Copper(II)-catalyzed conversion of aryl/heteroaryl boronic acids, boronates, and trifluoroborates into the corresponding azides: Substrate scope and limitations

Article abstract of DOI:10.1055/s-0029-1218683

We report the copper(II)-catalyzed conversion of organoboron compounds into the corresponding azide derivatives. A systematic series of phenylboronic acid derivatives is evaluated to examine the importance of steric and electronic effects of the sub-stituents on reaction yield as well as functional group compatibility. Heterocyclic substrates are also shown to participate in this mild reaction while compounds incorporating B-C(sp³) bonds are unreactive under the reaction conditions. The copper(II)-catalyzed boronic acid-azide coupling reaction is further extended to both boronate esters and potassium organotrifluoroborate salts. The method described herein complements existing procedures for the preparation of aryl azides from the respective amino, triazene, and halide derivatives and we expect that it will greatly facilitate copper- and ruthenium-catalyzed azide-alkyne cycloaddition reactions for the preparation of diversely functionalized 1-aryl- or 1-heteroaryl-1,2,3- triazoles derivatives. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0029-1218683

NIH Public Access
Author Manuscript
Synthesis (Stuttg). Author manuscript; available in PMC 2010 November 1.
Published in final edited form as:
Synthesis (Stuttg). 2010 May 1; 2010(9): 1441–1448. doi:10.1055/s-0029-1218683.
Copper(II)-Catalyzed Conversion of Aryl/Heteroaryl Boronic
Acids, Boronates, and Trifluoroborates into the Corresponding
Azides: Substrate Scope and Limitations
Kimberly D. Grimes, Amol Gupte, and Courtney C. Aldrich
Center for Drug Design, Academic Health Center, University of Minnesota, Minneapolis, MN 55455,
USA, Fax +1(612)6265173; aldri015@umn.edu
Abstract
We report the copper(II)-catalyzed conversion of organoboron compounds into the corresponding
azide derivatives. A systematic series of phenylboronic acid derivatives is evaluated to examine the
importance of steric and electronic effects of the substituents on reaction yield as well as functional
group compatibility. Heterocyclic substrates are also shown to participate in this mild reaction while
3
compounds incorporating B–C(sp ) bonds are unreactive under the reaction conditions. The copper
(II)-catalyzed boronic acid–azide coupling reaction is further extended to both boronate esters and
potassium organotrifluoroborate salts. The method described herein complements existing
procedures for the preparation of aryl azides from the respective amino, triazene, and halide
derivatives and we expect that it will greatly facilitate copper- and ruthenium-catalyzed azide–alkyne
cycloaddition reactions for the preparation of diversely functionalized 1-aryl- or 1-heteroaryl-1,2,3-
triazoles derivatives.
Keywords
azides; organoboron compounds; copper catalysis; 1,2,3-triazoles
Aryl and heteroaryl azides have emerged as valuable building blocks for the construction of
1,2
1,2,3-triazoles in combinatorial drug discovery. Traditionally, aryl azides have been
prepared from the corresponding amines via diazotization. Moses and co-workers reported an
improved diazotization procedure utilizing tert-butyl nitrite and azidotrimethylsilane under
mild conditions while Bräse and co-workers described a mild synthesis of aryl azides from
3,4
aryltriazenes. Ma and Zhu demonstrated the synthesis of both aryl and vinyl azides from the
5
respective aryl halides by proline–copper(I)-catalyzed coupling with sodium azide. Pinhey
and co-workers first reported the conversion of arylboronic acids into aryl azides using
stoichiometric lead(IV) acetate and mercury(II) acetate to afford intermediate aryl–lead
6,7
species, which were then reacted with sodium azide in dimethyl sulfoxide. In a significant
advance, Guo, Liu, and co-workers recently showed that simple arylboronic acids can be
8
elaborated to aryl azides using copper salts at room temperature. Herein, we disclose our
results that examine in much greater detail the substrate scope of this powerful transformation
including the influence of ligands, impact of electronics of the substrate on coupling efficiency,
functional group compatibility, utilization of boronate esters and organotrifluoroborate salts,
and examination of various heterocyclic substrates. Our results demonstrate the broad
applicability and identify limitations of the copper(II)-catalyzed boronic acid–azide coupling
reaction.
Correspondence to: Courtney C. Aldrich.

Grimes et al.
Page 2
The copper(II)-catalyzed synthesis of aryl azides from arylboronic acids is based on the Chan–
Lam coupling of arylboronic acids with N–H containing heteroarenes, anilines, phenols, and
amides that employs stoichiometric copper(II) acetate in aprotic solvents and requires a base
9,10
such as pyridine or triethylamine.
Since sodium azide is sparingly soluble in most aprotic
organic solvents, we examined protic solvents and discovered that in contrast to the Chan–Lam
coupling conditions, the azidation of 4-cyanophenylboronic acid (1q) proceeds efficiently in
water, methanol, and ethanol (Table 1). Additionally, we found removal of the obligate base
(pyridine or triethylamine) in the Chan–Lam coupling improved the yield (Table 1, entries 4
and 7). The ligands 2,2′-bipyridine (Bipy) and 1,10-phenanthroline (Phen) were also evaluated
as these have been shown to enhance copper(II)-catalyzed cross-coupling reactions; however,
these provided no major benefit in terms of isolated yield or enhancement in the reaction rate
(Table 1, entries 5 and 6). If the reaction was conducted under an aerobic atmosphere to
reoxidize Cu(0) back to Cu(II), then copper could be used catalytically (Table 1, entry 8). Our
standardized reaction conditions involved vigorously stirring a 0.2 M solution of arylboronic
acid in methanol with sodium azide (1.5 equiv) and copper(II) acetate (0.1 equiv) at 55 °C for
1–3 hours under air (Table 1, entry 9). The higher temperature used herein allow the reactions
to reach completion in 1–3 hours, but the reaction can also be performed at room temperature
if the reaction time is extended to 24 hours, which is preferred when working on scales larger
8
than several millimoles. The reaction was conveniently followed colorimetrically as the dark
brown solution turned light green as the reaction reached completion.
