(±)Methanodibenzodiazocine tethered [C-H]δ+ functional site: Study towards benzoin condensation and Baylis-Hillman reactions

Article abstract of DOI:10.1007/s12039-015-0837-7

Abstract New heterocyclic ring systems consisting of (±) methanodibenzodiazocine and imidazolium/ benzimidazolium salts were synthesized in very good yield. Subsequently, these halide salts were subjected to the anion exchange reaction with KPF₆ to yield the corresponding azolium salts in excellent yield. The possible applications of these newly prepared salts were investigated in homogeneous catalysis. Remarkable changes in the catalytic activity were observed by varying the bulkiness of N-substituent at imidazole. Catalytic activity of these newly prepared salts was tested for the benzoin condensation reaction. Exclusive formation of benzoin products were observed in good yield. Similarly, the dimerization of cyclohexen-1-one to Baylis-Hillman type product, 2-(3-oxocyclohexyl)-2-cyclohexen-1-one was studied. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-015-0837-7

c
J. Chem. Sci. Vol. 127, No. 5, May 2015, pp. 821–831. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0837-7
( )Methanodibenzodiazocine tethered [C-H]δ+ functional site: Study
towards benzoin condensation and Baylis-Hillman reactions
ARRURI SATHYANARAYANA and GANESAN PRABUSANKAR∗
Department of Chemistry, Indian Institute of Technology Hyderabad, ODF Estate, Yeddumailaram,
Telengana, India 502 205
e-mail: prabu@iith.ac.in
MS received 18 June 2014; revised 25 November 2014; accepted 3 December 2014
Abstract. New heterocyclic ring systems consisting of ( ) methanodibenzodiazocine and imidazolium/
benzimidazolium salts were synthesized in very good yield. Subsequently, these halide salts were subjected to
the anion exchange reaction with KPF6 to yield the corresponding azolium salts in excellent yield. The possible
applications of these newly prepared salts were investigated in homogeneous catalysis. Remarkable changes in
the catalytic activity were observed by varying the bulkiness of N-substituent at imidazole. Catalytic activity
of these newly prepared salts was tested for the benzoin condensation reaction. Exclusive formation of benzoin
products were observed in good yield. Similarly, the dimerization of cyclohexen-1-one to Baylis-Hillman type
product, 2-(3-oxocyclohexyl)-2-cyclohexen-1-one was studied.
Keywords. Tröger’s base; Imidazolium salt; N-Heterocyclic carbenes; C-H bond activation.
and benzimidazolium salts, TB[CH2-Im-CH2-Ar]+2 2X−,
where Ar = C6F5, X = Br (4a), PF6 (4b); Ar = 1-
Naphthyl, X = Cl (5a), PF6 (5b) and TB[CH2-BIm-
CH2-Ar]+2 2X−, where BIm = Benzimidazolium; Ar =
Mes, X = Br (6a), PF6 (6b); Ar = C6F5, X = Br
(7a), PF6 (7b); Ar = 1-Naphthyl, X = Cl (8a), PF6
(8b). Molecules 4a–8a and 4b–8b were fully charac-
terized. The possible applications of these newly pre-
pared salts were demonstrated in the carbon-carbon
bond formation reactions.
1. Introduction
The cationic imidazolium systems with rigid or unusual
molecular platforms are uncommon and such molecu-
lar assemblies have been employed as organocatalyst
or model systems to simulate the biological processes
that take place in the living organism. Namely, the
supramolecular system based on a tetrathiafulvalene
calix[4]pyrrole and a dicationicbisimidazoliumquinone
to understand the artificial ion mediated electron tran-
sfer process;1 highly selective water soluble anthrace-
neimidazolium receptors for sensing CT-DNA2 and
DNA.3 Besides, the ( )-2,8-dimethylene-6H,12H-5,11-
methanodibenzo[b,f ][1,5]diazocine, known as Tröger’s
base (TB) linked with imidazolium salts, TB[CH2-Im-
CH2-Mes]2+2X−, where Im = imidazolium; Mes =
2,4,6-trimethylbenzene; X = Br (3a), PF6 (3b) and
BF4 (3c) were used as organocatalysts for synthesis
of 2-hydroxy-1,2-diferrocenyl-ethan-1-one and 1,2-
diferrocenylethanedione from ferrocenealdehyde.4a
This synthetic methodology is proved to be the most
efficient synthetic protocol to derive 1,2-diferroceny-
lethanedione using organocatalysts.
2. Experimental
2.1 General remarks
All manipulations were carried out under nitrogen using
Schlenk vacuum line techniques and argon glove box.
The solvents were purchased from commercial sources
and purified according to standard procedures.6 1, 2 and
3 were synthesized by previously reported methods.4a
Starting materials were purchased from commercial
sources and used without further purification. FT-
IR measurement (neat) was carried out on a Bruker
Alpha-P Fourier transform spectrometer. The UV−Vis
spectra were measured on a T90+ UV-Visible spec-
trophotometer. NMR spectra were recorded on Bruker
Ultrashield-400 MHz spectrometers at 25◦C unless
otherwise stated. Chemical shifts are given relative to
Me4Si and were referenced to the solvent resonances as
Although the importance of the century old TB skele-
ton has been a subject of focus in last several decades,
the examples based on Tröger’s base tethered hetero-
cyclic ring systems are still rare.4,5 Thus, in this paper
we report a series of Tröger’s base attached imidazolium
∗For correspondence
821

822
Arruri Sathyanarayana and Ganesan Prabusankar
internal standards. High-resolution mass spectra (HR- then the reaction mixture was stirred at 60◦C for 4–48 h
MS) were recorded on an Agilent 6538 UHD Q-TOF. under nitrogen atmosphere. After completion of reac-
The crystal structure of 2 was measured on an Oxford tion, the expected product was formed as an insoluble
Xcalibur 2 diffractometer. Data were collected at 150 K. solid, which was filtered then washed with acetone (2 ×
The structure of 2 was solved by direct methods using 5 mL) and diethyl ether (3 × 5 mL). Finally the free
the SIR-97 program7 and refined with a full matrix least- flowing solid was dried for 2–3 h under high vacuum.
