Cobalt bis(acetylacetonate)–tert-butyl hydroperoxide–triethyl-silane: A general reagent combination for the Markovnikov-selective hydrofunctionalization of alkenes by hydrogen atom transfer

Article abstract of DOI:10.3762/bjoc.14.201

We show that cobalt bis(acetylacetonate) [Co(acac)₂], tert-butyl hydroperoxide (TBHP), and triethylsilane (Et₃SiH) constitute an inexpensive, general, and practical reagent combination to initiate a broad range of Markovnikov-selective alkene hydrofunctionalization reactions. These transformations are believed to proceed by cobalt-mediated hydrogen atom transfer (HAT) to the alkene substrate, followed by interception of the resulting alkyl radical intermediate with a SOMOphile. In addition, we report the first reductive couplings of unactivated alkenes and aryldiazonium salts by an HAT pathway. The simplicity and generality of the Co(acac)₂–TBHP–Et₃SiH reagent combination suggests it as a useful starting point to develop HAT reactions in complex settings.

Full text of DOI:10.3762/bjoc.14.201

Cobalt bis(acetylacetonate)–tert-butyl hydroperoxide–triethyl-
silane: a general reagent combination for the Markovnikov-
selective hydrofunctionalization of alkenes by
hydrogen atom transfer
Xiaoshen Ma1 and Seth B. Herzon*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 2259–2265.
1Department of Chemistry, Yale University, New Haven, Connecticut
06520, United States and 2Department of Pharmacology, Yale School
of Medicine, New Haven, Connecticut 06520, United States
doi:10.3762/bjoc.14.201
Received: 22 May 2018
Accepted: 19 July 2018
Published: 28 August 2018
Email:
Seth B. Herzon* - seth.herzon@yale.edu
This article is part of the thematic issue "Cobalt catalysis".
Guest Editor: S. Matsunaga
* Corresponding author
Keywords:
HAT; hydrogen atom transfer; hydrofunctionalization
© 2018 Ma and Herzon; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
We show that cobalt bis(acetylacetonate) [Co(acac)2], tert-butyl hydroperoxide (TBHP), and triethylsilane (Et3SiH) constitute an
inexpensive, general, and practical reagent combination to initiate a broad range of Markovnikov-selective alkene hydrofunctional-
ization reactions. These transformations are believed to proceed by cobalt-mediated hydrogen atom transfer (HAT) to the alkene
substrate, followed by interception of the resulting alkyl radical intermediate with a SOMOphile. In addition, we report the
first reductive couplings of unactivated alkenes and aryldiazonium salts by an HAT pathway. The simplicity and generality of the
Co(acac)2–TBHP–Et3SiH reagent combination suggests it as a useful starting point to develop HAT reactions in complex settings.
Introduction
Many powerful methods to effect alkene hydrogenation [1-4] mation [32], hydroheteroarylation [28,33-35], hydroarylation
and Markovnikov-selective hydroheterofunctionalization (H–X [36-38], and cross-coupling [37]). Many of these transformat-
addition, X = O [5-9], I [3], Br [3], Se [3], S [8-10], Cl [8,11], F ions have found applications in synthesis [6,39-47]. Although
[12,13], and N [8,14-17]) by metal-mediated hydrogen atom mechanistically-complex [28] the outcome of these reactions
transfer (HAT) [18-21] are now known. Additionally, methods can be rationalized as initiating by HAT to the alkene, to form
to achieve carbon–carbon bond formation to alkenes by HAT the kinetically- and thermodynamically-favored alkyl radical
have been developed (e.g., reductive coupling [22-28], formal intermediate, which may be in equilibrium with the correspond-
hydromethylation [29], cycloisomerization [8,30,31], hydrooxi- ing metal alkyl complex. This radical then undergoes addition
2259

Beilstein J. Org. Chem. 2018, 14, 2259–2265.
to a second reagent (SOMOphile) to form the functionalized these studies, methallyl p-methoxybenzoate (3a) was used as
product (Scheme 1).
substrate (Table 1).
