Fluoroboric acid adsorbed on silica-gel (HBF4-SiO2) as a new, highly efficient and reusable heterogeneous catalyst for thia-Michael addition to α,β-unsaturated carbonyl compounds

Article abstract of DOI:10.1016/j.tetlet.2008.04.144

Fluoroboric acid adsorbed on silica-gel (HBF₄-SiO₂) has been found to be a new and highly efficient heterogeneous catalyst for thia-Michael addition to α,β-unsaturated carbonyl compounds under solvent-free conditions. In the case of

Full text of DOI:10.1016/j.tetlet.2008.04.144

Tetrahedron Letters 49 (2008) 4272–4275
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Fluoroboric acid adsorbed on silica-gel (HBF4–SiO2) as a new, highly
efficient and reusable heterogeneous catalyst for thia-Michael addition
to a,b-unsaturated carbonyl compounds
*
Gaurav Sharma, Raj Kumar, Asit K. Chakraborti
Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research (NIPER), Sector 67, S.A.S. Nagar 160 062, Punjab, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Fluoroboric acid adsorbed on silica-gel (HBF4–SiO2) has been found to be a new and highly efficient
heterogeneous catalyst for thia-Michael addition to a,b-unsaturated carbonyl compounds under
solvent-free conditions. In the case of 1,3-diaryl-2-propenones, the reactions are best carried out in
MeOH. The rate of thia-Michael addition was dependent on the steric hindrance at the b-carbon of the
a,b-unsaturated carbonyl substrate as well as surrounding the thiol moiety and was exploited for
selective thia-Michael addition during intermolecular competition between two enones with a common
thiol and between two aryl/alkyl thiols for a common enone. The methodology finds application for one-
pot syntheses of 2,3-dihydro-1,5-benzothiazepines.
Received 20 January 2008
Revised 17 April 2008
Accepted 26 April 2008
Available online 30 April 2008
Keywords:
Thia-Michael addition
Thiol
a,b-Unsaturated carbonyl
Benzothiazepines
HBF4–SiO2
Ó 2008 Elsevier Ltd. All rights reserved.
Heterogeneous catalyst
Thia-Michael addition to a,b-unsaturated carbonyl compounds
is an important transformation. It is a key step in biosynthesis1
and in the synthesis of bioactive compounds2 and provides a
means to protect the olefinic double bond of a,b-unsaturated
carbonyl groups.2b The resultant 2-sulfido carbonyl compounds
undergo copper(I)-induced3 or oxidative thermolytic2b elimination
of the sulfur moiety for easy regeneration of the a,b-unsaturated
carbonyl groups. The b-sulfido carbonyl compounds serve as start-
ing materials for b-acylvinyl cation4 and homoenolate5 equiva-
lents. Thus, there have been significant efforts towards the
development of methodologies for thia-Michael addition.6
In continuation of our efforts7 to develop newer and better
methodologies for thia-Michael addition reactions, we were influ-
enced by the tight legislation on maintenance of greenness in syn-
thetic pathways and processes, that is, to prevent generation of
waste, avoid use of auxiliary substances (e.g., solvents, additional
reagents) and minimise energy requirements.8 Catalysis plays a
critical role in the design, development and implementation of
green chemistry9 and solid acids are leading contenders as envi-
ronmentally friendly catalysts.10
for various organic transformations.11 In this investigation, we
present HBF4–SiO212 as a new, highly efficient and reusable hetero-
geneous catalyst for thia-Michael addition reactions to a,b-unsat-
urated carbonyl compounds (Table 1).
Excellent results were obtained in each case. The rate of the
thia-Michael addition was influenced by the electronic and steric
factors associated with the thiols and the a,b-unsaturated car-
bonyl compounds. The reactions with aryl thiols were, in general,
faster compared to those of aryl alkyl and alkyl thiols. Compari-
son of the reactions of 1a with 1b, and those of 1d with 1e–g
reveals that the introduction of an alkyl/aryl substituent at the
b-carbon of the a,b-unsaturated carbonyl compounds decreases
the rate of the reaction. An excellent result was obtained during
the reaction of an a,b-unsaturated carboxylic acid (entry 33) that
is susceptible to polymerisation in the presence of an acid
catalyst. There has been only one previous report of thia-Michael
addition of such substrates.13 Reactions of b-substituted a,b-
unsaturated acid/ester 1j–l with 2a afforded 87%, 77% and 74%
yields, respectively (Table 1, entries 35–37). The catalyst can be
recovered and reused after reactivation without loss of the cata-
lytic activity.
