Rhodium-Complex-Catalyzed [2+2+2] Cycloaddition of Diynes and Carbonyl Compounds

Abstract

A Rh-BINAP complex was used to catalyze the hetero-[2+2+2] cycloaddition of symmetrical 1,6-diynes and carbonyl moiety of ketoesters, a diketone, and an aldehyde to give bicyclic a-pyrans, which were readily transformed into monocyclic compounds via the following electrocyclic ring opening. In the reaction of an unsymmetrical 1,6-diyne and a 1,7-diyne, a-pyrans with a quaternary carbon stereocenter were obtained in moderate to excellent ee using a chiral rhodium catalyst.Transition-metal-catalyzed cycloaddition is a powerful tool for the construction of cyclic systems from unsaturated acyclic precursors. 1 In particular, the cycloaddition using carbon-heteroatom multiple bonds as unsaturated motifs is very important because it gives various types of heterocyclic compounds. 2 For example, [2+2+2] cycloaddition of alkynes with nitriles is a well-established protocol for the synthesis of pyridine derivatives. 3 Carbonheteroatom double bonds in the cumulated systems, such as carbon dioxide 4 and isocyanates, 5 are very reactive and could be incorporated into [2+2+2] cycloaddition. Compared with these examples of C(sp)-heteroatom multiple bonds, however, those of C(sp 2 )-heteroatom double bonds have been scarce: Ni-and Ru-catalyzed reactions are the only examples to the best of our knowledge. Tsuda and Saegusa pioneeringly reported Ni-catalyzed [2+2+2] cycloaddition of diynes with aldehydes, 6 and recently Louie disclosed an improved protocol of the same reaction using Ni-imidazolylidene complex. 7 Yamamoto and Itoh reported Ru-catalyzed cycloaddition of diynes with tricarbonyl compounds, which gave monocyclic products via the electrocyclic ring-opening reaction of [2+2+2] cycloadducts. 8Recently, we found that Rh complexes are active catalysts in the cycloaddition of diynes with alkenes. 9 These results prompted us to investigate Rh-catalyzed cycloaddition of diynes with carbon-oxygen double bonds, in place of carbon-carbon double bonds. 10,11 We chose dibenzyl 2,2-di(but-2-ynyl)malonate (1a) and methyl pyruvate (2a) as model substrates; [2+2+2] cycloaddition was examined in the presence of five mol% of Rh-(rac-BINAP) catalyst, which was prepared in situ from [Rh(cod) 2 ]BF 4 and rac-BINAP, and pretreated with hydrogen gas for the exclusion of 1,5-cyclooctadiene (COD) (Scheme 1). 12 As a preliminary experiment, the reaction was conducted in an NMR tube; diyne 1a was immediately consumed even at room temperature and almost quantitative formation of the bicyclic a-pyran 3aa was ascertained by 1 H NMR and 13 C NMR analyses. 13 These results mean that the Rh catalyst realized hetero-[2+2+2] cycloaddition of a diyne with carbonyl moiety of the ketoester. Purification of a-pyran 3aa failed because of its susceptibility to electrocyclic ring opening, 6-8,14 therefore, we isolated and fully characterized the product after complete conversion of the primary product 3aa into the secondary product 4aa at room temperature. Scheme 1The catalytic activity in the present reaction was dependent upon the ligands of the Rh complex...