Under our standard reduction conditions, alkene 3a was trans-
formed to isobutyl p-methoxybenzoate (4a) in 86% yield (entry
1, Table 1). Inspired by the methods of Boger [12] and Hiroya
[13], a number of fluorination reagents were examined to
achieve hydrofluorination. Although no product was observed
using SelectFluor®, diethylaminosulfur trifluoride (DAST), or
tosyl fluoride (see Supporting Information File 1, Table S1,
entries 2–4), N-fluorobenzenesulfonimide (NFSI) provided the
desired hydrofluorination product 4b in 36% yield (Table 1,
entry 2). Carreira and co-workers reported the first hydrochlori-
nation reaction via a cobalt-catalyzed HAT process [11]. By
utilizing p-toluenesulfonyl chloride (TsCl) and p-toluene-
Scheme 1: General mechanism of alkene hydrofunctionalization via
HAT.
A wide range of manganese, cobalt, or iron-based complexes sulfonyl bromide (TsBr) under our conditions, the desired
containing diverse supporting ligands have found use in these hydrochlorination and hydrobromination products 4c and 4d
reactions. To the best of our knowledge, the iron oxalate–sodi- were obtained in 92% and 95% yields, respectively [3] (Table 1,
um borohydride system, introduced by Boger and co-workers entries 3 and 4). To our knowledge, the formation of 4d repre-
[8], is the only reagent combination shown to accommodate a sents the first Markovnikov-selective alkene hydrobromination
broad range of SOMOphiles. However, the cobalt–salen com- by an HAT pathway. Attempts to extend this reaction to
plexes that are commonly employed [10,11,13,15,16,30- hydroiodination using related reagents, p-toluenesulfonyl
32,36,37,48] contain many different ligand architectures [21], iodide, N-iodosuccinimide, or molecular iodine failed to
and often need to be prepared by multistep sequences. Here we provide the expected product (see Supporting Information
report a uniform set of reaction conditions to achieve a broad File 1, Table S1, entries 9–11) [3]. Surprisingly, diiodomethane,
range of HAT hydrofunctionalization reactions using the simple possessed the desired reactivity and the hydroiodination prod-
reagents cobalt acetoacetonate [Co(acac)2], tert-butyl hydroper- uct 4e was isolated in 89% yield (Table 1, entry 5) [3]. Ethyl
oxide (TBHP), and triethylsilane (Et3SiH). The practicality and iodoacetate, iodoacetonitrile, and 1,2-diiodoethane were also
generality of this system should motivate its application in syn- effective, but the yields of 4e and conversion of 3a were lower
thesis.
(see Supporting Information File 1, Table S1, entries 13–15).
Mukaiyama and co-workers’ pioneering Markonikov-selective
alkene hydration reaction [5,49-54] proceeds using dioxygen as
Results and Discussion
In 2014, we reported the reduction of alkenyl halides (e.g., 1, the oxygen atom source. Exposure of the alkene 3a to similar
Scheme 2) utilizing Co(acac)2, TBHP, and two reductants, tri- conditions provided the tertiary alcohol 4f in 69% yield
ethylsilane and 1,4-dihydrobenzene (DHB) [2]. Mechanistic (Table 1, entry 6). Inspired by Girijavallabhan and co-workers’
studies showed that Et3SiH participates in the formation of a report [10], we were able to trap the tertiary alkyl radical with
cobalt hydride intermediate that delivers a hydrogen atom to the S-phenyl benzene thiosulfonate (PhSO2SPh, Table 1, entry 7)
less-substituted position of the alkene. The resulting alkyl and Se-phenyl 4-methylbenzenesulfonoselenoate (TsSePh,
radical is believed to abstract a second hydrogen atom from Table 1, entry 8) [3] to afford the corresponding products 4g
DHB to generate the reduced product [2]. This mechanism and 4h in 96% and 89% yields, respectively. Carreira and
separates the alkyl radical formation and functionalization steps co-workers reported the hydroazidation of alkenes using
by employing two different reagents. Accordingly, we investi- cobalt–salen complexes as hydrogen atom transfer agents and
gated the application of this system in other HAT reactions. In para-toluenesulfonyl azide as an azide source [16,48,55]. After
Scheme 2: Reduction of the alkenyl chloride 1 by HAT.
2260

Beilstein J. Org. Chem. 2018, 14, 2259–2265.
