Versatile synthesis of cadalene and iso-cadalene from himachalene mixtures: Evidence and application of unprecedented rearrangements

Article abstract of DOI:10.1016/j.cclet.2020.03.008

From a mixture of α-, β- and γ-himachalenes extracted from waste wood of Atlas cedar (Cedrus atlantica), cadalene (1,6-dimethyl-4-isopropylnaphthalene) and iso-cadalene (1,6-dimethyl-3-isopropylnaphthalene) were produced in two steps with up to 71percent ± 5percent yield through the ar-himachalene intermediate using I₂ and/or AlCl₃ as reagents. The selectivity is shown to sharply depend on the operating conditions: while I₂/AlCl₃ in dichloromethane promotes the formation of cadalene, the formation of iso-cadalene is favored in the presence of AlCl₃ in cyclohexane. The bicyclic aromatic compounds were thus obtained through unique rearrangements involving sequential C–C bond cleavage/formation and hydride transfer processes. In the absence of AlCl₃ or I₂, dihydrocurcumene was also found to be formed with up to 70percent selectivity. A tentative mechanism is proposed and discussed.

Full text of DOI:10.1016/j.cclet.2020.03.008

Journal Pre-proof
Versatile synthesis of cadalene and iso-cadalene from himachalene
mixtures: evidence and application of unprecedented rearrangements
Mustapha Ait El Had, Abdelouahd Oukhrib, Mohamed Zaki, Martine
Urrutigo¨ıty, Ahmed Benharref, Remi Chauvin
PII:
S1001-8417(20)30127-3
https://doi.org/10.1016/j.cclet.2020.03.008
CCLET 5506
DOI:
Reference:
To appear in:
Chinese Chemical Letters
Received Date:
Revised Date:
Accepted Date:
18 December 2019
19 February 2020
3 March 2020
Please cite this article as: Ait El Had M, Oukhrib A, Zaki M, Urrutigo¨ıty M, Benharref A,
Chauvin R, Versatile synthesis of cadalene and iso-cadalene from himachalene mixtures:
evidence and application of unprecedented rearrangements, Chinese Chemical Letters
(2020), doi: https://doi.org/10.1016/j.cclet.2020.03.008
This is a PDF file of an article that has undergone enhancements after acceptance, such as
the addition of a cover page and metadata, and formatting for readability, but it is not yet the
definitive version of record. This version will undergo additional copyediting, typesetting and
review before it is published in its final form, but we are providing this version to give early
visibility of the article. Please note that, during the production process, errors may be
discovered which could affect the content, and all legal disclaimers that apply to the journal
pertain.
© 2019 Published by Elsevier.

Communication
Versatile synthesis of cadalene and iso-cadalene from himachalene mixtures:
evidence and application of unprecedented rearrangements
Mustapha Ait El Hada, Abdelouahd Oukhriba*, Mohamed Zakia*, Martine Urrutigoïtyb,c, Ahmed
Benharrefa, Remi Chauvinb,d

aLaboratoire de Chimie Biomoléculaire, substances naturelles et Réactivité (URAC 16), Faculté des Sciences Semlalia, Université Cadi Ayyad, B.P. 2390,
Marrakech, Morocco
bCNRS, LCC (Laboratoire de Chimie de Coordination), BP 44099, 31077 Toulouse Cedex 4, France
cINP-ENSIACET, 4 allée Emile Monso 31030 Toulouse Cedex, France
dUniversité de Toulouse, UPS, ICT-FR 2599, 31062 Toulouse Cedex 9, France
Graphical Abstract
A versatile selective valorization of an abundant essential oil mixture from Atlas cedar is disclosed: it relies on unprecedented
redox-controlled skeletal rearrangements in aromatic chemistry, occurring under green conditions.
ARTICLE INFO
ABSTRACT
Article history:
From a mixture of α-, β- and γ-himachalenes extracted from waste wood of Atlas cedar (Cedrus
atlantica), cadalene (1,6-dimethyl-4-isopropylnaphthalene) and iso-cadalene (1,6-dimethyl-3-
isopropylnaphthalene) were produced in two steps with up to 71%±5% yield through the ar-
himachalene intermediate using I2 and/or AlCl3 as reagents. The selectivity is shown to sharply
depend on the operating conditions: while I2/AlCl3 in dichloromethane promotes the formation
of cadalene, the formation of iso-cadalene is favored in the presence of AlCl3 in cyclohexane.
