Metal-arene complexes reaction with nucleophiles

The attachment of a transition metal to an unsaturated hydrocarbon ligand transforms the reactivity properties of the ligand. Whereas the classic chemistry of alkenes and arenes is that of electrophilic addition and substitution reactions, the classic reactivity of multihapto-complexes is their reactions with nucleophiles. Such complexes need not be cationic to be good electrophiles, but it helps, and not surprisingly, the most powerful electrophilic multihapto-complexes have positive charge stabilized by the metal. 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Some Schrock-type carbene complexes, i.e. high-valent, electron-deficient, nucleophilic complexes of early transition metals, can undergo C-H insertion reactions with simple alkanes or arenes. This reaction corresponds to the reversal of the formation of these carbene complexes by elimination of an alkane (Figure 3.36). 

Nucleophilic aromatic substitution of activated substrates (pentafluoropyridine, octafluorotoluene, hexafluorobenzene, metal arene jc-complexes) by anions of diethyl cyanomethylphosphonate has been achieved. The reaction is carried out with satisfactory yields in DMF, MeCN, or THF at room temperature in the presence of NaH, CsF, KjtX), or CS2CO3." In a similar manner, 3,6-dihalopyridazines react with sodium diethyl cyanomethylphosphonate in refluxing THF to give phosphonosubstituted pyridazines in 22-68% yields. ... 

The synthetic methodology outlined in this chapter is fundamentally different than that described elsewhere in this book. It is based on a transition metal acting as an electron-donor (i.e., a 71 base) for the aromatic ligand. Thus, reactions with electrophiles are promoted, and the resulting arenium systems are stabilized such that a nucleophile may then be added. In this regard, the reactions outlined in this chapter complement the more established q -arene methodology. Whereas the chromium q -arene complexes have been known for over 40 years, viable q -arene complexes have been known for the past 15, and only recently has an alternative to osmium emerged. Ultimately the impact that 71 bases... 

The use of the temporary complexation of the aromatics by Cr(CO)3 is illustrated on the scheme below. The electrophilic reactions are more difficult, and the nucleophilic reactions are easier than with the free arene. In addition, the nucleophilic reactions on the ring or on the arene substituent benefit from the stereospecificity of the exo (trans) attacks, with respect to the metal, due to the bulk of the Cr(CO)3 fragment. When the arene has been functionalized by temporary complexation to Cr(CO)3, it can be removed from the metal using solar-light irradiation. All these properties have been largely exploited due to the stability and ease of synthesis of these complexes. These properties will now be detailed (see also Chap. 21.2). 

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