Allylic reduction

The 3-acetoxymethylazepine 18 formed by RCM of the corresponding allyl acetate 17 underwent palladium-catalysed allylic reduction to give almost exclusively the 3-exo-methylene isomer 19 <07JOC2674>. 

The order of reactivity of this Ru/silane combination to various functional groups differs greatly from that of its Pd/silane/ZnCh analog. While the latter is very useful for allylic reductions and essentially useless for unsaturated esters, the Ru-based system exhibits opposite reactivity. This complementary che-moselectivity is illustrated by the reduction of cinnamyl cinnamate (Scheme 59), a substrate containing both an allylic carboxylate and an a, -unsaturated ester.Each of these can be reduced separately by silicon hydride and the appropriate transition metal catalyst. 

In l-chloro-2-hydroperfluorocyclobutene and cyclopentene, direct reaction of hydride substitutes hydrogen for chlorine (1), while allylic reduction leads to replacement of fluorine (2) (equation 6, Table 3). ... 

Elimination. 1,4-Dienes are prepared from a-(benzothiazol-2-yl)methyl ketones by way of allylation, reduction of the carbonyl group with NaBH, and treatment with DIBALH-MgBr. The newly formed double bond of the major product has a fZj-configuration. 

Carboxylic acids, esters, amides, nitriles, nitro groups and most aromatic nuclei are not reduced under ionic hydrogenation conditions (133). An organosiloxane, polymethylhydrosiloxane [9004-73-3] (PMHS), is most economically favored for large-scale reductions. Polymethylhydrosiloxane is a versatile low cost hydride transfer reagent having a hydride equivalent weight of 60. Reactions are catalyzed by Pd or dibutyltinoxide. The choice of reaction conditions leads to chemoselective reduction, eg, allyl reductions in the presence of ketones and aldehydes (134—136). Esters are reduced to... 

Subsequent to this, the 6- and 6 -anchored PS-PEG MOP ligands 140 and 141 have been prepared and the catalytic activity in asymmetric tr-allylic reduction of their corresponding allylpalladium complexes compared and contrasted. The 6-anchored ligand was found to be the best and the 6 -analog the worst. This has been attributed to the fact that the 6 -position on a MOP ligand is in close proximity with the substrate moiety and thus the effect of attaching a sterically demanding tether at this position may well be to reduce catalytic activity. 

We then focused our studies on the elaboration of C12, C13, C15 -stereocluster and the most appropriate strategy appeared to be the asymmetric dihydroxylation of polyenes such as ester 13. When treated under Sharpless conditions, supplemented with an additional osmium source (0.6 mol%), a chemoselective dihydroxylation occurred only at the more electron rich unsaturation and the corresponding diol was isolated in 70% yield and 86% ee (Scheme 10). The latter could be converted into carbonate 47 in 79% yield before being submitted to a pallado-catalyzed allylic reduction under O Doherty conditions, which dehvered 6-hydroxyester 48 in reasonable yield. Protection of the hydroxyl group with a sUyl ether was followed by a second asymmetric dihydroxylation of the y,8-unsaturation with AD-Mix a, supplemented with additional osmium salt (2mol%) to increase the reactivity of the catalytic system. To our great pleasure, a complete... 

A Study that compares the hydride donor characteristics of PMHS and BU3S11H has shown remarkable differences between the twoJ As mentioned before, BusSnH is capable of reducing a large variety of polyfunctional allylic heterosubstituents. However, its reaction with compounds possessing a relatively acidic hydrogen a to the aUylic unit leads to subsequent j8-hydride elimination from the 7r-allylpalladium intermediate to yield the corresponding diene. Conversely, when PMHS was employed no /3-hydride elimination side products interfered with the allylic reduction reaction (Scheme 14). 

Moreover, the reactivity differences between the tin and silicon hydrides can be exploited for functional-group differentiation. For example, in the presence of Pd(0), tri-butyltin hydride reduces a,/3-unsaturated aldehydes and ketones rapidly but silicon hydrides are unable to do so. Thus, the treatment of a mixture of an allylic acetate and an unsaturated ketone with tin hydride and Pd(0) catalyst results in total conjugate reduction of the latter and unreacted allylic acetate. In contrast, employment of silicon hydride provides complementary chemoselectivity allylic reduction was completed before any reduction of the Michael acceptor could be detected (Scheme 15). 

Basavaiah [75] reported in 1992 that allylic reduction of allyl acetates with a a-CN substituent yielded the vinyl cyanide products in complete Z-selectivity (Fig. 36). One decade later, KabaUca reported that Pd-catalyzed allylic substitution of similar substrates using P-T0IBF3K also led to the products as an exclusive Z-isomer (Fig. 37) [76]. The CN group was necessary for Z-selectivity and the substrate scope was limited in both systems. 

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