Table 1: Markovnikov Hydrofunctionalization of 3a.a
entry
1
x
y
SOMOphile
–
z
solvent
product
yieldb
86%
5.00 5.00
2.50 10.0
2.50 10.0
3.75 10.0
3.75 10.0
10.0 10.0
–
n-propanol
4a
4b
4c
4d
4e
4f
2
3
4
5
6
NFSI
TsCl
TsBr
CH2I2
O2
2.50
CH2Cl2
36%
92%
95%
89%
69%
2.50 n-propanol
2.50 n-propanol
15.0
CH2Cl2
–
n-propanol
7
2.50 10.0
PhSO2SPh
2.50 n-propanol
2.50 n-propanol
96%
4g
8
9
2.50 10.0
1.00 10.0
TsSePh
89%
79%
4h
4i
p-ABSA
5.00
1.50
CH3CN
CH2Cl2
10
0
6.25
92%
5a
7a
2261

Beilstein J. Org. Chem. 2018, 14, 2259–2265.
Table 1: Markovnikov Hydrofunctionalization of 3a.a (continued)
11
12
3.75 10.0
2.50 n-propanol
60%
6a
6b
4j
3.75 10.0
2.50 n-propanol
48%
4k
66%
4.7:1
rrb
13
14
0
0
5.00
5.00
5.00
1.00
CH2Cl2
CH3CN
6c
6d
4l
50%
4m
aFor detailed reaction conditions, see Supporting Information File 1. bIsolated yields after purification by flash-column chromatography. brr = ratio of
regioisomers.
careful optimization of the azidation reagent (p-acetamido- steric and electronic properties were examined. These experi-
benzenesulfonyl azide (p-ABSA)) and additive equivalents (see ments revealed that this coupling is compatible with a broad
Supporting Information File 1, Table S1, entries 19–28), the range of functional groups and aryl substitution patterns (7d–j,
tertiary alkyl azide 4i was obtained in 79% yield (Table 1, 52–97%). The naphthylazo derivative 7k was obtained in 69%
entry 9).
yield using 1-naphthyldiazonium tetrafluoroborate. However,
aryldiazonium salts bearing nitro substituents were not compati-
To broaden the scope of C–N coupling process via HAT, we in- ble with this HAT coupling. For example, the use of p-nitroben-
vestigated other nitrogen-containing SOMOphiles in the HAT zendiazonium tetrafluoroborate failed to provide the expected
reaction. Employing 4-methoxyphenyldiazonium tetrafluoro- coupling product 7l. This may be due to alternative reaction
borate (5a) in our HAT conditions, the alkyl aryl azo product 7a pathways involving reduction of the nitro substituent [17].
was obtained in 92% yield (Table 1, entry 10). Due to the
importance of azo compounds in synthetic organic chemistry, Additional carbon–carbon bond formation strategies were also
industrial dyes, and medicinal chemistry [56,57], we investigat- examined using the Co(acac)2 HAT system. Sulfonyl oximes 6a
ed the scope of this transformation (Scheme 3). By varying the and 6b [32] afforded the carbon–carbon coupled products 4j
alkene substitution pattern, we determined that the coupling of and 4k in 60% and 48% yields, respectively (Table 1, entries 11
tertiary radicals is more efficient than secondary radicals. For and 12, respectively). Recently, our laboratory reported a
example, methallyl p-methoxybenzoate (3a) and prenyl formal intermolecular hydroheteroarylation using N-methoxy
p-methoxybenzoate (3c, not shown) afforded the azo com- heteroarenium salts by Co(acac)2-mediated HAT [33,34]. In the
pounds 7a and 7c in 92% and 91% yields, respectively. By original reports, 36 discrete unactivated alkenes were coupled
comparison the yield of the azo product using allyl p-methoxy- with 38 different heteroarenium salts under mild conditions
benzoate (3b, now shown) as substrate was somewhat lower [33,34]. A representative example comprises the coupling of
(77%). Diazonium salts bearing substituents with different methallyl p-methoxybenzoate (3a) with N-methoxypyridinium
2262

Beilstein J. Org. Chem. 2018, 14, 2259–2265.
Scheme 3: Substrate scope of alkyl-aryl azo compound synthesis via HAT. Conditions: alkene (0.250 mmol), diazonium salt (1.50 equiv), Co(acac)2
(1.00 equiv), TBHP (1.00 equiv), Et3SiH (6.25 equiv), CH2Cl2 (0.2 M), argon, 15–120 min. All yields are isolated yields after flash-column chromatog-
raphy. aReaction conducted on 1.00 mmol scale.
2263

Beilstein J. Org. Chem. 2018, 14, 2259–2265.
