Friedel–Crafts Ring-Coupling Reactions Promoted by Tungsten Dearomatization Agent

Abstract

The complexes TpW(NO)(PMe 3 )(L), where L = phenol, N,N-dimethylanilinium, or naphthalene, undergo protonation followed by addition of an aromatic nucleophile. The addition of aromatic molecules occurs at the para carbon of the phenol or aniline ring or the beta carbon of the naphthalene. The addition occurs anti to the metal fragment, as determined by X-ray crystallography. In the case where L = phenol or N,N-dimethylanilinium, treatment of the bound arene with an electrophilic heteroatom followed by an aromatic nucleophile sets two stereocenters, with both additions occurring anti to the metal. The resultant ligands have been removed from the metal by oxidative decomplexation using ceric ammonium nitrate.

Reactivity and Functionalization of Naphthalene and Anthracene Complexes of {TpW(NO)(PMe3)}

“…The stabilization energy of two benzene rings is greater than that of one naphthalene fragment, making this the more favorable carbocation and substitution on the middle ring the preferred product in most cases. 3 In naphthalene, initial addition of electrophiles typically occurs at the α carbon, or C1.…”

“…3 Isomerization gives the thermodynamically more stable addition at the β carbon, or C2 (Scheme 2). useful insights into the preference for α vs. β substitution.…”

“…3 The authors posited that carbocation stability could facilitate intramolecular isomerization prior to deprotonation (Scheme 3). 7 While addition reactions to naphthalene are rarely observed because of the high energetic cost of breaking aromaticity, addition reactions of anthracene are more favorable.…”

“…Aromaticity is also evident in the ring current observed in NMR spectroscopy. 3 The delocalized π electrons in a ring system produce a magnetic field, detectable during NMR experiments. Nuclei that lie above or below the plane of the aromatic ring will be shielded by the induced magnetic field and appear upfield in NMR spectra.…”

“…37 Naphthalene and anthracene can form complexes with metal tricarbonyl and cyclopentadienyl fragments, but these ligands tend to be labile as they are able to adopt η 4 and η 2 binding modes as the naphthalene slips off the metal, increasing aromaticity in the unbound portion of the system. 35,38 For example (η 6 -naphthalene)Cr(CO) 3 can be used as a starting material for the substitution of other arenes on the chromium fragment, 38 including a report of styrene (Scheme 11).…”