Asymmetric aerobic

Although the chiral ketoiminatomanganese(lll) complexes were reported to catalyze the asymmetric aerobic alkene epoxidations, an aldehyde such as pivalaldehyde is required as a sacrihcial reducing agent. Groves reported that the dioxo(porphyrinato)ruthenium complexes 31, prepared with m-chloroperoxyben-zoic acid, catalyzed the aerobic epoxidation without any reductant. " On the basis of these reports, Che synthesized the optically active D4-porphyrin 35 and applied it to the truly aerobic enantioselective epoxidation of alkenes catalyzed by the chiral frani-dioxo (D4-porphyrinato)ruthenium(Vl) complex. The dioxoruthenium complex catalyzed the enantioselective aerobic epoxidation of alkenes with moderate to good enantiomeric excess without any reductant. In the toluene solvent, the turnovers for the epoxidation of T-(3-methylstyrene reached 20 and the ee of the epoxide was increased to 73% ee. 

Chiral A-salicylidene vanadyl carboxylates are efficient catalysts for asymmetric aerobic oxidation of a-hydoxy esters and amides with divergent substituents. These catalysts have been explored for the kinetic resolution of secondary alcohols also. The stereochemical origin of the almost total asymmetric control has been probed.262... 

Trend, R. M., Ramtohul, Y. K., Ferreira, E. M., Stoltz, B. M. Palladium-catalyzed oxidative Wacker cyclizations in nonpolar organic solvents with molecular oxygen A stepping stone to asymmetric aerobic cyclizations. Angew. Chem., Int. Ed. Engl. 2003,42, 2892-2895. 

In 2003, Stoltz at CalTech described a palladium-catalyzed oxidative Wacker cyclization of o-allylphenols such as 55 in nonpolar organic solvents with molecular oxygen to afford dihydrobenzofurans such as 56.44 Interestingly, when (-)-sparteine was used in place of pyridine, dihydrobenzofuran 56 was produced asymmetrically. The ee reached 90% when Ca(OH)2 was added as an additive. Stoltz considered it a stepping stone to asymmetric aerobic cyclizations. In 2004, Mufiiz carried out aerobic, intramolecular Wacker-type cyclization reactions similar to 55—>56 using palladium-carbene catalysts.45 Hiyashi et al. investigated the stereochemistry at the oxypalladation step in the Wacker-type oxidative cyclization of an o-allylphenol. Like o-allylphenol, o-allylbenzoic acid 57 underwent the Wacker-type oxidative cyclization to afford lactone 58.47... 

Table 3.1 Asymmetric aerobic oxidative biaryl coupling reactions reported by Nakajima.

Scheme 3.4 Proposed mechanism for the asymmetric aerobic oxidative hiaryl coupling reactions.

Scheme 3.5 An overall catalytic cycle proposed for the asymmetric aerobic oxidative biaryl coupling catalyzed by chiral Cu-diamine complex.

Table 3.3 Asymmetric aerobic oxidative biatyl coupling reactions reported by Sekar.

In 2000, Katsuki and co-workers applied the chiral chloro nitrosyl Ru -(salen) complex developed in their own group to asymmetric aerobic oxidative biaryl coupling reactions (Scheme 3.14). The reaction was found to proceed smoothly in air under irradiation with a halogen lamp as the light source at room temperature. Examination of a series of Ru (salen) complexes revealed that the combination of (R,R)-diamine unit and (R)-BINOL scaffold in the catalyst is important for achieving higher enantioselectivity. The absolute configuration of the major product is determined by the chirality of the BINOL scaffold whereas the structural variation in the diamine part shows little influence on asymmetric induction. Under the optimal conditions, several 2-naphthols with a substituent at the C6 position of the naphthalene... 

Tanaka, H., Nishikawa, H., Uchida, T., etal. (2010). Photopromoted Ru-Cataly zed Asymmetric Aerobic Sulfide Oxidation and Epoxidation Using Water as a Proton Transfer Mediator, J. Am. 

Egami, H. and Katsuki, T. (2009). Iron-Catalyzed Asymmetric Aerobic Oxidation oxidative CoupUng of 2-Naphthols, J. Am. Chem. Soc., 131, pp. 6082-6083. 

Scheme 18 Asymmetric aerobic oxidation/double [3-1-2] cycloaddition of azomethine ylides with cyclopentadiene...

Chiral N-sulfonyldiamine ligands are used to create effective chiral bifunctional amidoiridium catalysts for the asymmetric aerobic oxidation of meso- and prochiral diols to give up to >99% ee of hydroxyl ketones and 50%ee oflactones. " These catalysts can be also applied for an efficient oxidative kinetic resolution of racemic secondary alcohols affording R enantiomers with >99% ee and with 46—50% yields. 

Scheme 5.12 Asymmetric aerobic epoxidation of olefins with chiral ruthenium complex 15.

Resting cell of G. candidum, as well as dried cell, has been shown to be an effective catalyst for the asymmetric reduction. Both enantiomers of secondary alcohols were prepared by reduction of the corresponding ketones with a single microbe [23]. Reduction of aromatic ketones with G. candidum IFO 5 767 afforded the corresponding (S)-alcohols in an excellent enantioselectivity when amberlite XAD-7, a hydro-phobic polymer, was added to the reaction system, and the reduction with the same microbe afforded (R)-alcohols, also in an excellent enantioselectivity, when the reaction was conducted under aerobic conditions (Figure 8.31). 

