Palladium complexes, 1,10-phenanthroline

It has been discovered that styrene forms a linear alternating copolymer with carbon monoxide using palladium II—phenanthroline complexes. The polymers are syndiotactic and have a crystalline melting point - 280° C (59). Shell Oil Company is commercializing carbon monoxide a-olefin plastics based on this technology (60). 

Helquist et al. [129] have reported molecular mechanics calculations to predict the suitability of a number of chiral-substituted phenanthrolines and their corresponding palladium-complexes for use in asymmetric nucleophilic substitutions of allylic acetates. Good correlation was obtained with experimental results, the highest levels of asymmetric induction being predicted and obtained with a readily available 2-(2-bornyl)-phenanthroline ligand (90 in Scheme 50). Kocovsky et al. [130] prepared a series of chiral bipyridines, also derived from monoterpene (namely pinocarvone or myrtenal). They synthesized and characterized corresponding Mo complexes, which were found to be moderately enantioselective in allylic substitution (up to 22%). 

Hydrosilylation of dienes accompanied by cyclization is emerging as a potential route to the synthesis of functionalized carbocycles. However, the utility of cycliza-tion/hydrosilylation has been Umited because of the absence of an asymmetric protocol. One example of asymmetric cycUzation/hydrosilylation has been reported very recently using a chiral pyridine-oxazoUne ligand instead of 1,10-phenanthroline of the cationic palladium complex (53) [60]. As shown in Scheme 3-21, the pyridine-oxazoUne Ugand is more effective than the bisoxazoUne ligand in this asymmetric cyclization/hydrosilylation of a 1,6-diene. 

It has been found that A-tosyl aziridines undergo oxidative addition to palladium complexes to form azapalladacyclobutanes <06JA15415>. Reaction of aziridine 95 with Pd2(dba)3 and 1,10-phenanthroline provides the palladacycle 96 in 45% isolated yield. This compound is an air stable solid. Treatment the palladacycle 96 with catalytic Cul is believed to open the palladacycle to form a copper intermediate, which cyclizes to cyclopentyl alkylpalladium intermediate 97. Loss of Cul then provides the product palladacycle 97 as an air stable solid. Several different aziridines were examined in this reaction. Only a limited set of olefin substituted aziridines provided the azapalladacyclobutanes (e.g. 96). 

Similarly, a water-soluble palladium complex of a sulfonated phenanthroline ligand catalyzed the highly selective aerobic oxidation of primary and secondary alcohols in an aqueous biphasic system in the absence of any organic solvent (Figure 1.8) [40]. The liquid product could be recovered by simple phase separation, and the aqueous phase, containing the catalyst, used with a fresh batch of alcohol substrate, affording a truly green method for the oxidation of alcohols. 

It is reasouable to expect that the use of rigid 60° angular buildiug blocks with 180° liuear linking units would provide triangular structures. Indeed, this has been demonstrated by the self-assembly of 4,7-phenanthroline and the linear phenyl-bridged bis-palladium complex, which yields the triangle (26). ... 

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]. 

An improved method for the synthesis of hydrogen peroxide from carbon monoxide, water and oxygen catalyzed by palladium complexes in presence of a quinone co-catalyst is described. The use of 1,4-naphthoquinone and 1,10-phenanthroline as palladium ligand resulted in a marked catalyst stabilization against deactivation processes such as polynuclear species formation and Pd-black precipitation, which are very fast operating in the absence of quinone. 

In particular, the palladium complex with 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, was selected as the most efficient catalyst. The presence of the methyl substitution in 2,9-positions was necessary to avoid the formation of binuclear complexes like 1 (detected using 1,10-phenanthroline as palladium ligand), which are stable in reaction conditions and are responsible of the fast irreversible deactivation of the catalyst. 

The use of palladium complexes witli 1,10-phenanthroline in the presence of a catalytic amount of a suitable quinone, provides a stable and efficient catalyst for the synthesis of hydrogen peroxide from carbon monoxide, oxygen and water. 

A number of bidentate ligands containing pyridine and other azine binding sites were described, involving five-membered [262], six-membered [263] and macrocyclic [262] chelate rings. All these ligands afford SRPCs active in type 1 Mizoroki-Heck reactions. Similar activity was recorded for phenanthroline-derived palladium complexes [264]. 

Indeed, direct measurements of the rates of insertion of CO and ethylene into alkyl-metal olefin and acylmetal olefin complexes show that the insertion of ethylene into the metal-acyl linkage is faster than the insertion of ethylene into the metal-alkyl linkage. Comparisons of these rates for insertions into cationic palladium complexes containing phenanthroline and bis-diphenylphosphinopropane as ancillary ligand have been made by Brookhart and co-workers. These reactions are shown in Equations 9.69 and 9.70. A summary of the barriers for insertion is provided in Table 9.2. The rate of insertion of ethylene into the metal-acyl bond is orders of magnitude faster than the rate of insertion of ethylene into the metal-alkyl bond. - - ... 

In addition, Durand et al. have recently used an axially chiral monodentate phosphine, such as (I j-Ph-BINEPINE (Scheme 2.62), as a chiral resolving agent of a racemic palladium complex prepared from a 2-ferrocenyl-l,10-phenanthroline ligand. The reaction evolved through a DKR process, leading after recrystallisation to the isolation of only one of the two possible diastereoisomers. 

Palladium complexes have also been used to generate vinyl ethers through transetherification reactions [103], In addition to simple substrates, this chemistry has been extended to steroidal substrates (Scheme 2.75) [104]. The catalyst was a phenanthroline complex of palladium acetate, and ethyl vinyl ether served as the source of the vinyl fragment. The reaction conditions were mild, but the dmation of the reaction was long (4 d). 

An interesting approach to the preparation of enamides entailed a formal vinyl transfer from vinyl ethers (Scheme 3.122) [130]. The reaction was catalyzed by palladium complexes bearing a phenanthroline derivative as the solubilizing/stabilizing ligand. Several secondary amides including cyclic and acyclic substrates were screened, and... 

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