The polycondensation of 1a in the presence of 2 as an initiator and 18‐crown‐6 proceeded in a chain‐polymerization manner in sulfolane at 120 °C to yield poly(ether sulfone) having defined molecular weights and low polydispersities.

We describe the nickel-catalyzed hydrocarboxylation of ynamides with CO 2 and H 2 O to afford a variety of α-amino-α,β-unsaturated esters with high regioselectivities. Selective α-carboxylation of ynamide with this catalytic protocol is unexpected in view of the electronic bias of ynamide and is in sharp contrast to our previous study in which a stoichiometric amount of Ni(0) was used to form a β-carboxylated product exclusively. We revealed that this unexpected C−C bond formation was induced by the combination of Zn and MgBr 2 .Homogeneous nickel catalysis has emerged in the past few decades as a highly efficient tool for organic synthesis.1 The intrinsic chemical properties of some nickel catalysts to construct a C−C bond have been applied to incorporation of CO 2 , which is an abundant, non-toxic and renewable carbon source. 2 Especially, reactions using compounds bearing a C−C double bond or triple bond as a coupling partner of CO 2 enable transformation of these building blocks into carboxylic acid derivatives without prefunctionalization such as halogenation or metalation. [3][4][5] Since all carbons constituting an unsaturated bond are potentially reactive, controlling the regioselectivity is a common concern of these promising reactions employing CO 2 . Recently, Martin's group has developed regioselective hydrocarboxylation of alkyl aryl acetylenes with CO 2 employing alcohol as a proton source. 4f They speculated that an equilibrium would exist between two possible nickelalactones derived from alkyne and CO 2 and that steric repulsion between an alkyl group and alcohol would be a key factor for preferential formation of carboxylic acid A behind B (Scheme 1). Their proposed mechanism let us assume another strategy for nickel-catalyzed regioselective hydrocarboxylation of a C−C triple bond with CO 2 in which the selectivity would be engendered by taking advantage of an electronic factor. Herein, we report a nickel-catalyzed regioselective hydrocarboxylation of ynamide, which is known to have an electronic bias on the C−C triple bond, by use of H 2 O and CO 2 .6 Consequently, we observed an interesting selectivity switching triggered by additives. Scheme 2. Our previous work: Nickel-mediated hydrocarboxylation of ynamide with CO2We previously described a nickel-mediated regioselective synthesis of functionalized β-amino-α,β-unsaturated esters from ynamides 1 and CO 2 .7 The selectivity was controlled by a regioselective oxidative cyclization step in which a negative β-carbon of an ynamide forms a C−C bond with CO 2 on Ni(0) (Scheme 2). The resulting β-carboxylated products were shown to be precursors of enantio-enriched β-amino acid derivatives by conducting Rh-catalyzed asymmetric hydrogenation. As a next step, we commenced to develop a regioselective catalytic hydrocarboxylation of ynamides. Recently, Sakaki and Tsuji et al. reported a nickel-catalyzed double carboxylation of alkyne by the use of CO 2 in the presence of Zn/MgBr 2 as reducing agents. 4g We applied their reaction conditions to catalytic ...

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A good combination between a continuously irradiating microwave system and recyclable Ru(0) nanoparticle for ligand-free Suzuki–Miyaura coupling.