Synthesis of Esters from Primary Alcohols

When primary alcohols were heated with a catalytic amount of complex 1, in the presence of a catalytic amount of base, an unusual dehydrogenative coupHng to form esters with evolution of Hj was observed [11]. Thus, refluxing a hexanol (bp 157 °C) solution containing complex 1 and KOH (0.1 mol% each) under argon in 

Further improvement in the reaction was possible by exclusion of the need for base. Thus, reaction of complex 7 with KO Bu resulted in the formation of the coordinatively unsaturated, 16e Ru(ll) neutral complex 8. Indeed, 8 is an excellent catalyst for the dehydrogenative coupling of alcohols to esters and the reaction proceeds with liberation of Hj under neutral conditions [llj. Table 1.2 provides a few examples. GC analysis of the reaction mixtures indicated that aldehydes were formed only in trace amounts. This catalytic reaction provided a new green pathway for the synthesis of esters directly from alcohols. Considerably less efficient methods had been reported previously for this transformation [34]. 

Conditions 0.01 mmol catalyst 8, lOmmol alcohol, and toluene (2 ml) were refluxed (115 °C) rmder 

Recently, Gusev and coworkers [37] reported the PNN mthenium pincer complex 14 with an aliphatic backbone, which displayed impressive catalytic activity in the presence of a base (KO Bu, 0.5 mol%) for the conversion of simple alcohols such as ethanol and propanol. Although the reactions worked at temperatures below 100 °C, good conversions required heating at higher temperatures (Table 1.3, 

Though in moderate yield, the reported conversion of ethanol to ethyl acetate by complex 14 and a related osmium dimer complex [37] by Gusev generated interest in the catalytic synthesis of ethyl acetate from ethanol because it is a widely used fine chemical. Very recently, Beller et al. [38] screened the catalytic activity of various known pincer complexes for this transformation and found thatTakasago s complex 15, known as Ru-MACHO catalyst, is very efficient and the reaction in the presence of a base resulted in very good TON (Table 1.4). 

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