Palladium acetate irradiations

In a related study, the precursor 41 could be amiulated either by irradiation or by treatment with palladium acetate in acetic acid to provide indolocarbazoles 42 and 43 in yields of 37% and 55%, respectively (Scheme 8). Both products were eventually deprotected efficiently to give 44 and transformed further under reductive conditions to staurosporinone 45, the aglycone of 8, Alternatively, a shorter route encompassing deprotection of 41, followed by cychzation by irradiation in the presence of iodine and subsequent reduction, gave 45 in an even better overall yield (98T6909). 

In a recent communication, the parent system 2 has been obtained in poor yields (16% and 10%, respectively) from the double cyclization of N-diphenyl-1,3-phenylenediamine either by using two equivalents of palladium acetate in refluxing acetic acid or by irradiation in methanol in the presence of a catalytic amount of iodine (00SC3651). All the available approaches sununarized so far were marred by harsh reactioi conditions or troublesome-to-prepare starting materials, leading to low overall yields of the desired products and difficulty in introducing sensitive substituents. 

The Suzuki reaction has been successfully used to introduce new C - C bonds into 2-pyridones [75,83,84]. The use of microwave irradiation in transition-metal-catalyzed transformations is reported to decrease reaction times [52]. Still, there is, to our knowledge, only one example where a microwave-assisted Suzuki reaction has been performed on a quinolin-2(lH)-one or any other 2-pyridone containing heterocycle. Glasnov et al. described a Suzuki reaction of 4-chloro-quinolin-2(lff)-one with phenylboronic acid in presence of a palladium-catalyst under microwave irradiation (Scheme 13) [53]. After screening different conditions to improve the conversion and isolated yield of the desired aryl substituted quinolin-2( lff)-one 47, they found that a combination of palladium acetate and triphenylphosphine as catalyst (0.5 mol %), a 3 1 mixture of 1,2-dimethoxyethane (DME) and water as solvent, triethyl-amine as base, and irradiation for 30 min at 150 °C gave the best result. Crucial for the reaction was the temperature and the amount of water in the... 

Palladium acetate, [PdO —02CCH3)2l3, possesses a unique quality that makes it attractive for solid state decomposition studies as well as technological applications. It can be spin-coated from solution to form a homogeneous, apparently amorphous solid film. This provides large uniform areas over which we can study the effects of various irradiation sources on the chemical nature of the film. The bulky structure of palladium acetate, shown in Figure 1 (8), may offer a partial explanation of the molecule s ability to achieve an amorphous metastable phase upon rapid evaporation of solvent. 

Photothermal decomposition of palladium acetate by scanned cw Ar+ laser irradiation produces metal features that exhibit pronounced periodic structure as a function of laser power, scan speed, substrate and beam diameter, as shown in Figures 3 and 4. The periodic structure is a function of the rate at which the film is heated by absorption of the incident laser radiation coupled with the rate at which the heat of the decomposition reaction is liberated. This coupling generates a reaction front that outruns the scanning laser until quenched by thermal losses, the process to be repeated when the laser catches up and reaches unreacted material. Clearly, such a thermal process is also affected by the thermal conductivity of the substrate, the optical absorption of the substrate in those cases where the overlying film is not fully absorbing,... 

Table I. Evolved Gases From 2 MeV He+ Irradiation of 0.9 nm Palladium Acetate Film. Molecules arranged in order of decreasing abundance within each group...

Thermal decomposition of palladium acetate, either by laser irradiation or conventional means, leads to complete volatilization of the organic components. The purity of the ion beam-irradiated samples is significantly improved by heating the samples in hydrogen at 300 °C after removal of unirradiated palladium acetate. 

The electronic stopping power of the 2 MeV Ne+ ions in the palladium acetate films is much larger than that of 2 MeV He ions. The most obvious difference between the effects of the two ions is in the appearance of the films at the high dose limit. A 0.90 nm thick palladium acetate film exposed to 2 MeV Ne+ ion irradiation until no further spectroscopic changes occur looks black, compared with the metallic silvery films produced in the He ion irradiation. However... 

Ne+ ion irradiation of a 0.13 nm thick film produces a metallic silvery film. A plot of the infrared COO vibrations as a function of fluence in Figure 10 shows that the intensity decreases with approximately the same functional dependence as in the He ion irradiation, but at a dose that is 17 times lower. In addition, a new band appears at 1616 cm-1, peaking at a dose of — 1.7x1012 ions/cm2, then decreasing rapidly to the same level as the original acetate bands. This may represent the formation of some monodentate acetate species as the palladium acetate trimers are cleaved. In situ infrared spectra of the He ion-irradiated films show a similar band of much smaller relative intensity. 

In a recent article by Botella and Najera, controlled mono- and double-Heck arylations in water catalyzed by an oxime-derived palladacycle were described [22], When the reaction was carried out under microwave irradiation at 120 °C in the presence of dicyclohexylmethylamine with only 0.01 mol% of palladium catalyst (palladium acetate or palladacycle), monoarylation took place in only 10 min with a very high turnover frequency (TOF) of > 40000 (Scheme 6.3). As regards diarylation, 1 mol% of the palladacycle catalyst and 2 equivalents of iodobenzene had to be utilized to obtain moderate to good yields of diarylated product. Whereas microwave heating at 120 °C provided a 31% yield after 10 min, a 66% isolated yield of product was obtained by heating the reaction mixture under reflux for 13 h at 100 °C. Here, the... 

