Chemoselective oxidation of alcohols to aldehydes and ketones by iodosobenzene/(salen) chromium complex

Article abstract of DOI:10.1055/s-2003-40852

Various benzylic and allylic alcohols were selectively oxidized to their corresponding aldehydes and ketones by a new Cr(III)(salen) complex I as the catalyst and PhIO as terminal oxidant. The oxidation requires relatively shorter reaction time and produces excellent yields.

Full text of DOI:10.1055/s-2003-40852

LETTER
1391
Chemoselective Oxidation of Alcohols to Aldehydes and Ketones by
Iodosobenzene/(Salen) Chromium Complex
C
hemoselectiv
u
e
O
xidation
n
of
A
lcoholsg Soo Kim,* Dong Won Kim
Department of Chemistry and Center for Chemical Dynamics, Inha University, Incheon 402-751, South Korea
Fax +82(32)8675604; E-mail: sungsoo@inha.ac.kr
Received 22 May 2003
essary for the reaction. Various allyic and benzylic alco-
hols are oxidized according to Scheme 1. The oxidation
reactions were carried out under the mild homogeneous
conditions in CH2Cl2 at r.t. The in situ formation of oxo-
metal complex CrV=O(salen) is responsible for the oxida-
tion of the various alcohols. The oxidation of organic
compounds by CrV=O complex have been previously re-
Abstract: Various benzylic and allylic alcohols were selectively
oxidized to their corresponding aldehydes and ketones by a new
Cr(III)(salen) complex I as the catalyst and PhIO as terminal oxi-
dant. The oxidation requires relatively shorter reaction time and
produces excellent yields.
Key words: benzylic alcohol, allylic alcohol, Cr(salen), oxidation,
chemoselectivity
ported by various authors3,8,9
.
Several catalytic processes using transition metal com-
plexes have been reported using a stoichiometric amount
of terminal oxidant. Salen ligands are recognized as effi-
cient auxiliaries and many metallosalen complexes have
been found to serve as excellent catalyst for various or-
ganic reactions.1 Oxidation of alcohols to aldehyde and
ketone is one of the most important functional group
transformations in organic synthesis. The enantioselective
oxidation of numerous olefins via metal catalysis2 has
been the subject of interest during the last twenty years.
The achiral chromium(III)3 and manganese(III)4 com-
plexes of the salen ligand are the effective catalysts for the
epoxidation of various olefins with iodosobenzene as the
terminal oxidant. Chiral (salen)manganese catalysts5,6
were combined with iodosylmesitiylene, sodium hy-
pochlorite, or m-chloroperbenzoic acid for the enantiose-
lective epoxidation of cis- and trans-olefins. E-b-
methylstyrene, a trans-olefin7, undergo asymmetric ep-
oxidation utilizing (salen)chromium complex/iodosoben-
zene with triphenylphosphine oxide as the additive. Very
recently selective (salen)chromium(III) catalyzed oxida-
tions of a series of allylic and benzylic alcohols to corre-
sponding carbonyl compounds8,9 have been carried out
with iodosobenzene and iodosobenzene diacetate as the
oxidant, respectively. We now report the oxidation of
these alcohols with a new (salen)chromium complex that
shows much improved result than the previous one.
Figure 1 N,N-Ethylenebis(3,5-dichlorosalicylideneiminato)
chromium(III) chloride.
Table 1 Oxidation of Alcohols with [Cr(III)Salen]Cl/PhIO with
and without 4-Phenyl-pyridine-N-oxide (PPNO)a
Entry
Substrate
Additive Time
Yieldb
1
PPNO
PPNO
0.5 h
0.5 h
92%
93%
OH
2
1 h
1 h
87%
86%
OH
a Reaction conditions: substrate (1 mmol), PhIO (1.5 mmol),
[Cr(III)salen] (0.1 mmol), CH2Cl2 (5 mL).
b Isolated yield.
Results and Discussion
N,N-Ethylenebis (3,5-dichlorosalicylideneiminato) chro-
mium(III) chloride (Figure 1), I was synthesized. I was
tested with or without additive for the oxidative process.
Table 1 indicates that addition of additive may not be nec-
Synlett 2003, No. 10, Print: 05 08 2003.
Art Id.1437-2096,E;2003,0,10,1391,1394,ftx,en;U06203ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
Scheme 1 Catalytic cycle for the oxidation of alcohols to carbonyl
compounds by (salen)Cr(III)/PhIO.

