Nitro-assisted Br?nsted acid catalysis: Application to a challenging catalytic azidation

Article abstract of DOI:10.1021/jacs.5b06055

A cocatalytic effect of nitro compounds is described for the B(C₆F₅)₃·H₂O catalyzed azidation of tertiary aliphatic alcohols, enabling catalyst turnover for the first time and with a broad range of substrates. K

Full text of DOI:10.1021/jacs.5b06055

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Nitro-Assisted Brønsted Acid Catalysis: Application to a Challenging
Catalytic Azidation
Marian Dryzhakov,‡ Malik Hellal,‡ Eléna Wolf, Florian C. Falk and Joseph Moran*
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ISIS & icFRC, Université de Strasbourg & CNRS, 8 allée Gaspard Monge, 67000 Strasbourg, France
Supporting Information Placeholder
rapid reaction with catalytic turnover for the first
time.7,8,9,10 Kinetic, electronic and spectroscopic investi-
gations suggest that hydrogen bonding between the nitro
compound and the acid generates aggregates that are the
kinetically competent Brønsted acid catalysts. Taken
more broadly, weak H-bond accepting co-catalysts may
constitute a new general approach to modulating
Brønsted acid catalysis in solution.
ABSTRACT: A co-catalytic effect of nitro compounds is
described for the B(C6F5)3ŸH2O catalyzed azidation of
tertiary aliphatic alcohols, enabling catalyst turnover for
the first time and with a broad range of substrates. Ki-
netic investigations into this surprising effect reveal that
nitro compounds induce a switch from first order con-
centration dependence in Brønsted acid to second order
concentration dependence in Brønsted acid and second
order dependence in the nitro compounds. Kinetic, elec-
tronic and spectroscopic evidence suggests that higher
order hydrogen-bonded aggregates of nitro compounds
and acids are the kinetically competent Brønsted acid
catalysts. Specific weak H-bond accepting additives may
offer a new general approach to accelerating Brønsted
acid catalysis in solution.
We initially became intrigued by Pocker’s report that
the hydrochlorination of olefins proceeds more rapidly
and displays unusual second order concentration de-
pendence in HCl when the reaction is carried out in
nitromethane.11,12 Though it was concluded that coop-
erative effects between two molecules of HCl were re-
sponsible for the observed concentration dependence, no
efforts were made to uncover the role of nitromethane, a
well-known weak dual H-bond acceptor.13,14 Speculating
that the solvent might play a more active role, we elect-
ed to investigate the effect of nitro compounds in a
Brønsted acid catalyzed reaction where the acid catalyst
and reagents could be independently varied. We selected
the azidation of tertiary aliphatic alcohols with TMSN3,
a useful but sluggish and low-yielding transformation
that thus far has not been achieved with catalytic turno-
ver.7,8 We first investigated the azidation of alcohol 1a
catalyzed by B(C6F5)3ŸH2O, an easily weighable solid
Lewis acid hydrate that is a strong Brønsted acid.15 Grat-
ifyingly, the use of nitromethane as solvent led to imme-
diate formation of silyl ether 2a followed by complete
conversion of 2a to azide 3a in <1 h at room temperature
in 80% isolated yield (eq 1).16
Hydrogen bond catalysis and Brønsted acid catalysis are
arguably the most common activation modes in synthet-
1
ic and biological reactions. One of the simplest ways to
increase the acidity, and hence the catalytic ability, of
such catalysts is to cause them to interact with other
hydrogen bond donating groups,2 an approach well
known to lead to dramatic rate acceleration and impres-
sive selectivity within the molecular scaffolds of en-
zymes, enzyme mimics and some organocatalysts.3,4 In
solution, aggregation of two or more hydrogen bond
donating molecules is likewise known to cause rate ac-
celeration, but the strict concentration dependence
means that the catalyst must effectively also be the sol-
vent.5 In contrast, intermolecular interactions between
hydrogen bond donating catalysts and weak hydrogen
bond accepting additives can occur at low concentra-
tions but typically attenuate the acidity of the former,
leading to slower but better controlled catalysis.6 Herein,
we report a case where weak hydrogen bond accepting
additives dramatically increase the rate of a Brønsted
acid catalyzed reaction. Specifically, electron-rich nitro
compounds, even when present as substoichiometric
additives, act as co-catalysts in the Brønsted acid cata-
lyzed azidation of tertiary aliphatic alcohols, enabling a
B(C6F5)3•H2O (5 mol%)
TMSN3 (3 equiv)
OTMS
Me
N3
OH
Me
Ph
Ph
(1)
Ph
Me
MeNO2, 23 °C, 1 h
Me
2a
Me
3a
80% yield
Me
1a
Direct submission of silyl ether 2a to the reaction con-
ditions also yielded 3a with similar efficiency. Attempts
in eleven other common polar and non-polar aprotic
solvents either did not result in any reaction or produced
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Table 1. Substoichiometric catalytic dehydroazidation of
tertiary aliphatic alcohols.a,b
silyl ether 2a but gave <5% azide 3a after 1 h, though
1
2
3
4
5
6
7
8
detectable amounts (<10%) of 3a were found in CH2Cl2
and (CH2Cl)2. In contrast, all five other solvents tested
that contained a nitro group led to rapid progression of
the reaction after 1 h, even though in some cases their
dielectric constants are much lower than nitromethane
(Table S1).17 Azidation could even be carried out in
‘unreactive’ solvents when nitro compounds were em-
ployed as additives. To our delight, 50 mol% p-
nitroanisole was an effective co-catalyst for the azida-
tion of alcohol 1e in benzene, giving 65% isolated yield
after 24 h compared to <5% in the absence of co-catalyst
(eq 2).
