Palladium-Catalyzed Hydrohalogenation of 1,6-Enynes: Hydrogen Halide Salts and Alkyl Halides as Convenient HX Surrogates

Abstract

Difficulties associated with handling H and CO in metal-catalyzed processes have led to the development of chemical surrogates to these species. Despite many successful examples using this strategy, the application of convenient hydrogen halide (HX) surrogates in catalysis has lagged behind considerably. We now report the use of ammonium halides as HX surrogates to accomplish a Pd-catalyzed hydrohalogenation of enynes. These safe and practical salts avoid many drawbacks associated with traditional HX sources including toxicity and corrosiveness. Experimental and computational studies support a reaction mechanism involving a crucial E-to-Z vinyl-Pd isomerization and a carbon-halogen bond-forming reductive elimination. Furthermore, rare examples of C(sp)-Br and -Cl reductive elimination from Pd(II) as well as transfer hydroiodination using 1-iodobutane as an alternate HI surrogate are also presented.

Iridium‐Catalyzed Hydrochlorination and Hydrobromination of Alkynes by Shuttle Catalysis

“…Catalytic hydrobromination of alkynes represents an efficient approach to synthesize vinyl bromides.H owever, current reaction conditions for this process mainly rely on the use of either corrosive and gaseous HBr or in situ generated HBr, [17] which is not ideal for laboratory-scale synthesis. Recently,a na lternative method using at ransfer hydrofunctionalization strategy was reported by the groups of Lautens [18] and Oestreich. [19] They achieved the hydrobromination of 1,6-enynes and alkynes through the transfer of HBr from Et 3 N·HBr and 1-(2-bromoethyl)-1,4-dihydro-1,1'-biphenyl, respectively.I nt his context, we were delighted to see that our protocol for the transfer hydrochlorination using tert-butyl chloride could also be applied to the transfer hydrobromination using tert-butyl bromide as the source of HBr.…”

Iridium‐katalysierte Hydrochlorierung und Hydrobromierung von Alkinen durch Shuttlekatalyse

“…Die gegenwärtigen Bedingungen fürd ieses Verfahren beruhen je-doch hauptsächlich auf der Verwendung von ätzendem und gasfçrmigem HBr oder in situ erzeugtem HBr, [17] was fürdie Synthese im Labormaßstab nicht ideal ist. Kürzlich wurde von Lautens [18] und Oestreich [19]…”

Recent Development in Palladium-Catalyzed Domino Reactions: Access to Materials and Biologically Important Carbo- and Heterocycles

“…A combination of experiment and theory has provided insight into the unprecedented reaction mechanism that involved a formal anti-alkyne hydropalladation step resulting from a crucial E-to Z-vinyl-palladium(II) isomerization (Scheme 119). 181 Scheme 119. Synthesis of halogenated pyridinones.…”