Table 2 Chemoselective reduction of compounds 1a and 7a with different
6 (a) T. Tsuda, T. Fuhii, K. Kawasaki and T. Saegusa, J. Chem. Soc.,
Chem. Commun., 1980, 1013; (b) E. C. Ashby, J. J. Lin and A. B. Goel,
J. Org. Chem., 1978, 43, 183; (c) M. F. Semmelhack and R. D. Stauffer,
J. Org. Chem., 1975, 40, 3619; (d) R. K. Baeckman, Jr. and R. Michalak,
J. Am. Chem. Soc., 1974, 96, 1623; (e) W. S. Mahoney, D. M. Brestensky
and J. M. Struker, J. Am. Chem. Soc., 1988, 110, 291; (f) B. H.
Lipshutz, W. Chrisman, K. Noson, P. P. Papa, J. A. Sclafani, R. W.
Vivian and J. K. Keith, Tetrahedron, 2000, 56, 2779; (g) N. Ravasio,
M. Antenori, M. Gargano and P. Mastrorilli, Tetrahedron Lett., 1996,
37, 3529; (h) B. H. Lipshutz, J. Keith, P. P. Papa and R. Vivian,
Tetrahedron Lett., 1998, 39, 4627; (i) A. Mori, A. Fujita, H. Kajiro,
Y. Nishihara and T. Hiyama, Tetrahedron, 1999, 55, 4573; (j) A. Mori,
A. Fujita, H. Kajiro, Y. Nishihara and T. Hiyama, Chem. Commun.,
1997, 2159.
7 (a) R. W. Goetz and M. Orchin, J. Am. Chem. Soc., 1963, 85, 2782;
(b) S. B. Kadin, J. Org. Chem., 1966, 31, 620; (c) L. Mordenti, J. J.
Brunet and P. Caubere, J. Org. Chem., 1981, 46, 192; (d) T. Tsuda, T.
Hayashi, H. Satomi, T. Kawamoto and T. Saegusa, J. Org. Chem., 1986,
51, 537; (e) K. E. Kim, S. B. Park and N. M. Yoon, Synth. Commun.,
1988, 18, 89; (f) C. Bianchini, E. Farnetti, M. Graziani, M. Peruzzini
and A. Polo, Organometallics, 1993, 12, 3753; (g) B. C. Ranu and S.
Samanta, J. Org. Chem., 2003, 68, 7130.
Lewis acids using PMHS. A dash (—) indicates no reaction
Entry 1
Entry 7
Lewis acid
Time/h
Yield (%)
Time/h
Yield (%)
B(C6F5)3
AlCl3
0.5
10
10
10
10
10
10
10
81
—
—
10
—
15
—
—
1.0
10
10
10
10
10
10
10
83
—
—
20
—
0
Ti(OiPr)4
TMSOTf
BF3·Et2O
ZnCl2/Pd(0)
ZrCl4
—
—
CeCl3·7H2O
a,b-unsaturated ketones were reduced with good yields (Table 1).18
Surprisingly, the reagent system was found to be highly chemose-
lective. For instance, the reagent system reduced selectively the
a,b-carbon–carbon double bonds in the a,b,c,d-diunsaturated
compounds 4a and 9a. Easily reducible aromatic halides (entries
2 and 8), nitro compounds (entries 5 and 8), nitriles (entry 6)
and terminal alkenes (entries 3 and 10) were also tolerated by this
unique reagent, which therefore provides an attractive alternative
system for chemoselective reductions.
8 (a) I. Ojima and T. Kogure, Organometallics, 1982, 1, 1390; (b) R. E.
Harmon, J. L. Parsons, D. W. Cooke, S. K. Gupta and J. Schoolgenberg,
J. Org. Chem., 1969, 34, 3684; (c) D. A. Evans and G. C. Fu, J. Org.
Chem., 1990, 55, 5678; (d) H. J. Liu and B. Ramani, Synth. Commun.,
1985, 15, 965.
9 E. Keinan and D. Perez, J. Org. Chem., 1987, 52, 2576.
10 (a) P. L. Maux, V. Massonneau and G. Simonneaux, J. Organomet.
Chem., 1985, 284, 101; (b) T. Keno, T. Kimura, Y. Ohtsuka and T.
Yamada, Synlett, 1999, 96.
When attempts were made to improve the yields by increasing
the amount of the catalyst B(C6F5)3 to 1 mol%, the result was
a complex mixture of saturated and unsaturated compounds,
with partial reduction of the ketone moiety to the methylene
functionality. For comparison, entries 1 and entry 7 were studied
with several other Lewis acids [viz., AlCl3, Ti(OiPr)4, TMSOTf,
BF3·Et2O, ZnCl2 (in the presence of 2.0 mol% Pd(PPh3)4 and
1 eq. ZnCl2), ZrCl4 and CeCl3·7H2O] with PMHS, following a
similar protocol (Table 2). The reaction with zinc chloride and
TMSOTf looked promising but suffered from lower yields and
longer reaction times than the PMHS–B(C6F5)3 system. Even
when the amount of the catalysts AlCl3, Ti(OiPr)4, BF3·Et2O,
ZrCl4 and CeCl3·7H2O were increased from 0.5 mol% to
20 mol%, the reactions did not proceed. Thus the PMHS–B(C6F5)3
system was found to be superior than other PMHS–Lewis acid
combinations.
