D.K. Wolgemuth et al.
Catalysis Communications 150 (2021) 106275
Fig. 2. Aziridination of olefins using 5 mol% [MnTMPyP4]I
5
with, 3 mmol olefin, 0.3 mmol chloramine-T and 2 mL of buffer. The yields shown in red are in 75 mM
acetate buffer at pH 4, black are phosphate buffer at pH 7, and blue bicarbonate buffer are at pH 10. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
ratio to a 1:1, stoichiometric ratio of styrene:chloramine-T resulted in no
further improvement of the product yields (entry 11). We do note an
increase in the product yield when this ratio is inverted to 1:2 styrene:
chloramine-T, where this modification resulted in an improved the
yield of N-tosyl-2-phenylaziridine to 56% (entry 12). Further increases
in the chloramine-T concentration in these reactions where there is a 1:3
or 1:5 styrene:chloramine-T ratio produced no further increase in re-
action yield (52% and 53%, respectively). Similar trend in the optimized
N-transfer agent has been previously observed by Zhang with cobalt and
iron catalytic systems, but the underlying reason for this optimized
concentration is currently unknown [14,29]. Other side products are
also observed in these reactions. Under conditions similar to those in
Table 1, entry 12 a range of other styrene-derived products were
generated at low yield, which may be due to product degradation or
aberrant activation pathways (c.f. Fig. S1 in the supporting materials).
UV–visible absorption data (c.f. Fig. S2 in the supporting materials)
shows a spectroscopic shift in the absorption intensity of two prominent
transitions associated with Mn-TMPyP4, where the isolated catalyst
shows nearly equal intensity in the 420 and 460 nm signals. Upon
addition of chloramine-T to this sample, a systematic reduction in the
intensity of the 430 nm transition and an increase in the 460 nm feature
is recorded. This absorption shifts maximized at a 1:2 ratio of Mn-
TMPyP4 to chloramine-T, where additional chloramine-T once again
lowers the differences in intensity of these spectral features, which could
indicate that a 1 to 2 stoichiometry of Mn-TMPyP4 to chloramine-T is
required to activate this catalyst for aziridination in a similar fashion to
Zdilla et al. [28]
(Fig. 2). Anecdotally, there appears to be more side products generated
from the aziridination of styrene than the aliphatic olefins, which may
contribute to the disparate yields observed for the substituted styrene
trials. Aziridines have traditionally shown instability at both low and
high pH values, where this strained 3-member heterocycle is susceptible
to ring opening under both conditions [30]. Specifically the low yields
generally observed at pH 10, maybe due to instability of the aziridine
unit at this pH. The aziridination products of the para-substituted sty-
renes show some weak correlation to the inductive and resonance effects
associated with their substituents. However, it is unclear if this corre-
lation is associated with a distinctive mechanistic step (or steps) within
the reaction, or do these effects impact the relative stability of the
aziridination products at different pH values. More detailed kinetic
studies are required to directly connect the electronics of the substrate
with the catalysis described herein. In general, the non-styrene olefins
generated more product at pH 4, than other pH values. Once again, this
is likely due to the stability of the product rather than some mechanistic
dependence on the pH. The only outlier to this trend is the aziridination
product of cyclopentene, which seems to have some additional stability
over its counterparts. One possibility that this molecule has slightly
more stability at high pH due to the inaccessibility to the aziridine
moiety in this more congested 5-atom ring system.
Finally manganese(III)-TMPyP4 was added to 3 different DNA sam-
ples (salmon teste DNA, calf thymus DNA, and pUC19 plasmid DNA) to
generate a DNA hybrid catalyst system for asymmetric aziridinations.
Manganese(III)-TMPyP4 is a known “groove-binder” to B-DNA samples,
[31] and several metal-TMPyP4 catalysts have been successfully
adapted to perform chiral catalytic transformations using duplex DNA
[21–23,24]. When approximately 2 mg of DNA is added as part of the
optimized catalytic trial described in the data above (1:2 styrene:
chloramine-T ratio with 5 mol% manganese(III)-TMPyP4 in buffered
aqueous media at pH 7), the yield of these reactions was lowered
significantly. With salmon teste DNA (stDNA), calf thymus DNA
(ctDNA), and plasmid DNA (pUC19), the product yield was 8%, 19%,
Under these optimized conditions, alternative olefins where reacted
with chloramine-T in a 1:2 olefin:chloramine-T ratio with 5 mol%
manganese(III)-TMPyP4 in buffered aqueous media at pH 4, 7, and 10.
All olefins studied here were converted to aziridination products with a
wide-range of measured yields. Overall production of the aziridination
product seems to be favored under slightly acidic (pH 4) to neutral pH
(
pH 7), where generally less aziridine products were recovered at pH 10
3