Dimerization, alkyne, metal

When desired vinylidene-mediated pathways are not sufficiently favorable. Group 9 metal catalysts can access a set of typical side-reaction pathways. Alkyne dimerization to give conjugated enynes or higher oligomers is often observed. Polysubstituted benzenes resulting from [2 + 2 + 2] alkyne cyclotrimerization are also common coproducts. Fortunately, the selectivity of rhodium and iridium catalysts can often be modulated by the variation of spectator ligands. 

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],... 

Although alkyne dimerization reactions have been observed with a variety of transition metal catalysts, selectivity is highly dependent not only on the metallic center but also on the ancillary ligands and the substrates. 

Terminal alkynes can undergo several types of interaction with ruthenium centres. In addition to the formation of ruthenium vinylidene species, a second type of activation provides alkynyl ruthenium complexes via oxidative addition. When these two types of coordination take place at the same metal centre, the migration of the alkynyl ligand onto the Ca atom of the vinylidene can occur to form enynyl intermediates, which upon protonation by the terminal alkyne lead to the formation of enynes corresponding to alkyne dimerization... 

Many of the complexes discussed in the previous sections are catalysts for alkyne oligomerization. In fact, alkyne dimerization and trimerization (see Cyclodimerization -tri-merization Reactions) at a cobalt center is recognized as one of the most synthetically useful catalytic reactions mediated by a homogeneous transition metal complex. The cobalt complexes most useful and extensively studied are CpCoL2, where L is CO, alkene, diene, or phosphine. The complex types... 

Fig. 25 Rare-earth metal catalysts for alkene and alkyne dimerizations [201-204]...

Compounds 33 and 34 are readily formed from 31 by direct reaction with CpCo(CO)2. A possible reaction sequence is formation of the triene, 31, from alkyne dimerization followed by reaction with the cobalt species to give the three complexes. Reaction of 34 with alkynes yielded only cyclobutadiene complexes alkyne metathesis was not observed, probably since the carbon-to-metal bonds are too strong. 

The insertion of alkynes into metal-hydride bonds occurs during a number of catalytic processes, including alkyne hydrogenation, hydrosilylation, silylformylation, hydroesterification and dimerization. This insertion chemistry is more complex mechanistically than the insertions of olefins into metal hydrides. In some cases, ds addition products have... 

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]. 

It has been fully documented that transition metal-vinylidene complexes play an important role in many catalytic processes. " Some relevant examples are alkene and enyne metathesis, alkyne dimerization and metathesis polymeri-... 

Ir(Tp ")N2] (Tp " = doubly metalated hydrotris(3-mesitylpyrazol-l-yl)borate) reacts with ethylene affording [Ir(Tp ")(C2H4)], whereas the reaction with acetylene yields, upon alkyne dimerization, [Ir(Tp ") (H2C=CHC=CH)], the enyne being coordinated through the C—C double bond (Scheme 55). ... 

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]. 

The analogous dimerization of alkynes over Fe(C0)5 is not applicable, so clearly a different route towards alkynylated derivatives of 25 was needed. Comparison of 25 to cymantrene suggests that metallation of the hydrocarbon ligand should be the route of choice for the synthesis of novel substituted cyclobutadienes. In the literature, addition of organolithium bases (MeLi, BuLi) to the CO ligands with concomitant rearrangement had been observed [25]. But the utilization of LiTMP (lithium tetramethylpiperidide, Hafner [26]) or sec-BuLi as effectively non-nucleophilic bases led to clean deprotonation of the cyclobuta-... 

Carbon Dioxide 0=C=0 The reactions of the metallocene sources 1-6 with carbon dioxide depend strongly on the metal and ligands used. Complex 1 gives, by elimination of half of the alkyne, the dimer 93, which forms the titanafuranone 94 after aerial oxidation [49]. 

Complex condensation products are obtained by reaction of the alkyne complex with excess of 3,3-dimethyl-l-butyne, which yields two isomeric products of formulas Ru3(CO)6[HC2C(Me)3-COCH2CMe3][HC2CMe3]2 (125). The X-ray structure of one of those adducts (Fig. 19) shows that both dimerization of two alkyne molecules and the insertion of carbon monoxide into the alkyne metal bonds have occurred. The Ru-Ru distances of 2.820,2.828, and 2.686 A in the ring are of interest. The value of 2.686 A is one of the shortest found in a... 

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. 

Reduction of (312) has been found to afford the dimer (313) which upon heating rearranged to yield the unprecedented di(benzopentalene) complex (314). The regio- and stereo-specificity of the conversion (313) into (314) implies a metal-mediated pathway for the process (see Scheme 100). The first observable cis-bis(alkyne)cyclobutadiene rearrangement [see (315) to (316)] has been reported. 

The first complexes to be described were obtained from reactions between MCI2CP2 and metallated alkynes and initially formulated as the dimers M(/r-C=CR)Cp 2. Further studies have clarified the nature of these complexes together with those formed from RC CC CR (R = Me, Bu, Ph, SiMe3) and MCp2. The several structural types obtained contain 1 /1,1 /2, 2/1, and 2/2 ratios of MCp2 to diyne, as shown in Scheme 48. The nature of the complexes formed depends on diyne substituent and metal. 

Many examples of complexes containing enynyl ligands are known from reactions of 1-alkynes with various metal complexes two coordination sites are necessary for this reaction. Displacement of the ligand often results in a catalytic cycle of head-to-head dimerization of the alkyne. The protonation may occur by solvent, e.g., MeOH, in other cases acid is required, e.g., CF3CO2H. Coupling... 

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... 

In our initial studies on the [5+2] cycloaddition, several metal catalysts were screened. Rhodium(I) systems were found to provide the optimum yields and generality [26]. Since the introduction of this new reaction in 1995, our group and others have reported other catalyst systems that can effect the cycloaddition of tethered VCPs and systems. These new catalysts thus far include chlororhodium dicarbonyl dimer ( [RhCl(CO)2]2 ) [27], bidentate phosphine chlororhodium dimers such as [RhCl(dppb)]2 [28] and [RhCl(dppe)]2 [29], and arene-rhodium complexes [(arene)Rh(cod)] SbFs [30]. [Cp Ru(NCCH3)3] PFg has also been demonstrated to be effective in the case of tethered alkyne-VCPs [31], but has not yet been extended to intermolecular systems or other 2n -components. 

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