Azetinone formation is not competitive with intermolecular reactions of a β-lactam-4-ylidene

Abstract

3-Phenoxy-1-phenyl-2-azetidinon-4-ylidene (β-lactam-4-ylidene) 2a, was generated by thermolysis of a spiro-fused β-lactam oxadiazoline precursor (1). Fast 1,2-H migration, a characteristic reaction of singlet carbenes that would convert 2a to 3-phenoxy-1-phenyl-3-azetin-2-one (4a) could not be demonstrated. Added 1, 3-diphenylisobenzofuran (6) did not afford the [4 + 2] cycloadduct (7) expected from 4a but, instead, the isomeric E- and Z-4-[1-(2-benzoylphenyl)-1-phenyl]-methylene-3-phenoxy-1-phenylazetidin-2-ones (9). Those compounds can be rationalized as the products of rearrangement of first-formed [2 + 1] adducts of 6 and the ylidene. The structure of the Z isomer of 9 was established by means of single crystal X-ray diffraction. Generation of 2a in methanol-d4, either neat or 3.3 M in benzene, afforded the isomeric products of carbene insertion into the OD bond of methanol-d4. Structural isomers expected from addition of methanol-d4 to 4a could not be detected. The results suggest that 1,2-H migration in 2a is relatively slow, with kH ≤ 1.4 × 106 s−1 at 100 °C, which is slower than analogous rearrangement of benzylchlorocarbene by 200-fold or more.