Reactions of Propargyl Compounds Containing a Cyclobutyl Group Induced by a Ruthenium Complex

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Abstract

Reactions of [Ru]Cl ([Ru]={Cp(PPh3)2Ru}; Cp=cyclopentadienyl) with three alkynyl compounds, 1, 5, and 8, each containing a cyclobutyl group, are explored. For 1, the reaction gives the vinylidene complex 2, with a cyclobutylidene group, through dehydration at CδH and CγOH. With an additional methylene group, compound 5 reacts with [Ru]Cl to afford the cyclic oxacarbene complex 6. The reaction proceeds via a vinylidene intermediate followed by an intramolecular cyclization reaction through nucleophilic addition of the hydroxy group onto Cα of the vinylidene ligand. Deprotonation of 2 with NaOMe produces the acetylide complex 3 and alkylations of 3 by allyl iodide, methyl iodide, and ethyl iodoacetate generate 4 ac, respectively, each with a stable cyclobutyl group. Dehydration of 1 is catalyzed by the cationic ruthenium acetonitrile complex at 70 °C to form the 1,3-enyne 7. The epoxidation reaction of the double bond of 7 yields oxirane 8. Ring expansion of the cyclobutyl group of 8 is readily induced by the acidic salt NH4PF6 to afford the 2-ethynyl-substituted cyclopentanone 9. The same ring expansion is also seen in the reaction of [Ru]Cl with 8 in CH2Cl2, affording the vinylidene complex 10, which can also be obtained from 9 and [Ru]Cl. However, in MeOH, the same reaction of [Ru]Cl with 8 affords the bicyclic oxacarbene complex 12 a through an additional cyclization reaction. Transformation of 10 into 12 a is readily achieved in MeOH/HBF4, but, in MeOH alone, acetylide complex 11 is produced from 10. In the absence of MeOH, cyclization of 10, induced by HBF4, is followed by fluorination to afford complex 13. Crystal structures of 6 and 12 a′ were determined by single-crystal diffraction analysis.

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