Anti-Markovnikov hydroazidation of activated olefins via organic photoredox catalysis

Article abstract of DOI:10.1055/s-0039-1690691

Organic azides serve as synthetically useful surrogates for primary amines, a functional group which is ubiquitous in bioactive and medicinally relevant molecules. Historically, the formal hydroazidation of simple activated olefins and styrenes has proven

Full text of DOI:10.1055/s-0039-1690691

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© Georg Thieme Verlag Stuttgart · New York
2019, 30, A–E
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N. P. Onuska et al.
Cluster
Synlett
Anti-Markovnikov Hydroazidation of Activated Olefins via
Organic Photoredox Catalysis
Nicholas P. R. Onuska
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0
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2-
0
0
8
8-1
2
7
X
Me
catalyst
(1 mol%)
Megan E. Schutzbach-Horton
José L. Rosario Collazo
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0
0
0-0
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0
3-
0
1
6
9-1
0
8
8
Me
Me
David. A. Nicewicz*
0
0
0
0-0
0
0
3-
1
1
9
9-9
8
7
9
tBu
BF4
N
Ph
H
tBu
R
R
31 examples
22–98% yield
Department of Chemistry, The University of North Carolina at
Chapel Hill, Chapel Hill, NC, 27599-3290, USA
nicewicz@unc.edu
R
R
N3
TMSN3 (1.25 eq.)
TFE, 465 nm LEDs, N2
complete anti-Markovnikov selectivity
iPr
iPr
Published as part of the Cluster ‘9th Pacific Symposium on Radical
Chemistry’
- terminal styrenes
- substituted styrenes
- vinyl ethers
(20 mol%)
SH
iPr
Received: 17.08.2019
Accepted after revision: 11.09.2019
Published online: 24.09.2019
following protonation and requires the use of excess HN3.6
Compounded with the well-documented hazards associat-
ed with the use of HN3, including its explosive and toxic na-
ture, the direct reaction of alkenes with hydrazoic acid is
not a realistic solution to the preparation of organic azides
in a small-scale laboratory setting.
DOI: 10.1055/s-0039-1690691; Art ID: st-2019-r0433-c
Abstract Organic azides serve as synthetically useful surrogates for
primary amines, a functional group which is ubiquitous in bioactive and
medicinally relevant molecules. Historically, the formal hydroazidation
of simple activated olefins and styrenes has proven difficult due to the
inherent propensity of these compounds to oligomerize. Herein is dis-
closed a method for the anti-Markovnikov hydroazidation of activated
olefins, catalyzed by an organic acridinium salt under irradiation from
blue LEDs. This method is applicable to a variety of substituted and ter-
minal styrenes and several vinyl ethers, yielding synthetically versatile
hydroazidation products in moderate to excellent yield.
Several transition-metal-mediated and radical hy-
droazidation reactions have been reported in recent years
(Scheme 1). In 2005, Carreira and co-workers reported a
hydroazidation reaction of simple unactivated olefins by
using a cobalt Schiff base complex that was prepared in situ
in the presence of a substoichiometric quantity of hydrop-
eroxide oxidant, silane, and tosyl azide.7 While this method
is mild and tolerant of a variety of functional groups, exam-
ples utilizing styrenes or other types of electron-rich olefin
substrates are limited. Electrophilic nitrogen-centered
azide radicals engage in anti-Markovnikov addition reac-
tions with unactivated olefins.8–11 Azide radicals may be
generated using hypervalent azido-iodine compounds in
combination with a copper catalyst or photoredox catalyst,
affecting the hydroazidation of simple olefins.12,13 Earlier
this year, Xu and co-workers reported the generation of
azide radicals using a benziodoxole organocatalyst in the
presence of water and trimethylsilylazide (TMSN3).14 This
reaction proceeds smoothly with a variety of mono-, di-
and tri-substituted olefins bearing a number of potentially
sensitive functional groups. However, more electron-rich
olefins, including substituted and unsubstituted styrenes,
enol ethers and enamines proved unreactive under the op-
timized conditions.
