Synthesis of Sterically Hindered Biaryls by Zr‐Mediated Co‐cyclotrimerization of Alkynes

Abstract

Treatment of zirconacyclopentadienes with ortho‐substituted arylpropynoates in the presence of stoichiometric amounts of CuCl or NiBr2(PPh3)2 represents a novel approach to the synthesis of biaryls, formed in good yields. The CuCl‐mediated reaction proceeded through two reaction mechanisms, also affording Dewar benzenes together with the corresponding biaryls. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Transition Metal-Free Trans Hydroboration of Alkynoic Acid Derivatives: Experimental and Theoretical Studies

“…Concentration in vacuo yielded a yellow oil which, upon purification by column chromatography (1:10, ethyl acetate:hexanes), yielded the corresponding ester. Esters 1a , 1b , 1c , 1d , 1e , 1f , 1g , 1i , 1j , 1k , 1l , 1m , 1o , 1p , 1q , 1r , 1s , 1v , 1w , 1x , 1y , 1z , and 6 are known compounds. Compound 1n was purchased commercially and used directly.…”

Mechanistic Studies of Transition-Metal-Catalyzed [2 + 2 + 2] Cycloaddition Reactions

“…The reaction of different zirconacyclopentadienes with alkynes bearing at least one electron-withdrawing group attached to the triple bond to afford [2 + 2 + 2] cycloaddition reactions in the presence of CuCl has been studied by the same and other research groups. Haloterphenyl derivatives, 217 haloterphenyl compounds with chiral backbones, 218 polyacenes, 219 highly substituted phenylferrocenes, 220 and sterically hindered biaryls 221 were prepared using this process.…”

“…Kotora et al also used [NiX 2 (PPh 3 ) 2 ] (X = Cl or Br) for the cycloaddition of zirconacyclopentadienes with alkynes such as ethynylferrocenes 220 and arylpropionates. 221 In the case of ferrocenyl derivatives, no electron-withdrawing group was required in the third alkyne to make the process efficient.…”

Palladium-Catalyzed Dimerization of Bis(2-biphenylyl)acetylene toward Sterically Hindered Acephenanthrylene

“…5,6 While alkylated terminal and internal alkynes react rather smoothly, 7 harsher conditions and prolonged reactions times are necessary for bis(aryl)acetylenes due to higher steric demand, mostly resulting in poor yields. 8 Even fewer reports exist for successful cyclization of ortho-substituted diphenylacetylenes, albeit with low conversions. 9 To overcome these limitations, we initially focused on conditions for the trimerization of the sterically demanding bis(2-biphenylyl)acetylene 2.…”

“…This intermediate reacts either in a [4 + 2] cycloaddition to a metallanorbornadiene, liberating the arene, or via alkyne insertion to a metallacycloheptatriene, followed by reductive elimination (Scheme ). , While alkylated terminal and internal alkynes react rather smoothly, harsher conditions and prolonged reactions times are necessary for bis­(aryl)­acetylenes due to higher steric demand, mostly resulting in poor yields . Even fewer reports exist for successful cyclization of ortho -substituted diphenylacetylenes, albeit with low conversions …”