A,u>-Diynes

Hydrosilation of an alkenylsilane in the presence of a catalyst provides a route to five-, six- and seven-membered rings but is of little use for the smaller ones. Tin-substituted cyclohexa-2,5-dienes result directly from the a,[Pg.575]

Related co-cyclotrimerizations of two alkyne molecules with limited isocyanates have also been achieved using cobalt and nickel catalysts. With respect to intramolecular versions, two examples of the cobalt(I)-catalyzed cycloaddition of a,u>-diynes with isocyanates have been reported to afford bicyclic pyri-dones only in low yields, although 2,3-dihydro-5(lff)-indolizinones were successfully obtained from isocyanatoalkynes and several silylalkynes with the same cobalt catalysis [19]. On the other hand, the ruthenium catalysis using Cp" RuCl(cod) as a precatalyst effectively catalyzed the cycloaddition of 1,6-diynes 21 with 4 equiv. of isocyanates in refluxing 1,2-dichloroethane to afford bicyclic pyridones 25 in 58-93% yield (Eq. 12) [20]. In this case,both aryl and aliphatic isocyanates can be widely employed. 

Substituted benzocyclobutenes, indans, and tetralins can be synthesized in good yield by the related co-oligomerization of a,cobalt-catalysed reaction of a,a>-diynes is their reac-... 

Reactions between [ppn][Au(acac)2] and buta-l,3-diyne give [ppn][Au(C= CC=CH)2]. Similarly, (AuCl)2(/U-dppm) affords Au(C=CC=CH)2 (ju-dppm) while serendipitous use of a stoichiometric amount of Cul catalyst resulted in migration of dppm from gold to copper and formation of the curious bis(diynyl) species Cu3(/U3-I)(/.i3-C=CC=CAuC=CC=CH)(/r -dppm)3 (31). Direct reaction between [ppn][Au(C CC=CH)2] and [Cu2(/L-dppm)2(NCMe)2]" " afforded the dumbbell complex 32. 

The diyne complexes Co2(/u.-fj -RC2C=CR)(CO)6 (R = Ph, Fc) give irreversible reduction waves even at 213 K which indicates that fast chemical reactions follow the electrochemical production of the corresponding radical anions [Co2(jU-)j -RC2C=CR)(C0)6]. The ESR spectra of the anion radical generated in situ were not consistent with the presence of two different Co centers. In the case of the ferrocenyl-substituted complex, two distinct oxidation waves separated by 70 mV are observed, which indicates a modest degree of interaction between the Fc cores through the cluster. 

In thf the complexes Ni2 ( u.- j -PhC2R)Cp2 (including R = Ph, C CPh) undergo irreversible oxidation processes near -I-0.7 V (vs SCE, FcH /FcH " -l-O.ll V, FcH/FcH" " -I-0.56 V) which results in the formation of deposits on the electrode surface. The anodic sweep indicates the presence of a reversible reduction near — 1.30 V attributed to a Ni2-centered reduction and the formation of [Ni2(M-7 -PhC2R)Cp2] Further reduction results in decomposition of the complexes, and the liberation of the alkyne or diyne ligand, as evidenced by two characteristic alkyne/diyne reductions at very negative potentials. ... 

A 3 2 mixture of cis-trans isomers is obtained from the addition of secondary amines to butadiyne in dioxane . The ratio remains constant during the course of the reaction signifying that the isomers are formed in this ratio. This, coupled with the second-order kinetics observed and large negative values for the activation entropy (AS — 50 e.u.), led to the postulation of a mechanism involving ratedetermining attack by the amine on the diyne, followed by stereochemical equilibration of the dipolar ion and proton transfer, as illustrated in Scheme 7. 

Bicyclization of linear diynes (30) to produce bicyclic metallacyclopentadienes (31) followed by their conversion into tra/u,traRr-l,2-bis(alkylidene)cycloalkanes (32) is a highly attractive synthetic operation primarily because (32) can now serve as the diene component in the Diels-Alder reaction (equation 26). 

Very nearly quantitative reaction occurs within a few hours. Moreover, tin hydrides containing two organic groups add under the same conditions to terminal C=CH groups, giving polymers of the type —(SnR2CH2CHR )u—, and to oc,co-dienes and, co-diynes, giving linear and cyclic polymers of the types 438... 

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