Nitrenium Salts in Lewis Acid Catalysis

Article abstract of DOI:10.1002/anie.201915547

Molecular compounds featuring nitrogen atoms are typically regarded as Lewis bases and are extensively employed as donor ligands in coordination chemistry or as nucleophiles in organic chemistry. By contrast, electrophilic nitrogen-containing compounds are much rarer. Nitrenium cations are a new family of nitrogen-based Lewis acids, the reactivity of which remains largely unexplored. In this work, nitrenium ions are explored as catalysts in five organic transformations. These reactions are the first examples of Lewis acid catalysis employing nitrogen as the site of substrate activation. Moreover, these compounds are readily accessed from commercially available reagents and exhibit remarkable stability toward moisture, allowing for benchtop transformations without the need to pretreat solvents.

Full text of DOI:10.1002/anie.201915547

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Title: Nitrenium Salts in Lewis Acid Catalysis
Authors: Meera Mehta and Jose Manuel Goicoechea
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Nitrenium Salts in Lewis Acid Catalysis
Meera Mehta* and Jose M. Goicoechea*
Abstract: Molecular compounds featuring nitrogen atoms are
typically regarded as Lewis bases and are extensively employed as
donor ligands in coordination chemistry or as nucleophiles in organic
chemistry. By contrast, electrophilic nitrogen-containing compounds
are much rarer. Nitrenium cations are a new family of nitrogen-based
Lewis acids, the reactivity of which remains largely unexplored. In this
work, nitrenium ions are explored as catalysts in five organic
transformations. These reactions are the first examples of Lewis acid
catalysis employing nitrogen as the site of substrate activation.
Moreover, these compounds are readily accessed from commercially
available reagents and exhibit remarkable stability toward moisture,
allowing for benchtop transformations without the need to pretreat
solvents.
on the cyclic(alkyl)(amino)nitrenium ions and explored their
reactivity in FLP chemistry.[37,38] This display of Lewis acidity
prompted us to investigate these ions in Lewis acid catalysis.
Figure 1. N-heterocyclic nitrenium cations.
Lewis acid catalysts function by accepting electron density
from
a nucleophilic substrate, activating the substrate for
subsequent reactivity. Some of the earliest reports of Lewis acid
catalysts were based on titanium and zirconium.[39] After this work,
there was a movement to use boron and aluminium based
compounds for similar chemistry.[40,41] Generally, moving away
from metal-based systems to main group catalysts reduces
operational costs, particularly when separation of the catalyst
from the product is necessary. Perhaps the most widely employed
system in this category is tris(pentafluorophenylborane) [B(C6F5)3;
BCF] due to its high Lewis acidity and ubiquitous use in frustrated
Lewis pair (FLP) chemistry.[42–45] It is worth noting however, that
catalytic reactions employing BCF are often performed under inert
conditions given its propensity to form a stable adduct with water
and undergo further hydrolysis at elevated temperatures.[45–47]
Herein, we prepared the nitrenium salts 1 and 2 (Scheme 1), and
probed their Lewis acidity in an effort to benchmark them relative
to more conventional Lewis acids. Next, we employed these salts
to affect the Friedel-Crafts dimerization of 1,1-diphenylethylene,
hydrodefluorination of 1-fluoroadamantane, deoxygenation of
ketones, transfer hydrogenation of olefins, and dehydrocoupling
of alcohols and amines with silanes.
Traditional examples of main-group Lewis acid catalysts involve
group 13 and 14 elements as the central locus of reactivity, as
they feature a low-lying orbital for substrate activation.[1] Over the
last decade there has been increasing interest in studying
pnictogens, often considered Lewis basic given the availability of
an energetically accessible lone pair, in this unconventional role.
