Diarylmethane synthesis through Re2O7-catalyzed bimolecular dehydrative Friedel-Crafts reactions.

Youwei Xie,Qi Qin,Paul E. Floreancig

doi:10.1039/c8sc03570a

Abstract

This manuscript describes the application of Re2O7 to the syntheses of diarylmethanes from benzylic alcohols through solvolysis followed by Friedel-Crafts alkylation. The reactions are characterized by broad substrate scope, low catalyst loadings, high chemical yields, and minimal waste generation. The intermediate perrhenate esters are superior leaving groups to chlorides and bromides in these reactions. The polarity and water sequestering capacity of hexafluoroisopropyl alcohol are critical to the success of these processes. Re2O7 is a precatalyst for HOReO3, which serves as a less costly and easily handled promoter for these reactions. Oxorhenium catalysts selectively activate alcohols in the presence of similarly substituted acetates, indicating a unique chemoselectivity and mechanism in comparison to Brønsted acid catalysis.

Highlights

A report in 2007 from the American Chemical Society Green Chemistry Institute Pharmaceutical Roundtable Research Grant Program identi ed “OH activation for nucleophilic substitution” as a process for which reactions are currently used but better reagents are preferred.[1]
The Hall group made a notable advance in this area by showing[3] that benzylic alcohols bearing electron-withdrawing groups such as –CF3 or –CO2Me ionize efficiently in the presence of a ferrocenyl boronic acid catalyst in (CF3)2CHOH (HFIP) and CH3NO2 to form benzylic cations that react with arenes in Friedel–Cra s reactions
Ghorai and co-workers showed[18] in related processes that Re2O7-mediated Friedel–Cra s reactions with highly activated benzylic alcohols in CH3CN are signi cantly slower than the reactions that we have studied, indicating that the efficiencies of our processes can be attributed to the solvent

Summary

Introduction

Crystalline Re2O7 can be used to effect ionization of 3,5-bis-tri uoromethylbenzyl alcohol (sm 1⁄4 +0.43 for the CF3 group), though this reaction requires heating for 2 d to achieve a moderate product yield. Ghorai and co-workers showed[18] in related processes that Re2O7-mediated Friedel–Cra s reactions with highly activated benzylic alcohols in CH3CN are signi cantly slower than the reactions that we have studied, indicating that the efficiencies of our processes can be attributed to the solvent.

Results

Conclusion