We have been engaged recently in developing heterogeneous
catalysts derived from clays, silica and silica-supported protic acids
The difference in the reaction rates encouraged us to test the
efficiency of this catalyst for a few representative intermolecular
competition studies (Scheme 1). A 75:25 (GCMS) selectivity was
observed during the reaction of an equimolar mixture of 1a and
1b with 2a and exemplified the control of selectivity by the steric
* Corresponding author. Tel.: +91 172 2214683; fax: +91 172 2214692.
E-mail address: akchakraborti@niper.ac.in (A. K. Chakraborti).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.04.144

G. Sharma et al. / Tetrahedron Letters 49 (2008) 4272–4275
4273
Table 1 (continued)
Table 1
HBF4–SiO2-catalysed thia-Michael addition to a,b-unsaturated carbonyl compoundsa
Entry
Enone
Thiol
Time (min)
Yield 3b,c,d (%)
85f
82f
88f
88f
77f
O
O
28
29
30
31
32
1g
1g
1g
1g
1g
2a
2b
2c
2e
2i
45
60
45
60
180
(R)ArSH (1)
HBF -SiO (1 mol%)
1
1
R
R
2
2
4
2
SAr(R)
R
R
rt, neat or MeOH
1
3
O
1h
1i
33
34
2a
2a
15
10
82
Entry Enone
Thiol
Time (min) Yield 3b,c,d (%)
OH
O
R
SH
78g
O
2
1a
OMe
HO
1
2
3
4
1a
1a
1a
1a
a: R = H
5
10
5
90
90
95
90
b: R = Me
c: R = NO2
d: R = F
O
35
36
2a
2a
240
240
87h
77
10
1j
SH
SH
Ph
Ph
5
1a
15
15
88
2e
MeO
O
O
6
1a
89
1k
2f
HO
7
8
1a
1a
20
85
78
SH
2g
O
37
2a
240
78h
1l
180
SH 2h
a
The a,b-unsaturated carbonyl compound (2.5 mmol) was treated with the thiol
(2.75 mmol, 1.1 equiv) in the presence of HBF4–SiO2 (1 mol %) at room temperature
(ꢀ25 °C, except for entries 10, 25–27, 34, 35 and 37) under neat conditions (except
for entries 28–32).
SH
2i
9
1a
60
77
b
Isolated yield of the corresponding thia-Michael adduct obtained after chro-
matographic purification.
10
2a
240
70e
c
All the products were characterised by analysis of the spectral data (IR, 1H and
O
1b
13C NMR and MS).
d
All new compounds gave satisfactory elemental analysis.
The reaction was carried out at 80 °C.
The reaction was carried out in methanol.
The reaction was carried out at 0 °C.
e
O
1c
f
g
11
12
13
14
15
16
1c
1c
1c
1c
1c
1c
2a
2b
2c
2e
2f
5
5
5
10
10
10
95
90
95
90
90
86
h
The reaction was carried out at 50 °C.
2g
influence of the substituent at the b-position of the a,b-unsatu-
rated carbonyl compound. The different steric factors of the thiol
moiety in controlling the selectivity were demonstrated during
the reaction of 1a with equimolar mixtures of 2a and 2i (73:27
selectivity in favour of 2a) and 2g and 2h (65:35 selectivity in
favour of 2g), respectively.
