Bis(sym-collidine)bromine(I) hexafluorophosphate as oxidant

Article abstract of DOI:10.1016/S0040-4039(00)01598-7

Primary and secondary alcohols in solution in methylene chloride are oxidised with bis(sym-collidine)bromine(I) hexafluorophosphate in good yields to the carbonyl compounds. For secondary and tertiary alcohols in which one of the substituents is a 4-methoxyphenyl group the oxidation takes place by cleavage of the phenyl-sp³ carbon bond and formation of bromoanisole and carbonyl compounds. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01598-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8881–8885
Bis(sym-collidine)bromine(I) hexafluorophosphate as oxidant
G e´ rard Rousseau* and Sylvie Robin
Laboratoire des Carbocycles (Associ e´ au CNRS), Institut de Chimie Mol e´ culaire d’Orsay, B aˆ t. 420,
Universit e´ de Paris-Sud, 91405 Orsay, France
Received 23 June 2000; accepted 15 September 2000
Abstract
Primary and secondary alcohols in solution in methylene chloride are oxidised with bis(sym-col-
lidine)bromine(I) hexafluorophosphate in good yields to the carbonyl compounds. For secondary and
tertiary alcohols in which one of the substituents is a 4-methoxyphenyl group the oxidation takes place
3
by cleavage of the phenyl-sp carbon bond and formation of bromoanisole and carbonyl compounds.
©
2000 Elsevier Science Ltd. All rights reserved.
The search for non-metallic oxidation reagents is still a major goal in chemical synthesis.
1
Numerous compounds have already been used such as the Dess–Martin periodinane. However,
in this field, the best known oxidation methods is still the utilisation of dimethylsulfoxide in the
presence of catalysts, such as the Moffatt and Swern oxidations. Among all these methods, the
reactivity of positive halogen reagents such as sodium or calcium hypochlorite or NBS has been
1
2
examined. We can also cite bis(quinuclidine)bromine(I) reagents used in the presence of
additives.
During our studies concerning the reactivity of bis(collidine)bromine(I) hexafluorophosphate
3
(
BBH), we observed that this reagent could act as an oxidant. The simplicity of the reaction
conditions used with BBH prompted us to examine this reaction in more detail. These
oxidations were conducted in methylene chloride at room temperature in the presence of 1.7
4
equivalents of BBH. Our results are reported in Table 1. The reactions were monitored by TLC.
The yields were calculated after purification of the products by liquid chromatography on silica
gel. The structure of the products were determined in general by comparison with authentic
samples and confirmed from their spectral data. With the exceptions of tetrahydropyran-2-
*
Corresponding author. Fax: 33 1.69.15.62.78; e-mail: grousseau@icmo.u-psud.fr
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01598-7

8882
Table 1
Oxidation of alcohols with BBH

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methanol (entry p) and the 4-alkoxybenzylalcohols (entries d and e), the carbonyl compounds
were obtained in high yields. The fact that tetrahydropyran-2-methanol did not react is
probably due to its higher oxidation potential compared with the other alcohols studied.
With 4-alkoxybenzylalcohols we observed the formation of 4-alkoxy-1-bromobenzene. These
latter reactions implied the cleavage of a CꢀC bond and formation of acetaldehyde, probably
by the mechanism reported in Eq. (1). This cleavage was not observed when the benzylalco-
hol was substituted in the ortho position by an alkoxy group (entry g). In this case, bromina-
tion in the para position to the methoxy group was easier (entry f) than the oxidation of the
primary alcohol which occurred only when an excess of BBH was used (entry g). Competi-
tion between oxidation of the benzyl alcohol and its cleavage was observed in one case
(
entry h). Except for these particular cases, BBH appears to be a good reagent for oxidising
5
primary and secondary alcohols into carbonyl compounds.
(1)
Even if there are some reports in the literature concerning the cleavage of benzyl alcohols
+
6
+ 7
+ 8
using reagents which generate F , Cl , or Br , this reaction was not studied as a method
to prepare carbonyl compounds. Reaction of 4-methoxybenzyl alcohols with BBH at room
9
temperature led cleanly to a mixture of bromoanisole and carbonyl compounds. The prod-
ucts were purified by liquid chromatography over silica gel, and identified from their spectral
1
0
data. Our results are reported in Table 2. This reaction occurred with secondary and
tertiary alcohols leading to aldehydes and ketones, respectively. Bromoetherification was
observed (entry e) when the alcohol was unsaturated. 6-Methoxy-1-tetralols (entries f and g)
furnished interesting intermediates by a very simple method. 5-Methoxy-1-indanol only
formed a mixture of unidentified products under the same conditions.
In conclusion, we have shown that bis(collidine)bromine(I) hexafluorophosphate is an inter-
esting reagent for the oxidation of alcohols. It also allows the transformation of 4-methoxy-
benzylalcohols into carbonyl compounds. Since these alcohols can be prepared from
4
-bromoanisole, this could also be a new method for protecting carbonyl compounds (Eq.
(
2)).
(2)

