Reaction of an oxaruthenacycle with DMAD. Stoichiometric transformations of 2,6-xylenol to allylic phenols and benzopyransvia sp3 C–H bond cleavage reaction

Abstract

Insertion of a dimethyl acetylenedicarboxylate (DMAD) into the Ru–C bond in a cycloruthenated complex Ru[OC₆H₃(2-CH₂)(6-Me)-κ²O,C](PMe₃)₄ (2) has been achieved to give a seven-membered oxaruthenacycle Ru[OC₆H₃{2-CH₂C(CO₂Me)C(CO₂Me)}(6-Me)-κ²O,C](PMe₃)₃ (3) in 47% yield. The molecular structure of 3 by X-ray analysis shows an agostic interaction between the ruthenium and one of the benzylic methylene protons. Complex 3 shows fluxional behaviour in solution and the variable temperature NMR studies suggest this fluxionality to be responsible for the turnstile rotation of three PMe₃ ligands and the rotation of the α-methoxycarbonyl group. Heating of a toluene solution of 3 at 100 °C for 2 h results in the 1,3-H shift reaction in 3 to give a κ¹O,η³-C,C′,C″ allylic complex Ru[OC₆H₃{2-CHC(CO₂Me)CH(CO₂Me)}(6-Me)-κ¹O,η³C,C′,C″](PMe₃)₃ (6) (80–90%), whose molecular structure is revealed by X-ray analysis. Acidolyses of 3 and 6 give 2-[(Z)-2′,3′-bis(methoxycarbonyl)allyl]-6-methylphenol (7) (88%) and 2-[(Z)-2′,3′-bis(methoxycarbonyl)propenyl]-6-methylphenol (8) (47%), respectively, and iodolyses of 3 and 6 produce 2,3-bis(methoxycarbonyl)-8-methyl-4H-benzopyran (9) (24%) and 2,3-bis(methoxycarbonyl)-8-methyl-2H-benzopyran (10) (48%), respectively.