A systematic series of para-substituted phenylboronic acids was initially examined to explore
the influence of electron-donating and -withdrawing substituents on coupling efficiency (Table
2). Remarkably, all compounds underwent coupling providing 40–98% yields of the
corresponding aryl azides. While coupling efficiency did not correlate with electron-donating
11
ability as measured by the Hammett substituent constant σ , we observed that arylboronic
p
acids with strongly electron-withdrawing groups reacted more slowly, requiring three hours to
reach completion and compounds with electron-donating groups, such as methoxy, were
complete within one hour. In the case of volatile azides, such as phenyl azide (2e), 1-azido-4-
fluorobenzene (2g), and 1-azido-4-(trifluoromethyl)benzene (2p), low isolated yields were
obtained, therefore these were isolated as the corresponding triazole derivatives 3e, 3g, and
3p, respectively, by reacting the azides with methyl propiolate at room temperature. Overall,
the results highlight the functional group compatibility of this reaction. The impact of sterics
was examined with 2-(pivaloylamino)phenylboronic acid (1v) that provided 2v in 94% yield
while biphenyl-2- (1u), -3- (1t), and -4-boronic acid (1c) provided 2u, 2t, and 2c, respectively,
in yields ranging from 40–50% illustrating the relative lack of sensitivity to sterics (Table 2).
Next, arylboronic acid pinacol esters were evaluated. 4-Cyanophenylboronic acid pinacol ester
(4q) furnished azide 2q in 81% yield (Table 3, entry 1), while 2q was obtained in 98% yield
from the corresponding boronic acid derivative 1q (Table 2, entry 17). The successful coupling
of amino- and dimethylamino-substituted phenylboronic acid pinacol esters4w and 4x to afford
azides 2w and 2x further highlights the functional group compatibility of this reaction (Table
3). Examination of heterocyclic substrates revealed some limitations to this reaction as 4-
azidopyrazole 2y and 5-azidoindole 2z were isolated in only modest yields.
Heterocyclic boronic acids were also examined including pyridine, pyrimidine, quinoline, and
isoquinoline 5a–d and the corresponding azides 6a–d were isolated in yields ranging from 34–
3
42% (Table 4). Finally, several substrates containing B–C(sp ) bonds were examined including
cyclopropyl 5e, cyclohexyl 5f, and benzyl 5g; however, no coupling was observed with any of
these substrates demonstrating the limitations of the current method, which is restricted to
2
substrates with B–C(sp ) bonds.
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Grimes et al.
Page 3
Potassium organotrifluoroborate salts have emerged as attractive reagents in organic synthesis
due to their increased chemical stability, excellent reactivity, and improved physicochemical
12
properties. Therefore we examined whether this valuable class of organoboron reagents
would participate in the copper(II)-catalyzed cross-coupling reaction with sodium azide. A
subset of the aryltrifluoroborates (Table 5) was selected in order to directly compare the results
with the corresponding boronic derivatives from Table 1. The aryltrifluoroborates were
competent substrates, but provided lower yields of the azides in all cases and were slightly less
reactive than the respective arylboronic acids.
The Chan–Lam coupling of arylboronic acids with azides as first disclosed by Guo, Liu, and
co-workers represents a powerful and more environmentally friendly method for the
8
preparation of various aryl and heteroaryl azides. Another major advantage of this method is
the compatibility with the conditions for the copper(I)-catalyzed azide–alkyne cycloaddition
(CuAAC) reaction, thus enabling one to sequentially perform both reactions in the same vessel.
13,14
Notably, the Chan–Lam coupling is a copper(II) catalytic cycle while the CuAAC reaction
requires a copper(I) cycle. At the completion of the copper(II)–boronic acid–azide coupling,
the addition of an alkyne and a reducing agent (aqueous solution of sodium ascorbate) results
in a facile CuAAC reaction. The product azides in general are nonpolar and can be easily
purified by filtration through a short silica gel column to remove all inorganic salts and
potentially unreacted organoboron substrates. In almost all cases, except for heterocyclic
compounds, the reaction as monitored by thin-layer chromatography resulted in complete
consumption of the polar organoboranes followed by the formation of the more nonpolar aryl
azide with no observed byproducts. The mass balance for compounds that resulted in less than
90% yield is likely due to losses obtained during purification of these volatile low molecular
weight (MW < 150) azide derivatives. While we primarily investigated the coupling of
arylboronic acids with sodium azide using catalytic copper(II) salts, the stoichiometric version
may be preferred in common parallel synthesis reactors due to lack of turnover of the air
headspace. Additionally, stoichiometric copper is more desirable for volatile organic azides as
reaction can be done in a sealed flask.
The proposed catalytic cycle for the Chan–Lam coupling of organoboranes with the azide ion
is shown in Scheme 1 and consists of an initial transmetalation to afford a R–Cu(II)
intermediate. In the second step, the azide anion coordinates to the R–Cu(II) complex providing
R–Cu(II)–N that subsequently undergoes reductive elimination to afford the organic azide
3
and Cu(0). Oxidation of Cu(0) to Cu(II) by molecular oxygen completes the catalytic cycle.
The successful copper-catalyzed coupling of aryl iodides with sodium azide as reported by Zhu
and Ma, demonstrated that aryl–Cu(II)–azido complexes could undergo reductive elimination
5
and immediately suggested the viability of the Chan–Lam coupling with the azide anion.
Reoxidation of Cu(0) to Cu(II) is slow and can be rate-limiting if the reaction vessel is sealed.
Alternatively, it is plausible that oxidation of the aryl–Cu(II)–azido complex to a Cu(III)
species would serve to promote reductive elimination to afford the product aryl azide and a Cu
(I) species, which could be rapidly oxidized to Cu(II). This mechanism has been proposed for
15
the Chan–Lam coupling. Furthermore, the first Cu(III) intermediate, which was identified
16
from conjugate addition to an enone, has been recently spectroscopically characterized.