O
1d
17
18
19
20
21
1d
1d
1d
1d
1d
2a
2b
2e
2f
5
5
10
10
10
85
79
81
82
78
2g
We next planned to elaborate the applicability of this method-
ology in the selectivity of thia- versus aza-Michael addition
reactions. Thus, various chalcones were treated with 2-
aminothiophenol (2j) and 4-aminothiophenol (2k) in the presence
of HBF4–SiO2 (Table 2). In each case, selective thia-Michael addi-
tion to the a,b-unsaturated carbonyl moiety took place to form
the corresponding adduct 4. In addition, no competitive imine
formation was observed.14
O
1e
22
23
24
1e
1e
1e
2a
2b
2e
25
35
60
90
88
84
O
The chemoselective formation of the thia-Michael adducts in
excellent yields during the reactions of 1,3-diaryl-2-propenones
with 2j gave us impetus to extend this protocol for the synthesis
of benzothiazepines (Scheme 2).15
We were delighted to observe that the cyclodehydration of the
intermediate thia-Michael adducts 4 occured efficiently leading to
the corresponding 2,3-dihydro-1,5-benzothiazepines 5 in high
yields in a one-pot reaction under reflux in MeOH (Table 3).
In conclusion, we have reported HBF4–SiO2 as a highly efficient
heterogeneous reusable catalyst for chemoselective thia-Michael
1f
Ph
25
26
27
1f
1f
1f
2a
2b
2e
240
300
360
90e
85e
82e
Ph
O
1g
Ph

4274
G. Sharma et al. / Tetrahedron Letters 49 (2008) 4272–4275
O
O
H N
2
1a
H N
2
SH
2a (2.75 mmol)
2j
O
S
SPh
SPh
SPh
HBF -SiO (1 mol%)
3aa
2.5 mmol
+
4
2
+
N
S
2
1
+
i)
Ar
Ar
O
1
1
O
neat, RT, 5 min
2
O
Ar
Ar
2
(3aa:3ba::75:25)
Ar
Ar
4
5
1b
Scheme 2. One-pot synthesis of 2,3-dihydro-1,5-benzothiazepines 5.
3ba
2.5 mmol
SH
O
Table 3
2a
HBF4–SiO2-catalysed one-pot synthesis of 2,3-dihydro-1,5-benzathiazepines by
1a (2.5 mmol)
reaction of 1,3-diaryl-2-propenones with 2ja
HBF -SiO (1 mol%)
3aa
+
O
2.75 mmol
+
4
2
ii)
Entry
1,3-Diaryl-2-propenone
Time (h)
Yield 5b,c (%)
SH
neat, RT, 5 min
O
2i
(3aa:3ai::73:27)
2.75 mmol
2
1
SNapth
R
R
3ai
1
2
3
4
5
6
R1 = R2 = H
4
5
4
5
6
6
81
89
82
88
89
87
O
R1 = H; R2 = Cl
R1 = Cl; R2 = H
R1 = H; R2 = NO2
R1 = H; R2 = OMe
R1 = OMe; R2 = H
SH
2g
1a (2.5 mmol)
S
2.75 mmol
HBF -SiO (1 mol%)
3ag
+
O
4
2
iii)
+
a
The 1,3-diaryl-2-propenone (2.5 mmol) in MeOH (3 mL) was treated with 2j
(2.75 mmol, 1.1 equiv) in the presence of HBF4–SiO2 (1 mol %) under reflux.
neat, RT, 5 min
SH
2h
2.75 mmol
b
Isolated yield of the corresponding 2,3-dihydro-1,5-benzothiazepine 5 obtained
(3ag:3ah::65:35)
after chromatographic purification.
All of products were characterised by analysis of spectral data (IR, 1H and 13C
c
S
NMR and MS).
3ah
Scheme 1. Selective thia-Michael addition reaction during intermolecular
competition studies.
Acknowledgement
The author thanks OPCW, The Hague, The Netherlands, for
financial support.
Table 2
HBF4–SiO2-catalysed thia-Michael addition of 2j and 2k to 1,3-diaryl-2-propenonea
Entry
1,3-Diaryl-2-propenones
Thiol
Time
(min)
Yield 4b,c
(%)
References and notes
1. Fujita, E.; Nagao, Y. Bioorg. Chem. 1977, 6, 287–309.
O
2. (a) Fluharty, A. L. In The Chemistry of the Thiol Group, Part 2; Patai, S., Ed.; Wiley
Interscience: New York, 1974; p 589; (b) Trost, B. M.; Keeley, D. E. J. Org. Chem.