The bicyclic aromatic compounds were thus obtained through unique rearrangements involving
sequential C-C bond cleavage/formation and hydride transfer processes. In the absence of AlCl3
or I2, dihydrocurcumene was also found to be formed with up to 70% selectivity. A tentative
mechanism is proposed and discussed.
Received 18 December 2020
Received in revised form 20 February 2020
Accepted 24 February 2020
Available online
Keywords:
Aromatic reactivity
Cadalenes
Dihydrocurcumene
Himachalenes
Rearrangements
Plants constitute a sustainable source of complex chemical molecules exploited in perfume, food, cosmetic and pharmaceutical
i—nd—ustr—y [1-4]. Among renewable resources, valuable products occurring in relatively high concentration in biomass include fatty acids,
 Corresponding authors.
E-mail addresses: oukhrib@gmail.com (A. Oukhrib), zaki.smc@gmail.com (M. Zaki), chauvin@lcc-toulouse.fr (R. Chauvin).

their methyl esters or triglycerides (vegetable oils), and terpenes. As relatively small stereo-chemically defined molecules, terpenes are
widely used as key building blocks for the industrial hemi-synthetic production of fragrances, flavors or pharmaceuticals [5,6].
The essential oil of Atlas cedar (Cedrus atlantica) is a unique raw material for the perfume industry [7]. The main component of the
essence (75%) is a mixture of three isomeric bicyclic sesquiterpenes, the α-, β- and γ-cis-himachalenes 1a-c, the reactivity of which, and
derivatives thereof, having been extensively studied [8-14]. Previous studies have thus shown that himachalene mixtures can be
converted to ar-himachalene 2 in the presence of various reagents [15-22]. In this context, we have been interested in synthesizing other
aromatic sesquiterpenes such as cadalene 3 and iso-cadalene 4, occurring as biomarkers in various sediments [23-25], and particularly
abundant in fossil resins, plant resins and essential oils produced by terrestrial plants [26].
Very few efforts have been dedicated to the total or hemi-synthesis of these sesquiterpenes, more particularly iso-cadalene, the total
synthesis or chemical modification of which had not been reported. Long after Ruzicka and Seidel's report on the synthesis of cadalene
from carvone [27], other routes were reported to occur in low yields (up to 27%) [28-30], the shortest one consisting in a one-step
dehydrogenation of cadinene [31]. Nevertheless, the value of cadalene derivatives is readily illustrated by their biological activities,
such as the inhibitory effect of 7-hydroxy-3-methoxy-cadalene on 4-(methylinitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced
lung tumorigenesis in A/J mice [32], anti-inflammatory and analgesic properties of cadalen-15-oic acid [33], or anticancer cytotoxicity
of 7-hydroxy-3-methoxy-cadalene [34].
The hereafter presented results pertain to the development of an unprecedented strategy for the synthesis of cadalene and iso-cadalene,
i.e., from the abundant and inexpensive natural feedstock of the α-, β-, γ-himachalene mixture from the essential oil of Atlas cedar.
The α-, β-, γ-himachalene mixture was first converted to ar-himachalene 2 by various dehydrogenation agents (selenium, Ni, Pd/C,
chloranil (tetrachloro-1,4-benzoquinone) [35-40]. A solvent-free procedure, using catalytic amount of Pd/C (0.025 wt%) at 160 °C for
12 h, was found to afford ar-himachalene 2 with a quantitative yield (Scheme 1). The process was further developed and applied on a
multi-gram scale (up to 1 kg), demonstrating the industrial feasibility of the transformation.
In an attempt at Friedel-Crafts acylation of 2 with acetyl chloride (AcCl) in the presence of aluminum chloride in dichloromethane
(DCM) at room temperature, formation of small amounts of cadalene 3 was observed. In the absence of AcCl under the same
conditions, the reaction unexpectedly led to cadalene 3 and dihydrocurcumene 5 with 89% conversion and a selectivity of 21% and
20%, respectively (Table 1, entry 1). Gas chromatography (GC) analysis indicated the formation of ca. 48% of unidentified by-
products.
Scheme 1. Two-step synthesis of cadalene and iso-cadalene from a himachalene mixture.
