Of the variety of synthetic transformations mediated by transition metals, catalytic processes involving cleavage of C À C single bonds are among the most difficult to achieve. [1] Hence, their development remains an important challenge in organic chemistry. We recently found that the bond between the carbonyl carbon and the a-carbon atom of a cyclobutanone can be catalytically cleaved by a rhodium(i) complex. [2] Phenolic hydroxy groups have also been reported to direct intramolecular functionalization of unreactive bonds such as CÀH bonds. [3] Thus we anticipated that CÀC bond cleavage would be facilitated by appropriate placement of a phenol group in the substrate molecule. Here we report new lactoneforming reactions by the rhodium-catalyzed CÀC bond cleavage of a cyclobutanone having a pendant phenol group.
Cyclobutanone 1 a, which has an o-substituted phenol group at the 3-position, [4] was heated to 140 8C in m-xylene in the presence of a rhodium complex prepared in situ from [Rh(cod) 2 ]BF 4 (5 mol %, cod 1,5-cyclooctadiene) and cyclohexyldiphenylphosphane (PCyPh 2 , 12 mol%) [5] under an argon atmosphere. The substrate 1 a was consumed after 210 min. Chromatographic purification afforded a mixture of saturated and unsaturated six-membered-ring lactones (4 a and 5 a, 68:32) in 76 % combined yield (Scheme 1). [6] The reaction occurred at 140 8C even under an atmosphere of carbon monoxide to give an analogous result. [7] Scheme 1. Reaction of 1 a to give six-membered-ring lactones and the postulated mechanism for this reaction.