Allenes as carbon nucleophiles in intramolecular attack on (pi-1,3-diene)palladium complexes: evidence for trans carbopalladation of the 1,3-diene

Reaction of allene-substituted cyclohexa- and cyclohepta-1,3-dienes with [PdCl(2)(PhCN)(2)] gave eta(3)-(1,2,3)-cyclohexenyl- and eta(3)-(1,2,3)-cycloheptenylpalladium complexes, respectively, in which C-C bond formation between the allene and the 1,3-diene has occurred. Analysis of the (pi-allyl)palladium complexes by NMR spectroscopy, using reporter ligands, shows that the C-C bond formation has occurred by a trans carbopalladation involving nucleophilic attack by the middle carbon atom of the allene on a (pi-diene)palladium(II) complex. The stereochemistry of the (pi-allyl)palladium complexes was confirmed by benzoquinone-induced stereoselective transformations to allylic acetates.     

Palladium acetate-catalyzed cyclization reaction of 2,3-allenoic acids in the presence of simple allenes: an efficient synthesis of 4-(1'-bromoalk-2'(Z)-en-2'-yl)furan-2(5H)-one derivatives and the synthetic application

We have realized a cyclization reaction of 2,3-allenoic acids 1 in the presence of simple alkyl- or aryl-substituted allenes 3. In this reaction, the cyclic oxypalladation of 2,3-allenoic acid with Pd(II) would afford the furanonyl palladium intermediate 2, which could be trapped by the simple allene to afford a pi-allylic intermediate anti-9. This intermediate anti-9 could be nucleophilically attacked by Br- to yield 4-(1'-bromoalk-2'(Z)-en-2'-yl)furan-2(5H)-one derivatives Z-5 and Pd(0). The in-situ formed Pd(0) was efficiently converted to the catalytically active Pd(II) species by benzoquinone in HOAc. The functional groups, such as malonate, acetoxyl, and phthalic amide in allene 3, are tolerable under the current conditions. High efficiency of chirality transfer was observed when optically active 2,3-allenoic acids were used, which reveals that the formation of the intermediates 2 was a highly stereoselective anti-oxypalladation process. The highly selective formation of Z-isomer may be explained by face-selective coordination of allene 3 with the palladium atom in intermediate 2: the palladium atom coordinates to the terminal C=C double bond of allene 3 from the face opposite to the substituent group to avoid the steric congestion. The products Z-5 could be further elaborated via the S(N)2 nucleophilic substitution with amine or sodium benzenesulfinate, the reduction of the C-Br bond by NaBH(4), and the CuBr.SMe(2)-catalyzed S(N)2'-substitution with CH(3)MgBr.     

Unexpected reversal of regioselectivity in the palladium-catalysed addition of the tin-germanium bond to allenes

Whereas the addition of tributyl(trimethylgermyl)stannane to 1-alkynes in the presence of tetrakis(triphenylphosphine)palladium proceeds regiospecifically, the direction of its addition to allenes is dependent on the structure of the allene.     

Palladium‐Catalyzed Insertion of an Allene into an Aminal: Aminomethylamination of Allenes by C?N Bond Activation

A new and atom‐economic palladium‐catalyzed aminomethylamination of allenes with aminals by C N bond activation is described. This direct and operationally simple method provides a fundamentally novel approach for the synthesis of 1,3‐diamines. Mechanistic studies suggest that a unique cationic π‐allylpalladium complex containing an aminomethyl moiety is generated as a key intermediate through the carbopalladation of the allene with a cyclometalated palladium–alkyl species.     

Palladium‐Catalyzed Insertion of an Allene into an Aminal: Aminomethylamination of Allenes by CN Bond Activation

A new and atom‐economic palladium‐catalyzed aminomethylamination of allenes with aminals by C? N bond activation is described. This direct and operationally simple method provides a fundamentally novel approach for the synthesis of 1,3‐diamines. Mechanistic studies suggest that a unique cationic π‐allylpalladium complex containing an aminomethyl moiety is generated as a key intermediate through the carbopalladation of the allene with a cyclometalated palladium–alkyl species.     

Synthesis of allenes from allylic alcohol derivatives bearing a bromine atom using a palladium(0)/diethylzinc system

A general and efficient synthesis of allenes using a palladium(0)/diethylzinc system is described. Treatment of mesylates or trichloroacetates of (E)- or (Z)-2-bromoalk-2-en-1-ols with diethylzinc in the presence of a catalytic amount of palladium(0) affords allenes bearing an aminoalkyl, alkyl, or aryl substituent(s) in good to high yields. No transfer of chirality from the stereogenic center carrying the mesyloxy group to the allene was observed.     

