01-05-03 05:15
No 395516
An interesting approach to aldehydes from both benzyl and alipathic alcohols......
http://www.orgsyn.org/orgsyn/prep.asp?pr
"Oxammonium salts 1 have been used extensively either in stoichiometric or in catalytic amounts13 for the oxidation of primary and secondary alcohols to the corresponding carbonyl derivatives.
The catalytic procedure described here allows a fast, cheap, and highly selective conversion of primary alcohols into aldehydes, using sodium hypochlorite as the oxidant in a two-phase (dichloromethane–water) system. Aqueous sodium hypochlorite is buffered at pH 8.6–9.5 to ensure the presence of hypochlorous acid in the organic layer.14
Oxammonium salt 1, the effective oxidant species, is continuously generated from nitroxyl radical 2 by hypochlorous acid in the organic phase. Radical 2 is one of the most stable radicals known, and is easily prepared from the inexpensive triacetoneamine 3.15 16 17 18 The oxidation is very exothermic; for this reason scale-up of the reaction needs a very efficient cooling system to maintain the temperature in the optimum 0–15°C range. One one-hundredth (0.01) molar equivalent of nitroxyl radical 2 is generally used, but on this reaction scale the amount of catalyst can be reduced to 0.002 molar equivalent, without substantially affecting the reaction time. Sodium hypochlorite is used in only slight excess and is entirely consumed, an unusual occurrence for reactions carried out under aqueous, organic two-phase conditions.19 20
Conversion of saturated, primary alkyl and aryl alkyl alcohols into the corresponding aldehydes can be achieved by this method provided that the alcohols are entirely dissolved in the organic phase. Relatively unstable protective groups are not affected, as in the oxidation of the acetonide of 1,2,6-hexanetriol, whereas conjugated and isolated double bonds give rise to side reactions that considerably decrease selectivities and yields.4 Some examples of aldehydes synthesized with this method are reported in Table I. Under the same conditions, secondary alcohols are oxidized to ketones. Addition of catalytic amounts of quaternary onium salts allows fast and total conversion of primary alcohols and aldehydes into carboxylic acids making this methodology very versatile.4
TABLE I
OXIDATION OF PRIMARY ALCOHOLS TO ALDEHYDES
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Alcohol
Isolated Yield (%)
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1-Heptanol
88
1-Octanol
92
1-Nonanol
92
1-Undecanol
93
85
Benzyl alcohol
90
p-Nitrobenzyl alcohol
89
m-Nitrobenyl alcohol
88
75
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When the limitations outlined above are considered, the procedure described here appears to be easier and cheaper than most methods in the condensed phase known to date.21 Furthermore, alkali halides are almost the only contaminants in the waste water, making the scale up of this method very attractive."
Saludos, from Latin America
(Hive Addict)
01-06-03 17:06
No 395820
Does anybody have a visual aid version of the mechanism?
(Hive Bee)
01-08-03 21:39
No 396458
Ok, so the oxonium salts method is a bit involved hence in March's 5th edition page 1514 under section 19-3 "Oxidation or Dehydrogenation of alcohols to aldehydes and Ketones"
(by Catalytic Dehydrogenation) "For the conversion of primary alcohols to aldehydes, dehydrogenation catalysts have the advantage over strong oxidizing agents that further oxidation to the carboxylic acid is prevented. Copper chromite is the agent most often used, but other catalysts (e.g., silver and copper) have also been employed. Many ketones have also been prepared in this manner. Catalytic dehydrogenation is more often used industrially than as a laboratory method. However, convenient laboratory procedures using copper oxide (93), Raney Nickle(94), and palladium acetate ( under phase -transfer conditions),(95) have been reported".
Ref.
(93) Sheikh,M.Y. ; Eadon, G.Tetrahedron Lett. 1972,257
(94) Krafft, M.E.;Zorc, B.J. J. Org. chem.,1986,51,5482
(95) Choudary,B.M.; Reddy,N.P.; Kantam,M.L.; Jamil, Z. Tetrahedron Lett., 1985,26,6257
Edit; see also a fine find by ..... Post 393415 (Aurelius: "Aldehyde synthesis using Raney Nickel", Chemistry Discourse)
Saludos, from Latin America