Alkynes bridged dimer

Reactions of acetylenes with M2Xg(THT)3 yield monomeric, ligand-bridged dimeric and metal-metal-bound dimeric complexes depending on the alkyne substituents. An alkyne-bridged dimer, Ta2Cl6(RC=CR)(THF)2, with a Ta-Ta distance of 268 pm, results for R = CMes . 

Heterocyclic compounds Heterocyclic C=N systems, such as benzoxazole and related thia-zoles, react with complex 2b to yield the ring-expanded adducts, e. g. complexes 90 and 91, by formal C-X (X = O, S) bond cleavage and coupling with the alkyne. In the case of benzisoxazole, the alkyne is not coupled but eliminated, and ring-enlargement of the ben-zisoxazole leads to the N-bridged dimer 92 [48]. 

Z-selective cross-coupling of terminal alkynes with isocyanides to exclusively yield (Z)-l-aza-l,3-enyne products has been achieved for the first time using a constrained geometry catalysts [Y(C5Me4Si(Me2)NR)(Alk) (THF)X] (Aik = R, dimethylaminobenzyl R = lBu, Ph, 2,4,6-Me3C6H2. The intermediate was shown to be the acetylide-bridged dimer [Y(C5Me4Si (Me2)NR)(p-C=CPh)]2.41 [24]... 

Pauson and Khand discovered the very important class of alkyne/alkene/CO cyclization catalytic reactions catalyzed, once again, by Co2(CO)g see Pauson-Khand Reaction). This reaction produces a, /3-unsaturated cyclopentanones (equation 23). With unstrained alkenes the reaction works best in a stoichiometric setting, but with strained alkenes like norbomadiene the reaction can be made catalytic. These reactions have been fairly extensively studied, and the reaction proceeds through an alkyne-bridged Co2(CO)6 dimer species. Unsymmetrical alkynes lead to mixtures of the various substituted a, /3-unsaturated cyclopentanone products. 

C(OMe)C6H4-o-C=CPh (CO)j leads directly to the formation of a chrysene derivative via the formal dimerization of the carbene ligand. A plausible explanation for the formation of the final product involves a doubly alkyne-bridged dinuclear complex, alkyne insertions into metal-carbene bonds, and coupling of the carbene carbons. 

Similar reactions can lead to the formation of two P-C bonds. For example, Pd-catalyzed addition of 7 (a privileged substrate which is often more reactive than other H-phosphonates) to terminal alkynes led to dehydrogenative double cis phosphonylation (Scheme 9). A Pd(II)/Pd(IV) cycle was proposed. Reaction of [Pd(allyl)Cl]2 with 7 gave chloro-bridged dimer 8. Alkyne insertion into the Pd-P bond was then proposed to give alkenyl intermediate 9. P-H oxidative addition would then yield hydride 10, whose reaction with 7 was proposed to lead to P-C bond formation and loss of H2 [18]. 

The iodo-bridged complexes 12 and 34 (formed by a simple halogen metathetic reaction of the corresponding chloro-bridged dimers with Nal in acetone, for example) both react with alkynes to afford heterocyclic products. 12 leads to a range of indoles 40 in a one-pot procedure, ... 

The reaction of nickelocene with primary phosphines has allowed the preparation of the phosphido-bridged dimers 114 deprotonation and methylation of the phosphide moiety results in the formation of the methylated derivatives 115 (Scheme 31). Analogous reactions with 2-phosphinophenolates give Ni(2-phosphinophenolato)Cp complexes. Reaction of nickelocene with 1-norbornyl lithium and alkynes leads to NiCp(l-norbornyl)(77 -RC=CR) (R = Me, SiMcs, Ph, CH20Me, CH2NMc2, GH2OH). These complexes have been characterized by and NMR and mass spectroscopy. " ... 

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