Diynes, intramolecular coupling

In Section 9.2, intermolecular reactions of titanium—acetylene complexes with acetylenes, allenes, alkenes, and allylic compounds were discussed. This section describes the intramolecular coupling of bis-unsaturated compounds, including dienes, enynes, and diynes, as formulated in Eq. 9.49. As the titanium alkoxide is very inexpensive, the reactions in Eq. 9.49 represent one of the most economical methods for accomplishing the formation of metallacycles of this type [1,2]. Moreover, the titanium alkoxide based method enables several new synthetic transformations that are not viable by conventional metallocene-mediated methods. 

Interestingly, seven-membered conjugated diyne complex 16 is produced by the acid-promoted Nicholas reaction of the dicobalt-coordinated bispropargylic complex (Equation (7)). The cyclization proceeds via an intramolecular coupling reaction between a propargyiic cation and an alkene produced after dehydration. The molecular structure of seven-membered diyne complex 16 is confirmed by X-ray analysis, although the decomplexation is not successfully carried out. 

The catalytic intramolecular coupling of two C=C bonds at a ruthenium site leads to cyclization reactions. For example, although generally less reactive than a,co-diynes or enynes, 1,6-dienes react with [RuC12(COD)] in 2-propanol, leading to exo-methylenecyclopentanes in excellent yields [13] (Eq. 8). The mechanism suggests the formation of the ruthenacyclopentane(hydrido) intermediate 19. 

Two closely related intramolecular couplings to form ten-membered 1,5-diynes have been reported [149,150] (Scheme 4-44). 

The reductive intramolecular coupling of a,a>-diynes is catalyzed by Pd(0) in the presence of P(o-tol)3, Et3SiH, and acetic acid. It is thought that a hydrido species, [Pd(H)(OAc)L2], initiates the reaction. The mechanistic scheme resembles that outlined in Scheme 19. The method is quite general and applicable to a wide range of substrates. ... 

The Ti-promoted intramolecular alkyne-alkyne coupling of tethered diynes was extensively developed earlier with TiCp2 derivatives as reagents65 66 (Equation (1) of Scheme 22). Bicyclic titanacyclopentadienes fused to... 

The Pd-catalyzed electro-cleavage of the C—O bond of allyl aryl ether proceeds smoothly in a DMF-Bu4NBp4-(Mg)-(Stainless Steel) system, giving depro-tected products in 73 99% yield [437]. The sp-sp intermolecular coupling reaction with the Pd water-soluble catalyst prepared in situ from Pd(II) acetate and sul-fonated triphenylphosphine in an MeCN-H2O system yields diynes in 45 65% yields [438]. Similarly, the sp -sp coupling of 2-iodophenols or 2-iodoanilines and terminal alkynes followed by intramolecular cyclization gives indol and furan... 

Pd/tppts-catalysed intermolecular sp-sp couplings to afford diynes and sp2-sp coupling of 2-iodophenols or iodoanilines with alkynes followed by intramolecular cyclization to benzofurans and indoles, respectively, with moderate to good yields in H20/CH3CN solvent mixtures have also been described.517 8... 

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