M. R. Heinrich et al.
Encouraged by the overall observation that the presence
of oxygen is acceptable with iron(II) salts in the reaction
mixture, we finally investigated the possibility to recycle ni-
trogen monoxide from waste gas. In our experiments a simu-
lated stream of waste gas containing 5 vol% of NO was gen-
erated by passing air through a reaction vessel (vessel A,
Scheme 4), in which NO was produced from the reduction
affected by lower concentrations of NO. Favorably, the pro-
cess is not significantly disturbed by NO2 resulting from the
reaction of NO with oxygen, and the secondary products
formed from NO2 in aqueous solution.[35] In the overall con-
text, industrial processes such as the chemical etching of sili-
con wafers by concentrated nitric acid are especially inter-
esting since gas mixtures with an NO content of up to 20%
are produced.[36,37] Owing to the very unsteady evolution of
NO, denitrification is particularly difficult by known selec-
tive catalytic reduction (SCR)[38] or selective non-catalytic
reduction (SNCR)[39] methods. NO fixation and later conver-
sion, as is done in the carbonitrosation protocol, might
therefore be a valuable alternative.
In summary, we have described a new type of Meerwein
arylation that is able to produce b-aryloximes by incorpora-
tion of nitrogen monoxide. Until now, demanding precursors
such as organocobalt complexes were mainly used for radi-
cal nitrosation reactions,[17] and carbonitrosations have only
been described as intramolecular reactions employing orga-
nonitrites.[19] With the new procedure, b-aryloximes are
readily accessible from activated alkenes and arenediazoni-
um salts in one step. For non-activated alkenes, additional
functional groups, such as acetates, are required in allylic
positions. b-Aryloximes represent versatile synthetic build-
ing blocks of natural products[40] and numerous synthetic ap-
plications have been reported. Besides the well-known
Beckmann[41] and Neber[25a] rearrangements, the reduction
of compounds such as 9e–9h to aromatic amino acids[42,43] is
a further example. Owing to the low sensitivity of the car-
bonitrosation reaction towards oxygen, we have been able
to demonstrate its applicability for recycling of nitrogen
monoxide occuring as waste gas. Current research is direct-
ed towards an extension of the process to industrial scale.
Scheme 4. Carbonitrosation using a simulated stream of waste gas.
of nitrous acid with potassium iodide. The gas mixture was
then passed through a solution of iron(II) sulfate (vessel B),
which led to the absorption of NO mainly by reversible for-
mation of iron(II) NO complexes.[32] Denitrification by com-
plexation of NO with iron(II) complexes is known and has
been evaluated as a part of the BioDeNOx process.[33] Since
the formation of iron(II) NO complexes in vessel B can
easily be observed by their brown color (c.f. classical
“brown-ring” probe),[32c] we used a third vessel C to monitor
that most of the NO had been absorbed in vessel B. In the
following step, the NO-containing solution from vessel B
was used in the carbonitrosation reaction. For this purpose,
the alkene and the diazonium salt were added without any
further modification.
The formation of the oximes 9e, 9k, and 9m demon-
strates that Meerwein-type carbonitrosation reactions are
capable of recycling NO from waste gases. Oxime 9e, for
which the best yields had been obtained with pure NO and
iron(II) sulfate under air (Figure 1), now turned out to be
the least well-suited for the recycling reaction. As the con-
centration of NO in the reaction mixture is significantly
lower in the recycling experiments than in all other carboni-
trosations described before (Figure 1 and Scheme 2), the oli-
gomerization of the alkene starts to occur as an undesired
side process. Owing to the fact that radical oligomerization
is far more pronounced for acrylates than for non-activated
olefins such as allyl acetate,[34] it appears comprehensible
that carbonitrosations involving activated alkenes are more
Experimental Section
Representative procedure for the synthesis of b-aryloximes (Scheme 2):
FeSO4·7H2O (3.00 mmol, 834 mg) was added to a mixture of DMSO and
H2O (10:1 v/v) (50 mL) containing NaOAc (2.00 mmol, 164 mg) and
AcOH (2.00 mmol, 0.160 mL). The resulting solution was flushed with ni-
trogen (ca. 2 min) and afterwards nitrogen monoxide (>99 vol%, from a
pressure cylinder) was bubbled through the solution for 15 s. The alkene
(30.0 mmol) was added to the stirred mixture and a solution of the arene-
diazonium tetrafluoroborate (1.00 mmol) in DMSO (3 mL) was added
dropwise over 3 min. The mixture was stirred for another 2 min, the re-
maining nitrogen monoxide was removed through a stream of nitrogen,
and ascorbic acid (5.00 mmol, 881 mg) was added. After stirring for an
additional 30 min, the solution was diluted with water (50 mL) and ex-
tracted with ethyl acetate (4ꢃ50 mL). The combined organic phases were
washed with saturated aqueous NaCl and dried over Na2SO4. Removal of
the solvents under reduced pressure and purification by column chroma-
tography on silica gel gave the desired products.
Acknowledgements
The financial support of our research by the Deutsche Bundesstiftung
Umwelt (DBU) and the FAU Erlangen-Nꢀrnberg is gratefully acknowl-
9308
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 9306 – 9310