Hydroisomerization of hexacyclo[9.2.1.02,10.03,8.04,6.05,9]tetradecanes to diamantane induced by ionic liquids

Article abstract of DOI:10.1016/j.catcom.2019.105756

Hydroisomerization of endo-endo-, exo-exo-, exo-endo-, and endo-exo-, hexacyclo[9.2.1.0^2,10.0^3,8.0^4,6.0^5,9]tetradecanes to diamantane was performed for the first time in 80–96% yields under the action of ionic liquids (ILs), [Et₃NH]⁺[Al₂Cl₇]^? and [BMIM]⁺[Fe₂Cl₇]^?. Ionic liquids have multiple functions, promoting the following reactions of hexacyclotetradecanes: hydrogenation, dehydrogenation, and skeletal rearrangement; simultaneously, they serve as HCl donors.

Full text of DOI:10.1016/j.catcom.2019.105756

Accepted Manuscript
Hydroisomerization
of
hexacyclo[9.2.1.02,10.03,8.04,6.05,9]tetradecanes to diamantane
induced by ionic liquids
Rishat I. Aminov, Aida N. Akshieva, Ravil I. Khusnutdinov
PII:
S1566-7367(19)30218-3
https://doi.org/10.1016/j.catcom.2019.105756
105756
DOI:
Article Number:
Reference:
CATCOM 105756
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
13 May 2019
9 July 2019
11 July 2019
Please cite this article as: R.I. Aminov, A.N. Akshieva and R.I. Khusnutdinov,
Hydroisomerization of hexacyclo[9.2.1.02,10.03,8.04,6.05,9]tetradecanes to diamantane
induced by ionic liquids, Catalysis Communications, https://doi.org/10.1016/
j.catcom.2019.105756
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ACCEPTED MANUSCRIPT
2
,10 3,8 4,6 5,9
Hydroisomerization of hexacyclo[9.2.1.0 .0 .0 .0 ]tetradecanes to
diamantane induced by ionic liquids
*
Rishat I. Aminov, Aida N. Akshieva, Ravil I. Khusnutdinov
khusnutdinovri47@gmail.com
Institute of Petrochemistry and Catalysis, Russian Academy of Sciences
pr. Oktyabrya 141, Ufa, 450075, Russia
*
Corresponding author.

ACCEPTED MANUSCRIPT
ABSTRACT
Hydroisomerization
of
endo-endo-,
exo-exo-,
exo-endo-,
and
endo-exo-,
2
,10 3,8 4,6 5,9
hexacyclo[9.2.1.0 .0 .0 .0 ]tetradecanes to diamantane was performed for the first time in
+
-
+
-
8
0-96% yields under the action of ionic liquids (ILs), [Et NH] [Al Cl ] and [BMIM] [Fe Cl ] .
3 2 7 2 7
Ionic liquids have multiple functions, promoting the following reactions of
hexacyclotetradecanes: hydrogenation, dehydrogenation, and skeletal rearrangement;
simultaneously, they serve as HCl donors.
Keywords: diamantane, ionic liquid, hydroisomerization [4+4]-dimers of norbornadiene