7. Ishikawa, H.; Colby, D. A.; Seto, S.; Va, P.; Tam, A.; Kakei, H.;
Rayl, T. J.; Hwang, I.; Boger, D. L. J. Am. Chem. Soc. 2009, 131,
4904–4916. doi:10.1021/ja809842b
methyl sulfate (6c) to form the hydropyridylation product 4l in
66% yield and as a 4.7:1 ratio of regioisomers (Table 1,
entry 13). We also demonstrated that the alkyl radical
generated from the HAT process can be trapped by (η6-
benzene)manganese tricarbonyl hexafluorophosphate (6d) to
provide the reductive coupling product 4m in 50% yield
(Table 1, entry 14).
8. Leggans, E. K.; Barker, T. J.; Duncan, K. K.; Boger, D. L. Org. Lett.
2012, 14, 1428–1431. doi:10.1021/ol300173v
9. Ma, X.; Herzon, S. B. J. Org. Chem. 2016, 81, 8673–8695.
doi:10.1021/acs.joc.6b01709
10.Girijavallabhan, V.; Alvarez, C.; Njoroge, F. G. J. Org. Chem. 2011, 76,
6442–6446. doi:10.1021/jo201016z
11.Gaspar, B.; Carreira, E. M. Angew. Chem., Int. Ed. 2008, 47,
5758–5760. doi:10.1002/anie.200801760
Conclusion
In summary, we have demonstrated that under a consistent set
of conditions, the Co(acac)2–TBHP–Et3SiH system effects a
diverse array of Markovnikov-selective hydrofunctionalization
reactions of unactivated alkenes (H–X addition, X = H, F, Cl,
Br, I, O, S, Se, N, and C). We have also reported the first reduc-
tive coupling reactions of alkenes and aryldiazonium salts under
HAT conditions. These transformations proceed in high regio-
selectivity and efficiency. Further efforts will focus on
expanding the alkene scope and exploring the site-selectivity in
polyene substrates.
12.Barker, T. J.; Boger, D. L. J. Am. Chem. Soc. 2012, 134,
13588–13591. doi:10.1021/ja3063716
13.Shigehisa, H.; Nishi, E.; Fujisawa, M.; Hiroya, K. Org. Lett. 2013, 15,
5158–5161. doi:10.1021/ol402696h
14.Kato, K.; Mukaiyama, T. Chem. Lett. 1992, 21, 1137–1140.
doi:10.1246/cl.1992.1137
15.Waser, J.; Carreira, E. M. J. Am. Chem. Soc. 2004, 126, 5676–5677.
doi:10.1021/ja048698u
16.Waser, J.; Nambu, H.; Carreira, E. M. J. Am. Chem. Soc. 2005, 127,
8294–8295. doi:10.1021/ja052164r
17.Gui, J.; Pan, C.-M.; Jin, Y.; Qin, T.; Lo, J. C.; Lee, B. J.; Spergel, S. H.;
Mertzman, M. E.; Pitts, W. J.; La Cruz, T. E.; Schmidt, M. A.;
Darvatkar, N.; Natarajan, S. R.; Baran, P. S. Science 2015, 348,
886–891. doi:10.1126/science.aab0245
Supporting Information
18.Eisenberg, D. C.; Norton, J. R. Isr. J. Chem. 1991, 31, 55–66.
doi:10.1002/ijch.199100006
Supporting Information File 1
Detailed experimental procedures and characterization data
for all new compounds.
19.Gansäuer, A.; Shi, L.; Otte, M.; Huth, I.; Rosales, A.; Sancho-Sanz, I.;
Padial, N. M.; Oltra, J. E. Hydrogen Atom Donors: Recent
Developments. In Radicals in Synthesis III; Heinrich, M.; Gansäuer, A.,
Eds.; Topics in Current Chemistry; Springer: Berlin Heidelberg, 2012.
doi:10.1007/128_2011_124
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-14-201-S1.pdf]
20.Hoffmann, R. W. Chem. Soc. Rev. 2016, 45, 577–583.
doi:10.1039/C5CS00423C
21.Crossley, S. W. M.; Obradors, C.; Martinez, R. M.; Shenvi, R. A.
Chem. Rev. 2016, 116, 8912–9000. doi:10.1021/acs.chemrev.6b00334
22.Choi, J.; Tang, L.; Norton, J. R. J. Am. Chem. Soc. 2007, 129,
234–240. doi:10.1021/ja066325i
Acknowledgements
Financial support from the National Science Foundation (CHE-
1151563) is gratefully acknowledged.