In a related attempt at aromatic iodination with AlCl3/I2 under the same conditions [41], cadalene 3 was formed with a 76%
selectivity for a total conversion of the starting material, without any trace of 5 (GC analysis showed the formation of ca. 24% of
unidentified by-products).
Further tests were performed by varying the reagent ratios and nature of the solvent (Table 1).
In all cases, the substrate underwent a conversion ranging from 10% to 100%, with a chemoselectivity depending strongly on the
reaction parameters. With respect to the above disclosed first results (entries 1 and 2), variation of the reagent stoichiometry (I2 and
AlCl3) caused a slight decrease in the selectivity in cadalene 3 while keeping high conversions (entries 3-5). Notably, using a two-fold
excess of AlCl3 (vs. I2), formation of minor amounts (6%) of iso-cadalene 4 was observed (entry 5). By contrast, in the absence of
AlCl3, only 3 and 5 were produced, with 70% selectivity in the latter product resulting from the opening of the seven-membered ring of
ar-himachalene 2 (entry 6).
The effect of solvent was then considered. Changing DCM by dichloroethane (DCE) just gave lower yields in cadalene 3. However,
changing DCM by cyclohexane with two molar equivalents of AlCl3 and I2 was found to promote the formation of 72% of by-products,
with 14% selectivity in iso-cadalene 4 (entry 7). In the absence of AlCl3, with 2 equiv. of I2, the conversion dropped to 10%, with a
mere 2% selectivity in dihydrocurcumene 5 (entry 8).
In the presence of 2 equiv. of AlCl3 only, a 91% conversion and reversed selectivity in iso-cadalene 4 (66%) were obtained, along
with 25% of 5 (entry 9). Changing cyclohexane by hexane resulted in both lower conversion and lower yield in 4. With 1 equiv. of
AlCl3 (entry 10), a lower conversion of 60% was obtained, albeit with a globally higher 70% selectivity in dihydrocurcumene 5, a
valuable natural product also extracted from various sources [42-46]
In the presence of I2, the use of other Lewis acids (FeCl3, ZnCl2, Et2OBF3, Bi(OTf)3) did not induce any conversion of 2. The same
absence of conversion was observed using any Lewis acid in other solvents such as THF, MeCN or EtOH.
The weakly polar products 3, 4 and 5 were purified by column chromatography on silica gel using hexane as eluent (to a ca. 85%
purity level only for 4). NMR data of 3 and 4 are similar, except for the secondary isopropyl CH proton resonating at higher field in iso-
cadalene 4 (3.06 ppm) than in cadalene 3 (3.80 ppm, due to the deshielding by the closer fused benzenic ring).
Although no quantitative kinetic measurement was carried out, qualitative TLC monitoring showed that under working conditions
(entries 1-10 in Table 1), conversion of ar-himachalene 2 was almost complete in ca. 8 h.

Table 1
Variation of the reaction conditions (Scheme 1).a
Entry
AlCl3 (equiv.)
I2 (equiv.)
Solvent
Conv. (%)
Selectivity (%)
unidentified
3
21
76
67
57
59
30
3
4
0
0
0
0
6
0
14
0
66
30
5
20
0
0
0
1
2
3
4
5
6
7
8
1
2
1
1
2
0
2
0
2
1
0
2
1
2
1
2
2
2
0
0
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
89
100
89
88
85
100
95
10
91
60
48
24
33
31
20
0
72
98
9
0
CH2Cl2
70
11
2
25
70
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
0
0
0
9
10
0
a Reaction condition: 9.9 mmol of substrate 1a-c in 40 mL of solvent, over12 h at r.t.
Structural assignment of 3 was confirmed by aromatic nitration with HNO3/H2SO4 (Scheme 2). Among various products, the dinitro
derivative 6 was isolated with 65% yield after crystallization from EtOAc. The cadalenic structure 6 was confirmed by X-ray diffraction
analysis of selected crystals, the data being identical to those previously reported (P1, triclinic space group, CCDC: 887079) [47].
Scheme 2. Synthesis of dinitro-cadalene 6 and diacetyl-iso-cadalene 7.
Likewise, the structure of iso-cadalene 4 was confirmed by X-ray diffraction analysis of single crystals of the aromatic diacetylation
product 7 obtained in 60% yield by treatment with AlCl3/AcCl in DCM at room temperature (P21/n monoclinic space group, CCDC:
1476547) [48].
