Rhodium Complex-Catalyzed Intramolecular Ene-Type Reaction of Allenynes

Abstract

Wilkinson’s catalyst [RhCl(PPh₃)₃] catalyzed a cyclo­isomerization of allenynes which possess geminal methyls on the allene terminus and an intramolecular ene-type reaction proceeded to give various cross-conjugated trienes. Thermal Diels-Alder reaction of the obtained triene was examined to give a bicyclic compound in a high yield.

Key words

alkynes - allenes - catalysis - ene reactions - rhodium

References

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12

A typical experimental procedure is as follows (Table [1] , entry 9): Under an argon atmosphere, RhCl(PPh₃)₃ (3.7 mg, 4.0 µmol) was placed in a flask. A degassed toluene solution (4 mL) of allenyne 1a (42.5 mg, 0.200 mmol) was added, then the mixture was stirred at 90 °C (bath temperature) for 5 h. Solvent was removed under a reduced pressure, then the resulting crude products were purified by thin layer chromatography to give pure 2a (30.8 mg, 0.145 mmol, 73%). 2a: IR (neat) 3025, 2970, 2818, 1493, 1445 cm^-1; ¹H NMR (300 MHz, CDCl₃) δ = 1.96 (dd, J = 0.9 Hz, 1.5 Hz, 3 H), 4.30 (d, J = 3.2 Hz, 2 H), 4.58 (s, 2 H), 4.96-4.99 (m, 1 H), 5.08-5.11 (m, 1 H), 5.75 (t, J = 3.2 Hz, 1 H), 6.53 (s, 1 H), 7.12-7.35 (m, 5 H); ¹³C NMR δ = 23.2, 65.2, 65.8, 115.2, 124.0, 126.1, 126.8, 128.1, 128.9, 131.9, 136.5, 140.5, 143.5; HRMS found m/z 212.1208, calcd for C₁₅H₁₆O: 212.1201. The stereochemistry of 2a was determined by the NOESY spectrum in which NOEs between H^a and phenyl protons, and H^b and Me protons were observed (Figure [1] ).

13

Using 0.5 mol% catalyst, 2a was obtained in 14% yield and 1a (58%) was recovered.