Cyclization diynes

A variation of this process is the Bergnan cyclization, where an ene-diyne cyclizes to a biradical and then aromatizes, for example ... 

Cationic palladium complex 121 reductively coupled enynes (Eq. 20) using trichlorosilane as the stoichiometric reductant [71]. This combination of catalyst and silane afforded silylated methylenecyclopentanes such as 122 in good yield from enynes such as 123. Attempts to develop an enantioselective version of this reaction were not successful [71]. When enediyne 124 was cyclized in the presence of trichlorosilane, the reaction favored enyne cycli-zation 126 by a 3 1 ratio over diyne cyclization to 125 (Eq. 21). In contrast, when the more electron-rich dichloromethylsilane was used as the reductant, diyne cyclization product 125 was preferred in a ratio of 4 1 [71]. Selectivities of up to 10 1 for enyne cyclization were observed, depending on the substrate employed [72],... 

Scheme 28 Scope and selectivity of the diyne cyclization catalyzed by 145...

Scheme 30 Scope and selectivity of diyne cyclization catalyzed by cationic Pt species 151...

An interesting Pd-induced diyne cyclization-carbonylation sequence has been discovered by Chiusoli [117]. Thus, diyne 162 is transformed as shown into a mixture of ( )- and (Z)-bis(alkylidene)pyrrolidines 163 and 164. 

Table 1 Nickel-catalyzed diyne cyclization/hydrosilylation...

Diyne cyclization/hydrosilylation catalyzed by 4 was proposed to occur via a mechanism analogous to that proposed for nickel-catalyzed diyne cyclization/hydrosilylation (Scheme 4). It was worth noting that experimental evidence pointed to a silane-promoted reductive elimination pathway. In particular, reaction of dimethyl dipropargylmalonate with HSiMc2Et (3 equiv.) catalyzed by 4 led to predominant formation of the disilylated uncyclized compound 5 in 51% yield, whereas slow addition of HSiMe2Et to a mixture of the diyne and 4 led to predominant formation of silylated 1,2-dialkylidene cyclopentane 6 (Scheme 5). This and related observations were consistent with a mechanism involving silane-promoted G-H reductive elimination from alkenylrhodium hydride species Id to form silylated uncyclized products in competition with intramolecular carbometallation of Id to form cyclization/hydrosilylation products (Scheme 4). Silane-promoted reductive elimination could occur either via an oxidative addition/reductive elimination sequence involving an Rh(v) intermediate, or via a cr-bond metathesis pathway. 

The mechanism of these new enyne with diyne cyclizations has been studied using the deuterated enyne 163 [69]. D1 in dideuterated 163 migrates, but D2 does not. The deuterated (fT)-enyne 164 cocyclizes with the diyne 165 to give 166 with migration of D. However, no migration of D was observed in the cyclized product 168 by the reaction of (Z)-enyne 167 with 165. 

Cyclization of diynes to iminocyclopentadienes. 1,6-1,8-Diynes cyclize with 2,6-dimethylphenyl isocyanide when treated with Ni(COD)2, 1 equiv., to form bicy-... 

An illustration of the preparation of six-membered rings by enyne cycloisomerizations is found in Trost s total synthesis of (-t-)-cassiol (113) (Scheme 6-19) [44]. The key step of this synthesis involved conversion of enyne 111 to 1,4-diene 112. Although a mixture of diastereomers is produced, the offending stereocenter is not found in the natural product, allowing both diastereomers of 112 to be used. A reductive diyne cyclization (114 115) was recently described as the key step in a total synthesis of ( )-siccanin (116) [45]. Hydropalladation of the terminal alkyne, insertion of the internal alkyne, hydride transfer to palladium, and reductive elimination are proposed to account for the observed reaction. 

Hydrostannylation. Allenyl alcohols are converted to homoallylic alcohols in which the hydroxy group and the tin atom are separated by four bonds. 1,6-Diynes cyclize on hydrostannylation. ... 

Trost, B.M. and Rudd, M.T. (2003) Chemoselectivity of the ruthenium-catalyzed hydrative diyne cyclization total synthesis of (+ )-cylindricine C, D, and E. Organic Letters, 5, 4599-4602. 

The diyne cyclizations discussed so far involve the formation of an aromatic system together with the biradical. Surprisingly, though, a new kind of diyne- to -biradical cyclization has been discovered only recently, where the transannular interaction of two triple bonds in a diazacy-... 

Scheme 14.11 Tandem zirconocene-mediated diyne-cyclization and regioselective opening of zirconacyclopentadienes.

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