Iron-Catalyzed Intramolecular Perezone-Type [5 + 2] Cycloaddition: Access to Tricyclo[6.3.1.01,6]dodecane

Article abstract of DOI:10.1021/acs.orglett.8b00989

An iron-catalyzed perezone-type [5 + 2] cycloaddition toward tricyclo[6.3.1.0^1,6]dodecane scaffolds is presented, furnishing cycloadducts with two new C-C bonds and three to four stereogenic centers generated with typically good yields and excellent diastereoselectivities. Two independent conditions, catalytic FeCl₃/PhCO₃tBu (0.5 equiv/1 equiv) and stoichiometric FeCl₃(2 equiv), have been respectively manifested to be applicable for various substrates. Mechanistically, the reaction may proceed via iron-mediated generation of a carbon cationic center, which triggers subsequent [5 + 2] cyclization. Derivation of the cycloadduct was conducted to testify the applicability of this method.

Full text of DOI:10.1021/acs.orglett.8b00989

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Iron-Catalyzed Intramolecular Perezone-Type [5 + 2] Cycloaddition:
Access to Tricyclo[6.3.1.01,6]dodecane
Yongjiang Liu,† Xiao Wang,† Song Chen,† Shaomin Fu,† and Bo Liu*,†,‡
†Key Laboratory of Green Chemistry &Technology of Ministry of Education, College of Chemistry, Sichuan University, Chengdu
610064, China
‡State key Laboratory of Natural Medicines, China Pharmaceutical University, Nanjing 210009, China
S
* Supporting Information
ABSTRACT: An iron-catalyzed perezone-type [5 + 2] cycloaddition toward tricyclo[6.3.1.01,6]dodecane scaffolds is presented,
furnishing cycloadducts with two new C−C bonds and three to four stereogenic centers generated with typically good yields and
excellent diastereoselectivities. Two independent conditions, catalytic FeCl3/PhCO3tBu (0.5 equiv/1 equiv) and stoichiometric
FeCl3(2 equiv), have been respectively manifested to be applicable for various substrates. Mechanistically, the reaction may
proceed via iron-mediated generation of a carbon cationic center, which triggers subsequent [5 + 2] cyclization. Derivation of the
cycloadduct was conducted to testify the applicability of this method.
harmaceutical ingredients and natural products embodying
diverse tricyclo[6.3.1.01,6]dodecane subunits have been
floating throughout the literature with a remarkable range of
biological activities. As highlighted in Figure 1, campylopin (1)
base A (4), isolated from Aconitum coreanum, was reported to
exhibit antiarrhythmic activities.2e
P
Synthetically, the above core tricyclo[6.3.1.01,6]dodecane
motif could be accessible via intramolecular perezone-type [5
+ 2] cycloaddition.3 Although the identical strategy has been
extensively implemented to forge tricyclo[5.3.1.01,5]undecane
subunits and related natural product syntheses, further
expansion to the tricyclo[6.3.1.01,6]dodecane skeleton was not
successful (Scheme 1a). To date, only two cases of intra-
molecular perezone-type [5 + 2] cycloaddition toward
tricyclo[6.3.1.01,6] dodecane have been documented, including
LiClO4/TMSOTf-promoted cyclization from the Grieco
group4 and anionic oxidation enabled ring formation by the
Yamamura group.5 Both studies resulted in either moderate
yields or very limited substrate scope (Scheme 1b). This
insufficiency can be ascribed to the relatively slower cyclization
rate to form a six-membered ring than that in robust formation
of a five-membered ring, which hinted at a synthetic challenge
and deserved further insightful investigation. Recently, our
group became captivated by tricyclo[6.3.1.01,6]dodecane
subunits on account of their exquisite structure and potent
biological activities. We now present our discovery on an iron-
catalyzed intramolecular perezone-type [5 + 2] cycloaddition
toward tricyclo[6.3.1.01,6]dodecane compounds with satisfac-
tory yields, diastereoselectivities, and broad substrate scope
(Scheme 1c).
