Cationic palladium complexes, 1,10-phenanthroline

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. 

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. - - ... 

The cationic palladium complex [(dppp)Pd(H20)2] catalyzes the carbonylation of styrene under CO/H2 to give l,5-diphenylpentan-3-one and l,5-diphenylpent-l-en-3-one (40) (5 95), indicating that insertion of a second styrene molecule into the Pd—COR bond is faster than metal-acyl bond hydrogenolysis and hydroformylation. A crystallographically characterized dimer, [(dppp)Pd(/i-OH)] has only low activity. The carbonylation of styrene with Pd(OAc)2/p-tolS03H/benzoquinone/L is ligand-dependent if L = phenanthroline, a syndiotactic copolymer is produced whose regio-selectivity indicates chain-end control of the insertion process (Scheme 25). 

R. A. Widenhoefer developed a mechanistic study on the cyclization-hydrosilylation of various functionalized dienes catalyzed by cationic palladium phenanthroline or oxazoline complex (60). Kinetic, deuterium-labeling, and low temperature nuclear magnetic resonance studies permitted establishing a... 

Compared to the CO insertion, fewer reports have appeared on the olefin insertion into an acyl palladium. Direct observation of this process has been reported very recently by Rix and Brookhart [89-92] using cationic l,10-phenanthroline-Pd(II) complexes. They have investigated the microscopic steps responsible for the alternating copolymerization of ethene with CO using the same 1,10-phenanthroline system. On the basis of the kinetic and thermodynamic data, they proposed an accurate model for the polymer chain growth. In support stepwise isolation of the intermediates have been accomplished by norbornene as a substrate where symmetrical bidentate nitrogen ligands were used [93-95]. 

A second feature to be noted is the fact that isocyanate is apparently liberated much more easily from the cationic complex than from the neutral one. This is to be compared with the palladium-phenanthroline system discussed in paragraph 6.3.2., were the presence of an acid is essential to promote the formation of the isocyanate. We recall that, in the absence of an acid, complex (A) (or (B) as previously discussed, see paragraph 6.3.2.) yields products which may derive from CO deinsertion from a coordinated isocyanate complex, a reaction which is known to be easy for mononuclear complexes [225]. The presence of an acid apparently favours isocyanate decoordination before any following reaction can occur. 

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