ACCEPTED MANUSCRIPT
1
. Introduction
Diamondoids (adamantane, diamantane, triamantane) represent a separate
class of hydrocarbons possessing a unique structure and unusual properties.
Adamantane derivatives have found application as antiviral and anti-Parkinson
drugs [1-5] and in the manufacture of light- and heat-resistant polymers [6-8].
The second representative of the diamondoid homologous series, diamantane
4
,12 2,7 6,11
(
pentacyclo[7.3.1.1 .0 .0 ]tetradecane) 1, is the raw material for the synthesis
of heat-resistant synthetic lubricating oils and transmission fluids. It is promising
for the preparation of pharmaceuticals and solvent-resistant polymeric materials
and rubbers [9-13].
The known synthetic routes to diamantane 1 are based on the skeletal
rearrangement of strained, thermodynamically less stable polycyclic hydrocarbons
С Н [14-18]. In particular, the [4+4]-dimer of norbornadiene (NBD),
1
4
20
2,12 3,8 4,6 5,9 11,13
heptacyclo[8.4.0.0 .0 .0 .0 .0 ]tetradecane 2 (binor-S), is the most
convenient starting compound for the preparative synthesis of diamantane 1.
Hydrocarbon 2, which has the composition С H , is converted to the precursor of
14
16
diamantane 1 by hydrogenation under drastic conditions (200°С and 305 atm of
H ) in the presence of a platinum catalyst (Н PtCl or PtO ) in glacial acetic acid.
2
2
6
2
The hydrogenation of 2 is accompanied by cyclopropane ring opening giving rise
to a mixture of three C H hydrocarbons 3а-с [19, 20]. According to published
1
4
20
data, C H hydrocarbons 3a-c isomerize in the presence of superacid catalysts
1
4
20
such as B(OSO CF ) , CF3SO H-SbF (1:1), CF SO H-B(OSO CF ) (1:1) [21],
2
3 3
3
5
3
3
2
3 3
NaBH /CF SO H [22], or zeolite Y in the NaH form [23] to give diamantane in a
4
3
3
yield of up to 99% (Scheme 1).
Scheme 1. Isomerization of 3а-с to diamantane.
At the end of the 20th century, inorganic ionic liquids (ILs) started to be
used in the synthetic practice. Ionic liquids are unique objects for chemical
research; they are widely employed in catalysis and organic synthesis. Inorganic
ionic liquids are known to possess Brønsted or Lewis acidity or superacidity. In

ACCEPTED MANUSCRIPT
particular, superacid properties are inherent in melts containing AlCl , which
3
makes them attractive for catalysis [24-27]. According to the works [28–30], the
+
-
ionic liquid [Et NH] [Al Cl ] is characterized by high catalytic activity in the
3
2
7
skeletal isomerization of cyclohexane to methylcyclopentane and polycyclic
hydrocarbons of composition С12-15H18-22 to derivatives of adamantane and
diamantane.
2
. Experimental
2
.1. General procedures and materials
1
13
H and С NMR spectra were measured on a Bruker Avance-III 400
1
13
Ascend instrument (400 MHz for Н and 100 MHz for С in CDCl ). Mass spectra
3
were run on a Shimadzu GCMS-QP2010Plus mass spectrometer (SPB-5 capillary
column, 30 m  0.25 mm, helium as the carrier gas, temperature programming
ꢀ
о
о
from 40 to 300 С at 8 °C/min, evaporation temperature of 280 С, ion source
о
temperature of 200 С, and ionization energy of 70 eV). The elemental composition
of the samples was determined on a Carlo Erba 1106 elemental analyzer. The
course of the reaction and the purity of the products were monitored by gas liquid
chromatography on a Shimadzu GC-9A, GC-2014 instrument [2 m 3 mm column,
SE-30 silicone (5%) on Chromaton N-AW-HMDS as the stationary phase,
о
temperature programming from 50 to 270 С at 8 °C/min, helium as the carrier gas
(47 mL/min)].
2
.2. General procedure for the preparation of saturated polycyclic hydrocarbons
4
-7.
A glass reactor was charged with the Pd/C catalyst (0.5 g) and hydrocarbon
(5 g) dissolved in hexane (15 mL). Hydrogenation of norbornadiene dimers was
carried out at room temperature (1 atm H ). After completion of the reaction, the
2
reaction mixture was filtered through a silica gel layer (elution with hexane).
2
.3. Preparation of the ionic liquids
The ionic liquids were prepared by direct reactions of metal halides with
+
-
+
-
-
[
Et NH] Cl , [Et DH] Cl , EMIM-Cl, or BMIM Cl.
3
3
+
-
A glass reactor (V=50 mL) was charged under argon with [Et NH] Cl
3
+
-
-
(
[Et DH] Cl , EMIM-Cl, or BMIM Cl) (10 mmol) and metal chloride (Al (III), Fe
3
(
III), Zn (II), Sn (II), Cu (II)) (10–20 mmol). The reaction was carried out with
о
continuous stirring at 70 С for 3 h. In the case of reactions involving copper (II)

