Surprising Outcomes of Classic Ring‐Expansion Conditions Applied to Octaethyloxochlorin, 3. Schmidt‐Reaction Conditions

Abstract

The Schmidt reaction (treatment of a ketone with sodium azide and a mineral acid) is an alternative to the Beckmann rearrangement to expand a cyclic ketone to a lactam. However, when applied toward the conversion of a synthetic porphyrin into a derivative containing a nonpyrrolic building block, this approach failed to generate the expected lactam. Instead, by using sulfuric acid as a catalyst, a novel heptaethyl‐2‐hydroxy‐chlorin‐3‐one was formed, structurally characterized, and its mechanism of formation deduced. The work demonstrates how the extraordinary structural stability of the porphyrin macrocycle redirects the reactivity patterns of classic ring‐expansion reactions. By using hydrochloric acid as a catalyst, a somewhat regioselective chlorination of the meso‐positions of the oxochlorin was observed. The halogenation sites were determined spectroscopically and by X‐ray crystallography of selected derivatives. The regioselectivity of the halogenation was computationally rationalized. This method is superior to alternative halogenation methods for the regioselective generation of 5‐chloro‐, 10‐chloro‐, and 5,10‐dichlorooxochlorins.

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

“…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] .…”

Azides and Porphyrinoids: Synthetic Approaches and Applications. Part 1—Azides, Porphyrins and Corroles

“…The Schmidt reaction conditions, involving the treatment of ketones with sodium azide in an acidic medium, are usually used to prepare lactams from cyclic ketones by ring expansion and is an alternative to the Beckmann rearrangement. However, when Brückner and co-workers treated the octaethylporphyrin derivative P144 under the Schmidt reaction conditions a α-hydroxyoxochlorin derivative was isolated (P145), or meso-chlorinated products (P146), instead of the expected lactams [93]. The reaction with the oxochlorin P144 required the use of a large excess of NaN 3 and conc.…”

“…The attempt to perform the Schmidt reaction with conc. hydrochloric acid instead of sulfuric acid led to the formation of meso-chlorinated products P146a-f [93]. The treatment of 10-bromo-5,15-diarylporphyrins P147 with sodium azide allowed for the preparation of the corresponding azidoporphyrin derivative as an intermediate, in high yields, through an aromatic nucleophilic substitution.…”

Oxochlorin frameworks confining a β-hydroxyketone moiety

“…A range of functionalization reactions of oxochlorin 6 were reported. 15 a ,19 h ,20 Most important in the context of this contribution, three independent reports described the two-step oxidation of β-oxochlorin 6 to produce meso -hydroxylated derivative 9 , with the meso -hydroxy group located adjacent to the oxo-functionality (Scheme 3). 19 h ,20 a , b Other isomers of the meso -hydroxyoxochlorins have also become known, but these carry the meso -hydroxy functionality at remote positions from the β-oxo-functionality.…”