Asymmetric trans-stilbene

Scheme 4-51. Asymmetric epoxidation of trans-stilbene with catalyst 135. Substrate (E )-stilbene. Substrate ( )-4,4 -diphenylstilbene. Reprinted with permission by Am. Chem. Soc., Ref. 105a, 106.

A very simple yet elegant method for efficient epoxidation of aromatic and aliphatic alkenes was presented by Beller and coworkers [63, 64], FeCl3 hexahydrate in combination with 2,6-pyridinedicarboxylic add and various organic amines gave a highly reactive and selective catalyst system. An asymmetric variant (for epoxidations of trans-stilbene and related aromatic alkenes) was published recently [65] using N-monosulfonylated diamines as chiral ligands (Scheme 3.7). 

Cavallo et al. from (+)-dihydrocarvone and evaluated in the asymmetric epoxida-tion of several silyl enol ethers [32]. Enantiomeric excess up to 74% was achieved in the epoxidation of the TBDMS trans-enol ether of desoxybenzoin with the fluoro ketone 19d (30 mol% of the ketone catalysts). In earlier work Solladie-Cavallo et al. had shown that the fluoro ketones 19a and 19e can be used to epoxidize trans-stilbene with up to 90% ee (30 mol% ketone catalyst) [33], Asymmetric epoxidation of trans-methyl 4-para-methoxycinnamate using ketone 19e as catalyst is discussed in Section 10.2. 

As summarized in Table 10.4, further studies by Armstrong et al. revealed that asymmetric epoxidation of trans-stilbene can also be effected by other bicydic ketones (25-28) carrying electron withdrawing substituents in both the a-position and on the 1-bridge [35],... 

In the course of their exploration of structure-activity relationships for ketone catalysts, Denmark et al. noted that oxoammonium salts such as 29-33 are very efficient catalysts of the epoxidation of olefins [34a]. Unfortunately, enantiomeric excesses achieved with this class of ketone catalyst have not yet exceeded 40% (30, epoxidation of tram-fl-rn eth yI styrene . With the fhiorinated oxoammonium catalyst 23 already mentioned, however, 58% ee was achieved in the asymmetric epoxidation of trans-stilbene [34b]. 

Fig. 10.1 Selected chiral sulfides and results obtained using alkylation/ deprotonation catalytic methodology for the asymmetric synthesis of trans-stilbene oxide, dr = trans cis solvents and additives vary.

The successful example of catalytic asymmetric chiral sulfonium epoxidation was reported with enantiomeric excess (43%) of trans-stilbene oxide (3.71) by the reaction of 4-chlorobenzaldehyde with benzyl bromide in acetonitrile at room temperature by using 0.5 equiv. of optically active sulfide 3.70 (with exo-OH group). Powdered KOH was used as a base. f... 

The first use of an enantiomerically pure oxaziridinium salt to catalyze asymmetric epoxidation (trans stilbene oxide produced with 33% ee using 61 (Figure 13)) was reported by Lusinchi and co-workers in 1993 <1993TL7271> Subsequently, it was reported that phenylcyclohexene is converted to the corresponding epoxide with just 5% ee using stoichiometric quantities of 61 <1999T141>. 

By employing polymer-bound alkaloid derivatives, heterogeneous catalytic asymmetric dihydroxylation has been achieved with good to excellent enantioselectivities in the dihydroxylation of trans-stilbene. These polymers can be recovered and reused while both the yields and the optical purities of diols were maintained. 

Another breakthrough came several years later, when the photoadduct 84 of trans stilbene with chiral bornyl methyl fumarate 82 was obtained with a high diastereomeric excess [60], Here again, a model involving an approach of the reagents in parallel planes was proposed to explain the observed stereoselectivity (Scheme 19). In an attempt to increase the observed de, the cycloaddition reaction of dibornyl fumarates was examined, but a far lower selectivity was observed. On this basis, a multistep process was proposed with control of the asymmetric induction by the rate of cyclization of the 1,4-biradical intermediates. The nature of the substituents, however, the complexity of the reaction mixture, and the low chemical yields of the chiral adducts are major limitations for synthetic applications [61],... 