squares method on F2 using the SHELXL-97 program.8,9
2.4 General Procedure for the synthesis
of the azolium PF−6 salts, 4b–8b
2.2 Synthesis of TB-[CH2-BIm]2 (2)
2 was synthesized by modifying the previously
reported procedure. Cooled (−15◦C) trifluoroacetic
acid (40 mL) was added to 4-(methyl benzimida-
zole) aniline (2.000 g, 8.957 mmol) at −15◦C then
paraformaldehyde (0.430 g, 14.332 mmol) was added to
the reaction mixture at same temperature and stirred
for 40 h at room temperature. The reaction progress
was monitored by TLC. After completion of the reac-
tion, the reaction mixture was poured into crushed ice
(120 g), then 30% aq. ammonia solution (40 mL) was
added to the reaction mixture (until pH = 9–10) then
Imidazolium or benzimidazolium halide (1 eq.), was
dissolved in hot water and KPF6 (2.5 eq.) was dissolved
in normal water separately then both were mixed under
stirring to produce a white precipitate immediately. The
resulted white precipitate was filtered to get crude com-
pound, which was washed with methanol, dissolved in
acetone, dried over sodium sulphate, filtered and sol-
vent was evaporated under reduced pressure to give an
analytically pure compound.
extracted with dichloromethane (3 × 30 mL) and the 2.5 Synthesis of TB-[CH2-Im-CH2 −C5F5]+2 2Br− (4a)
organic extract was washed with brine solution, dried
over an anhydrous Na2SO4, organic solvent was evap-
orated under reduced pressure to result crude gummy
compound. Crude compound was purified by column
Reaction mixture was stirred at 60◦C for 4 h. White
solid. Yield: 80% (based on 1). M.p. >250◦C (decom.).
1H NMR(DMSO-d6, 400 MHz) δ 9.48 (s, 2H, Im
NCHN), 7.84 (s, 2H, ImH), 7.78 (s, 2H, ImH), 7.24–
7.22 (d, JH,H = 7.6 Hz, 2H, ArH), 7.17–7.15 (d,
JH,H = 8 Hz, 2H, ArH), 7.05 (s, 2H, ArH), 5.63 (s, 4H,
C5F5-CH2-Im), 5.29 (s, 4H, Im-CH2-Ar), 4.64–4.60 (d,
JH,H = 16.8 Hz, 2H, exoCH2), 4.19 (s, 2H, NCH2N),
4.11–4.07 (d, JH,H = 17.2 Hz, 2H, endoCH2) ppm.
13C NMR is not clear due to 13C &19F coupling. FT-
IR (neat): 3051 (w), 2945 (w), 1561 (w), 1526 (m),
1507 (s), 1429 (w), 1351 (w), 1161(m), 1126 (m),
1035 (m), 1014 (m), 967 (m) cm−1. Anal. Calcd for
C37H26Br2F10N6: C, 49.14; H, 2.90; N, 9.29. found: C,
49.2; H, 3.0; N, 9.3.
chromatography (100–200 silica gel) and the desired
product 2 was eluted by 10% MeOH:DCM mixture.
Yellow solid yield: 80% (based on 4-(methyl benzim-
1
idazole) aniline). M.p. 219–222◦C. H NMR (CDCl3,
400 MHz) δ 7.84 (s, 2H, BIm NCHN), 7.81–7.79 (m,
2H, ArH), 7.28–7.20 (m, 6H, ArH), 7.06–7.04 (d,
JH,H = 8 Hz, 2H, ArH), 7.00–6.97 (dd, JH,H = 10,
8.4 Hz, 2H, ArH), 6.67 (s, 2H, ArH), 5.16 (s, 4H, BIm
CH2Ar), 4.59–4.55 (d, JH,H = 16.8 Hz, 2H, exoCH2),
4.21 (s, 2H, NCH2N), 4.05–4.01 (d, JH,H = 16.8 Hz,
2H, endoCH2) ppm. 13C NMR (CDCl3, 100 MHz) δ
147.90 (ArC), 143.75 (ArC), 142.98 (ArC), 133.74
(ArC), 131.11 (ArC), 128.21 (ArC), 126.25 (ArC),
125.57 (ArC), 125.51 (ArC), 122.96 (ArC), 122.16
(ArC), 120.29 (ArC), 109.81 (ArC), 66.53 (NCH2N),
58.40 (NCH2Ar), 48.14 (BIm-CH2-Ar) ppm. FT-IR
(neat): 3051 (w), 2924 (m), 2853 (w), 1613 (w), 1494
(s), 1450 (m), 1356 (m), 1325 (m), 1288 (m), 1262 (m)
1204 (m), 1110 (m), 742 (s) cm−1. HRMS: m/z [M +
H]+ calcd for C31H26N6: 483.2297; found: 483.2258.
