Surprising Outcomes of Classic Ring‐Expansion Conditions Applied to Octaethyloxochlorin, 2. Beckmann‐Rearrangement Conditions

Abstract

The step‐wise conversion of synthetic porphyrins to derivatives that contain a non‐pyrrolic building block is an appealing method to generate functionalized porphyrinoids. Applied to octaethyloxochlorin oxime, a Beckmann rearrangement is potentially suited to generate a nitrogen‐expanded porphyrinoid. Unexpectedly, this reaction led to a ring‐expansion by an oxygen atom. The mechanism – an abnormal Beckmann reaction, followed by an intramolecular ring‐closing reaction and hydrolysis – was conclusively derived by trapping and structural characterization of the key secochlorin intermediate. The structural and UV/Vis spectral changes observed upon ring‐expansion of the oxochlorin are discussed. Other Beckmann‐rearrangement conditions failed to produce ring‐expanded products altogether. This work demonstrates in many ways how the extraordinary structural stability of the porphyrin macrocycle redirects the reactivity patterns of classic ring‐expansion reactions, which outlines the limits of the “breaking and mending of porphyrins” approach toward pyrrole‐modified porphyrins.

Structural and Photophysical Characterization of All Five Constitutional Isomers of the Octaethyl‐β,β′‐dioxo‐bacterio‐ and ‐isobacteriochlorin Series

“…The chemical properties of the OEP‐derived oxo‐derivatives – primarily those of oxochlorin 5 – with respect to reduction, [21] carbonyl C ‐methylation, [18b] N‐ methylation, [22] meso ‐deuteration, [23] osmylation, [24] and thionation [25] reactions were studied. We also found oxochlorin 5 to be a starting material for the preparation of a pyrrolinone‐expanded product [8h,26] . The group of Stolzenberg, and others, studied the metal complexes of 5 (specifically, their Co(II), [27] Ni(II), Cu(II), [28] Zn(II), Al(III)(OH), Mg(II), and Fe(III)Cl [29] complexes) [21a,30] .…”

“…For details to the structural determinations, see Supporting Information. Structures of OEP [35] and 5 [8h] were reported previously and are included for comparison; all other this work.…”

“…The Pt(II) complex of 5 was used as an optical oxygen sensor [31] . We reported the reactivity of the free‐base and Ni(II) complex of oxochlorin 5 , their oximes, [8h] the free‐base N ‐oxide, [8b] and meso ‐chlorides [8c] . Fewer studies of the dioxo‐isomers were published, but a low‐resolution structure of free‐base dioxobacteriochlorin 9 is known; [25] we recently studied the insertion of Co(II), Ni(II), Cu(II), Zn(II), Pd(II), Ag(II), Cd(II), and Fe(III) into 9 and determined the solid‐state structures of its Ni(II), Cu(II), Pd(II), and Ag(II) comple‐xes [9f,g] .…”

Synthesis and Properties of Carbaporphyrin and Carbachlorin Dimethyl Esters Derived from Cyclopentanedialdehydes

“…Temoporfin ( 3 ) is being used for the PDT treatment of squamous cell carcinomas, while talaporfin ( 4 ) has been approved for lung cancer . Chlorins commonly have electronic absorptions above 650 nm, and this feature is beneficial in PDT applications as light penetration in bodily tissues is more effective at longer wavelengths. , Unfortunately chlorin analogues have received very little attention, although structures of this type could potentially be equally effective.…”

Photophysical Exploration of Two Isomers of Octaethyltrioxopyrrocorphin

“…An alternative way to modify the porphyrinic cross-conjugated double bonds is to introduce β-oxo-functionalities into β-alkylporphyrins. One simple method is their treatment with hydrogen peroxide when dissolved in concentrated sulfuric acid. − This classic, 90-year-old reaction, when applied to octaethylporphyrin ( OEP ), generates one β-oxochlorin, five isomers of the corresponding β,β’-dioxo-chromophores, as well as two out of the four possible β,β′,β″-trioxopyrrocorphins ( 3-O 2,7,12 and 3-O 2,7,18 ), in addition to other products. − The chemical, coordination, and physical properties of β-oxochlorin have long been investigated. ,− More recently, the chemistry and photophysical properties of the dioxo-derivatives were the subject of several studies, revealing the large impact of the β-oxo-substituents on the electronic properties of the porphyrinic chromophore. ,− Remarkable regiochemical influences were revealed in porphyrinic chromophores carrying multiple β-oxo-substituents, ,,,, including the meso -arylporphodilactones. − …”