B(C6F5)3•H2O (1 or 5 mol%)
RNO2 (50 mol% or as solvent)
OH
N3
R1
R3
R1
R3
TMSN3 (3 equiv)
R2
R2
23 °C
1a-y
3a-y
N3
Br
80% (R = Bn, 3a)
95%, 93%c,d,e (R = Me, 3b)
N3
Me
Me
R
9
Et
85% (R = C8H17, 3c)
2 Et
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87%, 65%c,d (R = H, 3e)
90% (R = Br, 3f)
R
N3
64% (3d)
Me
89% (R = OTIPS, 3g)
69%d (R = OTES, 3h)
44%f (R = NO2, 3i)
2 Me
N3
n-Bu
n-Bu
Me
HN
N3
81%, 92%c,d (n = 1, 3j)
52% (n = 3, 3k)
75%, 70%c,d (3l)
B(C6F5)3•H2O (1 mol%)
p-OMeC6H4NO2 (50 mol%)
TMSN3 (3 equiv)
Me
n Me
N3
OH
Me
Ph
N3
N3
N3
N3
(2)
Ph
2
2
Me
C6H6, 23 °C, 24 h
Me
3e
65% yield
Me
Ph
Me
Me
Me
Me
2 Me
Me
2 Me
Me
Me
1e
Me
Me
46%g (3m)
Me N3
73% (3o)
58% (3p)
70% (3n)
A wide variety of tertiary aliphatic alcohols were sur-
veyed using nitromethane as solvent or using 50 mol%
nitroanisole in C6H6 (Table 1). Moderate to excellent
yields could be obtained in most cases. Reactions were
completed in 5-60 min when nitromethane was the sol-
vent. Reactions employing 50 mol% p-nitroanisole in
C6H6 completed in <24 h and were typically less effi-
cient due to faster competing elimination in that solvent.
Notably, acid-sensitive functional groups such as OTIPS
(3g) and OTES (3h) ethers survive under the reaction
conditions. Nitro-containing substrate 3i reacts even in
the absence of nitro additive. Azidation dominates over
intramolecular carbocation capture for alcohols bearing
pendant indoles (3j, 3k). No bis-azidation was observed
in alcohols bearing alkenes (3m-3s). Finally, diols un-
dergo azidation selectively at the tertiary position even
when competing cyclization is possible (3x, 3y). The
utility of the method is emphasized by the fact that, de-
spite the apparent simplicity of the azides reported here-
in, 18 of 25 are new compounds.
H
N3
Me
N3
Me Me
H
H
Me
Me
Me
Me
Me
H
Me
Me
Me
Me
Me
Me
53% (3q)
58% (3r)
74%, 42%c,d (3s)
Me
N3
N3
Me
n-Bu N3
N3
Me
Me
Me
77%, 31%c,d (3t)
87%, 81%c,d (3u)
45% (3v)
OH N3
92% (3w)
N3
HO
Me
3 Me
Me
5 Me
n-Bu
64%h (3x)
70% (3y)
aConditions: Alcohol (2 M in MeNO2), 5–60 min. bIsolat-
c
ed yields after silica gel chromatography. 50 mol% of p-
d
e
nitroanisole, C6H6, 24 h. 1 mol% B(C6F5)3ŸH2O. Yield
estimated by 1H NMR using 1.0 equiv of anisole as internal
standard. Neat, 1 h. Reaction carried out at 0 °C. Reac-
tion carried out at 90 °C.