2.6 Synthesis of TB-[CH2-Im-CH2 −C5F5]+2 2PF6− (4b)
White solid. Yield: 92% (based on 4b). M.p. 206 –
1
208◦C. H NMR(DMSO-d6, 400 MHz) δ 9.35 (s, 2H,
Im NCHN), 7.78–7.74 (d, 4H, ImH), 7.19–7.16 (m,
4H, ArH), 7.02 (s, 2H, ArH), 5.61 (s, 4H, Ar-CH2-Im),
5.24 (s, 4H, Im-CH2-Ar), 4.65–4.60 (d, JH,H = 17.2
Hz, 2H, exoCH2), 4.20 (s, 2H, NCH2N), 4.12–4.07 (d,
JH,H = 17.2 Hz, 2H, endoCH2) ppm. 13C NMR is not
clear due to 13C &19F coupling. 31P NMR (DMSO-d6,
161 MHz) δ − 126.29 to −152.64 (septet) ppm. FT-IR
2.3 General procedure for the synthesis
of the azolium halides, 4a–8a
An oven dried Schlenk tube was charged with 1 or 2 (neat): 3163 (w), 1659 (w), 1562 (w), 1511 (m), 1159
(1 eq.), aryl methyl halide (2.2 eq.) and dry acetone, (m), 1128 (m), 1039 (w), 831 (s) cm−1.Anal. Calcd for

Tröger’s-base derivative of azolium salts
823
C37H26F22N6 P2: C, 42.96; H, 2.53; N, 8.12. found: C, C, 56.61; H, 4.22; N, 8.80. Found: C, 56.5; H, 4.4;
43.0; H, 2.4; N, 7.9.
N, 8.7.
2.7 Synthesis of TB-[CH2-Im-CH2-(1-Naphthyl)]2+
2Cl− (5a)
2.9 Synthesis of TB-[CH2-BIm-CH2-Mes]+2 2Br− (6a)
Reaction mixture was stirred at 60◦C for 6 h. White
Reaction mixture was stirred at 60◦C for 48 h. Pale solid. Yield: 91% (based on 2). M.p. >360◦C (decom.).
brown solid. Yield: 60% (based on 1). M.p. 215– 1H NMR(DMSO-d6, 400 MHz) δ 9.50 (s, 2H, BIm
1
217◦C. H NMR(DMSO-d6, 400 MHz) δ 9.57 (s, 2H, NCHN), 8.07–7.94 (m, 4H, ArH), 7.68–7.63 (m, 4H,
Im NCHN), 8.13-8.11 (m, 2H, ArH), 8.02–8.00 (d, ArH), 7.36–7.02 (m, 10H, ArH), 5.67 (s, 4H, Mes-
JH,H = 6 Hz, 4H, ArH), 7.80 (s, 4H, ArH), 7.62– CH2-Ar), 5.57 (s, 4H, BIm-CH2-Ar), 4.58–4.54 (d,
7.53 (m, 8H, ArH), 7.21–7.19 (d, JH,H = 8 Hz, 2H, JH,H = 14.4 Hz, 2H, exoCH2), 4.14 (s, 2H, NCH2N),
ArH), 7.14–7.12 (d, JH,H = 8.4 Hz, 2H, ArH), 6.97 4.04–4.00 (d, JH,H = 14.8 Hz 2H, endoCH2), 2.29
(s, 2H, ArH), 5.94 (s, 4H, Im-CH2-Nap), 5.27 (s, (s, 6H, Mes-paraCH3), 2.23 (s, 12H, Mes-orthoCH3)
4H, Im-CH2-Ar), 4.59–4.55 (d, JH,H = 16.8 Hz, 2H, ppm.13C NMR(DMSO-d6, 100 MHz) δ 148.19 (ArC),
exoCH2), 4.17 (s, 2H, NCH2N), 4.05-4.01 (d, JH,H
=
141.33 (NCHN), 138.73 (ArC), 138.30 (ArC), 131.64
16.8 Hz, 2H, endoCH2) ppm. 13C NMR (DMSO-d6, (ArC), 130.93 (ArC), 129.53 (ArC), 129.33 (ArC),
100 MHz) δ 148.45(ArC), 136.42 (ImNCN), 133.48 128.32 (ArC), 126.82 (ArC), 126.73 (ArC), 126.54
(ArC), 130.43 (ArC), 130.01 (ArC), 129.81 (ArC), (ArC), 125.84 (ArC), 125.18 (ArC), 114.00 (ArC),
128.98 (ArC), 128.47 (ArC), 128.14 (ArC), 128.08 113.91 (ArC), 65.77 (NCH2N), 57.87 (NCH2Ar),
(ArC), 127.26 (ArC), 127.20 (ArC), 126.88 (ArC), 49.31 (BIm-CH2-Ar), 45.29 (BIm-CH2-Mes), 20.67
126.50 (ArC), 125.72 (ArC), 125.35 (ArC), 123.11 (Mes-paraCH3), 19.34 (Mes-orthoCH3) ppm. FT-IR
(ArC), 122.93 (ArC), 122.69 (ArC), 65.86 (NCH2N), (neat): 3123 (w), 2951 (m), 2920 (m), 2855 (w), 1612
57.95 (NCH2-Ar), 51.56 (Im-CH2-Ar), 50.02 (Im- (m), 1560 (s), 1493 (s), 1430 (m), 1190 (m) cm−1.
CH2-Nap) ppm. FT-IR (neat): 2922 (w), 1554 (m), 1510 Anal. Calcd for C51H52Br2N6: C, 67.40; H, 5.77; N, 9.5.
(w), 1492 (w), 1141 (w), 825 (s) cm−1. Anal. Calcd for found: C, 67.3; H, 5.9; N, 9.3.
C45H40Cl2N6: C, 73.46; H, 5.48; N, 11.42. found: C,
73.6; H, 5.4; N, 11.3.
2.10 Synthesis of TB-[CH2-BIm-CH2-Mes]2+
2PF−6 (6b)
2.8 Synthesis of TB-[CH2-Im-CH2-(1-Naphthyl)]2+
2PF−6 (5b)
Yellowish white solid. Yield: 89% (based on 6a). M.p.