This work has been expanded to include compounds of
phosphorus, [2–11] arsenic,[12] antimony,[13–20] and bismuth.[21–23] By
contrast, the reactivity of nitrogen-containing compounds as
Lewis acids has been investigated to a much lesser extent, largely
due to the dearth of such species. Chloramines, nitrenes,
diazocarboxylates, azides, and diazonium salts have been shown
to possess N-centered electrophilic behavior. However, further
reactivity of these compounds with nucleophiles often led to
unstable intermediates and subsequent decomposition rather
than adduct formation. [24–28]
Nitrenium cations, nitrogen-based Lewis acids that are
isoelectronic with carbenes (R2C:), were first reported in 1996 by
Boche et al.[29] It was found that bis(amino) ligands were
necessary to isolate stable crystalline ions, as previously
observed by Arduengo with imidazol-2-ylidenes.[30] Related
silyene, germylene, phosphenium and arsenium ions have also
been reported.[12,31–34] In 2011, Gandelman demonstrated that
nitrenium cations can serve as ambiphilic ligands on rhodium and
that they behave as weak σ-donors but considerable π-
acceptors.[35] Stoichiometric investigations of these ions with
Lewis bases were also undertaken by Gandelman, demonstrating
that the Lewis acidic centre is primarily located on the internal
nitrogen (Figure 1).[36] In 2018, Stephan and co-workers reported
In line with literature reports, compound 1 was prepared in
moderate yield, by oxidation of diaminonapthalene and
subsequent methylation.[48] Noted in both this report by Bohle et
al. and the crystallographic data acquired by us, a weak
interaction between the Lewis acidic nitrogen centre and the
trifluoromethanesufonate anion could be observed.
Scheme 1. Synthesis of nitrenium salts 1 and 2.
[a]
Dr. Meera Mehta and Prof. Dr. Jose M. Goicoechea
Department of Chemistry
University of Oxford
Chemistry Research Laboratory, Mansfield Rd., Oxford, OX1 3TA
E-mail: meera.mehta@chem.ox.ac.uk;
jose.goicoechea@chem.ox.ac.uk
Counter-ion exchange with K[B(C6F5)4] yielded an analytically
pure sample of the novel salt 2 (Scheme 1). While 1H and 13C
NMR spectroscopy showed little change, single crystal X-ray
diffraction studies revealed the absence of any significant cation-
anion interaction in compound 2, shown in Figure 2.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201915547
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yield using standard bench techniques and wet solvent. This
transformation serves to highlight both the unique stability and
selectivity of 2 relatively to BCF.
Figure 2. Molecular structure of 2. Anisotropic displacement ellipsoids set at
50% probability. Hydrogen atoms have been omitted for clarity, fluorine atoms
pictured as spheres of arbitrary radius.
The Gutmann-Beckett test and global electrophility index DFT
investigations were undertaken to assess the relative Lewis
acidity of 1 and 2.[49,50] Both studies revealed them to be weak
Lewis acids (see Supporting Information). Undeterred by these
results, we focused our efforts on reactivity by studying 1 and 2 in
five benchmark catalytic reactions (Figure 3).
First, the Friedel-Crafts dimerization of 1,1-diphenylethylene
(3a) was examined. It was found that compound 1 did not act as
a catalyst for this transformation even at 50 °C over 24h, whereas
compound 2 afforded 1-methyl-1,3,3-triphenyl-2,3-dihydro-1H-
indene (3b) in 20% conversion as determined by 1H NMR
spectroscopy. Under similar conditions, employing BCF as a
catalyst resulted in 48% conversion. However, catalyst 2 gave
similar conversion (26%) when stored on the bench for several
weeks and the Friedel-Crafts dimerization of 1,1-diphenylethylene
(3a) could be performed in air without the use of pre-dried
solvents (Karl Fisher titrations of CH2Cl2 revealed 21.7 ppm of
H2O).
Next, the hydrodefluorination of 1-fluoroadamantane (4a) was
examined. Catalyst 1 was able to affect this transformation;
however complete conversion was observed only at 50 °C over
24h. By contrast, 2 gave complete conversion to adamantane (4b)
after 5 hours under ambient conditions. This reaction was
monitored by 19F NMR spectroscopy by probing the signal
corresponding to 4a (–128 ppm) which diminished over time
accompanied by the appearance of a resonance for Et3SiF (–175
ppm). BCF also gave complete conversion under similar
conditions. We suspect that for this particular transformation all
three catalysts act as initiators for silylium catalysis, consistent
with a recent report by Stephan and Oestreich.[51]
Figure 3. Catalytic reactions explored with the nitrenium salts 1, 2 and BCF.
Conversions determined by 1H or 19F NMR spectroscopy. Isolated yields given
in parenthesis.
Following these observations, catalysts 1 and 2 where
investigated in the transfer hydrogenation of 1,1-diphenylethylene
(3a) with 1,4-cyclohexadiene to yield ethane-1,1-diyldibenzene
(3c). Again, the counter-ion played a significant role in the
catalytic activity of 1 relative to 2, with 1 being unable to catalyze
this reaction while 2 yielded complete conversion within 5 hours.
