Ex situ generation of sulfuryl fluoride for the synthesis of aryl fluorosulfates

Article abstract of DOI:10.1021/acs.orglett.7b02522

A convenient transformation of phenols into the corresponding aryl fluorosulfates is presented: the first protocol to completely circumvent direct handling of gaseous sulfuryl fluoride (SO₂F₂). The proposed method employs 1,1'-sulfonyldiimidazole as a precursor to generate near-stoichiometric amounts of SO₂F₂ gas using a two-chamber reactor. With NMR studies, it was shown that this ex situ gas evolution is extremely rapid, and a variety of phenols and hydroxylated heteroarenes were fluorosulfated in good to excellent yields.

Full text of DOI:10.1021/acs.orglett.7b02522

Letter
pubs.acs.org/OrgLett
Ex Situ Generation of Sulfuryl Fluoride for the Synthesis of Aryl
Fluorosulfates
Cedrick Veryser,‡ Joachim Demaerel,‡ Vidmantas Bieliunas, Philippe Gilles,
̅
and Wim M. De Borggraeve*
Molecular Design and Synthesis, Department of Chemistry, KU Leuven, Celestijnenlaan 200F, Box 2404, 3001 Leuven, Belgium
S
* Supporting Information
ABSTRACT: A convenient transformation of phenols into the correspond-
ing aryl fluorosulfates is presented: the first protocol to completely
circumvent direct handling of gaseous sulfuryl fluoride (SO2F2). The
proposed method employs 1,1′-sulfonyldiimidazole as a precursor to
generate near-stoichiometric amounts of SO2F2 gas using a two-chamber
reactor. With NMR studies, it was shown that this ex situ gas evolution is
extremely rapid, and a variety of phenols and hydroxylated heteroarenes
were fluorosulfated in good to excellent yields.
ince 2014, aryl fluorosulfates have sparked enormous
three strategies, the appropriate phenol (or phenolate) is
Sinterest as they give access to a broad and powerful set of combined with fluorosulfonic anhydride,5a,6−8,16 sulfuryl
chloride fluoride,17 or sulfuryl fluoride18 in the presence of a
applications. This is primarily due to the specific reactivity of
base at low temperature. Unfortunately, these approaches either
require highly toxic and/or expensive reagents or make use of
complicated and nonreliable reaction procedures (e.g., gas
condensation) which often results in low yields.
Recently, Sharpless and others described a robust, reliable,
and easy-to-execute synthesis of aryl fluorosulfates.1,2,19 These
compounds were prepared from phenols and sulfuryl fluoride
in the presence of a base, typically triethylamine. This finding
has been foundational for the renewed interest in the
fluorosulfate chemistry, considering that all newly discovered
applications make use of this methodology.
The SO2F2 gas is generally introduced using a balloon, filled
from a pressurized lecture bottle. Other methods rely on the
use of a stock solution of sulfuryl fluoride.10 Although sulfuryl
fluoride gas is produced on an industrial scale and widely used
as a fumigant, its impact on human health and the environment
should not be neglected.20 The time-weighted average exposure
limit for sulfuryl fluoride has been set at 5 ppm by the
Occupational Safety and Health Administration, indicating that
long-term exposure is harmful for human well-being.20,21 The
gas was also recently identified as a greenhouse gas with a
global warming potential of 4800 relative to carbon dioxide and
an atmospheric lifetime of 36 years.22 Moreover, the limited
number of suppliers makes it challenging and expensive to
obtain a gas cylinder, especially when taking into account the
costs of transportation and disposal. Despite the inherent
drawbacks of this reagent, all reported procedures make
(in)direct use of pressurized sulfuryl fluoride gas bottles and
the fluorosulfate group.
First, the S(VI)−F bond conveys onto the sulfur center an
electrophilic behavior very different from other sulfonyl halides.
