Oxidative dearomatization catalytic

Hypervalent iodine induced oxidative dearomatization of orf/io-substituted phenolic substrates in the intramolecular mode has been realized as an enantioselective reaction. In particular, Kita and coworkers have developed the enantioselective spirocyclization reaction of the orfho-substituted phenolic substrates 275 using chiral aryliodine(III) diacetate 276 having a rigid spirobiindane backbone (Scheme 3.115) [346]. Similar enantioselective oxidative spirocyclization reactions of the ort/io-substituted phenolic substrates under catalytic conditions in the presence of chiral iodoarenes or chiral quaternary ammonium iodide catalysts are discussed in Sections 4.1.6 and 4.4. 

The oxidative dearomatization of appropriately substituted phenolic substrates resulting in intramolecular cyclization with the formation of spirocyclic products represents one of the most powerful synthetic tools in modern organic synthesis (Seetion 3.1.11). Kita and coworkers were the first to report a catalytic variant of the oxidative spirocyclization reaction based on the in situ regeneration of a [bis(trifluoroacetoxy)iodo]arene from iodoarene using mCPBA as a terminal oxidant [2]. In a representative example, the oxidation of... 

Scheme 9.1 Catalytic asymmetric conjugate addition/oxidative dearomatization.

Rudolph, A., Bos, P H., Meetsma, A., Minnaard, A. J., Feringa, B. (2011). Catalytic asymmetric conjugate addition/oxidative dearomatization towards multifunctional spirocyclic compounds. Angewandte Chemie International Edition, 50, 5834-5838. 

Deprotonation of a pyridinylmethylenic proton of pyridine- and bipyridine-based pincer complexes can lead to dearomatization. The dearomatized complexes can then activate a chemical bond (H-Y, Y = H, OH, OR, NH2, NR2, C) by cooperation between the metal and the ligand, thereby regaining aromatization (Figure 1.1). The overall process does not involve a change in the metal s oxidation state [6-8]. In this chapter, we describe the novel, environmentally benign catalytic synthesis of esters, amides, and peptides that operate via this new metal-ligand cooperation based on aromatization-dearomatization processes. 

The initial realization involved a catalytic system of 4-tolyl iodide and mCPBA as terminal oxidant, which generates a hypervalent iodine reagent that promotes the formation of a cationic nitrogen. Subsequent nucleophilic attack of the anisole ring engages in C-N bond formation and ultimately in dearomatization. 

Phenolic oxidations are pivotal steps frequently involved in the biosynthesis of natural products, which possess a variety of important biological activities. Therefore, a continuing interest exists in such transformations, in particular in asymmetric oxidative protocols. Kita et al. performed asymmetric dearomatization of naphthols 43 mediated by chiral hypervalent iodine(III) reagents, 33 and 45 having a rigid spirobiindane backbone (Scheme 20) [66, 67]. A series of other ortho-functionalized spirobiindane reagents of type 46 were synthesized. Intramolecular oxidative substitution of 43 afforded five-membered spirolactone 44 with good levels of enantioselectivity (up to 92% ee). Conformationally flexible iodoarenes employed in this study produced almost racemic products. Catalytic use of these chiral catalysts with wCPBA as cooxidant afforded the chiral spirolactones without detrimental effects on the ee values. 

In 2008 Kita developed a procedure for the asymmetric dearomatization of naph-thols via the formation of ortho-spirolactones using a C2-symmetrical chiral precatalyst 36 (Figure 19.11) and co-oxidant mCPBA with acetic acid [115] (Scheme 19.16). This reaction proceeds via the mCPBA/acetic acid-mediated oxidation of precatalyst 36 to the hypervalent iodine(III) active catalytic species 37. Ishihara has taken this work further with the development of the conformationally more flexible chiral organo-iodine precatalyst 38 [116], offering increased enantioselectivity at... 

An interesting system for the catalytic aerobic oxidation of benzyl alcohol was developed by Albrecht and co-workers. A porphyrin rhodium(iii) complex [(IMe)2Rh(TPP)]Cl (TPP = meso-tetraphenyl porphyrin) with two apical NHC ligands led to porphyrin distortion and dearomatization. This coordi-natively saturated complex was catalytically active in the oxidation of benzyl alcohol. It was demonstrated that its catalytic activity was imparted by NHC dissociation, as facile cleavage of the Rh-NHC bond was promoted by the strong trans effect of the distal NHC ligand and by the distorted and partially dearomatized porphyrin. 

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