Chromene-Annulated Bacteriochlorins

Abstract

Syntheses and optical properties of mono- and bis-chromene-annulated bacteriochlorins are described. Known monochromene-annulated meso-(pentafluorophenyl)chlorin is susceptible to a regioselective OsO4-mediated dihydroxylation, generating two monochromene-annulated trihydroxybacteriochlorin stereoisomers: either the newly introduced vic-cis-diol functionality is on the same side as the vic-cis-diol moiety the chromene-annulation was based on or on the opposite side. Treatment of the two isomers with heat or base generates different sets of bis-chromene-annulated bacteriochlorin stereo- and regioisomers. Detailed 1D and 2D (1)H and (19)F NMR spectroscopic investigations allowed the characterization of the isomers that formed. The regioselectivity of the second annulation reaction was rationalized computationally on steric grounds. The bacteriochlorin-type optical spectra of the mono- and bis-chromene-annulated bacteriochlorins are modulated as a result of the annulation, with each isomer possessing a unique spectrum, attributed to the effects the regiochemically distinct annulations have on the conformation of the chromophore. The formation of a bis-chromene-annulated chlorin from the bacteriochlorins is also described, including its X-ray crystal structure, revealing some details of the metrics of the chromene-annulated moiety. The vic-diol functionality of monochromene-annulated trihydroxybacteriochlorins is also susceptible to oxidation and ring-expansion reactions, generating chromene-annulated pyrrole-modified chlorins incorporating oxazolone and morpholine moieties. The work expands the body of work on the synthesis and optical fine-tuning of meso-aryl-substituted bacteriochlorins.

Stepwise Preparation of meso‐Tetraphenyl‐ and meso‐Tetrakis(4‐trifluoromethylphenyl)bacteriodilactones and their Zinc(II) and Palladium(II) Complexes

“…The facility of the oxidation of meso ‐tetrakis(pentafluorophenyl)porphyrin 2c , supported by several of our findings,[28c], as well as those of others,[25a], [28b] is puzzling as this porphyrin was described to be more electron‐poor and robust towards oxidative degradations . This may be, in part, the result of the better solubility of 2c vs. 2a , but we suggest that intrinsic electronic factors also play a major role.…”

“…We deem these routes to be generally applicable to a range of meso ‐arylporphyrins (in as far as both key reaction steps, the OsO 4 ‐mediated dihydroxylation and pyrroline diol to oxazolone conversion steps were shown to be applicable to a range of meso ‐arylporphyrins/chlorins). [17a], [28c], [31a], [38b] The metal insertion step bacteriodilactone → [bacteriodilactonato]Zn(II) and Pd(II) was also described, whereby we find that the metal insertions require significantly harsher metal insertion conditions than the corresponding porphyrins, likely the result of the significantly reduced Lewis‐basicity of the lactone‐modified macrocycle. The two regioisomeric products 3 3,12 and 3 3,13 could not be separated in their free base or metallated forms, but their relative ratios could be determined by 1 H NMR spectroscopy.…”

β‐meso‐Annulated meso‐Tetraarylchlorins: A Study of the Effect of Ring Fusion on Chlorin Conformation and Optical Spectra

“…The E‐isomer of trihydroxybacteriochlorin 62 under similar reaction conditions only afforded a single compound 65 . This molecule underwent two known oxidative transformations of diol chlorins to pyrrole‐modified porphyrins to furnish examples of chromene‐annulated compounds 66 and 67 (Scheme ) …”

“…also extended the preparation of chromene chlorins into the bacteriochlorin domain. They reported the synthesis of two examples of mono‐chromene‐annulated trihydroxybacteriochlorin diastereomers 61 and 62 (Scheme ) . In one example, the newly introduced vic‐ cis ‐diol functionality and the annulated chromene are on the same side of the porphyrin plane (Z‐isomer, 61 ) and in the diastereomer, these groups are located on opposite sides (E‐isomer, 62 ).…”

“…As annulation is known to increase π‐overlap by imposing the meso ‐phenyl group into greater co‐planarity with the porphyrin plane, this would result in modification of the chromophore conformation and thus a bathochromically shifted optical spectra can be expected. The mono‐annulated trihydroxybacteriochlorin 61 : λ max (log ϵ)=729 nm (5.02) (Table , entry 36) exhibited more intense Q y band and 24 nm red‐shifted bacteriochlorin‐like UV‐Visible spectrum compared to the parent non‐annulated meso‐ pentafluorophenyl tetrahydroxy bacteriochlorin [λ max (log ϵ)=705 (4.74) nm] . This data also shows that mono‐annulated bacteriochlorin triol 61 displayed ∼66 nm bathochromically shifted optical spectra relative to its precursor chromene‐annulated chlorin 57 .…”