Regioselective Dihalohydration Reactions of Propargylic Alcohols: Gold-Catalyzed and Noncatalyzed Reactions

Visible-Light-Induced Oxidative α-keto-Dichlorination of Arylalkynes by CuCl2 at Room Temperature

A visible light-induced oxidative α-keto-dichlorination of terminal and internal aryl alkynes was developed to form dichloroacetophenones (DCAPs) and dichlorophenyl-acetophenones (DCPAPs), respectively, by using CuCl₂ as a photoredox catalyst in the presence of air at room temperature (without using any exogenous photocatalyst). Here, photoexcited CuCl₂ underwent ligand-to-metal charge transfer to generate a Cl radical, which readily added to the alkynes to form DCAPs or DCPAPs in the presence of O₂ . This α-keto-dichlorination reaction is a green and mild protocol as it produced water as the only by-product. Moreover, the evaluation of green chemistry metrics indicated that the E-factor (mass of wastes/mass of products) of the current α-keto-chlorination method is around 10.1 times lower than that of a literature-reported photochemical method. The Eco Scale value (score 55, which on a scale of 0-100 indicates an acceptable synthesis) signifies that this process is simple, highly efficient, eco-friendly, and cost-effective.

Tunable Lewis Basicity and Nucleophilicity of Water against α,α-Dihalo-β-acetoxyketones for the Selective Synthesis of α-Haloenones and 1,2-Diketones

The discovery and systematic study of tunable yet competitive nucleophilicity and Lewis basicity of water against novel building blocks α,α-dihalo-β-acetoxyketones (possessing a tertiary acetate) have been reported. This distinct reactivity resulted in the formation of two competitive and different products 1,2-diketones and α-haloenones from a common intermediate α,α-dihalo-β-acetoxyketones through the nucleophilicity and Lewis basicity of water, respectively. A systematic study to understand the effect of temperature and amount of water on the product distribution revealed that a lower temperature in combination with a higher amount of water shows a high preference for 1,2-diketones over α-haloenones. Measuring the dielectric constant (permittivity, ε) of various reaction media at various temperatures and a correlation with the experimental observations suggested that the reaction media with a higher dielectric constant exhibit the nucleophilic character and hence show a preference for 1,2-diketones over α-haloenones.

Dihalohydration of Alkynols: A Versatile Approach to Diverse Halogenated Molecules

In this paper we outline how dihalohydration reactions of propargylic alcohols can be used to access a wide variety of useful halogenated building blocks. A novel procedure for dibromohydration of alkynes has been developed, and a selection of dichloro and dibromo diols and cyclic ethers were synthesized. The dihalohydration of homo-propargylic alcohols provides a useful route to 3-halofurans, which were shown to readily undergo cycloaddition reactions under mild conditions. Finally, a novel ring expansion of propargylic alcohols containing a cyclopropylalkyne provides access to halogenated alkenylcyclobutanes.

Intercepting the Gold-Catalysed Meyer-Schuster Rearrangement by Controlled Protodemetallation: A Regioselective Hydration of Propargylic Alcohols

The regioselective gold‐catalysed hydration of propargylic alcohols to β‐hydroxy ketones can be achieved by diverting the gold‐catalysed Meyer–Schuster rearrangement through the addition of a protic additive with a pK_a of 7–9 such as p‐nitrophenol, boric acid or a boronic acid. This provides an interesting alternative to an aldol reaction when combined with the straightforward addition of an alkyne to an aldehyde or ketone. The gold‐catalysed reaction of an electron‐deficient, sterically hindered propargylic alcohol with a boronic acid led to the formation of an unusually stable cyclic boron enolate.