ACCEPTED MANUSCRIPT
sulfate, CuSO (0.05 mmol) was added to the prepared ionic liquid, and the
4
mixture was stirred for additional 1 h at room temperature.
2
.4. Preparation of diamantane.
A glass reactor (V=50 mL) was charged under argon with hydrogenated
norbornadiene dimer (4-7) (1 mmol) and the ionic liquid prepared in advance (3
о
mmol). The reaction was carried out with continuous stirring at 50 С for 6 h. Then
the reactor was cooled down to room temperature, and the reaction mixture was
extracted with petroleum ether and filtered through a silica gel layer (elution with
petroleum ether).
The products were separated by column chromatography: hydrocarbon 9 and
diamantane 1 were eluted with hexane, while chlorinated derivatives 8 and 10 were
eluted with a hexane–ethyl acetate mixture (10:1).
о
1
Diamantane 1. White crystals; mp 244–245 С. H NMR (400 MHz,
1
3
4
9
CDCl ): δ = 1.72–1.80 (m, 20H). С NMR (100 MHz, CDCl ): δ = 25.95 (С , С ),
3
3
3
5
8
10
13
14
1
2
6
7
11
12
3
7.64 (С , С , С , С , С , С ), 38.37 (С , С , С , С , С , С ). MS (EI, 70 eV):
+
m/z (%) = 188 [M] (100), 189 (15), 187 (18), 159 (10), 145 (8), 131 (23), 130
18), 117 (12), 105 (13), 93 (12), 92 (11), 91 (28), 77 (15), 67 (8). Calcd for
C H : С, 88.29; H, 11.71; found С, 88.75; H, 11.25.
endo-exo-4-Chloropentacyclo[8.2.1.1 .0 .0 ]tetradecane 8. Colorless
oil; H NMR (400 MHz, CDCl ): δ = 0.79 (d, J = 10.4 Hz, 1H), 1.33 (s, 3H), 1.42–
(
1
4
20
5,8 2,9 3,7
1
3
1
2
.45 (m, 3H), 1.54 (d, J = 10.4 Hz, 1H), 1.93–2.03 (m, 2H), 2.11 (d, J = 9.6 Hz,
13
H), 2.17 (s, 2H), 2.24 (s, 3H), 2.58 (s, 1H), 3.52 (s, 1H). C NMR (100 MHz,
1
2
11
14
10
1
CDCl ): δ = 23.95 (C ), 24.05 (C ), 37.12 (C ), 38.77 (C ), 38.80 (C ), 41.82
3
6
5
3
9
2
13
10
(
C ), 41.86 (C ), 42.35 (C ), 44.92 (C ), 47.78 (C ), 52.12 (C ), 54.22 (C ), 54.38
7
4
(
C ), 72.25 (C ). MS (EI, 70 eV): m/z (%) = 222[M]+ (100). Calcd for C H Cl:
1
4
19
C, 75.48; H, 8.60; Cl, 15.92; found C, 75.38; H, 8.54; Cl, 16.08.
2
,12 3,7 4,11 5,9 10,14
Heptacyclo[6.6.0.0 .0 .0 .0 .0 ]tetradecane 9. White crystals; mp
o
1
1
1
(
64–165 C. H NMR (400 MHz, CDCl ): δ = 1.22–1.28 (m, 6H), 1.48–1.53 (m,
3
13
6
5
7
12
14
0H). C NMR (100 MHz, CDCl ): δ = 42.69 (С ), 50.99 (С , С , С , С ), 53.19
3
1
2
3
4
8
9
10
+
С , С , С , С , С , С , С ). MS (EI, 70 eV): m/z (%) = 184 (100) [M] , 142 (5),
29 (6), 118 (13), 117 (29), 116 (8),115 (10), 106 (16), 105 (15), 104 (18). Calcd
for C H : C, 91.25; H, 8.75; found C, 91.36; H, 8.64.
1
1
4
16
5
,8 2,9 3,7
exo-exo-4-Chloropentacyclo[8.2.1.1 .0 .0 ]tetradecane 10. Colorless
oil. H NMR (400 MHz, CDCl ): δ = 0.91 (d, J = 10 Hz 1H), 1.04–1.12 (m, 2H),
1
3
1
4
.35–1.44 (m, 3H), 1.53 (d, J = 10 Hz, 1H), 1.73 (d, J = 6.8 Hz, 1H), 1.92–2.03 (m,
13
H), 2.10 (s, 1H), 2.17 (s, 2H), 2.18 (s, 1H), 2.59 (s, 1H), 3.51 (s, 1H). C NMR
1
2
11
14
10
(100 MHz, CDCl ): δ = 29.14 (C ), 29.36 (C ), 34.81 (C ), 37.13 (C ), 38.46
3