1
182–184◦C. H NMR(DMSO-d6, 400 MHz) δ 9.38 (s,
Brownish white solid. Yield: 68% (based on 5a). M.p. 2H, BIm NCHN), 8.08–8.06 (d, JH,H = 8.4 Hz, 2H,
1
156–158◦C. H NMR(DMSO-d6, 400 MHz) δ 9.30 (s, ArH), 7.93–7.90 (d, JH,H = 8.4 Hz, 2H, ArH), 7.72–
2H, Im NCHN), 8.08–8.02 (m, 6H, ArH), 7.77–7.75 7.68 (t, JH,H = 7.6, 8.0 Hz, 2H, ArH), 7.65–7.61 (t,
(d, JH,H = 9.6 Hz, 4H, ArH), 7.64–7.53 (m, 8H, JH,H = 8.0, 7.6 Hz, 2H, ArH), 7.21–7.19 (d, JH,H
=
ArH), 7.20–7.13 (m, 4H, ArH), 6.93 (s, 2H, ArH), 8.4 Hz, 2H, ArH), 7.14–7.12 (d, JH,H = 8.0 Hz, 2H,
5.90 (s, 4H, Im-CH2-Ar), 5.23 (s, 4H, Im-CH2-Nap), ArH), 7.04 (s, 4H, MesArH), 7.01 (s, 2H, BImH),
4.62–4.57 (d, JH,H = 17.2 Hz, 2H, exoCH2), 4.19 5.64 (s, 4H, BIm-CH2-Ar), 5.53 (s, 4H, BIm-CH2-
(s, 2H, NCH2N), 4.07–4.03 (d, JH,H = 16.8 Hz, Mes), 4.59–4.55 (d, JH,H = 16.8 Hz, 2H, exoCH2), 4.15
2H, endoCH2) ppm.13C NMR (DMSO-d6, 100 MHz) (s, 2H, NCH2N), 4.04–4.00 (d, JH,H = 17.2 Hz, 2H,
δ 148.45 (ArC), 136.32(Im NCN), 136.19 (ArC), endoCH2), 2.31 (s, 6H, Mes-paraCH3), 2.23 (s, 12H,
133.43 (ArC), 130.36 (ArC), 129.80 (ArC), 128.92 Mes-orthoCH3) ppm. 13C NMR (DMSO-d6, 100 MHz)
(ArC), 128.43 (ArC), 127.94 (ArC), 127.16 (ArC), δ 148.26 (ArC), 141.21 (ArC), 138.76 (ArC), 138.28
127.02 (ArC), 126.72 (ArC), 126.43 (ArC), 125.63 (BIm NCN), 131.64 (ArC), 130.94 (ArC), 129.50
(ArC), 125.33 (ArC), 123.12 (ArC), 123.07 (ArC), (ArC), 129.22 (ArC), 128.30 (ArC), 126.78 (ArC),
122.69 (ArC), 122.61(ArC), 65.81 (NCH2N), 57.87 126.73 (ArC), 126.68 (ArC), 126.45 (ArC), 125.67
(NCH2Ar), 51.62 (Im-CH2-Ar), 50.03(Im-CH2-Nap) (ArC), 125.17 (ArC), 113.89 (ArC), 113.79 (ArC),
ppm. 31P NMR (DMSO-d6, 161 MHz) δ − 130.99 to 65.72 (NCH2N), 57.83 (NCH2Ar), 49.29 (BIm-CH2-
−157.34 (septet) ppm. FT-IR (neat): 3154 (w), 2922 Ar), 45.09 (BIm-CH2-Mes), 20.60 (Mes-paraCH3),
(w), 1554 (m), 1510 (w), 1492 (m), 1453 (w), 1207 (m), 19.20 (Mes-orthoCH3) ppm. 31P NMR (DMSO-d6,
1141 (m), 825 (s) cm−1. Anal. Calcd for C45H40F12N6P2: 161 MHz) δ − 130.98 to −157.33 (septet) ppm. FT-IR

824
Arruri Sathyanarayana and Ganesan Prabusankar
(neat): 2915 (w), 1612 (w), 1562 (m), 1492 (w), 1428 ArH), 6.28 (s, 4H, BIm-CH2-Ar), 5.60 (s, 4H, BIm-
(w), 1377 (w), 1185 (w), 828 (s), 742 (m) cm−1. Anal. CH2-Nap), 4.58–4.54 (d, JH,H = 17.2 Hz, 2H, exoCH2),
Calcd for C51H52F12N6P2: C, 58.96; H, 5.04; N, 8.09. 4.17 (s, 2H, NCH2N), 4.03–3.99 (d, JH,H = 16.8,Hz,
Found: C, 58.8; H, 5.2; N, 8.1.
2H, endoCH2) ppm. 13C NMR (DMSO-d6, 100 MHz)
δ 148.08 (ArC), 142.77 (ArC), 133.44 (BIm NCN),
131.33 (ArC), 130.86 (ArC), 130.34 (ArC), 129.58
(ArC), 129.26 (ArC), 129.01 (ArC), 128.93 (ArC),
128.85 (ArC), 128.31 (ArC), 127.37 (ArC), 127.14
(ArC), 127.04 (ArC), 126.80 (ArC), 126.66 (ArC),
126.42 (ArC), 125.53 (ArC), 125.18 (ArC), 122.92
(ArC), 114.13 (ArC), 114.02 (ArC), 65.69 (NCH2N),
57.81 (NCH2Ar), 49.28 (BIm-CH2-Ar), 48.23 (BIm-
CH2-Nap) ppm. FT-IR (neat): 3052 (w), 2925 (m), 2853
(w), 1656 (w), 1613 (m), 1559 (s), 1493 (s), 1439 (m),
1370 (m), 1205 (m), 1188 (s), 1016 (m), 789 (s), 744 (s)
cm−1. Anal. Calcd for C53H44Cl2N6: C, 76.16; H, 5.31;
N, 10.05. found: C, 76.3; H, 5.1; N, 10.1.
