Palladium n

In contrast to binary carbonyl studies, co-condensation of Pd atoms with at 4.2—10 °K has been shown by matrix isolation i.r. and Raman spectroscopy to lead to only three species [Pd(N2) ] (n = 1—3). Structural assignments were made on the basis of isotope studies, and force constants calculated. 

Halogeno-phosphine and -phosphite complexes, cis- and trans-isomers of [PdCl2(PMe2-o-tolyl)2] have been prepared 

Group VI Donors. O-donor ligands. Few complexes of palladium with the soft OH ligand are known in view of HSAB considerations. However, stable 

Thermogravimetric curves for solid K2[Pd(C204)2],3H20 and other transition-metal oxalates indicate that the thermal stability of the anhydrous complexes decreases with increase in electron affinity of the central metal ion. AH values were obtained for both dehydration and decomposition. Subsequent studies showed carbon dioxide as the only gaseous product, the decomposition occurring via electron transfer from a 304 ligand to the central palladium.  

Sagara, H. Kobayashi, and K. Ueno, Bull. Chem. Soc. Japan, 1973, 46, 484. 

Miscellaneous. Hynes and Moran have carried out kinetic studies on substitution reactions of Pt dithiolate complexes with CN and with several dithiolate ions. In the reactions with the bidentate ligands the mechanism is similar to that proposed for the corresponding reactions of the nickel and palladium complexes, and the order of reactivities is Ni Pd Pt = 10 10 1. Evidence for an unusual mechanism in some reactions of Pt complexes has been reported in the case of the process 

PalladiumCn).—Reference has been made above to several publications - in which both platinum(ii) and palladium(ii) complexes have been studied. 

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The successful implementation of this strategy is shown in Scheme 4. In the central double cyclization step, the combined action of palladium(n) acetate (10 mol %), triphenylphosphine (20 mol %), and silver carbonate (2 equiv.) on trienyl iodide 16 in refluxing THF results in the formation of tricycle 20 (ca. 83 % yield). Compound 20 is the only product formed in this spectacular transformation. It is noteworthy that the stereochemical course of the initial insertion (see 17—>18) is guided by an equatorially disposed /-butyldimethylsilyl ether at C-6 in a transition state having a preferred eclipsed orientation of the C-Pd a bond and the exocyclic double bond (see 17). Insertion of the trisubstituted cycloheptene double bond into the C-Pd bond in 18 then gives a new organopal-... 

Scheme 12. Trost s palladium(n)-catalyzed Alder ene cyclization process.

Because the Sonogashira coupling process outlined in Scheme 18 is initiated by the in situ reduction of palladium(n) to palladium(o), it would be expected that palladium(o) catalysts could be utilized directly. Indeed, a catalytic amount of tetrakis(triphenylphosphine)-... 

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

To a mixture of vinyl bromide (40 mmol) and the catalyst dichloro-[(R)-Af,N-dimethyl-l-[(.S)-2-(diphenylphosphino)ferrocenyl]ethylamine]-palladium(n) (0.2 mmol) was added an ethereal solution of [a-(trimethyl-silyl)benzyl]magnesium bromide (0.6-1 m, 80 mmol) at —78 °C. The mixture was stirred at 30 °C for 4 days, and then cooled to 0 °C and hydrolysed with dilute aqueous HC1 (3 m). The organic layer was separated, and the aqueous layer was re-extracted with ether. The combined organic extracts were washed with saturated sodium hydrogen carbonate solution and water, and dried. Concentration and distillation gave the chiral allylsilane (79%, 66% ee), b.p. 55°C/0.4mmHg. 

Independently, Caddick et al. reported microwave-assisted amination of aryl chlorides using a palladium-N-heterocyclic carbene complex as the catalyst (Scheme 99) [lOlj. Initial experiments in a domestic microwave oven (reflux conditions) revealed that the solvent is crucial for the reaction. The Pd source also proved very important, since Pd(OAc)2 at high power in DMF gave extensive catalyst decomposition and using it at medium and low power gave no reaction at all. Pd(dba)2/imidazohum salt (1 mol% catalyst loading) in DME with the addition of some DMF was found to be suitable. Oil bath experiments indicated that only thermal effects are governing the amination reactions. 

