Reactions enantioselective vinylation

The literature presents a large number of examples concerning the use of known oxazolidinones as chiral auxiliaries in many kinds of reactions. Rare is the use of A-amino derivatives of oxazolidinones, which were used to synthesise new A-acylhydrazones 207. Radical addition reactions occurred with high diastereoselectivity <00JA8329>. The use of glycolate oxazolidinones 210 proved to be efficient for the enantioselective preparation of a-alkoxy carboxylic acid derivatives . Photochemical reaction of vinyl... 

Figure 3.49. Chiral ligands that exhibit high enantioselectivity in the nickel-catalyzed Grignard cross-coupling reaction of vinyl bromide (or chloride) with 1-phenylethylmagne-sium chloride.

This chemistry was extended to a catalytic enantioselective alkenylation and phenylation of aldehydes and a-ketoesters. Using CuF-DTBM-SEGPHOS complex, products were obtained with excellent enantioselectivity from a wide range of aldehydes including aromatic and aliphatic aldehdyes, [Eq. (13.26)]. Previously catalytic enantioselective vinylation and phenylation are restricted using the corresponding zinc reagents. The active nucleophile is proposed to be an alkenyl or phenyl copper, based on NMR studies. The chiral CuF catalyst can also be applied to a catalytic enantioselective aldol reaction to ketones... 

In a similar investigation, transesterification reactions of vinyl acetate with alcohols in [BMIM]BF4 and [BMIM]PF6 in the presence of immobilized lipases CALB and PS-C were found to proceed with higher enantioselectivities than in THF or toluene, with the best result again being observed with [BMIM]PF6 (280). 

In order to obtain high enantioselectivities it seems to be important that the reactions proceed via cationic intermediates (e.g. 24) to disfavor the partial dissociation of the chiral ligand. For this reason, silver salts are added to reactions of vinyl-iodides. These reactions are best performed in Al-methylpyrrolidone (NMP) as a solvent. As the examples shown in Scheme 9 demonstrate, both c/s-decalin [12] and cA-hydrindane [13] derivatives can be obtained in useful yields and enantiomeric purities. 

This titanocene-catalyzed procedure was immediately extended by Gansauer et al. to the enantioselective opening of meso-epoxides by employing substoichiometric quantities of titanocene complexes with chiral hgands [58-60]. It has also been applied by this group in racemic form not only for reductive epoxide openings and intermolecular additions to a,P-unsaturated carbonyl compounds, but also to achieve 3-exo, 4-exo, and 5-exo cycliza-tions, as well as tandem cyclization addition reactions featuring vinyl radicals (Scheme 9) [8,9,44,46,57,61-65]. 

Very recently, Lu and coworkers successfully applied aminocatalysis via the enamine intermediate to the Michael addition of cyclic ketones to vinyl sulfones 181 [56]. In the presence of the cinchonidine-derived primary amine salt 179, the Michael reactions between vinyl sulfones 181 and cyclic ketones 180 proceeded smoothly, affording the desired adduct 182 in very high yield and with excellent enantioselectivity (up to 97% ee) (Scheme 9.63). They also successfully applied this methodology to the synthesis of sodium cyclamate. However, this protocol gave poor yields and ee values for acyclic ketones. 

Recently, Jorgensen reported the first example of a catalytic enantioselective vinylic substitution reaction (Scheme 11.14). With a bulky 1-adamantylcarbonyl group modified phase-transfer catalyst lOd as the catalyst, the reaction between alkyl cyclopentanone-2-carboxylates (53a) with (ZJ-P-cholro-l-phenylpropenone (63a) proceeded smoothly, affording the product 64a with Z/f > 95 5 and 94% ee [50]. As for the trisubstituted alkene 64b, the a-iodine atom was tolerated in the catalytic reaction. 

Scheme 2.28 Enantioselective Michael reaction using vinyl phosphonates as Michael acceptors.

A series of aminal-pyrrolidine-derived organcatalysts was synthesised by Alexakis and coworkers. ° Initially, this family of aminal catalysts was evaluated for catalysing the Michael addition of aldehydes to nitro-olefins. When Ik was used, the products obtained had moderate to good stereoselectivities (up to 79% ee) at ambient temperature (Scheme 9.17). The reaction of vinyl-sulfone with linear aldehydes yielded more satisfactory results. Impressive reactivity and good enantioselectivities were obtained when only 10 mol% of Ik was used and representative examples are presented in Scheme 9.17. The stereocontrol exhibited by the catalyst was possibly associated with the strong interaction between the sulfonyl group and the aminal catalyst. 

