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.