Synthesis and Functionalization of Allenes by Direct Pd‐Catalyzed Organolithium Cross‐Coupling

Abstract

Ap alladium-catalyzed cross-coupling between in situ generated allenyl/propargyl-lithium species and aryl bromides to yield highly functionalizeda llenes is reported. The direct and selective formation of allenic products preventing the corresponding isomeric propargylic product is accomplished by the choice of SPhos or XPhos based Pd catalysts. The methodology avoids the prior transmetalation to other transition metals or reverse approaches that required prefunctionalization of substrates with leaving groups,r esulting in af ast and efficient approach for the synthesis of tri-and tetrasubstituted allenes.E xperimental and theoretical studies on the mechanism showc atalyst control of selectivity in this allene formation.

Acylboronates in Polarity-Reversed Generation of Acyl Palladium(II) Intermediates

“…After 15 min, the signals at 41.9 ppm, which corresponded to XPhos oxide, and 27.4 ppm appeared. The latter was assigned to the oxidative addition palladium complex III , which exhibited a chemical shift consistent with previous literature reports . After 1 h, a new signal at 25.9 ppm appeared and its intensity continued to rise while the resonance of III slowly disappeared.…”

Asymmetric hydroalkylation of alkynes and allenes with imidazolidinone derivatives: α-alkenylation of α-amino acids

“… 18 Formation of free allene as an electrophile coupling partner in this reaction is possible by protonation of the lithiated allenyl species. 19 Also, the deprotonation of the imidazolidinone would afford the corresponding lithium enolate ( III ). Then, the nucleophilic attack of III to the central allene carbon dictates the alkene geometry anti relative to the aryl group.…”

“…These experiments confirm the presence of dilithiated allenyl species II which is in equilibrium with the alkyne, allene, and monolithiated allene moieties. 19,20 …”

“…These experiments conrm the presence of dilithiated allenyl species II which is in equilibrium with the alkyne, allene, and monolithiated allene moieties. 19,20 The C]O stretching absorption corresponding to the amide carbonyl group in 1a ($1709 cm À1 ) was decayed upon addition of one equivalent of LiHMDS and a new peak at $1666 cm À1 was formed which corresponds to the enolate of 1a. Progress of the reaction showed the appearance of a new C]O stretching absorption bond which is attributed to the product ($1712 cm À1 ).…”

Pd-catalyzed sp–sp3 cross-coupling of benzyl bromides using lithium acetylides

“…organozinc, –boron, –silicon, or –tin reagents. Having successfully tamed the extreme reactivity of organolithium compounds 12 in batch reactions, these palladium-catalyzed C–C bond formation reactions proceed fast (5–60 min) with high conversion and selectivity, 13 and can be performed at room temperature, 14 with a nickel catalyst, 15 without solvent, 16 under air, 16 on ‘water’, 17 at low temperature 18 and have found application in stereoselective natural product synthesis. 19 Recently, we reported a Sonogashira coupling employing lithium acetylides.…”