Palladium-Catalyzed Tail-to-Tail Reductive Dimerization of Terminal Alkynes to 2,3-Dibranched Butadienes

Metal-catalyzed Markovnikov-type selective hydrofunctionalization of terminal alkynes

Metal-catalyzed highly Markovnikov-type selective hydrofunctionalization of terminal alkynes provides a straightforward and atom-economical route to access 1,1-disubstituted alkenes, which have a wide range of applications in organic synthesis. However, the highly Markovnikov-type selective transformations are challenging due to the electronic and steric effects during the addition process. With the development of metal-catalyzed organic synthesis, different metal catalysts have been developed to solve this challenge, especially for platinum group metal catalysts. In this perspective, we review homogeneous metal-catalyzed Markovnikov-type selective hydrofunctionalization of terminal alkynes according to the classified element types as well as reaction mechanisms. Future avenues for investigation are also presented to help expand this exciting field.

Pd(II)-catalyzed hydroarylations/hydroalkenylations of terminal alkynes: regioselective synthesis of allylic, homoallylic, and 1,3-diene systems

A Pd-catalyzed regioselective hydroarylation of terminal alkynes containing a heteroatom has been developed via carbopalladation for the synthesis of allylic ethers, amines, and homoallylic alcohols. Moreover, hydroalkenylation of alkynes produces a variety of stereodefined 1,4-dienes with high regioselectivity. The important features of the present protocol are that it is highly regioselective, operationally rapid, and scalable with a huge substrate scope using only 3 mol% of PdCl2(PPh3)2 catalyst in the presence of a mild base KOAc.

Synthesis of Five-Membered Cyclic Phosphinic Acids via the [4C+1P] Cyclization of 1,3-Dienes with a Combination of PBr3 and P(OMe)3 as the P(III) Source

An efficient method for the synthesis of 1-hydroxy-2,5-dihydrophosphole 1-oxides, a type of five-membered P-containing heterocyclic compound, is presented. The reaction was carried out through a [4C+1P] cyclization of 1,3-dienes with a combination of PBr3 and P(OMe)3 as the P(III) source. To compare with the reported methods, the protocol reported herein not only is much milder and more rapid but also displays a broad substrate scope and affords the products in high yields (50-94%). In addition, the reaction could be reliably scaled up at the gram-scale level and was demonstrated to be a versatile platform for flexible derivatization. Consequently, this method provides a general and reliable way for the synthesis of five-membered phosphole derivatives.

Palladium-Catalyzed Cycloisomerization of 2-Ethynylbiaryls to 9-Methylidene Fluorenes

A palladium-catalyzed cycloisomerization of 2-ethynylbiaryls to 9-methylidene fluorenes is described for the first time. The cycloisomerization of 2-ethynylbiaryls proceeded smoothly in the presence of weak acid at low temperature to afford 9-methylidene fluorenes in satisfactory to high yields. This new type of cycloisomerization of 2-ethynylbiaryls is operationally simple and scalable and exhibits high functional-group tolerance. Various synthetically useful functional groups, such as halogen atoms, as well as formyl, acetyl, methoxycarbonyl, cyano, and nitro groups, remain intact during the cycloisomerization of 2-ethynylbiaryls.