The oxidation of silyl enol ethers 111 with palladium(n) acetate is a convenient nnethod for the preparation of synthetically useful 2,6-disubstituted 2,3-dihydro-4-pyridones 112 <95TL(36)9449>. 

A silver(i) complex having the heterosubstituted 3-methyl-l-(2-pyridylmethyl)imidazol-2-ylidene ligand, [Ag(carbene)2] [I/Agl2] 37, was reported, which was further reacted to give a series of palladium(n) carbene complexes that were demonstrated to be active catalysts toward Heck, Suzuki, and Sonogashira coupling reactions.87... 

When Knochel and his co-workers attempted to use [PdC CF CN ] and related palladium(n) complexes as catalysts in the reactions of dialkylzincs with alkyl iodides, they observed the formation of the halogen-zinc exchange405 or cyclization406 products only. A recent paper of Zhou and Fu demonstrated that palladium complexes can also be used in the coupling reactions of alkylzinc bromides with alkyl iodides, bromides, chlorides, and... 

Utilizing more reactive discrete palladium-N-heterocyclic carbene (NHC) complexes (for example, Pd(carb)2) or in situ generated palladium/imidazolium salt complexes (1 mol% ligand A), Caddick and coworkers were able to extend the rapid amination protocols described above to electron-rich aryl chlorides (Scheme 6.61) [128],... 

The dimeric 2,3-dihydro-l,2,4-oxadiazole palladium(n) complex 182 (Equation 27) reacts with aqueous methylamine to liberate the ligand 183 <2003JCD2544>. A similar process has also been applied to platinum(lv)-bound complexes 184, using pyridine to liberate the 2,3-dihydro-l,2,4-oxadiazole <2000JA3106>. Reduction of the platinum(iv) complexes 184 (Equation 28) gives the corresponding platinum(n) complexes 185 <2001IC264>. 

The addition of 1-alkynes to a,/3-unsaturated ketones in water is catalyzed by a palladium(n)/phosphine combination. Deuteration studies suggest that this reaction proceeds via carbometallation of the olefinic moiety by an alkynylpalladium intermediate (Equation (193)).1... 

The palladium-catalyzed arylation of 2-phenylphenols and naphthols shows an interesting feature of arylation of C-H bonds, leading to the formation of an (aryl)(aryloxy)palladium(n) intermediate.65,65a,65b The phenolates are suitable as precoordinating groups. The reaction of 2-hydroxybiphenyl with an excess of iodobenzene occurs regioselectively at the two ortho-positions of phenyl group under palladium catalysis (Equation (57)). In the case of 1-naphthol, the peri-position is phenylated (Equation (58)). 

The palladium(n)-catalyzed direct carbonylation proceeds with remarkable site selectivity to afford a variety of five- or six-membered benzolactams from A-alkyl-cc-arylalkylamines in a phosphine-free catalytic system using Pd(OAc)2 and Cu(OAc)2 in an atmosphere of CO gas containing air (Equation (90)).118... 

Rhodium(i) complexes are excellent catalysts for the 1,4-addition of aryl- or 1-alkenylboron, -silicon, and -tin compounds to a,/3-unsaturated carbonyl compounds. In contrast, there are few reports on the palladium(n) complex-catalyzed 1,4-addition to enones126,126a for the easy formation of C-bound enolate, which will result in /3-hydride elimination product of Heck reaction. Previously, Cacchi et al. described the palladium(n)-catalyzed Michael addition of ArHgCl or SnAr4 to enones in acidic water.127 Recently, Miyaura and co-workers reported the 1,4-addition of arylboronic acids and boroxines to a,/3-unsaturated carbonyl compounds. A cationic palladium(n) complex [Pd(dppe)(PhCN)2](SbF6)2 was found to be an excellent catalyst for this reaction (dppe = l,2-bis(diphenyl-phosphine)ethane Scheme 42).128... 

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