(Arene)Cr(CO)3 Complexes: Aromatic Nucleophilic Substitution

Abstract

Transition metals coordinated to the pi system of arenes function as strong electronwithdrawing groups and favor nucleophilic addition to the arene to give cyclohexadienyl-metal complexes. The regioselectivity of addition is influenced in subtle and indirect ways, compared to the powerful effect on both reactivity and regioselectivity of electron-withdrawing groups attached to the sigma bond system of the arene. The intermediate cyclohexadienyl complexes can be processed in several ways, leading to several distinct synthesis methods. Useful reactivity is known for several metal systems and each is discussed for specific reaction types. The presence of a fluoro or chloro substituent on the arene allows an effective analog of classical S N Ar reactivity, with a wide range of nucleophiles including simple amines, alkoxides, and carbon anions. Another general process is the addition/protonation protocol, where a hydrogen substituent on the arene is replaced by a nucleophile. This opens new questions of regioselectivity since the typical arene ligand has several hydrogens which are candidates for substitution. The formation of five-and six-membered rings by both general mechanisms are known. Less general indirect mechanisms (e.g., tele substitution) have also been defined.

Catalytic SNAr Hydroxylation and Alkoxylation of Aryl Fluorides

“…Nucleophilic substitutions on aromatic rings can proceed via a concerted mechanism [25, 26] or a stepwise mechanism that involves an intermediate (benzyne or Meisenheimer adduct, Scheme 6). [6, 27] Although S N Ar reactions of metal η 6 ‐arene complexes are thought to proceed via an addition‐elimination process (pathway c), [9d,e,j,n] definitive experimental evidence for the proposed reaction intermediate, a Meisenheimer‐type adduct, is still lacking.…”

“…Since the first chromium(0) η 6 ‐arene complex was reported by Fischer and Hafner in 1955, [8] chemists have made considerable progress in S N Ar reactions of aromatic compounds, that is, umpolung aromatic substitution, by taking advantage of the electron‐withdrawing effect of η 6 ‐coordination between metals and arenes [9] . In standard S N Ar reactions, substitution reactions of η 6 ‐coordinated arenes are widely accepted as proceeding stepwise [9d,e,j,n] via Meisenheimer‐type intermediates. However, experimental evidence for such intermediates has yet to be obtained; instead, η 5 ‐complexes bearing a nucleophile and a leaving group at different positions have been reported [10] .…”

η2-Arene Binding at High-Spin Fe(I) Enabled by a Sterically Accommodating Tris(pyrazolyl)hydroborate Ligand

“…If sterically accessible, η 6 binding is the norm, as it preserves the aromaticity of the arene. With electron-deficient metal fragments, η 6 coordination can imbue reactivity with nucleophiles (e.g., group 6 (CO) 3 M­(arene) complexes) . In some cases, ring slippage to η 4 coordination occurs, as in the reduction of [(η 6 -C 6 Me 6 ) 2 Ru] 2+ to (η 6 -C 6 Me 6 )­Ru­(η 4 -C 6 Me 6 ) .…”

Trifluoromethyl-Substituted Benzocyclobutenone and Benzocyclobutenedione: The Structure Anomaly of (Benzocyclobutenedione)tricarbonylchromium Complexes

“…(Arene)­tricarbonylchromium complexes have attracted interest, because their chemical properties significantly differ from those of the uncoordinated arenes . The most prominent reasons for this are a strong electron withdrawal by the tricarbonylchromium group, which is comparable to that of a p -nitro substituent at an arene, − and the facial differentiation, which eliminates one plane of symmetry, allowing for stereoselective applications of (arene)­tricarbonylchromium complexes in stoichiometric as well as catalytic asymmetric natural product synthesis. − …”