Key words photooxidation, alkenes, azides, radicals, regioselectivity
The ubiquity of nitrogenous functional groups in indus-
trially and medicinally important organic molecules cannot
be overstated. Given their known importance, the develop-
ment of reactions that can introduce nitrogen-based func-
tionality to a molecular scaffold has been a well-traversed
area of research. Organic azides serve as versatile precur-
sors to primary amines following a simple hydrogenation or
Staudinger reduction. Outside of their utility as amine sur-
rogates, the biological stability and diverse reactivity of
azides has led to the development of a variety of bioconju-
gation methods that hinge on so-called azide ‘click chemis-
try’ – typically in the form of copper-catalyzed [3+2] Huis-
gen type cycloadditions or Staudinger ligation processes.1–5
Organic azides are typically prepared via SN2 type sub-
stitution reactions, utilizing an inorganic azide source (of-
ten sodium azide) and a primary or secondary alkyl halide.
However, this limits the scope of possible azide products to
those that can be tracked to commercially available or syn-
thetically viable alkyl halide precursors. The direct reaction
of a hydrazoic acid (HN3) across olefins is known, but often
limited to substrates that produce stabilized carbocations
To our knowledge, methods for the direct anti-Mar-
kovnikov hydroazidation of activated alkenes are nonexis-
tent. As such, we sought to develop a hydroazidation reac-
tion that would operate smoothly on these more electron-
rich substrates. A methodology that would enable the hydro-
azidation of styrenyl substrates is particularly interesting
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–E
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

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N. P. Onuska et al.
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Synlett
R2
PREVIOUS WORK
metal-catalyzed hydroazidation - Carreira et al., 2005
R1
Ph
R3
– e–
Ph
tBu
N
CO2K
disfavored
favored
OH
6 mol%
R
R
tBu
N3
R
R2
R2
R2
Nu
Nu
R'
R'
Nu
R1
R
Co(BF4)2•6H2O (6 mol%)
Nu
R1
R1
H
tBuOOH (30 mol%), TsN3
– Nu
– Nu
R3
R3
R3
silane, EtOH, rt
radical hydroazidation
Scheme 2 Regioselectivity in nucleophilic addition to alkene radical cations
O
Gagosz et al., 2017
OH
O
I
N3
Reaction development began with the use of indene as a
substrate in the presence of 5 mol% of acridinium photooxi-
dant tBu-Acr-BF4, 20 mol% of diphenyl disulfide, and 2.0
equivalents of sodium azide in trifluoroethanol (TFE, 0.1
M), a solvent commonly employed in our past reactions
that was also thought to act as a proton source (Scheme 3).
Tetrabutylammonium tetrafluoroborate (TBA BF4; 2.0
equiv) was added under the assumption that the tetrabu-
tylammonium cation would serve as a phase-transfer
agent, helping to solubilize azide ions. Following irradiation
for 18 hours, the desired hydroazidation product was
formed in 30% yield (based on 1H NMR analysis using HMD-
SO as an internal standard). Along with the desired product,
a nearly equal amount of the corresponding thiol-ene prod-
uct, resulting from addition of a thiyl radical formed via ho-
molysis of diphenyl disulfide to the olefin, was observed.
When acetonitrile (MeCN) was used as the solvent under
otherwise identical conditions, only alkene starting materi-
al was observed following irradiation. By exchanging the
hydrogen atom transfer catalyst from diphenyl disulfide to
the bulkier 2,4,6-triisopropylthiophenol (TRIP-SH), the de-
sired product was formed in 75% yield with no formation of
thiol-ene byproducts. Further optimization showed that
H
OBn
R
N3
R
R
CuBr2, DCE, 60 °C
R
Yu et al., 2019
Ir[III] catalyst, TMSN3 (5 eq.)
DMAP, H2O, blue LEDs
R
R
H
N3
R
OR
R
O
TMSN3 (1.8 eq.),
H2O, DCM, rt
Xu et al., 2019
O
I
(0.1 eq.)
OH
Scheme 1 Recent work in alkene hydroazidation
given the known biological activity of phenylethylamine
derivatives.15
Based on previous work from our group focused on
alkene hydrofunctionalization reactions, we envisioned
that photoredox catalysis may be a useful tool to develop a
methodology enabling the anti-Markovnikov hydroazida-
tion of this class of substrates.
