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.
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