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Abstract
Iron porphyrin methoxy complexes, of the general formula [Fe(porphyrin)(OCH3)], are able to catalyze the reaction of diazo compounds with alkenes to give cyclopropane products with very high efficiency and selectivity. The overall mechanism of these reactions was thoroughly investigated with the aid of a computational approach based on density functional theory calculations. The energy profile for the processes catalyzed by the oxidized [FeIII(Por)(OCH3)] (Por = porphine) as well as the reduced [FeII(Por)(OCH3)]− forms of the iron porphyrin was determined. The main reaction step is the same in both of the cases, that is, the one leading to the terminal-carbene intermediate [Fe(Por)(OCH3)(CHCO2Et)] with simultaneous dinitrogen loss; however, the reduced species performs much better than the oxidized one. Contrarily to the iron(III) profile in which the carbene intermediate is directly obtained from the starting reactant complex, the favored iron(II) process is more intricate. The initially formed reactant adduct between [FeII(Por)(OCH3)]− and ethyl diazoacetate (EDA) is converted into a closer reactant adduct, which is in turn converted into the terminal iron porphyrin carbene [Fe(Por)(OCH3)(CHCO2Et)]−. The two corresponding transition states are almost isoenergetic, thus raising the question of whether the rate-determining step corresponds to dinitrogen loss or to the previous structural and electronic rearrangement. The ethylene addition to the terminal carbene is a downhill process, which, on the open-shell singlet surface, presents a defined but probably short-living diradicaloid intermediate, though other spin-state surfaces do not show this intermediate allowing a direct access to the cyclopropane product. For the crucial stationary points, the more complex catalyst [Fe(2)(OCH3)], in which a sterically hindered chiral bulk is mounted onto the porphyrin, was investigated. The corresponding computational data disclose the very significant effect of the porphyrin skeleton on the reaction energy profile. Though the geometrical features around the reactive core of the system remain unchanged, the energy barriers become much lower, thus revealing the profound effects that can be exerted by the three-dimensional organic scaffold surrounding the reaction site.
Highlights
Iron porphyrins are recognized as having a fundamental importance in chemistry and biology; among others, they play a key role for their function as a heme in the case of cytochromes
We recently described the use of the iron(III) porphyrin methoxy complex [Fe(1)(OCH3)] (Chart 1), bearing suitable chiral C2 symmetrical moieties onto the porphyrin core, as catalysts for cyclopropanation reactions showing high turnover number (TON) and turnover frequency (TOF) values, as well as high diastereo and enantioselectivity.[9]
In a previous paper9b we showed that the behavior of the [Fe(1)(OCH3)] complex can be safely reproduced in calculations by the corresponding single-stranded porphyrin model complex [Fe(2)(OCH3)] (FP-2) (Chart 1)
Summary
Iron porphyrins are recognized as having a fundamental importance in chemistry and biology; among others, they play a key role for their function as a heme in the case of cytochromes. Three of the four FP−EDA complexes, in particular, the radical or diradicaloid species, are not directly connected to the transition states corresponding to the dinitrogen loss but give rise, in the first reaction step, to three intermediate structures, 1osINT1−, 3INT1−, and 5INT1−, in which the distance between the EDA C2 and Nα (dC2−Nα) atoms is still a bond distance (∼1.45 Å) In these structures the interaction between EDA and iron is already significant (dC2−Fe has shortened to a bond distance, 2.22−2.25 Å), and the stability order of the various spin states is reversed, the brokensymmetry solution of the singlet 1osINT1− and the triplet 3INT1− being the most stable ones and 5INT1− being the least stable. The decrease is significant in terms of the relative free energy (∼4 kcal/mol), and the barrier approaches the value of 30 kcal/mol as the computed free energy barrier from the reactant complex 6FP-2-EDA Though this barrier represents a significant improvement with respect to the initial value of 37 kcal/mol, it remains much higher than in the case of the reaction catalyzed by [FeII(Por)(OCH3)]− (FP−).
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