Boyer's Reaction and Transetherification: Mechanism and New Perspectives

Abstract

The progress and stereochemistry of Boyer's reaction were analyzed using several simple, chiral, alcoholic substrates, a variable amount of BiBr3 and different solvents. Basic solvents inhibit the reaction, while cyclohexane works very well; thus, it was our choice for the present study. In contrast to previous works, BiBr3 behaves as a true catalyst, being not consumed during the reaction. Although poisoning of the catalyst occurs to some extent, it does not prejudice the reaction yields (>90%). Gas chromatography/mass spectrometry (GC-MS) monitoring of the reaction revealed that, for example, in the presence of alcohol , isomeric ethers transetherificate to . We propose a unifying mechanistic model for both Boyer's and transetherification reactions, in which the electronic properties of n-adducts intermediates, formed by combination of bismuth(III) of BiBr3 and oxygen atoms of alcohols and ethers, play the key role for both the reactivity and the stereochemical outcome of the reaction.

Ring-closing C–O/C–O metathesis of ethers with primary aliphatic alcohols

“…3, pathway d, purple arrow), protonation of the electron-rich anisole moiety provides the reactive key intermediate VII. An intramolecular nucleophilic additionand-elimination sequence gives the desired product B and regenerates the catalyst, a process similar to previously reported C-O/O-H σ-bond metathesis reactions between aromatic ethers and aliphatic alcohols 10,[17][18][19] . It is impossible to confirm one pathway while completely ruling out the other three based solely on the reaction outcome, since all pathways result in the same products and by-products.…”

“…Transetherification (Fig. 1b ) between an easily available ether and an alcohol provides a promising alternative to unsymmetrical ethers by direct exchange of “RO–” in the inert functionality of “RO–R” 10 , a reaction that has been frequently applied in total syntheses 11 , materials science 12 , polymer chemistry 13 , biomass conversion 14 , etc. However, it suffers from limited scope due to the sole reaction mode available for both aliphatic (Fig.…”

“…1c) and aromatic ethers (Fig. 1d) via acid catalysis 10,[15][16][17][18][19] , transition-metalcatalyzed reversible cleavage of C(sp 2 )-O bonds 20 , as well as recently burgeoning techniques such as electrocatalysis 21 and photocatalysis 14 . These methodologies proceed exclusively through ether activation, resulting in a formal C-O/O-H σ-bond metathesis.…”