Palladium retro-allylation

A procedure has been developed for the palladium-catalysed a-arylation of amides by aryl bromides using the zinc enolates of the amides. The reaction works well with bromoarenes carrying a variety of ring substituents and with bromopyridine. In addition, the reaction has been shown to be effective with morpholine amides to give products which are precursors for aldehydes and ketones.39 A new method has been reported for the allylation of aryl halides using homoallyl alcohols as the allyl source the palladium-catalysed reaction, which may be both stereo- and regio-sepecific, uses a retro-allylation reaction to form a a-allyl(aryl)palladium intermediate.40... 

Advantage of Palladium-Catalyzed Allylation via Retro-allylation... 

Homoallylic alcohol 9 is readily prepared by palladium-catalyzed directed cyanoboration followed by Suzuki-Miyaura cross-coupling (Scheme 5.15) [15]. Palladium-catalyzed allyl transfer from 9 to aryl bromide results in removal of the hydroxymethyl directing group and concomitant formation of 3,4-diaryl-2-butenenitrile albeit the lack of regio- and stereospecificity. The overall transformation proposes a concept of intramolecular reaction with a carbon tether removable through retro-allylation. 

Enantioselective ring-opening of a norbornenol derivative by means of a palladium catalyst complexed with chiral phosphoramidite ligand 16 affords cyclo-hexenyl ketone with 64%ee (Scheme 5.24), which represents the first example of palladium-catalyzed enantioselective retro-allylation. 

Palladium-catalyzed retro-allylation of homoallylic alcohols is extended to retro-propargylation of homopropargylic alcohols [20]. Despite the rigid and linear structure of the triple bond, the reactions of aryl halides with homopropargylic alcohols necessitate slightly modified conditions to afford allenylated arenes (Scheme 5.25). Not only gewi-disubstituted allenes but also tri- and tetrasubstituted ones are accessible. The reactions with diastereomerically pure homopropargylic alcohols threo- and erythro-17 result in stereospecific transfer to afford anti- and syn-18, respectively (Scheme 5.26). 

Palladium-catalyzed reactions of aryl halides with 3,4-alkadien-l-ols were reported by Oh in 2002 (Scheme 5.27) [21]. The finding is an interesting pioneering work on palladium-catalyzed carbon-carbon bond cleavage of homoallylic alcohols. They proposed a mechanism that does not involve retro-allylation but P-carbon elimination. 

A diorganonickel complex generally undergoes reductive elimination more rapidly than the corresponding palladium complex. Nickel can mediate retro-allylation, which is involved in allylation of allylic carbonate with homoallylic alcohols for efficient synthesis of 1,5-hexadienes [25]. A combination of Ni(cod)2 (cod = 1,5-cyclooctadiene) and triethyl phosphite catalyzes the allylation reaction of Boc-protected cinnamyl alcohol (Boc = f-butoxycarbonyl) with homoallylic alcohols (Scheme 5.34). The reactions with alkyl-substituted homoallylic alcohols (R = alkyl) are not regiospecific but are sterically controlled. The highest linear... 

It has been reported that a retro-oxidative cyclization process proceeds in catalytic C-C bond cleavage reactions. For example, a six-membered cyclic allylic carbonate 38 underwent a palladium-catalyzed decarboxylative C-C bond cleaving reaction to afford dienyl aldehyde 40 (Scheme 7.12) [15]. It is proposed that oxidative addition of the allylic carbonate to palladium(O) followed by elimination of carbon dioxide generates the palladacycle 39. Subsequent retro-oxidative cyclization produces the diene and aldehyde functionalities. 

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