1
74
H. Sajiki et al. / Tetrahedron Letters 44 (2003) 171–174
1
3
aromatic carbonyls using a Pd/C–ethylenediamine
complex catalyst [Pd/C(en)]. 2.5% Pd/Fib and 5% Pd/
C(en) catalysts both exhibit no catalyst activity toward
the hydrogenolysis of benzyl ethers (entry 10 and Ref.
3. Zhou, Y.; Chen, W.; Itoh, H.; Naka, K.; Ni, Q.; Yamane,
H.; Chujo, Y. Chem. Commun. 2001, 2518–2519.
4. The reported procedure in Ref. 1b; 350 mg of silk fibroin
was boiled for 8 min in 100 mL of 0.1N AcOH contain-
ing 100 mg of PdCl2 and the resulting chelate was
1
2). While aromatic ketones and aldehydes were easily
transformed into the corresponding benzyl alcohols by
the 10%Pd/C(en)-catalyzed hydrogenation, 2.5% Pd/
Fib catalyst was entirely inactive toward the hydro-
genation of aromatic carbonyl groups (entries 1–6).
Further, the hydrogenation using 2.5% Pd/Fib catalyst
also perfectly tolerates aromatic chlorides (entries 6 and
) and bromides (entries 7 and 8). Needless to say,
Pd/C(en) or commercial Pd/C-catalyzed chemoselective
hydrogenation with retention of aromatic halides could
not be accomplished. Consequently, 2.5% Pd/Fib cat-
alyst can be applied to the development of a chemose-
lective hydrogenation method of olefin and azide
groups which distinguishes the aromatic ketone, alde-
hyde, chloride and bromide functionalities although the
Pd/C(en) or commercial Pd/C-catalyzed hydrogenation
cannot tolerate these functionalities.
2
reduced under hydrogen (80 kg/cm ) in an autoclave.
13
5. Robson, R. M. In Handbook of Fiber Chemistry; 2nd ed.;
Lewin, M.; Pearce, E. M., Eds.; New York: Marcel
Dekker, 1998.
6
. Mita, K.; Ichimura, S.; James, T. C. J. Mol. Evol. 1994,
8, 583–592.
3
14
9
7
. Since no black turbidity or verging on black solution was
observed during the deposition of Pd(0) onto the silk
fibroin, it can be presumed that the formation of Pd(0)
occurs only on the fibroin.
1
5
8
. Day, R. A.; Underwood, A. L. Quantitative Analysis;
Prentice-Hall: Englewood Cliffs, 1967.
9. Nash, T. Biochemistry 1953, 55, 416–421.
1
0. 2.5% Pd/Fib was prepared using MeOH solution of
Pd(OAc)2 (2.5% as Pd metal of the weight of the silk
fibroin) and the content of Pd was estimated as 100% of
absorption. Furthermore, the assignment of Pd content%
was made on the ash content of the 2.5% Pd/Fib deter-
mined with elemental analysis basis which was corrected
by subtracting the value obtained in a blank consisting of
the silk fibroin. The average Pd content% of three times
analyses was 2.7%.
At present, detailed mechanistic studies have not been
undertaken and the exact process of the chemoselectiv-
ity is unclear. It has been proposed that the formation
of Pd metal particles on the surface of silk fibroin
reduced the active surface area of the catalyst. There-
fore, the catalyst activity of Pd/Fib was partially
diminished.
1
1. For reviews, see: (a) Rylander, P. N. Hydrogenation
Methods, Academic Press: New York, 1985; (b)
Hudlicky, M. Reductions in Organic Chemistry, 2nd ed.;
American Chemical Society: Washington, DC, 1996; (c)
Nishimura, S. Handbook of Heterogeneous Catalytic
Hydrogenation for Organic Synthesis, Wiley-Interscience:
New York, 2001.
In summary, the present study provides indication that
the rapid reduction of the silk-fibroin conjugated
Pd(OAc) proceeded using MeOH as a reductant at
room temperature. The Pd/Fib catalyst displays good
chemoselectivity in the hydrogenation of olefins and
azides in the presence of aromatic carbonyls and/or
halogens or an O-benzyl protective group. This catalyst
provides a simple and practical protocol for chemose-
lective hydrogenation and reinforces the versatility of
aromatic carbonyls and halides in organic synthesis.
2
1
1
1
2. Sajiki, H.; Hattori, K.; Hirota, K. J. Org. Chem. 1998,
6
3, 7990–7992.
3. Hattori, K.; Sajiki, H.; Hirota, K. Tetrahedron 2001, 57,
817–4824.
4
4. The present hydrogenation also entirely tolerates aro-
matic iodides, such as 3,5-diiodosalicylaldehyde and 2-
iodobiphenyl.
1
5. It is well known that aromatic chlorides are much less
reactive than aromatic bromides and iodides and, hence,
the dechlorination of aromatic chloride cannot readily be
achieved and the hydrodechlorination reactions are very
frequently incomplete, see: (a) Hudlicky, M. In Compre-
hensive Organic Synthesis; Trost, B. M.; Fleming, I., Eds.;
Pergamon: Oxford, 1991; Vol. 8, pp. 895–922; (b)
Nishimura, S. Handbook of Heterogeneous Catalytic
Hydrogenation for Organic Synthesis, Wiley-Interscience:
New York, 2001; pp. 623–637; (c) Hara, R.; Sato, K.;
Sun, W.-H.; Takahashi, T. Chem. Commun. 1999, 845–
846; (d) Sajiki, H.; Kume, A.; Hattori, K.; Hirota, K.
Tetrahedron Lett. 2002, 43, 7247–7250.
References
1
. (a) Akabori, S.; Sakurai, S.; Izumi, Y.; Fujii, Y. Nature
1
1
956, 178, 323–324; (b) Izumi, Y. Bull. Chem. Soc. Jpn.
959, 32, 932–936, 936–942 and 942–945; (c) Akamatsu,
A.; Izumi, Y.; Akabori, S. Bull. Chem. Soc. Jpn. 1961, 34,
067–1072; (d) Akamatsu, A.; Izumi, Y.; Akabori, S.
Bull. Chem. Soc. Jpn. 1961, 35, 1706–1711.
. (a) Iuchi, K.; Yamada, H.; Teramoto, T. Japan Kokai
1
2
7
5/05,405 (Cl. 19C2), 21 Jan 1975, Appl. 73/56,627, 17
May 1973; Chem. Abstr. 1975, 82, 141951r; (b) Zhang, S.;
Xu, Y.; Liao, S. Cuihua Xuebao 1986, 7, 364–370; Chem.
Abstr. 1987, 106, 86642c.