Loading Phlorins with Fullerenes by a [4 + 2]-Cycloaddition Reaction: Regulation of the Regioselectivity by Pyrrole Linkage Modes

Abstract

Sulfolenopyrrole-based normal and N-confused phlorins have been constructed to address the seldom touched phlorin functionalization and simultaneously explore the effect of the pyrrole linkage modes (αα, αβ) on the [4 + 2] cycloaddition reaction. The common sulfolenophlorin 1 contains two sulfolenopyrroles with the same reactivity upon tautomerization and undergoes stepwise [4 + 2]-cycloaddition with fullerene to furnish monoadduct 1-C60 and bisadduct 1-2C60 with a total yield up to 76%. By contrast, the presence of the confused pyrrole in 2 fixes the π-system owing to the low tendency to tautomerize and enables the two sulfolenopyrroles to exhibit in different fashions (i.e., normal NH-type and imino-type). Notably, under milder conditions (120 °C), the monofullerenoadduct 2-C60 forms rapidly and has been isolated from the [4 + 2] cycloaddition reaction of 2 and fullerene as the predominant fraction, accompanied by a trace amount of bisadduct 2-2C60. Raising the temperature to 140 °C did not improve the yield of 2-2C60. The structural analysis of 2-C60 indicates the attachment of fullerene at the iminopyrrole part. The high regioselectivity in the [4 + 2] cycloaddition of the imino-type sulfolenopyrrole unit has been rationalized thermodynamically by the DFT calculation on the relative energy of the two diene intermediates.

Aromatic Ring-Fused Dipyrrins: Programmed [4 + 2]-Cycloaddition Pathway with Regio-selectivity upon Alkylamino-Substitution

“…Our early work on the [4 + 2]-cycloaddition reaction of sulfolenophlorins also demonstrates the higher reactivity of imino-type sulfolenopyrrole compared to the normal NH-type sulfolenopyrrole. 31 According to the prediction, more energy is required to extrude SO 2 from normal sulfolenopyrrole, which is consistent with the high temperature (at 190 °C) that triggered further cycloaddition of 2a-NQ with C 60 (Scheme S1). In addition, owing to the substitution of the alkylamino group, 2a−d-dienes display elevated highest occupied molecular orbital (HOMO) levels around −4.85 to −4.90 eV, which are much higher than that of 1-diene (−5.16 eV) (Figures 4d and S30).…”

“…Again, the diene obtained by releasing SO 2 from imino-type sulfolenopyrrole is energetically more favorable (18.4–21.8 kcal/mol) than the other isomer (Figures c and S29). Our early work on the [4 + 2]-cycloaddition reaction of sulfolenophlorins also demonstrates the higher reactivity of imino-type sulfolenopyrrole compared to the normal NH-type sulfolenopyrrole . According to the prediction, more energy is required to extrude SO 2 from normal sulfolenopyrrole, which is consistent with the high temperature (at 190 °C) that triggered further cycloaddition of 2a-NQ with C 60 (Scheme S1).…”

“…The unique sulfolenopyrrole features a sulfolene unit fused at β,β-positions, − and can undergo the cheletropic reaction to extrude SO 2 under heating. As a result, the reactive s-cis type diene will be generated in situ, which can trap the dienophiles through the [4 + 2]-cycloaddition reaction to build the six-member ring. ,− Incorporation of sulfolenopyrrole(s) into oligopyrroles proves to be an efficient way for peripheral annulations through [4 + 2] cycloaddition. − In 2018, Kräutler, Li, and co-workers prepared sulfolenodipyrrin, to which the fullerenes were efficiently loaded through a programmed [4 + 2]-cycloaddition reaction . Thus, sulfolenodipyrrins are expected to be potential building blocks in the construction of aromatic ring-fused dipyrrins.…”

Diels–Alder cycloadditions of fullerene: Advances in mechanistic theory

“…Herein, we present an overall picture of the mechanisms of fullerene cycloadditions with a focus on developments reported in recent years 20,34–39 . We first discuss the mechanism of the cycloaddition for hollow fullerene, and then move on to the influence of encapsulated ions, clusters, and molecules on the mechanism and regioselectivity of the DA fullerene reaction from a theoretical standpoint 21,40–43 .…”