Conversion of Alcohols to Aldehydes and Ketones by Oxidation of Trialkoxyaluminum with Pyridinum Chlorochromate (PCC),
Bull. Korean Chem. Soc., 1998, Vol. 19, No. 7, pp 724Experimental procedureThe following experimental procedure is illustrative. To an oven-dried, nitrogen-flushed 100-mL RB flask, fitted with a septum inlet, a magnetic stirring bar, and a reflux condenser leading to a mercury bubbler, a 1.10 M solution of aluminum hydride (18.2mL, 20 mmol) in THF was injected and the solution was kept at 0°C with aid of an ice-water bath. The mixture was stirred at 0°C and a 3.0 M solution of 1-octanol (20mL, 60 mmol) was added dropwise with a syringe. After the addition was complete, the mixture was then allowed to room temperature and stirred for 3 h. In another oven-dried, nitrogen-flushed 500-mL RB flask, fitted with a septum inlet, a magnetic stirring bar, and reflux condenser leading to a mercury bubbler, are placed powdered PCC (26g, 120 mmol) and methylene chloride (200mL). To the well-stirred suspension, a solution of trioctylaluminum in THF thus prepared was added with the aid of a double-ended needle. The mixture was stirred at room temperature for 1 h. Then, ethyl ether (200mL) was added and the mixture was filtered through a column containing Florisil
®. The solid residue in the flask was triturated with ethyl ether (3 × 50mL) and filtered through the same Florisil column. The combined filtrate was concentrated and distilled to afford 6.04g of pure octanal (78%); bp 170-172°C/761 mmHg. The purity was further confirmed by GC analysis.
A small scale of same reaction (trioctylaluminum, 1 mmol) was also performed and tridecane was added as an internal standard. The product aldehyde was analyzed by GC with use of a Carbowax TAP capillary column (25 m) to show 97% octanal formation.
Alkyl group of (RO)3Al Product
Ph-CH2- BA* (98%)
4-Me-Ph-CH2- 4-Me-BA (97%)
4-MeO-Ph-CH2- 4-MeO-BA (97%)
4-Cl-Ph-CH2- 4-Cl-BA (98%)
*BA = Benzaldehyde
Conversion of Carboxylic Acids into Aldehydes by Oxidation of Alkoxyaluminum Intermediate with Pyridinum Chlorochromate or Pyridinum Dichromate, Bull. Korean Chem. Soc., 1998, Vol. 19, No.7, pp 730
Experimental procedure
This method provides another convenient procedure for the direct conversion of carboxylic acids to corresponding aldehydes. The following procedure for the reaction of hexanoic acid is representative. An oven-dried, 250-mL RB flask with sidearm, equipped with a magnetic stirring bar and a reflux condenser, was attached to a mercury bubbler. The flask was flushed with dry nitrogen and then maintained under a static pressure of nitrogen. The flask was charged with hexanoic acid (6.97g, 60 mmol) and 30mL of THF. The flask was immersed in an ice-water bath and a pre-cooled 1.0 M solution of aluminum hydride (30mL, 30 mmol) in THF was added dropwise with vigorous stirring. After the complete evolution of the hydrogen, the ice-water bath was removed and the reaction mixture was stirred for 30 min at room temperature.
To a well-stirred suspension of PCC (14.3g, 66 mmol) in methylene chloride (100mL) taken in a 500-mL RB flask equipped as described above, is added dropwise the above solution of alkoxyaluminum intermediate in THF using a cannula. The mixture was stirred for 12 h at room temperature. The small portion of this mixture was transfered to a vial and dodecane was added as an internal standard. GC analysis using a Carbowax 20 M capillary column (20 m) showed a presence of hexanal in a yield of 96%. The reaction mixture was diluted with 200mL of diethyl ether and the supernatant liquid is filtered through Florisil® (100g) contained in a 300-mL sintered glass funnel. The solid residue is washed with diethyl ether (3 × 50mL) and passed through the same Florisil column. The filtrate was concentrated and distilled to afford pure hexanal (4.93g, 82% yield); bp 129-130°C (754 mmHg).
Analogous procedures are used for the synthesis of the other aldehydes listed in Table 1. In the case of PDC as an oxidant used, actually the same procedure was adopted except the oxidation time.
Cinnamic acid -> Cinnamaldehyde, 95% w/PCC (98% w/PDC), reaction time: 12h;
Benzoic acid -> Benzaldehyde, 85%* w/PCC (99% w/PDC), reaction time: 6h;
o-Toluic acid -> o-Tolualdehyde, 99% w/PCC (99% w/PDC), reaction time: 6h;
m-Toluic acid -> m-Tolualdehyde, 98% w/PCC (99% w/PDC), reaction time: 6h;
p-Toluic acid -> p-Tolualdehyde, 99% w/PCC (99% w/PDC), reaction time: 6h;
p-Anisic acid -> p-Anisaldehyde, 98% w/PCC (98% w/PDC), reaction time: 6h;
4-Chlorobenzoic acid -> 4-Chlorobenzaldehyde, 99% w/PCC (98% w/PDC), reaction time: 6h.
*Isolated yield
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