2.11 Synthesis of TB-[CH2-BIm-CH2 − C5F5]2+
2Br− (7a)
Reaction mixture was stirred at 60◦C for 48 h. Brown-
ish white solid. Yield: 72% (based on 2). M.p.
>360◦C(decom.). 1H NMR(DMSO-d6, 400 MHz) δ
11.11 (s, 2H, BIm NCHN), 7.72–7.61 (m, 4H, ArH),
7.55–7.45 (m, 4H, ArH), 7.37–7.13 (m, 4H, ArH),
6.98–6.95 (m, 2H, ArH), 6.00 (s, 4H, BIm-CH2-
Ar), 5.67–5.61 (m, 4H, BIm-CH2-PFP), 4.54–4.49 (d,
JH,H = 16.8 Hz, 2H, exoCH2), 4.08 (s, 2H, NCH2N),
4.06–4.04 (d, JH,H = 16.8 Hz, 2H, endoCH2) ppm.
FT-IR (neat): 3117 (w), 2947 (w), 1657 (w), 1611 (w),
1560 (w), 1504 (s), 1425 (m), 1347 (m), 1206 (m), 1122
(m), 1032 (m), 963 (m), 748 (m) cm−1. Anal. Calcd for
C45H30Br2F10N6: C, 53.80; H, 3.01; N, 8.37. Found: C,
53.8; H, 3.0; N, 8.4.
2.14 Synthesis of TB-[CH2-BIm-CH2-(1-Naphthyl)]2+
2PF−6 (8b)
White solid. Yield: 80% (based on 8a). M.p. 202–
1
204◦C. H NMR(DMSO-d6, 400 MHz) δ 9.82 (s, 2H,
2.12 Synthesis of TB-[CH2-BIm-CH2 − C5F5]2+
2PF−6 (7b)
BIm NCHN), 8.09–7.98 (m, 10H, ArH), 7.67–7.59
(m, 8H, ArH), 7.57–7.49 (m, 4H, ArH), 7.26–7.24
(d, JH,H = 8.0 Hz, 2H, ArH), 7.15–7.13 (d, JH,H
=
8.4 Hz, 2H, ArH), 6.99 (s, 2H, ArH), 6.26 (s, 4H,
BIm-CH2-Ar), 5.57 (s, 4H, BIm-CH2-Nap), 4.58–4.54
(d, JH,H = 16.8 Hz, 2H, exoCH2), 4.16 (s, 2H,
NCH2N), 4.03–3.99 (d, JH,H = 16.8 Hz, 2H, endoCH2)
ppm. 13C NMR (DMSO-d6, 100 MHz) δ 148.38 (ArC),
142.52 (ArC), 133.45 ( BIm NCHN), 131.40 (ArC),
130.89 (ArC), 130.35 (ArC), 129.65 (ArC), 129.01
(ArC), 128.95 (ArC), 128.85 (ArC), 128.40 (ArC),
127.47 (ArC), 127.40 (ArC), 127.13 (ArC), 126.98
(ArC), 126.86 (ArC), 126.55 (ArC), 126.44 (ArC),
125.54 (ArC), 125.23 (ArC), 122.80 (ArC), 114.08
(ArC), 113.94 (ArC), 65.74 (NCH2N), 57.83 (NCH2-
Ar), 49.35 (BIm-CH2-Ar), 48.20 (BIm-CH2-Nap) ppm.
31P NMR (DMSO-d6, 161 MHz) δ −130.98 to −157.33
(septet) ppm. FT-IR (neat): 3146 (w), 2917 (w), 2851
(w), 1613 (w), 1563 (m), 1493 (m), 1443 (w), 1188
(w), 1012 (w), 831 (s), 744 (m) cm−1. Anal. Calcd for
C53H44F12N6P2: C, 60.34; H, 4.20; N, 7.97. found: C,
60.3; H, 4.1; N, 8.1.
Brownish white solid. Yield: 83% (based on 7a). M.p.
1
185–187◦C. H NMR(DMSO-d6, 400 MHz) δ 9.96 (s,
2H, BIm NCHN), 8.00–7.95 (t, JH,H = 8.4, 8.4 Hz,
4H, ArH), 7.74–7.70 (t, JH,H = 7.6, 8.0 Hz, 2H, ArH),
7.66–7.62 (t, JH,H = 8.0, 7.6 Hz, 2H, ArH), 7.30–7.28
(m, 2H, ArH), 7.16–7.14 (d, JH,H = 8.4 Hz, 2H, ArH),
7.10 (s, 2H, ArH), 5.95 (s, 4H, BIm-CH2-Ar), 5.60 (s,
4H, BIm-CH2-PFP), 4.61–4.57 (d, JH,H = 16.8 Hz, 2H,
exoCH2), 4.17 (s, 2H, NCH2N), 4.08–4.04 (d, JH,H
=
16.8 Hz, 2H, endoCH2) ppm. 31P NMR (DMSO-d6,
161 MHz) δ − 131.04 to −157.39 (septet) ppm. FT-
IR (neat): 2923 (w), 2854 (w), 1612 (m), 1565 (s),
1510 (w), 1450 (w), 1299 (m), 1124 (m), 1028 (s), 831
(s) cm−1. Anal. Calcd for C45H30F22N6P2: C, 47.63; H,
2.66; N, 7.41. found:C, 47.5; H, 2.8; N, 7.4.