In 2017, Oestreich reported that 10 mol% of BCF affected this
transfer hydrogenation after 16 hours to give 92% conversion to
give ethane-1,1-diyldibenzene (3c), however a more decorated
cyclohexadiene was employed as an H2 source.[52] Unlike the
observations made by Oestreich, in the absence of substrate
catalyst 2 did not dehydrogenate 1,4-cyclohexadiene to yield
benzene. Consistent with the observations made for reaction A
and C, catalyst 2 retained catalytic activity using standard bench
techniques.
1 and 2 were also explored in the catalytic deoxygenation of
benzophenone (5a) with triethylsilane to give diphenylmethane
(5b) and the hexamethyldisiloxane. Consistent with the reactivity
noted for reaction A (Figure 3) it was found that 1 did not catalyze
this reduction even at 50 °C (only starting material was observed
1
in the H NMR spectrum). Catalyst 2 gave complete conversion
within 5 hours under ambient conditions. Interestingly, Stephan
and co-workers reported that in the same transformation BCF
gave 9% conversion to diphenylmethane (5b) and 91%
conversion to (benzhydryloxy)triethylsilane at 50 °C.[8] As noted
previously, catalyst 2 was able to affect this transformation in high
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Finally, both 1 and 2 were explored as catalysts for the
dehydrocoupling of phenol (6a) with triethylsilane on the bench to
whereas moderate conversion was observed with a para-
fluoro subsistent (9a). Para-trifluoromethane substitution of
substrate 10a resulted in 73% conversion to 10b, with no
evidence of C–F bond activation. This is consistent with the
observed C–F bond activation for 1-fluoroadamantane being a
result of silylium catalysis, a process not possible in air.
Furthermore, electron donating groups like para-methyl of 12a
were also tolerated to give 12b. Similar to 10a, carbonyl 13a
was reduced in moderate yield to give 13b with no
dehalogenation. Benzophenone derivatives functionalized
with methoxy, cyano or amino groups displayed little to no
conversion to the corresponding deoxygenated products. The
presence of competing donors, either in the form of a counter-
ion (as with compound 1) or functional groups on the substrate,
resulted in lower conversions.
give triethyl(phenoxy)silane (6b). As expected, catalyst
2
displayed high activity with complete conversion observed by 1H
NMR spectroscopy after 5 hours under ambient conditions.
Similar reactivity has been reported for BCF by Piers and co-
workers.[53] While the air stability of compound 1 has been noted
in the literature previously, it is interesting to note the naked
nitrenium cation retains this property on exchange of the
trifluoromethansulfonate ion for [B(C6F5)4]–. Thus, no
decomposition of 2 could be observed in CD2Cl2 over several
weeks in air. Furthermore, no decomposition of either catalyst 1
or 2 could be observed by NMR spectroscopy during catalytic
reactions A–E (Figure 3).
BCF mediated hydrosilylation of carbonyls was first reported
by Piers and co-workers in 1996,[54] however there are few
examples which result in the complete reduction of the carbonyl
to a methylene group.[8,55,56] Given the benchtop activity observed
for nitrenium ion 2 to affect this reaction, we were interested in
expanding this work. We found the hexamethyldisiloxane by-
product was not trivial to remove after this reduction and were
interested in moving to a protocol with an easier work-up.
Triethylsilane was replaced with polymethylhydrosiloxane
(PMHS), a milder reducing agent with greater bench-top stability
commonly used in organic and industrial laboratories. Upon
deoxygenation of benzophenone (5a) to diphenylmethane (5b)
with 10 mol% of 2 the polymeric silane cross-links to form a gel
(see Supporting Information). Using the violet colour of the
catalyst as an indicator, the siloxane gel was washed until clear
allowing for the isolation of the final product in 98% yield.
Table 2. Transfer hydrogenation reactions of alkenes with catalytic amounts
of 2.
Entry
substrate[a]
conv.(%)[b]
produc
t
1
2
3
4
5
6
7
8
3a: R1/R2 = Ph, R3/R4 = H
>99(89)
3c
14a: R1 = 3-ClPh, R2 = Ph, R3/R4 = H
15a: R1/R4 = Ph, R2/R3 = H
91
0
14b
15b
16b
17b
18b
19b
20b
16a: R1/R3 = Ph, R2/R4 = H
0
Table 1. Deoxgenation reactions of ketones with catalytic amounts of 2.