ACCEPTED MANUSCRIPT
1
6
5
3
9
2
13
(
(
C ), 41.64 (C ), 41.84 (C ), 43.40 (C ), 43.84 (C ), 47.75 (C ), 55.42 (C ), 57.35
1
0
7
4
C ), 58.83 (C ), 71.95 (C ). MS (EI, 70 eV): m/z (%) = 222 [M]+ (100). Calcd
for C H Cl: C, 75.48; H, 8.60; Cl, 15.92; found: C, 75.56; H, 8.51; Cl, 15.93.
14
19
3
. Result and Discussion
This communication describes the first synthesis of diamantane by
hydroisomerization of С Н hydrocarbons containing two hydrogen atoms less
1
4
18
than diamantane, namely, endo-endo- 4, endo-exo- 5, exo-exo- 6, and exo-endo-
2
,10 3,8 4,6 5,9
hexacyclo[9.2.1.0 .0 .0 .0 ]tetradecanes
7
(hydrogenated
[4+2]
norbornadiene dimers). The reaction is induced by inorganic ionic liquids
containing Al (III), Fe (III), Zn (II), Sn (II), and Cu (II) chlorides.
In the presence of ILs, hydrocarbon 4 is converted to a mixture of three products:
5
,8 2,9 3,7
diamantane 1, endo-exo-chloropentacyclo[8.2.1.1 .0 .0 ]tetradecane 8, and
2
,12 3,7 4,11 5,9 10,14
[
4π+4π] NBD dimer, heptacyclo[6.6.0.0 .0 .0 .0 .0 ]tetradecane 9 (Table
1
).
Table
1
Hydroisomerization
of
endo-endo-
2
,10 3,8 4,6 5,9
a
hexacyclo[9.2.1.0 .0 .0 .0 ]tetradecane 4 in the presence of ionic liquids.
b
Yield (%)
Entry
ILs
Ratio 4 : IL
1
8
9
+
-
-
-
1
2
3
4
[Et NH] [AlCl ]
1:3
1:1
3:1
1:3
-
4
100
50
31
-
-
7
3
4
+
[Et NH] [AlCl ]
3
4
+
[Et NH] [AlCl ]
-
5
3
4
+
-
[EMIM] [Al Cl ]
28
45
2
7
+
-
5
[EMIM] [Al Cl ]
1:1
22
-
23
2
7
+
-
-
-
-
6
7
8
9
[EMIM] [Al Cl ]
3:1
1:3
1:1
3:1
1:3
1:2
1:1
3:1
1:3
11
64
25
21
80
68
32
44
8
35
32
61
-
49
-
2
7
+
[Et NH] [Al Cl ]
3
2
7
+
[Et NH] [Al Cl ]
14
10
-
3
2
7
+
[Et NH] [Al Cl ]
3
2
7
+
-
1
1
1
1
1
0
1
2
3
4
[Et NH] [Al Cl ] -CuSO
20
24
54
-
3
2
7
4
4
4
4
+
-
[Et NH] [Al Cl ] -CuSO
5
3
2
7
+
-
[Et NH] [Al Cl ] -CuSO
14
8
3
2
7
+
-
[Et NH] [Al Cl ] -CuSO
3
2
+
7
-
[BMIM] [FeCl ]
79
-
4

