SCHEME 1
Ra p id Defu n ction a liza tion of Ca r bon yl
Gr ou p to Meth ylen e w ith
P olym eth ylh yd r osiloxa n e-B(C6F 5)3
†
S. Chandrasekhar,* Ch. Raji Reddy, and
B. Nagendra Babu
BF3Et2O or CF3CO2H12 besides a few others.13 The
majority of the known procedures used for defunction-
alization are nonchemoselective and require harsh reac-
tion conditions.
Organic Chemistry Division-I, Indian Institute of Chemical
Technology, Hyderabad-500 007, India
srivaric@iict.ap.nic.in
Received J une 12, 2002
All of these methods, while offering some advantages,
also suffer from disadvantages. Most of these methods
are generally restricted to aromatic systems, are some
times harsh and need a pyrophoric hydride source for
reduction, require longer reaction hours with careful
workup procedures for quenching the excess reagent, and
are often associated with low yields. The usefulness of
polymeric hydride source polymethylhydrosiloxane
(PMHS), a coproduct of the silicone industry, as an
excellent reduction reagent is well demonstrated in
several recent publications.14,15 The quest to find newer
activators for this rather inert polymer resulted in
identification of tris(pentafluorophenyl)borane as an
excellent catalyst for activation of PMHS. B(C6F5)316 is a
relatively unexplored Lewis acid. This combination of
PMHS-B(C6F5)3 is found to be a versatile carbonyl
defunctionalization system with very short reaction times
(Scheme 1). Interestingly, this combination establishes
a powerful “catalytic switch”, viz., our initial studies
using ZnCl2 as an activator resulted in reduction of
ketone to alcohol,17 whereas this new catalyst promoted
the reduction of the same substrate to methylene group.18
Abstr a ct: The polymethylhydrosiloxane-B(C6F5)3 combi-
nation is found to be a versatile carbonyl defunctionalization
system under mild and rapid conditions. For the first time,
B(C6F5)3 has been used as a nonconventional Lewis acid
catalyst to activate PMHS. Aromatic and aliphatic carbonyl
compounds were effectively reduced to give the correspond-
ing alkanes in high yields.
Defunctionalization of organic functional groups is an
equally desirable achievement as compared to function-
alization. There is a great need to discover new method-
ologies for defunctionalization especially for conversion
of polyfunctional natural products to useful building
blocks and bioactive molecules. Available literature
speaks of only a few protocols for removal of a certain
functional group, viz., the carbonyl group can be defunc-
tionalized to a methylene group by Clemensen1 or Wolff-
Kishner reduction,2 both of which require very drastic
reaction conditions. The hydroxyl group can be removed
by a Barton-McCombie procedure,3 wherein highly
malodorous xanthate and Bu3SnH are required. Some
other methods known in the literature include catalytic
hydrogenation4 and reaction involving use of PtO2,5 HI-
phosphorus,6 BH3,7 Zn/HCl/HgCl2/H2O,8 NaBH4-CF3-
CO2H,9 NaCNBH3-BF3Et2O,10 LAH-AlCl3,11 Et3SiH-
(12) (a) Smonou, I. Tetrahedron Lett. 1994, 35, 2071. (b) Fry, J . L.;
Orfanopoulos, M.; Adlington, M. G.; Dittman, W. P.; Silverman, S. B.
J . Org. Chem. 1978, 43, 374. (c) West, C. T.; Donelly, S. J .; Kooistra,
D. A.; Doyle, M. P. J . Org. Chem. 1973, 38, 2675. (d) Smith, C. N.;
Ambler, S. J .; Steggler, D. J . Tetrahedron Lett. 1993, 34, 7447. (e)
Kursanov, D. N.; Parnes, Z. N.; Loim, N. M. Synthesis 1974, 633.
(13) (a) Brieger, G.; Fu, T.-H. J . Chem. Soc., Chem. Commun. 1976,
757. (b) Karaman, R.; Fry, J . L. Tetrahedron Lett. 1989, 30, 4931. (c)
Ram, S.; Spicer, L. D. Tetrahedron Lett. 1988, 29, 3741. (d) J axa-
Chamiec, A.; Shah, V. P.; Kruse, L. I. J . Chem. Soc., Perkin Trans. 1
1989, 1705. (e) Larock, R. C. Comprehensive Organic Transformations,
2nd ed.; Wiley & Sons: 1999, p 61. (f) Lipowitz, J .; Bowman, S. A. J .
Org. Chem. 1973, 38, 162.
† IICT communication no. 020101.
(1) (a) Clemmensen, E. Chem. Ber. 1914, 47, 51, 681. (b) Vedejs, E.
Org. React. 1975, 22, 401.
(2) (a) Kishner, J . J . Russ. Phys. Chem. Soc. 1911, 43, 582. (b) Wolff,
C. Liebigs Ann. 1912, 394, 86. (c) Todd, D. Org. React. 1948, 4, 378.
(d) Minlon, H. J . Am. Chem. Soc. 1949, 71, 3301.
(3) (a) Barton, D. H. R.; McCombie, S. W. J . Chem. Soc., Perkin
Trans 1 1975, 1574. (b) Barton, D. H. R. Tetrahedron 1986, 42, 2329.
(4) (a) Lee, W. Y.; Park, C. H.; Kim, H. J .; Kim, S. J . Org. Chem.
1994, 59, 878. (b) Lee, W. Y.; Park, C. H.; Kim, Y. D. J . Org. Chem.
1992, 57, 4074.
(5) Rao, A. V. R.; Mahendale, A. R.; Reddy, K. B. Tetrahedron Lett.
1982, 23, 2415.
(6) (a) See ref 4b. (b) Reimschneider, R.; Kassahn, H. Chem. Ber.
1959, 92, 1705. (c) Bradsher, C.; Vingiello, F. J . Org. Chem. 1948, 13,
786.
(7) (a) Kelly, T. R.; Kim, M. H. J . Am. Chem. Soc. 1994, 116, 7072.
(b) Breuer, E. Tetrahedron Lett. 1967, 20, 1849.
(8) Lee, W. Y.; Park, C. H.; Kim, E. H. J . Org. Chem. 1994, 59, 4495.
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(14) (a) Chandrasekhar, S.; Reddy, Ch. R.; Rao, R. J .; Rao, J . M.
SynLett 2002, 349. (b) Chandrasekhar, S.; Reddy, Ch. R.; Rao, R. J .
SynLett 2001, 1561 and references therein.
(15) For an exhaustive review on PMHS, see: (a) Lawrence, N. J .;
Drew, M. D.; Bushell, S. M. J . Chem. Soc., Perkin Trans. 1 1999, 3381.
Also see: (b) Nitzsche, S.; Wick, M. Angew. Chem. 1957, 69, 96. (c)
Mimoun, H.; Laumer, J . Y. S.; Giannini, L.; Scopelliti, R.; Floriani, C.
J . Am. Chem. Soc. 1999, 121, 6158. (d) Verdaguer, X.; Lange, U. E.
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Lawrence, N. J . Synlett 1994, 833. (g) Drew, M. D.; Lawrence, N. J .;
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Chem. 1999, 64, 2582 and references therein.
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(11) (a) Paquette, L. A.; Maleczka, R. E., J r. J . Org. Chem. 1992,
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(17) Chandrasekhar, S.; Reddy, Y. R.; Ramarao, C. Synth. Commun.
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(18) Polymethylhydrosiloxane in the presence of AlCl3 reduced aryl
carbonyl group to methylene; see ref 13d.
10.1021/jo0204045 CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/20/2002
9080
J . Org. Chem. 2002, 67, 9080-9082