Figure 1. Structures of tricyclo[6.3.1.01,6]dodecane and related natural
products.
and its analogues, for example, function as the selective
antagonists of the neuronal α7 nAChR receptors, which hold
promise as an agent in the treatment of Alzheimer’s disease.1
Aphidicolin (2), a DNA polymerase inhibitor, shows antiviral
activity against human herpes viruses.2a,b Lappaconitine hydro-
bromide (3), a C18-diterpenoid alkaloid, possesses remarkable
anti-inflammatory activity and is eight times more potent than
cocaine as an anesthetic in the rabbit cornea model.2c,d Guaufu
Received: March 28, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00989
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Progress in Synthetic Reactions Involving
Perezone-Type [5 + 2] Cycloaddition
Table 1. Screening of Reaction Conditions
b
c
oxidant
yield
a
entry Lewis acid (x equiv) temp (°C)
(2.5 equiv)
(%)
1
2
3
4
5
6
7
8
9
InBr3(2.0)
−40
−40
−78 to rt
−78 to rt
CM
CM
CM
NR
NR
20
BBr3 (2.0)
ZrCl4 (2.0)
Et2AlCl (2.0)
LaCl3·LiCl (2.0)
FeCl3 (2.0)
SnCl4(2.0)
−78 to rt
−78
−78
0
20
SnCl4 (2.0)
FeCl3 (2.0)
FeCl3·6H2O (2.0)
Fe(OTf)3 (2.0)
Fe(acac)3 (2.0)
Fe(OTf)2 (2.0)
FeCl3 (0.2)
FeCl3 (0.2)
FeCl3 (0.2)
FeCl3 (0.2)
FeCl3 (0.2)
FeCl3 (0.5)
FeCl3 (0.5)
FeCl3 (0.3)
FeCl3 (0.5)
45
d
0
83
10
11
12
13
14
15
16
17
18
19
20
0
35
0
CM
NR
NR
CM
CM
22
0
0
0
O2 (balloon)
(iPrO)2
Because the corresponding intramolecular cycloaddition
toward tricyclo[5.3.1.01,5]undecane has been well developed,3a
we focused our attention on intramolecular perezone-type [5 +
2] cycloaddition toward tricyclo[6.3.1.01,6]dodecane. Initially,
we screened a number of Lewis acid catalysts, but they gave
poor results (Table 1, entries 1−5; see the Supporting
Information (SI) for more details). We then discovered that
iron has been extensively applied as an abundant and
environmentally benign catalyst or promoter in organic
synthesis,6 especially in several cycloaddition reactions.7 FeCl3
(2 equiv) was then tested and found to promote the desired
cycloaddition product 2a at −78 °C, albeit in 20% yield (Table
1, entry 6). Although SnCl4 also gave a comparable yield (20%)
under identical conditions, FeCl3 could give a higher product
yield than SnCl4 at elevated temperature (Table 1, entries 7−
9). In addition, higher catalyst loading did not increase the
yield, and lower loading reduced the yield (see the SI for
details). Furthermore, FeCl3·6H2O delivered cycloadducts in
35% yield (Table 1, entry 10), indicating the need for
anhydrous FeCl3. Iron(III) triflate, a stronger Lewis acid than
FeCl3, proved inefficient, generating a complex mixture. Other
iron(III) and iron(II) Lewis acids could not catalyze this
cycloaddition reaction under the conditions we tested (Table 1,
entries 11−13; see the SI for details).
Next, we screened various additives to determine whether or
not they might assist FeCl3 and thus reduce the amount of
iron(III) catalyst needed. Interestingly, we discovered that
oxidizing additives strongly influenced the cycloaddition with
use of 0.2 equiv of FeCl3 as the catalyst. Of the various oxidants
(2.5 equiv) tested, only DDQ, CAN, and PhCO3tBu afforded
the desired cycloadducts in respective yields of 62%, 22%, and
66% (Table 1, entries 14−18; see the SI for details). Prudent
augmentation of FeCl3 to 50 mol % could enable higher
reactivity (79% yield, Table 1, entry 19). The optimal yield of
81% was achieved by using 0.5 equiv of FeCl3 and 1 equiv of
PhCO3tBu (Table 1, entry 20).8 Delightfully, the reaction
performed on a 2.3 g scale proceeded smoothly (Table 1, entry
21). Using either FeCl3 or PhCO3tBu on its own did not
support the reaction (Table 1, entries 22 and 23). Notably, our
extensive study on the transformation of 1,4-benzoquinones 2′
to 2a′ with various catalysts was unsuccessful,9 although several
studies achieved intermolecular cycloaddition reactions using
0
0
CAN
0
DDQ
62
0
PhCO3tBu
PhCO3tBu
PhCO3tBu (1.0)
PhCO3tBu (1.0)
66
0
79
e
0
81
f
21
22
23
0
63
0
18
0
PhCO3tBu
NR
a
b
Reactions were performed on a 0.19 mmol scale. 2.5 equiv of
c
oxidant was added unless otherwise stated. Isolated yield after full
conversion of 1a (TLC control). NR, no reaction; CM, complex
mixture. Defined as conditions A. Defined as conditions B. Gram
scale (2.3 g of 1a).