Owing to these unique properties, the fluorosulfate moiety is
inert toward most nucleophiles but reacts cleanly with either
amines (by merit of protons solvating F−) or aryl silyl ethers
(metathesizing into a diaryl sulfate and an extremely strong Si−
F bond). This set of Lewis base mediated “click” reactions was
recently disclosed by Sharpless and co-workers and baptized as
sulfur(VI) fluoride exchange (SuFEx) chemistry.1 The SuFEx
click reaction between aromatic bis(fluorosulfates) and bis(silyl
ethers) can, for example, be applied for the synthesis of
poly(aryl sulfates), sulfate-backboned analogs to polycarbonates
which show promising mechanical properties.2 SuFEx chem-
istry has also found intriguing applications in selective and
orthogonal postpolymerization modifications3 as well as in
biomolecular and peptide chemistry.4
Second, besides being a robust connector handle, fluoro-
sulfates are also excellent pseudohalides in transition-metal-
catalyzed cross coupling reactions. Many applications have
followed, including Suzuki−Miyaura,5 Negishi,6 or Stille−
Migita coupling,6,7 alkoxycarbonylation,8 and Buchwald−
Hartwig amination.9 Consequently, they are often considered
as efficient triflate surrogates, albeit at a much lower production
cost. Furthermore, aryl fluorosulfates are also versatile
intermediates as they can be converted into aryl fluorides,10
aryl sulfamate esters,11 and others12 or can even be used for the
synthesis of anhydrous tetraalkylammonium fluoride salts.13
Historically, aryl fluorosulfates are mainly synthesized via
four different approaches.14 The first method relies on the
pyrolysis of arenediazonium fluorosulfate salts.15 In the other
Received: August 15, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02522
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
thus are always associated with high cost and risks of explosion
and leakage.
Table 1. Optimization of Reaction Conditions
In summary, there is an urgent need to further develop new,
convenient, and inherently safe methodologies to produce
fluorosulfates, preferably starting from inexpensive and readily
available commodity chemicals. Here, we report the first
procedure for the on-demand ex situ generation of sulfuryl
fluoride from a solid precursor for the synthesis of aryl
fluorosulfates.
Our investigation began by scanning literature to identify
potential sulfuryl fluoride precursors or surrogates. Inspired by
the report of Prakash, sulfuryl chloride fluoride was selected as
our first candidate.23 Therein, the SO2ClF gas could selectively
be formed from sulfuryl chloride and a fluoride source in acidic
medium and subsequently be isolated after gas condensation,
albeit in low yield. We assumed that translating this work to a
two-chamber reactor24 would significantly improve this
method, as the generated gas can migrate to an adjacent
chamber, where it is directly consumed thus avoiding
intermediate isolation. This reactor was originally designed
and commercialized by the Skrydstrup group.25 We have
recently used this setup for the ex situ generation of carbon
monoxide and sulfur dioxide for applications in organic
synthesis.26
After an extensive optimization study (see Supporting
Information (SI)), the model substrate 4-fluoro-4′-hydroxybi-
phenyl was successfully converted into its corresponding aryl
fluorosulfate 1 and isolated in 95% yield. Unfortunately, under
the optimized reaction conditions (4 equiv of SO2Cl2 and 6
equiv of KF in 0.6 mL of HCOOH) the transformation of the
electron-rich 4-hydroxyanisole resulted in multiple chlorinated
byproducts. Considering sulfuryl chloride’s known behavior as a
chlorinating agent in electrophilic aromatic substitutions, we
speculate that the volatile SO2Cl2 and/or the SO2ClF also
participate in this type of reaction. A similar observation was
reported when sulfuryl chloride was used for the synthesis of
aryl chlorosulfates.18a
In order to eliminate the formation of chlorinated
byproducts, we started looking for alternative nonvolatile
precursors not containing chloride. In this respect, 1,1′-
sulfonyldiimidazole (SDI) seemed an attractive sulfuryl fluoride
precursor, as it is a commercially available solid. It can also
easily be synthesized and isolated by precipitation on gram scale
from sulfuryl chloride and imidazole (see SI). As shown before,
acidic conditions turn the imidazolium substituent into an
excellent leaving group,27 and it was postulated that a 2-fold
displacement by fluoride would generate pure SO2F2 gas.
First, SDI was evaluated under the previously optimized
reaction conditions. Gratifyingly, full conversion toward the
desired fluorosulfate 1 was observed after 18 h (Table 1, entry
1). Next, the reaction conditions were further modified to
minimize the required amount of precursor. In formic acid, the
number of equivalents of SDI could be reduced to 3.0, without
affecting the conversion (entry 2). Further decreasing the
amount of SDI resulted in incomplete conversion (entry 3).
