[3 + 2]-Cycloadditions with Porphyrin β,β′-Bonds: Theoretical Basis of the Counterintuitive meso-Aryl Group Influence on the Rates of Reaction

Abstract

Removal of a β,β′-bond from meso-tetraarylporphyrin using [3 + 2]cycloadditions generates meso-tetraarylhydroporphyrins. Literature evidence indicates that meso-tetraphenylporphyrins react more sluggishly with 1,3-dipoles such as ylides and OsO 4 (in the presence of pyridine) than meso-tetrakis-(pentafluorophenyl)porphyrin. The trend is counterintuitive for the reaction with OsO 4 , as this formal oxidation reaction is expected to proceed more readily with more electron-rich substrates. This work presents a density functional theory−based computational study of the frontier molecular orbital (FMO) interactions and reaction profile thermodynamics involved in the reaction of archetypical cycloaddition reactions (a simple ylide, OsO 4 , OsO 4 •py, OsO 4 •(py) 2 , and ozone) with the β,β′-double bonds of variously fluorinated meso-arylporphyrins. The trend observed for the Type I cycloaddition of an ylide is straightforward, as lowering the LUMO of the porphyrin with increasing meso-phenyl-fluorination also lowers the reaction barrier. The corresponding simple FMO analyses of Type III cycloadditions do not correctly model the reaction energetics. This is because increasing fluorination leads to lowering of the porphyrin HOMO−2, thus increasing the reaction barrier. However, coordination of pyridine to OsO 4 preorganizes the transition state complex; lowering of the energy barrier by the preorganization exceeds the increase in repulsive orbital interactions, overall accelerating the cycloaddition and rationalizing the counterintuitive experimental findings.

meso-Tetrahexyl-7,8-dihydroxychlorin and Its Conversion to ß-Modified Derivatives

“…The OsO 4 -mediated dihydroxylation of meso-tetrahexylporphyrin 7 took place under standard conditions that are also applicable to octaalkylporphyrins and mesotetraphenylporphyrin: 1-2 equiv OsO 4 in CHCl 3 /pyridine (optionally applied in two aliquots) over several days at ambient temperature, followed by the reductive cleavage of the initially formed osmate ester (Scheme 2). We found that the osmylation takes place at a comparable, or possibly slightly lower rate, as in meso-tetraphenylporphyrin, a finding in line with our expectations for the electron-rich porphyrin 7 [26].…”

“…This versatile method is suitable for converting β-alkylporphyrins 1 or meso-arylporphyrins 3 to their corresponding 7,8-dihydroxychlorins 2 and 4, respectively (Scheme 1) [18][19][20][21][22][23][24]. The regiochemistry and other mechanistic aspects of this reaction are well understood [25,26]. Importantly, the diol functionality in the dihydroxychlorins can be used as a synthetic Curiously, the chemistry of meso-alkylporphyrins (5) (Figure 1), although known for decades [31][32][33], has been far less studied, by a wide margin, compared to that of their βalkyl-or meso-aryl analogues [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50].…”

“…(Scheme 1) [18][19][20][21][22][23][24]. The regiochemistry and other mechanistic aspects of this reaction a well understood [25,26]. Importantly, the diol functionality in the dihydroxychlorins ca be used as a synthetic handle for a number of subsequent functional grou transformations, generating a host of chlorins and chlorin analogues [19,20,24,[27][28][29][30].…”

Understanding the Nonlinear Hammett Relationship in Osmylation of Olefins with OsO4–Amine Ligands: Importance of Singlet-Diradical Character

“…11 However, the calculated HOMO−LUMO data for the osmylation reaction do not support this assumption (see Table S6). 43 In this paper, we explore various factors that can influence the activation energies and derive a scheme that can nicely rationalize all of the experimental observations by Sharpless.…”

“…As the basicity of the amine-type ligand decreases, the position of the minimum points (see the black circled points in the purple, gray, green, and orange plots in Figure A) of four curved Hammett plots accelerated by amine-type ligands shifts to the right, indicating a more electron-withdrawing styrene. The general observation that the β,β′-double bond of meso -tetrakis(pentafluorophenyl)porphyrin (T F PP) is more reactive than the corresponding bond(s) in meso -tetraphenylporphyrin (TPP) appears to have the same underlying cause. − …”