Palladium-catalyzed Trimerization of Strained Cycloalkynes: Synthesis of Decacyclene

Abstract

Palladium-catalyzed cyclotrimerization was applied to three strained cycloalkynes. Pd(PPh₃)₄ and Pt(PPh₃)₄-catalyzed cyclotrimerizations of cyclohexyne (2) afforded dodecahydrotriphenylene (3) in 64% and 62% yields, respectively, but subjecting cyclopentyne to the same conditions failed to afford isolable amounts of the cyclotrimer. Finally, decacyclene (15), a putative C₆₀-fullerene precursor, was obtained in 23% yield by Pd₂(dba)₃-catalyzed cyclotrimerization of acenaphthyne (14)_.

Key words

arynes - transition metals - palladium

References

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3

In this paper we use the term ‘highly strained’ to refer to mono- or polycyclic alkynes in which the triple bond is contained in a ring of six or fewer atoms.

16

Dodecahydrotryphenylene (3): A solution of 2-(trimethylsilyl)cyclohexenyltriflate (1, 160 mg, 0.53 mmol) in dry CH₃CN (2 mL) was added to a suspension of Pd(PPh₃)₄ (61 mg, 0.053 mmol) and finely powdered, anhydrous CsF (161 mg, 1.06 mmol) in dry CH₃CN (1 mL). After stirring for 12 h at room temperature under Ar atmosphere, the solvent was evaporated in vacuo and the residue was chromatographed (SiO_2, hexane) to yield trimer 3 (27 mg, 64%).

25

Decacyclene (15): A solution of 13 (160 mg, 0.175 mmol)
in dry CH₃CN (1.5 mL) was added to a suspension of Pd₂(dba)₃·CHCl₃ (9 mg, 0.0087 mmol) in dry CH₃CN (1 mL) in a Schlenk flask containing molecular sieves. Then n-Bu₄NF solution (175 mL, 0.175 mmol) was added dropwise at 60 ºC. At the end of the addition the heating was suppressed and the mixture was stirred until cooled to room temperature. The solvent was evaporated in vacuo and the residue was solved in CH₂Cl₂ and washed with H₂O. The organic phase was dried over anhyd Na₂SO₄ and evaporated in vacuo. The residue was chromatographed (SiO₂, hexane) to yield decacyclene (15, 10 mg, 23%).