d
e
f
unmasked p-benzoquinones and aromatic alkenes toward
bicyclo[3.2.1]octane or using intramolecular cycloaddition
reactions with unmasked p-benzoquinones as substrates toward
tricyclo[5.3.1.01,5]undecane.3a Our results reveal the partic-
ularity of intramolecular perezone-type [5 + 2] cycloaddition
toward tricyclo[6.3.1.01,6]dodecane.
From the reaction conditions shown in Table 1, we selected
the conditions in entry 9 as conditions A and those in entry 20
as conditions B and then examined the substrate scope of the
[5 + 2] cycloaddition under these two sets of conditions.
Conditions A and B were compatible with a wide range of
substrates with different chain lengths, various substitutions on
the chain tether, and diverse alkenes (Scheme 2). Under both
conditions A and B, the reaction could be extended to afford
cycloadduct 2b as well with the tricyclo[5.3.1.01,5]undecane
motif, indicating general applicability. Even after the methoxyl
ketal was changed to an allyloxyl ketal, both conditions proved
effective at generating 2c in comparable yield (79%, 72%) with
excellent dr (>20:1). Furthermore, chain substitution adjacent
to the benzoquinone ring proceeded with remarkable stereo-
chemical control during cycloaddition, furnishing compounds
2d and 2e with decent diastereoselectivity. However,
substitution distant from the benzoquinone ring did not
proceed with satisfactory diastereoselectivity, such as in the
reaction to generate 2f (dr = 1:1). Notably, the substrates scope
could be extensively broadened to diverse alkenes, including
B
DOI: 10.1021/acs.orglett.8b00989
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. [5 + 2] Cycloaddition toward
Scheme 3. Derivatization of Compounds Obtained Using
Iron-Catalyzed Intramolecular Perezone-Type [5 + 2]
Cycloaddition
a
Tricyclo[6.3.1.01,6]
hydrazone formation gave compound 5 whose structure was
confirmed via NMR analysis and X-ray crystallography. In
addition, photoirradiation of 2a produced compound 6 after
skeleton rearrangement of a diradical intermediate. The
structure of 6, confirmed by X-ray crystallography, was of
potential value for accessing trachylobanes with vasorelaxant
activity.11
A tentative mechanism for the present Fe-catalyzed [5 + 2]
cycloaddition is depicted in Scheme 4a. The reaction
commenced with complexation of substrate 1a with FeCl3 to
configure a five-membered didentated intermediate I. Under
Scheme 4. Plausible Mechanism and Stereochemical
Analysis of Product
a
b
Isolated yield. The E-alkene of 1g,l,n were submitted to this
reaction.
trans-disubstituted, gem-disubstituted, and tri- and tetrasub-
stituted alkenes, leading to cycloadduct products featuring
several contiguous tertiary or quaternary stereogenic centers
(2g−j,l−o) with 46−93% yields and 6:1 to >20:1 diaster-
eoselectivities. After that, we were curious about how various
substituents on the benzoquinones would impact the efficiency
of cycloaddition. Gratifyingly, adding a methyl group on the
benzoquinone maintained the reactivity (2k, 80−87% yield,
>20:1 dr). Furthermore, varying the methyl substituent on
benzoquinone to allyl or chloride resulted in the corresponding
cycloadducts 2q and 2r with comparable yields (82−90%) and
decent diastereoselectivities (17:1 to >20:1).10 Surprisingly,
even compound 2p, with four contiguous quaternary carbon
centers, could be assembled as a single diastereomer.
