Enantioselectivity allylation

S ]2 -selective reactions between primary allylic substrates and otganocoppet reagents testiU in the creation of new Chirality in previously aChital molecules, and it is tempting to try to take advantage of this for the development of enantioselective allylic substitution reactions. 

Reagents of type 1 are the most important and exhibit the highest reactivity towards carbonyl compounds. The reactivity can be further tuned by altering the substitution on titanium. Reagents of type 2 show lower reactivity, but higher selectivities, but have, so far, only been used occasionally (Section 1.3.3.3.8.2.1.2.). Reagents of type 3, derived from chiral alcohols, accomplish efficient enantioselective allyl transfer (Section 1.3.3.3.8.2,3.3.). 

Fig. 2.21 Chiral binap-based catalysts or catalyst precursors for the enantioselective allylic alkylation of aUylphosphates...

A Case History Enantioselective Allylic Alkylation Catalysed by Pd Nanoparticles... 

Since Pd complexes are well-known catalysts for enantioselective allylic substitution reactions, here the catalytic behaviour of palladium NPs for this reaction is examined (Scheme 1). One example involving a chiral phosphite with a carbohydrate backbone, able to coordinate firmly at the surface of NPs together with oxygen atoms capable to interact weakly with this surface, is presented. In particular. 

The majority of catalytic enantioselective allylation reactions involve the chiral Lewis-acid-catalysed additions of allylsilanes or allylstannanes to carbonyl compounds. Monothiobinaphthol has been used by Woodward et al. as a chiral promoter in the enantioselective catalytic allylation of aryl ketones with impure Sn(allyl)4, prepared from allyl chloride, air-oxidised magnesium and SnCl4. Therefore, the allylation of arylketones in these conditions was achieved very efficiently, since the corresponding allylic alcohols were formed in... 

The synthesis in Scheme 13.49 features use of an enantioselective allylic boronate reagent derived from diisopropyl tartrate to establish the C(4) and C(5) stereochemistry. The ring is closed by an olefin metathesis reaction. The C(2) methyl group was introduced by alkylation of the lactone enolate. The alkylation is not stereoselective, but base-catalyzed epimerization favors the desired stereoisomer by 4 1. 

Enantioselective allylations of a-nitro ketones and a-nitro esters with allyl acetates are carried out in the presence of 2 equiv of alkali metal fluorides (KF, RbF, CsF) and 1 mol% palladium catalysts prepared in situ from Pd2(dba)3-CHC13 and chiral phosphine ligands. Moderate enantio-selectivity (ca 50% ee) is reported for allylation of cx-nitroketones (Eq. 5.60). The highest selectivity (80% ee) is observed for allylation of the reaction of tert-butyl ester (Eq. 5.61).93... 

Asymmetric synthesis of tricyclic nitro ergoline synthon (up to 70% ee) is accomplished by intramolecular cyclization of nitro compound Pd(0)-catalyzed complexes with classical C2 symmetry diphosphanes.94 Palladium complexes of 4,5-dihydrooxazoles are better chiral ligands to promote asymmetric allylic alkylation than classical catalysts. For example, allylic substitution with nitromethane gives enantioselectivity exceeding 99% ee (Eq. 5.62).95 Phosphi-noxazolines can induce very high enatioselectivity in other transition metal-catalyzed reactions.96 Diastereo- and enantioselective allylation of substituted nitroalkanes has also been reported.9513... 

As practiced in the preceding syntheses by Evans and Nishiyama and Yamamura, the A-ring fragment 43 is formed through substrate-directed vinylogous aldol reaction of the Brassard-type diene 19 and the chiral aldehyde 42, which is prepared using Brown s protocols for enantioselective allylation [53], followed by hydroxy-directed nnn-diastereoselective reduction of the C3 ketone (Me4NB(OAc)3H) [41],... 

It is noteworthy that ZnEt2 has been used as a base in enantioselective allylic substitutions. A remarkable increase in ee was observed when ZnEt2 was used instead of KH, NaH, LiH, LDA, or BuLi in the Pd-catalyzed alkylations of allylic acetates by enolates of malonic esters and related compounds.403 In contrast, application of ZnEt2 was not as very effective as in similar iridium-catalyzed allylic alkylations.404... 

Accordingly, chiral 7r-allylpalladium complex 204 was found to catalyze enantioselective allylation of imines with good to high selectivity (Equation (98)).4S7c 457e 457g 4S71... 

A novel procedure for the synthesis of an indole skeleton 81 was developed by Mori s group (Scheme 13).16e,16f Enantioselective allylic amination of 78 with A-sulfonated < r/ < -bromoaniline 79 followed by Heck cyclization of 80 provided chiral indoline 81. The treatment of a cyclohexenol derivative 78 with 79 in the presence of Pd2(dba)3-GHGl3 and ( )-BINAPO gave compound 80 with 84% ee in 75% yield. Total syntheses of (—)-tubifoline, (—)-dehydrotubifoline, and (—)-strychnine were achieved from compound 80. 

Allylation of aromatic aldehydes with allyl trimethylsilane catalyzed by chiral acyloxyborane gives good results. In contrast, the results are normally poor for aliphatic aldehydes.89 Costa et al.90 introduced another enantioselective allylation procedure aiming to overcome this problem. In the following example, the enantioselective allylation of aldehyde octanal was carried out using... 

Scheme 7. The simplest type of enantioselective allylic alkylation which occurs on treatment of an allylic substrate with a metal derivative, together with a stabilized nucleophile (R = H, alkyl or aryl X" = leaving group [M] = metal catalyst Nu" = nucleophile L = coordinating atom).

Scheme 2.36. Enantioselective allylation with diorganozinc reagents.

Scheme 27 Enantioselective allylation, crotylation, and lert-prenylation of substituted isatins via iridium-catalyzed transfer hydrogenation...

This chapter describes the development of iridium-catalyzed, enantioselective allylic substitution. It is organized to focus on how modifications to the catalyst, combined with mechanistic insights, have provided the foundation for a steady... 

The scope of allylic electrophiles that react with amines was shown to encompass electron-neutral and electron-rich ciimamyl methyl carbonates, as well as furan-2-yl and alkyl-substituted allylic methyl carbonates. An ort/io-substituted cinnamyl carbonate was found to react with lower enantioselectivity, a trend that has been observed in later studies of reactions with other nucleophiles. The electron-poor p-nitrocinnamyl carbonate also reacted, but with reduced enantioselectivity. Allylic amination of dienyl carbonates also occur in some cases with high selectivity for formation of the product with the amino group at the y-position over the s-position of the pentadienyl unit [66]. Arylamines did not react with allylic carbonates under these conditions. However, they have been shown to react in the presence of the metalacyclic iridium-phosphoramidite catalysts that are discussed in Sect. 4. 

---