1975, 40, 2013; (c) Kumar, A.; Salunkhe, R. V.; Rane, R. A.; Dike, S. Y. J. Chem.
Soc., Chem. Commun. 1991, 485–486.
3. Cohen, T.; Mura, A. J., Jr.; Shull, D. W.; Fogel, E. R.; Ruffner, R. J.; Falck, J. R. J. Org.
Chem. 1976, 41, 3218–3219.
2
1
R
R
1
2
3
4
5
6
7
8
9
R1 = R2 = H
2j
2j
2j
2j
2j
2j
2k
2k
2k
2k
2k
45
45
40
45
75
75
60
45
45
60
60
83
88
86
90
81
87
82
84
87
80
81
R1 = H; R2 = Cl
R1 = Cl; R2 = H
R1 = H; R2 = NO2
R1 = H; R2 = OMe
R1 = OMe; R2 = H
R1 = R2 = H
4. Bakuzia, P.; Bakuzis, M. L. F. J. Org. Chem. 1981, 46, 235–239.
5. Cherkauskas, J. P.; Cohen, T. J. Org. Chem. 1992, 57, 6–8.
6. Recent examples: (a) Chu, C.-M.; Gao, S.; Sastry, M. N. V.; Kuo, C.-W.; Lu, C.; Liu,
J.-T.; Yao, C.-F. Tetrahedron 2007, 63, 1863–1871; (b) Hussain, S.; Bharadwaj, S.
K.; Chaudhuri, M. K.; Kalita, H. Eur. J. Org. Chem. 2007, 374–378; (c) Chu, C.-M.;
Huang, W.-J.; Lu, C.; Wu, P.; Liu, J.-T.; Yao, C.-F. Tetrahedron Lett. 2006, 47,
7375–7380; (d) Karodia, N.; Liu, X.; Ludley, P.; Pletsas, D.; Stevenson, G.
Tetrahedron 2006, 62, 11039–11043; (e) Maria, M.; Stefan, T. Lett. Org. Chem.
2006, 3, 794–797; (f) Kumarraja, M.; Pitchumani, K. J. Mol. Catal. A: Chem. 2006,
256, 138–142; (g) Pore, D. M.; Saodagar, M. S.; Desai, U. V.; Thopate, T. S.;
Wadagaonkar, P. P. Tetrahedron Lett. 2006, 47, 9325–9328; (h) Khan, A. T.;
Ghosh, S.; Choudhary, L. H. . Eur. J. Org. Chem. 2006, 2226–2231; (i) Firouzabadi,
H.; Iranpoor, N.; Jafarpour, M.; Ghaderi, A. Tetrahedron Lett. 2006, 249, 98–102;
(j) Zahouily, M.; Mounir, B.; Charki, H.; Mezdar, A.; Bahlaouan, B.; Ouammou,
M. ARKIVOC 2006, xiii, 178–186; (k) Yang, L.; Xu, L.-W.; Xia, C.-G. Tetrahedron
Lett. 2005, 46, 3279–3282; (l) Krishnaveni, N. K.; Surendra, K.; Rao, K. R.
Chem. Commun. 2005, 669–671; (m) Chu, C.-M.; Gao, S.; Sastry, M. N. V.; Yao,
C.-F. Tetrahedron Lett. 2005, 46, 4971–4974; (n) Zhang, H.; Zhang, Y.; Liu, L.; Xu,
H.; Wang, Y. Synthesis 2005, 2129–2136; (o) Ranu, B. C.; Dey, S. S.; Samanta, S.
ARKIVOC 2005, iii, 44–50; (p) Firouzabadi, H.; Iranpoor, N.; Jafari, A. A. Adv.
Synth. Catal. 2005, 347, 655–661; (q) Firouzabadi, H.; Iranpoor, N.; Jafari, A. A.
Synlett 2005, 299–303.