ACCEPTED MANUSCRIPT
+
-
-
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
[BMIM] [FeCl ]
1:1
3:1
1:3
1:1
3:1
1:3
1:1
3:1
1:3
1:1
3:1
1:3
1:1
1:3
1:1
1:3
1:1
3
12
82
35
6
42
-
12
18
-
4
+
[BMIM] [FeCl ]
4
+
-
-
-
-
-
-
-
-
-
-
[BMIM] [Fe Cl ]
18
50
51
26
64
58
90
86
6
2
7
+
[BMIM] [Fe Cl ]
15
12
20
10
10
2
2
7
+
[BMIM] [Fe Cl ]
2
7
+
[EMIM] [Fe Cl ]
54
26
3
2
7
+
[EMIM] [Fe Cl ]
2
7
+
[EMIM] [Fe Cl ]
2
7
+
[Et NH] [Fe Cl ]
6
3
2
7
+
[Et NH] [Fe Cl ]
11
11
5
3
3
2
7
+
[Et NH] [Fe Cl ]
55
4
3
2
7
+
[Et NH] [Zn Cl ]
56
-
3
2
5
+
-
-
-
-
[Et NH] [Zn Cl ]
3
2
3
2
5
+
[Et NH] [Sn Cl ]
1
16
-
7
3
2
5
+
[Et NH] [Sn Cl ]
2
8
3
2
5
+
[Et NH] [Cu Cl ]
2
5
7
3
2
5
+
-
[Et NH] [Cu Cl ]
˂1
-
8
3
2
5
a
o
Reaction conditions: 50 C, 6 h.
b
Determined by GC using C H as the internal standard.
1
2
26
It was shown experimentally that the highest yield of diamantane 1 attained
with ionic liquids based on SnCl , ZnCl , and CuCl does not exceed 5%, which
2
2
2
might be attributable to the decrease in the Lewis acidity in the series
AlCl >FeCl >ZnCl >SnCl >CuCl . Compound 8 results from the addition of
3
3
2
2
2
hydrogen chloride present in the ionic liquid to compound 4.
The formation of hydrocarbon 9, С H , which is [4+4] norbornadiene
1
4
16
2
,12 3,7 4,11 5,9 10,14
dimer, heptacyclo[6.6.0.0 .0 .0 .0 .0 ]tetradecane, is more unusual [31,
2]. The pathway to 9 obviously includes dehydrogenation and skeletal
isomerization.
As can be seen in Table 1 (entry 10, 17), the highest yield of diamantane 1
3
о
(
[
80-82%) is attained under conditions that include 50 С, 6 h, and the presence of
+
-
Et NH] [Al Cl ] and copper(II) sulfate or the ionic liquid based on 1-butyl-3-
methylimidazolium chloride (BMIM-Cl) and iron(III) chloride ([BMIM] [Fe Cl ]
3
2
7
+
-
2
7
). A high yield of diamantane is observed when IL is present in a threefold excess
over hydrocarbon 4. Note that this is the first example of using iron compounds in
diamantane synthesis.
Considering the presence of CuSO , it was found [33, 34] that AlCl -CuSO
4
3
4
mixtures show higher catalytic activity towards n-pentane isomerization than AlCl3
alone. Thus, copper(II) sulfate being added to an ionic liquid gives rise to a more
efficient catalytic system. It is known that ionic liquids, which are polar, represent
media in which salts easily dissociate into cations and anions, and, therefore, a
complex is formed between the components directly during the synthesis of the

