5982
M. Sridhar et al. / Tetrahedron Letters 52 (2011) 5980–5982
Table 2 (continued)
Entry
R–OH
Hydrosilane
Et3SiH
Product
Reaction time (h)
1.5
Yield (%)a
92
N OH
N OSiEt3
13
Ph
Ph
Ph
Ph
NOH
CH=NOH
NOSiEt3
CH=NOSiEt3
14
15
Et3SiH
Et3SiH
2.5
2.0
76
85
MeO
MeO
S
16
Et3SiH
2.0
85
CH=NOH
NOH
CH=NOSiEt3
NOSiEt3
S
17
Et3SiH
2.0
82
18
19
20
21
22
PhOH
PhOH
PhOH
PhOH
PhOH
Et3SiH
Ph3SiH
(i-Pr)3SiH
(n-Oct)3SiH
Et2(Me)SiH
PhOSiEt3
PhOSiPh3
PhOSi(i-Pr)3
PhOSi(n-Oct)3
PhOSi(Me)Et2
1.5
1.0
1.5
2.0
1.0
90
83
86
85
92
OH
OH
OSiPh3
OSiPh3
23
24
a
Et3SiH
Et3SiH
2.5
2.5
82
80
Isolated yields. All products gave satisfactory 1H and 13C NMR, IR and mass spectral data.
Acknowledgements
H.
J.R. and B.C.R. are thankful to CSIR, New Delhi for the award of
research fellowships. C.N. is thankful to UGC for the award of re-
search fellowship.
.
[R3SiH]+
R3Si+
[InBr3]
[InBr3]
References and notes
1. Greene, T. W.; Wuts, P. G. M. Protective groups in organic synthesis, third ed.;
John Wiley & Sons: New York, 1991.
2. Xianqi, W.; William, W. E.; Bosnich, B. Chem. Commun. 1996, 2561–2612.
3. Biffis, A.; Braga, M.; Basato, M. Adv. Synth. Catal. 2004, 346, 451–458.
4. Hara, K.; Akiyama, R.; Takakusagi, S.; Uosaki, K.; Yohino, T.; Kagi, H.; Sawamura,
M. Angew. Chem., Int. Ed. 2008, 47, 5627–5630.
5. Raffa, P.; Evangelisti, C.; Vitulli, G.; Salvadori, P. Tetrahedron Lett. 2008, 49, 3221–3224.
6. Chung, M.-K.; Orlova, G.; Goddard, J. D.; Schlaf, M.; Harris, R.; Beveridge, T. J.;
White, G.; Hallett, F. R. J. Am. Chem. Soc. 2002, 124, 10508–10518.
InBr3
.
InBr3
R3SiH
.
R3Si
H2
H
H2
.
.
R' OH
H
R' O
7. Barton, D. H. R.; Kelly, M. J. Tetrahedron Lett. 1992, 33, 5041–5044.
8. Maifeld, S. V.; Miller, R. L.; Lee, D. Tetrahedron Lett. 2002, 43, 6363–6366.
9. (a) Field, L. D.; Messerle, B. A.; Rehr, M.; Soler, L. P.; Hambley, T. W.
Organometallics 2003, 22, 2387–2395; (b) Luo, X.-L.; Crabtree, R. H. J. Am.
Chem. Soc. 1989, 111, 2527–2535; (c) Parish, R. V.; Blackburn, S. N.; Haszeldine,
R. N.; Setchfield, J. H. J. Organomet. Chem. 1980, 192, 329–338.
10. (a) Ito, H.; Watanabe, A.; Sawamura, M. Org. Lett. 2005, 7, 1869–1871; (b)
Rendler, S.; Plefka, O.; Karatas, B.; Auer, G.; Fröhlich, R.; Mück-Lichtenfeld, C.;
Grimme, S.; Oestreich, M. Chem. Eur. J. 2008, 14, 11512–11528; (c) Lorenz, C.;
Schubert, U. Chem. Ber. 1995, 128, 1267–1269; (d) Schmidt, D. R.; O’Malley, S. J.;
Leighton, J. L. J. Am. Chem. Soc. 2003, 125, 1190–1191.
R'O-OR'
RO-SiR3
.
