Dearomatizing agents

Such dearomatization of the arene ligand activates it toward an electrophilic addition. Thus, osmium(ll) was used as a dearomatization agent for the direct 10/3-alkylation of /3-estradiol 70 (equation 29). When the tautomeric mixture 71 72 was placed in acidic methanol and reprecipitated, a 3 1 equilibrium ratio of the phenolic 71 and dienone 72 tautomers was observed . This intermolecular Michael addition to the C(10) position of the aromatic steroid was unprecedented. 

A general review on rhenium complexes containing poly(pyrazolyl)borates appeared in 1998.110 In 2004, an overview of the progress involving group VII elements and actinide was presented by Santos and coworkers,111 who very recently described also rhenium and technetium complexes anchored by scorpionates prepared for radiopharmaceutical application.112 Keane and Harman described exhaustively the coordination chemistry of a new generation of Tt-basic dearomatization agents M(Tp)(L)(7t-acid). 113 Jones et al.114 reported a review on the role of unstable alkane... 

DFT methods have been employed to characterize bonding and charge distribution in [W(Tp)NO(L)J, and also in the analogous Re and Mo species. Structural details illustrate the ability of W, Re, and Mo species to dearomatize complexed benzene, which is extensive for all but the most weakly bound species with L = NCMe and CO. Re and W dearomatizing agents have been indicated as economic alternatives to osmium dearomatizing agents.195... 

Synthetic applications of the dearomatization agent pentaammineosmium(II) 01T8203. 

Of the handful of transition metal systems that are known to form stable q complexes with aromatic molecules [8-14], only d octahedral metal complexes have been shown to enhance the reactivity of the aromatic ligand toward electrophiles to date [15]. For nearly a decade, despite our best efforts, this mode of arene activation was known only for the pentaammineosmium(II) system. However, in the past few years a new generation of dearomatization agents have been developed based on a careful matching of the d /d reduction potential of rhe-... 

This chapter will initially cover several aspects of dihapto-coordination of aromatic molecules, including the scope of the dearomatization agent and the aromatic substrate. The primary focus of this work, however, will be the fundamental organic reactions of these complexes with electrophiles and the subsequent reactions of those products. Several applications of this methodology will also be illustrated. Owing largely to its earlier discovery, the majority of the organic transformations reviewed will be with osmium(II), however, recent arene transformations promoted with rhenium(I) and molybdenum(O) will also be discussed, with an emphasis on differences in reactivity compared to those of osmium. 

Treatment of isonicotinamide with LDA at 40°C and addition of an acylating or alkylating agent were reported in 2005 to form in good yield, a dearomatized product with spirocyclic p-lactam structure (Scheme 88), [195]... 

Benzimidazoles, too, are readily acylated and may also be converted by acylating agents into quaternary salts. In one such reaction the benzimidazole is eventually partially dearomatized to (68) on reaction with bis(l-chloroformyl)diphenylamine (Scheme 23) (76CC48). Under similar conditions imidazole is only acylated. 

Oil paints, terpene-lf ee (M-LL05) oils (linseed oil, wood oil, soya oil), natural resins and modified natural resins, mineral pigments, wetting agents, flow improvers, solvents isoaliphatics (dearomatized) covering varnishes, primers... 

Chiral oligotriazoles catalyze the reaction of quinohnes with acylating agents and nucleophiles to form dearomatized heterocycles in moderate... 

Arene oxidation leading to direct C—C bond formation allows rapid assembly of complex and ste-reochemically rich carbocyclic ring systems. Crucial to the success of this approach is the identification of carbon nucleophiles that are stable in the presence of oxidation agents typically used to effect arene dearomatization. Enolates and enol ethers are problematic as these species undergo rapid oxidation under mild conditions [62]. Stabilized enolates (such as those derived from activated methylenes) exhibit greater compatibility with oxidation conditions and have been used as nucleophilic participants in intramolecular oxidative dearomatizations initiated by [Fe(CN)g] and PIDA to afford spirocyclic cyclohexadienones [63, 64]. Detailed mechanisms for these reactions have not been defined so it is unclear whether bond formation occurs through ionic or radical intermediates. 

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