By using a strongly oxidizing acridinium salt photore-
dox catalyst, alkene cation radicals may be generated in cat-
alytic quantities via photoinduced electron transfer (PET)
upon irradiation with blue light. The removal of an electron
from the alkene -system renders the resulting cation radi-
cal electrophilic and able to react with a suitably paired nu-
cleophilic partner. Following interception of the cation rad-
ical by a nucleophile, a hydrogen atom transfer event facili-
tated by a hydrogen atom donor co-catalyst can trap the
resulting benzylic radical, affording formal hydrofunction-
alization products.16,17 Since the regioselectivity of nucleop-
hile addition is dictated by the formation of the more stable
radical species, this process proceeds with anti-Mar-
kovnikov selectivity in nearly all cases (Scheme 2).
The generation of reactive cation radical and neutral
radical species in catalytic qualities renders this chemistry
tolerant of a variety of synthetically useful functional han-
dles. Our group has leveraged the reactivity of PET-generat-
ed alkene cation radicals to develop formal anti-Mar-
kovnikov hydroacetoxylation, hydroamination, and hydro-
etherification reactions, among others.17–21 Based on this
body of work, we envisioned that a formal hydroazidation
reaction could be conceived through a similar reaction
manifold.
H
conditions
N3
tBu-Acr-BF4 (1 mol%)
465 nm LEDs
18 h, N2
entry
azide source
co-catalyst
solvent
yield
1*
2*
3
NaN3 (2.0 eq.)
NaN3 (2.0 eq.)
Ph2S2 (20 mol%)
TFE (0.1 M)
30%
N.R.
75%
75%
Ph2S2 (20 mol%) MeCN (0.1 M)
TMSN3 (2.0 eq.) TRIP-SH (20 mol%) TFE (0.1 M)
TMSN3 (1.25 eq.) TRIP-SH (20 mol%) TFE (0.1 M)
4
* = with 1.0 eq. TBABF4, 5 mol% catalyst
H
TRIP-SH (20 mol%)
TMSN3 (1.25 eq.)
Me
O
O
Me
O
O
tBu-Acr-BF4 (1 mol%)
N3
465 nm LEDs
18 h, N2
Me
tBu-Acr-BF4
TRIP-SH
entry
solvent
yield
iPr
iPr
5
6
7
8
TFE:MeCN (9:1, 0.1 M) 91%
TFE:MeCN (7:1, 0.1 M) 79%
TFE:MeCN (3:1, 0.1 M) 75%
Me
Me
HS
tBu
BF4
N
Ph
tBu
TFE (0.1 M)
97%
iPr
Scheme 3 Reaction optimization
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–E

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N. P. Onuska et al.
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the loadings of acridinium photocatalyst and TMSN3 could
be lowered to 1 mol% and 1.25 equivalents, respectively,
with no adverse effect on the yield of the hydroazidation
product.
Interestingly, in a series of experiments utilizing various
TFE/MeCN solvent mixtures and isosafrole as the substrate,
the yield of the desired hydroazidation product decreased
from 91% to 75% as the percentage of acetonitrile in the sol-
vent mixture was increased from 10% to 25% (Scheme 3, en-
tries 5–8).
under the optimized reaction conditions, including those
bearing benzoyl (22), benzyl (24), and silyl (25) protecting
groups. Aromatic ester product 27 was isolated in 70% yield
with no transesterification observed. A more oxidizable
naphthalene-derived substrate also afforded the hydroazi-
dation product 26 in good yield. Substrates containing po-
tentially labile benzylic C–H bonds were converted into the
desired products 10, 17, and 28 in excellent yield and no
functionalization was detected at these reactive C–H sites.
Notably, a substrate containing a terminal alkene was con-
verted into the anti-Markovnikov hydroazidation product
30 in 71% isolated yield with no functionalization of the un-
activated alkene detected, highlighting the complementari-
ty of this method to other known radical hydroazidation re-
actions. Heterocyclic quinoline and thiophene substrates
were functionalized to give the corresponding azide prod-
ucts 31 and 32, in 55% and 65% yield, respectively. Terminal
styrene derivatives underwent the desired transformation
in poor to moderate yields. Notably, diphenyl disulfide was
identified as a more efficient hydrogen atom transfer cata-
lyst than TRIP-SH for these substrates. Due to the lack of
substituents in the -position, these styrenes are prone to
oligomerization via radical mechanisms. To combat this, a
less sterically hindered HAT catalyst must be employed to
accelerate the rate of HAT versus oligomerization. Oxidiz-
able vinyl ethers were also competent reactants, with prod-
ucts 6 and 7 isolated in 58% and 79% yield. Product 7 is
formed following elimination of the tertiary alcohol in hy-
droazidation product of substrate 8 during chromatography.
