Allylic substitutions reviews

An important variant for transition metal-catalyzed carbon-nitrogen bond formation is allylic substitution (for reviews, see1,la lh). Nucleophilic attack by an amine on an 7r-allyl intermediate, generated from either an allylic alcohol derivative,2 16,16a 16f an alkenyloxirane,17-19,19a-19d an alkenylaziridine19,19a 19d, or a propargyl alcohol derivative,21,21a 21d gives an allylic amine derivative. 

Strategies centered on reductive introduction of the fluoroolefin via a geminal difluoro allylic array have been reviewed [66]. In an introductory example to this synthetic approach, Okada et al. [67] developed a completely stereoselective synthesis of Z)-2,5-syn 2-alkyl-4-fluoro-5-hydroxy-3-alkenoic acids through the Cu(l)-mediated allylic substitution reaction of trialkylaluminum with the (E)-4,4-difluoro-5-hydroxyallylic alcohol derivative (61) (Scheme 21). Reaction... 

The use of C2-symmetric chiral bis(oxazolines) in allylic substitutions has also been reviewed.22... 

Finally, allylic substitution reactions with chiral copper catalysts have been reviewed.13 Little mechanistic detail of these reactions is known. 

An enormous amount of work in the area of transition-metal-catalyzed asymmetric allylic substitution has been done over the last few years. Since the publication of the original chapter [1], a number of books [2-5], reviews [6-12], and accounts [13,14] covering this field have been published. This update will cover the recent developments from 1999 to early 2003. 

Allylic substitutions are among the most frequently used Pd-catalyzed CC-bond forming processes in organic synthesis (for reviews see [77-79]). Since tetrahedral carbon centers are formed, high levels of enantioselectiv-ity can be obtained if chiral Pd catalysts are applied. On the other hand allylic substitutions are well suited for establishing sequentially catalyzed processes, either if both allylic positions can subsequently be reacted or if the shifted olefin moiety can set the stage for further Pd-catalyzed bond forming processes. 

A primary amino function in aminocyclopropanes (413) can be removed easily by diazotation (for a review see Ref. 594). In the absence of any steric restrictions, displacement of the diazonium group led mainly to ring-opened allylic substitution products (579)3.4.11.53.84.86.144,147.595 (equation 150). Substituted cyclopropanes (580) are obtained only to a very small amount they were formed mainly (> 92 %) with inversion of configuration (see also Ref. 218). 

Another complexity with allylic substrates is the potentitil tor formation of allylic isomers. Reviews by De Wolfe and Young (93). Figiderc and Franck (94). and Magid (951 are relevant to this question. Scheme 2.2 illustrates the consequences of Sx I and SET in the prototype substituted allylic system, erotyl/o -methylallyl. The new bond I onnation. at either end of the allylic system, could be subject to a memory effect due to restricted reorientation during the lifetime of a radical or ion pair. Alternatively, competition between Ss2 and S, 2 could lead to isomer mixtures (Scheme 2.3). In early work (96, rciictions of crotyl and o -methylallyl chlorides with PhMeBr led to mixtures (9 II I l 75 25) which... 

For reviews on Ir-catalyzed asymmetric allylic substitution reactions (a) H. Miyabe and Y. Takemoto, Synlett, 2005,1641 (b) R. Takeuchi and... 

Four reviews on allylic substitution reactions have been published. The first deals with the enantioselective allylic substitutions by carbon nucleophiles, in the presence of both palladium and non-palladium catalysts. The second reviews stere- 0 oselective allylic substitution reactions forming asymmetric C-C, C-N, and C-O bonds. The third review covers new developments in metal-catalysed asymmetric 0 allylic substitution reactions with heteroatom-centred nucleophiles. Several applications of this new methodology are included. Finally, the catalytic 5 2 and 5 2 reactions of allylic alcohols, most of which occur with a very high ee, have been reviewed. ... 

A Stereoselective Synthesis volume of the Science of Synthesis series has reviewed the alkylation and allylation of carbonyl and imino groups, as well as the enantio-selective addition of metal alkynylides to imino and carbonyl compounds. Recent advances in Favorskii rearrangement, Sonogashira reactions, and catalytic enantio-selective allylic substitutions with carbon nucleophiles have been highlighted. 

Allyl ligands are reactive groups in catalytic processes, such as allylic substitution (Chapter 20) and transition-metal-catalyzed additions of allyl groups to carbonyl compounds (Chapter 12). They are also intermediates in a variety of catalytic processes involving dienes, including the hydroamination (Chapter 16) and telomerization of dienes (Chapter 22). They are also formed by cleavage of the allylic C-H bonds of olefins to form intermediates in allylic oxidation chemistry and as stable species that inhibit the polymerization of a-olefins (Chapter 22). The synthesis, structure, and occurrence of -rj -allyl complexes have been reviewed. ... 

Cyclization reactions (Scheme 12) have played an important role in Pd-catalyzed allylic substitution chemistry, and the area was reviewed in 1989 In many cases, the same principles apply as in intermolecular variant of the reaction. The first example of a Pd-catalyzed cyclization involving allylic substitution was reported by Trost and Verhoeven in 1977. " It involved the cyclization of allyl acetate 75 to give the cis-fused product 76. 

Pd-catalyzed allylic substitution reactions can also be performed using water-soluble phosphine ligands, including TPPTS 132, as shown by the reaction of nitroester 48 with allyl acetate 133 to give the substitution product 134 (Scheme 24). The use of water-soluble palladium catalysts has been the subject of a review.t Water-soluble catalysts have also been applied to supported Uquid phase reactions. A silica bead supports a thin film of polar solvent in which the palladium complex resides.t The substrates and product reside in the bulk organic phase and can be decanted from the glass bead catalyst at the end of the reaction. 

Asymmetric variants of the Pd-catalyzed allylic substitution reaction have enjoyed considerable attention since the first stoichiometric example reported in 1973J The reaction has been the subject of several reviews, and in some cases, the levels of enantioselec-tivity have reached in excess of 99% ee. 

Over the last few years, the Ir-catalyzed allylic substitution has been investigated in several laboratories and was found to be well suited for applications in organic synthesis. Using this reaction, branched chiral allylic derivatives can be prepared with high selectivity from simple achiral monosubstituted allylic substrates (Scheme 11.1). The reaction has been carried out with C, N, O, and S nucleophiles. The very broad range of nucleophiles is impressive. Several reviews have appeared on the subject [1]. At present, the best catalysts are prepared from [Ir(cod)Cl]2 (cod, cycloocta-1,5-diene) [2, 3] or [Ir(dbcot)Cl]2 (dbcot, dibenzocyclooctatetraene) [4] and a chiral phosphoramidite by base-induced C-H activation. Reliable experimental procedures have been developed for the reaction [5]. 

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