Palladium complexes monodentate

The a-arylation of carbonyl compounds (sometimes in enantioselective version) such as ketones,107-115 amides,114 115 lactones,116 azlactones,117 malonates,118 piperidinones,119,120 cyanoesters,121,122 nitriles,125,124 sul-fones, trimethylsilyl enolates, nitroalkanes, esters, amino acids, or acids has been reported using palladium catalysis. The asymmetric vinylation of ketone enolates has been developed with palladium complexes bearing electron-rich chiral monodentate ligands.155... 

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

The last possibility for ester formation (20, Figure 12.15) comprises the reductive elimination of esters from acyl-alkoxy-palladium complexes 17, formed by deprotonation of the alcohol adducts 16. Clearly, it requires cis coordination of the alkoxide and acyl fragment. Since monodentates have a preference for ester formation, it was thought that this mechanism was very unlikely. 

Styrene characteristically yields the branched acid in the presence of palladium and monodentate phosphine ligands,132 142 and in the [Fe(CO)5]-promoted process.143 Palladium with certain bidentate phosphines, in turn, produces more linear acid.142 Asymmetric hydrocarboxylations with palladium complexes and chiral ligands with enantiomeric excesses up to 84% have been reported.144 145... 

Several catalytic systems have been investigated for hydroamination of unsaturated bonds [16]. Takahashi et al. reported the telomerization of 1,3-dienes in the presence of an amine leading to octadienylamine or allylic amines when palladium catalysts are used in association with monodentate or bidentate phosphine ligands, respectively [17]. Dieck et al. demonstrated the beneficial effect of addition of an amine hydroiodic salt in the hydroamination reaction of 1,3-dienes in which the allylic amines are produced via an intermediate Jt-allyl palladium complex [18]. Coulson reported the Pd-catalyzed addition of amines to allenes where dimerization is incorporated [4]. This reaction presumably proceeds via a cyclic palladium intermediate in which the Pd activates the olefinic bond for nucleophilic attack the reactions are therefore different from pronucleophilic additions. 

In batch processes, the monodentate catalysts showed lower activity compared to their bidentate analogs. The activity per palladium center was constant upon increasing the dendrimer generation of the dendritic Pd(allyl) complexes, indicating that all active sites act as independent catalysts. In addition, the selectivity between the E- and Z-products was similar to that induced by analogous mononuclear palladium complexes. Although a considerable amount of the branched product was observed, the authors did not put forward an explanation for its formation. 

Others have investigated the kinetics of amination reactions mediated by catalyst systems employing the new electron-rich monodentate ligands. In particular, Hartwig has shown that for catalysis by a 1 1 palladium to Xn tert-butyl)phosphine system, a mechanism in which oxidative addition of aryl chlorides follows coordination of base to the palladium competes with the standard nonanionic pathway. Finally, Caddick, Cloke, and coworkers have studied amination reactions of aryl chlorides performed by palladium complexes of N-heterocyclic carbene ligands. They found the rate to be limited by the oxidative addition step, which occurs first through the dissociation of an NHC ligand. 

Palladium complexes bearing chiral monodentate phosphine ligands 592 (R = H, Ph) are effective catalysts for asymmetric alkoxycarbonylation of allyl phosphates <1997TL8227>. 

The asymmetric hydrosilylation of 1-alkenes giving highly optically active 2-tri-chlorosilylalkanes is catalyzed by palladium complexes with chiral monodentate binaphthyl phosphine (MOP) ligands. ... 

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