Palladium-Catalyzed Interceptive Decarboxylative Addition of Allyl Car­bonates with Carbonyl Group

Abstract

The first palladium-catalyzed interceptive decarboxylative 1,4-addition of allyl carbonates with squarates is reported. Interestingly, the C-3 carbonyl group of N-substituted isatins undergoes smooth decarboxylative 1,2-addition with allyl carbonates. This transformation offers a straightforward method for the synthesis of spiro-oxepane-fused 2-oxindole.The development of efficient methods to construct organic frameworks continues as a challenge in both academic as well as industrial research applications. Since the ground-breaking work of Tsuji, 1 Saegusa, 2 and Trost, 3 palladium-catalyzed decarboxylative allylation has emerged as an important method for the construction of new carbon-carbon bonds. Intensive research efforts have been devoted to transition-metal-catalyzed intramolecular decarboxylative allylation by many research groups. 4 Recent advances toward interceptive decarboxylative allylation include the reactions of allyl-β-keto esters, 5 allylcarbonates, 6 allyl carbamates, 7 and allyl diphenylglycinate esters 8 with electrophilic olefins. Hayashi and coworkers have extended the palladium-catalyzed IDcA to malonate-derived valerolactones. 9 We have recently disclosed our success on palladiumcatalyzed allylation reactions of isatylidenes 10 and heptafulvenes. 11 Nucleophilic allylation of carbonyl groups have attracted attention of a number of organic chemists. 12 Yamamoto et al. reported the first palladium-catalyzed allylation of aldehydes using bis-π-allylpalladium intermediates. 13 Allylation of carbonyl groups using iridium, 14 indium, 15 zinc, 16 and other metal reagents 17 have been extensively studied. In 2004, Schaus and co-workers reported the palladium(0)-catalyzed decarboxylative aldol reaction of allyl-β-keto esters and aldehydes. 18 But there are only limited reports on palladium-catalyzed intermolecular decarboxylative coupling with carbonyl groups. 9c,8b This fact combined with our interest in allylation using π-allyl-palladium complex prompted us to investigate its reactivity on activated ketones such as squarates and isatins. Herein, we report the catalytic interceptive decarboxylative 1,4-addition of allylcarbonates with squarates and 1,2-addition of carbonyl group of acenaphthenequinone and isatins.Squarates, fascinating and versatile C-4 synthons, offer the prospect of serving as useful starting materials for the synthesis of a wide variety of compounds. 19 The monoaddition and two-fold addition to the carbonyl group of squarates by organolithium and Grignard reagents to synthesize polycyclic compounds are well documented in the literature. 20 But there are only few reports on palladiumcatalyzed reactions involving squarates. 19d,21 Our initial studies involved the treatment of dibutyl squarate (1a) and diallylcarbonate 2a in the presence of Pd(PPh 3 ) 4 (5 mol%) in THF at room temperature. The reaction afforded 1,4-addition product 2,4-bis(allyloxy)-3,4-dibutoxycyclobut-2-enone (3aa) in 22% yield (Scheme 1). The structure of the 1,4-addition product was characterized by usual ...