Palladium-catalyzed diastereoselective synthesis of homoaldol equivalent products

A palladium-catalyzed reaction of easily accessible 3-(pinacolatoboryl)allyl acetates and aldehydes provides facile access to synthetically useful homoaldol equivalent products with high diastereoselectivity. The reaction presumably proceeds via allylation of aldehydes with α-acetoxy allylboronates that produced in situ by reductive elimination from allylic gem-palladium/boryl intermediates.     

Dihydrofurocoumarin and dihydrofurodihydropyrid-2-one derivatives via palladium catalysed cascades involving aryl/heteroaryl/vinyl iodides and allene followed by acid catalysed cyclisation

Mono 3-(2′-arylallyl) derivatives of 4-hydroxycoumarin 1a,b, 4-hydroxy-6-methyl-pyran-2-one (3) and 6-hydroxy-1,4-dimethyl-1,2-dihydropyrid-2-one-3-carbonitrile (4) are produced in 3-component cascades involving aryl/heteroaryl/vinyl iodides and allene (1 atm) using Pd(PPh₃)₄/Cs₂CO₃/MeCN/80 °C or Pd₂(dba)₃/tris(2-furyl)phosphine/K₂CO₃/DMF/80 °C as the catalyst system. 4-Hydroxy-2-quinolone (2) afforded a mixture of mono- and bis-allylation products under these conditions. Mono C-allylation products 5a-e and 15a-e undergo facile acid catalysed cyclisation to afford dihydrofurocoumarins 11a-e and dihydrofurodihydropyrid-2-ones 16a-e in good overall yield.     

Enantioselective synthesis of N-heterocycles via intramolecular Pd(0)-catalysed allylic amination

An efficient and stereoselective synthesis of pyrrolidine-, piperidine-, and azepane-type N-heterocycles is described by the intramolecular Pd(0)-catalysed cyclisation of amino allylic carbonates. The use of chiral ligands gave the corresponding heterocyclic derivatives having er values that were from moderate to good.     

Palladium-catalyzed cross-couplings of allylic phosphates

A range of palladium-catalyzed cross-coupling reactions can be performed using allylic phosphates as electrophiles. Both conventional heating and microwave irradiation can be used.     

Hydrogen-bond directed palladium-catalyzed allylic substitution of cyclic substrates

The regioselectivity of the Pd-catalyzed allylic substitution of cyclic substrates possessing an adjacent amide functional group was investigated. With imide-like nucleophiles, six-membered ring substrates were found to proceed with a high level of regiodirection whereas hydrogen-bond directed addition was not a significant factor in the five-membered ring substrates.     

N-arylation of carbamate-protected glycine derivatives via palladium catalysis

A synthesis of N-aryl and N-heteroaryl amino acid derivatives using palladium catalysis is described. Several carbamate-protected glycine derivatives react with aryl and heteroaryl halides using a palladium/Xantphos catalyst system to access the desired synthons.     

Pd-catalyzed allylic alkylation of thioamides

This work describes the first Pd-catalyzed allylic alkylation of thioamides. Various thioamides were efficiently α-allylated in high yields and with excellent selectivity for monoallylation under mild reaction conditions. The process not only provides a facile method for the synthesis of functionalized thioamides, but also presents a useful transformation for synthetically important thioamides.     

Isotope effects and the mechanism of palladium-catalyzed allylic alkylation

The palladium-catalyzed allylic alkylation reaction of 1,1-dimethylallyl acetate with dimethyl malonate is studied by a combination of isotope effects and theoretical calculations. A large ¹³C isotope effect of ≈1.037 is observed at the tertiary carbon, while small isotope effects are observed at the olefinic carbons. These results support rate-limiting ionization of a η²-Pd complex. The observed isotope effects are compared with predictions from calculational models employing either solvent models or ionization of an amidinium ion. The calculated transition structures are notably η² in character, and the implications of this observation are discussed.     

