Amorati Riccardo, Lucarini Marco, Mugnaini Veronica, Pedulli Gian Franco, Minisci Franceso, Recupero Francesco, Fontana Francesca, Astolfi Paola, Greci Lucedio
Dipartimento di Chimica Organica A. Mangini, Università degli Studi di Bologna, Via S. Donato 15, 40127 Bologna, Italy.
J Org Chem. 2003 Mar 7;68(5):1747-54. doi: 10.1021/jo026660z.
Bond dissociation enthalpies (BDE) of hydroxylamines containing alkyl, aryl, vinyl, and carbonyl substituents at the nitrogen atom have been determined by using the EPR radical equilibration technique in order to study the effect of the substituents on the O-H bond strength of these compounds. It has been found that substitution of an alkyl group directly bonded to the nitrogen atom with vinyl or aryl groups has a small effect, while substitution with acyl groups induces a large increase of the O-H BDE value. Thus, dialkyl hydroxylamines have O-H bond strengths of only ca. 70 kcal/mol, while acylhydroxylamines and N-hydroxyphthalimide (NHPI), containing two acyl substituents at nitrogen, are characterized by BDE values of ca. 80 and 88 kcal/mol, respectively. Since the phthalimide N-oxyl radical (PINO) has been recently proposed as an efficient oxidation catalyst of hydrocarbons or other substrates, the large BDE value found for the parent hydroxylamine (NHPI) justifies this proposal. Kinetic studies, carried out in order to better understand the mechanism of the NHPI-catalyzed aerobic oxidation of cumene, are consistent with a simple kinetic model where the rate-determining step is the hydrogen atom abstraction from the hydroxylamine by cumylperoxyl radicals.
为了研究取代基对这些化合物中O-H键强度的影响，采用电子顺磁共振自由基平衡技术测定了氮原子上含有烷基、芳基、乙烯基和羰基取代基的羟胺的键解离焓（BDE）。已发现，直接与氮原子相连的烷基被乙烯基或芳基取代时影响较小，而被酰基取代则会使O-H BDE值大幅增加。因此，二烷基羟胺的O-H键强度仅约为70 kcal/mol，而氮上含有两个酰基取代基的酰基羟胺和N-羟基邻苯二甲酰亚胺（NHPI）的BDE值分别约为80和88 kcal/mol。由于最近有人提出邻苯二甲酰亚胺N-氧基自由基（PINO）是一种有效的烃类或其他底物的氧化催化剂，母体羟胺（NHPI）的BDE值较高证明了这一观点。为了更好地理解NHPI催化的异丙苯需氧氧化机理而进行的动力学研究，与一个简单的动力学模型一致，该模型中速率决定步骤是过氧异丙苯自由基从羟胺中夺取氢原子。
