The double protonation of dihapto-coordinated benzene complexes enables dearomatization using aromatic nucleophiles

Abstract

Friedel-Crafts Arylation (the Scholl reaction) is the coupling of two aromatic rings with the aid of a strong Lewis or Brønsted acid. This historically significant C–C bond forming reaction normally leads to aromatic products, often as oligomeric mixtures, dictated by the large stabilization gained upon their rearomatization. The coordination of benzene by a tungsten complex disrupts the natural course of this reaction sequence, allowing for Friedel-Crafts Arylation without rearomatization or oligomerization. Subsequent addition of a nucleophile to the coupled intermediate leads to functionalized cyclohexenes. In this work, we show that by coordinating benzene to tungsten through two carbons (dihapto-coordinate), a rarely observed double protonation of the bound benzene is enabled, allowing its subsequent coupling to a second arene without the need of a precious metal or Lewis acid catalyst.

Tungsten-anisole complex provides 3,6-substituted cyclohexenes for highly diversified chemical libraries

“…Compounds WTp(NO)(PMe 3 )(Br), 1, 2, and [WTp(NO) (PMe 3 )(5,6-η 2 -4H-1,3-dimethoxybenzenium)](L-DBTH) were prepared according to previous literature procedures with some modifications (10,22,62). Compounds 5D-8D, 13D, 15D, 78D, 19D, 8P, 28P, 27D-30D, 32D-37D, cis-38D, cis-46D, 51D, 61-65, 70, and 72-74 have previously been published (9). Synthetic details, DFT calculations, and spectroscopic and crystallographic characterizations of compounds are provided in the Supplementary Materials.…”

“…Starting with a racemic mixture of the anisole complex (1), the organic compound isolated is identical, regardless of which pathway (P or D) it is derived from, and yields appear to be comparable. However, if the initial anisole complex is enantioenriched, then the P series and D series of reactions would yield opposite enantiomers (9). Unfortunately, in contrast to what is observed for racemic anisole complex (1) (vide supra), powders of enantioenriched 1 were often found to contain a mixture of 1P and 1D diastereomers.…”

“…While the "dearomatization" of benzene has been widely studied (5)(6)(7)(8), the double protonation/triple nucleophilic addition sequence described herein allows for unparalleled access to structural variability. A portion of this study has been previously communicated (9).…”