Asymmetric synthesis of all-carbon quaternary spirocycles via a catalytic enantioselective allylic alkylation strategy

Rapid access to enantioenriched spirocycles possessing a 1,4-dicarbonyl moiety spanning an all-carbon quaternary stereogenic spirocenter was achieved using a masked bromomethyl vinyl ketone reagent. The developed protocol entails an enantioselective palladium-catalyzed allylic alkylation reaction followed by a one-pot unmasking/RCM sequence that provides access to the spirocyclic compounds in good yields and selectivities.     

Pd-catalyzed nucleophilic allylic alkylation of aliphatic aldehydes by the use of allyl alcohols

Under catalysis of Pd(OAc)₂-(P-n-Bu)₃, Et₂Zn promotes a variety of allyl alcohols to undergo nucleophilic allylation of aliphatic aldehydes and ketones at room temperature and provides homoallyl alcohols in 60-90 and ca. 60% isolated yield, respectively. The reaction is irreversible and kinetically controlled, and unique regio- and stereoselectivities observed for the allylation with unsymmetrically substituted allyl alcohols are discussed.     

A study of catalyst selectivity with polymer bound palladium phosphine complexes on various solid phase synthesis supports

A peptide based ligand system is synthesized on six different supports and then examined for the ability of its palladium complex to catalyze the addition of dimethylmalonate to 3-acetoxycyclopentene in six different solvents. The results on support are correlated to the results observed with the catalyst system dissolved in solution.     

Coordination mode for turn-based phosphine ligands: the origin of selectivity in Pd catalysis

A series of experiments was performed to determine the nature of the catalyst in peptide-derived phosphine ligands. The selectivity of the catalyst system was determined with four ligands that are diastereomeric at the phosphine containing amino acid. Additionally, a series of monophosphine ligands was synthesized and screened to determine if the active catalysts are derived from a phosphine-amide complex.     

Direct palladium/carboxylic acid-catalyzed allylation of anilines with allylic alcohols in water

The direct activation of C-O bonds in allylic alcohols in water as a suspension medium by palladium complexes has been accelerated by carrying out the reactions in the presence of a carboxylic acid. The palladium-catalyzed allylation of anilines using allylic alcohols directly gave allylic anilines in good yields.     

Synthesis of fused heterocycles with a benzazepinone moiety via intramolecular Heck coupling

The preparation of fused heterocycles with a benzazepinone moiety was realised via an intramolecular Heck coupling reaction either at position 2 or at position 3 of the heterocyclic system. This method allowed the synthesis of the pyrrolo[2,3-c]azepinone core and Paullone derivatives.     

Silver-promoted cross-coupling of substituted allyl(trimethyl)silanes with aryl iodides by palladium catalysis

A ligand-free Pd-catalyzed cross-coupling of substituted allyl(trimethyl)silanes with aryl iodides enabled by silver salts was developed. This reaction delivered allylic arenes chemoselectively and regioselectively. The study suggested that the reaction might proceed through oxidative addition of ArI to Pd(0) followed by halide abstraction to give an electrophilic complex ArPdX, which further reacted with allyl(trimethyl)silanes via electrophilic addition/desilylation/reductive elimination to afford the allyl-aryl coupling products.     

Oxidative cross-coupling of allyl(trimethyl)silanes with aryl boronic acids by palladium catalysis

The first oxidative cross-coupling of allylsilanes with aryl boronic acids has been developed by palladium catalysis. The reaction between β-substituted allyl(trimethyl)silanes and a wide range of aryl boronic acids afforded allylarenes in moderate to good yields and excellent selectivity. On the basis of experimental results and literature reports, it was suggested that the reaction might start from transmetalation of aryl boronic acid with AgOAc followed by transmetalation with Pd(II) to give an arylpalladium acetate complex as a key intermediate. This intermediate underwent either electrophilic addition/desilylation or transmetalation with allylsilane and subsequent reductive elimination to give the final product.     

