Formal Halo‐Meyer–Schuster Rearrangement of Propargylic Acetates through a Novel Intermediate and an Unexampled Mechanistic Pathway

• Published in: Chemistry : a European journal Vol. 25; no. 42; pp. 9816 - 9820
• Main Authors: Sadhukhan, Santu, Baire, Beeraiah
• Format: Journal Article
• Language: English
• Published: WEINHEIM Wiley 25.07.2019; Wiley Subscription Services, Inc
• Subjects: Acetates; Acetic acid; alpha,alpha-dihalo-beta-acetoxyketones; alpha-haloenones; cascade strategy; Chemistry; Chemistry, Multidisciplinary; Lewis base; Physical Sciences; propargylic acetates; Reagents; Science & Technology; Selectivity; Stereoselectivity; Temperature dependence; water; alpha-dihalo-beta-acetoxyketones; FURANS; MILD APPROACH; cascade strategy; ALKYNES; water; propargylic acetates; HYDRATION; alpha-haloenones; ALCOHOLS; KETONES; FACILE SYNTHESIS; alpha; CONDENSATION; CYCLIZATION; alpha,alpha-dihalo-beta-acetoxyketones
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201901856

A formal, highly stereoselective halo‐Meyer–Schuster rearrangement of inactivated propargylic acetates to (Z)‐α‐haloenones has been reported, under metal free conditions. This cascade process involves a new class of intermediate, i.e., α,α‐dihalo‐β‐acetoxyketones and mechanism to generate the α‐haloenones, employing water as Lewis base. The outcome of the reaction is temperature‐dependent, as room temperature, selectively provides α,α‐dihalo‐β‐acetoxyketones whereas reactions at 100 °C give direct access to α‐haloenones. Either type of product can be obtained in excellent yield. A suitable rationale for the observed high Z‐selectivity for α‐haloenones (based on conformational population) and distinct reaction rates for various N‐halosuccinimide (NXS) reagents (based on C−X bond strengths) has also been provided. Metal‐free synthesis: A highly stereoselective, formal halo‐Meyer–Schuster rearrangement of inactivated propargylic acetates to (Z)‐α‐haloenones has been developed, which uses metal‐free conditions. This cascade process involves a new class of intermediates, α,α‐dihalo‐β‐acetoxyketones and mechanism to generate the α‐haloenones, employing water as a Lewis base. The outcome of the reaction is temperature‐dependent, that is, reactions at room temperature, selectively provide α,α‐dihalo‐β‐acetoxy ketones whereas reactions at 100 °C give direct access to α‐haloenones.