Minor improvements were achieved when the amount of
potassium fluoride was increased or when the reaction was
performed at 40 °C (entries 4−5). However, when formic acid
was replaced by trifluoroacetic acid, the number of equivalents
of SDI and KF could be significantly reduced to 1.5 and 4.0,
respectively. These conditions furnished fluorosulfate 1 in an
isolated yield of 96% (entries 6−9). Again, product formation
was hampered when the amount of SDI and/or KF were
b
entry
SDI [equiv]
KF [equiv]
acid
yield [%]
1
2
3
4.0
3.0
2.0
2.0
2.0
2.0
2.0
1.5
1.5
1.5
1.3
1.5
1.5
6.0
6.0
6.0
6.0
8.0
6.0
4.0
4.5
4.0
3.0
6.0
4.0
4.0
HCOOH
HCOOH
HCOOH
HCOOH
HCOOH
TFA
>99
>99
81
c
4
82
5
89
6
>99
>99
>99
7
TFA
8
TFA
d
9
TFA
>99 (96)
10
11
TFA
93
89
84
94
TFA
e
12
f
TFA
13
TFA
a
Reaction conditions: Chamber A: 1,1′-sulfonyldiimidazole (SDI, 1.5
equiv), KF (4.0 equiv), and trifluoroacetic acid (TFA, 0.6 mL) at rt for
18 h. Chamber B: 4-fluoro-4′-hydroxybiphenyl (0.5 mmol), Et3N (2.0
b
equiv) in dichloromethane (4 mL) at rt for 18 h. Determined by 19
F
c
NMR using trifluorotoluene as internal standard. Chamber A was
heated to 40 °C. Isolated yield. Reaction run for 2 h. Reaction run
d
e
f
for 8 h.
further decreased (entries 10−11). The optimization study was
finalized by investigating the influence of the reaction duration.
After 2 and 6 h, the yield was 84% and 94%, respectively
(entries 12−13). We hypothesize that fluorosulfation is the
rate-limiting step instead of the SO2F2 generation from SDI.
13C NMR confirmed that SDI completely decomposed in less
than 30 s under the optimized reaction conditions (see SI). It
was also shown that under these circumstances, the actual
pressure inside the vessel remained well under the maximally
allowed internal pressure (see SI).
With the optimized conditions in hand, we first tried to
convert 4-hydroxyanisole into its corresponding aryl fluoro-
sulfate 2 as this transformation was unsuccessful with sulfuryl
chloride as a precursor. To our delight, the starting material was
fully consumed under these reaction conditions and yielded the
desired aryl fluorosulfate 2 in 91% isolated yield (Scheme 1).
This clearly showed the usefulness of 1,1′-sulfonyldiimidazole
as a sulfuryl fluoride precursor, as no byproducts were formed.
In order to further explore the scope of this methodology, a
broad and diverse set of phenol derivates was investigated
(Scheme 1). First, monosubstituted electron-rich and -deficient
phenols were successfully converted into their corresponding
aryl fluorosulfates (3−11). These results illustrate that the
electronic properties of the phenol derivates do not significantly
influence the fluorosulfation reaction. Even sterically hindered
aryl fluorosulfates were furnished in excellent yields (9, 10, 11,
and 15). Also, naturally occurring phenols, such as eugenol,
vanillin, and raspberry ketone turned out to be suitable
substrates for this transformation (12, 13, and 14), while D-α-
tocopherol and β-estradiol produced only moderate conversion
under the optimized reaction conditions. Gratifyingly, when the
triethylamine dichloromethane system was substituted by N,N-
diisopropylethylamine (DIPEA) in acetonitrile,28 the corre-
sponding aryl fluorosulfates were acquired in near-quantitative
B
DOI: 10.1021/acs.orglett.7b02522
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Synthesis of Aryl Fluorosulfates through ex Situ Generation of Sulfuryl Fluoride in a Two-Chamber Reactor
a
Reaction conditions: Chamber A: 1,1′-sulfonyldiimidazole (1.5 equiv), KF (4.0 equiv) in TFA. Chamber B: (hetero)aryl alcohol (1.0 mmol), Et3N
b
c
(2.0 equiv) in DCM. Reaction details and pressure profile: see SI and instructional video. Chamber B: DIPEA (3.0 equiv) in MeCN. Chamber B:
DIPEA (4.0 equiv) in MeCN. Chamber B: (hetero)aryl alcohol (0.5 mmol), Et3N (4.0 equiv) in DCM. Chamber B: (hetero)aryl alcohol (0.5
mmol), DIPEA (6.0 equiv) in MeCN.
d
e
yields (15 and 16). These slightly modified conditions were
also required for the fluorosulfation of paracetamol and L-
tyrosine methyl ester (17 and 18). Next, two bicyclic phenol
derivates were successfully tested (19 and 20). For the reaction
of hydroquinone and phenolphthalein, 3.0 equiv of SDI and 4.0
equiv of triethylamine were added, resulting in the exclusive
formation of the corresponding bis(fluorosulfates) (21 and 22).
