Palladium complexes amidines

The metal-bound RCN group is also activated on coordination towards nucleophilic attack by alcohols, thiols or amines to give stable N-bonded iminoether, iminothioether and amidine complexes respectively.332 Several cationic cyanobenzylpalladium(II) complexes have been prepared, and the reactivity of the CN group towards nucleophiles has been studied.333,334 The palladium complex (97) reacts with aromatic amines to give chelated amidino complexes (98) and the reaction has been studied kinetically.333 In this case intermediates with the nitrile group bonded side-on are considered to be involved. 

Table 3.5 Asymmetric allyic alkylation catalysed by the palladium complex of amidine ligand...

Dauwe and Buddrus [12] tested a series of chiral amines for their ability to discriminate between the enantiomers of several acidic compounds. Strong basic amidines, as shown in Figure 6-8, turned out to be suitable for the resolution of even weak acidic compounds such as phenols, barbiturates and alcohols. The same group [13] developed chiral palladium complexes with diamines for discrimination of a-amino acids and determination of enantiomeric excess resulting from asymmetric synthesis. 

The protocols for the utilization of ketone-derived silyl enol ethers in Tsuji-Trost reactions were preceded by a report of Morimoto and coworkers on the enantioselective allylation of sUyl ketene acetals 88. Without external activation, they reacted with the allylic substrate 19d in the presence of the palladium complex derived from the amidine ligand 89 to give y,5-unsaturated esters 90 in moderate chemical yield but high enantiomeric excess (Scheme 5.29) [46]. Presumably, the pivalate anion hberated during the oxidative addition functions as an activator of the silyl ketene acetal. The protocol is remarkable in view of the fact that asymmetric allylic alkylations of carboxylic esters are rare. Interestingly, the asymmetric induction originates from a ligand with an uncomplicated structure. The protocol seems however rather restricted with respect to the substitution pattern of allylic component and sUyl ketene acetal. 

Pd(0)/phosphine complexes, or their precursors, in the presence of a suitable co-base, have also been shown to promote, in good yields (66-100%), the formation of allylic carbamates from various primary and secondary aliphatic amines, pressurized C02 and allylic chlorides, in THF, at ambient temperature [87a]. The choice of the added co-base (Base), used for generating the carbamate salt RR NC02 (BaseH)+, was found to be critical for high yields of O-allylic urethanes. The use of a guanidine (CyTMG) or amidine (DBU) base was optimal for this system (see also Section 6.3.1). ft is assumed that this chemistry passes catalytically through a mechanism similar to that illustrated in Scheme 6.19. This involves nucleophilic attack by carbamate anion on a (tt-allyl) palladium species, formed by the oxidative addition of the allylic chloride to a palladium(O) intermediate. 

Wanniarachchi, YA. and Slaughter, L.M. (2007) One-step assembly of a chiral palladium bis (acyclic diaminocarbene) complex and its unexpected oxidation to a bis(amidine) complex. 

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