Partial Reduction of Esters to Aldehydes Using a Novel Modified Red-Al Reducing Agent

Experimental

General. All glassware used was dried thoroughly in an oven, assembled hot, and cooled under a stream of dry nitrogen prior to use. All reactions and manipulation of air-and moisture-sensitive materials were carried out using standard techniques for handling air sensitive materials. All chemicals were commercial products of the highest pure which were further purified by standard methods before use. THF was dried over sodium-benzophenone and distilled. GC analyses were performed on a Yonglin, Acme 6000MFID chromatograph using a HP-5 (5%-diphenyl-95%-dimethylsiloxane copolymer, 30 m) capillary column. All GC yields were determined with the use of naphthalene as internal standard and authentic mixture.

Preparation of New Reducing Reagent (cis-2,6-Dimethyl Morpholine-Modified Red-Al). A dry and argon-flushed flask, equipped with a magnetic stirring bar and a septum, was charged with cis-2,6-dimethylmorpholine (6.81 mL, 55 mmol) and 79 mL THF. After cooling to 0 ℃, Red-Al (14.3 mL, 3.5 M in toluene, 50 mmol) was added dropwise and stirred for 1 h at same temperature to give a colorless homogeneous solution. The concentration of cis-2,6-dimethylmorpholine- modified Red-Al solution in THF-toluene was measured gasometrically by hydrolysis of an aliquot of the solution with a hydrolyzing of aqueous 1 N HCl at 0 ℃.

Partial Reduction of Esters to Corresponding Aldehydes. The following experimental procedure for the partial reduction of ethyl benzoate to benzaldehyde is representative. A dry and argon-flushed flask, equipped with a magnetic stirring bar and a septum, was charged with ethyl benzoate (0.07 mL, 0.5 mmol) and THF (5 mL). After cooling to 0 ℃, cis-2,6-dimethylmorpholine-modified Red-Al (2.5 mL, 0.4 M 1.0 mmol) was added dropwise and the mixture was stirred for 30 min at the room temperature. The reaction was stopped aqueous 1 N HCl (5 mL) and the product was extracted with diethyl ether (10 mL). The ether layer was dried over anhydrous magnesium sulfate. GC analysis showed a 98% yield of benzaldehyde. All products in Table 1 were confirmed through comparison with GC data of authentic sample.