Polymeric DABCO-bromine complex: a mild oxidant for the preparation of ketones and aldehydes

Article abstract of DOI:10.1016/j.tetlet.2006.07.006

A stable, shelf-ready polymeric oxidant was prepared from the addition of Br₂ to DABCO in CCl₄. This material was used to convert simple primary and secondary alcohols to the corresponding aldehydes and ketones in biphasic CH₂Cl₂/H₂O.

Full text of DOI:10.1016/j.tetlet.2006.07.006

Tetrahedron Letters 47 (2006) 6635–6636
Polymeric DABCO–bromine complex: a mild oxidant
for the preparation of ketones and aldehydes
John A. Struss,* William D. Barnhart, Maria R. Velasco and Apryl Bronley-DeLancey
The University of Tampa, 401 W. Kennedy Blvd., Tampa, FL 33606-1490, United States
Received 18 May 2006; revised 3 July 2006; accepted 5 July 2006
Available online 28 July 2006
Abstract—A stable, shelf-ready polymeric oxidant was prepared from the addition of Br2 to DABCO in CCl4. This material was
used to convert simple primary and secondary alcohols to the corresponding aldehydes and ketones in biphasic CH2Cl2/H2O.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
PDB behaves as a stable ionic polymer and is insoluble
and unreactive in conventional nonalcoholic organic
solvents. However, in the presence of additional DAB-
CO, PDB behaves as a mild oxidant toward simple alco-
hols. In fact, in the absence of additional DABCO, PDB
is entirely unreactive. BQBB has demonstrated increased
reactivity in the presence of a pyridinium catalyst in bi-
phasic CH2Cl2/H2O and is selective for the oxidation of
2° alcohols over 1°.10 With this in mind, we set out to
study the reactivity of the PDB system under BQBB
reaction conditions. That is, we investigated (a) the pos-
sibility of using a pyridinium catalyst in the absence of
additional DABCO, (b) the reactivity of PDB in bipha-
sic CH2Cl2/H2O, and (c) obtain preliminary data
regarding the 1°/2° selectivity of PDB by means of a ser-
ies of oxidations. We plan to explore the utility of PDB
in the presence of other reactive functional groups in a
forthcoming paper.
Hypervalent halogen complexes have enjoyed a good
deal of attention recently as alternatives to Swern,1
chromate,2 and other mild oxidants.3 In particular, the
Dess–Martin,4 2-iodoxybenzoic acid (IBX),5 and related
reagents6 have shown a wide range of applications.
Herein, we reinvestigate the oxidative properties of a
complex prepared by the addition of 2 M equiv of Br2
to 1,4-diazabicyclo[2.2.2]octane (DABCO) in carbon
tetrachloride.7 Tetrameric DABCO–bromine complexes
prepared in chloroform have been reported and fully
characterized.8 These complexes contain
a linear
arrangement of three DABCO subunits in which pro-
tonated nitrogen forms hydrogen bonds to nearby nitro-
gen lone pairs. The source of these hydrogens is unclear,
however, it has been suggested that they result ‘from
radical oxidation of the solvent by bromine or oxy-
gen’.8b When molecular bromine is added to DABCO
in CCl4, the characteristics of the resulting complex
are quite different than those observed for the tetrameric
complex. In the absence of a labile hydrogen atom, we
feel that the complex is most likely an ionic polymer
containing alternating DABCO–bromine–DABCO [N–
Br–N]+ linkages with Br3 acting as the counter ion.9
À
X-ray crystallography data of a similar complex pre-
pared from quinuclidine (bis(quinuclidine)bromine(I)
bromide; BQBB) supports a linear N–Br–N arrange-
ment.9 We will, therefore, refer to this material as poly-
meric DABCO bromine complex, or PDB, throughout
the remainder of this letter.
2. Results and discussion
*
Using 2- and 1-pentanol as representative small molecu-
lar weight alcohols and PDB as the oxidant, a series of
Corresponding author. Tel.: +1 813 253 3333; fax: +1 813 258
7881; e-mail: jstruss@ut.edu
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.07.006

