COMMUNICATIONS
alcohols were oxidized to the corresponding substituted
benzaldehydes in excellent yields (entries 1 7). Thiomethyl,
triethysiloxy, amino, and amido groups remained intact under
the present reaction conditions. Allylic and nonconjugated
aliphatic alcohols were also oxidized in high yields to a,b-
unsaturated aldehydes and nonconjugated aliphatic alde-
hydes, respectively (entries 8 12). Complete stereochemical
retention was observed for geraniol (entry 9), whereas a slight
isomerization was observed for nerol (entry 10). In these
reactions, the aldehydes produced were not oxidized further
to carboxylic acids, thus indicating the potential utility of the
2a/DBU system as a selective oxidant. A limitation was
observed in the oxidation with 2-phenylethanol, which could
not be efficiently converted into phenylacetaldehyde (en-
try 13). In this reaction, a complex mixture was formed with
only moderate recovery of the starting alcohol. It seems that
the acidic a-proton of the initially formed phenylacetaldehyde
is readily abstracted by DBU, causing undesired reactions.[16]
Secondary alcohols were oxidized to the corresponding
ketones in high yields (entries 14 20). It is worth noting that
2,2,2-trifluoro-1-phenylethanol could be cleanly oxidized to
a,a,a-trifluoroacetophenone, though a longer reaction time
was necessary (entry 16).[17] When 2c (Ar¼ p-Tol) was used in
place of 2a, the trifluoromethyl ketone was not formed under
all the same conditions.
2a (0.9 equiv)
DBU (1.0 equiv)
Ph
Ph
OH
OH
CH2Cl2, RT, 30 min
OH
O
9 (98%)
8
Scheme 2. The intramolecular competitive oxidation of alcohol 8 by the
2a/DBU system.
promises facile conversion of alcohols into carbonyl com-
pounds with high efficiency and selectivity under mild
conditions. The remarkable oxidizing ability of this new
system is derived from the steric effect of the ortho-tolyl
groups attached to the BiV center. The present system also has
a practical advantage over the previously reported BiV-based
oxidation systems in that the carbonyl products can be easily
isolated by simple filtration following treatment on a short
column. Further studies on the use of this method in other
organic transformations are currently in progress.
Experimental Section
Oxidation of alcohol: DBU (46 mg, 0.30 mmol) was added to a stirred
suspension of tris(2-methylphenyl)bismuth dichloride (2a;[19] 166 mg,
0.30 mmol) and alcohol (0.27 mmol) in toluene (3 mL) at room temper-
ature, and the resulting mixture was monitored by TLC at several intervals.
After the alcohol was consumed, the precipitated complex 5 was filtered off
through a thin celite bed. The filtrate was concentrated under reduced
pressure to leave an oily residue, which contained the desired carbonyl
product in a high state of purity (> 90%). The residue was then passed
through a short silica gel column (710 mm2 î 18 45 mm of Wako-gel C-200,
about 50 mL of hexane/EtOAc (5/1) as eluent) to afford the carbonyl
compound in the yields listed in Table 2. Complex 5 was recovered
quantitatively and could be converted into tris(2-methylphenyl)bismuth-
ane by treatment with 2.5 equiv of o-TolMgBr in THF.
To estimate the relative reactivity of allylic, benzylic, and
nonconjugated alcohols toward the 2a/DBU system, both
intermolecular and intramolecular competitive oxidations
were examined. As summarized in Table 3, the 2a/DBU
system oxidized cinnamyl and benzylic alcohols preferentially
5: M.p. 181 1848C (decomp); 1H NMR (400 MHz, CDCl3, TMS): d ¼ 1.63
1.74 (m, 6H), 1.92 (tt, J ¼ 5.9, 5.9 Hz, 2H), 2.58 (s, 6H), 2.76 2.79 (m, 2H),
3.19 (t, J ¼ 5.9 Hz, 2H), 3.39 (t, J ¼ 5.9 Hz, 2H), 3.43 3.45 (m, 2H), 7.31 (dd,
J ¼ 7.6, 7.3 Hz, 2H), 7.44 (dd, J ¼ 7.6, 7.3 Hz, 2H), 7.48 (d, J ¼ 7.6 Hz, 2H),
8.49 (d, J ¼ 7.6 Hz, 2H), 10.84 ppm (brs, 1H); IR (KBr): n˜ ¼ 3217, 3126,
3038, 2980, 2937, 2856, 1651, 1589, 1470, 1448, 1383, 1323, 1292, 1271, 1207,
1159, 1109, 984, 845, 756, 743, 706, 606, 536, 436, 430 cmꢀ1; elemental
analysis (%) calcd for C23H31BiCl2N2: C 44.89, H 5.08, N 4.55; found: C
44.85, H 4.99, N, 4.51.