In 2006, Tan and co-workers [88] synthesized a series of chiral imidazolines and found 38 was suitable catalyst for asymmetric BH reactions of vinyl alkyl ketones with aryl aldehydes, which afforded the BH adducts in moderate to high yields and with up to 78% ee in toluene. Recently, Bugarin and Connell [89] applied the Fu s chiral DMAP 39 as catalyst in the MgL-mediated enantioselective BH reaction of cyclopentenone and aldehydes, giving the adducts in good yields and enantioselectivities. 

The same study was performed with chiral enantiopure ligands to investigate the enantioselective version of the reaction, giving vinylic P-stereogenic phosphine boranes (Scheme 6.18). ... 

By analogy to the DYKAT reactions with vinyl epoxides, Trost et al. have developed the Pd-eatalysed asymmetric dynamic kinetic allylic amination and acyl migration of vinyl aziridines with benzoyl imido carboxylates. As shown in Scheme 2.51, the process afforded the corresponding protected vicinal diamines in high yields and enantioselectivities of up to 93% ee. This methodology was demonstrated to be a platform for the formal synthesis of (-I- )-balanol and analogues. 

A few studies reporting an asymmetric Diels-Alder reaction involving vinyl-boronates have been hitherto published. A catalytic asymmetric cycloaddition has been developed using 3- 3-boronyl-propenoyl)-l,3-oxazolidin-2-ones as dienophiles and a chiral titanium catalyst in the presence of molecular sieves [57]. Adducts were obtained as single isomers in good yield and high enantioselectivity (>93% ee) (Scheme 9.25). Cyclopentadiene gave a mixture of endo and exo isomers (-95 5). 

For the catalytic asymmetric FC reaction, the application of chiral titanium complexes of BINOL derivatives was first realized by Mikami and coworkers in the reaction of electron-rich aryl and vinyl ethers with fluoral [245]. For the FC reaction of anisole, it was found that the catalytic activity and enantioselectivity of BINOL-Ti catalysts were critically influenced by the substituents of BINOL derivatives. The electron-withdrawing bromo atoms at 6,6 -positions of BINOL turned out to be beneficial to the catalysis, the trifluoroethanol derivatives were obtained in high yield (89%) and up to 90% ee with a p o isomer ratio of 4 1. The FC reaction of vinyl ether with fluoral catalyzed by BINOL-TiCb (10 mol%) gave a mixture of allylic alcohols in which the major isomer was usually the Z-alkene. The increase of bulkiness of silyl group is favorable for the formation of FC products with very high enantiomeric excess (Scheme 14.105). 

A recent study revealed that the rate of aza-Michael-initiated ring-closure reaction between vinyl selenone 8 and primary amines 9 forming aziridines 10 (Scheme 2) was accelerated significantly in water [25]. The reactions in case of many substrates were accomplished in 3-7h as against over 24h when carried out in toluene. An asymmetric aziridination under the reaction conditions in the presence of either catalyhc or stoichiometric amounts of (R)-BINOL gave excellent yields of aziridines, but very poor enantioselectivity (ee 18% and 10% in two reactions investigated) was observed. 

This type of complex also efficiently induced the intra- and intermolecu-lar [4+2] and intramolecular [5+2] cycloaddition reactions shown in Scheme 8.10. However, these eatalysts were not effective for the mtermolecular [5+2] cycloaddition reaction. The [(IPr)RhCl(COD)]/AgSbF6 system has several attractive features, such as mild reaction conditions (15-20 °C) combined with high yields (up to 99%) and high turnover numbers (up to 1900). This [(NHC)Rh]-based system also efficiently eatalysed the diastereoselective tandem hetero-[5+2] cyeloaddition/Claisen rearrangement reaction of vinylic oxiranes with all nes. A complete ehirality transfer in the tandem process was observed, thus providing a novel and efficient method to access [3.1.0] bicyclic targets in an enantioselective manner. ... 

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