Khavrutskii Ilja V, Musaev Djamaladdin G, Morokuma Keiji
Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia, 30322.
J Am Chem Soc. 2003 Nov 12;125(45):13879-89. doi: 10.1021/ja0343656.
Structural properties of the acylperoxo complexes [(Salen)Mn(III)RCO(3)] (2) and [(Salen)Mn(IV)RCO(3)] (3), the critical intermediates in the Kochi-Jacobsen-Katsuki reaction utilizing organic peracids or O(2)/aldehydes as oxygen source, have been studied with the density functional theory. Four distinct isomers, cis(O,N), cis(N,O), cis(N,N), and trans, of these complexes have been located. The isomer 2-cis(O,N) in its quintet ground state, and nearly degenerate isomers 3-cis(O,N) and 3-cis(N,O) in their quartet ground states are found to be the lowest in energy among the other isomers. The O-O bond cleavage in the cis(O,N), cis(N,O), and trans isomers of 2 and 3 has been elucidated. In complex 3, the O-O bond is inert. On the contrary, in complex 2, the O-O bond cleaves via two distinct pathways. The first pathway occurs exclusively on the quintet potential energy surface (PES) and corresponds to heterolytic O-O bond scission coupled with insertion of an oxygen atom into an Mn-N(Salen) bond to form 2-N-oxo species; this pathway has the lowest barrier of 14.9 kcal/mol and is 15.6 kcal/mol exothermic. The second pathway is tentatively a spin crossover pathway. In particular, for 2-cis(O,N) and 2-cis(N,O) the second pathway proceeds through a crucial minimum on the seam of crossing (MSX) between the quintet and triplet PESs followed by heterolytic O-O cleavage on the triplet PES, and produces unusual triplet 2-cis(O,N)- and 2-cis(N,O)-oxo ([(Salen)Mn(V)(O)RCO(2)]) species; this pathway requires 12.8 kcal/mol and is 1.4 kcal/mol endothermic. In contrast, for the 2-trans isomer, spin crossing is less crucial and the O-O cleavage proceeds homolytically to generate 2-trans-oxo [(Salen)Mn(IV)(O)] species with RCO(2) radical; this pathway, however, cannot compete with that in 2-cis because it needs 21.9 kcal/mol for activation and is 15.3 kcal/mol endothermic. In summary, the O-O cleavage occurs predominantly in the 2-cis complexes, and may proceed either through pure high spin or spin crossover heterolytic pathway to produce 2-cis-oxo and 2-N-oxo species.
酰基过氧配合物[(Salen)Mn(III)RCO(3)] (2)和[(Salen)Mn(IV)RCO(3)] (3)的结构性质，这两种配合物是利用有机过酸或O(2)/醛作为氧源的高知-雅各布森-胜木反应中的关键中间体，已采用密度泛函理论进行了研究。已确定了这些配合物的四种不同异构体，即顺式(O,N)、顺式(N,O)、顺式(N,N)和反式。发现处于五重态基态的异构体2-顺式(O,N)，以及处于四重态基态的几乎简并的异构体3-顺式(O,N)和3-顺式(N,O)在其他异构体中能量最低。已阐明了2和3的顺式(O,N)、顺式(N,O)和反式异构体中的O-O键断裂情况。在配合物3中，O-O键是惰性的。相反，在配合物2中，O-O键通过两种不同的途径断裂。第一种途径仅发生在五重态势能面(PES)上，对应于异裂O-O键断裂并伴有一个氧原子插入Mn-N(Salen)键形成2-N-氧代物种；该途径具有14.9 kcal/mol的最低势垒，且放热15.6 kcal/mol。第二种途径初步认为是自旋交叉途径。具体而言，对于2-顺式(O,N)和2-顺式(N,O)，第二种途径通过五重态和三重态势能面之间交叉缝(MSX)上的一个关键极小值，然后在三重态势能面上进行异裂O-O断裂，并生成不寻常的三重态2-顺式(O,N)-和2-顺式(N,O)-氧代([(Salen)Mn(V)(O)RCO(2)])物种；该途径需要12.8 kcal/mol，且吸热1.4 kcal/mol。相比之下，对于2-反式异构体，自旋交叉不太关键，O-O断裂以均裂方式进行，生成带有RCO(2)自由基的2-反式氧代[(Salen)Mn(IV)(O)]物种；然而，该途径无法与2-顺式中的途径竞争，因为它需要21.9 kcal/mol的活化能，且吸热15.3 kcal/mol。总之，O-O键断裂主要发生在2-顺式配合物中，并且可以通过纯高自旋或自旋交叉异裂途径进行，以生成2-顺式氧代和2-N-氧代物种。