The scope was finalized by the synthesis of five heteroaryl
fluorosulfates (23−27). It is worth noting that, under the
applied reaction conditions, anilines, aliphatic alcohols, and
aliphatic amines were tolerated, as only aromatic hydroxyl
groups reacted with sulfuryl fluoride (3, 16, and 18). This
observation is in agreement with the results reported by the
Sharpless group.1
electrophilic functional handles will increase as they find their
way into many more applications, by merit of their leaving
group ability or as SuFEx partners.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b02522.
Experimental procedures, characterization for all com-
pounds, decomposition study, and pressure profile
(PDF)
Instructional video (AVI)
The developed method is easily scaled up by simply using a
larger two-chamber reactor. This was illustrated by synthesizing
compound 11 on 5 gram scale. After an aqueous acid/base
wash, the desired aryl fluorosulfate was achieved in 96% yield
(see SI).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wim.deborggraeve@kuleuven.be.
ORCID
In conclusion, we have demonstrated a new, practical, and
efficient way of transforming phenols into their corresponding
aryl fluorosulfates in good to excellent yields. The proposed
method relies on ex situ generation of sulfuryl fluoride gas from
cheap and readily available commodity chemicals in a two-
chamber reactor. This provides a convenient means of
transforming common phenolic substances, including some
drug-like and naturally occurring compounds, into reactive
aromatic intermediates. Furthermore, it is easily scaled up as
evidenced by the preparation of analytically pure 2-
bromophenyl fluorosulfate on multigram scale using only
extractive workup as the purification step. Further implementa-
tion of this promising chemistry within the larger research
community can herewith be accelerated, where otherwise it
might remain less explored due to the cumbersome handling of
gaseous reagents. We speculate that the demand for these
Cedrick Veryser: 0000-0001-9076-3623
Wim M. De Borggraeve: 0000-0001-7813-6192
Author Contributions
‡C.V. and J.D. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to Karel Duerinckx (KU Leuven) for the
assistance with NMR measurements and to Dirk Henot (KU
Leuven) for the elemental analysis. C.V., J.D., and P.G. thank
FWO (PhD Fellowship of the Research Foundation - Flanders)
for fellowships received. W.M.D.B. thanks KU Leuven for
financial support via Project OT/14/067. The Hercules
C
DOI: 10.1021/acs.orglett.7b02522
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Chem. Eng. Data 1976, 21, 386−387. (d) Hedayatullah, M.; Guy, A.;
Denivelle, L. Phosphorus Sulfur Relat. Elem. 1980, 8, 125−126.
(19) (a) Ishii, A.; Yasumoto, M. Method for Producing Fluorosulfuric
Acid Ester. US 2011/0201825A1, August 18, 2011. (b) Ishii, A.;
Ishimaru, T.; Yamazaki, T.; Yasumoto, M. Process for Producing
Fluorosulfuric Acid Aromatic-ring Esters. WO 2013/002040A1,
January 3, 2013.
Foundation is acknowledged for supporting the purchase of a
400 MHz NMR spectrometer through project AKUL1311. V.B.
and W.M.D.B. are grateful for funding received from the
European Union’s Horizon 2020 research and innovation
program under the Marie Skłodowska-Curie grant agreement
No. 721290.
(20) Tsai, W.-T. J. Environ. Sci. Health C 2010, 28, 125−145.
(21) Schneir, A.; Clark, R. F.; Kene, M.; Betten, D. Clin. Toxicol.
2008, 46, 850−854.
REFERENCES
■
(1) Dong, J.; Krasnova, L.; Finn, M. G.; Sharpless, K. B. Angew.
Chem., Int. Ed. 2014, 53, 9430−9448.