Although related rearrangements have been previously reported, none of the proposed mechanistic processes are directly relevant to
the present observations [49,50]. A possible specific mechanism accounting for the formation of 3, 4 and 5 is therefore proposed
(Scheme 3).
In DCM, the formation of cadalene 3 would start by electrophilic iodination of the less hindered aromatic CH site of 2 to the
cyclohexadienyl cation intermediate A. Deprotonative rearrangement of A would then give the tricyclic spiro-cyclopropane C, via a
transition state B of carbene-alkene type. Aromaticity recovery and steric relaxation would act as synergic driving forces for I–/H+
dissociation and cyclopropane ring opening to the exocyclic alkene D. Oxygen of air or I2 would finally play the role of more classical
reagents (such as DDQ or chloranil) for oxidative aromatization of D to cadalene 3 [35-40].

4
Tetrahedron
Scheme 3. Proposed mechanism for the formation of cadalene 3, iso-cadalene 4 and dihydrocurcumene 5 from ar-himachalene 2. Indicated
energy changes ∆E are in kcal/mol: they were calculated in the gas phase at the DFT level (without zero-point correction). For more detail, see
the Supporting information.
In cyclohexane, traces of HCl, e.g., resulting from the reaction of traces of water with AlCl3, would trigger the reversible formation
of the cyclohexadienylium salt E. Deprotonative cleavage of the benzylic C1-C11 bond of E would then give the spiro-cyclobutane G,
via a transient cyclohexadiene-cyclopropane-carbenoid intermediate F-AlCl3 (stabilized form of the free carbene F: see exploratory
DFT calculations in the Supplementary Data, and references [51-53] for Al-NHC complexes). Under the synergic driving forces of
steric relief and aromaticity recovery, dehydrogenation of G would then lead to the exocyclic alkene deproto-H or/via the
corresponding tertiary carbocation H. After isomerization to the π-conjugated endocyclic alkene, the dihydro-naphthalene bicycle
would readily undergo oxidative aromatization to iso-cadalene 4, mediated by either air or carbenium L. This carbenoid route is also in
line with the long-evidenced carbene intermediates in thermal rearrangements of polycyclic aromatics [54,55], and in particular the
related thermal rearrangement of azulenes to naphthalenes [56,57].
Although steric relaxation of cadalene 3 to iso-cadalene 4 might be envisaged by thermal rearrangement under completely different
conditions [54-57], the isolation of 3 in up to 76% yield allows ruling out thermodynamic control (calculation shows that 4 is 2.4
kcal/mol lower in energy than 3, Supplementary Data).
In default of AlCl3, the competing formation of dihydrocurcumene 5 can result from the opening of the cyclohexadienyl ring of E or
its isomer L, playing the role of hydride acceptors allowing the parallel ultimate oxidation steps to 3 or 4 (Scheme 2). Noteworthy is the
long benzylic C1-C11 distance (1.70 Å) in the calculated equilibrium geometry of L (Supplementary Data), which can thus be regarded
as the intermediate of an ipso-substitution process by H+ [58]. The carbenium L would be reduced to 5 by serving as a co-oxidant in the
conversion of D or H to 3 or 4, always formed simultaneously.
As a dramatic solvent effect, the use of cyclohexane instead of DCM is found to prevent the formation of cadalene 3, while favoring
the formation of 4 and 5 (Table 1). This is accounted for by the lack of hetero-polar reactivity of iodine I2 and C–I bonds in apolar
medium at the respective steps 2 –>A and C –>D.
A convenient synthesis of cadalene 3 and iso-cadalene 4 from a mixture of isomeric α-, β- and γ-himachalenes isolated from essential
oil of Atlas cedar waste wood has thus been disclosed. The strategy involves two steps: after quantitative dehydrogenation of
himachalenes, treatment of the resulting ar-himachalene 2 with I2 and/or AlCl3 affords 3 and 4 in proportions depending on the
operating conditions. While the use of AlCl3 in the presence of I2 in DCM orientates the selectivity to cadalene 3 (isolated in up to 76%
yield), the presence of AlCl3 in the absence of I2 in cyclohexane allows iso-cadalene 4 to be isolated in up to 66% yield. Last but not
least, it is remarkable that in the absence of AlCl3 or I2 dihydrocurcumene 5 can form with up to 70% selectivity. Beyond preliminary
DFT calculations of equilibrium structures just happening to not be in thermodynamic contradiction with the proposed mechanism
(Supplementary Data), further claim would require to be confirmed or refined by calculations of transition states, which was out of the
scope of this work. Nevertheless, the solvent- and Lewis acid-controlled versatility of the reaction outcome from the same cheap
himachalene material is a typical illustration of the concept of dual roles design in organic synthesis, beyond general Green Chemistry
principles [59].
