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