Oxidative dimerization, chiral phenols

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

Other radical-based transformations are ruthenium-catalyzed oxidative dimerizations of phenols [263] and reductive dimerizations [264], The isomerization of chiral c/s-epoxides to tram-epoxides catalyzed by 2-10 mol% TpRu(py)2Cl proceeds at 100 °C in 95-98% yields with inversion of configuration [265], A radical or SN2 mechanism was discussed for this process. 

In nature, the oxidative dimerization of phenols is controlled by enzymes, as is demonstrated by the axial chirality of the 6,8 -coupled juglone derivative isodiospyrin. In synthesis, however, phenol oxidation only proceeds in high yields when the enzymatic reaction control is replaced by substituent control, that is, if all but one of the positions with high spin density in the radical (ortho-and para positions) are blocked. 

Wynberg studied stereochemistry of the McMurry reductive dimerization of camphor in detail (64). In Scheme 37, A and B are homochiral dimerization products derived by the low-valence Ti-promoted reduction, while C and D are achiral heterochiral dimers. The reaction of racemic camphor prefers homochiral dimerization (total 64.9%) over the diastereomeric heterochiral coupling (total 35.1 %). Similarly, as illustrated in Scheme 38, oxidative dimerization of the chiral phenol A can afford the chiral dimers B and C (and the enantiomers) or the meso dimer D. In fact, a significant difference is seen in diastereoselectivity between the enaritiomerically pure and racemic phenol as starting materials. The enantiomerically pure S substrate produces (S,S)-B exclusively, while the dimerization of the racemic substrate is not stereoselective. In the latter case, some indirect enantiomer effect assists the production of C, which is absent in the former reaction. Thus, it appears that, even though the reagents and reaction conditions are identical, the chirality of the substrate profoundly affects the stability of the transition state. 

Bringmann and co-workers have shown that modest control of atrop-selectivity can be achieved upon oxidative intermolecular coupling (dimerization) of the chiral phenol 205 [77]. In the synthesis of magistophorenes A and B (206a/b), the chiral precursor 205 afforded both isomers with a slight preference for the B series when exposed to di-tert-butyl peroxide (DTBP) (Scheme 51). 

Scheme 51. Di-tert-butyl peroxide-mediated oxidative dimerization of a chiral phenol precursor to the magistophorenes.

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

Using a different dimerization method, namely phenolic oxidation, chiral substrates react in a more stereoselective manner than under reductive conditions. The choice of oxidizing reagent may drastically affect the stereochemical outcome of the reaction. Thus, when potassium hexacyanoferrate(III) is used (17 )-l,2,3,4-tetrahydro-6-methoxy-l,2-dimethyl-7-isoquino-linol couples to give a mixture of atropisomers 3 in 38 % yield and with a d.r. (M)I(P) of 45 553,4. Only one single atropisomer, namely (A/)-3, is formed, in a 66% yield by anodic oxidation, which is attributed to electrode surface effects3. 

Other enantioselective reactions. Several asymmetric reactions worth mentioning are the Cu-cataly/.ed allylic oxidation in the presence of 105, 106, or 107- - with t-butyl perbenzoate, oxidation of sulfides (/-BuOOH-Ti ) in the presence of a 4,4 -dimer of B-aromatic l-hydroxyestrane,-" the reductive amination by chiral t-butylsulfinamidc,- the glyoxylate ene reaction promoted by Yb(OTf), and ent-l ) C-arylation ol phenols with aryllcad reagents under the influence of brucine,- and the C—H bond insertion by Rh-carbenoids."-"... 

The oxidation of ort/to-alkylphenols 49 with iodine(V) derivatives of type 42 containing chiral oxazoline moieties led to asymmetric [4+2] Diels-Alder dimerizations. The <9ri/io-alkylphenols 49 were transformed into ort/io-quinol dimers 50 with significant levels of asymmetric induction (up to 77% ee) (Scheme 21) [71], Similar substrates 51 were subjected to hydroxylative phenol dearomatization to give ort/io-quinol products 53 (Scheme 22) [72], The protocol was devised making use of the chiral iodoarene 52 in combination with mCPBA however, an... 

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