Probing the Compound I-like Reactivity of a Bare High-Valent Oxo Iron Porphyrin Complex: The Oxidation of Tertiary Amines
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- 作者：Barbara Chiavarino ; Romano Cipollini ; Maria Elisa Crestoni ; Simonetta Fornarini ; Francesco Lanucara ; Andrea Lapi
- 刊名：Journal of the American Chemical Society
- 出版年：2008
- 出版时间：March 12, 2008
- 年：2008
- 卷：130
- 期：10
- 页码：3208 - 3217
- 全文大小：152K
- 年卷期：v.130,no.10(March 12, 2008)
- ISSN：1520-5126
文摘
The mechanisms of oxidative N-dealkylation of amines by heme enzymes including peroxidasesand cytochromes P450 and by functional models for the active Compound I species have long been studied.A debated issue has concerned in particular the character of the primary step initiating the oxidationsequence, either a hydrogen atom transfer (HAT) or an electron transfer (ET) event, facing problems suchas the possible contribution of multiple oxidants and complex environmental effects. In the present study,an oxo iron(IV) porphyrin radical cation intermediate 1, [(TPFPP)
+FeIV=O]+ (TPFPP = meso-tetrakis(pentafluorophenyl)porphinato dianion), functional model of Compound I, has been produced as a barespecies. The gas-phase reaction with amines (A) studied by ESI-FT-ICR mass spectrometry has revealedfor the first time the elementary steps and the ionic intermediates involved in the oxidative activation. Ionicproducts are formed involving ET (A+, the amine radical cation), formal hydride transfer (HT) from theamine ([A(-H)]+, an iminium ion), and oxygen atom transfer (OAT) to the amine (A(O), likely a carbinolamineproduct), whereas an ionic product involving a net initial HAT event is never observed. The reaction appearsto be initiated by an ET event for the majority of the tested amines which included tertiary aliphatic andaromatic amines as well as a cyclic and a secondary amine. For a series of N,N-dimethylanilines the reactionefficiency for the ET activated pathways was found to correlate with the ionization energy of the amine. Astepwise pathway accounts for the C-H bond activation resulting in the formal HT product, namely a primaryET process forming A+, which is deprotonated at the 
-C-H bond forming an N-methyl-N-arylaminomethylradical, A(-H), readily oxidized to the iminium ion, [A(-H)]+. The kinetic isotope effect (KIE) for protontransfer (PT) increases as the acidity of the amine radical cation increases and the PT reaction to thebase, the ferryl group of (TPFPP)FeIV=O, approaches thermoneutrality. The ET reaction displayed by 1with gaseous N,N-dimethylaniline finds a counterpart in the ET reactivity of FeO+, reportedly a potent oxidantin the gas phase, and with the barrierless ET process for a model (P)+FeIV=O species (where P is theporphine dianion) as found by theoretical calculations. Finally, the remarkable OAT reactivity of 1 withC6F5N(CH3)2 may hint to a mechanism along a route of diverse spin multiplicity.
+FeIV=O]+ (TPFPP = meso-tetrakis(pentafluorophenyl)porphinato dianion), functional model of Compound I, has been produced as a barespecies. The gas-phase reaction with amines (A) studied by ESI-FT-ICR mass spectrometry has revealedfor the first time the elementary steps and the ionic intermediates involved in the oxidative activation. Ionicproducts are formed involving ET (A+, the amine radical cation), formal hydride transfer (HT) from theamine ([A(-H)]+, an iminium ion), and oxygen atom transfer (OAT) to the amine (A(O), likely a carbinolamineproduct), whereas an ionic product involving a net initial HAT event is never observed. The reaction appearsto be initiated by an ET event for the majority of the tested amines which included tertiary aliphatic andaromatic amines as well as a cyclic and a secondary amine. For a series of N,N-dimethylanilines the reactionefficiency for the ET activated pathways was found to correlate with the ionization energy of the amine. Astepwise pathway accounts for the C-H bond activation resulting in the formal HT product, namely a primaryET process forming A+, which is deprotonated at the -C-H bond forming an N-methyl-N-arylaminomethylradical, A(-H), readily oxidized to the iminium ion, [A(-H)]+. The kinetic isotope effect (KIE) for protontransfer (PT) increases as the acidity of the amine radical cation increases and the PT reaction to thebase, the ferryl group of (TPFPP)FeIV=O, approaches thermoneutrality. The ET reaction displayed by 1with gaseous N,N-dimethylaniline finds a counterpart in the ET reactivity of FeO+, reportedly a potent oxidantin the gas phase, and with the barrierless ET process for a model (P)+FeIV=O species (where P is theporphine dianion) as found by theoretical calculations. Finally, the remarkable OAT reactivity of 1 withC6F5N(CH3)2 may hint to a mechanism along a route of diverse spin multiplicity.