Palladium-catalyzed reactions of allenes

Carbon-carbon bond formation in palladium(II)-catalyzed allylic oxidation: a novel oxidative carbocyclization of allene-substituted olefins

A new efficient palladium(II)-catalyzed oxidative carbocyclization has been developed. It was found that allene-substituted olefins 1 cyclized in the presence of 1 mol % Pd(O2CCF3)2 and p-benzoquinone (2 equiv) to give bicyclic ring systems 2 in good to excellent yields. The cyclization constitutes a new type of carbon-carbon bond forming reaction between an allene and an olefin under oxidative conditions.

Reversed stereoselectivity in iodohydroxylation of allenyl sulfides. An efficient synthesis of (Z)-3-organosulfur-2-iodo-2-propenols

[reaction: see text] It is observed that the iodohydroxylation of 1,2-allenyl sulfides with I(2) and water in aqueous acetone showed Z-selectivity, which is opposite to that of the iodohydroxylation of 1,2-allenyl sulfoxides.

Allenes as carbon nucleophiles in palladium-catalyzed reactions: observation of anti attack of allenes on (pi-allyl)palladium complexes

In the palladium-catalyzed cyclization of allenic allylic esters using Pd(dba)2 as catalyst, it was shown that the allene acts as a carbon nucleophile. Intermediates were isolated and stereochemical studies established that the double bond of the allene has attacked the (pi-allyl)palladium intermediate on the face opposite to that of palladium.

Efficient synthesis of 4-(2'-alkenyl)-2,5-dihydrofurans and 5,6-dihydro-2H-pyrans via the Pd-catalyzed cyclizative coupling reaction of 2,3- or 3,4-allenols with allylic halides

In the absence of a base, palladium(II) catalysts, such as PdCl(2), PdCl(2)(CH(3)CN)(2), Pd(OAc)(2), and [(pi-C(3)H(5))PdCl](2), can catalyze the cyclizative coupling reaction of 2,3- or 3,4-allenols with allylic halides in DMA at room temperature to provide 2,5-dihydrofurans and 5,6-dihydro-2H-pyrans, respectively, in moderate to good yields. Under similar reaction conditions, nonsubstituted 2,3-allenol 1s affords bimolecular cyclizative coupling product 5s as the major product. The scope of the reaction and its mechanism have been studied briefly. On the basis of the experimental results, the transformation was believed to proceed via a divalent palladium-catalyzed pathway.

Tuning the reaction paths in palladium(0)-catalyzed coupling-cyclization reaction of beta-amino allenes with organic halides: a substituent switch

[reaction: see text] Substituent effects on the allene moiety and the N-protecting group were found to be the dominant factor in determining the reaction paths in the Pd(0)-catalyzed coupling-cyclization reaction of beta-amino allenes with organic halides.

Additions of allenyl/propargyl organometallic reagents to 4-oxoazetidine-2-carbaldehydes: novel palladium-catalyzed domino reactions in allenynes

Metal-mediated carbonyl allenylation and propargylation of 4-oxoazetidine-2-carbaldehydes were investigated in aqueous environment. Different propargyl bromide and metal promoters showed varied regio- and stereoselectivities on product formation. In addition, an unprecedented one-pot stereoselective synthesis of beta-chlorinated allylic alcohols, which can also be considered as functionalized allylsilanes, has been developed, which involves tin(IV) chloride-mediated reaction of propargyltrimethylsilane and 4-oxoazetidine-2-carbaldehydes. Some of the resulting coupling products were submitted to transition metal catalyzed reactions, such as the allenic Pauson-Khand and palladium-catalyzed reactions, leading to novel fused or bridged tricyclic beta-lactams. Remarkably, a novel domino process, namely the allene cyclization/intramolecular Heck reaction was found. A likely mechanism for the cascade reaction should involve an intramolecular cyclization on a (pi-allyl)palladium complex and a Heck-type reaction.

