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1.2.3.5.1 Intramolecular Addition of Malononitriles to Alkynes under Neutral Conditions

DOI: 10.1055/sos-SD-001-00206

Takacs, J. M.; Vayalakkada, S.; Jiang, X.Science of Synthesis, (20011299.

The intramolecular palladium(II) acetate catalyzed addition of malononitriles to alkynes proceeds in good yield under mild, neutral conditions (Scheme 40).[‌38‌] The use of ethanol as the reaction solvent or as a component of the solvent mixture is essential; the reaction does not proceed at all in aprotic solvents such as toluene and tetrahydrofuran. In addition, additives have a marked influence on the reaction. Without any additive, the palladium(II) acetate catalyzed cyclization of 146 proceeds, but requires relatively high catalyst loading (20mol%), high temperature (70°C) and gives only a low yield of 147. In the presence of triphenylphosphine, the reaction proceeds at room temperature, but the yield is not improved. When oct-1-ene is added (20mol%), the yield improves, but the higher reaction temperature is still required. Using cycloocta-1,5-diene as the additive (20mol%), the reaction proceeds in high yield at room temperature and with less palladium(II) acetate catalyst (5mol%). Cycloocta-1,5-diene is thought to act as a chelating ligand for palladium(0), but the use of [Pd2(dba)3]CHCl3 and 1,1-bis(diphenylphosphino)ferrocene gives only a low yield of 147, and no reaction is observed using [Pd2(dba)3]CHCl3 and Pd(PPh3)4. In the case of (5-phenylpent-4-ynyl)malononitrile (146, R1=Ph), a small amount of the regioisomeric cyclization product 2-phenylcyclohex-2-ene-1,1-dicarbonitrile is obtained, and in the case of [5-(trimethylsilyl)pent-4-ynyl]malononitrile (146, R1=TMS), 2-[(trimethylsilyl)methylene]cyclopentane-1,1-dicarbonitrile (147, R1=TMS) is obtained as a 91:9 mixture of Z- and E-isomers.

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