Microwave-Assisted Silylation and Silyl Ether Cleavage
1103
Experimental
Natural kaolinitic clay was obtained from the Padappakara mine (Quilon district, Kerala, India) and
was puri®ed and characterized [7] by FTIR, XRD, UV, EPR, SEM, EDX, and AAS. Composition: (i)
determined by wet chemical analysis: SiO2 67:45%, Al2O3 22:2%, Fe2O3 6:1%, TiO2
3:45%, K 0:8%; (ii) determined by electron dispersive X-ray (EDX) analysis: SiO2 62:8%,
Al2O3 24:92%, Fe2O3 7:5%, TiO2 3:79%, K 0:4%. Natural kaolinitic clay was supplied
by Dr. Lalithambika, RRL, Trivandrum, India and used as obtained without any pretreatment or
activation.
Solvents were distilled before use. All chemicals were of analytical grade. A microwave oven
(Kelvinator, T-37 model) with 100% ef®ciency at 760 W and 2450 MHz was used.
Silylation
A mixture of 10 mmol alcohol, 5 mmol HMDS, and 1 mmol LiClO4 in a beaker covered with a watch
glass was irradiated for the speci®ed time (Table 1). After completion of the reaction, 10 cm3
petroleum ether were added, and the catalyst was removed by ®ltration and washed twice with 10 cm3
petroleum ether. Removal of the solvent under reduced pressure gave the desired product. The results
of elemental analyses (C, H, Cl, Si) agreed with the calculated values within experimental error.
4-Chlorobenzyl trimethylsilyl ether (2i; C10H15ClOSi)
Oil; IR (neat): ꢁ 670, 710, 770, 810, 842, 874, 1014, 1087, 1160, 1252, 1330, 1450, 1491, 1540,
2860, 2910, 2951 cm 1; 1H NMR (300 MHz, ꢂ, CDCl3): 0.2 (s, 9H, Si(CH3)3), 4.7 (s, 2H, CH2), 7.3
(d, 2H, Ar-H), 7.5 (d, 2H, Ar-H) ppm.
Deprotection of silyl ethers
A mixture of 5 mmol TMS ether and 100g natural kaolinitic clay moistened with H2O was irradiated
with microwaves for the time period speci®ed in Table 1. After completion of the reaction, the product
was extracted three times with 5 cm3 of diethyl ether. The ether layer was dried over anhydrous
Na2SO4. Removal of the solvent furnished a crude product which was further puri®ed by column
chromatography (petroleum ether:ethyl acetate 9:1).
3,4-Methylenedioxybenzyl alcohol (1k; C8H8O2)
M.p.: 58ꢀC; IR (KBr): ꢁ 766, 803, 860, 919, 936, 1015, 1034, 1098, 1123, 1191, 1245, 1329, 1413,
1
1490, 1503, 1604, 2907, 3314 cm 1; H NMR (300 MHz, ꢂ, CDCl3): 2.49 (s, 1H, OH), 4.49 (s, 2H,
CH2), 5.9 (s, 2H, OCH2O), 6.7 (s, 2H, Ar-H), 6.80 (s, 2H, Ar-H) ppm.
References
[1] Greene TW, Wuts PGM (1991) Protective groups in Organic Synthesis, 2nd edn. Wiley, New York
[2] Bruynes CA, Jurriens TK (1982) J Org Chem 47: 3966 and References cited therein
[3] Coossy J, Pale P (1987) Tetrahedron Lett 6039
[4] a) Morita T, Okomoto Y, Sakurai H (1980) Tetrahedron Lett 21: 835; b) Renga JM, Wange PC
(1985) Tetrahedron Lett 26: 1175; c) Sweeley CC, Bentley R, Makita M, Wells WW (1963) J Am
Chem Soc 85: 2497; d) Firouzabadi H, Korimi B (1993) Synth Commun 23: 1633; e) Morita T,
Okomoto Y, Sakurai H (1981) Synthesis 745; f) Murata S, Nayori R (1980) Tetrahedron Lett
21: 767