Palladium allyl compounds

An important method of preparation of palladium allyl compounds is the reaction of PdX4 or PdX2 with olefins ... 

Palladium catalyzes allylation of carbonyl compounds with various ally lie compounds using In-InCl3 in aqueous media (Eq. 8.66).158 Various allylic compounds can be effectively applied via the formation of TT-allylpalladium(II) intermediates and their transmetalation with indium in the presence of indium trichloride in aqueous media. 

Palladium oxazoline compounds (e.g., (47)) have been used to catalyze the cyclization/hydro-silylation of functionalized 1,6-dienes (Scheme 31). With R = Pr1, >95% diastereomeric excess and 87% ee was achieved at low temperature. Changing the ligand bulk with R = Bu1 gave a higher ee value, but poorer diastereoselectivity. A range of functional groups can be tolerated at both the allylic and terminal alkene positions.135-137... 

Some synthetically useful isomerization reactions of alkenes, other than nitrogen- or oxygen-substituted allylic compounds, were reported by the use of a catalytic amount of transition metal complexes. The palladium complex, /ra r-Pd(C6HsCN)2Gl2, effectively catalyzed the stereoselective isomerization of /3,7-unsaturated esters to a,/3-unsaturated esters (Equation (26)). 

The reaction starts with an oxidative addition of an allylic compound to palladium(O) and a Tt-allyl-palladium complex forms. Carboxylates, allyl halides, etc. can be used. In practice one often starts with divalent palladium sources, which require in situ reduction. This reduction can take place in several ways, it may involve the alkene, the nucleophile, or the phosphine ligand added. One can start from zerovalent palladium complexes, but very stable palladium(O) complexes may also require an incubation period. Good starting materials are the 7t-allyl-palladium intermediates ... 

Palladium-mediated substitutions of silylated allylic compounds are not subject to steric hindrances. The silicon atom exerts control over the reaction site snch thaty-substitution results [64]. While the electrophiles are unavoidably disjoint with respect to the acceptor silicon, one of the two limiting forms, i.e., the a-silylcarbenium ion, is much more unfavorable (a-a arrangement) and therefore its population is expectedly low. 

In 1971 only two complexes of palladium(I) had been identified.65 Although the area has grown significantly, the relative paucity of palladium cluster compounds can be attributed, in part, to the surprising weakness of palladium-carbon monoxide bonds and in particular those where CO is bound terminally. In this chapter the chemistry of palladium(I) and clusters of palladium in other oxidation states will be considered. However, complexes containing organic ligands such as allyl and cyclopentadienyl will not be dealt with as this area has been reviewed recently in a companion volume.66... 

In a sense the tr-allyl compounds of the transition metals can be regarded as the simplest of the sandwich molecules. Bis(jr-allyl)nickel, the best known of such complexes, has been shown by x-ray crystallography (104,105) to have a staggered arrangement of tr-allyl moieties and hence a C2h molecular conformation. The electronic structure of the ground state of bis(jr-allyl)nickel has been investigated by both semiempirical (47) and ab initio (274,275) methods, and a semiempirical computation has been performed on bis(7r-allyl)palladium (47). 

An excellent review21 outlining the mechanism of diphenylphosphinobenzoic acid-based palladium-catalysed asymmetric substitution of allyl compounds with nucleophiles has been published. The mechanistic model developed for these reactions allows one to predict the stereochemistry of the product(s). 

The Tsuji-Trost Reaction (or Trost Allylation) is the palladium-catalyzed allylation of nucleophiles such as active methylenes, enolates, amines and phenols with allylic compounds such as allyl acetates and allyl bromides. 

This chemistry has been extended to produce palladium(O) intermediates (48). Much of the chemistry is similar to that of the Pt analogues except that the palladium(O) complexes are more unstable and difficult to isolate. A reaction characteristic of palladium is the addition of allyl compounds to form cationic allyl complexes, Equation 16. This has been postulated (49) as a key step in the mechanism for Pd(diphos)2 catalyzed reactions of allyl compounds. 

