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DOI: 10.1055/sos-SD-002-00214

Kündig, E. P.; Pache, S. H.Science of Synthesis, (20032163.

Arene exchange reactions in (arene)tricarbonylmetal complexes are equilibrium reactions, and a large excess of the incoming arene may be required to drive the reaction to the product. Equilibria are reached faster in the presence of coordinating solvents. For chromium, elevated temperatures are required for exchange (>140°C in tetrahydrofu­ran)[‌75‌] and concomitant decomposition of complexes under these conditions is a problem for all but simple alkylarenes. For this reason, arene exchange in [Cr(CO)3(arene)] complexes is not a useful synthetic method except for labile complexes such as tricarbonyl(naphthalene)chromium(0) (14).[‌27‌] Complex 14 and other tricarbonylchromium complexes of fused aromatics undergo intramolecular haptotropic rearrangements on heating to ca.100°C.[‌76‌‌79‌] Theoretical analysis for the degenerate rearrangement of 14 favors a reaction pathway via an exocyclic intermediate.[‌80‌] In the absence of a Lewis basic solvent, arene exchange in 14 is about 20 times slower than the intramolecular rearrangement. However, Lewis basic solvents considerably accelerate arene exchange. In tetrahydrofuran it occurs slowly already at ambient temperature, and rapidly on heating to 80°C. The mechanism involves haptotropic slippage of the naphthalene ligand (change from η6- to η4- or η2-coordination), thus facilitating the dissociation and coordination of the new arene.[‌81‌] The mild conditions are particularly suited for diastereoselective complexation reactions (see Section A variant is the in situ generation of the naphthalene complex under conditions of arene exchange.[‌82‌] Facile arene exchange also occurs with tricarbonyl(1-methylpyrrole)chromium(0) (see Section 2.4.9).[‌83‌] This has not found widespread application yet because this complex is not easy to handle (it is pyrophoric and is best handled under an inert atmosphere in a glove box). Mild arene exchange also occurs upon generation of a benzylic cation (see Section,[‌84‌] or catalyzed by iodine (the reaction presumably passes via formation of a labile 17-electron complex).[‌85‌] This has not been used much but may have considerable potential. Finally, an unusual synthesis of tricarbonyl(styrene)chromium(0) (20) consists of co-condensing chromium atoms (see Section 2.4.1) with styrene at 196°C followed by warm up under a carbon monoxide atmosphere.[‌86‌] With reference to the stepwise displacement of naphthalene in bis(naphthalene)chromium(0) (2, M=Cr) by carbon monoxide,[‌14‌] the intermediacy of the styrene sandwich complex is likely.

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Meeeeeeeeee(η8-eeeeeeeeeeeeeeeeeeeee)eeeeeeeeee(8) (8, M=Me; M8=MMM):[‌88‌]

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