Palladium allylic transposition

Both the palladium- and the nickel-catalysis enables the use of allylic acetate as starting reagent. In the two approaches a zinc compound is evoked as key intermediate, though its formation has been demonstrated only indirectly. In the two methods allylic transposition is observed. The authors have then concluded that these electrochemical allylation reactions closely parallel the chemical allylation reactions involving allylzinc intermediates. 

Electrophilic transition metals, particularly palladium(II) salts, catalyze a number of heteroatom allylic transposition processes38 by a mechanism which almost certainly involves nucleophilic attack of a heteroatom on a metal-bound alkene (Scheme 10), often termed cyclization-induced rearrangement . 

Allylimidates underwent a clean Claisen-type rearrangement in the presence of 5 mol % palladium chloride (equation 28),40 as did allyl carbamates39 and S-allylthioimidates (equation 29).41 This S to N rearrangement has found application particularly in the synthesis of pyrimidines, systems for which thermal S to N allylic rearrangements were generally ineffective (equation 30).42 Finally, 0-allyI S-methyl dithiocarbonates cleanly underwent palladium(ll)-catalyzed O to S allylic transposition (equation 31 ).43... 

Scheme 12.6). Stereoselective reduction and chromatographic separation afforded diastereomerically pure derivative 32 in 94% ee. Removal of the silicon protecting group, followed by acetylation of the two secondary alcohols, set the stage for an elegant palladium-catalyzed allylic transposition that provided compound 33 with...

The research groups of Trost and Stille have demonstrated in related studies that allylic stannanes undergo palladium-catalysed cross-coupling with allylic acetates °° or allylic bromides to furnish 1,5-dienes in variable yields. The stannane undergoes predominant or complete allylic transposition during coupling with either type of substrate (e.g. Scheme 96). 

The reaction of allyl bromides with allyltin reagents, catalysed by palladium or zinc chloride, gives cross-coupled products without allylic transposition in the allyl halide partner, but with predominant allylic rearrangement from the tin partner. Similar unsymmetrical cross-couplings of allylstannanes with allyl acetates have also been observed (Scheme 19). ... 

The first example of chemoenzymatic DKR of allylic alcohol derivatives was reported by Williams et al. [37]. Cyclic allylic acetates were deracemized by combining a lipase-catalyzed hydrolysis with a racemization via transposition of the acetate group, catalyzed by a Pd(II) complex. Despite a limitation of the process, i.e. long reaction times (19 days), this work was a significant step forward in the combination of enzymes and metals in one pot Some years later, Kim et al. considerably improved the DKR of allylic acetates using a Pd(0) complex for the racemization, which occurs through Tt-allyl(palladium) intermediates. The transesterification is catalyzed by a lipase (Candida antarctica lipase B, CALB) using isopropanol as acyl acceptor (Scheme 5.19) [38]. 

Their original option was to use isomer i, because less sterical hindrance for the complexation of the double bond with the palladium was expected (Scheme 30). When no reaction occurred with several ligands, 275 was chosen, which produces the same (allyl)palladium complex. Trost assumed that preliminary complexation of the palladium to the alkyne and intramolecular transposition to the double bond as the reason for this surprising result. Indeed, when ii, where the alkyne is absent, was treated under identical conditions as 275, no reaction occurred (Scheme 30). 

With this bicyclic intermediate available in sizeable amounts, ready advance to 111 could be conveniently accomplished prior to annulation of the second five-membered ring (Scheme XIV). 1,3-Carbonyl transposition was realized by complete eradication of the original carbonyl by Ireland s method [60] followed by allylic oxidation. Application of the Piers cyclopentannulation protocol [61] to 111 made 113 conveniently available. Introduction of a methyl group into ring B was brought about by treatment of the kinetically derived enol triflate [62] with lithium dimethylcuprate [63], Hydrolysis of 114 gave the dienone, which was directly transformed into 115 by oxidation of its silyl enol ether with palladium acetate in acetonitrile [64],... 

Very little information is available about the effect of catalysts towards the thio-Claisen transposition. Meyers and his group had noticed this, and screened a variety of additives. Palladium(II) and nickel(II) complexes (10mol%) significantly accelerated the rearrangement of hindered keteneaminothioacetals, which could be performed at 25 °C, instead of heating to 140 °C without catalyst [114]. Some variation of the diastereoselectivity was observed. The authors proposed that palladium(II) coordinates to the allylic double bond. The effect of CeCh has also been reported [115]. 

Related to the Overman esterification is the Overman rearrangement. The latter allows the conversion of readily available allylic alcohols into allylic amines by a two-step synthesis involving the rearrangement of an allylic trichloroacetimidate to an allylic trichloroacetamide with clean 1,3-transposition of the alkenyl moiety. Although this rearrangement can be thermally driven, addition of palladium salts lower the needed temperature. Chiral ligands such as proline-based diamines would lead to enantioenriched material. ... 

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