meso-Arylporpholactones and their Reduction Products

Abstract

The rational syntheses of meso-tetraaryl-3-oxo-2-oxaporphyrins 5, known as porpholactones, via MnO 4 − mediated oxidations of the corresponding meso-tetraaryl-2,3dihydroxychlorins ( 7) is detailed. Since chlorin 7 is prepared from the parent porphyrin 1, this amounts to a 2-step replacement of a pyrrole moiety in 1 by an oxazolone moiety. The stepwise reduction of the porpholactone 5 results in the formation of chlorin analogues, meso-tetraaryl-3-hydroxy-2oxachlorin ( 11) and meso-tetraaryl-2-oxachlorins (12). The reactivity of 11 with respect to nucleophilic substitution by O-, N-, and S-nucleophiles is described. The profound photophysical consequences of the formal replacement of a pyrrole with an oxazolone (porphyrin-like chromophore) or (substituted) oxazole moiety (chlorin-like chromophore with, for the parent oxazolochlorin 12, red-shifted Q x band with enhanced oscillator strengths) are detailed and rationalized on the basis of SAC−CI and MNDO-PSDCI molecular orbital theory calculations. The single crystal X-ray structures of the porpholactones point at a minor steric interaction between the carbonyl oxygen and the flanking phenyl group. The essentially planar structures of all chromophores in all oxidation states prove that the observed optical properties originate from the intrinsic electronic properties of the chromophores and are not subject to conformational modulation.

Expansion of a Pyrrole in meso‐Tetra­phenylporphyrin to a Pyrazine Imide Moiety­ Using a Beckmann Rearrangement

“…The free base possesses a typical porphyrin‐like optical spectrum. In this respect, this system is similar to the optical properties of porpholactones and their metal complexes 28…”

“…The free‐base pyrazinoporphyrin imide 11 exhibits a typical free‐base porphyrin‐like UV/Vis spectrum (see Figure 4). This duality, in which the metal complexes appear metallochlorin‐like but the free‐base compounds look porphyrin‐like, is similar to the optical properties of porpholactones 28. After removing the central metal “protecting group”, the addition of acid to a solution of 11 changes its UV/Vis spectrum in a manner that is consistent with the protonation of the inner imine nitrogens.…”

Ytterbium(III) Porpholactones: β‐Lactonization of Porphyrin Ligands Enhances Sensitization Efficiency of Lanthanide Near‐Infrared Luminescence

“…Iterative extended Hückel calculations by Gouterman and co‐workers suggested a porphyrinic electronic structure with the π orbitals on both the oxa and oxo oxygen atoms present in three of the four porphyrin orbitals and a resulting symmetry change 5b. d This perturbation of the molecular orbitals of the lactone moiety would result in changes in the HOMO, LUMO, and LUMO+1 of the porphyrin, which correspond to S 0 , S 1 , and S 2 levels, respectively. Since the electron‐withdrawing lactone moiety would stabilize HOMO or HOMO−1 more than LUMO and LUMO+1, it is not surprising that in Yb‐1 a the Q(0,0) band, which is assigned to S 0 –S 1 transitions, is redshifted by 26 nm and the T 1 state is lower in energy (Δ E =2546 cm −1 ) compared to Yb‐1 b .…”

Indachlorins: Nonplanar Indanone‐Annulated Chlorin Analogues with Panchromatic Absorption Spectra between 300 and 900 nm

“…Both the spectral resemblance of the β,β′‐lactone moiety to a double bond and its bathochromic effect in chlorin‐type chromophores were observed before 25c. 26b, 32b, 38…”

“…The similarity of the conformational effects of a pyrrolic β,β′‐double bond and a lactone moiety was amply demonstrated before 25c. 26b, 32b, 37…”