ACS Catalysis
Letter
forming under operative conditions. When equimolar concen-
depending on the nature of the active species (i.e., neutral vs
ion pair), which can be purposely generated and made
operative under finely adjusted reaction conditions. The
reported system demonstrates that primary functional groups
can be controlled by specifically targeting their intrinsic
chemical bonds to open divergent reaction pathways. More
specifically, in the case of aldehydes, we have shown that
addressing either the CO or C−H bond can provide nitriles
and amides in high yields and selectivities under mild reaction
conditions. On the basis of preliminary mechanistic inves-
II
trations of Fe Br and TBAB were mixed in an acetonitrile
2
solution at 60 °C, the formation of an ion pair 4 was observed
23
(
Scheme 3, panel F). The molecular identity of 4 could be
confirmed by X-ray structure analysis (Scheme 3, panel I) and
high-resolution mass spectrometry measurements as tetra-n-
III
butylammonium iron tetrabromide (i.e., [Fe Br ][N-
4
(
C H ) ]). Similar species have been reported in the
4 9 4
24
literature and may result from the facile oxidation of iron(II)
bromide in solution to a thermodynamically stable
tetrabromoferrate(III) anion. Its formation may be influenced
by various parameters such as solvent polarity, temperature,
the excess of halide anions, or other coordinating function-
alities. Ion pair 4 can also be prepared in 90% yield by mixing
equimolar amounts of Fe Br and TBAB (Scheme 3, panel
tigations, we propose that nitrile formation involves a neutral
III
Fe Br species acting as a Lewis acid that activates the
3
aldehyde. This promotes the nucleophilic attack of TMS-N3
and leads to the desired product after hydrolysis. In contrast,
amide synthesis involves an in situ generated ion pair
2
5
III
3
III
G). The catalytic relevance of ion pair 4 in the established
protocol for amide synthesis was first examined using 4-
methylbenzaldehyde as a substrate in the absence of added
TBAB (Scheme 3, panel H). As expected, the exposure of the
reaction mixture to light (i.e., blue LEDs) led to the formation
of the corresponding amide 2l in 32% yield after 2 h and
further extending the reaction time to 4 h produced 77% of the
product. Likewise, other substrates could also be converted by
[Fe Br ][N(C H ) ]. This species may then operate as a
4
4
9 4
molecular platform capable of generating and transferring an
electrophilic nitrene radical to the aldehyde C−H bond en
route to the desired product. Further studies aiming to obtain
more in-depth mechanistic information and extend this
protocol to other functional groups are currently underway.
ASSOCIATED CONTENT
sı Supporting Information
■
4
to the expected products in yields similar to those obtained
*
when using a mixture of FeBr and TBAB (Scheme 3, panel L).
2
In contrast, employing ion pair 4 as a catalyst for nitrile
synthesis failed to produce 3q (Scheme 3, panel J), thus
confirming disparities in terms of reactivity and product
General considerations, experimental methods and
graphic data for 4 (CCDC 2049687) (PDF)
III
selectivity between [Fe Br ][N(C H ) ] 4 and parent neutral
4
4
9 4
III
Fe X salts, which were all found to produce the desired nitrile
3
under similar reaction conditions (Scheme 3, panel K).
Complementarily, using equimolar quantities of 4 and FeBr3
in the protocol for nitrile synthesis also hampered product
formation (Scheme 3, panel M). This may be attributed to a
decreased Lewis acidity of the tetravalent iron center. Finally,
■
Corresponding Author
Christophe Werlé − Max Planck Institute for Chemical
III
II
an implication of Fe Br3 and Fe Br2 salts in the
interconversion of amides to nitriles and vice versa could be
excluded (Scheme 3, panel N).
Energy Conversion, 45470 Mulheim an der Ruhr, Germany;
̈
Therefore, it is postulated in Scheme 4 that the formation of
amides and nitriles proceeds through two different reaction
networks consistent with the preliminary control experiments
Authors
Basujit Chatterjee − Max Planck Institute for Chemical
Energy Conversion, 45470 Mulheim an der Ruhr, Germany;
̈
For amide formation (Scheme 4, panel A), FeBr is proposed
2
to first react with TBAB to produce the ion pair catalyst 4. This
species then activates TMS-N with the contribution of an
3
Soumyashree Jena − Max Planck Institute for Chemical
external stimulus such as light (λmax = 455 nm) to provide a
Energy Conversion, 45470 Mulheim an der Ruhr, Germany;
̈
20a,b,26
reactive iron−nitrene radical intermediate.
Subsequently,
Ruhr University Bochum, 44801 Bochum, Germany
its insertion into the aldehyde C−H bond generates an N-
silylamide derivative, which offers the corresponding primary
amide after hydrolysis. In contrast, when subjected to the
standard set of reaction conditions for nitrile formation, FeBr3
may react with the Lewis basic aldehyde O-donor center,
which increases the electrophilicity of the adjacent carbon
center (Scheme 4, panel B). This induces the nucleophilic
attack of TMS-N3 at the electron-deficient carbon atom,
resulting in an intermediate species A, conceivably of
Vishal Chugh − Max Planck Institute for Chemical Energy
̈
Thomas Weyhermuller − Max Planck Institute for Chemical
̈
27
iminodiazonium character, which provides the corresponding
nitrile after the liberation of N and Me Si−OH.
2
3
Author Contributions
B.C. and S.J. contributed equally to this work.
#
CONCLUSION
■
In summary, we have developed an iron-based catalyst system,
allowing for unique molecular control over product formation
Notes
The authors declare no competing financial interest.
7
182
ACS Catal. 2021, 11, 7176−7185