Palladium-catalyzed three-component assembling of allenes, organic halides, and arylboronic acids

An efficient method for the construction of two carbon-carbon bonds in a regio- and stereoselective fashion via palladium-catalyzed assembling of allenes, organic halides, and arylboronic acids is described. Organic halides (RI = C(6)H(5)I, o-, m-, and p-CH(3)OC(6)H(4)I, p-C(2)H(5)OCOC(6)H(4)I, p-CH(3)COC(6)H(4)I, p-CH(3)C(6)H(4)I, p-CH(3)C(6)H(4)Br, p-CH(3)C(6)H(4)Cl, p-NO(2)C(6)H(4)I, p-NO(2)C(6)H(4)Br, p-NO(2)C(6)H(4)Cl, p-IC(6)H(4)Cl, 1-iodonaphthalene, 2-iodothiophene, 3-iodo-2-cyclopenten-1-one, 3-iodo-5,5-dimethyl-2-cyclohexen-1-one, C(6)Eta(5)(Br)C=CH(2) and ICH(2)CO(2)C(2)H(5)), and arylboronic acids (ArB(OH)(2), Ar = C(6)H(5), p-CH(3)OC(6)H(4), m-NO(2)C(6)H(4), p-FC(6)H(4), 1-C(10)H(7), and o-, m-, and p-CHOC(6)H(4)) undergo Suzuki-type three-component assembling with 1,1-dimethylallene to give the corresponding allylic derivatives, (CH(3))(2)=CRCH(2)Ar, in DMF at 70 degrees C in the presence of CsF using Pd(dba)(2) as the catalyst. Higher yields of products were obtained for aryl iodides than for the corresponding aryl bromides and chlorides. This three-component assembling is highly regioselective, with the organic group on halides adding to the middle carbon and the aryl group on arylboronic acids to the unsubstituted terminal carbon of allenes. Monosubstituted allenes 1b-e (cyclopentylallene, cyclohexylallene, tert-butylallene, and n-butylallene) also undergo similar assembling reaction with organic halides and arylboronic acids to afford the corresponding products 7a-i with high regio- and stereoselectivity. Based on the known palladium chemistry, a mechanism is proposed to account for the catalytic reaction and the stereochemistry.

Carbon-carbon bond formation in regio- and stereoselective palladium-catalyzed cyclization of allene-substituted conjugated dienes

Regio- and stereoselective palladium-catalyzed reactions of allene-substituted 1,3-dienes 1 in acetic acid at room temperature lead to cyclization with formation of a carbon-carbon bond between the middle carbon of the allene and the terminal carbon of the 1,3-diene. Two different types of reactions, both that constitute 1,4-carboacetoxylations of the 1,3-diene, have been developed. In one of the reactions, Pd(II) catalyzes the oxidation of 1 to bicyclic compounds 2, and in the other, Pd(0) catalyzes the transformation of 1 to bicyclic compounds 3. The products 2 are useful for further synthetic transformations and undergo Diels-Alder reactions with dienophiles to give polycyclic ring systems.

Palladium(II)-catalyzed S(N)2' reactions of alpha-allenic acetates. Stereoconvergent synthesis of (Z,E)-2-bromo-1,3-dienes

The reaction of acetylated alpha-allenic alcohols with LiBr in the presence of 1.5 mol % of Pd(OAc)(2) provides easy access to substituted (Z,E)-2-bromo-1,3-dienes in good yields with excellent diastereoselectivity. Both secondary and tertiary acetates as well as terminal and nonterminal allenes were studied to investigate the scope and the limitations of the reaction. A mechanism is proposed to clarify how a diastereomeric mixture of the starting compound is transformed into a single diastereomer of the product.

Palladium(II)-catalyzed tandem reaction of intramolecular aminopalladation of allenyl N-tosylcarbamates and conjugate addition

A divalent palladium-catalyzed tandem cyclization-coupling reaction of allenyl N-tosylcarbamates and acrolein was developed. The reaction gave aldehyde-functionalized 2-oxazolidinones in one step with high regioselectivity. A mechanism involving intramolecular aminopalladation of allene, followed by insertion of alkene and C-Pd bond protonolysis, is proposed. [reaction: see text]

Highly efficient synthesis of 1,1-difluoro-2-substituted-1,3-dienes and dienynes from gem-difluorohomoallenyl bromide via palladium-catalyzed cross-coupling reactions

[reaction: see text] A stable gem-difluoroallenylindium 1 is readily transformed to a potentially bifunctional electrophile 2, which can then undergo a transition-metal-promoted isomerization-cross-coupling cascade to yield diene 3 and dienyne 4 in very high yields.