f
g
h
Electronic effects and kinetic concentration dependen-
cies were studied to probe the origin of the accelerating
effect of the nitro compounds. A study of the initial rate
of the azidation of silyl ether 2e as a function of the
electronics of the nitro co-catalyst (100 mol%) in ben-
zene shows that electron-rich nitrobenzenes have a much
larger promoting effect than electron-poor analogs (Fig-
ure 1). This trend is opposite to the expected outcome if
small changes in dielectric constant due to the additive
were responsible for the faster rates, since p-
methoxynitrobenzene (ε = 27.5) is less polar than nitro-
benzene (ε = 36.1).17 The method of initial rates was
used to determine the order of the individual compo-
nents for the reaction of 2e in benzene. The slow back-
ground reaction that occurs in the absence of nitro addi-
tive is first order in acid (0.98±0.02). In contrast, reac-
tions in benzene in the presence of nitromethane reveal
a second-order (1.96±0.04) rate dependence on the
concentration of nitro compound, an approximately
second-order (1.8±0.1) rate dependence on the concen-
tration of acid, an approximately first-order (0.83±0.02)
rate dependence on the concentration of silyl ether and a
zero-order (-0.025±0.002) rate dependence on the con-
centration of TMSN3, demonstrating that the nitro com-
pound is a co-catalyst and induces a change in the con-
centration dependence of the Brønsted acid.
IR experiments show evidence of hydrogen bonding
between the OH of B(C6F5)3ŸH2O and the nitro com-
pounds. The magnitudes of the changes in frequency and
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B(C6F5)3•H2O (1 mol%)
TMSN3 (2 equiv)
RNO2 (100 mol%)
Ph
OTMS
Me
Ph
N3
1
2
3
Me
Me
Me
C6H6 (2.0 M), 23 °C
2e
3e
4
5
6
7
8
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Figure 1. Initial rate dependence on the electronics of the
nitro compound. Acid loading was reduced to slow the
reaction for GC-MS analysis, resulting in a smaller rate
increase relative to the background reaction.
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broadness of the OH stretch increase with greater elec-
tron-rich character of the nitro compound, correlating
with their co-catalytic activity. The B(C6F5)3ŸH2O OH
stretch at 3546 cm-1 in toluene shifts to 3523, 3480 and
3465 cm-1 in the presence of p-CF3-nitrobenzene,
PhNO2 and p-nitroanisole, respectively. The observed
peak broadening in the case of electron-rich nitro com-
pounds indicates that a larger number of intermolecular
interactions between B(C6F5)3ŸH2O and the nitro com-
pounds are accessible, an observation characteristic of
aggregation (Figure 2, a). NMR experiments (1H, 19F,
29Si and 11B) in C6D6 also support the existence of hy-
drogen-bonding between B(C6F5)3ŸH2O and nitro com-
pounds. Most notably in the 19F NMR, the signal corre-
sponding to the para-F resonance of B(C6F5)3ŸH2O
shifts significantly in the presence of nitro compounds
(see SI). The influence of nitro compounds on the acti-
vation of TMSN3 by B(C6F5)3ŸH2O can be observed by
monitoring the azide stretch of the latter at 2136 cm-1.
Mixing catalytic amounts of B(C6F5)3ŸH2O and 1 equiv
TMSN3 in toluene results in a slight decrease of intensi-
ty of the peak at 2136 cm-1 and the appearance of a new
peak at 2200 cm-1. In the presence of PhNO2, the new
peak shifts slightly to 2207 cm-1. The peak grows with
increasing concentration of B(C6F5)3ŸH2O at the ex-
pense of the peak at 2136 cm-1 (Figure 2, b). No interac-
tions between PhNO2 and TMSN3 were observed in the
absence of acid. Mixing all three components had no
effect on the N-O stretch of PhNO2, ruling out the pos-
sibility that the nitro compound becomes silylated and
functions as a silyl transfer catalyst. Taken together, the
kinetic, electronic and spectroscopic evidence suggests
that mixed hydrogen-bonded aggregates of Brønsted
acid and nitro compound catalyze the reaction, and that
the catalytically competent Brønsted acid likely con-
tains two molecules of each component. We suspect
Figure 2. Stacked IR spectra showing the (a) effect of
various nitro compounds on the OH stretch of
B(C6F5)3ŸH2O (b) effect of B(C6F5)3ŸH2O and nitrobenzene
on the azide stretch of TMSN3.
that similar phenomenon may be responsible for the
second order concentration dependence on HCl in Pock-
er’s olefin hydrochlorination in nitromethane11 and may
account for the high specificity for that solvent in other
acid-catalyzed reactions.18
In conclusion, we have uncovered an unprecedented
example of co-catalysis between nitro compounds and
Brønsted acids, resulting in the first azidation of tertiary
aliphatic alcohols with catalytic turnover. Our results
serve as a reminder that ‘simple’ Brønsted acid catalysis
can be surprisingly complex in solution, illustrating the
need for mechanistic investigations. Furthermore, ag-
gregation between weak H-bond acceptors and Brønsted
acids may constitute a new modular and tunable ap-
proach to accelerating Brønsted acid catalysis in solu-
tion. Theoretical calculations are underway to gain in-
sight into the nature of the aggregated species. Ongoing
experimental efforts are aimed at applications to other
reactions and the design of second-generation co-
catalysts.
