- C-O Bond Activation as a Strategy in Palladium-Catalyzed Cross-Coupling
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The activation of strong C-O bonds in cross-coupling catalysis can open up new oxygenate-based feedstocks and building blocks for complex-molecule synthesis. Although Ni catalysis has been the major focus for cross-coupling of carboxylate-based electrophiles, we recently demonstrated that palladium catalyzes not only difficult C-O oxidative additions but also Suzuki-Type cross-couplings of alkenyl carboxylates under mild conditions. We propose that, depending on the reaction conditions, either a typical Pd(0)/(II) mechanism or a redox-neutral Pd(II)-only mechanism can operate. In the latter pathway, C-C bond formation occurs through carbopalladation of the alkene, and C-O cleavage by β-carboxyl elimination. 1 Introduction 2 A Mechanistic Challenge: Activating Strong C-O Bonds 3 Exploiting Vinylogy for C-Cl and C-O Oxidative Additions 4 An Alternative Mechanism for Efficient Cross-Coupling Catalysis 5 Conclusions and Outlook.
- Becica, Joseph,Leitch, David C.
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p. 641 - 646
(2020/12/14)
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- Scalable and Chemoselective Synthesis of ?-Keto Esters and Acids via Pd-Catalyzed Carbonylation of Cyclic β-Chloro Enones
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The Pd-catalyzed carbonylation of cyclic β-chloro enones using simple phosphine ligands is described. Screening identified P(Me)(t-Bu)2 as the most general ligand for an array of chloro enone electrophiles. The reaction scope has been evaluated on a milligram scale across 80 examples, with excellent reactivity observed in nearly every case. Carbonylation can be achieved even in the presence of potentially sensitive or inhibitory functional groups, including basic nitrogens as well as aryl chlorides or bromides. Twenty examples have been run on a gram scale, demonstrating scalability and practical utility. Using P(Me)(t-Bu)2, the reaction rate depends on both nucleophile and electrophile identity, with completion times varying between 3 and >18 h under a standard set of conditions. Switching to P(t-Bu)3 for the carbonylation of 3-chlorocyclohex-2-enone with methanol results in a dramatic rate increase, enabling effective catalysis with kinetics consistent with rate-limiting mass transfer. Stoichiometric oxidative addition of 3-chlorocyclohex-2-enone and 3-oxocyclohex-1-enecarbonyl chloride to both Pd[P(t-Bu)3]2 and Pd(PCy3)2 has enabled characterization and isolation of several potential catalytic intermediates, including Pd-vinyl and Pd-acyl species supported by P(t-Bu)3 and PCy3 ligands. Monitoring the oxidative addition of 3-chlorocyclohex-2-enone to Pd(PCy3)2 by NMR spectroscopy indicates that coordination of the alkene precedes oxidative addition. As a result of these studies, methyl 3-oxocyclohex-1-enecarboxylate has been synthesized via Pd-catalyzed carbonylation of 3-chlorocyclohex-2-enone in 90% yield on a 60 g scale with only 0.5 mol % catalyst loading.
- Kaplan, Justin M.,Hruszkewycz, Damian P.,Strambeanu, Iulia I.,Nunn, Christopher J.,Vangelder, Kelsey F.,Dunn, Anna L.,Wozniak, Derek I.,Dobereiner, Graham E.,Leitch, David C.
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supporting information
p. 85 - 96
(2018/09/25)
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- Simple Routes to Sarkomycin
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Two synthetic routes to sarkomycin (6) are demonstrated.The first involves a 3-carbon annelation to form the spirocyclopentenone (2) followed by regiospecific γ-alkylation and subsequent manipulation of the side chain in 15 to give the sarkomycin ester adduct 18.The second route employs the itaconate-anthracene adduct 20 as the C-5 synthon in a tandem Michael addition-Dieckmann condensation between the anion derived from 20 and methyl acrylate.The reaction furnishes the diester 22, which, upon selective decarboxylation, gives rise to the sarkomycin precursors 18 and 23 (1:3).Flash vacuum pyrolysis of either isomer 18 or 23 yields (+/-)-sarkomycin ester 7 which is then hydrolyzed to the acid 6.
- Kodpinid, Montree,Siwapinyoyos, Tiwa,Thebtaranonth, Yodhathai
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p. 4862 - 4865
(2007/10/02)
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