Enynes homodimerization

Initial attempts at carbocydization of the enyne 82 with Wilkinson s catalyst under an atmosphere of 1,3-butadiene in refluxing toluene furnished only trace amounts of product, which was attributed to the preference for the enyne to undergo homodimerization in a highly stereoselective manner (Tab. 12.8, entry 1 ds 19 1) [38]. Nonetheless, the coordinatively unsaturated (AgOTf-modified) catalyst furnished the [4-i-2-1-2] product in excellent yield (entry 2). The ability to alter the product distribution in this manner prompted further examination of various other silver salts. 

Trost and co-workers found that the Pd(OAc)2 tris(2,6-dimethoxyphenyl)phosphine (TDMPP) system catalyzes the homodimerization of terminal alkynes 1 to produce conjugated enynes 2 (Scheme This method is very simple and efBcient reactions are... 

More recently, Gevorgyan and Yamamoto proposed a Pd-catalyzed trimerization of alkynes as a sequential approach to the multisubstituted benzenes 17 and 18 (Scheme In the first step of the sequence, a donor alkyne 1 would undergo either homodimerization to produce 2 or cross-coupling with an acceptor alkyne 3 to form an enyne 4. Enynes 2 and 4, after completion of the first step of the sequence, would undergo [4 + 2] benzannelation with the enynophile 16 to afford the tetra- or pentasubsdtuted benzenes 17 and 18, respectively (Scheme 6) (for the Pd-catalyzed [4 + 2] benzannelation of conjugated enynes, see Sect. IV.10.2). 

The Pd(0)-catalysed 4 -i- 2-cross-benzannulations of conjugated enynes (167) with diynes (166) and triynes yield substituted benzene derivatives (168) (Scheme 64). " Monosubstituted conjugated enynes readily undergo 4 - - 2-homodimerization in the presence of Pd catalyst to produce 4,o - and 2,6-disubstituted styrenes. ... 

Homodimerization. The Pd-catalyzed [4+2]-benzannelation of conjugated enynes was first described by Yamamoto s group in 1996. It was shown that cyclodimerization of 2-substituted enynes 2.206 produces only one isomer of 1,4-disubstituted benzene 2.207 with 77-82% yield (Scheme 2.71) [3,126]. Since that time this reaction has been studied by several groups and has become a useful synthetic method for constructing polysubstituted benzenes [2, 3, 127, 128]. 

The reactivity of other related enynes such as 1-alkylenyne and dialkylenyne is much lower. Attempts at homodimerization did not... 

---