23.Choi, J.; Pulling, M. E.; Smith, D. M.; Norton, J. R. J. Am. Chem. Soc.
2008, 130, 4250–4252. doi:10.1021/ja710455c
ORCID® iDs
24.Li, G.; Han, A.; Pulling, M. E.; Estes, D. P.; Norton, J. R.
J. Am. Chem. Soc. 2012, 134, 14662–14665. doi:10.1021/ja306037w
25.Lo, J. C.; Yabe, Y.; Baran, P. S. J. Am. Chem. Soc. 2014, 136,
1304–1307. doi:10.1021/ja4117632
Xiaoshen Ma - https://orcid.org/0000-0002-0713-2896
Seth B. Herzon - https://orcid.org/0000-0001-5940-9853
References
1. Iwasaki, K.; Wan, K. K.; Oppedisano, A.; Crossley, S. W. M.;
Shenvi, R. A. J. Am. Chem. Soc. 2014, 136, 1300–1303.
doi:10.1021/ja412342g
26.Lo, J. C.; Gui, J.; Yabe, Y.; Pan, C.-M.; Baran, P. S. Nature 2014, 516,
343–348. doi:10.1038/nature14006
27.Kuo, J. L.; Hartung, J.; Han, A.; Norton, J. R. J. Am. Chem. Soc. 2015,
137, 1036–1039. doi:10.1021/ja511883b
28.Lo, J. C.; Kim, D.; Pan, C.-M.; Edwards, J. T.; Yabe, Y.; Gui, J.; Qin, T.;
Gutiérrez, S.; Giacoboni, J.; Smith, M. W.; Holland, P. L.; Baran, P. S.
J. Am. Chem. Soc. 2017, 139, 2484–2503. doi:10.1021/jacs.6b13155
29.Dao, H. T.; Li, C.; Michaudel, Q.; Maxwell, B. D.; Baran, P. S.
J. Am. Chem. Soc. 2015, 137, 8046–8049. doi:10.1021/jacs.5b05144
30.Crossley, S. W. M.; Barabé, F.; Shenvi, R. A. J. Am. Chem. Soc. 2014,
136, 16788–16791. doi:10.1021/ja5105602
2. King, S. M.; Ma, X.; Herzon, S. B. J. Am. Chem. Soc. 2014, 136,
6884–6887. doi:10.1021/ja502885c
3. Ma, X.; Herzon, S. B. Chem. Sci. 2015, 6, 6250–6255.
doi:10.1039/C5SC02476E
4. Obradors, C.; Martinez, R. M.; Shenvi, R. A. J. Am. Chem. Soc. 2016,
138, 4962–4971. doi:10.1021/jacs.6b02032
5. Isayama, S.; Mukaiyama, T. Chem. Lett. 1989, 18, 573–576.
doi:10.1246/cl.1989.573
31.Gaspar, B.; Carreira, E. M. Angew. Chem., Int. Ed. 2007, 46,
4519–4522. doi:10.1002/anie.200700575
6. Ishikawa, H.; Colby, D. A.; Boger, D. L. J. Am. Chem. Soc. 2008, 130,
420–421. doi:10.1021/ja078192m
32.Gaspar, B.; Carreira, E. M. J. Am. Chem. Soc. 2009, 131,
13214–13215. doi:10.1021/ja904856k
2264

Beilstein J. Org. Chem. 2018, 14, 2259–2265.
33.Ma, X.; Herzon, S. B. J. Am. Chem. Soc. 2016, 138, 8718–8721.
doi:10.1021/jacs.6b05271
License and Terms
34.Ma, X.; Dang, H.; Rose, J. A.; Rablen, P.; Herzon, S. B.
J. Am. Chem. Soc. 2017, 139, 5998–6007. doi:10.1021/jacs.7b02388
35.Bordi, S.; Starr, J. T. Org. Lett. 2017, 19, 2290–2293.
doi:10.1021/acs.orglett.7b00833
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/4.0). Please note
that the reuse, redistribution and reproduction in particular
requires that the authors and source are credited.