R1 = H; R2 = NO2
R1 = Cl; R2 = H
R1 = H; R2 = OMe
R1 = OMe; R2 = H
10
11
a
The 1,3-diaryl-2-propenone (2.5 mmol) was treated with the thiol (2.75 mmol,
1.1 equiv) in the presence of HBF4–SiO2 (1 mol %) at room temperature (ꢀ25 °C) in
MeOH (3 mL).
b
Isolated yield of the corresponding thia-Michael adduct 4 obtained after chro-
matographic purification.
All the products were characterised by analysis of spectral data (IR, 1H and 13C
c
NMR and MS).
addition to a,b-unsaturated carbonyl compounds.16 The methodo-
logy finds application in a one-pot synthesis of 2,3-dihydro-1,5-
benzothiazepines.
7. (a) Garg, S. K.; Kumar, R.; Chakraborti, A. K. Tetrahedron Lett. 2005, 46, 1721–
1724; (b) Garg, S. K.; Kumar, R.; Chakraborti, A. K. Synlett 2005, 1370–1374; (c)
Khatik, G. L.; Kumar, R.; Chakraborti, A. K. Org. Lett. 2006, 8, 2433–2436; (d)

G. Sharma et al. / Tetrahedron Letters 49 (2008) 4272–4275
4275
Sharma, G.; Kumar, R.; Chakraborti, A. K. J. Mol. Catal. A: Chem. 2007, 263, 143–
148.
respectively). Representative experimental procedure for the intermolecular
competition reaction (Scheme 1): To magnetically stirred mixture of 1a
a
8. Clark, J. H. Green Chem. 1999, 1–8.
(0.24 g, 2.5 mmol), 3-methyl-2-cyclohexen-1-one 1b (0.27 g, 2.5 mmol) (2) and
2a (0.30 g, 2.75 mmol, 1.1 equiv) was added HBF4–SiO2 (0.05 g, 0.025 mmol,
1 mol %) and the reaction mixture was stirred at rt (ꢀ25–30 °C) for 5 min. The
reaction mixture was diluted with Et2O (2 mL), filtered through a plug of
cotton (to separate the catalyst), and the cotton plug was washed with Et2O
(2 Â 1 mL). The combined ethereal filtrates were concentrated and the isolated
reaction mixture, without further purification, was subjected to GCMS which
indicated 75:25 selectivity in favour of the formation of 3aa. Representative
experimental procedure for the intra-molecular competition of thia- vs aza-
Michael addition reactions (Table 2, entry 1): To a magnetically stirred mixture
of 1,3-diphenylpropenone (1g) (0.21 g, 1 mmol) and 2-aminothiophenol (2j)
(0.14 g, 1.1 mmol, 1.1 equiv) in MeOH (3 mL) was added HBF4–SiO2 (0.02 g,
0.01 mmol, 1 mol %) at rt (ꢀ25–30 oC). After 45 min, the reaction mixture was
diluted with EtOAc (5 mL), filtered through a plug of cotton (to separate the
catalyst), and the cotton plug was washed with EtOAc (2 Â 1 mL). The
combined filtrates were concentrated, and the solid residue crystallised
(EtOH) to afford 0.27 g (83%) of 3-(2-amino-phenylsulfanyl)-1,3-diphenyl-
propan-1-one (4a) as the sole product indicating exclusive selectivity for thia-
Michael addition over aza-Michael addition. One-pot synthesis of 2,3-dihydro-
2,4-diphenyl-1,5-benzothiazepine (Table 3, entry 1): To a magnetically stirred
mixture of 1g (0.21 g, 1 mmol) and 2j (0.14 g, 1.1 mmol, 1.1 equiv) in MeOH
(3 mL) was added HBF4–SiO2 (0.02 g, 0.01 mmol, 1 mol %) and the
reaction mixture was stirred under reflux until completion of the reaction
(4 h: TLC, IR). The reaction mixture was diluted with EtOAc to dissolve the
product (5 mL), filtered through a plug of cotton (to separate the catalyst), and
the cotton plug was washed with EtOAc (2 Â 1 mL). The combined filtrates
were concentrated and purification was accomplished by column
chromatography (silica gel #60-120) to afford 2,3-dihydro-2,4-diphenyl-1,5-
benzothiazepine (5a) (0.25 g, 81%) as a yellow solid identical (mp, IR, 1H and
13C NMR, and MS) with an authentic sample.15 Spectral data of new compounds.
