Chiral hypervalent iodine catalyst

The first ever chiral hypervalent iodine compound, diphenyliodonium tartrate, was reported in 1907 by Pribram [12]. However, the use of hypervalent iodine compounds in asymmetric synthesis has only been explored over the last decade. Access to asymmetric reactions can be obtained either through the use of chiral hypervalent iodine reagents or by using achiral hypervalent iodine compounds in combination with chiral ligands. Hypervalent iodine compounds are utilized in these reactions either in stoichiometric amounts or as catalysts. Various asymmetric transformations are achieved with moderate to excellent enantioselectivity with these reagents. This review is divided into different sections based on the type of transformations involved ... 

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 Wirth developed the first example of the a-oxysulfonylation of ketones using hypervalent iodine catalysts with mCPBA and TsOH (Scheme 19.17) [117]. The best results were achieved with precatalysts containing esters with two chiral centers however, long reaction times of up to 2-4 days were needed. The reaction is proposed to proceed through a chiral Koser-type chiral hypervalent iodine active catalyst (ArlOTs) formed from the reaction of the precatalyst 39 with mCPBA and TsOH. 

Ketones can be a hydroxylated in good yields, without conversion to the enolates, by treatment with the hypervalent iodine reagents162 o-iodosobenzoic acid163 or phenyliodoso acetate PhI(OAc)2 in methanolic NaOH.164 The latter reagent has also been used on carboxylic esters.165 02 and a chiral phase transfer catalyst gave enantioselective a hydroxylation of ketones, if the a position was tertiary.166... 

Sulphides are oxidized readily by most hypervalent iodine reagents. IOB alone is not suitable, since mixtures of sulphoxides and sulphones are formed under drastic conditions. However, in the presence of catalytic amounts of p-toluenesulphonic acid [49] or benzeneseleninic acid [50] various sulphides were cleanly oxidized to sulphoxides in excellent yields. Using a chiral catalyst asymmetric oxidation was highly successful [51]. 

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. 

In the presence of a chiral catalyst such as rhodium(II) (5)-/V l,8-naphthanoyl-tert-leucinate dimer, Troc-amino indane was produced with 56% yield and 2.57 1 enantiomeric ratio. In contrast to other methods, no hypervalent iodine reagent (typically used stoichiometrically or in excess and forming iodobenzene as by-product) is required for oxidation of the amine component. However, a slight excess of the aromatic alkane component (5 equiv) must be used to achieve good conversions. The reactivity of rhodium nitrenes generated from 2,2,2-trichloroethyl-/V-tosyloxycarbamate with aliphatic alkanes is similar to the one observed with metal nitrenes obtained from the oxidation of sulfamate with hypervalent iodine reagent. Troc-protected amino cyclohexane and cyclooctane were obtained, respectively, in 73 and 62% yields when 2 equiv of alkanes was used, whereas yields up to 85% were observed with 5 equiv (eq 3). 

Alkyl-4-oxy-3,4-dihydroisocoumarins are enantioselectively prepared by oxylactonization ofo-(alk-l-enyl)benzoates promoted by the in situ-generated chiral lactate-based hypervalent iodine(III) catalysts (13EJ07128). Chemoenzymatic synthesis of 3,4-dialkyl-3,4-dihydroisocoumarins involves one-pot dynamic kinetic reductive resolution processes catalyzed by E. co/i/alcohol desidrogenase. This strategy consists in the bioreduction of various racemic ketones to the corresponding enantiopure alcohols followed by intramolecular acidic cyclization (Scheme 71) (130L3872). 

MacMillan et al. studied the reactivity of transient enamines formed with organocatalysts with hypervalent iodine reagents in the presence of metal catalysts (Scheme 26.5a). They found that Cu(l) salts can be used in combination with chiral imidazolidinones to perform a-electrophilic trifluoromethylation [47], arylation [48], and vinylation [49] of aldehydes. As suggested by the authors, the most likely mechanism involves a copper-mediated I-O bond cleavage furnishing a highly... 

To date, this chemistry remains rather unexplored with respect to the development of related transition metal catalyses [43]. Still, difunctimialization of alkenes with hypervalent iodine reagents has been explored extensively over the past few decades [44], and there are important recent contributions that indicate that hypervalent iodines can indeed serve as suitable chiral reagents or catalysts for enantio-selective oxidation of alkenes [43-46]. 

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