17a: R1/R2 = Ph, R3 = Ph, R4 = H
18a: R1/R2 = Ph, R3 = 4-BrPh, R4 = H
19a: R1/R2 = 4-ClPh, R3/R4 = Cl
20a: R1/R2 = Ph, R3/R4 = Ph
84
46
0
Entry
substrate[a]
conv.(%)[b]
product
0
[a]
2 (10 mol%, 10 mg) was added to a solution of olefin (0.1 mmol) and 1,4-
1
2
3
4
5
6
7
8
5a: R1/R2 = H
>99 (98)
97
5b
cyclohexadiene (14 mg, 0.2 mmol) in DCM (0.7 mL) at ambient temperature on
the bench. Determined by 1H NMR spectroscopy (CD2Cl2, based on olefin).
Isolated yields given in parenthesis.
[b]
7a: R1 = Cl, R2 = H
8a: R1 = Br, R2 = H
9a: R1 = F, R2 = H
10a: R1 = CF3, R2 = H
11a: R1/R2 = Cl
7b
89
8b
Transfer hydrogenation, formal transfer of an H2 molecule,
from an organic reagent allows for reduction of a substrate without
the inconvenience and hazards of using gaseous H2. We were
interested in exploring the utility of air stable catalyst 2 to mediate
this reaction (Table 2). Yet again, to increase conversion a 10
mol% catalyst loading was used. As expected, 1,1-
diphenylethylene (3a) gave complete conversion to 3c upon
benchtop reduction with catalyst 2. Additionally, meta-chloro
substitution in compound 14a was tolerated and gave high
conversion to 14b. By contrast, moving to 1,2-diaryl substituted
alkenes like cis- and trans-stilbene gave no conversion. This
transfer hydrogenation is thought to go through FLP- like
activation of the 1,4-cyclohexadiene resulting in protonation of the
olefin to give a carbocation and nitrenium-hydride complex. Thus,
the stilbene derivatives would yield the less stable secondary
carbocation, whereas 1,1-diarylsubsituted would yield a more
stable tertiary carbocation. This mechanism is also consistent as
66
9b
73
10b
11b
12b
13b
98
12a: R1 = CH3, R2 = H
13a: R1 = CH2Br, R2 = H
61
64
[a] 2 (10 mol%, 10 mg) was added to a solution of ketone (0.1 mmol) and PMHS
[b]
(17 mg, 0.3 mmol) in DCM (1.2 mL) at ambient temperature on the bench.
Determined by 1H or 19F NMR spectroscopy (CD2Cl2, based on ketone). Isolated
yields given in parenthesis.
In order to establish functional group tolerance, the
deoxygenation of ketones with 10 mol% catalyst loading of
compound 2 was explored (Table 1). It was found that both
chloro- and bromo- substituents were well tolerated (7a, 8a,
11a) to give products 7b, 8b and 11b in high conversion,
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no reaction is observed when nitrenium cation 2 is exposed to 1,4-
cyclohexadiene in the absence of substrate. Both tri-substituted
alkenes 17a and 18a were converted to 17b and 18b in high to
moderate yields. Finally, both tetrasubstituted alkenes 19a and
20a showed no conversion to 19b and 20b, presumably due to
steric encumbrance.
Acknowledgements
M. M. gratefully acknowledges the Royal Society for funding
(NF170051). We also thank the University of Oxford for access to
Chemical Crystallography and Advanced Research Computing
facilities, and Elemental Microanalysis Ltd. (Devon) for elemental
analyses.
Table 3. Dehydrocoupling reactions of alcohols and diphenylamine with
catalytic amounts of 2.
Keywords: Nitrenium ions • Lewis acids • Catalysis • Ketone
deoxygenation • Transfer hydrogenation
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substrate (RE1H)[a]
PhOH (6a)
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HSiEt3
)
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similar reaction conditions both the electron-withdrawing
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in the case of 22a and 23a, were well tolerated and afforded 21b,
22b, or 23b in high conversion. Finally, diphenylamine (24a) was
dehydrocoupled with triethylsilane to give 24b in complete
conversion.
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10.1002/anie.201915547
Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
Meera Mehta* and Jose M. Goicoechea*
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Nitrenium Salts in Lewis Acid
Catalysis
We explore the use of nitrenium salts as Lewis acid catalysts in five organic
transformations. Despite their weakly acidic character, these species proved
competent catalysts in a number of benchmark transformations. The moisture-
stability of the salts allows for the reactions to be carried on the benchtop without
the need for pre-dried solvents.
This article is protected by copyright. All rights reserved.