61699-83-0Relevant articles and documents
Structural effects on interconversion of oxygen-substituted bisketenes and cyclobutenediones
Fu, Nanyan,Allen, Annette D.,Kobayashi, Shinjiro,Tidwell, Thomas T.,Vukovic, Sinisa,Matsuoka, Takeshi,Mishima, Masaaki
, p. 1768 - 1773 (2008/09/18)
(Graph Presented) 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 OCH 2CH2O (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-O 2NC6H4O). 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 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).