ASSOCIATED CONTENT
Supporting Information. Table S1, additional control
experiments, kinetic data, IR data, experimental procedures
and characterization data for all new compounds.
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(10) For catalytic azidation of allylic alcohols and benzylic silyl
AUTHOR INFORMATION
1
2
3
4
ethers, respectively, see: (a) Rueping, M.; Vila, C.; Uria, U. Org. Lett.
2012, 14, 768–771. (b) Sawama, Y.; Nagata, S.; Yabe, Y.; Morita, K.;
Monguchi, Y.; Sajiki, H. Chem. Eur. J. 2012, 18, 16608–16611.
(11) (a) Pocker, Y. J. Chem. Soc. 1960, 1292–1297. (b) Pocker, Y.;
Stevens, K. D.; Champoux, J. J. J. Am. Chem. Soc. 1969, 91, 4199–
4205. (c) Pocker, Y.; Stevens, K. D. J. Am. Chem. Soc. 1969, 91,
4205–4210.
Corresponding Author
moran@unistra.fr
Notes
5
‡These authors contributed equally.
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7
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(12) For a report on rate acceleration of zeolite catalysis in nitro-
methane, see: Haw, J. F.; Nicholas, J. B.; Xu, T.; Goguen, P. W.
Nature 1997, 389, 832–835.
The authors declare no competing financial interest.
ACKNOWLEDGMENT
(13) For spectroscopic observation of intermolecular hydrogen-
bonding with nitro compounds, see: (a) Baitinger, W. F.; Schleyer, P.
von R.; Murty, T. S. S. R.; Robinson, L. Tetrahedron 1964, 20, 1635–
1647. (b) Ungnade, H. E.; Roberts, E. M.; Kissinger, L. W. J. Phys.
Chem. 1964, 68, 3225–3228. (c) Etter, M. C.; Urbanczyk-Lipkowska,
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8415–8426. (d) Laurence, C; Berthelot, M.; Lucon, M.; Morris, D. G.
J. Chem. Soc., Perkin Trans. 2 1994, 491–493.
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We thank Dr. Marco Cecchini for fruitful discussions. This
work was supported in part by grants from LabEx CSC and
the Marie Curie Actions FP7-PEOPLE Program (CIG-
2012-326112). M.D. thanks the Région Alsace. M.H.
thanks the Marie Curie Actions FP7-PEOPLE Program
(IEF-2012-326113). E.W. thanks the French government
for a MRT fellowship. F.C.F. thanks the French Embassy
to Germany for a fellowship.
(14) For a seminal example of dual hydrogen bond catalysis of a
nitroalkene, see: Okino, T.; Hoashi, Y.; Takemoto, Y. J. Am. Chem.
Soc. 2003, 125, 12672–12673.
(15) B(C6F5)3ŸH2O has a pKa of 8.4 in MeCN and has been studied
in detail, see: (a) Danopoulos, A. A.; Galsworthy, J. R.; Green, M. L.
H.; Cafferkey, S.; Doerrer, L. H.; Hursthouse, M. B. Chem. Commun.
1998, 2529–2530. (b) Bergquist, C.; Bridgewater, B. M.; Harlan, C.
J.; Norton, J. R.; Friesner, R. A.; Parkin, G. J. Am. Chem. Soc. 2000,
122, 10581–10590. (c) Beringhelli, T.; Maggioni, D.; D'Alfonso, G.
Organometallics 2001, 20, 4927–4938. (d) Di Saverio, A.; Focante,
F.; Camurati, I.; Resconi, L.; Beringhelli, T.; D'Alfonso, G.; Donghi,
D.; Maggioni, D.; Mercandelli, P.; Sironi, A. Inorg. Chem. 2005, 44,
5030–5041.
(16) TfOH and Bi(OTf)3 were also effective catalysts, but
B(C6F5)3ŸH2O provided the optimal yield by minimizing competing
elimination and silyl ether cleavage (Table S1). No reaction was
observed in the absence of acids. Lewis acids (eg. Bi(OTf)3) and
protic compounds can form strong Brønsted acids in situ. The addi-
tion of 20 mol% Proton Sponge completely inhibits azidation by
Lewis acids and by Brønsted acids.
(17) (a) Reichardt, C. Solvents and Solvent Effects in Organic
Chemistry, 3rd updated and enlarged ed.; Wiley-VCH: Weinheim,
2003. (b) Laurence, C.; Nicolet, P.; Dalati, M. T.; Abboud, J.-L. M.;
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