Direct allylation of rhodanine 49 (Scheme 13) under Pd(0)-catalysis with cinnamyl ethyl carbonate affords the /V-allylated compound 50. However, allylation with cinnamyl bromide and a base is not regioselective, producing a mixture of 50 and sulfide 51. Sulfide 51 isomerizes to 50 under palladium catalysis (N > S), thus indicating that Pd(0)-catalyzed allylation of 49 is thermodynamically controlled (93T1465). 

Likewise, in the case of the oxygenated analogs, the combination of the aqueous medium, the water-soluble phosphine tppts (199), palladium(II) acetate, and cinnamyl acetate plus DBU changed the outcome of the reaction dramatically. Thus, from 214, 219, and 224, only sulfides 218,223 and 226 were formed. The sulfides are the products of kinetic control (see the conversion of 218 into the mixture of N-l, N-3, and N-l/N-3 allylated compounds) they separate from the reaction aqueous medium. And this fact, together with the lower temperature required when working in water/ acetonitrile (17/3), permits isolation of the sulfides instead of the thermodynamically more stable /V-allylated compounds. 

As will be highlighted in the following examples, the most interesting feature of this strategy lies in the possibility of generating the dipole component in situ under mild, neutral conditions by simple exposure of a properly designed allylic substrate to a suitable palladium catalyst (for an updated, general review of Pd-catalyzed cyclizations of allylic compounds, see [17]). 

Palladium forms a more extensive series of 7r-allyl complexes than any other transition metal. In many cases these molecules are thermally stable, air-stable crystalline sohds. In addition to stoichiometric 7r-allyl compounds, many palladium 7r-allyls are prepared under catalytic conditions as intermediates in palladium-catalyzed organic synthetic reactions. 

Rhodium and palladium allyl complexes form from the reaction of a Tl(III)allyl compound with a metal halide ... 

The carbonylation of allylic compounds by transition metal complexes is a versatile method for synthesizing unsaturated carboxylic acid derivatives (Eq. 11.22) [64]. Usually, palladium complexes are used for the carbonylation of allylic compounds [65], whereas ruthenium complexes show characteristic catalytic activity in allylic carbonylation reactions. Cinnamyl methyl carbonate reacts with CO in the presence of a Ru3(CO)i2/l,10-phenanthroline catalyst in dimethylformamide (DMF) to give methyl 4-phenyl-3-butenoate in excellent yield (Eq. 11.23) [66]. The regioselectivity is the same as in the palladium complex-catalyzed reaction. However, when ( )-2-butenyl methyl carbonate is used as a substrate, methyl ( )-2-methyl-2-butenoate is the major product, with the more sterically hindered carbon atom of the allylic group being carbo-nylated (Eq. 11.24). This regioselectivity is characteristic of the ruthenium catalyst [66]. 

Cyclopropenes react with palladium(II) compounds to give n allyl complexes 1,2,3-triphenylcyclopropene forms the ring-opened product 310 (equation 93), whilst its 3-methyl and tetraphenyl counterparts give " indenyl complexes 309. 

The palladium-mediated allylation proceeds via an initial oxidative addition of an allylic substrate to Pd(0). The resultant TC-allylpalladium(II) complex A is electrophilic and reacts with carbon nucleophiles generating the Pd(0) complex B, which undergoes ligand exchange to release the product and restart the cycle for palladium. With substituted allylic compounds, the palladium-catalyzed nucleophilic addition usually occurs at the less substituted side. 

Cleavage of the C-O bond in various allylic substrates by oxidative addition to M(0) species gives r 3-allylic complexes, which undergo nucleophilic attack to produce allylic compounds catalytically. A base is needed in most cases to remove HOX and to drive the catalytic cycle. There are a lot of synthetic reactions utilizing allylic oxygen bond cleavage catalyzed by palladium complexes [6, 7, 19-21]. 

Tab, 8.11 Indium-mediated palladium-catalyzed allylation of benzaldehyde with allylic compounds... 

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