Unfortunately, a further survey on substrates with heteroatom
on tethered alkyl chain does not furnish the desired [5 + 2]
cycloproduct (see the SI for details).
The [5 + 2] cycloadducts constructed by our approach
consist of several functional groups that could be exploited for
further transformations (Scheme 3). In the presence of base,
Horner−Wadsworth−Emmons reaction of compound 2a with
the phosphate 3 selectively transformed the ketone on the
bridge into a Z-alkene, affording compound 4, the structure of
which was confirmed by NOE experiments. Moreover, selective
C
DOI: 10.1021/acs.orglett.8b00989
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
conditions A, excess FeCl3 was capable of extracting OMe to
form the anionic [MeO-FeCl3]− and the cationic intermediate
III. Alternatively, under conditions B, Fe(III) complex I might
be initially oxidized to Fe(IV) species II.12 The Fe(IV) species
II held stronger Lewis acidity, which was beneficial to eliminate
a CH3O group on the masked benzoquinone, leading to the
cationic III and the anionic [CH3O-Fe(IV)Cl3L]−. The above
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: chembliu@scu.edu.cn.
ORCID
Bo Liu: 0000-0002-6235-1573
Notes
alkoxyl Fe(IV) species was supposed to be unstable and would
The authors declare no competing financial interest.
13
̇
undergo SET to give rise to CH3−O: →Fe(III)Cl3. This
radical Fe(III) species would further act on substrate 1a via
ligand exchange to release the radical CH3O•, which was
subsequently captured by tBuO• to result in CH3OOtBu and
regenerate intermediate I. Accordingly, reactions in both
conditions might share the same pathway involving the
intermediate III, which could lead to the following stepwise
electrophilic [5 + 2] cycloaddition to give the intermediate IV.
Final demethoxylation aided by H2O and FeCl3 could furnish
the cycloadduct 2a.14
Furthermore, we rationalized the stereochemistry of the
cycloadducts (Scheme 4b). Exposure of compound 1 under
conditions A or B results in the intermediate III or III′. The
presence of [1, 3] axial repulsion in the chair conformation of
the cyclohexane in the transition state favors the pathway
involving the intermediate III, which harbors a less bulky RS at
the axial position. Otherwise, more steric repulsion in
intermediate III′ resulted in the minor cycloadduct. In the
pathway involving intermediate III, the first intramolecular
cyclization generates intermediate VI′ through a cationic 6-exo-
trig mode, in which steric repulsion between R2 and RL drives
single bond rotation to furnish intermediate VI. A second
intramolecular cyclization gives intermediate IV, which
subsequently undergoes demethoxylation under FeCl3 to
furnish compound 2 as the major cycloadduct.
ACKNOWLEDGMENTS
■
We acknowledge financial support from the NSFC (21672153)
and the Open Fund of State Key Laboratory of Natural
Medicines in China Pharmaceutical University
(SKLNMKF201810). We also thank the Analytical & Testing
Center of Sichuan University for NMR and X-ray recording.
REFERENCES
■
(1) (a) Wang, F.-P.; Yan, L.-P. Tetrahedron 2007, 63, 1417.
(b) Manchand, P. S.; White, J. D.; Wright, H.; Clardy, J. J. Am.
Chem. Soc. 1973, 95, 2705.
(2) (a) Selwood, D. L.; Challand, S. R.; Champness, J. N.; Gillam, J.;
Hibberd, D. K.; Jandu, K. S.; Lowe, D.; Pether, M.; Selway, J.; Trantor,
G. E. J. Med. Chem. 1993, 36, 3503. (b) Wang, F.-P.; Chen, Q.-H.;
Liang, X.-T. In The Alkaloids: Chemistry and Biology; Cordell, G. A.,
Ed.; Elsevier Science: New York, 2009; Vol. 67, pp 1−78. (c) Guo, R.
