# The First Example of the Friedel–Crafts Cyclization Leading to (10-Hydroxy-9,10-dihydroanthr-9-yl)phosphonium Salts without the Expected Bradsher Dehydration

## TL;DR

A chemical reaction unexpectedly produces new phosphonium salts instead of expected products, revealing a novel cyclization mechanism.

## Contribution

The first example of Friedel–Crafts cyclization forming (10-hydroxy-9,10-dihydroanthr-9-yl)phosphonium salts without Bradsher dehydration is reported.

## Key findings

- Cyclization forms cis/trans isomers and conformers of phosphonium salts.
- Electron-rich phosphines yield all four stereoisomers in NMR spectra.
- DFT, NCI, and FT-IR analyses support the observed reaction mechanism.

## Abstract

The reaction of (ortho-acetalaryl)arylmethanols with various phosphines PR1R2R3 (R1 = R2 = R3 = Ph; R1 = R2 = Ph, R3 = Me and R1 = R2 = Me, R3 = Ph) under acidic conditions (e.g., HCl, HBF4, TsOH) unexpectedly led to the formation of (10-hydroxy-9,10-dihydroanthr-9-yl)phosphonium salts instead of the corresponding anthryl phosphonium salts. The cyclization occurred according to the Friedel–Crafts mechanism but without the usually observed Bradsher dehydration, giving cyclic products in the form of cis/trans isomers and their conformers. In case of electron-rich and less-hindered dimethylphenylphosphine, all four stereoisomers were recorded in 31P{1H} NMR spectra, while for the other phosphines, only the two most stable cis/trans stereoisomers were detected. This study was supported by DFT and NCI calculations in combination with FT-IR analysis.