Abstract
The development and application of a transfer hydrochlorination reagent based on a trichlorinated bicyclo[3.1.0]hexane core that transfers two molecules of HCl per molecule of surrogate to a π-basic substrate under B(C6F5)3 catalysis is reported. Lewis acid-assisted chloride abstraction followed by thermal electrocyclic cyclopropyl-to-allyl cation ring opening releases ring strain as a previously unexploited driving force.
Highlights
Metal-free transfer hydrochlorination of internal C–C triple bonds with a bicyclo[3.1.0]hexanebased surrogate releasing two molecules of hydrogen chloride†
Lewis acidassisted chloride abstraction followed by thermal electrocyclic cyclopropyl-to-allyl cation ring opening releases ring strain as a previously unexploited driving force
When adapting this concept to transfer hydrobromination, we realized that activation of the surrogate was the easier the weaker the C(sp3)–X bond with X = I and Br as well as Cl. Overcoming this difficulty required higher temperatures and better stabilization of the intermediate carbenium ion with an additional phenyl group in the case of hydrobromination. These findings indicated that a transfer hydrohalogenation becomes increasingly complicated for the lighter halogen homologues and that a more sophisticated surrogate design might be necessary
Summary
Metal-free transfer hydrochlorination of internal C–C triple bonds with a bicyclo[3.1.0]hexanebased surrogate releasing two molecules of hydrogen chloride†. A broadly applicable surrogate with reduced acidity compared to HCl was introduced by Hammond and Xu.[10] a gold catalyst was typically required, a handful of more activated substrates reacted under metal-free conditions.
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