11 (a) E. Keinan and N. Greenspoon, J. Org. Chem., 1983, 48, 3545; (b) E.
Keinan and N. Greenspoon, J. Am. Chem. Soc., 1986, 108, 7314; (c) E.
Keinan and N. Greenspoon, Isr. J. Chem., 1984, 24, 82; (d) N. A. Cortese
and R. F. Rheck, J. Org. Chem., 1978, 43, 3985; (e) E. Keinan and
N. Greenspoon, Tetrahedron Lett., 1985, 26, 1353; (f) P. A. Aristoff,
P. D. Johnson and A. W. Harrison, J. Am. Chem. Soc., 1985, 107,
7967.
12 E. Yoshii, H. Ikeshima and K. Ozaki, Chem. Pharm. Bull., 1972, 20,
1827.
13 (a) J. Lipowitz and S. A. Bowman, Aldrichimica Acta, 1973, 6, 1;
(b) R. Q. Savers, W. J. Scheiber and S. O. Brewer, J. Am. Chem.
Soc., 1946, 68, 962; (c) A. L. Klyashchitskaya, G. N. Krasovskii and
S. A. Fridlyand, Gig. Sanit., 1970, 35, 28; A. L. Klyashchitskaya, G. N.
Krasovskii and S. A. Fridlyand, Chem. Abstr., 1970, 72, 124864.
14 For our contributions on PMHS, see: (a) S. Chandrasekhar, Y. R.
Reddy and C. Ramarao, Synth. Commun., 1997, 27, 2251; (b) S.
Chandrasekhar, Y. R. Reddy and Ch. R. Reddy, Chem. Lett., 1998,
29, 1273; (c) S. Chandrasekhar, M. V. Reddy and L. Chandraiah,
Synth. Commun., 1999, 3981; (d) S. Chandrasekhar and Md. Ahmed,
Tetrahedron Lett., 1999, 40, 9325; (e) S. Chandrasekhar, L. Chandraiah,
Ch. R. Reddy and M. V. Reddy, Chem. Lett., 2000, 780; (f) S.
Chandrasekhar, Ch. R. Reddy and Md. Ahmed, Synlett, 2000, 1655;
(g) S. Chandrasekhar, L. Chandraiah and M. V. Reddy, Synlett, 2000,
1351; (h) S. Chandrasekhar, Ch. R. Reddy and R. J. Rao, Tetrahedron,
2001, 57, 3435; (i) S. Chandrasekhar, Ch. R. Reddy and R. J. Rao,
Synlett, 2001, 1561; (j) S. Chandrasekhar, Ch. R. Reddy and B. N. Babu,
Tetrahedron Lett., 2003, 44, 2057; (k) S. Chandrasekhar, B. N. Babu,
Md. Ahmed, M. V. Reddy, P. Srihari, B. Jagadeesh and A. Prabhakar,
Synlett, 2004, 1303.
15 (a) D. J. Parks and W. E. Piers, J. Am. Chem. Soc., 1996, 118, 9440;
(b) J. M. Blackwell, K. L. Foster, V. H. Beck and W. E. Piers, J. Org.
Chem., 1999, 64, 4887; (c) J. M. Blackwell, D. J. Morrison and W. E.
Piers, Tetrahedron, 2002, 58, 8247; (d) G. Erker, Dalton Trans., 2005,
1883 and references therein.
16 (a) S. Chandrasekhar, G. Chandrashekar, B. N. Babu, K. Vijeender and
K. V. Reddy, Tetrahedron Lett., 2004, 45, 5497; (b) S. Chandrasekhar,
Ch. R. Reddy, B. N. Babu and G. Chandrashekar, Tetrahedron
Lett., 2002, 43, 3801; (c) S. Chandrasekhar, Y. S. Rao and N. R.
Reddy, Synlett, 2005, 1471; (d) S. Chandrasekhar, Ch. R. Reddy
and G. Chandrashekar, Tetrahedron Lett., 2004, 45, 6481; (e) For a
review on tris(pentafluorophenyl)borane, see our contribution to the
‘Electronic Encyclopedia of Organic Reagents for Organic Synthesis’,
S. Chandrasekhar, B. N. Babu and G. Chandrashekar, eEROS, 2005,
and references therein.
In conclusion, PMHS along with a catalytic amount of B(C6F5)3
was found to be a unique reagent system for the conjugate
reduction of Michael acceptors. Currently, development of a chiral
version19 of this novel reagent and further studies on the scope of
this reagent system are in progress.
References
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