H.; Guo, C. X.; He, D.; Zhao, D. K.; Shen, Y. Chin. J. Chem. 2017, 35,
1644. (d) Wang, F.-P.; Chen, Q.-H. In The Alkaloids: Chemistry and
Biology; Cordell, G. A., Ed.; Elsevier Science: New York, 2010; Vol. 69,
pp 1−577. (e) Wang, F.-P.; Liang, X.-T. In The Alkaloids: Chemistry
and Biology; Cordell, G. A., Ed.; Elsevier Science: New York, 2002;
Vol. 59; pp 1−280.
(3) For recent review article, see: (a) Liu, X.; Hu, Y.-J.; Fan, J.-H.;
Zhao, J.; Li, S.; Li, C.-C. Org. Chem. Front. 2018, 5, 1217. (b) Pellissier,
H. Adv. Synth. Catal. 2018, 360, 1551. (c) Wang, Y.; Yu, Z.-X. Acc.
Chem. Res. 2015, 48, 2288. (d) Ylijoki, K. E. O.; Stryker, J. M. Chem.
Rev. 2013, 113, 2244. (e) Jiao, L.; Yu, Z.-X. J. Org. Chem. 2013, 78,
6842. For selected research work published recently, see: (f) Liu, J.;
Wu, J.; Fan, J.; Yan, X.; Mei, G.; Li, C.-C. J. Am. Chem. Soc. 2018, 140,
5365. (g) He, C.; Hu, J.; Wu, Y.; Ding, H. J. Am. Chem. Soc. 2017, 139,
6098. (h) He, C.; Bai, Z.; Hu, J.; Wang, B.; Xie, H.; Yu, L.; Ding, H.
Chem. Commun. 2017, 53, 8435. (i) Liu, X.; Liu, J.; Zhao, J.; Li, S.; Li,
C.-C. Org. Lett. 2017, 19, 2742. (j) Wilkerson-Hill, S. M.; Sawano, S.;
Sarpong, R. J. Org. Chem. 2016, 81, 11132. (k) Hu, C.; Song, R.-J.; Hu,
M.; Yang, Y.; Li, J.-H.; Luo, S. Angew. Chem., Int. Ed. 2016, 55, 10423.
(l) D’ Erasmo, M. P.; Meck, C.; Lewis, C. A.; Murelli, R. P. J. Org.
Chem. 2016, 81, 3744. (m) Feng, J.-J.; Lin, T.-Y.; Wu, H.-H.; Zhang, J.
Angew. Chem., Int. Ed. 2015, 54, 15854. (n) Feng, J.-J.; Lin, T.-Y.; Wu,
H.-H.; Zhang, J. J. Am. Chem. Soc. 2015, 137, 3787. (o) Mei, G.; Liu,
X.; Qiao, C.; Chen, W.; Li, C.-C. Angew. Chem., Int. Ed. 2015, 54, 1754.
(p) Mei, G.; Yuan, H.; Gu, Y.; Chen, W.; Chung, L. W.; Li, C.-C.
Angew. Chem., Int. Ed. 2014, 53, 11051. (q) Hong, X.; Stevens, M. C.;
Liu, P.; Wender, P. A.; Houk, K. N. J. Am. Chem. Soc. 2014, 136,
17273. (r) Witten, M. R.; Jacobsen, E. N. Angew. Chem., Int. Ed. 2014,
53, 5912.
In conclusion, we have developed an iron-catalyzed intra-
molecular perezone-type [5 + 2] cycloaddition toward
tricyclo[6.3.1.01,6]dodecane. The efficacy of our approach,
coupled with the existence of various functional group handles
for further transformation in the tricyclo[6.3.1.01,6]dodecane
motif, will definitely enable its application in total synthesis of
related pharmaceutically useful compounds and natural
products.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00989.
Detailed experimental procedures, spectroscopic data, 1H
and 13C NMR spectra and X-ray crystallographic data of
new compounds (PDF)
(4) (a) Collins, J. L.; Grieco, P. A.; Walker, J. K. Tetrahedron Lett.
1997, 38, 1321. (b) Grieco, P. A.; Walker, J. K. Tetrahedron 1997, 53,
8975. Although the absolute configuration of product we proposed
was identical to that reported by Gerieco’s group, our NMR data were
not consistent with those of Gerieco’s group. The absolute
configuration of our products was confirmed by X-ray crystallography.
