specifically to eliminate the undesired dimerization of terminal alkynes. Various copper-free conditions have been developed in order to reduce the amount of diacetylene formation. The focus seems to have been on changing the ligand. 

Catalytic dimerization of terminal alkynes is also reported to give eneynes [10,11] or butatriene [12]. In both reactions, the activity and selectivity are sensitive to the substituents in the alkyne and tertiary phosphine ligands employed (Scheme 14.3). 

The dimerization of terminal alkynes is a straightforward process which is catalyzed by lanthanocenes of scandium [195, 196], yttrium [66, 197], lanthanum... 

Cationic hydridoosmium complexes catalyze the regio- and stereoselective dimerization of terminal alkynes. [OsH(N2)(PP3)]BPh4 [PP3 = P(CH2CH2PPh2)3] is a catalyst precursor for the dimerization of 1-phenylacetylene to give (Z)-l,4-diphenylbut-3-en-l-yne [252]. 

Dimerization of terminal alkynes is promoted by RhCl(PPh3)3(eq (41)) [50]. 

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