Structural Effects on Interconversion of Oxygen-Substituted Bisketenes and Cyclobutenediones

Cyclobutenediones 5 disubstituted with HO (a), MeO (b), EtO (c), i-PrO (d), t-BuO (e), PhO (f), 4-MeOC₆H₄O (g), 4-O₂NC₆H₄O (h), and 3,4-bridging OCH₂CH₂O (i) substituents upon laser flash photolysis gave the corresponding bisketenes 6a−i, as detected by their distinctive doublet IR absorptions between 2075 and 2106 and 2116 and 2140 cm^-1. The reactivities in ring closure back to the cyclobutenediones were greatest for the group 6b−e, with the highest rate constant of 2.95 × 10⁷ s^-1 at 25 °C for 6e (RO = t-BuO) in isooctane, were less for 6a (RO = OH, k = 2.57 × 10⁶ s^-1 in CH₃CN), while 6f−i were the least reactive, with the lowest rate constant of 3.8 × 10⁴ s^-1 in CH₃CN for 6h (RO = 4-O₂NC₆H₄O). The significantly reduced rate constants for 6f−i are attributed to diminution of the electron-donating ability of oxygen to the cyclobutenediones 5f−h by the ArO substituents compared to alkoxy groups and to angle strain in the bridged product cyclobutenedione 5i. The reactivities of the ArO-substituted bisketenes 6f−h in CH₃CN varied by a factor of 50 and gave an excellent correlation of the observed rate constants log k with the σ_p constants of the aryl substituents. Computational studies at the B3LYP/6-31G(d) level of ring-closure barriers are consistent with the measured reactivities. Photolysis of squaric acid (5a) in solution provides a convenient preparation of deltic acid (7).