36.Crossley, S. W. M.; Martinez, R. M.; Guevara-Zuluaga, S.;
Shenvi, R. A. Org. Lett. 2016, 18, 2620–2623.
doi:10.1021/acs.orglett.6b01047
37.Green, S. A.; Matos, J. L. M.; Yagi, A.; Shenvi, R. A.
J. Am. Chem. Soc. 2016, 138, 12779–12782.
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
doi:10.1021/jacs.6b08507
38.Green, S. A.; Vásquez-Céspedes, S.; Shenvi, R. A. J. Am. Chem. Soc.
2018. doi:10.1021/jacs.8b05868
(https://www.beilstein-journals.org/bjoc)
39.Shenvi, R. A.; Guerrero, C. A.; Shi, J.; Li, C.-C.; Baran, P. S.
J. Am. Chem. Soc. 2008, 130, 7241–7243. doi:10.1021/ja8023466
40.Schindler, C. S.; Stephenson, C. R. J.; Carreira, E. M.
Angew. Chem., Int. Ed. 2008, 47, 8852–8855.
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.14.201
doi:10.1002/anie.200803655
41.Jeker, O. F.; Carreira, E. M. Angew. Chem., Int. Ed. 2012, 51,
3474–3477. doi:10.1002/anie.201109175
42.Barker, T. J.; Duncan, K. K.; Otrubova, K.; Boger, D. L.
ACS Med. Chem. Lett. 2013, 4, 985–988. doi:10.1021/ml400281w
43.Leggans, E. K.; Duncan, K. K.; Barker, T. J.; Schleicher, K. D.;
Boger, D. L. J. Med. Chem. 2013, 56, 628–639.
doi:10.1021/jm3015684
44.Shigehisa, H.; Suwa, Y.; Furiya, N.; Nakaya, Y.; Fukushima, M.;
Ichihashi, Y.; Hiroya, K. Angew. Chem., Int. Ed. 2013, 52, 3646–3649.
doi:10.1002/anie.201210099
45.George, D. T.; Kuenstner, E. J.; Pronin, S. V. J. Am. Chem. Soc. 2015,
137, 15410–15413. doi:10.1021/jacs.5b11129
46.Ruider, S. A.; Sandmeier, T.; Carreira, E. M. Angew. Chem., Int. Ed.
2015, 54, 2378–2382. doi:10.1002/anie.201410419
47.Allemann, O.; Brutsch, M.; Lukesh, J. C.; Brody, D. M.; Boger, D. L.
J. Am. Chem. Soc. 2016, 138, 8376–8379. doi:10.1021/jacs.6b04330
48.Waser, J.; Gaspar, B.; Nambu, H.; Carreira, E. M. J. Am. Chem. Soc.
2006, 128, 11693–11712. doi:10.1021/ja062355+
49.Mukaiyama, T.; Isayama, S.; Inoki, S.; Kato, K.; Yamada, T.; Takai, T.
Chem. Lett. 1989, 18, 449–452. doi:10.1246/cl.1989.449
50.Inoki, S.; Kato, K.; Takai, T.; Isayama, S.; Yamada, T.; Mukaiyama, T.
Chem. Lett. 1989, 18, 515–518. doi:10.1246/cl.1989.515
51.Isayama, S.; Mukaiyama, T. Chem. Lett. 1989, 18, 569–572.
doi:10.1246/cl.1989.569
52.Isayama, S.; Mukaiyama, T. Chem. Lett. 1989, 18, 1071–1074.
doi:10.1246/cl.1989.1071
53.Kato, K.; Yamada, T.; Takai, T.; Inoki, S.; Isayama, S.
Bull. Chem. Soc. Jpn. 1990, 63, 179–186. doi:10.1246/bcsj.63.179
54.Isayama, S. Bull. Chem. Soc. Jpn. 1990, 63, 1305–1310.
doi:10.1246/bcsj.63.1305
55.Gaspar, B.; Waser, J.; Carreira, E. M. Synthesis 2007, 3839–3845.
doi:10.1055/s-2007-1000817
56.Hunger, K.; Mischke, P.; Rieper, W.; Zhang, S. Azo Dyes. Ullmann's
Encyclopedia of Industrial Chemistr; Wiley-VCH Verlag GmbH & Co.
KGaA, 2000. doi:10.1002/14356007.o03_o07.pub2
57.Acton, Q. A. Azo Compounds—Advances in Research and Application:
2013 Edition: ScholarlyBrief; ScholarlyEditions: Atlanta, GA, 2013.
2265