1392
S. S. Kim, D. W. Kim
LETTER
Simple benzyl (entries 1–3) and secondary benzylic (en- oxide (yield: 5%) for 6 h. The formation of epoxide9 has
tries 4 and 5) alcohols are smoothly and selectively con- been prohibited by employing iodosobenzene diacetate
verted into their corresponding aldehydes and ketones, instead of iodosobenzene as the oxidant. The formation of
respectively (Table 2). Secondary benzylic alcohols (en- ketones showed moderate to good yield without interfer-
tries 6–8) are also subject to slight steric effect compared ence of formation of epoxide. However, our system pro-
to benzyl alcohol (entry 1) that are indicated as longer re- duce high yield within lesser reaction time while the
action time. Benzylic alcohols (entries 1–8) over all dem- previous report9 indicated very long reaction time and
onstrate excellent reactivity in terms of reaction time and much less yield (refer to the yield and reaction time for en-
yield compared to the previous study.8 Previous oxidation tries 1, 4, 11, and 13 in Table 2). The chemoselectivity of
of benzylic alcohols with iodosobenzene(salen)chromium allylic alcohols against non-activated alcohol is similarly
complex gave poor to moderate yield (17–70%) of the compared as with benzylic/non-activated alcohols.
carbonyl compounds for reaction time of 14 h. Under the Equimolar quantity of 3,5,5-trimethyl-2-cyclohexen-1-ol
same reaction condition a substituted cyclohexanol was and 2-decanol are subject to the similar reaction condition
chosen as an aliphatic alcohol which was oxidized to cor- of the oxidative process. Here again, allylic structure
responding cyclohexanone with 20% yield for 72 h. Our shows considerably higher chemoselective trend than ali-
oxidation of benzylic alcohols (entries 1–8) indicate phatic 2-decanol (Scheme 3).
>90% yield within 2 h. Equimolar quantity of p-methoxy
benzyl alcohol and 2-decanol are oxidized to test the
OH
chemoselectivity of benzylic alcohol against non-activat-
ed alcohol. The product ratio indicates around 5:1 strong-
ly favoring for the formation of p-methoxy benzaldehyde
+ Cr(III)(Salen)
0.1 mmol
+
C6H5IO
1 mmol
+
against 2-decanone (Scheme 2).
0.5 mmol
O
OH
+
0.5 mmol
+
C6H5IO + Cr(III)(Salen)
p-MeOC6H4CH2OH +
CH2Cl2 (5mL)
r.t., 50 min
1 mmol
0.1 mmol
0.5 mmol
0.5 mmol
OH
+
O
97%
18%
CH2Cl2 (5mL)
r.t., 20 min
p-MeOC6H4CHO
Scheme 3
O
Oxidation of carveol (entry 14) takes much longer reac-
tion period (15 h) with good yield of corresponding ke-
tone. Only cyclohexen-1-ol (entry 15) undergoes dual
oxidative pathway to give ketone:epoxide = 9:1. The for-
mation of dual products may be due to the absence of ster-
ic effect that could help facilitate formation of the
epoxide. On the contrary, the similar allylic compound
with some steric hiderance (entry 12) gives rise to ketonic
product only.
99%
22%
Scheme 2
Allylic alcohols could be vulnerable to oxidations through
the formation of carbonyl compound or epoxide. Several
allylic alcohols (entries 9–13) undergo selective oxidation
to give carbonyl compounds in good to excellent yield
for relatively shorter reaction period. The oxidation of
various allylic alcohols with iodosobenzenechromi-
um(III)salen complex8 gave rise to poor to good yield of
the carbonyls (yield: 19–90%) plus small amount of ep-
Table 2 Oxidation of Alcohols with [Cr(III)(salen)]Cl Complex/PhIOa
Entry
1
Substrate
Product
Time
Yieldb,c
30 min (14 h)d
93% (58%)d
OH
O
2
3
15 min
30 min
96%
90%
OH
O
H3CO
O2N
H3CO
O2N
OH
O
Synlett 2003, No. 10, 1391–1394 © Thieme Stuttgart · New York

LETTER
Chemoselective Oxidation of Alcohols
1393
Table 2 Oxidation of Alcohols with [Cr(III)(salen)]Cl Complex/PhIOa (continued)
Entry
4
Substrate
OH
Product
O
Time
Yieldb,c
2 h (14 h)d
92% (64%)d
5
30 min
90%
OH
O
H3CO
H3CO
6
7
8
1 h
1 h
1h
95%
99%
91%
OH
OH
OH
O
O
O
9
1.5 h
1 h
81%
82%
OH
OH
O
10
O
11
12
1 h (6 h)d
40 min
83%e (90%)d
90%
OH
O
OH
O
13
14
30 min (14 h)d
95%e (76%)d
OH
O
15 h
76%
OH
O
15
30 min
ketone:epoxide
88% (9:1)
OH
O
a Substrate (1 mmol), PhIO (1.5 mmol), [Cr(III)salen]Cl (0.1mmol), CH2Cl2 (5 mL).
b Isolated yield.
c All the 1H NMR spectra of ketones and aldehydes exactly match with those tabulated in The Aldrich Library of 13C and 1H FT NMR Spectra
edited by Charles J. Pouchert, Jacqlynn Behnke.
d The values in the parentheses are the results of ref.8
e Less than 1% of benzaldehyde is identified.
Synlett 2003, No. 10, 1391–1394 © Thieme Stuttgart · New York