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Table 2
Reaction of 4-methoxybenzylalcohols with BBH
References
1
2
. Hudlicky, M. Oxidations in Organic Chemistry; ACS Monograph 186; American Chemical Society: Washington,
990.
. (a) Blair, L. K.; Hobbs, S.; Bagnoli, N.; Hsuband, L.; Badika, N. J. Org. Chem. 1992, 57, 1600–1603. (b) Blair,
L. K.; Parris, K. D.; Lee, O. F. D.; Jenkins, K. H.; Feese, R. F.; Belcher, T.; Badger, D.; Morris, D.; Kuhn, C.
J. Org. Chem. 1986, 51, 5454–5456.
1
3. (a) Homsi, F.; Rousseau, G. J. Org. Chem. 1999, 64, 81–85. (b) Robin, S.; Rousseau, G. Eur. J. Org. Chem. 2000,
in press. (c) Homsi, F.; Robin, S.; Rousseau, G. Org. Synth. 1999, 77, 206–211.
4
. Representative procedure: To a solution of alcohol (2 mmol) in methylene chloride (20 mL) was added
3c
bis(collidine)bromine(I) hexafluorophosphate (3.4 mmol, 1.64 g). After reaction of the alcohol (TLC) the
solvent was removed and the residue purified by chromatography over silica gel (hexanes/ether).
. This reaction can be written as follows:
5
6
+
−
6
+
−
+
−
6
alcohol+Br (collidine) PF “carbonyl compound+H-(collidine) Br +H-(collidine) PF
2
. (a) Stavber, S.; Kosir, I.; Zupan, M. J. Chem. Soc., Chem. Commun. 1992, 274–275. (b) Stavber, S.; Planinsek,
Z.; Kosir, I.; Zupan, M. J. Fluor. Chem. 1992, 59, 409–412.

8885
7
8
. (a) Shen, X.; van Heiningen, A. Can. J. Chem. 1992, 70, 1754–1761. (b) Piria, R. Justus Liebigs Ann. Chem. 1845,
6, 47–57.
5
. (a) Arnett, E. M.; Klingensmith, G. B. J. Am. Chem. Soc. 1965, 87, 1023–1031 and references cited therein. (b)
Umezawa, B.; Hoshino, O.; Hara, H.; Mitsubayashi, S. J. Chem. Soc. C 1970, 465–467. (c) Nakatani, M.;
Takahashi, K.; Watanabe, S.; Shintoku, A.; Hase, T. Bull. Chem. Soc. Jpn. 1974, 57, 1510 and references cited
therein. (d) Yamada, H.; Itoh, N.; Murakami, S.; Izumi, Y. Agric. Biol. Chem. 1985, 10, 2961–2968. (d) Shang,
M.; Okuda, R. K.; Worthen, D. Phytochemistry 1994, 37, 307–310.
9
. Representative procedure: To a solution of alcohol (2 mmol) in methylene chloride (10 mL) was added over 1 h
a methylene chloride solution (10 mL) of bis(collidine)bromine(I) hexafluorophosphate (2.4 mmol, 1.16 g). After
3c
stirring for 30 min at room temperature, the solvent was removed and the residue purified by chromatography
over silica gel (hexanes/ether).
0. Selected data. 4-(2-Bromo-5-methoxyphenyl)butanal: H NMR (250 MHz, CDCl ): 9.77 (m, 1H); 7.40 (d, J=11
1
1
3
Hz, 1H); 6.78 (d, J=4 Hz, 1H); 6.65 (dd, J=4 and 11 Hz, 1H); 3.80 (s, 3H); 2.75 (dd, J=7 and 11 Hz, 2H); 2.51
1
3
(
2
dd, J=2 and 11 Hz, 2H); 2.00 (m, 2H). C NMR: 202.0; 158.9; 141.4; 133.3; 116.0; 113.9; 113.3; 55.3; 42.9; 35.3;
1
2.1. 8-(2-Bromo-5-methoxy-phenyl)octan-5-one: H NMR (250 MHz, CDCl ): 7.42 (d, J=11 Hz, 1H); 6.78 (d,
3
J=5 Hz, 1H); 6.65 (dd, J=5 and 11 Hz, 1H); 3.78 (s, 3H); 2.71 (dd, J=7 and 11 Hz, 2H); 2.45 (q, J=10 Hz,
13
4
H); 1.92 (m, 2H); 1.57 (m, 2H); 1.34 (m, 2H); 0.92 (t, J=11 Hz, 3H). C NMR: 210.6; 158.8; 141.8; 133.0;
1
15.8; 114.7; 113.1; 55.2; 42.3; 41.6; 35.3; 25.7; 23.7; 22.2; 13.7.
.