Retro palladium-catalyzed reaction

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. 

Palladium-catalyzed carbonylative addition reactions of 2-iodophenols 3 to norbornadiene lead to coumarin systems 4 via stereoselective cis-exo addition and retro-Diels-Alder elimination of cyclopentadiene12, l3. Intramolecular versions of this reaction type have been reported34 35, which include the use of nickel36-42-4 3 and rhodium3 catalysts. 

Compound 2 was chosen as a direct precursor to cyclo-Ci since it should lose three anthracene molecules in a retro-Diels-Alder reaction under thermal conditions (Scheme 13-1). The synthesis of 2 (Scheme 13-2) started with the Diels-Alder reaction of anthracene and tra 5-dichloroethene [10], followed by dehydrochlorination and subsequently bromination to 10. The latter conversion was best achieved by simply adding elemental bromine to a solution of the vinyl anion formed with n-BuLi [11]. Palladium-catalyzed alkynylation of 10 with trimethylsilylacetylene in n-butylamine followed by deprotection with aqueous KOH in MeOH gave the diethynyl derivative 11 as very unstable crystals, which in one case exploded spontaneously. 

In contrast, palladium(0)-catalyzed coupling of the requisite starting oxazole for the synthesis of hippadine 219 with trimethylsilylacetylene at 80°C did not afford the expected oxazole-alkyne 220 (Fig. 3.64). Instead, they isolated the tricyclic furan 221 derived from a Diels-Alder retro-Diels-Alder reaction in 77% yield. Thermolysis of 221 at 320°C effected an intramolecular Diels-Alder reaction with concomitant desUylation. Subsequent DDQ oxidation of this product (not shown) then provided hippadine. 

Regioselective photocycloaddition of enol carbonate 59 with allene results in the formation of adduct 60 in 83% yield. The protecting group is removed under palladium-catalyzed conditions, and the resulting alcohol undergoes retro-aldol reaction and olefin isomerization to give the 1,5-diketone 62 in excellent yield. This compound contains the AB ring system of the taxane skeleton. 

The carbonylative cyclization of 2-iodophenols with norbornadiene or norbornene was also carried out. In 1989, Catellani s group studied the annulation of 2-iodophenol with norbornadiene to coumarin (Scheme 3.11a). ° In this method, the elimination of cyclopentadiene via retro-Diels-Alder reaction was involved in the formation of the terminal product. Fiaud and co-workers reported the palladium-catalyzed carbonylative cyclization of 2-iodophenol with norbornene in 1997 (Scheme 3.11b). They reported that the selectivity for the production of the two regioisomers can be controlled. Catellani and co-workers studied the intramolecular cyclization of ort/zo-iodophenyl 3-butenoate to 4-methylcoumarin with a palladium catalyst. 85% of 4-methylcoumarin was produced in the presence of benzonitrile and carbon monoxide, which is compulsory (Scheme 3.12a). Silva, Costa and their co-workers reported a tandem process for the synthesis of coumarins from 2-iodophenols and enoates with the assistance of a palladium catalyst (Scheme 3.12b). The general scope of this Heck-lactonization involves E- and Z-enoates as substrates. It was shown that this reaction is sensitive to steric hindrance around the double bound in... 

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. 

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). 

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. 

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... 

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