In closing, it is important to note the excellent functional group tolerance of aryl azides and
boronic acids. Thus, 1-azido-2-bromobenzene has been reported to successfully participate in
17
Suzuki cross-coupling with arylboronic acids. In an even more impressive example, Ham,
Molander, and co-workers demonstrated the chemoselective azidation of bromo- and
iodoaryltrifluoroborates to form (azidoaryl)trifluoroborates employing catalytic copper(I)
18
bromide followed by Suzuki cross-coupling of these bifunctional molecules. The use of a
copper(I) salt by these authors led to azidation of the halogen while the use of copper(II) salts
Synthesis (Stuttg). Author manuscript; available in PMC 2010 November 1.

Grimes et al.
Page 4
reported herein results in the azidation of the trifluoroborate moiety. These complimentary
reaction outcomes highlight the critical role of the copper oxidation state.
All commercial reagents (Sigma-Aldrich, Acros, Boron Molecular) were used as provided.
®
Flash chromatography was performed with an ISCO Combiflash Companion purification
1
system with pre-packed 4 g silica gel cartridges with the indicated solvent system. H
13
and C NMR spectra were recorded on a Varian 600 MHz spectrometer. Chemical shifts are
1
reported in ppm from an internal standard of residual methanol (δ = 3.31 ppm for H NMR
13
and δ = 49.1 ppm for C NMR). Proton chemical data are reported as follows: chemical shift,
multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet), coupling constant,
integration. FT-IR spectra were obtained on a Nicolet Protégé 460 ESP spectrometer. The
presence of an azido group was confirmed by a characteristic strong infrared absorption at
−1
2100–2150 cm . High-resolution mass spectra were acquired on an Agilent TOF II TOF/MS
instrument equipped with either an ESI or APCI interface. Azides that failed to ionize under
ESI or APCI conditions were treated with Ph P in THF at 55 °C for 2 h to form the
3
corresponding iminophosphoranes (R–N=PPh ), which were subsequently characterized by
3
HRMS. Melting points were measured on an electrothermal Mel-Temp manual melting point
apparatus and are uncorrected. CAUTION: Organic azides are potentially explosive substances
and the proposed synthesis employing elevated temperature should only be run on scales less
than 1 mmol.
Azide Synthesis; General Procedure
To the boronic acid (1.0 mmol, 1.0 equiv), pinacol boronate (1.0 mmol, 1.0 equiv), or potassium
organotrifluoroborate salt (1.0 mmol, 1.0 equiv) in MeOH (5 mL) were added NaN (1.5 mmol,
3
1.5 equiv) and Cu(OAc) (0.1 mmol, 0.1 equiv). The soln was vigorously stirred in an 8-mL
2
round-bottom vial at 55 °C under air. The mixture was concentrated onto Celite and purification
by flash chromatography (prepacked silica cartridges) afforded the title compounds.
1-Azido-4-methoxybenzene (2a)
Following the general procedure using 4-methoxyphenylboronic acid (1a) or potassium
trifluoro(4-methoxyphenyl)borate (7a) for 3 h and chromatography (10% EtOAc–hexane)
afforded 2a (110 mg, 74% from 1a; 105 mg, 70% from 7a) as a yellow oil; R = 0.6 (hexane–
f
EtOAc, 9:1).
−1
IR: 2109 cm .
1
H NMR (600 MHz, CD OD): δ = 3.76 (s, 3 H), 6.92 (d, J = 9.0 Hz, 2 H), 6.96 (d, J = 9.0 Hz,
3
2 H).
13
C NMR (150 MHz, CD OD): δ = 56.1, 116.4, 121.1, 133.7, 158.8.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for C H NOP:
2
3
25 22
384.1512; found: 384.1535.
1-Azido-4-phenoxybenzene (2b)
Following the general procedure using 4-phenoxyphenylboronic acid (1b) for 3 h and
chromatography (hexane) afforded 2b (182 mg, 86%) as a brown oil: R = 0.2 (hexane).
f
−1
IR: 2123 cm .
1
H NMR (600 MHz, CD OD): δ = 6.92–6.97 (m, 6 H), 7.06 (t, J = 7.2 Hz, 1 H), 7.27–7.30
3
(m, 2 H).
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Grimes et al.
Page 5
13
C NMR (150 MHz, CD OD): δ = 119.7, 121.4, 124.3, 124.5, 130.9, 136.4, 155.9, 158.7.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for C H NOP:
2
3
30 24
446.1668; found: 446.1707.
4-Azidobiphenyl (2c)
Following the general procedure using biphenyl-4-ylboronic acid (1c) for 3 h and
chromatography (hexane) afforded 2c (100 mg, 51%) as a yellow solid; mp 62–64 °C; R = 0.3
f
(hexane).
−1
IR: 2115 cm .
1
H NMR (600 MHz, CD OD): δ = 7.12 (d, J = 7.8 Hz, 2 H), 7.32 (t, J = 7.2 Hz, 1 H), 7.41 (t,
3
J = 7.2 Hz, 2 H), 7.58 (d, J = 7.8 Hz, 2 H), 7.63 (d, J = 7.8 Hz, 2 H).
13
C NMR (150 MHz, CD OD): δ = 120.6, 127.9, 128.5, 129.6, 130.1, 139.5, 140.7, 141.5.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for C H NP:
2
3
30 24
430.1719; found: 430.1756.
1-Azido-4-(hydroxymethyl)benzene (2d)
Following the general procedure using 4-(hydroxymethyl)phenylboronic acid (1d) or
potassium trifluoro[4-(hydroxymethyl)phenyl]borate (7d) for 3 h and chromatography (Et O)
2
afforded 2d (148 mg, 99% from 1d; 60 mg, 40% from 7d) as a yellow oil; R = 0.3 (hexane–
f
EtOAc, 7:3).
−1
IR: 2107 cm .
1
H NMR (600 MHz, CD OD): δ = 4.57 (s, 2 H), 7.03 (d, J = 8.4 Hz, 2 H), 7.37 (d, J = 8.4 Hz,
3
2 H).
13
C NMR (150 MHz, CD OD): δ = 64.7, 120.0, 129.6, 139.9, 140.5.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for C H NOP:
2
3
25 22
384.1512; found: 384.1512.
