Blake Jessie A, Ingold Keith U, Lin Shuqiong, Mulder Peter, Pratt Derek A, Sheeller Brad, Walton John C
Leiden Institute of Chemistry, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands.
Org Biomol Chem. 2004 Feb 7;2(3):415-20. doi: 10.1039/b313491a. Epub 2004 Jan 12.
Thermolyses of seven dialkyl, two alkyl-aryl and two diaryl O-benzyl ketoxime ethers, R(1)R(2)C[double bond, length as m-dash]NOCH(2)Ph, have been examined in three hydrogen donor solvents: tetralin, 9,10-dihydrophenanthrene, and 9,10-dihydroanthracene. All the oxime ethers gave the products expected from homolytic scission of both the O-C bond (viz., R(1)R(2)C[double bond, length as m-dash]NOH and PhCH(3)) and N-O bond (viz., R(1)R(2)C[double bond, length as m-dash]NH and PhCH(2)OH). The yields of these products depended on which solvent was used and the rates of decomposition of the O-benzyl oxime ethers were greater in 9,10-dihydrophenanthrene and 9,10-dihydroanthracene than in tetralin. These results indicated that a reverse radical disproportionation reaction in which a hydrogen atom was transferred from the solvent to the oxime ether, followed by [small beta]-scission of the resultant aminoalkyl radical, must be important in the latter two solvents. Benzaldehyde was found to be an additional product from thermolyses conducted in tetralin. This, and other evidence, indicated that another induced decomposition mode involving abstraction of a benzylic hydrogen atom, followed by [small beta]-scission of the resulting benzyl radical, became important for some substrates. Participation by minor amounts of enamine tautomers of the oxime ethers was shown to be negligible by comparison of thermolysis data for the O-benzyloxime of bicyclo[3.3.1]nonan-9-one, which cannot give an enamine tautomer, with that of the O-benzyloxime of cyclohexanone.
四氢萘、9,10-二氢菲和9,10-二氢蒽中，对七种二烷基、两种烷基芳基和两种二芳基O-苄基酮肟醚R(1)R(2)C=NOCH(2)Ph的热解反应进行了研究。所有肟醚均生成了O-C键均裂（即R(1)R(2)C=NOH和PhCH(3)）和N-O键均裂（即R(1)R(2)C=NH和PhCH(2)OH）所预期的产物。这些产物的产率取决于所使用的溶剂，并且O-苄基肟醚在9,10-二氢菲和9,10-二氢蒽中的分解速率比在四氢萘中更快。这些结果表明，在后面两种溶剂中，一个氢原子从溶剂转移到肟醚上，随后生成的氨基烷基自由基发生β-断裂的逆自由基歧化反应一定很重要。在四氢萘中进行热解时发现苯甲醛是一种额外产物。这以及其他证据表明，另一种诱导分解模式，即苄基氢原子被夺取，随后生成的苄基自由基发生β-断裂，对于某些底物变得很重要。通过比较双环[3.3.1]壬-9-酮的O-苄基肟（其不能生成烯胺互变异构体）和环己酮的O-苄基肟的热解数据，表明肟醚中少量烯胺互变异构体的参与可忽略不计。
