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Structural Effects on Interconversion of Oxygen-Substituted Bisketenes and Cyclobutenediones

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journal contribution
posted on 2008-03-07, 00:00 authored by Nanyan Fu, Annette D. Allen, Shinjiro Kobayashi, Thomas T. Tidwell, Sinisa Vukovic, Takeshi Matsuoka, Masaaki Mishima
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 6ai, 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 6be, 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 6fi 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 6fi are attributed to diminution of the electron-donating ability of oxygen to the cyclobutenediones 5fh 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 6fh in CH3CN varied by a factor of 50 and gave an excellent correlation of the observed rate constants log k with the σp 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).