Methyl 1-Phenyl-1H-1,2,3-triazole-4-carboxylate (3e)
Following the general procedure using phenylboronic acid (1e) for 1.5 h afforded crude 2e.
Methyl propiolate (3.0 equiv) and sodium ascorbate (0.1 equiv) were added and the mixture
was stirred at r.t. overnight. Chromatography (30% EtOAc–hexane) afforded 3e (130 mg, 64%)
as a white solid; mp 178–182 °C; R = 0.3 (hexane–EtOAc, 7:3).
f
1
H NMR (600 MHz, CD OD): δ = 3.96 (s, 3 H), 7.53 (t, J = 7.8 Hz, 1 H), 7.60 (t, J = 7.8 Hz,
3
2 H), 7.89 (d, J = 7.8 Hz, 2 H), 9.11 (s, 1 H).
13
C NMR (150 MHz, CD OD): δ = 52.8, 122.0, 127.9, 129.9, 131.2, 138.0, 141.4, 161.1.
3
+
HRMS (ESI+): m/z [M + H] calcd for C H N O : 204.0768; found: 204.0790.
10 10
3 2
1-Azido-4-(methylthio)benzene (2f)
Following the general procedure using 4-(methylthio)boronic acid (1f) or potassium trifluoro
[4-(methylthio)phenyl]borate (7f) for 3 h and chromatography (Et O) afforded 2f (162 mg,
2
98% from 1f; 110 mg, 67% from 7f) as a yellow oil; R = 0.6 (hexane–EtOAc, 9:1).
f
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Grimes et al.
Page 6
−1
IR: 2115 cm .
1
H NMR (600 MHz, CD OD): δ = 2.45 (s, 3 H), 6.98 (d, J = 8.4 Hz, 2 H), 7.28 (d, J = 8.4 Hz,
3
2 H).
13
C NMR (150 MHz, CD OD): δ = 16.4, 120.6, 129.6, 136.6, 138.6.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for C H NPS:
2
3
25 22
400.1283; found: 400.1296.
Methyl 1-(4-Fluorophenyl)-1H-1,2,3-triazole-4-carboxylate (3g)
Following the general procedure using 4-fluorophenylboronic acid (1g) for 1.5 h afforded crude
2g. Methyl propiolate (3.0 equiv) and sodium ascorbate (0.1 equiv) were added and the mixture
was stirred at r.t. overnight. Chromatography (20% EtOAc–hexane) afforded 3g (148 mg, 67%)
as a yellow solid; mp 185–188 °C; R = 0.4 (hexane–EtOAc, 7:3).
f
1
H NMR (600 MHz, CD OD): δ = 3.96 (s, 3 H), 7.35 (d, J = 8.4 Hz, 2 H), 7.93 (d, J = 8.4 Hz,
3
2 H), 9.10 (s, 1 H).
13
2
3
C NMR (150 MHz, CD OD): δ = 52.8, 117.9 (d, J = 24 Hz), 124.4 (d, J = 8.9 Hz),
3
C-F
C-F
1
128.2, 136.8, 141.5, 162.4, 164.5 (d, J = 247 Hz).
C-F
+
HRMS (ESI+): m/z [M + H] calcd for C H FN O : 222.0673; found: 222.0675.
10
9
3 2
1-Azido-4-iodobenzene (2h)
Following the general procedure using 4-iodophenylboronic acid (1h) for 3 h and
chromatography (hexane) afforded 2h (157 mg, 64%) as a yellow oil; R = 0.8 (hexane–EtOAc,
f
8:2).
−1
IR: 2125 cm .
1
H NMR (600 MHz, CD OD): δ = 6.87 (d, J = 7.2 Hz, 2 H), 7.70 (d, J = 7.2 Hz, 2 H).
3
13
C NMR (150 MHz, CD OD): δ = 89.0, 122.3, 140.2, 141.7.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for C H INP:
2
3
24 19
480.0373; found: 480.0417.
1-Azido-4-chlorobenzene (2i)
Following the general procedure using 4-chlorophenylboronic acid (1i) or potassium 4-
chlorophenyltrifluoroborate (7i) for 3 h and chromatography (Et O) afforded 2i (117 mg, 76%
2
from 1i; 70 mg, 46% from 7i) as a brown oil; R = 0.7 (hexane–EtOAc, 9:1).
f
−1
IR: 2106 cm .
1
H NMR (600 MHz, CD OD): δ = 7.04 (d, J = 8.4 Hz, 2 H), 7.37 (d, J = 8.4 Hz, 2 H).
3
13
C NMR (150 MHz, CD OD): δ = 89.0, 122.3, 140.2, 141.7.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for C H ClNP:
2
3
24 19
388.1016; found: 388.1024.
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Grimes et al.
Page 7
1-Azido-4-bromobenzene (2j)
Following the general procedure using 4-bromophenylboronic acid (1j) for 3 h and
chromatography (Et O) afforded 2j (143 mg, 72%) as a yellow oil; R = 0.7 (hexane–EtOAc,
2
f
9:1).
−1
IR: 2108 cm .
1
H NMR (600 MHz, CD OD): δ = 6.99 (d, J = 8.4 Hz, 2 H), 7.51 (d, J = 8.4 Hz, 2 H).
3
13
C NMR (150 MHz, CD OD): δ = 118.8, 122.0, 134.0, 141.0.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for C H BrNP:
2
3
24 19
432.0511; found: 432.0533.
4-Azidobenzamide (2k)
Following the general procedure using 4-carbamoylphenylboronic acid (1k) for 3 h and
chromatography (70% EtOAc–hexane) afforded 2k (131 mg, 81%) as a white amorphous solid;
mp 167–169 °C; R = 0.2 (hexanes–EtOAc, 6:4).
f
−1
IR: 2097 cm .
1
H NMR (600 MHz, CD OD): δ = 7.14 (d, J = 8.4 Hz, 2 H), 7.90 (d, J = 8.4 Hz, 2 H).