ACCEPTED MANUSCRIPT
catalytic system. The better promoting effect of CuSO can be attributed to higher
4
polarity.
The most efficient catalysts, ionic liquids based on aluminum(III) and
iron(III) chlorides, were used subsequently to carry out isomerization of other
hydrogenated hexacyclic norbornadiene dimers 5-7.
Diamantane 1 was obtained in the highest yield (96%) by isomerization of
2
,10 3,8 4,6 5,9
endo-exo-hexacyclo[9.2.1.0 .0 .0 .0 ]tetradecane 5. The reaction was
+
-
performed under the action of [Et NH] [Al Cl ] –CuSO system under the chosen
3
2
7
4
о
conditions: 50 С, 6 h, and 5 to IL molar ratio of 1 : 3 (Table 2).
Table
2
Hydroisomerization
of
endo-exo-hexacyclo-
2
,10 3,8 4,6 5,9
a
[9.2.1.0 .0 .0 .0 ]tetradecane 5 in the presence of ionic liquids.
b
Yield (%)
Entry
ILs
Ratio 5:IL
1
8
9
+
-
-
1
2
3
4
5
6
[BMIM] [Fe Cl ]
1:3
1:1
1:3
1:1
1:3
1:1
81
28
96
83
54
41
19
54
4
-
18
-
2
7
+
[BMIM] [Fe Cl ]
2
7
+
-
[Et NH] [Al Cl ] -CuSO
3
2
7
4
4
+
-
[Et NH] [Al Cl ] -CuSO
17
43
52
-
3
2
+
7
-
[Et NH] [Al Cl ]
3
-
3
2
7
+
-
[Et NH] [Al Cl ]
3
2
7
a
o
Reaction conditions: 50 C, 6 h.
b
Determined by GC using C H as the internal standard.
1
2
26
It
was
found
that
another
isomer,
exo-exo-
2
,10 3,8 4,6 5,9
hexacyclo[9.2.1.0 .0 .0 .0 ]tetradecane 6, is converted in the presence of
Et NH] [Al Cl ] –CuSO to give diamantane 1 in 85% yield (Table 3). The
+
-
[
3
2
7
4
highest yields of diamantane 1 (85-87%) are observed at the [6] : [IL] molar ratio
+
-
of 1 : 3 and in the presence of the [Et NH] [Al Cl ] ionic liquid, either with or
3
2
7
without copper sulfate. It is noteworthy that a lower ratio of IL to hydrocarbon 6
leads to the predominant formation of
5
,8 2,9 3,7
chloropentacyclo[8.2.1.1 .0 .0 ]tetradecane 10, resulting from the addition of
HCl to hydrocarbon 6 (Table 3).
Table
3
Hydroisomerization
of
exo-exo-
2
,10 3,8 4,6 5,9
a
hexacyclo[9.2.1.0 .0 .0 .0 ]tetradecane 6 in the presence of ionic liquids.

ACCEPTED MANUSCRIPT
b
Yield (%)
Entry
ILs
Ratio 6:IL
1
10
9
+
-
-
1
2
3
4
5
6
[BMIM] [Fe Cl ]
1:3
1:1
1:3
1:1
1:3
1:1
64
10
87
71
85
69
26
76
10
29
13
31
-
14
-
2
7
+
[BMIM] [Fe Cl ]
2
7
+
-
[Et NH] [Al Cl ] -CuSO
3
2
7
4
4
+
-
[Et NH] [Al Cl ] -CuSO
-
3
2
+
7
-
[Et NH] [Al Cl ]
-
3
2
7
+
-
[Et NH] [Al Cl ]
-
3
2
7
a
o
Reaction conditions: 50 C, 6 h.
b
Determined by GC using C H as the internal standard.
1
2
26
2
,10 3,8 4,6 5,9
exo-endo-Hexacyclo[9.2.1.0 .0 .0 .0 ]tetradecane 7 undergoes an
equally efficient isomerization to diamantane 1 in the presence of CuSO -pomoted
4
aluminate ionic liquids (Table 4).
Table
4
Hydroisomerization
of
exo-endo-
2
,10 3,8 4,6 5,9
a
hexacyclo[9.2.1.0 .0 .0 .0 ]tetradecane 7 in the presence of ionic liquids.
b
Yield (%)
Entry
ILs
Ratio 7:IL
1
10
9
+
-
-
1
2
3
4
5
6
[BMIM] [Fe Cl ]
1:3
1:1
1:3
1:1
1:3
1:1
58
19
83
40
72
43
40
67
15
55
13
45
2
14
2
2
7
+
[BMIM] [Fe Cl ]
2
7
+
-
[Et NH] [Al Cl ] -CuSO
3
2
7
4
4
+
-
[Et NH] [Al Cl ] -CuSO
5
3
2
+
7
-
[Et NH] [Al Cl ]
6
3
2
7
+
-
[Et NH] [Al Cl ]
12
3
2
7
a
o
Reaction conditions: 50 C, 6 h.
b
Determined by GC using C H as the internal standard.
1
2
26