R3Si
Scheme 3. Plausible mechanism for formation of a silyl ether.
yields. Earlier to this observation, Berberova15 observed the forma-
tion of symmetrical disiloxane with evolution of hydrogen from
electrochemical oxidation of a hydrosilane. In this reaction, hyd-
rosilane radical cation [R3SiH+Å] is produced in the initial step,
which eventually splits into silyl cation [R3Si+] and hydrogen [HÅ]
and reacts with air to form disiloxane. Since Lewis acids (metal ha-
lides) are known to induce single electron oxidations reactions,16
we believe that air oxidation of hydrosilane to siloxane involves
SET pathway as the initial step of the process (Scheme 1) and a
similar mechanism is possibly operative also in the present dehy-
drogenative O-silylation reaction. In our study, radical scavenger
(benzophenone) completely inhibited O-silylation of menthol with
triethylsilane under InBr3 catalysis(entry 2, Table 1), which asserts
that the present reaction proceeds through a radical mechanism
and here, we consider that the initial SET process between InBr3
and R3SiH possibly helps the splitting of R3SiH into R3SiÅ and HÅ rad-
icals, which react with alcohol forming silyl ether and hydrogen as
shown in Scheme 3.
11. Ojima, Y.; Yamaguchi, K.; Mizuno, N. Adv. Synth. Catal. 2009, 351, 1405–1411.
12. Kim, S.; Kwon, M. S.; Park, J. Tetrahedron Lett. 2010, 51, 4573–4575.
13. Sridhar, M.; Ramanaiah, B. C.; Narsaiah, C.; Swamy, M. K.; Mahesh, B.; Reddy,
M. K. K. Tetrahedron Lett. 2009, 50, 7166–7168.
14. Typical procedure for O-silylation with a hydrosilane using InBr3 as a catalyst: (À)-
Menthol (1.0 g, 6.4 mmol), triethylsilane (0.7 g, 6.4 mmol) and toluene (10 ml)
were taken into
a 50 ml two neck round-bottomed flask fitted with a
condenser, rubber septa and an argon balloon. InBr3 (0.1 g, 0.32 mmol) was
added to the contents in the flask under purge of argon and the mixture was
refluxed for 2 h under argon atmosphere. Progress of the reaction was by TLC
and after completion of the reaction, the reaction mixture was cooled to room
temperature and solvent was removed under reduced pressure. The crude
product was extracted with ethyl acetate (2 Â 5 ml) and the combined organic
layer was washed with water (1 Â 3 ml) and saturated NaCl solution (1 Â 3 ml)
and dried over anhyd. Na2SO4. Next, the solvent was removed under reduced
pressure and the crude product was purified by normal column
chromatography (Silica gel (60–120 mesh, hexane) to obtain
(menthyloxy)triethylsilane (01.36 g, 80%) in the form of a colourless oil and
it was characterized by the following spectral data: 1H NMR (300 MHz, CDCl3):
d = 3.33–3.39 (m, 1H), 2.16–2.24 (m, 1H), 1.81–1.86 (m, 1H), 1.55–1.64 (m, 2H),
1.25–1.39 (m, 1H), 1.06–1.12 (m, 1H), 0.93–0.97 (t, 9H, J = 7.82 Hz), 0.88–
0.91(m, 9H), 0.71–0.73(d, 3H, J = 6.84 Hz), 0.55–0.61(q, 6H, J = 7.82 Hz); 13C
NMR (75 MHz, CDCl3): d = 72.32, 50.20, 45.55, 34.54, 31.72, 25.05, 22.81, 22.35,
21.30,15.82, 6.98, 5.27; IR (neat):
t 2955, 2918, 2874, 1457, 1413, 1376,
In conclusion, we have shown an unprecedented and efficient
method for silylation of primary and secondary alcohols, phenols
and oximes with hydrosilanes using InBr3 as a catalyst under reflux
in toluene. In this method, bulky hydrosilanes also gave silyl ethers
in excellent yields.
1237,1107, 1008, 738 cmÀ1; EIMS (m/z,%): 270(M+), 255, 241, 185, 139, 115,
103; Exact mass observed for C16H34OSi: 270.2369 (calculated: 270.2379).
15. Berberova, N. T. Russ. J. Electrochem. 2000, 36, 174–182.
16. (a) Haynes, R. K. Aust. J. Chem. 1978, 31, 121–129; (b) Haynes, R. K. Aust. J. Chem.
1978, 31, 131–138.