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PALLADIUM-CATALYZED DECARBOXYLATIVE ALLYLATIONS OF ESTER ENOLATE EQUIVALENTS AND PALLADIUM-CATALYZED CYCLIZATIONS VIA CO2 AND SILYL ACTIVATION
Ariyarathna, Yamuna Kumari
Ariyarathna, Yamuna Kumari
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Abstract
Palladium-catalyzed decarboxylative allylation (DcA) has received much attention as an alternative C-C bond formation method to traditional metal-catalyzed cross-coupling reactions. Among various nucleophilic partners that undergo DcA, ester enolates are reported to be difficult to allylate and often demand harsher conditions. Herein we report the development of a mild and fast method that provides access to various types of α-allylated amides and esters via decarboxylative allylation of ester enolate equivalents. These amide and ester products undergo further transformations such as hydrolysis, reduction and nucleophilic addition reactions without pre-functionalization. Also enantioselective DcA and diastereoselective DcA of α,α-disubstituted amide enolates are extensively studied and reported. Rapid and efficient synthesis of complex molecules via multicomponent reactions (MCR) is a viable alternative method to time- and resource-consuming stepwise synthesis. In general, multicomponent reactions assemble three or more different reactive components into a multisubstituted product in a one-pot, batch-wise process. Also, this process allows the formation of multiple new bonds in a single operation. Herein we report the development of one-pot, three-component and four-component double decarboxylative allylation reactions to produce α- and γ-allylated amides. In these MCRs, benzylic amide enolates exhibited remarkable success over alkyl amide enolates due to stability differences between two nucleophiles. In the progress of transition metal-catalyzed allylation reactions, it is of great interest to activate allylic alcohols in situ to obtain π-allyl intermediates instead of using pre-activated allyl sources. Due to the inherently poor leaving ability of the hydroxyl group several attempts to activate allyl alcohols have been made using Lewis acids such as Ti(OPri)4, BEt3, BPh3, and SnCl2. Compared to these methods, activation of allyl alcohol using CO2, an inexpensive and readily available gas, is an economical choice. CO2 activates the allylic alcohol in 2-(1-hydroxyallyl)phenol substrate allowing formation of π-allyl palladium intermediate followed by intramolecular etherification to synthesize benzopyrans. Furthermore, we report a successful attempt to activate allyl alcohols by an adjacent silyl group to obtain benzopyrans.
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Date
2014-08-31
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University of Kansas
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Keywords
Organic chemistry, Allylations, Decarboxylative, Ester enolate equivalents, Meldrum's acid, multicomponent, palladium