Based on previous mechanistic investigations, the fol-
lowing mechanism is proposed. Following excitation by 465
nm light, the excited state of the acridinium catalyst engag-
es in photoinduced electron transfer with the alkene sub-
strate, yielding the corresponding alkene cation radical and
the reduced form of the catalyst. Nucleophilic addition of
azide ion generates a neutral radical species. Based on our
observations, it is not clear whether free azide ion is gener-
ated prior to addition to the alkene cation radical. It is pos-
sible that a termolecular transition state involving a solvent
molecule-assisted desilylation/nucleophilic addition is op-
erative or that an azide silicate species could be the nucleo-
phile. However, it is likely that some free azide ion is in
solution due to hydrolysis of TMSN3 by advantageous water
in the reaction mixture, as strict exclusion of water was not
maintained. The benzylic radical engages in hydrogen atom
transfer with TRIP-SH, generating the desired anti-Mar-
kovnikov hydroazidation product and a thiyl radical. This
thiyl radical then oxidizes the reduced form of the catalyst,
regenerating the ground state tBu-Acr-BF4 as well as a thio-
late anion. This anion is then protonated by solvent to yield
the starting co-catalyst and close the catalytic cycle.
Early spectroscopic investigations of the reaction be-
tween alkene cation radicals and various nucleophiles have
demonstrated that hydrogen bonding and solvent polarity
have profound effects on the rates of nucleophilic addition
to these reactive species.22,23 More specifically, previous
work from the Schepp group has demonstrated that the ad-
dition of azide anion to alkene radical cations is strongly af-
fected by hydrogen-bond attenuation of redox potentials.
The kinetics of the reaction between styrene cation radicals
and azide anion has been previously studied via flash pho-
tolysis transient absorption spectroscopy. In non-hydro-
gen-bonding solvents, such as acetonitrile, the azide ion
–
(N3 ) is oxidized by the alkene cation radical to yield azide
•
radical (N3 ) and the corresponding neutral alkene. The
azide radical then quickly equilibrates with a second equiv-
alent of azide ion to generate an inactive, non-nucleophilic
•–
N6 radical anion dimer with an estimated equilibrium
constant of ca. 200 M–1 (in MeCN), which is identifiable by
an absorption centered at 700 nm.24 However, when trifluo-
roethanol is used as the solvent, no absorption attributed to
the formation of N6•– is identifiable. Following flash photol-
ysis of TFE solutions of styrene derivatives in the presence
of azide ion, an absorption maxima between 350 and 380
nm is observed, indicating the formation of a benzylic radi-
cal resulting from nucleophilic addition of azide ion to the
alkene cation radical.25 Furthermore, the peak potential ob-
served via cyclic voltammetry for oxidation of azide ion is
shifted +0.5 V in TFE versus MeCN, indicating that hydrogen
bonding is capable of dramatically attenuating the redox
potential of this anionic species (E°peak (MeCN) = 0.275 V vs.
Fc, E°peak (TFE) = 0.802 V vs. Fc). When reacted in TFE solu-
tion, the alkene cation radical is no longer able to oxidize
the azide ion, and productive nucleophilic addition takes
place. This previous work is in good accordance with our
observations during reaction optimization.
With optimized conditions in hand, the scope of this
transformation with respect to alkene partners was ex-
plored (Scheme 4). -Substituted styrene derivatives were
found to be excellent substrates for this transformation,
with -methylstyrene affording the desired hydroazidation
product 9 in 98% yield (based on NMR analysis, using HMD-
SO as an internal standard). Simple alkyl-, aryl-, and chloro-
styrene derivatives were all smoothly converted into the
corresponding secondary azide products 11–14 in good
yield. A variety of phenolic substrates were well-tolerated
In conclusion, we have developed an organic photore-
dox anti-Markovnikov hydroazidation reaction of electron-
rich olefin substrates. By utilizing electrophilic cation radi-
cal intermediates, previously problematic hydroazidation
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–E

D
N. P. Onuska et al.
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Synlett
reactions involving activated olefins now proceed efficient-
ly and in high yields with low loadings of both photocata-
lyst and TMSN3. Furthermore, the transformation proceeds
in the absence of any transition metals using TMS-N3 as the
only stoichiometric reagent.26 This method fills the gap in
the literature with regard to alkene hydroazidation chemis-
try.