(22) (a) Papadimitriou, V. C.; Portmann, R. W.; Fahey, D. W.;
Muhle, J.; Weiss, R. F.; Burkholder, J. B. J. Phys. Chem. A 2008, 112,
̈
(2) (a) Dong, J.; Sharpless, K. B.; Kwisnek, L.; Oakdale, J. S.; Fokin,
V. V. Angew. Chem., Int. Ed. 2014, 53, 9466−9470. (b) Gao, B.; Zhang,
L.; Zheng, Q.; Zhou, F.; Klivansky, L. M.; Lu, J.; Liu, Y.; Dong, J.; Wu,
P.; Sharpless, K. B. Nat. Chem. 2017, DOI: 10.1038/nchem.2796.
(3) (a) Oakdale, J. S.; Kwisnek, L.; Fokin, V. V. Macromolecules 2016,
49, 4473−4479. (b) Li, S.; Beringer, L. T.; Chen, S.; Averick, S.
Polymer 2015, 78, 37−41.
12657−12666. (b) Muhle, J.; Huang, J.; Weiss, R. F.; Prinn, R. G.;
̈
Miller, B. R.; Salameh, P. K.; Harth, C. M.; Fraser, P. J.; Porter, L. W.;
Greally, B. R.; O’Doherty, S.; Simmonds, P. G. J. Geophys. Res. Atmos.
2009, 114, D05306. (c) Nie, Y.; Zheng, Q.; Liang, X.; Gu, D.; Lu, M.;
Min, M.; Ji, J. Environ. Sci. Technol. 2013, 47, 7934−7939.
(23) Prakash Reddy, V.; Bellew, D. R.; Prakash, G. K. S. J. Fluorine
Chem. 1992, 56, 195−197.
(4) (a) Baranczak, A.; Liu, Y.; Connelly, S.; Du, W.-G. H.; Greiner, E.
R.; Genereux, J. C.; Wiseman, R. L.; Eisele, Y. S.; Bradbury, N. C.;
Dong, J.; Noodleman, L.; Sharpless, K. B.; Wilson, I. A.; Encalada, S.
E.; Kelly, J. W. J. Am. Chem. Soc. 2015, 137, 7404−7414. (b) Chen, W.;
Dong, J.; Li, S.; Liu, Y.; Wang, Y.; Yoon, L.; Wu, P.; Sharpless, K. B.;
Kelly, J. W. Angew. Chem., Int. Ed. 2016, 55, 1835−1838. (c) Chen, W.;
Dong, J.; Plate, L.; Mortenson, D. E.; Brighty, G. J.; Li, S.; Liu, Y.;
Galmozzi, A.; Lee, P. S.; Hulce, J. J.; Cravatt, B. F.; Saez, E.; Powers, E.
T.; Wilson, I. A.; Sharpless, K. B.; Kelly, J. W. J. Am. Chem. Soc. 2016,
138, 7353−7364. (d) Yatvin, J.; Brooks, K.; Locklin, J. Angew. Chem.,
Int. Ed. 2015, 54, 13370−13373.
(5) (a) Clark, W. M.; Tickner-Eldridge, A. M.; Huang, G. K.;
Pridgen, L. N.; Olsen, M. A.; Mills, R. J.; Lantos, I.; Baine, N. H. J. Am.
Chem. Soc. 1998, 120, 4550−4551. (b) Pridgen, L. N.; Huang, G. K.
Tetrahedron Lett. 1998, 39, 8421−8424. (c) Hanley, P. S.; Ober, M. S.;
Krasovskiy, A. L.; Whiteker, G. T.; Kruper, W. J. ACS Catal. 2015, 5,
5041−5046. (d) Liang, Q.; Xing, P.; Huang, Z.; Dong, J.; Sharpless, K.
B.; Li, X.; Jiang, B. Org. Lett. 2015, 17, 1942−1945. (e) Zhang, E.;
Tang, J.; Li, S.; Wu, P.; Moses, J. E.; Sharpless, K. B. Chem. - Eur. J.
2016, 22, 5692−5697.
(24) The two-chamber reactor is commercially available under the
trade name COware at Sigma-Aldrich and Sytracks.
(25) (a) Hermange, P.; Lindhardt, A. T.; Taaning, R. H.; Bjerglund,
K.; Lupp, D.; Skrydstrup, T. J. Am. Chem. Soc. 2011, 133, 6061−6071.
(b) Friis, S. D.; Lindhardt, A. T.; Skrydstrup, T. Acc. Chem. Res. 2016,
49, 594−605.
