Product studies allylic substitution

Pd-catalyzed reactions proceed via 7i-allyl complexes, which at room temperature isomerize via a ti-o-ti rearrangement. As a consequence, branched as well as Z-and B-linear starting materials yield the same products, with memory effects being minimal at room temperature [16], The isomerization processes of (allyl)Ir complexes are usually slow, and accordingly any memory effects are pronounced. The configurational stability of (allyl) Ir intermediates of the Ir-catalyzed allylic substitution was studied by an investigation of substitutions at nonracemic allylic substrates (Scheme 9.5). 

When chlorination or bromination of alkenes is carried out in the gas phase at high temperature, addition to the double bond becomes less significant and substitution at the allylic position becomes the dominant reaction.153-155 In chlorination studied more thoroughly a small amount of oxygen and a liquid film enhance substitution, which is a radical process in the transformation of linear alkenes. Branched alkenes such as isobutylene behave exceptionally, since they yield allyl-substituted product even at low temperature. This reaction, however, is an ionic reaction.156 Despite the possibility of significant resonance stabilization of the allylic radical, the reactivity of different hydrogens in alkenes in allylic chlorination is very similar to that of alkanes. This is in accordance with the reactivity of benzylic hydrogens in chlorination. 

Substitution reactions of allylic substrates with nucleophiles have been shown to be catalyzed by certain palladium complexes [2, 42], The catalytic cycle of the reactions involves Jt-allylpalladium as a key intermediate (Scheme 2-22). Oxidative addition of the allylic substrate to a palladium(o) species forms a rr-allylpal-ladium(n) complex, which undergoes attack of a nucleophile on the rr-allyl moiety to give an allylic substitution product. The substitution reactions proceed in an Sn or Sn- manner depending on catalysts, nucleophiles, and substituents on the substrates. Studies on the stereochemistry of the allylic substitution have revealed that soft carbon nucleophiles represented by sodium dimethyl malonate attack the TT-allyl carbon directly from the side opposite to the palladium (Scheme 2-23). 

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... 

Asymmetric allylic substitution reactions have been studied for many years because they provide valuable chiral compounds. Regardless of the alkylating agent used, there are two major goals in these reactions (i) to minimize the amount of Sn2 products, and (ii) to maximize the enantiomeric purity of the Sn2 products. Various approaches have been investigated to achieve these goals. Recently, the efforts of several research groups have been focused on the... 

The first enantioselective, iridium-catalyzed allylic substitution was reported by Helmchen and coworkers soon after the initial report by Takeuchi. Helmchen studied catalysts generated from phosphinooxazoline (PHOX) ligands and [Ir(COD)Cl]2 for the reactions of sodium dimethylmalonate with cinnamyl acetates (Scheme 2) [50]. The alkylation products were isolated in nearly quantitative yield and were formed with ratios of branched-to-Unear products up to 99 1 and with enantioselectivities up to 95% ee. In this and subsequent studies with PHOX ligands [51,52], Helmchen et al. demonstrated that the highest yields and selectivities were obtained with a PHOX ligand containing electron-withdrawing substituents and... 

Mechanistic studies showed that metalacycle la is competent to be a catalyst in asymmetric allylic substitution reactions. The reaction of benzylamine with methyl ciimamyl carbonate catalyzed by a mixture of LI and [Ir(COD)Cl]2 occurs with an induction period and forms product in 84% yield and 95% ee, whereas the same reaction catalyzed by a mixture of metalacycle la and [Ir(COD)Cl]2 occurs without an induction period in just 2 hours to form the substitution product in 81% yield and 97% ee. The latter reaction was conducted with added [Ir(COD)Cl]2 to trap the -bound LI after dissociation. This ligand must dissociate to provide a site for oxidative addition of the allylic carbonate. 

Early work was focused to establish the preference for exo- vs endo-mode of cyclization. However, the absence of an effective method for generation of alkyl and/or aryl substituted silyl radicals made this task difficult. The reaction of prototype alkanesilane I with thermally generated t-BuO radicals at 145 °C after 4 h afforded a 48 % yield of unreacted starting material and 19 % yield of a six-membered cyclic product (Scheme 6.1) [1]. Moreover, EPR studies of the same reaction recorded the spectra at temperatures between —30 and 0°C, which were identified as the superimposition of two species having allylic-type (2) and six-membered ring (3) structures, respectively [2]. At higher temperatures radical 2 predominates therefore, the low yield detected in the product studies could derive from the extensive t-BuO attack on the allylic hydrogens. 

Substituted cycloalkenes usually react in the ring and not in the side chain. Internal alkenes with CH2 groups in both allylic positions yield a mixture of isomers, whereas terminal alkenes give primary alcohols as a result of allylic rearrangement. Later studies revealed, however, that the reactivity depends on both the structure of the alkene and reaction conditions.674 675 In alcoholic solutions, for example, racemic products are formed. Geminally disubstituted alkenes may exhibit a reactivity sequence CH > CH2 > CH3.675 676... 

In order to determine the degree of competition between both processes, the photo -stimulated reaction of the radical probe 3-bromo-2-tetrahydropyranyl allyl ether (83) with Ph2P ions in liquid ammonia was studied. In this reaction both the substitution 84 (20% yield) and the cyclized 85 (69% yield) substitution products were formed (equation 67)54,151. 

In general, the anodic oxidation of simple alkenes in nucleophilic solvents yields products resulting from both allylic substitution and oxidative addition of nucleophiles. Cyclohexene has been studied extensively as starting compound. The anodic oxidation of cyclohexene in methanol or acetic acid... 

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