Studies on the Preparation of Cyclamen Aldehyde and its Related Compounds

Part 2. Catalytic Reduction of p-Isopropyl-α-methylhydrocinnamic Acid by Formic Acid

Abstract

In 1938 POIZAT⁽³⁾ published the method of cyclamen aldehyde, by passing the vapors of p-isopropyl-α-methylhydrocinnamic acid and formic acid over MnO₂-pumice at 360° It was well known that some carboxylic acids were reduced to the corresponding aldehydes by treatment with formic acid in the presence of TiO₂⁽⁴⁾, MnO₂⁽³⁾, or MnO⁽⁵⁾ catalyst. More recently DAVIES et al.⁽⁶⁾ stated about the studies on the catalytic reduction of various carboxylic acids under pressure with TiO₂ as catalyst.
Guided with these literatures, the author attempted several investigations of the suitable catalysts for the preparation of cyclamen aldehyde. At first TiO₂, MnO₂, MnO and MnO-silicagel catalysts were tested using such organic acids as phenylacetic, salicylic and α-methylhydrocinnamic acid⁽⁷⁾, and then p-isopropyl-α-methylhydrocinnamic acid was converted to cyclamen aldehyde with MnO-silcagel which showed most favorable activity among, the four catalysts.
The experimental results were summarized as follows.
(1) Following to the DAVIS' study, the reduction of salicylic acid with' formic acid under pressure was attempted in the presence of TiO₂ as catalyst, but the expected aldehyde could not be obtained. The acid was only decarboxylated to phenol.
(2) Two MnO₂-pumice catalysts, prepared from potassium permanganate and manganese, nitrate, had little activties in the reduction of α-methylhydrocinnamic acid, as shown in Table 1.
(3) With MnO as catalyst; the reduction of phenylacetic acid yielded considerable amounts of by-products (Table 2), and the use of MnO-slicagel lessened the by-reactions, and markedly impoved the yield of phenylacetaldehyde (Table 3).
(4) p-Isopropyl-α-methylhydrocinnamic acid was reduced to cyclamen aldehyde with MnO-silicagel with the yield of 42% of the theoretical amount (68% based on the consuemd carboxylic acid).