Allylic substitution reactions, treatment

As shown in Scheme 2.20, selective lithiation of substrate 2-87 by treatment with LDA in THF at -78 °C triggers an intramolecular Michael/intermolecular aldol addition process with benzaldehyde to give a mixture of diastereomers 2-90 and 2-91. 2-91 was afterwards transformed into 2-92, which is used as a chiral ligand for Pd-catalyzed asymmetric allylic substitution reactions [29]. 

A different approach using alkene derivatives involved allylic substitution reactions. Ethyl cyclohex-3-en-l-carboxylate (7.51) reacted with N-bromosuccin-imide to give 7.52.26 This allowed reaction with potassium phthalimide to give 7.53 in an overall yield of 41%. Catalytic hydrogenation and treatment with methylamine led to ds-3-aminocyclohexane-l-carboxylic acid, 7.22.26... 

A first example of a combination of a Rh-catalyzed allylic substitution and a Pau-son-Khand annulation reaction has also been developed by the same group [222]. Thus, [RhCl(CO)dppp]2 is able to catalyze both transformations at different reaction temperatures. Treatment of the allylic carbonate 6-106 with the alkyne derivative 6-107 led to a diastereomeric mixture of 6-108 and 6-109 in 63-84% yield, with 6-108 as the main product (Scheme 6/2.23). 

Substitution reactions of allylic nitro compounds often lead to rearranged products, as in palladium(0)-catalysed aminations and alkylations. Thus treatment of the nitro ester 419 with piperidine in the presence of tetrakis(triphenylphosphine)palladium yields a mixture of the unrearranged and rearranged amines 420 (R = piperidin-l-yl) and 421... 

Stereoselective substitution reactions of chiral dienyl electrophiles have also been carried out. In analogy to the copper-promoted 8 2 reactions of simple allylic electrophiles [3], the corresponding 8 2 (1,3) substitutions of dienyl carbonates [43] have been reported to proceed with high anti selectivity. Interestingly, treatment of chiral dienyl acetal 63 with the Yamamoto reagent PhCu BFs gave rise to the formation of a 1 3 mixture of the anti-S l substitution product 64 and the syn-Sn2" (1,5) substitution product 65 (Eq. 4.28) [44]. A mechanistic explanation of this puzzling result has yet to be put forward, however. 

Hartwig and coworkers reported an approach to address this limitation involving tandem catalytic reactions. In this tandem process, sequential palladium-catalyzed isomerization of the branched isomer to the linear isomer, followed by iridium-catalyzed allylic substitution leads to the branched product with high enantiomeric excess [105]. More specifically, treatment of branched allylic esters with catalytic amounts of the combination of Pd(dba)2 and PPhs led to rapid isomerization of the branched allylic ester to the linear isomer, and the linear isomer underwent allylic substitution after addition of the iridium catalyst and nucleophile (Scheme 31). 

The known reactions of the perfluoroalkylsilver derivatives generally equate to nucleophilic substitution reactions and the oxidative perfluoroorganylation of group 12-16 elements.39,40 Normant and coworkers have reported the reaction of both carbon dioxide and allyl bromide with perfluoroisopropylsilver (Scheme l.ll).41 Heptafluoro-2-nitrosopropane has been prepared in good yield from perfluoroisopropylsilver by treatment with nitrosyl chloride.42... 

The evolution of HBr in the bromination reactions and the uptake of one bromine atom per ring indicate substitution at a secondary allylic carbon atom. The ease of oxidation and cross linking of the polymers and the presence of hydroxyl groups imply the intermediate formation of hydroperoxides on allylic carbon atoms. Treatment of the hydroxyl-containing polymer with benzoyl chloride indicates that the bulk of the hydroxyl groups are on secondary carbon atoms, since tertiary hydroxyl groups would tend to be replaced by chlorine. Although these results do not permit the elimination of structure B, it appears that the bulk of the structural units in the polycyclopentadiene corresponds to 1,2- addition (A). 

Occasionally, gew-dichlorocyclopropanes with phenyl or alkoxy substituents are transformed by strong base in aprotic or (rarely) protic solvents to allylic substitution products that do not obey at all the rules for cyclopropyl to allyl rearrangements with regard to their constitution and configuration. "These reactions are mechanistically distinct. They are usually initiated by base-induced elimination of hydrogen chloride to form phenyl- or alkoxy-substituted cyclopropenes, which are then intercepted by nucleophiles. ° These reactions are discussed in Section 2.B.2.I. Small structural differences can divert the reaction into one or the other reaction channel. Cyclopropane 25 on treatment at — 50°C with 2 equivalents of potassium /er/-butoxide in tetrahydrofuran in the presence of catalytic dicyclohexano-18-crown-6... 

Mixtures of epoxides and 3-substituted 2-methyl-4-penten-2-ols react to give homo-allylic alcohols, on treatment with In(OTf)3 (or TfOH). Under the reaction conditions the tertiary alcohols are decomposed to generate allylating agents while the epoxides undergo rearrangement to aldehydes, the transformed species then combine to furnish the observed products. ... 

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