3
13
C NMR (150 MHz, CD OD): δ = 120.0, 130.7, 131.6, 145.3, 171.8.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for
2
3
C H N OP: 397.1464; found: 397.1479.
25 21
2
4-Azidobenzaldehyde (2l)
Following the general procedure using 4-formylphenylboronic acid (1l) for 3 h and
chromatography (5% EtOAc–hexane) afforded 2l (124 mg, 84%) as a brown oil; R = 0.3
f
(hexane–EtOAc, 9:1).
−1
IR: 2115 cm .
1
H NMR (600 MHz, CD OD): δ = 7.22 (d, J = 8.4 Hz, 2 H), 7.91 (d, J = 8.4 Hz, 2 H), 9.90 (s,
3
1 H).
13
C NMR (150 MHz, CD OD): δ = 120.7, 132.7, 134.9, 147.9, 192.7.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for C H NOP:
2
3
25 20
382.1355; found: 382.1393.
4-Azidobenzoic Acid (2m)
Following the general procedure using 4-carboxyphenylboronic acid (1m) or potassium 4-
carboxyphenyltrifluoroborate (7m) for 3 h. The mixture was treated with 1 M HCl (10 mL)
and extracted with EtOAc (3 × 20 mL). The combined extracts were washed with sat. aq NaCl,
dried (Na SO ), and concentrated in vacuo. Recrystallization of the crude mixture (EtOAc, 10
2
4
mL) afforded 2m (150 mg, 92% from 1m; 121 mg, 75% from 7m) as a yellow solid; mp 109–
115 °C; R = 0.3 (hexanes–EtOAc, 7:3).
f
−1
IR: 2110 cm .
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Grimes et al.
Page 8
1
H NMR (600 MHz, CD OD): δ = 7.15 (d, J = 9.0 Hz, 2 H), 8.04 (d, J = 9.0 Hz, 2 H).
3
13
C NMR (150 MHz, CD OD): δ = 120.1, 128.9, 132.8, 146.4, 169.2.
3
−
HRMS (ESI–): m/z [M – H] calcd for C H N O : 162.0309; found: 162.0323.
7
4 3 2
Methyl 4-Azidobenzoate (2n)
Following the general procedure using 4-(methoxycarbonyl)phenylboronic acid (1n) for 3 h
and chromatography (10% EtOAc–hexane) afforded 2n (175 mg, 99%) as a yellow oil; R =
f
0.7 (hexane–EtOAc, 7:3).
−1
IR: 2117 cm .
1
H NMR (600 MHz, CD OD): δ = 3.89 (s, 3 H), 7.16 (d, J = 8.4 Hz, 2 H), 8.03 (d, J = 8.4 Hz,
3
2 H).
13
C NMR (150 MHz, CD OD): δ = 52.8, 120.2, 128.2, 132.6, 146.6, 167.9.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for
2
3
C H NO P: 412.1461; found: 412.1477.
26 22
2
4′-Azidoacetophenone (2o)
Following the general procedure using 4-acetylphenylboronic acid (1o) or potassium 4-
acetylphenyltrifluoroborate (7o) for 3 h and chromatography (10% EtOAc–hexane) afforded
2o (134 mg, 83% from 1o; 117 mg, 73% from 7o) as a brown solid; mp 40–45 °C; R = 0.3
f
(hexane–EtOAc, 9:1).
−1
IR: 2128 cm .
1
H NMR (600 MHz, CD OD): δ = 2.55 (s, 3 H), 7.12 (d, J = 8.4 Hz, 2 H), 7.98 (d, J = 8.4 Hz,
3
2 H).
13
C NMR (150 MHz, CD OD): δ = 26.6, 120.1, 131.6, 135.1, 146.6, 198.9.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for C H NOP:
2
3
26 22
396.1512; found: 396.1552.
Methyl 1-[4-(Trifluoromethyl)phenyl]-1H-1,2,3-triazole-4-carboxylate (3p)
Following the general procedure using 4-(trifluoromethyl)phenylboronic acid (1p) for 3 h
afforded crude 2p. Methyl propiolate (3.0 equiv) and sodium ascorbate (0.1 equiv) were added
and the mixture was stirred at r.t. overnight. Chromatography (20% EtOAc–hexane) afforded
3p (184 mg, 68%) as a yellow solid; mp 193–196 °C; R = 0.3 (hexane–EtOAc, 7:3).
f
1
H NMR (600 MHz, CD OD): δ = 3.97 (s, 3 H), 7.94 (d, J = 8.4 Hz, 2 H), 8.15 (d, J = 8.4 Hz,
3
2 H), 9.27 (s, 1 H).
13
3
C NMR (150 MHz, CD OD): δ = 52.8, 122.4, 128.2, 128.3 (q, J = 4.8 Hz), 132.4
3
C-F
2
(d, J = 33.0 Hz), 140.7, 141.7, 162.2; CF signal not observed.
C-F
3
+
HRMS (ESI+): m/z [M + H] calcd for C H F N O : 272.0641; found: 272.0647.
11 9 3
3 2
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Page 9
4-Azidobenzonitrile (2q)
Following the general procedure using 4-cyanophenylboronic acid (1q) or 4-
cyanophenylboronic acid pinacol ester (4q) for 3 h and chromatography (Et O) afforded 2q
2
(141 mg, 98% from 1q; 118 mg, 82% from 4q) as a light brown solid; mp 56–59 °C; R = 0.4
f
(hexane–EtOAc, 9:1).
−1
IR: 2112 cm .
1
H NMR (600 MHz, CD OD): δ = 7.23 (d, J = 8.4 Hz, 2 H), 7.74 (d, J = 8.4 Hz, 2 H).
3
13
C NMR (150 MHz, CD OD): δ = 109.3, 119.5, 121.2, 135.2, 146.8.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for C H N P:
2
3
25 19 2
379.1359; found: 379.1370.