Me
Me
catalyst
(1 mol%)
Me
tBu
BF4
N
Ph
H
tBu
R
R
R
R
N3
TMSN3 (1.25 eq.), TRIP-SH (20 mol%)
TFE (0.1 M), 465 nm LEDs, N2
vinyl ethers
* = 0.75 eq. (PhS)2 instead of TRIP-SH
terminal styrenes
Ph
N3
H
H
H
H
from:
H
Me
O
H
Ph
Ph
Ph
N3
N3
N3
N3
N3
OMe
OH
Ph
O
N3
tBu
Cl
Me
8
4
5
6
7
3
2
1
(24% yield)*
(43% yield)*
(22% yield)*
(49% yield)*
(49% yield)*
58% yield
79% yield
substituted styrenes
H
H
H
H
Cl
H
H
Me
Me
Me
Me
Me
N3
N3
N3
Me
12
76% yield
OMe H
N3
N3
N3
Ph
tBu
14
9
10
11
13
64% yield
98% yield*
47% yield
97% yield
51% yield
H
H
H
Ph
H
H
Me
N3
Me
Me
Me
N3
N3
N3
N3
N3
MeO
19
90% yield
20
15
17
18
75% yield
16
81% yield
OMe
80% yield
66% yield
44% yield
(single diastereomer)
H
H
H
H
H
H
Me
Me
Me
Me
Me
OTBS
BzO
N3
21
N3
N3
N3
N3
H
N3
PhO
BnO
TBDPSO
22
23
91% yield
24
91% yield
25
76% yield
26
62% yield
67% yield
(39% yield)
H
H
H
H
Me
Me
Me
Me
O
H
O
O
NPhth
Me
MeO
N3
N3
N3
N3
S
N3
MeO
N
N3
71% yield
27
70% yield
28
92% yield
29
87% yield
32
31
55% yield
O
30
(65% yield)
NMR yields given in parenthesis, all others isolated. Yields are average of two trials on 0.5 mmol scale.
RO
R2
R2
R2
R2
R2
TMS
N3
H
R1
R
TMS-N3
N3
N3
R1
R1
R1
R
S
R1
R3
R3
R3
R3
R3
Me
thiol-ene
S
Me
– e–
iPr
iPr
Me
HAT
tBu
N
R
*
Me
tBu
Ph
R
SH
S
iPr
iPr
iPr
Me
Me
photoredox
cycle
hydrogen atom
transfer cycle
+ e–
tBu
BF4
N
Ph
TRIP-SH
tBu
iPr
iPr
Me
R
S
S
R
iPr
Me
Me
iPr
iPr
hυ
iPr
H
tBu
BF4
N
Ph
tBu
sterically encumbered thiyl radical
iPr
Scheme 4 Substrate scope and proposed mechanism for photoredox alkene hydroazidation
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–E

E
N. P. Onuska et al.
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Synlett
Funding Information
ridin-10-ium tetrafluoroborate (0.01 equiv, 0.01 mmol) and
2,5,6-triisopropylthiophenol (0.10 mmol, 0.20 equiv).
For solid/non-volatile substrates, the substrate (0.50 mmol) was
then added. 2,2,2-Trifluoroethanol (5.0 mL) was added and the
vials were capped tightly with a Teflon-lined phenolic resin
septum cap (purchased through VWR international, Microliter
Product # 15-0060K). The reaction mixture was then sparged
by bubbling with nitrogen or argon for 5 minutes. Trimethyl-
silylazide (0.625 mmol, 1.25 equiv) was added by using a micro-
liter syringe. Prior to irradiation, vials were sealed with Teflon
tape and electrical tape to ensure maximal oxygen exclusion.
The reaction vial was then placed into the reactor and irradiated
for 18 hours unless otherwise noted.
This project was supported by Award No. R01 GM098340 from the
National Institute of General Medical Sciences. M.E.S.H. is grateful for
an NSF Graduate Fellowship. J.L.R.C. was supported by a National Sci-
ence Foundation REU SUROC Award to UNC (NSF-REU 1757413).
N
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Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0039-1690691.
S
u
p
p
orit
n
gInformati
o
n
S
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orti
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n
Following irradiation, the reaction mixture was concentrated
under reduced pressure and the desired products were isolated
by flash column chromatography (see the Supporting Informa-
tion substrate/product details for solvent information). Unless
otherwise noted, all reaction yields are reported as the average
of two separate trials (including chromatography).