(26) (a) Veryser, C.; Van Mileghem, S.; Egle, B.; Gilles, P.; De
Borggraeve, W. M. React. Chem. Eng. 2016, 1, 142−146. (b) Van
Mileghem, S.; De Borggraeve, W. M. Org. Process Res. Dev. 2017, 21,
785−787. (c) Veryser, C.; Steurs, G.; Van Meervelt, L.; De
Borggraeve, W. M. Adv. Synth. Catal. 2017, 359, 1271−1276.
(d) Van Mileghem, S.; Egle, B.; Gilles, P.; Veryser, C.; Van
Meervelt, L.; De Borggraeve, W. M. Org. Biomol. Chem. 2017, 15,
373−378.
(27) (a) Chou, T. S.; Becke, L. M.; O’Toole, J. C.; Carr, M. A.;
Parker, B. E. Tetrahedron Lett. 1996, 37, 17−20. (b) Uematsu, N.;
Hoshi, N.; Ikeda, M. J. Fluorine Chem. 2006, 127, 1595−1600.
(28) These conditions were adopted from ref 5e. In each case tested,
the combination of DIPEA/MeCN performed either equally well or
better than Et3N/DCM, in some cases presumably due to solubility.
However, when this modification was not necessary, DCM was kept as
solvent for its ease of evaporation.
(6) Roth, G. P.; Fuller, C. E. J. Org. Chem. 1991, 56, 3493−3496.
(7) Roth, G. P.; Sapino, C. Tetrahedron Lett. 1991, 32, 4073−4076.
(8) (a) Roth, G. P.; Thomas, J. A. Tetrahedron Lett. 1992, 33, 1959−
1962. (b) McGuire, M. A.; Sorenson, E.; Owings, F. W.; Resnick, T.
M.; Fox, M.; Baine, N. H. J. Org. Chem. 1994, 59, 6683−6686.
(c) Fang, W.-Y.; Leng, J.; Qin, H.-L. Chem. - Asian J. 2017, 12, 2323−
2331.
(9) (a) Hanley, P. S.; Clark, T. P.; Krasovskiy, A. L.; Ober, M. S.;
O’Brien, J. P.; Staton, T. S. ACS Catal. 2016, 6, 3515−3519. (b) Lim,
T.; Byun, S.; Kim, B. M. Asian J. Org. Chem. 2017, DOI: 10.1002/
ajoc.201700064.
(10) Schimler, S. D.; Cismesia, M. A.; Hanley, P. S.; Froese, R. D. J.;
Jansma, M. J.; Bland, D. C.; Sanford, M. S. J. Am. Chem. Soc. 2017, 139,
1452−1455.
(11) Ren, G.; Zheng, Q.; Wang, H. Org. Lett. 2017, 19, 1582−1585.
(12) (a) Chen, Q.; Yu, H.; Xu, Z.; Lin, L.; Jiang, X.; Wang, R. J. Org.
Chem. 2015, 80, 6890−6896. (b) Wang, X.-Y.; Leng, J.; Wang, S.-M.;
Asiri, A. M.; Marwani, H. M.; Qin, H.-L. Tetrahedron Lett. 2017, 58,
2340−2343.
(13) Cismesia, M. A.; Ryan, S. J.; Bland, D. C.; Sanford, M. S. J. Org.
Chem. 2017, 82, 5020−5026.
(14) Stang, P. J.; Hanack, M.; Subramanian, L. R. Synthesis 1982,
1982, 85−126.
(15) Lange, W.; Muller, E. Ber. Dtsch. Chem. Ges. B 1930, 63, 2653−
̈
2657.
(16) Boudakian, M. M.; Hyde, G. A.; Kongpricha, S. J. Org. Chem.
1971, 36, 940−942.
(17) Cramer, R.; Coffman, D. J. Org. Chem. 1961, 26, 4164−4165.
(18) (a) Firth, W. C. J. Polym. Sci., Part B: Polym. Lett. 1972, 10,
637−641. (b) Hedayatullah, M.; Guy, A.; Denivelle, L. C. R. Acad. Sc.
Paris 1974, 278, 57−59. (c) Falardeau, E. R.; DesMarteau, D. D. J.
D
DOI: 10.1021/acs.orglett.7b02522
Org. Lett. XXXX, XXX, XXX−XXX