2.13 Synthesis of TB-[CH2-BIm-CH2-(1-Naphthyl)]2+
2Cl− (8a)
Reaction mixture was stirred at 60◦C for 48 h.brownish
white solid. Yield: 61% (based on 2). M.p. 222–224◦C.
1H NMR(DMSO-d6, 400 MHz) δ 10.06 (s, 2H, BIm
NCHN), 8.13–8.11 (d, JH,H = 7.6 Hz, 2H, ArH), 8.05–
2.15 General procedure for catalytic
benzoin condensation
7.99 (m, 8H, ArH), 7.65–7.59 (m, 8H, ArH), 7.56–7.50 An oven-dried Schlenk tube was charged with mag-
(m, 4H, ArH), 7.29–7.27 (d, JH,H = 8.0 Hz, 2H, ArH), netic stirrer bar and catalyst (2.5 mol%), and then dried
7.15–7.13 (d, JH,H = 8.4 Hz, 2H, ArH), 7.03 (s, 2H, under vacuum for 5–15 min. After that, solvent (3 mL)

Tröger’s-base derivative of azolium salts
825
and distilled benzaldehyde (0.943 mmole) were added dried over an anhydrous sodium sulphate, the reaction
under nitrogen atmosphere at room temperature, and mass was concentrated under reduced pressure to get
then evacuated for a few seconds and refilled with nitro- crude compound. The crude compound was purified by
gen. After that, KOt Bu (5 mol%) was added to the reac- column chromatography (silica gel 100–200 mesh) the
tion mixture under nitrogen condition at room temper- desired compound was eluted by 8% EtOAc: pet-ether,
ature. The reaction progress was monitored by TLC. the product analyses werematched with reported values.11,22
After 20 h, the reaction mixture was diluted with water
(10 mL) and ethyl acetate (10 mL). The organic phase
3. Results and Discussion
was separated, washed with brine solution (7 mL), dried
over an anhydrous sodium sulphate, the reaction mass
was concentrated under reduced pressure to get crude
compound. The crude compound was purified by col-
umn chromatography (silica gel 100–200 mesh) the
desired compound was eluted in 10% EtOAc: pet-
ether, the product analysis were matched with reported
values.10
3.1 Synthesis and characterization
The derivatives of TB supported imidazolium and ben-
zimidazolium salts, 4–8 were synthesized by follow-
ing the synthetic procedure optimized for 1 and 3 in
very good yield (scheme 1).4a The yield of the prod-
uct ranges from 60 to 92%. Notably, 4–8 were isolated
without any quaternary ammonium salts at bridgehead
nitrogen atoms.12 Compounds 2 and 4–8 were fully
characterized. The 1H NMR chemical shift value of 3a–
2.16 General procedure for catalysts 3a, 3b and 3c
mediated Baylis-Hillman reaction
8a for imidazolium NCHN, proton fall in the range
1
An oven dried Schlenk tube was charged with magnetic from δ 9.17 to 11.11 ppm, while the H NMR chem-
stirrer bar and catalyst 3a or 3b or 3c (2.5–7.5 mol%), ical shift value of 3b–8b for NCHN, proton appears
then dried under vacuum for 5–15 min. After that, between δ 9.03 to 9.96 ppm. The endo and exo NCH2Ar
solvent (3 mL) and cyclohexenone (1.041 mmol) was proton chemical shift values of 3a–8a and 3b–8b are
added under nitrogen atmosphere at room temperature almost comparable. Except 3b and 8b, the NCH2N
then evacuated for few seconds and refilled with nitro- protons chemical shift values of 4b–7b are slightly
gen, after that KOt Bu (5–15 mol%) was added to the higher than that of corresponding halogen ion deriva-
reaction mixture under nitrogen condition at room tem- tives, 4a–7a. Furthermore the molecular skeleton of 3a
perature. The reaction progress was monitored by TLC. and 6a were confirmed by DEPT, HSQC and HMBC
After 4 h, the reaction mixture was diluted with water correlations, and the similar molecular skeleton also
(10 mL) and ethyl acetate (10 mL). The organic phase expected for other derivatives, 3b–8b, 3c, 4a, 5a, 7a,
was separated, washed with brine solution (7 mL), and 8a (see supporting information). Figure 1 presents
Scheme 1. Synthesis of 4–8.

826
Arruri Sathyanarayana and Ganesan Prabusankar
The structure of 2 was further confirmed by single
crystal X-ray diffraction technique (figure 2).13 Com-
pound 2 crystallized in the orthorhombic space group,
P22121. Selected bond lengths and bond angles are listed
in the caption of figure 2. Molecule 2 represents a rare
structurally characterized TB linked azole skeleton.4b
As shown in figure 2, the “V” shaped TB skeleton is
linked with benzimidazole through methylene group to
result the “Mexican hat” shape structure to the molecule.
The C(10)−N(2)− C(10) angle is 99.32(1)◦, which is
almost comparable with 2,8-dimethyl-5,11-methano-5,
6,11,12-tetrahydro-dibenzo-[b, f][1,5]diazocine (92.8–
97.4◦).14 TheN(3)−C(10)−C(4)angleis 111.5(4)◦. The
N(2)−C(1)− N(2) angle is 110.5(5)◦. N(9)−C(3) bond
distance is 1.289(7) Å. N(2)−C(1) bond distance is
1.429(6) Å. As shown in the packing diagram, molecule
is packed in a layer form through O−H· · · · N type of
hydrogen bonding between lattice water molecule and
bezimidazole nitrogen (figure 3). The donor-acceptor
bond distance of O−H· · · ·N is 2.913 Å.
