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Cycloaddition between a Transition-Metal Phenylallenylidene Complex and Allyl Alcohol

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journal contribution
posted on 30.04.1998 by Miguel A. Esteruelas, Angel V. Gómez, Ana M. López, Enrique Oñate, Natividad Ruiz
The allenylidene complex [Ru(η5-C5H5)(CCCPh2)(CO)(PiPr3)]BF4 (1) reacts with allyl alcohol to give the α,β-unsaturated alkoxycarbene derivative [Ru(η5-C5H5){C(OCH2CHCH2)CHCPh2}(CO)(PiPr3)]BF4 (2), which affords the alkoxyallenyl compound Ru(η5-C5H5){C(OCH2CHCH2)CCPh2}(CO)(PiPr3) (3) by deprotonation at −78 °C. At room temperature, in solution, complex 3 undergoes an intramolecular Diels−Alder reaction to form the tricyclic tetraenyl complex Ru(η5-C5H5)(9-phenyl-3,3a,4,4a-tetrahydronaphtho[2,3-c]-1-furanyl)(CO)(PiPr3) (4). This isomerization is highly stereospecific due to the chiral nature of 3 and gives a single pair of enantiomers of 4. For this reaction, first-order constants kobs were obtained in toluene-d8, which gave activation parameters of ΔH = 18 ± 2 kcal mol-1 and ΔS = −18 ± 3 cal K-1 mol-1. The structure of 4 was determined by an X-ray investigation, revealing a Ru−C(tricyclic) distance of 2.088(3) Å. Complex 4 reacts with HBF4 to give the tricyclic carbene compound [Ru(η5-C5H5)(9-phenyl-1,3,3a,4,4a,9a-hexahydronaphtho[2,3-c]-1-furanylidene)(CO)(PiPr3)]BF4 (5). Similarly, the reaction of 4 with DBF4 affords [Ru(η5-C5H5)(9-phenyl-9a-deutero-1,3,3a,4,4a-pentahydronaphtho[2,3-c]-1-furanylidene)(CO)(PiPr3)]BF4 (5-d1). At room temperature with chloroform and tetrahydrofuran as the solvents, complex 5 is unstable and evolves into the cationic acyclic alkoxycarbene derivative [Ru(η5-C5H5){C(OCH2[1-phenyl-3,4-dihydro-3-naphthyl])H}(CO)(PiPr3)]+ (6), which was isolated as the PF6 salt by addition of NaPF6. For the isomerization of 5 into 6, first-order constants kobs were obtained in chloroform-d, which gave activation parameters of ΔH = 20.8 ± 0.9 kcal mol-1 and ΔS = −10.6 ± 0.7 cal K-1 mol-1. At room temperature with methanol as the solvent, complex 6 loses the alkoxy group to afford the alcohol 3-hydroxymethyl-1-phenyl-3,4-dihydronaphthalene (7) and the organometallic methoxycarbene [Ru(η5-C5H5){C(OCH3)H}(CO)(PiPr3)]PF6 (8).