6636
J. A. Struss et al. / Tetrahedron Letters 47 (2006) 6635–6636
Table 1. Oxidation of secondary and primary alcohols using polymeric DABCO–bromine complex in biphasic CH2Cl2/H2O
Entry
Alcohol
Aldehyde or ketone
% Yield of aldehyde/
ketone after 2 and 20 ha,b
Unreacted alcohol (%)a,b
1
2
3
4
5
6
7
8
1-Pentanol
1-Octanol
Pentanal
Octanal
Isobutyraldehyde
Phenylethanal
Benzaldehyde
cis-Hex-2-enal
2-Pentanone
3-Pentanone
3-Methyl-2-butanone
2-Methylcyclopentanone
3-Methylcyclopentanone
Acetophenone
7.3 (41)
Trace (31)c
6.4 (28)
8.7 (27)
29 (85)
97 (92)
66 (85)
68 (89)
64 (86)
54 (97)
74 (84)
51 (93)
103 (97)d
84 (54)
93 (64)
84 (65)
80 (55)
Isobutyl alcohol
2-Phenylethanol
Benzyl alcohol
cis-Hex-2-en-1-ol
2-Pentanol
63 (14)
0.0 (0.0)
27 (8.3)
24 (0.0)
29 (0.0)
38 (trace)
20 (trace)
44 (0.0)
0.0 (0.0)
3-Pentanol
9
3-Methyl-2-butanol
2-Methylcyclopentanol
3-Methylcyclopentanol
1-Phenylethanol
But-3-en-2-ol
10
11
12
13
Methyl vinyl ketone
a Percent yields and recoveries were determined by GC.
b Yields and recoveries are given at reaction times of 2 and 20 h (in parentheses).
c ‘Trace’ refers to GC signal that was not quantifiable (approximately 0–3%).
d Values of >100% are indicative of analytical precision: approximately 3%.
oxidation reactions were performed under conditions
identical to those used for the BQBB/PTFA study.10
Surprisingly, in biphasic CH2Cl2/H2O, PDB is equally
reactive when oxidizing 2- and 1-pentanol with or with-
out the pyridinium catalyst. However, very little reactiv-
ity is observed under water free conditions. Thus, we
postulate that, unlike the BQBB system, PDB oxida-
tions are base promoted. This is evidenced by the reac-
tivity of the oxidant in the presence of additional
DABCO under water-free conditions.7 Under biphasic
conditions, water assumes the role of Bronsted base.
These data suggest that the mechanism may be bleach-
like in that carbonyl formation relies on an E2 elimina-
tion of HX from a hypohalite ester intermediate.
removed via Pasteur pipet and the aqueous layer was
extracted with three 2 mL portions of CH2Cl2. The com-
bined organic layers were diluted to a total volume of
25 mL with CH2Cl2 and analyzed by gas chromatogra-
phy using predetermined response factors.
References and notes
1. (a) Mancuso, A. J.; Huang, S. L.; Swern, D. J. Org. Chem.
1978, 43, 2480; (b) Mancuso, A. J.; Swern, D. Synthesis
1981, 165; (c) Mancuso, A. J.; Brownfan, D. S.; Swern, D.
J. Org. Chem. 1985, 50, 2198.
2. (a) Corey, E. J.; Suggs, J. W. Tetrahedron Lett. 1975, 2647;
(b) Banerji, K. K. J. Chem. Soc., Perkin Trans. 1978, 2,
639; (c) Coates, W. M.; Corrigan, J. R. Chem. Ind. 1969,
1594; (d) Corey, E. J.; Schmidt, G. Tetrahedron Lett. 1979,
399.
3. For a comprehensive review of oxidations in organic
chemistry see: Hudlicky, M. ACS Monograph 186: Oxi-
dations in Organic Chemistry; American Chemical Society:
Washington, DC, 1990.
4. (a) Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48, 4155;
(b) Dess, D. B.; Martin, J. C. J. Am. Chem. Soc. 1991, 113,
7277; (c) Parlow, J. J.; Case, B. L.; South, M. S.
Tetrahedron 1999, 55, 6785; (d) Nicolaou, K. C.; Sugita,
K.; Baran, P. S.; Zhong, Y.-L. J. Am. Chem. Soc. 2002,
124, 2221.
Knowing that no acid catalyst is required, we treated a
variety of alcohols with PDB under biphasic CH2Cl2/
H2O conditions and examined the mixtures by GC after
2 and 20 h (Table 1). As was observed in the earlier ac-
count, the reactions are well behaved with no apparent
over oxidation to the carboxylic acid and excellent mass
balances.7 This study reveals that secondary alcohols
generally react faster than primary alcohols (compare
entries 1–4 to 7–11; Table 1). Also, allylic alcohols (en-
tries 6 and 13) react very quickly and are entirely oxi-
dized within minutes. Benzylic alcohols (entries 5 and
12) do not appear to be as reactive as allylic substrates
although they were somewhat faster than secondary
alcohols. Overall, we can assign a preliminary ranking
regarding relative rates of oxidation as follows:
allylic ꢀ benzylic > secondary > primary.
5. (a) Frigero, M.; Santagostino, M. Tetrahedron Lett. 1994,
35, 8019; (b) DeMunari, S.; Frigero, M.; Satagostino, M.
J. Org. Chem. 1996, 61, 9272; (c) Moore, J. D.; Finney, S.
N. Org. Lett. 2002, 4, 3001.
6. (a) Stang, P. J.; Zhdankin, V. V. Chem. Rev. 1996, 96,
1123; (b) Stang, P. J.; Zhdankin, V. V. Chem. Rev. 2002,
102, 2523.
7. Blair, L. K.; Baldwin, J.; Smith, W. C. J. Org. Chem. 1977,
42, 1816.
3. Experimental
8. (a) Heravi, M. M.; Derikvand, F.; Ghassemzadeh, M.;
All oxidations were carried out using PDB prepared in
accordance to the earlier precedent.7 In a typical reaction,
0.55 mmol PDB (238 mg), 2 mL CH2Cl2, 2 mL H2O, and
1.0 mmol of alcohol were combined in a 5 mL WheatonÒ
V-vial equipped with a spin vane. At the appropriate
reaction times (2 or 20 h), 0.5 mmol bromobenzene
(internal standard) was added. The organic layer was
Neumuller, B. Tetrahedron Lett. 2005, 46, 6243; (b)
¨
Allwood, B. L.; Moysak, P. I.; Rzepa, H. S.; Williams,
D. J. J. Chem. Soc., Chem. Commun. 1985, 1127.
9. Blair, L. K.; Parris, K. D.; Hii, P. S.; Brock, C. P. J. Am.
Chem. Soc. 1983, 105, 3649.
10. Blair, L. K.; Hobbs, S.; Bagnoli, N.; Husband, L.; Badika,
N. J. Org. Chem. 1992, 57, 1600.