ACCEPTED MANUSCRIPT
A high yield of diamantane 1 (83%) is observed upon isomerization of the
+
–
exo-endo-dimer 7 in the [Et NH] [Al Cl ] – CuSO system (1:3) .
3
2
7
4
Attention is attracted by the structure of chlorinated derivatives 8 and 10,
which are formed from hydrocarbons 4 and 7, respectively, containing the most
shielded three-carbon rings. Obviously, this occurs via a skeletal rearrangement
the starting step of which is protonation of the cyclopropane ring with hydrogen
chloride. In particular, the formation of compound 8 from hydrocarbon 4 can be
depicted by the following scheme:
Scheme 2. Probable mechanism of formation of compound 8 from hydrocarbon 4.
Compound 10 is formed from hydrocarbon 7 by a similar pathway.
Upon interaction between HCl and hydrocarbons 4 and 7, the formation of
chlorine derivatives 8 and 10 accompanied by the skeletal rearrangement was not
unexpected. We observed the same transformation in the alkylation of benzene
+
-
with hydrocarbons 4 or 6 in the presence of the [Et NH] [Al Cl ] ionic liquid [35].
3
2
7
These results concerning isomerization of hexacyclic dimers to diamantane
are unexpected, as the molecules of hydrocarbons 4-7 (С H ) contain less
1
4
18
hydrogen atoms than diamantane. Most likely, components of the ionic liquid serve
as the main hydrogen donors. Meanwhile, it cannot be ruled out that the proper
hydrocarbons 4-7 can serve as additional sources of hydrogen, since they are partly
converted to hydrocarbon 9, which has the composition C H . It is unlikely that
1
4
16
HCl would serve as a hydrogen donor. Indeed, experiments on isomerization of
+
-
hydrocarbons 4-7 in the presence of the [Et ND] [Al Cl ] ionic liquid did not meet
3
2
7
with success: no dideuterodiamantane was detected. This implies that the major
hydrogen donors are the ethyl and butyl groups of triethylamine and 1-ethyl- and
1
-butyl-3-methylimidazolium cations incorporated in the ionic liquid.
4
. Conclusions

ACCEPTED MANUSCRIPT
Thus, new methods were developed for the preparation of diamantane in
high yield by hydroisomerization of partially hydrogenated hexacyclic
norbornadiene dimers: endo-endo-, exo-exo-, exo-endo-, and endo-exo-
2
,10 3,8 4,6 5,9
hexacyclo[9.2.1.0 .0 .0 .0 ]tetradecanes 4-7 induced by inorganic ionic
+
-
+
-
liquids, [Et NH] [Al Cl ] or [BMIM] [Fe Cl ] , under the following conditions:
5
3
2
7
2
7
о
0 С, 5-6 h, [4-7]:[IL] = 1:3.
It was found that the isomerizations of endo-endo-, exo-exo-, exo-endo-,
and endo-exo-hexacyclo[9.2.1.0 .0 .0 .0 ]tetradecanes 4-7 to diamantane
2
,10 3,8 4,6 5,9
were promoted by ionic liquids.
Acknowledgments
The structural studies of compounds were performed with the use of unique
equipment in "Agidel" Collective Usage Centre at the Institute of Petrochemistry
and Catalysis of RAS. The work was financially supported by the Scholarship of
the President of the Russian Federation to young scientists and postgraduates (SP-
1
1
601.2018.1) and carried out within the RF state assignment, reg. no. АААА-А19-
19022290009-3.
Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:/

ACCEPTED MANUSCRIPT
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Highlights




Ionic liquids catalyze diamantane synthesis
A new route to diamantane is based on skeletal rearrangement of C H hydrocarbon
14 18
The highest yields of diamantane are 80-96%
+
-
+
-
[Et NH] [Al Cl ] and [BMIM] [Fe Cl ] are the most efficient catalysts
3
2
7
2
7
Graphical abstract