Regioselectivity of allylic

To predict the regioselectivity of allylic oxidations of C-C double bonds by singlet oxygen, three general rules for hydrogen abstraction (% values in brackets) apply ... 

In addition to enantiocontrol, the problem of regiocontrol arises in these reactions. There are various factors that influence the regioselectivity of allylic substitutions [3,4,13, 36, 37, 38, 39]. Electronic effects exerted by the catalyst and the allylic substituents, steric interactions between the nucleophile, the allyl system and the catalyst, and the relative stabilities of the Ti-olefin complexes formed after nucleophilic addition, can all play a role. The relative importance of these factors varies with the catalyst, the substrate, the nucleophile, the solvent and other reaction parameters and is difficult to predict. 

Alvarez-IbaiTa. C. Csaky. A.G. Martin. M.El. Quiroga. M.L. Study of the asymmetrie synthesis of (Z)-y-substituted-0(,P-didehydroglutamate.s from V-alkylidene-glycinates. Tetrahedron 1999. 55 (23). 7319-7330. Yanagisawa, A. Yamada. Y. Yamamoto. H. Effect of crown ethers on the regioselectivity of allylation of benzaldehyde with allylic barium reagents. Synlett 1997. (9). 1090-1092. 

The regioselectivity of the allylic substitution process depends heavily on the identity of the metal and its coordination sphere. To bypass the issue of regioselectivity, most of the substrates used to demonstrate activity and enantioselectivity of catalysts for allylic substitution generate symmetrical allyl intermediates. However, many synthetic applications would require a regioselective substitution process involving unsymmetrical allyl intermediates. The regioselectivity of allylic substitution arises from the position of attack of the nucleophile on the allyl intermediate. 

Pertubation interactions between FMOs have been utilized to predetermine the regioselectivity of allylic and benzylic hydrogen-abstraction reactions. Thus, it is shown that, in unsaturated compounds containing two non-conjugated and non-equivalent r-systems, one system contributes mainly to the HOMO and the other to the HOMO-1. The important FMO interaction in these free-radical hydrogen-abstraction processes is SOMO-HOMO. Analysis of four representative examples corroborates empirical evidence and previously published studies/ ... 

Regioselective epoxidation of allylic and homo-allylic alcohols... 

The allyl-substituted cyclopentadiene 122 was prepared by the reaction of cyclopentadiene anion with allylic acetates[83], Allyl chloride reacts with carbon nucleophiles without Pd catalyst, but sometimes Pd catalyst accelerates the reaction of allylic chlorides and gives higher selectivity. As an example, allylation of the anion of 6,6-dimethylfulvene 123 with allyl chloride proceeded regioselectively at the methyl group, yielding 124[84]. The uncatalyzed reaction was not selective. 

Regioselectivity in reactions of heterocyclic anions of allyl-type with electrophiles 99CRV665. 

Regioselectivity of syntheses of heterocycles in reactions of allyl-type anions with electrophiles 99CRV665. 

For carbanionic addition, the relative negative charge and the electron densities in the 1- and 3-position in the HOMO of the ambident allylic anion determine, in addition to steric effects, the regioselectivity of the hydroxyalkylation. According to the allopolarization principle13 the following generalizations can be made ... 

Allylstannanes can be prepared by treatment of allyl halides with trialkyl- or triaryltin lithium reagents. Displacement of primary allyl halides tends to be regioselective for formation of the less substituted allylstannane, and takes place with useful retention of double-bond geometry14-16. 

In copper-catalyzed Grignard reactions of allylic carboxylates, high y regioselectivity is observed when copper(I) cyanide is used as catalyst16. 

The reaction of allylic halides 1 with the zinc cuprate (R2CuZnCl), prepared by treatment of LiCuR2 with 1 equivalent of anhydrous zinc(II) chloride, affords the SN2 product 2 with 98% regioselectivity, and the diastercoselectivity is nearly 100% am/75. 

An extensive study (6) of Sn displacement reactions of allyl halides using silyl anions/anionoids has provided the following regioselective alternatives ... 

Trost and Hachiya [140] studied asymmetric molybdenum-catalyzed alkylations. Interestingly, they noticed that the regioselectivity of this transformation performed with a non-symmetric allylic substrate varied according to the nature of the metal Pd-catalyzed substitutions on aryl-substituted allyl systems led to attack at the less substituted carbon, whereas molybdenum catalysis afforded the more substituted product. They prepared the bis(pyridylamide) ligand 105 (Scheme 55) and synthesized the corresponding Mo-complex from (C2H5 - CN)3Mo(CO)3. With such a catalyst, the allylic... 

Enantioselectivity (which is hnked to the regioselectivity of the attack of the nucleophile to the coordinated allyl) in the allylic amination of 1,3-diphenyl-allyl ethyl carbonate was also very low compared to the P-N system. This was attributed to the comparable fran -influence of P and NHC functionalities, leading to poor regioselec-tion of the two aUyl termini trans to the P and NHC ligands by the nucleophile [95],... 

The directive effect of allylic silyoxy groups has also been examined. The reactions are completely regioselective for 1,3-oxygen substitution. The reaction of... 

Epoxides of allylic alcohols exhibit chelation-controlled regioselectivity.128... 

Several strategies have been proposed to improve the regioselectivity of nitrile oxide cycloaddition. Kanemasa and coworkers have reported high-rate acceleration and regioselectivity in nitrile oxide cycloadditions to the magnesium alkoxides of allylic and homoallylic alcohols (Eq. 8.64)."... 

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