1-Azido-4-(methylsulfonyl)benzene (2r)
Following the general procedure using 4-(methylsulfonyl)phenylboronic acid (1r) for 3 h and
chromatography (10% EtOAc–hexane) afford 2r (166 mg, 84%) as a brown oil; R = 0.2
f
(hexane–EtOAc, 7:3).
−1
IR: 2132 cm .
1
H NMR (600 MHz, CD OD): δ = 3.11 (s, 3 H), 7.29 (d, J = 8.4 Hz, 2 H), 7.95 (d, J = 8.4 Hz,
3
2 H).
13
C NMR (150 MHz, CD OD): δ = 44.5, 120.9, 130.6, 138.2, 147.4.
3
+
HRMS (ESI+) (iminophosphorane adduct) : m/z [M − N + PPh + H] calcd for
2
3
C H NO PS: 432.1182; found: 432.1184.
25 22
2
1-Azido-4-nitrobenzene (2s)
Following the general procedure using 4-nitrophenylboronic acid (1s) for 3 h and
chromatography (Et O) afforded 2s (140 mg, 85%) as a yellow solid; mp 63–66 °C; R = 0.4
2
f
(hexane–EtOAc, 9:1).
−1
IR: 2128 cm .
1
H NMR (600 MHz, CD OD): δ = 7.27 (d, J = 8.4 Hz, 2 H), 8.28 (d, J = 8.4 Hz, 2 H).
3
13
C NMR (150 MHz, CD OD): δ = 120.9, 126.7, 146.2, 148.6.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for
2
3
C H N O P: 399.1257; found: 399.1251.
24 19
2 2
3-Azidobiphenyl (2t)
Following the general procedure using biphenyl-3-ylboronic acid (1t) for 3 h and
chromatography (hexane) afforded 2t (90 mg, 46%) as a colorless oil; R = 0.4 (hexane).
f
−1
IR: 2101 cm .
1
H NMR (600 MHz, CD OD): δ = 6.98 (d, J = 7.2 Hz, 1 H), 7.16 (s, 1 H), 7.30–7.34 (m, 2 H),
3
7.37 (m, 3 H), 7.52 (d, J = 8.4 Hz, 2 H).
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13
C NMR (150 MHz, CD OD): δ = 118.5, 118.8, 124.8, 128.1, 128.9, 130.0, 131.4, 141.4,
3
141.9, 144.4.
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for C H NP:
2
3
30 24
430.1719; found: 430.1757.
2-Azidobiphenyl (2u)
Following the general procedure using biphenyl-2-ylboronic acid (1u) for 3 h and
chromatography (hexane) afforded 2u (80 mg, 41%) as a colorless oil; R = 0.3 (hexane).
f
−1
IR: 2107 cm .
1
H NMR (600 MHz, CD OD): δ = 7.19 (t, J = 7.8 Hz, 1 H), 7.26 (d, J = 7.8 Hz, 1 H), 7.29–
3
7.32 (m, 2 H), 7.36–7.39 (m, 5 H).
13
C NMR (150 MHz, CD OD): δ = 120.1, 126.2, 128.5, 129.1, 130.0, 130.6, 132.3, 135.3,
3
138.4, 139.7.
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for C H NP:
2
3
30 24
430.1719; found: 430.1713.
1-Azido-N-pivaloylaniline (2v)
Following the general procedure using 2-(pivaloylamino)phenylboronic acid (1v) for 3 h and
chromatography (5% EtOAc–hexane) afforded 2v (207 mg, 95%) as a yellow oil; R = 0.5
f
(hexane–EtOAc, 9:1).
−1
IR: 2129 cm .
1
H NMR (600 MHz, CD OD): δ = 1.31 (s, 9 H), 7.11 (t, J = 7.8 Hz, 1 H), 7.17 (overlapping
3
d, J = 7.8 Hz, 1 H), 7.19 (overlapping t, J = 7.8 Hz, 1 H), 7.79 (d, J = 7.8 Hz, 1 H).
13
C NMR (150 MHz, CD OD): δ = 27.9, 40.7, 119.6, 126.2, 126.3, 127.2, 130.5, 133.2, 179.5.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for
2
3
C H N OP: 453.2090; found: 453.2116.
29 29
2
4-Azidoaniline (2w)
Following the general procedure using 4-aminophenylboronic acid pinacol ester (4w) for 24
h and chromatography (10% EtOAc–hexane) afforded 2w (127 mg, 95%) as a brown oil; R =
f
0.3 (hexanes–EtOAc, 9:1).
−1
IR: 2108 cm .
1
H NMR (600 MHz, CD OD): δ = 6.73 (d, J = 8.4 Hz, 2 H), 6.80 (d, J = 8.4 Hz, 2 H).
3
13
C NMR (150 MHz, CD OD): δ = 117.8, 120.8, 130.9, 146.6.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for C H N P:
2
3
24 21 2
369.1515; found: 369.1528.
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4-Azido-N,N-dimethylaniline (2x)
Following the general procedure using 4-(dimethylamino)phenylboronic acid pinacol ester
(4x) for 24 h and chromatography (15% EtOAc–hexane) afforded 2x (130 mg, 80%) as a yellow
solid; mp 35–39 °C; R = 0.3 (hexanes–EtOAc, 7:3).
f
−1
IR: 2087 cm .
1
H NMR (600 MHz, CD OD): δ = 2.88 (s, 6 H), 6.77 (d, J = 9.0 Hz, 2 H), 6.89 (d, J = 9.0 Hz,
3
2 H).
13
C NMR (150 MHz, CD OD): δ = 41.3, 115.4, 120.7, 129.9, 150.1.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for C H N P:
2
3
26 25 2
397.1828; found: 397.1796.
4-Azido-1-(tert-butoxycarbonyl)-1H-pyrazole (2y)
Following the general procedure using 1-(tert-butoxycarbonyl)-1H-pyrazole-4-boronic acid
pinacol ester (2y) for 1 h and chromatography (15% EtOAc–hexane) afforded 2y (50 mg, 24%)
as a yellow oil; R = 0.2 (hexane–EtOAc, 9:1).
f
−1
IR: 2119 cm .
