Bietti Massimo, Gente Giacomo, Salamone Michela
Dipartimento di Scienze e Tecnologie Chimiche, Università Tor Vergata, Via della Ricerca Scientifica, 1 I-00133 Rome, Italy.
J Org Chem. 2005 Aug 19;70(17):6820-6. doi: 10.1021/jo050883i.
A product and time-resolved kinetic study on the reactivity of tertiary arylcarbinyloxyl radicals bearing alpha-cyclopropyl and alpha-cyclobutyl groups has been carried out. Both the 1-cyclopropyl-1-phenylethoxyl (1.) and alpha,alpha-dicyclopropylphenylmethoxyl (2.) radicals undergo beta-scission to give cyclopropyl phenyl ketone as the major or exclusive product with rate constants higher than that measured for the cumyloxyl radical. It is proposed that in the transition state for beta-scission of 1. and 2., formation of the C=O double bond is assisted by overlap with the C-C bonding orbitals of the cyclopropane ring. With tertiary arylcarbinyloxyl radicals bearing alpha-cyclobutyl groups such as the 1-cyclobutyl-1-phenylethoxyl (4.) and 1-cyclobutyl-1-phenylpropoxyl (5.) radicals, the fragmentation regioselectivity is essentially governed by the stability of the radical formed by beta-scission. Accordingly, 4. undergoes exclusive C-cyclobutyl bond cleavage to give acetophenone, whereas with 5., competition between C-cyclobutyl and C-ethyl bond cleavage, leading to propiophenone and cyclobutylphenyl ketone in a 2:1 ratio, is observed.
开展了一项关于带有α-环丙基和α-环丁基的叔芳基羰基氧基自由基反应活性的产物及时间分辨动力学研究。1-环丙基-1-苯乙氧基自由基（1.）和α,α-二环丙基苯基甲氧基自由基（2.）均发生β-断裂，以高于枯基氧基自由基所测速率常数的速率常数生成环丙基苯基酮作为主要或唯一产物。有人提出，在1.和2.的β-断裂过渡态中，C=O双键的形成通过与环丙烷环的C-C键轨道重叠得到辅助。对于带有α-环丁基的叔芳基羰基氧基自由基，如1-环丁基-1-苯乙氧基自由基（4.）和1-环丁基-1-苯基丙氧基自由基（5.），断裂区域选择性基本上由β-断裂形成的自由基的稳定性决定。因此，4.发生唯一的C-环丁基键断裂生成苯乙酮，而对于5.，观察到C-环丁基和C-乙基键断裂之间的竞争，以2:1的比例生成苯丙酮和环丁基苯基酮。
