Enynes enyne cyclodimerization

Conjugated enynes cyclodimerize in the presence of Pd(PPh3)4 to give substituted benzenes. The mode of the cyclodimerization is strictly controlled as shown in Scheme 2, with 1,4-disubstituted benzenes obtained as the sole products. Though it has been... 

Conjugated enynes cyclodimerize in the presence of Pd(0) catalysts to give di- or trisubstituted benzene derivatives with high regioselectivity. This reaction could be carried out in an intramolecular fashion to give [njparacyclophanes. 

Yamamoto et al. used originally developed Pd-catalyzed enyne cyclodimerization for the construction of cyclophane skeleton. The intramolecular reaction of carbon-tethered bis-enynes selectively afforded para-cyclophanes in acceptable yield by formal [2 + 2 + 2] cycloaddition (Scheme 8.4) [5a]. When polyether-tethered substrates were used, the yield of the cycloadducts was improved using less Pd catalyst (Scheme 8.5) [5b]. 

Yamamoto group showed that conjugated enynes cyclodimerize in the presence of Pd(0) catalyst to form styrene derivatives 7 (Scheme 14.6) [4], The main feature of this reaction is its exclusive regiospecificity. Thus, formation of a single isomer 7 occurred upon reaction, whereas formation of isomeric products 8, as well as possible products of alkyne trimerization, was never detected. 

Arenes from enynes and diynes. A novel synthesis of alkynylarenes by the intermolecular cycloaddition of enynes and diynes has been developed. With enynes alone the products are 2,6-disubstituted styrenes which are formed by cyclodimerization. Note that benzynes undergo trimerization to give triphenylenes in situ, e.g., by treatment of o-trimethylsilylphenyl triflate with CsF in the presence of (PhjPl.Pd. ... 

Likewise when two alkyne molecules coordinate to a transition metal such as Co(I) with subsequent coupling of the C-C bond, oxidative cyclization takes place to give a metallacyclopentadiene. Further reaction of another alkyne molecule with the metallacyclopentadiene followed by reductive elimination liberates benzene derivatives. Thus cyclotrimerization of three alkyne molecules catalyzed by a cobalt complex [40,41] can be performed. If a nitrile is used as the second component, pyridine derivatives are obtained catalytically as shown in Scheme 1.13 [42]. The catalytic cyclotrimerization and cyclodimerization of alkynes and conjugated enynes have found extensive applications in synthesis of complex cyclic compounds such as steroid derivatives [43]. 

Passarella D, Giardini A et al (2001) Cyclodimerization of indol-2-ylacetylenes. An example of intermolecular enyne-alkyne cycloaddition. J Chem Soc, Perkin Trans 1 127-129... 

Cyclodimerizations of conjugated enynes 22a-22e in the presence of perfluoro-tagged Pd catalyst were reported by Saito et al. [Eq. (6)] [28]. Reactions of enynes 22a-22e were carried out in toluene/hexane/perfluorodecalin with 1 mol% of Pd2(dba)j as palladium source and 8 mol% of perfluoro-tagged phosphane 1 giving the desired products (23a-23e) in moderate to good yield (43-78%). Recovery of the perfluoro-tagged catalyst was possible up to four times. However, IPC atomic emission analysis of the fluorous phase indicated that the concentration of the palladium species decreased significantly. 

In this latter reaction mode, which is observed much more rarely than /3-dehydropal-ladation, a wide variety of ligands can be coupled to each other with the formation of new C—C, C—H, C— N, C—O, and C—Hal bonds. This section does not cover the numerous cascade couplings in which a number of successive intramolecular additions of 2 onto double bonds is eventually completed by /3-dehydropalladationt as well as the numerous [2 -I- 2 -I- 2] and [4 + 2] cyclotri- and cyclodimerizations of alkynes, enynes, and related compounds. " The Pd(0)-catalyzed Cope rearrangement also will not be considered here, as it proceeds via bis(i7 -allyl)palladium(ll) intermediates. The carbopalladation reactions of aUenes, which have been reviewed recently, are covered in Sect. IV.7. (For new examples see also refs. [10]-[12]). On the other hand, the numerous Pd-catalyzed formal [3 + 2] cycloadditions of trimethylenemethane (TMM) complexes may be classified as carbopalladations of alkenes without subsequent dehydropalladation. As the subject of this section has partially been covered in several newly published reviews, " the attention here will be on the most recent and interesting communications. 

The scope of the reaction discussed in Sect. B is somewhat limited since the only practical synthetic procedure consists of the cyclodimerization of the two identical enynes, and therefore the same substituents are attached at the phenyl group and the vinyl group of the reaction product. The attempted cross-annulation of conjugated enynes with alkynes or dienes turned out to be unsuccessful. However, the scope of the benzannulation reaction could be extended significantly by introducing conjugated... 

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]. 

Substituted enynes 2.208 are also capable of cyclodimerization to give 1,2,3-trisubstituted benzenes 2.209 with 86-100% yields (Scheme 2.72) [3,129]. 

Gevorgyan, V., Tando, K., Uchiyama, N. and Yamamoto, Y. (1998) An efficient route to 2,6-disubstituted styrenes via the palladium-catalyzed [4-1-2] cyclodimerization of conjugated enynes. Journal of Organic Chemistry, 63(20), 7021-7025. 

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