1
H NMR (600 MHz, CD OD): δ = 1.64 (s, 9 H), 7.73 (s, 1 H), 8.10 (s, 1 H).
3
13
C NMR (150 MHz, CD OD): δ = 26.8, 86.3, 91.3, 120.6, 136.8, 147.1.
3
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for
2
3
C H N O P: 444.1835; found: 444.1880.
26 26
3 2
5-Azido-1-(tert-butoxycarbonyl)-1H-indole (2z)
Following the general procedure using 1-(tert-butoxycarbonyl)-1H-indole-5-boronic acid
pinacol ester (1z) for 24 h and chromatography (10% EtOAc–hexane) afforded 2z (135 mg,
52%) as a brown oil; R = 0.7 (hexane–EtOAc, 8:2).
f
−1
IR: 2111 cm .
1
H NMR (600 MHz, CD OD): δ = 1.67 (s, 9 H), 6.59 (d, J = 4.2 Hz, 1 H), 6.99 (dd, J = 8.4,
3
2.4 Hz, 1 H), 7.25 (d, J = 2.4 Hz, 1 H), 7.64 (d, J = 4.2 Hz, 1 H), 8.11 (d, J = 8.4 Hz, 1 H).
13
C NMR (150 MHz, CD OD): δ = 28.5, 85.2, 108.0, 111.7, 116.6, 117.2, 128.3, 133.2, 134.1,
3
136.2, 150.8.
+
HRMS (ESI+) (iminophosphorane adduct): m/z [M − N + PPh + H] calcd for
2
3
C H N O P: 493.2039; found: 493.2007.
31 29
2 2
4-Azidopyridine (6a)
Following the general procedure using pyridin-4-ylboronic acid (5a) for 24 h and
chromatography (30% EtOAc–hexane) afforded 6a (41 mg, 34%); as a yellow oil R = 0.2
f
(hexanes–EtOAc, 7:3).
−1
IR: 2112 cm .
1
H NMR (600 MHz, CD OD): δ = 7.14 (d, J = 5.4 Hz, 2 H), 8.47 (br s, 2 H).
3
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Page 12
13
C NMR (150 MHz, CD OD): δ = 115.9, 151.5, 151.6.
3
+
HRMS (ESI+): m/z [M + H] calcd for C H N : 121.0509; found: 121.0509.
5
5 4
5-Azidopyrimidine (6b)
Following the general procedure using pyrimidin-5-ylboronic acid (5b) for 24 h and
chromatography (30% EtOAc–hexane) afforded 6b (44 mg, 36%) as a white solid; mp 52–57
°C; R = 0.3 (hexane–EtOAc, 7:3).
f
−1
IR: 2110 cm .
1
H NMR (600 MHz, CD OD): δ = 8.60 (s, 2 H), 8.92 (s, 1 H).
3
13
C NMR (150 MHz, CD OD): δ = 138.4, 149.1, 155.3.
3
+
HRMS (ESI+): m/z [M + H] calcd for C H N : 122.0461; found: 122.0475.
4
4 5
8-Azidoquinoline (6c)
Following the general procedure using quinolin-8-ylboronic acid (5c) for 24 h and
chromatography (15% EtOAc–hexane) afforded 6c (60 mg, 35%) as a white solid; mp 54–57
°C; R = 0.1 (hexane–EtOAc, 9:1).
f
−1
IR: 2120 cm .
1
H NMR (600 MHz, CD OD): δ = 6.98 (d, J = 7.8 Hz, 1 H), 7.15 (d, J = 8.4 Hz, 1 H), 7.32 (t,
3
J = 7.8 Hz, 1 H), 7.40 (dd, J = 8.4, 4.2 Hz, 1 H), 8.14 (d, J = 8.4 Hz, 1 H), 8.70 (d, J = 4.2 Hz,
1 H).
13
C NMR (150 MHz, CD OD): δ = 111.8, 117.0, 122.5, 128.7, 130.5, 137.6, 139.6, 145.8,
3
148.5.
+
HRMS (ESI+): m/z [M + H] calcd for C H N : 171.0665; found: 171.0677.
9
7 4
4-Azidoisoquinoline (6d)
Following the general procedure using isoquinolin-4-ylboronic acid (5d) for 24 h and
chromatography (10% EtOAc–hexane) afforded 6d (73 mg, 43%) as a white solid; mp 54–56
°C; R = 0.2 (hexane–EtOAc, 9:1).
f
−1
IR: 2123 cm .
1
H NMR (600 MHz, CD OD): δ = 7.75 (t, J = 7.8 Hz, 1 H), 7.83 (t, J = 7.8 Hz, 1 H), 8.09
3
(overlapping d, J = 9.0 Hz, 1 H), 8.10 (overlapping d, J = 8.4 Hz, 1 H), 8.39 (s, 1 H), 9.04 (s,
1 H).
13
C NMR (150 MHz, CD OD): δ = 122.6, 128.8, 130.0, 130.3, 130.5, 132.5 (2 C), 134.5,
3
149.8.
+
HRMS (ESI+): m/z [M + H] calcd for C H N : 171.0665; found: 171.0682.
9
7 4
Acknowledgments
This research was supported by a grant from the NIH (R01AI070219) and funding from the Center for Drug Design,
Academic Health Center, University of Minnesota to C.C.A.
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Page 13
References
1. Tron GC, Pirali T, Billington RA, Canonico PL, Sorba G, Genazzani AA. Med Res Rev 2008;28:278.
[PubMed: 17763363]
2. For some representative recent examples, see: (a) Gupte A, Boshoff HI, Wilson DJ, Neres J, Labello
NP, Somu RV, Xing C, Barry CE, Aldrich CC. J Med Chem 2008;51:7495. [PubMed: 19053762] (b)
Giffin MJ, Heaslet H, Brik A, Lin YC, Cauvi G, Wong CH, McRee DE, Elder JH, Stout CD, Torbett
BE. J Med Chem 2008;51:6263. [PubMed: 18823110] (c) Gill C, Jadhav G, Shaikh M, Kale R,
Ghawalkar A, Nagargoje D, Shiradkar M. Bioorg Med Chem Lett 2008;18:6244. [PubMed: 18930654]
(d) Klein M, Dinér P, Dorin-Semblat D, Doering C, Grotli M. Org Biomol Chem 2009;7:3421.
