Terminal alkynes dimerization

Scheme 10.19 Proposed catalytic cycles for terminal alkynes dimerization.

Alkynes undergo stoichiometric oxidative reactions with Pd(II). A useful reaction is oxidative carboiiyiation. Two types of the oxidative carbonyla-tion of alkynes are known. The first is a synthesis of the alkynic carbox-ylates 524 by oxidative carbonylation of terminal alkynes using PdCN and CuCh in the presence of a base[469], Dropwise addition of alkynes is recommended as a preparative-scale procedure of this reation in order to minimize the oxidative dimerization of alkynes as a competitive reaction[470]. Also efficient carbonylation of terminal alkynes using PdCU, CuCI and LiCi under CO-O2 (1 I) was reported[471]. The reaction has been applied to the synthesis of the carbapenem intermediate 525[472], The steroidal acetylenic ester 526 formed by this reaction undergoes the hydroarylalion of the triple bond (see Chapter 4, Section 1) with aryl iodide and formic acid to give the lactone 527(473],... 

The 2,3-alkadienyl acetate 851 reacts with terminal alkynes to give the 2-alkynyl-1,3-diene derivative 852 without using Cul and a base. In the absence of other reactants, the terminal alkyne 853 is formed by an unusual elimination as an intermediate, which reacts further with 851 to give the dimer 854. Hydrogenolysis of 851 with formic acid affords the 2, 4-diene 855[524]. 

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

Methylenative dimerization takes place when terminal alkynes are treated with the tita-nocene/methylidene/zinc halide complex generated from titanocene dichloride and CH2(ZnI)2. The process is believed to involve the formation of a titanacyclobutene intermediate [75],... 

Molecular scaffoldings with tetraethynylethenes (TEEs, 3,4-diethynylhex-3-ene-l,5-diynes) and trans-1,2-diethynylethenes [DEEs, (E)-hex-3-en-l,5-diynes] are at a particularly advanced stage.114,37 38 441 A collection of dose to one hundred partially protected and functionalized derivatives have been prepared in the meantime, providing starting materials for the perethynylated dehydroannulenes and expanded radialenes shown in Figure 6.136 441 TEEs and DEEs, as well as dimeric derivatives substituted at the terminal alkynes with donor (D, p-(dimethyl-... 

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. 

Hashmi et al. investigated a number of different transition metals for their ability to catalyze reactions of terminal allenyl ketones of type 96. Whereas with Cu(I) [57, 58] the cycloisomerization known from Rh(I) and Ag(I) was observed (in fact the first observation that copper is also active for cycloisomerizations of allenes), with different sources of Pd(II) the dimer 97 was observed (Scheme 15.25). Under optimized conditions, 97 was the major product. Numerous substituents are tolerated, among them even groups that are known to react also in palladium-catalyzed reactions. Examples of these groups are aryl halides (including iodides ), terminal alkynes, 1,6-diynes, 1,6-enynes and other allenes such as allenylcarbinols. This che-moselectivity might be explained by the mild reaction conditions. 

With the bulky metallo-organic Pd(II) catalyst 98, on the other hand, selective formation of 99 was possible here functional groups are tolerated that would react with an Ag(I) catalyst (for example, terminal alkynes, alkyl chlorides, alkyl bromides and alkyl iodides) [59]. With l,n-diallenyl diketones (100), easily accessible by a bidirectional synthesis, up to 52-membered macrocycles (101) could be prepared in an end-group differentiating intramolecular reaction (Scheme 15.26) [60], For ring sizes lager than 12 only the E-diastereomer is formed overall yields of the macrocydes varied between 17 and 38%. Only with tethers shorter than 11 carbon atoms could the Z-diastereomer of the products be observed, a stereoisomer unknown from the intermolecular dimerization reactions of 96. 

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.

Aryl acetylenes undergo dimerization to give 1-aryl naphthalenes at 180 °C in the presence of ruthenium and rhodium porphyrin complexes. The reaction proceeds via a metal vinylidene intermediate, which undergoes [4 + 2]-cycloaddition vdth the same terminal alkyne or another internal alkyne, and then H migration and aromatization furnish naphthalene products [28] (Scheme 6.29). 

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.

Chatani and coworkers published an efficient method for the Rh(I)-catalyzed anti-Markovnikov hydroamination of terminal alkynes using either primary or secondary amines [58]. This reactivity had been observed earlier in the course of their studies on hydrative alkyne dimerization (Equation 9.8). 

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]. 

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