Figure 1. UV-vis absorption spectra of 3–8 in DMSO
(2.64 × 10−5 M).
the optical spectra of the azolium salts in DMSO. The
UV−vis spectra of 5–8 demonstrate significant diver-
sity compared to 3–4. The absorption spectra of 3–4
are nearly identical. The absorption spectrum of 8b is
much stronger than 3–7 and 8a. Notably, the UV-vis
absorption spectra of 5–8 shows bathochromic shifts
compared to 3–4.
3.2 Application of 3–8 incarbon-carbon bond
formation reactions
In general, the organocatalyst-mediated benzoin con-
densation reactions were carried-out in THF using
KOt Bu to obtain the best output or to avoid the solubil-
ity issues.4a Unlike the conventional route, the present
Figure 2. The solid state structure of 2. Selected bond distances (Å)
and angles (◦): C(1)−N(2), 1.429(6); N(2)−C(16), 1.383(8); N(2)−C(5),
1.539(8); N(3)−C(3), 1.335(7); N(3)−C(6), 1.389(6); N(3)−C(10), 1.494(6);
C(8)−N(9), 1.413(9); N(9)−C(3), 1.289(7); N(2)−C(1)−N(2)#1, 110.5(5);
C(4)−C(10)−N(3), 111.5(4).

Tröger’s-base derivative of azolium salts
827
Figure 3. The molecular packing of 2 through O−H· · · ·N hydrogen bonding.
Table 1. Benzoin Condensation reactions mediated by catalysts 3a-3c.
Entry Cata,b
.
Solvent
Starting material conversion (%) I Yield (%)c II Yield (%)c
1
2
3
4
5
6
7
8
9
3a
3a
3a
3b
3b
3b
3c
3c
3c
Toluene
Fluorobenzene
1,4-Dioxane
Toluene
Fluorobenzene
1,4-Dioxane
Toluene
99
99
99
97
96
94
98
98
99
91
94
95
89
92
94
79
89
90
8
5
4
8
5
5
18
10
9
Fluorobenzene
1,4-Dioxane
a2.5% of catalyst loading,
bBase-KOt Bu,
cIsolated yields are 4–5% less than calculated.
Scheme 2. Catalysts 3a, 3c and 3a–8a mediated Benzoin
Condensation reaction.
Scheme 3. Benzoin Condensation reaction mediated by
catalyst 3a with different aryl aldehydes.
Table 2. Benzoin Condensation reaction mediated by cat-
alysts 3a–8a.
or 1,4-dioxane reaction medium can also reduce the
moisture absorbance in comparison with THF medium.
Catalysts 3a, 3b and 3c gave desired benzoin product
(I) as a major yield (from 79 to 94% yield) along with
Cannizzaro product (II)15 as a side product (from 5 to
18% yield) with the loading of 2.5 mol% catalyst. When
the catalyst loading was 1 mol%, the starting material
was consumed over a period of 4 days. Further incre-
ment in the catalyst loading (5 mol%) did not improve
the catalyst performance. Notably, the catalyst 3a in
1,4-dioxane gave I in excellent yield (95%). The
efficiency of starting material conversion for catalysts
Starting material
Entry Cat. conversion (%) I Yield (%)a II Yield (%)a
1
2
3
4
5
6
3a
4a
5a
6a
7a
8a
99
91
96
87
89
80
95
76
72
79
81
73
4
8
24
6
8
4
aIsolated yields are 4–5% less than calculated. Isolated
yields after column chromatography.
studies were accomplished using catalysts, 3a, 3b, 3a, 3b and 3c were nearly comparable (94 to 99%,
and 3c in toluene, fluorobenzene and 1,4-dioxane table 1). However, catalyst 3a in 1,4-dioxane is very
(table 1, scheme 2). Moreover, toluene, fluorobenzene selective towards benzoin condensation reaction.

828
Arruri Sathyanarayana and Ganesan Prabusankar
Table 3. Benzoin Condensation reaction with catalyst 3a (2.5 mol%), KOt Bu (7 mol%) as a base,
in 1,4-dioxane solvent at r.t., for 20 h.
Entry
Starting material
Product
Yield (%)a
1
2
72
69
3
4
83
68
5
6
66
68
7
8
9
78
82
0

Tröger’s-base derivative of azolium salts
829
Table 3. Continued.
Entry
Starting material
Product
Yield (%)a
10
11
85
0
12
64
aIsolated yields after column chromatography.
Therefore, catalysts 4a, 5a, 6a, 7a and 8a were tested
for the benzoin condensation reaction in 1,4-dioxane
(table 2). Among 3a–8a, 3a produced only I in excellent
yield (with starting material conversion of 94%, yield
of I is 94%), while 5a gave both I (72%) as well as II
(24%). Therefore 3a turned out to be the better catalyst
for benzoin condensation. Yield of the benzoin product
and the loading of catalyst are almost comparable with
literature.16
Subsequently, we have extended the benzoin conden-
sation on substituted aryl aldehydes using 3a as a cat-
alyst (scheme 3, table 3). For example, the 3-methoxy
benzaldehyde (83%, entry 1, table 3) gave better yield
than 2-methoxy benzaldehyde (69%, entry 2, table 3)
and 4-methoxy benzaldehyde (72%, entry 3, table 3).
Similarly, the 3,4-di substituted benzaldehydes were
converted in better yield (68%, entry 4 and 66%, entry
5, table 3). Notably, 3,4,5-trimethoxy benzaldehyde
(78%, entry 7, table 3) afforded better yield than 2,3,4-
trimethoxy benzaldehyde (68%, entry 6, table 3), which
can be explained by more steric hindrance around
aldehyde in 2,3,4-trimethoxy benzaldehyde than 3,4,5-
trimethoxy benzaldehyde. Moreover, the products of
entry 6 and 7 are considered to be the useful build-
ing blocks in many biologically important molecules.17
In general, these are derived from benzoin condensa-
tion reaction using toxic KCN as catalyst.18 Electron
rich aldehydes like 2-naphthaldehyde (entry 8, table 3)
produced 82% yield, while the indole 3-carboxaldehyde
(entry 9, table 3) and 2-nitro benzaldehyde (entry 11,
table 3) gave 0% yield. The zero conversion in entry
9 and 11 can be attribute to the deactivate nature of
NH or NO2 groups towards the reactive carbene site.
