Assessments on the transition metal salt-catalyzed -citronellal condensation reactions with alkyl alcohols

In this work, inexpensive and simple commercial transition metal salts were evaluated as catalysts in the acetalization of alkyl alcohols with beta-citronellal, a renewable origin substrate. After an initial screening, FeCl3 was the most active and selective catalyst among the various transition metal salts evaluated toward the beta-citronellal methyl acetal. The impacts of main reaction parameters such as time, temperature, catalyst load, and type of alcohol on conversion and selectivity of the reactions were investigated. Different iron salts were also investigated. It was demonstrated that both oxidation number and type of anion present in the salt play an essential role in this reaction. Notably, the dissolution of catalyst salts in solution triggered a decrease in the pH of the medium due to the hydrolysis (and or solvolysis) of the metal cation, impacting the conversion and reaction selectivity. The highest activity of FeCl3 was assigned to the greatest Lewis acidity strength, as demonstrated by the acidity measurements. This inexpensive, low-corrosive, and commercially affordable catalyst has advantages over traditional liquid mineral acid catalysts and provides an alternative route to synthesize alkyl terpene acetals.