See the SI for details.
(5) Maki, S.; Suzuki, T.; Kosemura, S.; Shizuri, Y.; Yamamura, S.
Tetrahedron Lett. 1991, 32, 4973.
(6) For recent review articles, see: (a) Bauer, I.; Knolker, H.-J. Chem.
Accession Codes
CCDC 1524256−1524259, 1524261, 1524263−1524264,
1524266, 1524269, and 1529719 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
̈
Rev. 2015, 115, 3170. (b) Furstner, A. ACS Cent. Sci. 2016, 2, 778.
̈
(c) Song, Y.; Tang, X.; Hou, X.; Bai, Y. Youji Huaxue 2013, 33, 76.
D
DOI: 10.1021/acs.orglett.8b00989
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
For a recent review article, see: (d) Wang, C.; Wan, B. Chin. Sci. Bull.
2012, 57, 2338.
(7) For selected research work published recently, see: (a) Wang, Q.;
Qi, X.; Lu, L.-Q.; Li, T.-R.; Yuan, Z.-G.; Zhang, K.; Li, B.-J.; Lan, Y.;
Xiao, W.-J. Angew. Chem., Int. Ed. 2016, 55, 2840. (b) Liu, C.-H.;
Zhuang, Z.; Bose, S.; Yu, Z.-X. Tetrahedron 2016, 72, 2752. (c) Wang,
C.; Wang, D.; Xu, F.; Pan, B.; Wan, B. J. Org. Chem. 2013, 78, 3065.
(d) Lane, T. K.; D’Souza, B. R.; Louie, J. J. Org. Chem. 2012, 77, 7555.
(8) ICP-OES analysis of our FeCl3 catalyst reveals it contains Cu, Ni,
Al, Ti, Cr, Zn, Mg, Ca, and Mn. However, the corresponding chloride
salts of these metals displayed no activity or very weak activity to
promote intramolecular [5 + 2] cycloaddition of 1a. See the SI for
details.
(9) We also exerted extensive study on the transformation of 1,4-
benzoquinones 2′ to cycloadduct 2a′ with various Lewis acids, but
failed. See the SI for details.
(10) Relative stereochemistry of the cycloadducts 2d, 2e, 2f, 2h, 2i,
2m, 2n, and 2p was determined by X-ray crystallography. The relative
configuration of other cycloadducts was confirmed by analogy
accordingly.
(11) (a) Baccelli, C.; Navarro, I.; Block, S.; Abad, A.; Morel, N.;
Quetin-Leclercq, J. J. Nat. Prod. 2007, 70, 910. (b) Martinsen, A.;
Baccelli, C.; Navarro, I.; Abad, A.; Quetin-Leclercq, J.; Morel, N. Vasc.
Pharmacol. 2010, 52, 63.
(12) The reaction involved an Fe(IV) process; see: (a) Uchiyama,
N.; Shirakawa, E.; Nishikawa, R.; Hayashi, T. Chem. Commun. 2011,
47, 11671. (b) Kshirsagar, U. A.; Regev, C.; Parnes, R.; Pappo, D. Org.
Lett. 2013, 15, 3174. (c) Deb, A.; Manna, S.; Maji, A.; Dutta, U.; Maiti,
D. Eur. J. Org. Chem. 2013, 2013, 5251. (d) Du, W. B.; Tian, L. X.; Lai,
J. S.; Huo, X.; Xie, X.; She, X. G.; Tang, S. C. Org. Lett. 2014, 16, 2470.
(e) Bonnot, F.; Molle, T.; Men
Favaudon, V.; Houee-Levin, C.; Nivier
134, 5120.
́
age, S.; Moreau, Y.; Duval, S.;
́
̀
e, V. J. Am. Chem. Soc. 2012,
(13) A similar TEMPO-Fe(III)Cl3 complex was reported by Hayton
et al.; see: Scepaniak, J. J.; Wright, A. M.; Lewis, R. A.; Wu, G.; Hayton,
T. W. J. Am. Chem. Soc. 2012, 134, 19350.
(14) For the related control experiments, see the SI for details.
E
DOI: 10.1021/acs.orglett.8b00989
Org. Lett. XXXX, XXX, XXX−XXX