Another interesting application of microwave irradiation is found in the selective dehalogenation of the iodinated indole alkaloid Plakohypaphorine F, described by E. Fattorusso et al. [117,118] (Scheme 32). The bis-halogenated compound was treated with potassium formate and palladium acetate under controlled microwave irradiation, resulting in selective deiodination. The choice of the solvent, in this case DMSO, was found to be crucial. 

The group of Harmata has explored a route that can be used to effectively couple aryl chlorides with methylphenylsulfoximines [100]. Using palladium acetate and rac-binap with a large excess of aryl chlorides as coupling partners and cesium carbonate as the base, yields of 10-94% were attained after one or two 1.5-hour irradiation periods at 135 °C. Switching to an aryl tri-flate, and using a surplus of the sulfoximines (five equivalents) furnished an impressive 94% yield (Scheme 32). 

The use of microwaves was first reported in 1996 for both homogeneous883 and solid-phase coupling reactions of arylboronic acids (Equation (208)).884 Microwave irradiation significantly increases the efficiency of ligandless palladium acetate, solid-phase coupling for combinatorial synthesis, solid-phase coupling using KF-y-alumina,... 

Palladium(II)-promoted alkenylation involving a-bromo sulfonamide has been utilized to construct the bridgehead bicyclic sultam 193 <04OL1313>. Treatment of 192 with palladium acetate in DMF containing K2CO3, tri-2-furylphosphine and 4A molecular sieves at 100 C furnishes 193. Subsequent bromination with NBS and elimination with DBU give rise to conjugated diene 194. When irradiated at 350 nM, 194 is isomerized via a two-photon process to the structurally novel spiro heterocycle 198. 

Reaction of organic halides with alkenes catalyzed by palladium compounds (Heck type reaction) is known to be a useful method for carbon-carbon bond formation at unsubstituted vinylic positions. The first reports of the application of MW methodology to this type of reaction were published by Hallberg et al. in 1996 [109] and by Diaz-Ortiz et al. in 1997 [llOj both used in triethylamine solutions. Later, Ville-min et al. studied the possibility of Heck coupling of iodoarenes with methyl acrylate in aqueous solution under pressurized conditions [111]. The reactions were conducted in a Teflon autoclave under the action of MW irradiation in the presence of palladium acetate, different phosphine ligands, and tetrabutylammonium hydrogen sulfate (TBAHS) as PTC catalyst, to afford the desired coupling products in 40 to 90% yield. 

More recently, Najera et al. performed a number of Heck reactions of deactivated aryl halides and styrenes under phosphane-free conditions using oxime-derived palladacycles or palladium acetate as catalyst [113], Coupling can be performed either vith chcyclohexylmethylamine as base and TBAB as PTC catalyst or in neat vater vith triethylamine in N,N-dimethylacetamide (DMA) solutions under the action of MW irradiation. 

Leadbeater et al. recently showed that the PTC procedure with MW irradiation can be used to prepare of biaryls using water, palladium acetate, and TBAB as solvent, catalyst, and phase-transfer agent, respectively [117]. The desired coupling products were obtained in good yield (60 to 90%). The reaction can, however, be performed equally well using MW and conventional heating methods. Although it... 

Epoxycydohexanone is an unusual substrate for the Heck reaction. The reactivity of this molecule under Heck coupling conditions is most likely attributed to its in-situ isomerization to 1,2-cyclohexanedione. The 1,2-diketone is subsequently reacting as an olefin via the enol tautomer. Thus, within 5 to 30 min of directed microwave irradiation of the aqueous PEG mixture, with less than 0.05% palladium acetate and no phosphine ligand, up to 72% C3-arylated product was isolated (Scheme 15.78) [148]. 

Treatment of MBH aeetates with stabilized ylide (ethoxycarbonylmethylene) triphenylphosphorane ean provide trisubstituted olefins with ( )-selectivity for ester-containing substrate, whereas a mixture of (Z)- and ( )-isomers is obtained for nitrile-substituted aeetate under thermal conditions or microwave irradiation (Seheme 3.91). " The reaetion rates and yields are significantly improved by employing mierowave irradiation. In addition, this reaetion also can proceed smoothly in the presence of palladium acetate (Scheme 3.91). " ... 

Hallberg et al. have shown that microwaves accelerate palladium-catalyzed reactions (e.g. Suzuki, Heck, Tsuji-Trost, Stille) in solution or with supported polymers [127]. Most recently, Villemin and Caillot have reported that, in the Suzuki reaction, the use of a ligand-free palladium catalyst, palladium acetate, without the use of solvent under microwave irradiation produces good yields of biphenyl products, one of which is shown in Equation 89 [128]. 

A combination of palladium acetate and tricyclohexylphosphine is an effective catalyst. The allyl transfer reactions proceed in the presence of the catalyst and cesium carbonate in refluxing toluene. Homoallylic alcohol 1 transfers the methallyl group to a variety of aryl halides (Scheme 5.9) [12b]. The reactions under microwave irradiation at 250 "C go to completion in 15 min [12c]. The catalyst loading can be reduced to 0.05 mol% in the reaction of ethyl chlorobenzoate under microwave heating. 

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