Novel Palladium-Catalyzed Diarylation and Dialkenylation Reactions of Norbornene Derivatives

Abstract

Norbornene and its derivatives are diarylated or dialkenylated by aryl-or alkenyltributylstannanes, a palladium catalyst, and chloroacetone or chloroacetonitrile as a reoxidant.Transition metal catalyzed ternary coupling reaction, i.e., insertion of alkynes or alkenes into a cross-coupling reaction, is one of the recent interests. 1 Insertion of alkynes or alkenes into a homocoupling reaction is considered to be included in a similar category. 2,3 We recently reported a palladium-catalyzed stereoselective difunctionalization reaction of some internal alkynes by organotributylstannanes using copper(II) chloride as a reoxidant. 4 In this reaction, however, only limited internal alkynes (diphenylacetylene and acetylenedicarboxylate) and aryltributylstannanes were tolerable. Alkenyl-and alkynyltributylstannanes did not give the addition products at all, in this reaction. In 1993, Kikukawa et al. reported the oxidative homocoupling of organostannanes using chloroacetone or chloroacetophenone as a reoxidant. 5 We attempted to employ chloroacetone as a reoxidant for our difunctionalization reaction and found that norbornene and its derivatives could be difunctionalized by alkenylstannanes as well as arylstannanes. We report here our results.To a tetrahydrofuran (THF) solution of dichlorobis(benzonitrile)palladium(II) (0.1 mmol) and triphenylphosphine (0.15 mmol) were added tributylphenylstannane (2.2 mmol), chloroacetone (4.0 mmol), and norbornene (1.0 mmol), successively. The resulting yellow solution was stirred at 50 °C for 29 h. The resulting brown mixture was diluted with hexane and washed with aqueous ammonium fluoride solution. The organic layer was washed with water, dried over anhydrous sodium sulfate, and concentrated to about 10 mL. The residual colorless solution was passed through a short silica gel column using hexane as the eluent. The elute was concentrated and the residual oil was subjected to silica gel column chromatography to provide exo-cis-2,3-diphenylbicyclo[2.2.1]heptane in 58% yield. The representative results are summarized in Table 1. Exo-cis products were the predominant or exclusive products in all cases. None of other isomers could be isolated. Chloroacetonitrile could be employed as a reoxidant as well as chloroacetone (run 2), whereas the following halides were not effective at all; bromoacetone, bromoacetonitrile, carbon tetrachloride, carbon tetrabromide, and chloroform. The alkenylations with (E)-alkenylstannanes were stereospecific, although not perfect, giving the 2,3-dialkenylbicyclo[2.2.1]heptanes mainly with retention of the geometries. The minor products if formed were the corresponding E,Zisomers. On the other hand, the reaction of (Z)-tributylstannylacrylate was interesting. The main product was exo-cis-2-[(Z)-methoxycarbonylethenyl]-3-[(E)-methoxycarbonylethenyl]bicyclo-[2.2.1]heptane and exo-cis-(Z,Z)-2,3-bis(2-methoxycarbonylethenyl)-bicyclo[2.2.1]heptane was not detected at all (run 3). Isomerization alkenylstannanes was never observed in any case. In every case, the major ...

Highly Efficient Phosphapalladacyclic Catalysts for the Hydroarylation of Norbornene

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