Moreover, N-Boc protected indole-3-carboxaldehyde
Scheme 4. Top: the proposed Breslow intermediate
through hydrogen bonding at enol form with TB nitrogen;
Bottom: unfavourable Breslow intermediate.
Scheme 5. Baylis-Hillman reaction mediated by catalysts
3a, 3b and 3c.
was not activated by catalyst 3a. 2-bromobenzaldehyde
gave poor yield (64%, entry 12, table 3) compared to
4-chlororbenzaldehyde (85%, entry 10, table 3).
Although the NHC plays major role in benzoin con-
densation reactions, the role of TB can’t be excluded.
For example, Wu et al. reported the TB mediated asym-
metric Mannich reaction via the TB stabilized enol
hydroxyl group by O−H· · · N hydrogen bonding.19 The

830
Arruri Sathyanarayana and Ganesan Prabusankar
Table 4. Baylis-Hillman reaction.
S. No.
Cat.
Loading mol% Base loading mol%
Solvent
Yield Time
1
2
3
4
5
6
7
8
9
3a
3a
3a
3a
3a
3a
3b
3c
5 mol%
2.5 mol%
7.5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
0
9 mol%
5 mol%
15 mol%
9 mol%
9 mol%
Without base
9 mol%
Fluorobenzene
Fluorobenzene
Fluorobenzene
Toluene
55%
52%
55%
0%
0%
0%
50%
48%
15%
4 h
24 h
4 h
4 h
4 h
24 h
4 h
4 h
1,4-Dioxane
Fluorobenzene
Fluorobenzene
Fluorobenzene
Fluorobenzene
9 mol%
10 mol%
Without cat.
24 h
similar mechanistic pathway was also proposed for the further evidence at present. The results are consistent
formationof 1,2-diferrocenylethanedione from ferrocene with reported yield of IV.22
aldehyde by 3a, 3b and 3c.4a Thus, the catalysts 3a, 3b,
3c and 4a–8a mediated benzoin condensation should
also proceed through Breslow intermediate, where the
hydrogen bonding should exist between enol form and
TB nitrogen (scheme 4), thus Breslow intermediate is
the driving force to enhance the reaction towards the
product.
4. Conclusions
We synthesized and characterized Tröger’s-base 4–8 in
good yields. The new molecules 3a–8a were used as
organocatalysts in benzoin condensation reactions. Cat-
alyst 3a was very selective towards benzoin condensa-
tion reaction (exclusive formation of I with 94% yield)
in 1,4-dioxane. Furthermore, 3a showed better activ-
ity for condensation of α, β-unsaturated cyclic ketone
(III) to Baylis-Hillman product, 2-(3-oxocyclohexyl)-
2-cyclohexen-1-one (IV) in fluorobenzene. In the cases
of benzoin condensation reactions, the catalyst loading
was appreciably low.10,23,24
Since 3a has shown the best activity in comparison
with other catalysts in benzoin condensation reaction,
the catalytic activity of former under mild condition for
theactivationofα,β-unsaturatedcyclicketone (III) was
tested (scheme 5). The catalytic conversion was car-
ried out in the presence of KOt Bu (5–15 mol%) using
selected solvents such as fluorobenzene, 1,4-dioxane
and toluene (table 4). Surprisingly, as observed for the
benzoin condensation, fluorobenzene showed the best
conversion rate with 5mol% catalyst and 9 mol% KOt Bu
within 4 h to produce the Baylis-Hillman product, 2-(3-
oxocyclohexyl)-2-cyclohexen-1-one (IV) (55%). Yield of
IV decreased when the same conversion was carried out
using 3b (50%) or 3c (48%). Similarly, the catalytic
transformation by 3a in toluene or in 1,4-dioxane did
not yield any product. When the catalyst loading was
decreased to 2.5 mol%, the reaction time increased to
24 h with 52% yield. Further increment in the cata-
lyst loading (7.5 mol%) did not improve the yield. It is
also important to note that the tertiary amines11 or ionic
liquids20 can also act as catalyst for Baylis-Hillman
reactions, hence we performed the 3a mediated Baylis-
Hillman reaction without base and the reaction did not
work. Since the alkoxy bases can also activate III to
produce trace amountof IV,21 the reaction was carried
out in the presence of KOt Bu without catalyst, which
resulted the formation of IV in 15% yield. The NHC
mediated cyclohexenone dimerization is very rare.20
Thus, we assume that both the Tröger’s bases as well
as imidazolium salts are responsible for the catalytic
activity of 3a, 3b or 3c. However, we cannot offer
Supplementary Information
CCDC 931490 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif
or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
+44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
NMR spectra and characterization table are available at
www.ias.ac.in/chemsci.
Acknowledgments
We thank CSIR (01(2529)/11/EMR-II) for the financial
support. AS thank UGC for the fellowship. We thank,
Dr. Natarajan Sampath, SCB, SASTRA University for
solving single crystal X-ray structure of molecule 2.

Tröger’s-base derivative of azolium salts
831
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Golovanov D G, Lyssenko K A and Kostyanovsky R G
2006 Tetrahedron: Asymmetry 17 2191; (g) Michon C,
Goncalves-Farbos M -H and Lacour J 2009 Chirality 21
809
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