Efficient Pd‐Catalyzed Allene Synthesis from Alkynes and Aryl Bromides through an Intramolecular Base‐Assisted Deprotonation (iBAD) Mechanism

An optimized ligand‐controlled palladium‐catalyzed allene synthesis starting from alkynes and aryl bromides giving rise to allene products in a simple and direct manner is described. The methodology is performed in an inter‐ and intramolecular fashion with unprecedented scope and excellent yields. Based on mechanistic investigations and on DFT calculations, the role played by the carboxylic additive (i.e., PivOH) in controlling the selectivity of the reaction is discussed, allowing us to propose an intramolecular base‐assisted deprotonation (iBAD) mechanism for this process.     

Synthesis of novel cyclic α-amino acid derivatives via a one-pot sequential Petasis reaction/palladium catalysed process

A novel one pot Petasis reaction/palladium catalysed process is described involving 2-iodo/bromo benzylamine, ethyl glyoxalate and aryl/heteroaryl boronic acids. The in situ generated carbinolamine/imine undergoes the Petasis reaction to afford 2 which reacts with carbon monoxide or allene (1 atm) in the presence of Pd(0) to generate acyl palladium or π-allyl palladium species which are intercepted intramolecularly by the proximal secondary amine to afford isoindolone/4-methylene-3,4-dihydroisoquinoline α-amino acid derivatives in good yield.     

Synthesis of triazolo- and tetrazolo-tetrahydroisoquinolines and isoquinolines via temperature controlled palladium catalysed allene/azide incorporation/intramolecular 1,3-dipolar cycloaddition cascades

A novel palladium catalysed three-component process is described involving an allene, an aryl iodide containing an activated alkene and TMSN₃. The cascade proceeds via a (π-allyl)palladium species which is intercepted by azide anion followed by stereoselective intramolecular 1,3-dipolar cycloaddition of the organoazide to afford substituted triazolo- and tetrazolo-tetrahydroisoquinolines in good yield. These latter products undergo extrusion of nitrogen to afford isoquinolines at 100 °C.     

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.     

The marked influence of steric and electronic properties of ancillary pyridylthioether ligands on the rate of allene insertion into the palladium–carbon bond

Neutral methyl- and acyl-palladium chloro complexes containing pyridylthioether ancillary ligands (R′N---SR) (R′=H, Me, Cl; R=Me, Et, i-Pr, t-Bu, Ph) have been synthesised and characterised by elemental analysis and spectroscopic methods. The reactivity of these complexes toward allene (allene=DMA=1,1-dimethylpropadiene; TMA=1,1,3,3-tetramethylpropadiene) insertion into the palladium–carbon bond has been studied by ¹H-NMR and UV–vis techniques. The rate of reaction appears to be strongly influenced by the steric and electronic properties of the ancillary ligand. The distortion induced by the substituent R′ in position 6 of the pyridine ring on the main coordination plane of the substrate (allowed by sulphur sp³ hybridisation) renders the substrate itself more prone to nucleophilic attack by the allene. The rate of allene insertion can further be enhanced by lowering the basicity of the chelating atoms in the N---S moiety which results in an increase of electrophilicity of the palladium core, so that the rate constants measured in the case of the complexes containing the ligand 6-chloro-2-phenylthiomethylpyridine (ClN---SPh) are by far the greatest observed so far for similar reactions. Furthermore, on the basis of the indications emerging from the exhaustive study on the behaviour of all the related pyridylthioether methyl complexes, an associative asynchronous bond making mechanism for the rate determining nucleophilic attack by allene is proposed.     

Palladium-catalyzed diastereoselective coupling of propargylic oxiranes with terminal alkynes

[reaction: see text] A diastereoselective coupling of propargylic oxiranes with terminal alkynes has been developed with use of a palladium catalyst. The stereochemistries of the resulting 4-alkynyl-substituted 2,3-allenols have been altered depending on the palladium catalyst. An optically active anti-substituted allene was synthesized from the reaction of an enantiomerically enriched propargylic oxirane without loss of chirality.     

Palladium-catalyzed carboxylative cyclization of α-allenyl amines in dense carbon dioxide

Carboxylative transformation of 2,3-allenic amines into 5-vinyl-1,3-oxazolidin-2-ones was promoted by palladium catalysts under a pressurized CO₂ condition. The presence of a stereogenic center adjacent to the allene group resulted in the formation of the corresponding cyclic urethane as a single trans-diastereomer.     

Four-component Pd-catalysed cascade/ring closing metathesis. Synthesis of heterocyclic enones

A palladium-catalysed four-component process is described involving carbon monoxide, allene and aryl/heteroaryl iodides generating (π-allyl) palladium species, which are intercepted by alkene tethered nitrogen nucleophiles to afford 1,6- and 1,7-dienones. Subsequent ring closing metathesis affords five- and six-membered N-heterocyclic enones. The N-heterocyclic enones are active dipolarophiles in 1,3-dipolar cycloaddition reactions as exemplified by azomethine ylide and nitrone cycloadditions.     

