Within the past years, several catalyst systems have
been discovered
that promote the carboxylation of terminal alkynes with formation
of propiolic acids. This reaction concept is particularly topical
in that it involves the functionalization of a C–H bond and
the incorporation of CO2 into an organic molecule without
need for strong bases or aggressive organometallic reagents. The present
article reviews the development of this new reaction type and critically
compares the reaction and catalyst concepts disclosed in the literature.
An effective protocol has been developed that allows the smooth protodecarboxylation of diversely functionalized aromatic carboxylic acids within 5-15 min. In the presence of at most 5 mol % of an inexpensive catalyst generated in situ from copper(I) oxide and 1,10-phenanthroline, even nonactivated benzoates were converted in high yields and with great preparative ease.
A catalyst system consisting of Pd(PPh3)4 and P(pTol)3 was found to effectively promote the intermolecular decarboxylative coupling of α‐oxocarboxylic acids with diallyl carbonate to give α,β‐unsaturated ketones. The key advantage of the new reaction protocol is that preformation of the allyl esters is not required. The reaction is believed to proceed via phosphane‐mediated decarboxylation of the α‐oxocarboxylates, leading to acyl anion equivalents that are allylated within the coordination sphere of the palladium catalyst. Under the reaction conditions, the double bond then migrates into conjugation with the carbonyl group.
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