Dimerization, alkyne, metal catalyzed

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. 

The cyclopropanation of alkenes, alkynes, and aromatic compounds by carbenoids generated in the metal-catalyzed decomposition of diazo ketones has found widespread use as a method for carbon-carbon bond construction for many years, and intramolecular applications of these reactions have provided a useful cyclization strategy. Historically, copper metal, cuprous chloride, cupric sulfate, and other copper salts were used most commonly as catalysts for such reactions however, the superior catalytic activity of rhodium(ll) acetate dimer has recently become well-established.3 This commercially available rhodium salt exhibits high catalytic activity for the decomposition of diazo ketones even at very low catalyst substrate ratios (< 1%) and is less capricious than the old copper catalysts. We recommend the use of rhodium(ll) acetate dimer in preference to copper catalysts in all diazo ketone decomposition reactions. The present synthesis describes a typical cyclization procedure. 

Most transition metal-catalyzed cross-coupling reactions also yield small quantities of the product of homocoupling of the nucleophilic reactant[16, 114,115]. In particular terminal alkynes [116, 117] or metalated terminal alkynes [118] readily dimerize to the corresponding 1,3-butadiynes (Scheme8.14). 

In the alkyne dimerization catalyzed by palladium systems, all proposed mechanisms account for an alkynyl/alkyne intermediate with cis addition of the alkynyl C-Pd bond to the alkyne in a Markovnikov fashion, in which the palladium is placed at the less-substituted carbon, both to minimize steric hindrance and to provide the most stable C-Pd bond (Scheme 2a). The reverse regioselectivity in the palladium-catalyzed dimerization of aryl acetylenes has been attributed to an agostic interaction between the transition metal and ortho protons of the aromatic ring in the substrate (Scheme 2b) [7, 8],... 

The coordinated alkene or alkyne ligand can be attacked by other alkene or alkyne molecule to accomplish some metal-catalyzed synthetically useful transformations. Typical examples include dimerization and polymerization of alkenes catalyzed by highly electrophilic cations [PdL2(MeCN)2] + (L = MeCN, PR3) (e.g. Scheme 8.35) [57], and Cope rearrangement of 1,5-hexadiene derivatives catalyzed by PdCl2 (Scheme 8.36) [58], It was proposed that the key step in these reactions was the C-C bond formation via attack of the external alkene at the alkene carbon which was made highly electron-dehcientby coordination to Pd(II) ion. 

Various situations are analyzed where the two metal centers play a role in one of the coordination modes A-E. There are many cases in which bimetallic catalysis can occur with the two metals acting cooperatively, for instance, in the dimerization of alkynes at two ruthenium metal centers, where a ruthenium-vinylidene species is formed, which is able to subsequently activate the second alkyne reactant through a C-H cleavage on the second ruthenium center. The coupling of these two moieties occurs on this dinuclear platform to provide the enyne product molecule. Examples are also presented where bimetallic catalysts cooperatively activate substituted alkynes in the catalyzed formation of heterocycles. 

Using late transition metals, trisubstituted pyrrole products have been assembled using a Rh-catalyzed regioselective head-to-tail alkyne dimerization of protected propargylic amine substrates (Scheme 15.98) [323]. Then, upon isolation of the resultant substituted enyne product, Au(III) intramolecular hydroamination with these protected amine substrates could be used to effectively prepare amine-functionalized pyrroles in up to 88% yield. The nature of the N-protecting group dramatically impacts the yields obtained in these reactions [323]. 

A number of actinide complexes have been investigated with respect to their catalytic activity in the intermolecular hydroamination of terminal alkynes with primary ahphatic and aromatic amines [98, 206-209]. Secondary amines generally do not react and the reaction is believed to proceed via an metal-imido species similar to that of group 4 metal complexes. The reaction of Cp 2UMc2 with sterically less-demanding aliphatic amines leads exclusively to the anti-Markovnikov adduct in form of the -imine (31) [207] however, sterically more demanding amines, e.g., t-BuNH2, result in exclusive alkyne dimerization. The ferrocene-diamido uranium complex 12 (Fig. 4) catalyzes the addition of aromatic amines very efficiently (32) [98]. 

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. 

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... 

Many Rh(I)-complexes are capable of dimerizing or oligomerizing alkynes to some degree. Seemingly small changes in reaction conditions can affect the stereo- and regio-selectivity of dimerization. The formation of (Z)-head-to-head dimers, however, can be indicative of metal vinylidene intermediates. This correlation was observed for Ru( 11)-catalyzed dimerizations (Chapter 10) and also holds true for the Rh(I)- and Ir (I)-catalyzed processes described herein. 

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... 

Metal-mediated cyclizations that rely on the initial complexation of an alkene or alkyne around a low oxidation state metal center are often sensitive to the presence of additional substituents (particularly electron-donating substituents), and relatively more stringent reaction conditions are often required for successful cychzation. This effect was noted in the Ni-catalyzed formal [4 -I- 4] cycloaddition reactions developed by Wender and Tebbe and is apparent when one compares the reported facility of Pd-catalyzed linear dimerization of 1,3-butadiene versus that of substituted 1,3-dienes. Similarly, the initial attempts at Pd-catalyzed cyclization of bisdiene 70a (Scheme 22) were rather disappointing. Using 0.05 equiv of [Pd(OAc)2/3 PhjP] (THF, 65 °C, 12 h), only a small... 

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