The complexation of an arene C 6 H 5 X to the tricarbonylchromium unit promotes addition of nucleophiles to the arene ring due to the strong electron-withdrawing ability of the Cr(CO) 3 group. If addition of the nucleophile Nu occurs at the carbon bearing the leaving group X, this is an overall ipso nucleophilic aromatic substitution S N Ar. If addition of the nucleophile takes place on another carbon, the anionic h 5 -X-cyclohexadienyltricarbonylchromium intermediates can be treated with acid giving h 4 -X-cyclohexadiene derivatives via chromium hydride intermediates. The driving force of these reactions is the elimination of HX. These reactions are called cine and tele S N Ar. In this chapter, an overview of these S N Ar substitutions is presented as well as recent developments relating to organic synthesis.
11.1Scheme 1. Effects of the Cr(CO) 3 entity on the arene ring.Modern Arene Chemistry. Edited by Didier Astruc 368 of negative or positive charges in a side-chain position, (ii) the acidification of the ring protons, which facilitates easy lithiation, and (iii) the large steric effect of the Cr(CO) 3 entity, which blocks one face of the arene and directs incoming reagents to the uncomplexed face of the cycle. The purpose of this chapter is to specifically cover nucleophilic reactions on the activated ring that do not disturb the coordination of the chromium to the arene, namely ipso, cine, and tele nucleophilic aromatic substitutions, and, therefore, open the way to possible double functionalizations that may offer rapid access to interesting synthetic building blocks.
11.1.2
Coverage and DefinitionsReactions in which coordination of the tricarbonylchromium unit to the p-system of an arene cycle 1 activates the ring towards addition of nucleophiles to give h 5 -cyclohexadienyltricarbonylchromium anionic species 2a-d are classified according to the regioselectivity of this addition. If the addition of the nucleophile occurs at the carbon bearing the leaving group, complex 2a can eliminate X À leading to an overall ipso nucleophilic aromatic substitution, S N Ar, affording complex 3a (Scheme 2, path a).If the addition of the nucleophile takes place at another carbon, the intermediates 2b-d can be oxidized to give the free disubstituted arenes 8b-d (Scheme 2, path b). No detailed mechanistic study of this oxidation has been reported in the literature, but it has been Scheme 2. Addition of a nucleophile to arenetricarbonylchromium complexes. 11.1 Introduction 369 Scheme 7. Ipso S N Ar: CaO bond formation. 11.2 Ipso Nucleophilic Aromatic Substitutions 373 Scheme 10. Ipso S N Ar: CaO bond formation. Scheme 11. Ipso S N Ar: CaO bond formation. Scheme 12. Ipso S N Ar: CaO bond formation. 11.2 Ipso Nucleophilic Aromatic Substitutions 375 Scheme 15. Intramolecular ipso S N Ar: CaO bond formation. Scheme 16. Intramolecular ipso S N Ar: CaO bond formation. 11.2 Ipso Nucleophilic Aromatic Substitutions 377 Scheme 27. Ipso S N Ar: CaC bond formation. Scheme 28. Ipso S N Ar: CaC bond formation. 11.2 Ipso Nucleophilic Arom...