Regiospecific introduction of four substituents to porphyrin systems at antipodal pyrrolenic positions

Abstract

Electrophilic bromination occurs regiospecifically at the antipodal pyrrolenic ring of free-base porphyrins bearing substituents that fix the aromatic delocalization pathway; this new feature of porphyrin chemistry is not observed in the reactions of the corresponding metalloporphyrins as the pre-requisite bond fixation is only possible in free-base compounds.

7,8,17,18-Tetrabromo-5,10,15,20-tetraphenylporphyrin, C44H26Br4N4

“…1; this is in agreement with the conclusion drawn by Crossley et al (1991). Thus, electrophilic bromination occurs regiospecifically at the antipodal pyrrole ring of free-base porphyrin.…”

“…Callot (1974) suggested that there existed one Br atom per pyrrole ring, but the results reported by Crossley et al (1991) indicated four Br atoms on the antipodal pyrrole tings. In order to resolve this discrepancy, we prepared the title compound, (I), following the literature method (Crossley et al, 1991) and obtained suitable specimens for crystal structure analysis by layering a solution of the title compound in CHC13 with petroleum/piperidine (333-363 K). …”

Partially and fully β-brominated Mn-porphyrins in P450 biomimetic systems: Effects of the degree of bromination on electrochemical and catalytic properties

“…Electron-withdrawing substituents in the b-positions of the porphyrin ring stabilize both HOMO and LUMO; bulky groups, however, cause severe distortions to the macrocycle and destabilize the HOMO more than the LUMO. In consequence, the HOMO-LUMO energy gap decreases [20,22,[27][28][29][30][31][32][33][34][35][36][37]67] and the Soret band is considerably shifted to lower energy; the Q bands are also shifted to lower energy but to a smaller extent. The magnitude of the red shift and the number of bromine groups on the porphyrin macrocycle showed a linear relationship for the series Mn(III) (Br x TCMPP)Cl (x = 0, 6, or 8).…”

“…The introduction of more than four halogens into these positions on the ring, for example, results in a pronounced decrease in the porphyrin basicity (associated with the electron-withdrawing effect of the b-halogens) and causes severe distortions on the macrocycle (steric effect) [1,[24][25][26][27][28][29][30]. The combined electronic and steric effects decrease the HOMO-LUMO (highest occupied molecular orbitallowest unoccupied molecular orbital) energy gap and consequently alter the reduction potential of the porphyrin [20,22,[28][29][30][31][32][33][34][35][36][37][38].…”

Effect of Substitution on the ESR Spectra and Electronic Ground State of ZnTPP Cation Radicals

“…[37] The UV/Visible spectrum and the resolved h.f. coupling features of species I at ambient temperatures are characteristic of a 2 A 2u electronic ground state while the featureless isotropic signal for species II is characteristic of a 2 A 1u state. Upon sudden lowering of the temperature to 77 K, the h.f. features disappeared and, instead, only two overlapping isotropic resonances for species I and II or II' were visible.…”