Palladium allene elimination reactions

The palladium-catalyzed elimination-cyclization reaction of biscarbamates 254 opens up a further route to nitrogen-substituted allenes (Scheme 8.68) [149]. This transformation proceeds for certain substitution patterns with surprisingly high regioselectivity, favoring allenes 256 with a terminally unsubstituted C=C bond. 

In 1999, Alper and Xiao developed a novel access to thiochroman-4-one derivatives by palladium-catalyzed carbonylative heteroannulation of o-iodothiophenols with allenes. The reaction afforded the thiochroman-4-ones in good to excellent isolated yields with high regioselectivity (Scheme 3.76). The catalytic heteroannulation may involve regioselective addition of the sulfur moiety of the reactants on the more positive end of the allene, atylpalladium formation, CO insertion, subsequent intramolecular cyclization, and then reductive elimination. 

A possible reaction mechanism shown in Scheme 7-10 includes (a) oxidative addition of the S-H bond to Pd(0), (b) insertion of the allene into the Pd-H bond to form the tt-allyl palladium 38, (c) reductive elimination of allyl sulfide, (d) oxidative addition of the I-aryl bond into the Pd(0), (e) insertion of CO into the Pd-C bond, (f) insertion of the tethered C=C into the Pd-C(O) bond, and (g) P-elimination to form 37 followed by the formation of [baseHjI and Pd(0). 

The palladium(0)-catalyzed cyclization of amide-allenes via a carbopalladation has been developed by several groups. The reaction proceeds through the carbopalladation of the allene moiety with an organopalladium species (R-Pd-X), generated by oxidative addition of R-X to palladium(O), and subsequent reductive elimination of the resultant 7r-allylpalladium intermediate.47,47a 47f... 

Addition of PhPdl to the allene triggers cyclopropyl ring opening to generate a cr-palladium species, which readily leads to a 1,3,5-triene through /3-elimination. From the observed diastereoselectivities, the reaction seemingly proceeds stepwise via the well-stabilized zwitterionic intermediate. 

An example of palladium-catalyzed furan synthesis utilizing allenes as starting materials was reported, in which 2,4-disubstituted-2,3-butadienoic acids and 1,2-propadienyl ketones were used and 2,4-disubstituted furans were produced. The reaction may proceed via a matched double oxypalladation-reductive elimination process <04CEJ2078>. In a similar cycloisomerization of substituted allenes to tri- and tetrasubstituted furans with regioselectivity, the allenes were produced in situ from acyloxy-, phosphatyloxy- and sulfonyloxy-substituted alkynylketones via a 1,2-migration of such substituents catalyzed by CuCl or AgBF <04AG(E)2280>. 

Due to the higher reactivity of the allene moiety toward hydropalladation in 1,6-allenynes, the reaction may proceed via a hydropalladation of the allene moiety of 146 affording a vinylic palladium intermediate 147. Subsequent intramolecular carbopalladation of the C-C triple bond moiety would lead to the 1,3-dienyl palladium formate 148. Releasing of CO2 and reductive elimination afford the final product 149 and Pd(0). Pd(0) would react with HCO2H to afford HC02PdH, which is the catalytically active species (Scheme 61) [36]. 

In this section, Pd(0)-catalyzed reactions of allenes with nucleophiles are treated, which are clearly different mechanistically from the reactions explained in the above. Attack of nucleophiles may occur at C-1, C-2, and C-3 carbons of the allenes 63. Among them, attack at C-3 to give 64 is predominant. Most importantly, reactions of allenes with pronucleophiles start by the oxidative addition of pronucleophiles to Pd(0) to generate H-Pd-Nu 65. The formation of 64 by hydro-carbonation can be explained in two ways in the case where Nu-H is the carbon pronucleophile. As one possibility, hydropalladation of one of the two double bonds occurs to afford the terminal palladium intermediate 66, which is stabilized by the formation of 7r-allyl complex 67, and reductive elimination provides the C-3 adduct 68. Another possibility is carbopalladation to generate 69, and subsequent reductive elimination provides 68. Of these two possibilities, the hydropalladation mechanism is preferable. 

The q -(Si-H)Pd(0) 43 was found to catalyze the hydrocarboxylation reaction of allenes, indicating that 43 worked as the palladium hydride complex 13 in solution via reversible oxidative addition/reductive elimination of the Si-H bond after dissociation of PPhj (Scheme 9.12) [21]. This result prompted us to investigate a... 

With the electTOTi-poor allenic esters, palladium(0) is able to catalyze the reaction without gold. The reactiOTi then is initiated at the other end, after oxidative addition of the aryl halide to the electrophilic palladium(II) species cycloisomerizes the allenic ester and then forms the product by reductive elimination. With o-alkynylbenzoates, the intermediate vinylgold species contains an enol ether substructure and is able to directly intercept the activated allyl donors, even in the absence of palladium. In both cases, by careful trace analysis (ICP), the presence of the other metal was excluded [78]. 

In the presence of o-iodothiophenol as the starting compound, it is conceivable that the reaction may proceed through oxidative addition of the o-iodothiophenol to palladium(O) followed by the insertion of allene to produce a a-allylpalladium intermediate. Reductive elimination of Pd(0) would give an iodothioether. Oxidative addition of the latter to Pd(0) and subsequent CO insertion and intramolecular cyclization would afford thiochroman-4-ones 48 and regenerate the catalyst (Scheme 13.31). 

The plausible mechanism of the catalytic reaction involves (1) oxidative addition (2) double bond insertion into the Pd-H bond with formation of / -allyl palladium intermediate and (3) reductive elimination (Scheme 8.23) [85]. In a similar way allene insertion into the Pd-H bond as a key-step of the catalytic cycle... 

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