1 research outputs found
α‑Alkylidene-γ-butyrolactone Formation via Bi(OTf)<sub>3</sub>‑Catalyzed, Dehydrative, Ring-Opening Cyclizations of Cyclopropyl Carbinols: Understanding Substituent Effects and Predicting <i>E</i>/<i>Z</i> Selectivity
A BiÂ(OTf)<sub>3</sub>-catalyzed ring-opening cyclization of (hetero)Âaryl
cyclopropyl carbinols to form α-alkylidene-γ-butyrolactones
(ABLs) is reported. This transformation represents different chemoselectivity
from previous reports that demonstrated formation of (hetero)Âaryl-fused
cyclohexa-1,3-dienes upon acid-promoted cyclopropyl carbinol ring
opening. ABLs are obtained in up to 89% yield with a general preference
for the <i>E</i>-isomers. Mechanistically, BiÂ(OTf)<sub>3</sub> serves as a stable and easy to handle precursor to TfOH. TfOH then
catalyzes the formation of cyclopropyl carbinyl cations, which undergo
ring opening, intramolecular trapping by the neighboring ester group,
subsequent hydrolysis, and loss of methanol resulting in the formation
of the ABLs. The nature and relative positioning of the substituents
on both the carbinol and the cyclopropane determine both chemo- and
stereoselective outcomes. Carbinol substituents determine the extent
of cyclopropyl carbinyl cation formation. The cyclopropane donor substituents
determine the overall reaction chemoselectivity. Weakly stabilizing
or electron-poor donor groups provide better yields of the ABL products.
In contrast, copious amounts of competing products are observed with
highly stabilizing cyclopropane donor substituents. Finally, a predictive
model for <i>E</i>/<i>Z</i> selectivity was developed
using DFT calculations