References and Notes
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1995, 23.
Example products
4-(2-Azidopropyl)-1,1′-biphenyl (11): Following irradiation,
the crude reaction mixture was concentrated under reduced
pressure and dry-loaded onto silica gel. The desired product
was isolated as a pale-yellow oil following column chromatogra-
phy (100% hexane to 1% EtOAc/hexane). Yield: 97% (n = 2).
1H NMR (600 MHz, CDCl3):  = 7.63 (dd, J = 26.1, 7.7 Hz, 4 H),
7.49 (t, J = 7.6 Hz, 2 H), 7.39 (t, J = 7.4 Hz, 1 H), 7.32 (d, J = 7.8 Hz,
2 H), 3.77 (h, J = 6.6 Hz, 1 H), 2.92 (dd, J = 13.7, 7.3 Hz, 1 H), 2.82
(dd, J = 13.7, 6.4 Hz, 1 H), 1.35 (d, J = 6.5 Hz, 3 H). 13C NMR (151
MHz, CDCl3):  = 140.90, 139.73, 136.91, 129.79, 128.84, 127.29,
127.10, 59.04, 42.24, 19.22. HRMS (APCI, positive mode): m/z
[M + H, – N2] calcd: 210.1277; found: 210.1278.
1-(2-Azidopropyl)-2-chlorobenzene (14): Following irradia-
tion, the crude reaction mixture was concentrated under
reduced pressure and dry-loaded onto silica gel. The desired
product was isolated as a pale-yellow oil following column chro-
matography (100% hexane to 3% EtOAc/hexane). Yield: 64% (n =
2).
1H NMR (600 MHz, CDCl3):  = 7.40 (dd, J = 7.2, 1.9 Hz, 1 H), 7.28
(dd, J = 7.1, 2.3 Hz, 1 H), 7.25–7.19 (m, 2 H), 3.85 (h, J = 6.7 Hz,
1 H), 3.00–2.84 (m, 2 H), 1.33 (d, J = 6.5 Hz, 3 H). 13C NMR (151
MHz, CDCl3):  = 135.70, 134.32, 131.85, 129.74, 128.42, 126.95,
57.57, 40.40, 19.43. MS (EI): m/z [M]+ calcd: 195.056; found:
195.05.
1-(2-Azidopropyl)-4-phenoxybenzene (23)
Following irradiation, the crude reaction mixture was concen-
trated under reduced pressure and dry-loaded onto silica gel.
The desired product was isolated as a clear oil following purifi-
cation by column chromatography (100% hexane to 1%
EtOAc/hexane). Yield: 91% (n = 2).
1H NMR (600 MHz, CDCl3):  = 7.38–7.30 (m, 2 H), 7.16 (d, J =
8.5 Hz, 2 H), 7.13–7.06 (m, 1 H), 7.01 (dd, J = 8.7, 1.1 Hz, 2 H),
6.96 (d, J = 8.5 Hz, 2 H), 3.77–3.47 (m, 1 H), 2.80 (dd, J = 13.8,
7.4 Hz, 1 H), 2.72 (dd, J = 13.8, 6.3 Hz, 1 H), 1.28 (d, J = 6.5 Hz,
3 H). 13C NMR (151 MHz, CDCl3):  = 157.31, 156.03, 132.62,
130.57, 129.72, 123.18, 118.96, 118.78, 59.14, 41.87, 19.16.
HRMS (APCI, positive mode): m/z [M + H, – N2] calcd: 226.1226;
found: 226.1227.
(23) Johnston, L. J.; Schepp, N. P. Pure Appl. Chem. 1995, 67, 71.
(24) Workentin, M. S.; Schepp, N. P.; Johnston, L. J.; Wayner, D. D. M.
J. Am. Chem. Soc. 1994, 116, 1141.
(25) Johnston, L. J.; Schepp, N. P. J. Am. Chem. Soc. 1993, 115, 6564.
(26) General Procedure (CAUTION: use care when handling TMS-
N3): A flame-dried 2-dram borosilicate vial (purchased from
Fisher Scientific, catalogue # 03-339-22D), equipped with a stir
bar, was charged with 3,6-di-tert-butyl-9-mesityl-10-phenylac-
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–E