In the future, the mechanism could also be refined by further identification of side products. Beyond the additional basic knowledge
brought to the field of aromatic chemistry, the results open prospects for further industrial exploitation of the essential oil of Atlas cedar.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgments
The authors are grateful to the Centro de Instrumentación Cientifica, Universidad de Granada and to Professor Rachid Chahboun
Karimi for NMR experiments.

References
[1] J. Gershenzon, N. Dudareva, Nat. Chem. Biol. 3 (2007) 408-414.
[2] J. Zhou, Y. Zhang, Cell Cycle 7 (2008) 1360-1370.
[3] C. Jordan, T. Cancer Cell 10 (2006) 253-254.
[4] B.T. Kawaraki, Prostrate 69 (2009) 827-837.
[5] N. Ravasio, F. Zaccheria, M. Guidotti, R. Psaro, Top. Catal. 27 (2004) 157-168.
[6] J.L.F. Monteiro, C.O. Veloso, Top. Catal. 27 (2004) 169-180.
[7] A. El Haib, A. Benharref, S. Parrès-Maynadié, et al., Tetrahedron Asymmetry 27 (2010) 1272-1277.
[8] E. Lassaba, A. Chekroun, A. Benharref, et al., Bull. Soc. Chim. Belg. 106 (1997) 281-288.
[9] A. Benharref, A. Chekroun, J.P. Lavergne, Bull. Soc. Chim. Fr. 128 (1991) 738-741.
[10] A. Chiaroni, C. Riche, A. Benharref, E. Lassaba, A. Baouid, Acta Cryst. C 52 (1996) 2504-2507.
[11] M. Loubidi, D. Agustin, A. Benharref, R.C.R. Poli, Chimie 17 (2014) 549-556.
[12] A. El Haib, A. Benharref, S. Parrès-Maynadié, et al., Tetrahedron: Asymmetry 22 (2011) 101-108.
[13] A. Chaudhary, P. Das, A. Mishra, Mol. Divers. 16 (2012) 357-366.
[14] A. Chaudhary, S. Sood, P. Das, et al., EXCLI J. 13 (2014) 1216-1225.
[15] R. Shankaranarayan, S.C. Bisarya, S. Dev, Tetrahedron 33 (1977) 1207-1210.
[16] Jr., R.K. Boeckman, D.M. Blum, B. Ganem, N. Halvey, Org. Synth. 58 (1978) 152-157.
[17] J. Daunis, R. Jacquier, H. Lopez, P. Viallefont, J. Chem. Res. (1981) o639-o649.
[18] B. Abouhamza, S. Allaoud, A. Karim, Molecules 6 (2001) M236.
[19] G.H. Jimenez-Alemana, T. Schöner, A.L. Montero-Alejo, W. Brandt, W. Boland, ARKIVOC (iii) (2012) 371-378.
[20] A. Chaudhary, S. Sood, P. Das, et al., EXCLI J. 13 (2014) 1216-1225.
[21] P. Teisseire, M. Plattier, Recherche 19 (1963) 153.
[22] J. Daunis, R. Jacquier, H. Lopez, O. Viallefont, J. Chem. Res. Synopses 2 (1981) 45.
[23] R. Alexander, R.I. Kagi, R.R. Singh, I.B. Sosrowidjojo, Org. Geochem. 21(1994) 115-120.
[24] A. de O. Gomes, D. de A. Azevedo, J. Braz. Chem. Soc. 14 (2003) 358-368.
[25] M. Bordoloi, V.S. Shukla, S.C. Nath, R.P. Sharma, Phytochemistry 28 (1989) 2007-2037.
[26] B.G.K.Van Aarssen, H.G. Cox, P. Hoogendoorn, J.W. de Leeuw, Geochem. Cosmochim. Acta 54 (1990) 3021-3031.
[27] L. Ruzicka, C.F. Seidel, Helv. Chim. Acta 5(1922) 369-375.