Table 3. Competitive oxidation of alcohols by 2a and Dess Martin
periodinane.[a]
2a/DBU or
Dess–Martin
RR'CHOH
+
EtOH
RR'C=O
+
MeCHO
CDCl3, RT, 15 min
[b]
¼
RR’CHOH
Ratio of RR’ O to MeCHO
2a/DBU
Dess Martin
Received: March 25, 2002 [Z18976]
98:2
96:4
91:9
Ph
OH
90:10
Ph
OH
OH
[1] a) S. V. Ley, A. Madin in Comprehensive Organic Synthesis, Vol. 7
(Ed.: B. M. Trost), Pergamon, New York, 1991, chap. 2.7, pp. 251 289;
T. V. Lee in Comprehensive Organic Synthesis, Vol. 7 (Ed.: B. M.
Trost), Pergamon, New York, 1991, chap. 2.8, pp. 291 303; G. Procter
in Comprehensive Organic Synthesis, Vol. 7 (Ed.: B. M. Trost),
Pergamon, New York, 1991, chap. 2.9, pp. 305 327; b) R. C. Larock,
Comprehensive Organic Transformations, Wiley, New York, 1999,
pp. 1234 1249, and references therein.
95:5
75:25
Ph
[a] Three equivalents each of RR’CHOH and ethanol were used. [b] De-
termined by H NMR spectroscopy.
1
[2] For recent examples with main group element-based oxidants, see
a) M. Frigerio, M. Santagostino, S. Sputore, G. Palmisano, J. Org.
Chem. 1995, 60, 7272 7276; b) T. Mukaiyama, J. Matsuo, M.
Yanagisawa, Chem. Lett. 2000, 1072 1073; c) K. C. Nicolaou, Y.-L.
Zhong, P. S. Baran, J. Am. Chem. Soc. 2000, 122, 7596 7597; d) M.
M¸lbaier, A. Giannis, Angew. Chem. 2001, 113, 4530 4532; Angew.
Chem. Int. Ed. 2001, 40, 4393 4394; e) G. Sorg, A. Mengel, G. Jung, J.
Rademann, Angew. Chem. 2001, 113, 4532 4535; Angew. Chem. Int.
Ed. 2001, 40, 4395 4397; f) L. De Luca, G. Giacomelli, A. Porcheddu,
Org. Lett. 2001, 3, 3041 3043; g) J. Matsuo, D. Iida, K. Tanaka, T.
Mukaiyama, Bull. Chem. Soc. Jpn. 2002, 75, 223 234.
in the presence of ethyl alcohol, and their selectivities were
found to be much higher than those attained by Dess Martin
periodinane.[18] The high chemoselectivity of benzylic versus
nonconjugated aliphatic alcohols was also observed in the
reaction of 1-phenyl-1,4-butanediol (8): When treated with
0.9 equiv of 2a/DBU, 8 was converted exclusively into 4-
hydroxy-1-phenylbutan-1-one (9) in 98% yield based on 2a
(Scheme 2). These results clearly demonstrate the synthetic
utility of the 2a/DBU system as a chemoselective oxidant.
In summary, we have established a new practical oxidation
method using the (o-Tol)3BiCl2/DBU binary system, which
[3] a) J. P. Kitchin in Organic Synthesis by Oxidation with Metal Com-
pounds (Eds.: W. J. Mijs, C. R. H. I. de Jonge), Plenum, New York,
1986, . 15, pp. 817 837; b) M. Postel, E. DuÊach, Coord. Chem. Rev.
3030
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