The dihalogenation of alkenes is one of the classic reactions in organic chemistry and a prime example of an electrophilic addition reaction. The often observed anti‐selectivity in this addition reaction can be explained by the formation of a haliranium‐ion intermediate. Although dihalogenations have been studied for more than a century, the development of reagent‐controlled, enantioselective dihalogenation has proved to be very difficult. Only recently, significant progress has been achieved. In this review, an overview on current method development in enantioselective dihalogenation is provided and mechanistic aspects that render this transformation challenging are discussed.
A transition‐metal‐free, redox‐neutral, organocatalytic C3‐alkenylation of pyrroles is reported. Readily available aldehydes were employed as alkenylating agent and the reaction tolerates several key functional groups. The E‐alkenylated products were isolated in moderate to exclusive selectivity. A one‐pot two‐fold alkenylation strategy is also developed for further downstream modifications. To show the applicability, synthetically challenging indolylpyrrole derivatives were synthesized using Cadogan cyclization.
Enantioselective iodocycloetherifications can be conducted using sterically highly demanding BINOL‐based phosphoric acid diesters as catalyst. To achieve highly enantioselective reactions, cocatalysis by triphenylphosphine selenide is necessary. With cocatalysis, good to excellent enantioselectivities can be achieved for a broad range of substrates using catalyst and cocatalyst loadings as low as 1 mol %. The triphenylphosphine selenide cocatalyst itself does not strongly influence diastereoselectivity, but leads to higher reactivity and, in combination with a BINOL‐based phosphoric acid diester, to good enantioselectivity.
Dihalogenation of alkenes is one of the classic reactions in organic chemistry and a prime example of an electrophilic addition reaction. Although being studied intensely, the development of reagent‐controlled, enantioselective dihalogenation is still challenging. Read more about current method development and mechanistic aspects in enantioselective dihalogenation in the Minireview by U. Hennecke et al. on page 4517.
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