Activation of H2 with allylpalladium(II) derivatives. Selective catalytic hydrogenation of allene to propene

The behaviour of allylpalladium(II) complexes in THF towards molecular hydrogen under mild experimental conditions has been studied. The decomposition to palladium metal and propane is discussed in terms of the fluxionality of the allyl moiety and the stability of a proposed PdH intermediate. Reaction of allylpalladium(II) complexes with H₂ and allene results in catalytic selective hydrogenation to give propene.     

A Versatile Synthesis of β‐Lactam‐Fused Oxacycles through the Palladium‐Catalyzed Chemo‐, Regio‐, and Diastereoselective Cyclization of Allenic Diols

Chemo‐, regio‐ and stereocontrolled palladium‐catalyzed preparations of enantiopure morpholines, oxocines, and dioxonines have been developed starting from 2‐azetidinone‐tethered γ,δ‐, δ,ε‐, and ε,ζ‐allendiols. The palladium‐catalyzed cyclizative coupling reaction of γ,δ‐allendiols 2 with allyl bromide or lithium bromide was effective as 8‐endo cyclization by attack of the primary hydroxy group to the terminal allene carbon to afford enantiopure functionalized oxocines; whereas the palladium‐catalyzed cyclizative coupling reaction of 2‐azetidinone‐tethered ε,ζ‐allendiols 4 furnished dioxonines 16 through a totally chemo‐ and regioselective 9‐endo oxycyclization. By contrast, the palladium‐catalyzed cyclizative coupling reaction of 2‐azetidinone‐tethered δ,ε‐allendiols 3 with aryl and alkenyl halides exclusively generated six‐membered‐ring compounds 14 a and 15 a . These results could be explained through a 6‐exo cyclization by chemo‐ and regiospecific attack of the secondary hydroxy group to the internal allene carbon. Chemo‐ and regiocontrol issues are mainly influenced by the length of the tether rather than by the nature of the metal catalysts and substituents. This reactivity can be rationalized by means of density functional theory calculations.     

Regio et stereochimie de la reduction de derives d'alcools propargyliques catalysee par un complexe de palladium (O)

Regio and stereoselectivity of the palladium catalyzed reduction of propargylic halides and esters is greatly influenced by the nature of the hydride donor as by the leaving group. Best results in allene formation are observed in the reduction by lithium triethyl-borohydride of mesylates and phosphates. In the former case, reaction occurs with anti displacement of the sulfonyl group.     

A new five-membered ring forming process based on palladium(0)-catalyzed arylative cyclization of allenyl enones

A palladium(0)/monophosphine catalyst promotes a novel arylative cyclization reaction of C1-, C2-, and C3-tethered allenyl enones with arylboronic acids to produce five-membered ring containing products. The regioselectivity of the process, associated with aryl group introduction into the allene moiety, depends on the length of the tether. This finding suggests that the cyclization reaction does not proceed through a carbopalladation pathway but rather via a route involving palladacycle-forming or "anti-Wacker"-type oxidative addition to the Pd(0) catalyst.     

Stereoselective palladium-catalyzed carbocyclization of allenic allylic carboxylates

Palladium(0)-catalyzed reaction of allene-substituted allylic carboxylates 3-8 employing 2-5 mol % of Pd(dba)(2) in refluxing toluene leads to the carbocyclization and elimination of carboxylic acid to give bicyclo[4.3.0]nonadiene and bicyclo[5.3.0]decadiene derivatives (12-17). The carbon-carbon bond formation is stereospecific, occurring syn with respect to the leaving group. Addition of maleic anhydride as a ligand to the above-mentioned procedures changed the outcome of the reaction, and under these conditions 3-5 afforded cycloisomerized products 21-23. The experimental results are consistent with a mechanism involving oxidative addition of the allylic carboxylate to Pd(0) to give an electron-deficient (pi-allyl)palladium intermediate, followed by nucleophilic attack by the allene on the face of the pi-allyl opposite to that of the palladium atom. Furthermore, it was found that the Pd(dba)(2)-catalyzed cyclization of the trans-cycloheptene derivative (trans-8) can be directed to give either the trans-fused (trans-17) or the cis-fused (cis-17) ring system by altering the solvent. The former reaction proceeds via a nucleophilic trans-allene attack on the (pi-allyl)palladium intermediate, whereas the latter involves a syn-allene insertion into the allyl-Pd bond of the same intermediate. The products from the carbocylization undergo stereoselective Diels-Alder reactions to give stereodefined polycyclic systems in high yields.     

Telomerization of allene with water catalyzed by palladium complexes

Conclusions The telomerization of allene with water catalyzed by palladium complexes leads to the preparation of 3-methyl-2-methylene-3-buten-1-ol and 2,3,6,7-tetramethylene-1-octanol in high yields.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2624–2626, November, 1985.