Kola (Cola acuminata) extract astringent, skin treatment Zinc oxide astringent, toners Kola (Cola acuminata) extract astringent, topical Aluminum chloride hexahydrate astringent, topical hexahydrate Aluminum chloride anhydrous astringent, veterinary medicine Lead acetate trihydrate asymmetric epoxidation, transition metal catalyzed trans-Stilbene a-terpineol precursor 2-(4-Methyl-3-cyclohexenyl)-2-propanol atmosphere protectant, casting magnesium alloys... 

One of the features of stilbene photochemistry is its essentially strong dependence on medium polarity and temperature the competition between fluorescence and trans-cis isomerization has been shown to be extremely sensitive to medium viscosity. Solvent polarity can affect both the dynamics and the pathway of the reaction. The dipolar character of asymmetrically substituted stilbenes and polarizability of the traws-stilbene transition state can explain the sensitivity of the photoisomerization rate to medium polarity [5, 6, 12, 31, 66-69]. 

The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 31 in combination with either iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trans-stilbene [130]. In the best result, using PhI(OAc)2 as oxidant, they obtained trans-stilbene oxide in 80% yield and 63% ee. More recently, Beller and coworkers have reexamined this catalytic system and found that asymmetric epoxidations could be performed using ruthenium catalysts 30 and 31 and 30% aqueous hydrogen peroxide [131-133]. A development of the pybox ligand led to ruthenium complex 32, which turned out to be the most efficient catalyst for asymmetric alkene epoxidation. Thus, using 5 mol% of 32 and slow addition of hydrogen peroxide, a number of aryl substituted alkenes were epoxidized in yields up >99% and enantioselectivity up to 84% (Scheme 2.25). 

Only low enantioselectivities (12%) were obtained for the epoxidation of trans-stilbene or tran -P-methylstyrene despite high catalyst loadings (20-3(K) mol%). During the past few years, new and better chiral ketones have been reported. In order to better describe the development of ketone catalysts for the asymmetric epoxidation, these will be divided into groups of different structural features. 

In 2002, Shing et al. reported glucose-derived ketones 391 and 392. Ketone 391 epoxidizes tran -stilbene with up to 71% ee [277]. A series of L-arabinose-derived ketones 393-399 followed, and up to 90% ee was obtained for tran -stilbene epoxide with ketone 396 [278], In the same year, Zhao et al. reported three fructose-derived ketones and aldehydes 400-402 for the asymmetric epoxidation [279], Aldehyde 402 achieved 94% ee for tran -stilbene. hi 2009, Davis et al. presented a variety of conformationally restricted ketones 403, prepared from A-acetyl-D-glucosamine which show useful selectivities with terminal olefins (styrene 81% ee. Fig. 7.19) [280]. 

On the other hand, the addition of a quaternary ammonium salt to the reaction medium accelerates the isomerization of the radical intermediate [36]. Thus, the epoxidation of ct j -stilbene in the presence of A -benzylquinine salt gives trans-stilbene oxide with 90% ee as major product (Table 6B.1, entry 24). This protocol provides an effective method for the synthesis of trans-epoxides. In contrast to the epoxidation of cts-di- and tri-substituted olefins for which complexes 11-13 are the catalysts of choice, the best catalyst for the epoxidation of tetra-substituted conjugated olefins varies with substrates (Table 6B.1, entries 27 and 28) [37]. The asymmetric epoxidation of 6-bromo-2,2,3,4-tetramethylchromene is well-promoted by complex 14 and that of 2-methyl-3-phenylindene, by complex 12a. 

As shown in cycle (b) in Scheme 10.1, the iminium-oxaziridinium pair can also effect catalytic asymmetric epoxidation of alkenes. Early work in this field by Bohe et al. included investigation of the norephedrine-derived oxaziridinium salt 34 (33% ee in the catalytic epoxidation of traws-stilbene [41] ee up to 61% was achieved when 34 was employed stoichiometrically [42]), or the L-proline-derived material 35 (39% ee in the epoxidation of trans-3-phenyl-2-propenol [43]). Rapid... 

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