The complex Pd-(-)-sparteine was also used as catalyst in an important reaction. Two groups have simultaneously and independently reported a closely related aerobic oxidative kinetic resolution of secondary alcohols. The oxidation of secondary alcohols is one of the most common and well-studied reactions in chemistry. Although excellent catalytic enantioselective methods exist for a variety of oxidation processes, such as epoxidation, dihydroxy-lation, and aziridination, there are relatively few catalytic enantioselective examples of alcohol oxidation. The two research teams were interested in the metal-catalyzed aerobic oxidation of alcohols to aldehydes and ketones and became involved in extending the scopes of these oxidations to asymmetric catalysis. 

In a very recent work, the Pd-catalysed cross-coupling reactions with arenediazonium salts under aerobic conditions in the presence of a chiral monothiourea ligand were reported (Scheme 25) [106]. Even if this Hgand bears four chiral centres, no test in asymmetric Heck-type reaction has been described so far. 

Oxidizing enzymes use molecular oxygen as the oxidant, but epoxidation with synthetic metalloporphyrins needs a chemical oxidant, except for one example Groves and Quinn have reported that dioxo-ruthenium porphyrin (19) catalyzes epoxidation using molecular oxygen.69 An asymmetric version of this aerobic epoxidation has been achieved by using complex (7) as the catalyst, albeit with moderate enantioselectivity (Scheme 9).53... 

Asymmetric induction has also been achieved in the cyclization of aliphatic alcohol substrates where the catalyst derived from a spirocyclic ligand differentiates enantiotopic alcohols and alkenes (Equation (114)).416 The catalyst system derived from Pd(TFA)2 and (—)-sparteine has recently been reported for a similar cyclization process (Equation (115)).417 In contrast to the previous cases, molecular oxygen was used as the stoichiometric oxidant, thereby eliminating the reliance on other co-oxidants such as GuCl or/>-benzoquinone. Additional aerobic Wacker-type cyclizations have also been reported employing a Pd(n) system supported by A-heterocyclic carbene (NHC) ligands.401,418... 

Bolm et al. (130) reported the asymmetric Baeyer-Villiger reaction catalyzed by Cu(II) complexes. Aerobic oxidation of racemic cyclic ketones in the presence of pivalaldehyde effects a kinetic resolution to afford lactones in moderate enan-tioselectivity. Aryloxide oxazolines are the most effective ligands among those examined. Sterically demanding substituents ortho to the phenoxide are necessary for high yields. Several neutral bis(oxazolines) provide poor selectivities and yields in this reaction. Cycloheptanones and cyclohexanones lacking an aryl group on the a carbon do not react under these conditions. 

The attractive (80) features of MOFs and similar materials noted above for catalytic applications have led to a few reports of catalysis by these systems (81-89), but to date the great majority of MOF applications have addressed selective sorption and separation of gases (54-57,59,80,90-94). Most of the MOF catalytic applications have involved hydrolytic processes and several have involved enantioselec-tive processes. Prior to our work, there were only two or three reports of selective oxidation processes catalyzed by MOFs. Nguyen and Hupp reported an MOF with chiral covalently incorporated (salen)Mn units that catalyzes asymmetric epoxidation by iodosylarenes (95), and in a very recent study, Corma and co-workers reported aerobic alcohol oxidation, but no mechanistic studies or discussion was provided (89). 

With the bisoxazoline hgand (S)-Phbox and CuCl, the asymmetric oxidative couphng of 2-naphthol and hydroxy-2-naphthoates resulted in an asymmetrically substituted 2,2 -binaphthol with ee s of up to 65% [260]. On the basis of the previous results obtained with this catalyst system, the asymmetric oxidative cross-coupling polymerization of 2,3-dihydroxynaphthalene [261] and methyl 6,6 -dihydroxy-2,2 -binaphthalene-7,7 -dicarboxylate [262] as well as the copolymerization of 6,6 -dihydroxy-2,2 -binaphthalene and dihexyl 6,6 -dihydroxy-2,2 -binaphthalene-7,7 -dicarboxylate with Cu diamine catalysts were carried out imder aerobic conditions, using O2 as the oxidant, and a cross-coupling selectivity of 99% was achieved [263]. 

In addition to the development of new catalysts and reaction conditions for aerobic oxidative heterocycUzation, considerable effort has been directed toward asymmetric transformations. Hosokawa and Murahashi reported the first example of asymmetric Pd-catalyzed oxidative heterocycUzation reactions of this type [157,158]. They employed catalytic [(+)-(Ti -pinene)Pd (OAc)]2 together with cocatalytic Cu(OAc)2 for the cycUzation of 2-allylphenol substrates however, the selectivity was relatively poor (< 26% ee). 

The possibility of asymmetric induction under the fluorous biphase conditions was first speculated upon by Horvath and Rabai [10], and this year has seen the first report of asymmetric catalysis in a fluorous biphase [69]. Two, C2 symmetric salen ligands (29a, b) with four C8Fi7 ponytails have been prepared (Scheme 5) and their Mn(II) complexes evaluated as chiral catalysts for the aerobic oxidation of alkenes under FBS-modified Mukaiyama conditions. Both complexes are active catalysts (isolated yields of epoxides up to 85%) under unusually low catalyst loadings (1.5% cf. the usual 12%). Although catalyst recovery and re-use was demonstrated, low enantioselectivities were observed in most cases. 

---