[28] N.S. Gill, F. Lions, J. Am. Chem. Soc. 72 (1950) 3468-3469.
[29] W.S. Johnson, A. Russell Jones, J. Am. Chem. Soc. 69 (1947) 792-794.
[30] E. de Barry Barnett, W. Cook, J. Chem. Soc. 22 (1933) 22-24.
[31] R.P. Linstead, K.O.A. Michaelis, S.L.S. Thomas, J. Chem. Soc. (1940) 1139-1147.
[32] J.H. Kim, H.J. Lee, G.S. Kim, et al., Cancer Lett. 213 (2004) 139-145.
[33] A.K. Banerjee, P.S. Poon, ARKIVOC 13 (2009) 108-115.
[34] H.Y. Lee, J.T. Kwon, M. Koh, M.H. Cho, S.B. Park, Bioorg. Med. Chem. Lett. 17 (2007) 6335-6339.
[35] T.C. Joseph, S. Dev, Tetrahedron Lett. 2 (1961) 216-222.
[36] B. Josshi, R. Seshadri, K. Charkravarti, S. Bhattacharyya, Tetrahedron 20 (1964) 2911-2919.
[37] T.C. Joseph, S. Dev, Tetrahedron 24 (1968) 3809-3827.
[38] R.C. Pandey, S. Dev, Tetrahedron 24 (1968) 3829-3839.
[39] J. Daunis, R. Jacquier, H. Lopez, P.J. Viallefont, Chem. Research (M) (1981) o639-o649.
[40] B. Abouhamza, S. Allaoud, A. Karim, Molecules 6 (2001) M236.
[41] R.L. Cobb, US patent 4551573, Nov. 5 (1985).
[42] R.D. Batt, S.N. Slater, J. Chem. Soc. (1949) 838-842.
[43] M.A. Al-Qudah, A.M. Al-Ghoul, I.N. Trawenh, et al., J. Biol. Active Prod. Nat. 4 (2014) 52-61.
[44] S.V. Nampoothiri, R.M. Philip, S. Kankangi, C.R. Kiran, A.N. Menon, J. Essent. Oil. Bear. Plants 18 (2015) 1051-1058.
[45] S. Sandeep, A. Kuanara, A. Akbar, et al., Indus. Crops Prod. 85 (2016) 229-240.
[46] H. Xiang, L. Zhang, Z. Yang, et al., Indus. Crops Prod. 108 (2017) 6.
[47] M. Chakkar, N. Oughris, A. Benharref, J.C. Daran, M. Berraho, Acta Cryst. Sect. E 68 (2012) o1893.
[48] A. Benharref, A. Oukhrib, M. Ait Elhad, et al., IUCr 1 (2016) x160703.
[49] G. Frater, U. Muller, Chem. Commun. (1988) 1198-1200.
[50] A. Scott, S.A. Snyder, D.S. Treitler, A.P. Brucks, J. Am. Chem. Soc. (2010) 14303-14314.
[51] A. Stasch, S. Singh, H.W. Roesky, M. Noltemeyer, H.G. Schmidt, Eur. J. Inorg. Chem. (2004) 4052-4055.
[52] V. Nestereov, D. Reiter, P. Bag, et al., Chem. Rev. 118 (2018) 9678-9842.
[53] C.H. Wang, Y.F. Lin, H.C. Tseng, et al., Eur. J. Inorg. Chem. (2018) 2232-2236.
[54] L.T. Scott, Acc. Chem. Res. 15 (1982) 52-58.
[55] L.T. Scott, M.M. Hashemi, T.H. Schultz, M.B. Wallace, J. Am. Chem. Soc. 113 (1991) 9692-9693.
[56] L.T. Scott, J. Org. Chem. 49 (1984) 3022-3024.
[57] C. Perrin, G.A. Skinner, J. Am. Chem. Soc. 93 (1971) 3389.
[58] J.R. Martyn, Ipso-Aromatic substitution, Ph. D. thesis, Christchurch, 1985, and references therein.
[59] (a) Z. Cao, Q. Zhu, Y.W. Lin, W.M. He, Chin. Chem. Lett. 30 (2019) 2132–2138;(b) K.J. Liu, J.H. Deng, J. Yang, et al., Green Chem. 22
(2020) 433-438;(c) C. Wu, X. Xin, Z.M. Fu, et al., Green Chem. 19 (2017) 1983-1989.