Cyclization of 1,6-diynes

Beyond palladium, it has recently been shown that isoelectronic metal complexes based on nickel and platinum are active catalysts for diyne reductive cyclization. While the stoichiometric reaction of nickel(O) complexes with non-conjugated diynes represents a robust area of research,8 only one example of nickel-catalyzed diyne reductive cyclization, which involves the hydrosilylative cyclization of 1,7-diynes to afford 1,2-dialkylidenecyclohexanes appears in the literature.7 The reductive cyclization of unsubstituted 1,7-diyne 53a illustrates the ability of this catalyst system to deliver cyclic Z-vinylsilanes in good yield with excellent control of alkene geometry. Cationic platinum catalysts, generated in situ from (phen)Pt(Me)2 and B(C6F5)3, are also excellent catalysts for highly Z-selective reductive cyclization of 1,6-diynes, as demonstrated by the cyclization of 1,6-diyne 54a.72 The related platinum bis(imine) complex [PhN=C(Me)C(Me)N=Ph]2Pt(Me)2 also catalyzes diyne hydrosilylation-cyclization (Scheme 35).72a... 

The stoichiometric reaction of low-valent rhodium salts with l, -diynes to afford rhodacyclopentadiene complexes is well established and has been reviewed.73 733 The first rhodium-catalyzed reductive cyclization of a non-conjugated diyne has been reported only recently.74 743 The stereochemical outcome of the rhodium-catalyzed hydrosilylation-cyclization is dependent upon the choice of catalyst. Whereas reductive cyclization of 1,6-diyne 54a catalyzed by Rh4(CO)i2 provides modest yields of the Z-vinylsilane 54c, exposure of 54a to Wilkinson s catalyst... 

Lee s group has also reported ruthenium-catalyzed carbonylative cyclization of 1,6-diynes. The noteworthy aspect of this cyclization is the unprecedented anti nucleophile attack on a 7i-alkyne complex bearing a ruthenium vinylidene functionality. A catalytic system based on [Ru(p-cymene)Cl2]2/P(4-F-C6H4)3/DMAP was active for the cyclization of 1,6-diyne 103 and benzoic acid in dioxane at 65 °Cto afford cydohexenylidene enol ester 104a in 74% yield after 24h [34]. Additional examples are shown in Scheme 6.35. 

A novel route to 2,3-dihydrothiophenes involved a titanocene-promoted carbene formation and subsequenct intramolecular cyclization onto a thiol ester <99SL1029>. Treatment of thioacetal 9 with the low-valent titanium complex 10 gave 2,3-dihydrothiophene 12 by intramolecular olefination of the thiol ester of titanium-carbene intermediate 11. Another metal-mediated cyclization onto the thiophene ring system involved the palladium-catalyzed cyclization of 1,6-diynes <99T485>. For example, treatment of thioether 1,6-diyne 13 with Pdlj in the presence of CO and Oj in methanol followed by treatment with base gave 14. 

Another reaction is reductive cyclization of 1,6-diynes and 1,6-enynes using hydrosilanes as a hydrogen source in AcOH. Triethylsilane is used as the hydrogen donor for this reaction [142], The reaction can be understood by the formation of vinylpalladium bond 350 via the insertion of alkyne 349 to the Pd—H bond in 328. Then intramolecular alkyne insertion gives vinylpalladium 351, which is hydro-genolysed with Si—H to give the 1,3-diene 352 by transmetallation and reductive... 

Reductive cyclization of 1,6-diynes.2 Cyclization of these substrates to 1,2-dialkylidenecyclopentanes can be effected with 1 and a phosphine as catalyst and triethylsilane (10-fold excess) in place of polymethylhydrosiloxane. 

Cyclization. In HO Ac the Ru-catalyzed cyclization of 1,6-diynes proceeds with loss of the terminal yp-carbon. If both triple bonds are internal, vtc-dialkylidenecyclopentanes are obtained. 1,7-Diynes seem to behave differently. 

Reductive cyclization of 1,6-diynes in the presence of triethylsilane and a catalyst prepared in situ from palladium(O) and acetic acid leads to dialkylidenecyclopentanes. Thus the diester 505 gave compound 506. ... 

In addition to cobalt catalysts, ruthenium catalysts were applied in [2 + 2 + 2] cycloaddition reactions as well. In 2001, Itoh and coworkers reported a ruthenium-catalyzed cyclization of 1,6-diynes with dicyanides to produce the desired bicyclic pyridines in good yields [28]. By applying Cp Ru(cod)Cl (Cp = pentamethylcyclopentadienyl) as the catalyst, good yields of the products can be achieved (Scheme 3.12). Meanwhile, they explored the catalyst system in cyclization of 1,6-diynes with electron deficient nitriles as well. The desired bicyclic pyridines can be isolated in moderate to high yields [29]. Later on, in 2005, Yamamoto and coworkers performed systematic studies on this... 

The complex [Cp Ru(MeCN)3PFg] and Hantzsch l,4-dihydropyridine as a H2 surrogate catalyse the cyclization of 1,6-diyne with terminal aryl groups to exocyclic... 

Scheme 35 Ru-catalyzed decarbonylative cyclization of 1,6-diynes to ej o-alkylidenecyclopentanes...

Variation of the electronic and steric nature of the ruthenium catalysts allow the complementary carboxylative cyclization of 1,6-diynes (Scheme 36) [150]. Lee and coworkers described how a variety of carboxylic acids condense with 1,6-terminal diynes in the presence of catalytic amounts of [Ru(p-cymene)Cl2l2. P(4-F-C6H5)3 and 4-dimethylaminopyridine to give cyclohexylidene enol carboxylates with exclusive ( )-selectivity. The proposed mechanism involves the initial formation of a ruthenium vinylidene species I followed by intramolecular cyclization induced by the nucleophilic attack of the carboxylate anion to afford a vinylmthenium species II. Final protonolysis furnished the product and turns the catalyst over. 

Exocyclic 1,3-dienylphosphine oxides can also be obtained by [Cp Ru (CH3CN)3]PF6-catalyzed hydrophosphinylative cyclization of 1,6-diynes via a H-atom transfer on the ruthenacyclopentatriene and subsequent intramolecular... 

Wu, W., Zhang, X.Y. and Kang, S.X. (2010) Rhodium-catalyzed selective [2-I-2-I-2] cyclizations of 1,6-diynes with monoynes leading to isoindolines and isobenzofu-rans. Chinese Chemical Letters, 21(1), 18-22. 

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