1394
S. S. Kim, D. W. Kim
LETTER
(3) Samsel, E. G.; Srinivasan, K.; Kochi, J. K. J. Am. Chem. Soc.
1985, 107, 7606.
(4) Srinivasan, K.; Michaud, P.; Kochi, J. K. J. Am. Chem. Soc.
1986, 108, 2309.
Typical experimental procedure is described for the oxi-
dation of benzyl alcohol with catalyst I. To a solution of
benzyl alcohol (1 mmol) in 5mL of CH2Cl2 was added
0.10 equivalents of catalyst I with stirring which was fol-
lowed by the addition of 1.5 equivalents of iodosoben-
zene. A color change from brown to dark green was
observed. After the mixture was stirred for the reaction
time, the solvent was evaporated and chromatographed on
Silica gel column by using ethyl acetate/hexane (2:8) mix-
ture as eluent. The solvent was removed by evaporation
and the products was identified by 1H NMR spectroscopy.
The catalyst I was prepared by using reported literature
method.8,10
(5) (a) Zhang, W.; Loebach, J. C.; Wilson, S. R.; Jacobsen, E. N.
J. Am. Chem. Soc. 1990, 112, 2801. (b) Jacobsen, E. N.;
Zhang, W.; Much, A.; Ecker, J. R.; Deng, L. J. Am. Chem.
Soc. 1991, 113, 7063. (c) Chang, S.; Galvin, J. M.;
Jacobsen, E. N. J. Am. Chem. Soc. 1994, 116, 6937.
(d) Brandes, B. D.; Jacobsen, E. N. J. Org. Chem. 1994, 116,
6937.
(6) (a) Irie, R.; Noda, K.; Ito, Y.; Matsumoto, N.; Katsuki, T.
Tetrahedron lett. 1990, 31, 7345. (b) Irie, R.; Ito, Y.;
Katsuki, T. Synlett 1991, 265. (c) Hamada, T.; Irie, R.;
Katsuki, T. Synlett 1994, 479.
(7) (a) O’Mahony, C. P.; McGarrigle, E. M.; Renehan, M. F.;
Ryan, K. M.; Kerrigan, N. J.; Bousquet, C.; Gilheany, D. G.
Org. Lett. 2001, 3, 3435. (b) Ryan, K. M.; Bousquet, C.;
Gilheany, D. G. Tetrahedron Lett. 1999, 40, 3613.
(8) Adam, W.; Gelalcha, F. G.; Saha-Möller, C. R.; Stegmann,
V. R. J. Org. Chem. 2000, 65, 1915.
In summary, benzylic alcohols are oxidized correspond-
ing aldehydes and ketones in yield of >90% for less
than 2 h. Allylic alcohols show the over 80% yield within
1.5 h. The control experiments indicates an excellent
chemoselectivity for the oxidation of benzylic and allylic
alcohols against non-activated alcohols.
(9) Adam, W.; Hajra, S.; Herderich, M.; Saha-Möller, C. R.
Org. Lett. 2000, 2, 2773.
(10) General procedure for preparation of catalyst I : To a
suspension of bis(3,5-dicholorosalicylidene)ethyl-
enediamine [0.775 g (1.9 mmol)] in 50 mL of absolute THF
was added suspension of anhydrous chromous chloride
[0.307 g (2.5 mmol)] in THF (30 mL) with vigorous stirring
under N2 atmosphere at room temperature (ca. 20 °C) for 1
h. Then the dark brown solution was allowed to reflux in the
presence of air for 2 h. The residue was suspended in water
(50 mL) and the undissolved yellowish brown color material
was collected by filtration and washed with water (3 × 10
mL). After filtration and concentration of the filtrate with
evaporator, brown material was precipitated on cooling
overnight, which was collected and dried under reduced
pressure to afford 0.86 g of the required complex I (yield;
93%), mp 320–325 ∞ C, IR (KBr): 3071, 1615, 1440, 1168,
869 cm–1. Anal. Calcd. for C16H10Cl5CrN2O2: C, 39.1; H,
2.05; N, 5.7. Found: C, 38.9; H, 2.2; N, 5.4.
Acknowledgement
The authors warmly thank Korea Science and Engineering Founda-
tion for the financial support (R01-2001-00057).
Reference
(1) (a) Canali, E. N.; Sherrington, D. C. Chem. Soc. Rev. 1999,
28, 85. (b) Daly, A. M.; Renehan, M. F.; Gilheany, D. G.
Org. Lett. 2001, 3, 663. (c) Kerrigan, N. J.; Langan, I. J.;
Dalton, C. T.; Daly, A. M.; Bousquet, C.; Gilheany, D. G.
Tetrahedron Lett. 2002, 43, 2107. (d) Baleizao, G.;Gigante,
B.; Sabater, M. J.; Garcia, H.; Corma, A. J. Mol. Catal. A:
Chem. 2002, 228, 279.
(2) Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H. Comprehensive
Asymmetric Catalysis II; Springer-Verlag: Berlin,
Heidelberg, 1999, Chap. 18.
Synlett 2003, No. 10, 1391–1394 © Thieme Stuttgart · New York