3-(4-Fluorophenylsulfanyl)-cyclohexanone (Table 1, entry 4): IR (neat) mmax 1712,
9. (a) Anastas, P. T.; Bartlett, L. B.; Kirchhoff, M. M.; Williamson, T. C. Catal. Today
2000, 55, 11–22; (b) Clark, J. H. Pure Appl. Chem. 2001, 73, 103–111.
10. Wilson, K.; Clark, J. H. Pure Appl. Chem. 2000, 72, 1313–1319.
11. (a) Kumar, R.; Kumar, D.; Chakraborti, A. K. Synthesis 2007, 299–303; (b)
Rudrawar, S.; Besra, R. C.; Chakraborti, A. K. Synthesis 2006, 2767–2771; (c)
Chakraborti, A. K.; Chankeshwara, S. V. Org. Biomol. Chem. 2006, 4, 2769–2771;
(d) Chankeshwara, S. V.; Chakraborti, A. K. J. Mol. Catal. A: Chem. 2006, 253,
198–202; (e) Chakraborti, A. K.; Kondaskar, A.; Rudrawar, S. Tetrahedron 2004,
60, 9085–9091; (f) Chakraborti, A. K.; Rudrawar, S.; Kondaskar, A. Org. Biomol.
Chem. 2004, 2, 1277–1280; (g) Chakraborti, A. K.; Gulhane, R. J. Chem. Soc.,
Chem. Commun. 2003, 1896–1897; (h) Chakraborti, A. K.; Gulhane, R.
Tetrahedron Lett. 2003, 44, 3521–3525.
12. Fluoroboric acid adsorbed on silica-gel is not commercially available and was
prepared following the procedure originally discovered by Chakraborti et al.11h
(Chakraborti, A. K.; Gulhane, R. Indian Patent, 266/DEL/2003 (10-3-2003)).
13. Gao, S.; Tzeng, T.; Sastry, M. N. V.; Chu, C.-M.; Liu, J.-T.; Lin, C.; Yao, C.-F.
Tetrahedron Lett. 2006, 47, 1889–1893.
14. Chakraborti, A. K.; Bhagat, S.; Rudrawar, S. Tetrahedron Lett. 2004, 45, 7641–
7644.
15. (a) Khatik, G. L.; Kumar, R.; Chakraborti, A. K. Synthesis 2007, 541–546; (b)
Khatik, G. L.; Kumar, R.; Chakraborti, A. K. Tetrahedron 2007, 63, 1200–1210.
16. Typical experimental procedure. Preparation of HBF4–SiO2:12
A magnetically
strirred suspension of silica-gel (26.7 g, 230–400 mesh, Sisco Research
Laboratories Pvt. Ltd, India) in diethyl ether (75 mL) was treated with 40% aq
HBF4 (3 g) for 3 h. The mixture was concentrated and the residue dried under
vacuum at 100 °C for 72 h to afford HBF4–SiO2 (0.5 mmol gÀ1). Representative
experimental procedure for thia-Michael addition to an a,b-unsaturated ketone. 3-
Phenylthiocyclohexanone 3aa (Table 1, entry 1): To a magnetically stirred
mixture of 2-cyclohexen-1-one (1a) (0.24 g, 2.5 mmol) and thiophenol (2a)
(0.30 g, 2.75 mmol, 1.1 equiv) was added HBF4–SiO2 (0.05 g, 0.025 mmol,
1 mol %) and the reaction mixture was stirred at rt (ꢀ25–30 °C) until
completion of the reaction (5 min: TLC, IR). The reaction mixture was diluted
with Et2O (2 mL), filtered through a plug of cotton (to separate the catalyst),
and the cotton plug was washed with Et2O (2 Â 1 mL). The combined ethereal
filtrates were concentrated, adsorbed on silica gel, charged on a column of
silica gel (60–120 mesh) and eluted with hexane followed by 1:10 EtOAc–
hexane to afford 3-phenylthiocyclohexanone (3aa) (0.46 g, 90%) as a colourless
oil, identical (IR, 1H and 13C NMR, and MS) with an authentic sample.7 In a
large-scale batch, 1a (5.0 g, 52 mmol) was treated with 2a (6.3 g, 57.2 mmol,
1.1 equiv) in the presence of HBF4–SiO2 (1.04 g, 0.52 mmol, 1 mol %) to afford
3aa (9.5 g, 89%) after work-up and purification. The cotton plug retaining the
recovered catalyst was put on a round bottomed flask (50 mL) and dried in a
rotary evaporator during which the catalyst separated out from the cotton. The
catalyst was activated on heating under reduced pressure (10 mm Hg) at 80 °C
for 24 h. Repetition of the reaction of 1a (2.5 g, 26 mmol) with 2a (3.1 g,
28.6 mmol, 1.1 equiv) in the presence of recovered HBF4–SiO2 (0.52 g,
0.26 mmol, 1 mol %) afforded 3aa (4.6 g, 86%) after work-up and purification.