Regioselective synthesis of trifluoromethyl group substituted allylic amines via palladium-catalyzed allylic amination

The palladium-catalyzed regioselective allylic amination of α-trifluoromethylated allyl acetate occurred using Pd(OAc)₂/DPPE and [Pd(π-allyl)(cod)]BF₄/DPPF. The selective formation of the γ-product was attained by Pd(OAc)₂/DPPE, while the α-product was obtained using [Pd(π-allyl)(cod)]BF₄/DPPF.     

Palladium-catalyzed asymmetric synthesis of allylic alcohols from unsymmetrical and symmetrical racemic allylic carbonates featuring C-O-bond formation and dynamic kinetic resolution

Described is the asymmetric synthesis of the allylic alcohols 11 (85% ee), 15 (99% ee), 17 (93% ee), 19 (61% ee), and 21 (69% ee) through a Pd-catalyzed reaction of the unsymmetrical carbonates rac-10, rac-12, rac-14, rac-16, rac-18, and rac-20, respectively, with KHCO₃ and H₂O in the presence of bisphosphane 6. Similarly the allylic alcohols 23 (99% ee) and 25 (97% ee) have been obtained from the symmetrical carbonates rac-22 and rac-24, respectively. Reaction of the meso-biscarbonate 26 with H₂O and Pd(0)/6 afforded alcohol 27 (96% ee), which was converted to the PG building block 32. The unsaturated bisphosphane 33 showed in the synthesis of alcohols 36, 37, and 39 a similar high selectivity as 6. The formation of alcohols 11, 15, and 17 involves an efficient dynamic kinetic resolution.     

Preparation and catalytic properties of NR-stabilized palladium colloids

Palladium colloids revealing narrow particle size distributions can be obtained by chemical reduction using tetra–alkylammonium hydrotriorganoborates. Combining the stabilizing agent [NR] with the reducing agent [BEt₃H^?] provides a high concentration of the protecting group at the reduction centre. Alternatively, NR₄X (X = halogen) may be coupled to the metal salt prior to the reduction step: addition of N(octyl)₄Br to Pd(ac)₂ in THF, for example, evokes an active interaction between the stabilizing agent and the metal salt. Reduction of NR-stabilized palladium salts with simple reducing agents such as hydrogen at room temperature yields stable palladium organosols which may be isolated in the form of redispersible powders. The anion of the palladium salt is crucial for the success of the colloid synthesis. Electron microscopy shows that the mean particle size ranges between 1.8 and 4.0 nm. An X–ray–photoelectron spectrscopic examination demonstrated the presence of zerovalent palladium. These palladium colloids may serve as both homogeneous and heterogeneous hydrogenation catalysts. Adsorption of the colloids onto industrially important supports can be achieved without agglomeration of palladium particles. The standard activity of a charcoal catalyst containing 5% of colloidal palladium determined through the cinnamic acid standard test was found to exceed considerably the activity of the conventional technical catalysts. In addition, the lifespan of the catalyst containing a palladium colloid, isolated from the reduction of [N(octyl)₄]₂PdCl₂Br₂ with hydrogen, is superior to conventionally prepared palladium/charcoal (Pd/C) catalysts. For example, the activity of a conventional Pd/C catalyst is completely suppressed after 38×10³ catalytic cycles per Pd atom, whereas the colloidal Pd/C catalyst shows activity even after 96times;10³ catalytic cycles.     

Microwave-assisted palladium-catalyzed allylation of aryl halides with homoallyl alcohols via retro-allylation

The palladium-catalyzed allylation of aryl halides with homoallyl alcohols via retro-allylation proceeds at 200-250 °C in a toluene-DMF mixed solvent using microwave heating. Even at such high temperatures, the regio- and stereospecificity of the allyl transfer reaction is still satisfactory. The amount of the palladium catalyst can be reduced to 0.5 or 0.05 mol %.