[PubMed: 19675896] (e) Maurya SK, Gollapalli DR, Kirubakaran S, Zhang M, Johnson CR, Benjamin
NN, Hedstrom L, Cuny GD. J Med Chem 2009;52:4623. [PubMed: 19624136] (f) Upadhayaya RS,
Kulkarni GM, Vasireddy NR, Vandavasi JK, Dixit SS, Sharma V, Chattopadhyaya J. Bioorg Med
Chem 2009;17:4681. [PubMed: 19457676]
3. Avermaria F, Zimmermann V, Bräse S. Synlett 2004:1163.
4. Barral K, Moorhouse AD, Moses JE. Org Lett 2007;9:1809. [PubMed: 17391043]
5. Zhu W, Ma D. Chem Commun 2004:888.
6. Morgan J, Pinhey JT. J Chem Soc, Perkin Trans 1 1990:715.
7. Huber ML, Pinhey JT. J Chem Soc, Perkin Trans 1 1990:721.
8. Tao CZ, Cui X, Li J, Liu AX, Liu L, Guo QX. Tetrahedron Lett 2007;48:3525.
9. Chan DMT, Monaco KL, Wang RP, Winters MP. Tetrahedron Lett 1998;39:2933.
10. Lam PYS, Clark CG, Saubern S, Adams J, Winters MP, Chan DMT, Combs A. Tetrahedron Lett
1998;39:2941.
11. Hansch C, Leo A, Taft RW. Chem Rev 1991;91:165.
12. Molander GA, Ellis N. Acc Chem Res 2007;40:275. [PubMed: 17256882]
13. Tornøe CW, Christensen C, Meldal M. J Org Chem 2002;67:3057. [PubMed: 11975567]
14. Rostovtsev VV, Green LG, Fokin VV, Sharpless KB. Angew Chem Int Ed 2002;41:2596.
15. Chan, DMT.; Lam, PYS. Recent Advances in Copper-Promoted C-Heteroatom Bond Cross-Coupling
Reactions with Boronic Acids and Derivatives. In: Hall, DG., editor. Boronic Acids. Wiley-VCH;
Weinheim: 2005.
16. Bertz SH, Cope S, Murphy M, Ogle CA, Taylor BJ. J Am Chem Soc 2007;129:7208. [PubMed:
17506552]
17. Pudlo M, Csányi D, Moreau F, Hajós G, Riedl Z, Sapi J. Tetrahedron 2007;63:10320.
18. Cho YA, Kim DS, Ahn HR, Canturk B, Molander GA, Ham J. Org Lett 2009;11:4330. [PubMed:
19736957]
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Page 14
Scheme 1.
Proposed catalytic cycle for Chan–Lam coupling of boronic acids with the azide anion
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Grimes et al.
Page 15
Synthesis (Stuttg). Author manuscript; available in PMC 2010 November 1.

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Page 16
Table 2
Scope of the Copper(II)-Catalyzed Azidation of Arylboronic Acids
a
b
σp
Entry
R
Product
2a
Yield (%)
1
2
3
4
4-OMe
4-OPh
4-Ph
−0.27
−0.03
−0.01
0
78 ± 5
92 ± 8
50 ± 1
98 ± 1
2b
2c
4-CH2OH
2d
5
H
0
3e
c
64
6
7
4-SMe
4-F
0
2f
97 ± 1
+0.06
3g
c
68
8
4-I
+0.18
+0.23
+0.23
+0.36
2h
2i
63 ± 2
73 ± 4
68 ± 6
84 ± 3
9
4-Cl
10
11
4-Br
2j
4-CONH2
2k
12
13
4-CHO
+0.42
+0.45
2l
81 ± 4
89 ± 3
4-CO2H
2m
14
4-CO2Me
+0.45
2n
99 ± 0
81 ± 3
15
16
4-COMe
4-CF3
+0.50
+0.54
2o
3p
c
68
17
18
4-CN
+0.66
+0.72
2q
2r
98 ± 1
86 ± 3
4-SO2Me
19
20
4-NO2
3-Ph
+0.78
2s
2t
85 ± 1
46 ± 1
d
–
21
22
2-Ph
d
2u
2v
40 ± 2
94 ± 2
–
2-NHCOt-Bu
d
–
a
b
c
11
Hammett substituent constants.
Isolated yield ± standard deviation from 2–3 independent experiments.
Due to volatility, the crude azides were not isolated, but reacted in situ with methyl propiolate to afford the corresponding triazoles 3. The isolated
yields are for the corresponding triazole products. See experimental section for details.
d
Not applicable.
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Page 17
Table 3
Scope of the Copper(II)-Catalyzed Azidation of Boronic Acid Pinacol Esters
a
Product Yield (
Entry
R
1
2q
81 ± 3
2
2w
94 ± 1
3
2x
83 ± 1
4
5
2y
2z
29 ± 5
51 ± 1
a
Isolated yield ± standard deviation from 2–3 independent experiments.
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Table 4
Scope of the Copper(II)-Catalyzed Azidation of Heteroaryl- and Alkylboronic Acids
a
Entry
R
Product Yield (%)
1
6a 42 ± 1
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a
Product Yield (%)
Entry
R
2
6b
40 ± 8
3
4
5
6c
6d
6e
34 ± 4
40 ± 6
0
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Page 20
a
Product Yield (%)
Entry
R
6
6f
0
7
6g
0
a
Isolated yield ± standard deviation from 2–3 independent experiments.
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Table 5
Scope of the Copper(II)-Catalyzed Azidation of Organofluoroborate Salts
Entry
R
Product Yie
1
2a
70
2
3
2d
2f
40
67
4
2i
46
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Entry
R
Product Yie
2m 75
5
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Entry
R
Product Yie
2o 73
6
Synthesis (Stuttg). Author manuscript; available in PMC 2010 November 1.