No decrease in the catalytic activity was observed after four consecutive uses
of the recovered catalyst (yield after third and fourth uses are 85% and 88%,
1588, 1489 cmÀ1 1H NMR (CDCl3, 300 MHz) d: 1.70–1.76 (m, 2H), 2.12–2.14
;
(m, 2H), 2.30–2.38 (m, 3H), 2.61–2.66 (m, 1H), 3.32–3.33 (m, 1H), 6.99–7.05
(m, 2H), 7.41–7.45 (m, 2H). 13C NMR (CDCl3, 75 MHz) d: 24.5, 31.6, 41.3, 47.3,
48.1, 116.5, 116.8, 128.4, 136.6, 161.7, 165.0, 209.1. Anal. Calcd for C12H13FOS:
C, 64.26; H, 5.84; S, 14.30. Found: C, 64.30; H, 5.88; S, 14.33. 3-(Naphthalen-2-
ylsulfanyl)-cyclohexanone (Table 1, entry 9): IR (neat) mmax 1751, 1712,
1628 cmÀ1 1H NMR (CDCl3, 300 MHz) d: 1.73–1.76 (m, 2H), 2.13–2.43 (m,
;
5H), 2.69–2.72 (m, 1H), 3.51–3.54 (m, 1H), 7.47–7.49 (m, 2H), 7.74–7.89 (m,
5H). 13C NMR (CDCl3, 75 MHz) d: 26.2, 31.8, 41.4, 46.6, 48.3, 127.1, 128.0,
128.2, 129.2, 130.5, 130.8, 132.7, 133.1, 134.1, 151.1, 209.2. MS (MALDI-TOF)
m/z 256.4 (MH+). Anal. Calcd for C16H16OS; C, 74.96; H, 6.29; S, 12.51. Found: C,
74.93; H, 6.32; S, 12.49. 3-(Naphthalen-2-ylsulfanyl)-1,3-diphenylpropan-1-one
(Table 1, entry 32): IR (KBr) mmax 1684, 1595, 1448 cmÀ1 1H NMR (CDCl3,
;
300 MHz) d: 3.57–3.74 (m, 2H), 5.08 (t, J = 6.9 Hz, 1H), 7.16–7.25 (m, 3H), 7.36–
7.46 (m, 7H), 7.53 (t, J= 7.2 Hz, 1H), 7.68–7.78 (m, 4H), 7.87 (d, J = 7.6 Hz, 2H).
13C NMR (CDCl3, 75 MHz) d: 45.2, 48.7, 122.7, 126.8, 126.9, 128.0, 128.1, 128.4,
128.6, 128.9, 129.0, 129.1, 130.4, 132.1, 132.3, 133.0, 133.8, 134.1, 141.7, 145.4,
197.5. MS (APCI) m/z 209.1, 249.2, 368.9 (MH+). Anal. Calcd for C25H20OS: C,
81.49; H, 5.47; S, 8.70. Found: C, 81.53; H, 5.49; S, 8.68.