- Chelate effect on the structure and reactivity of electron-rich palladium complexes and its relevance to catalysis
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In order to clarify the origin of the "chelate effect" in catalysis by palladium, complexes of Pr2P(CH2)niPr2P (n = 2, dippe; n = 3, dippp; n = 4, dippb), Ph2P(CH2)3PPh2 (dppp), and PiPr2nBu were prepared and their structures, dynamic properties, and reactivities were compared. Pd(dippe)2 1d is a coordinatively saturated complex, both in solution and in the solid state. X-ray characterization exhibits a distorted tetrahedral geometry. The dippe bite angle is 87.05°. The compound crystallizes in the orthorhombic space group Pnna with a = 16.713(3) A?, b = 17.561(3) A?, c = 11.116(2) A?, V = 3277(1) A?3, Z = 4. Pd(dippp)2 (1a) and Pd(dippb)2 (1e) are coordinatively unsaturated, trigonal complexes and are in equilibrium with the binuclear complexes LPd(η2-L)PdL, 1b and 1f, respectively. Whereas 1d does not exhibit dynamic behavior, 1a and 1e undergo fast, intramolecular phosphine exchange, a process which is not observed with 1b and 1f. The trigonal complexes (dippp)PdPiPr2Bu (1c) and (PiPr2nBu)3Pd were also prepared for comparison. The dippp complexes 1a-1c react with aryl chlorides to produce cis-(dippp)Pd(C6H4X)Cl as the major product and trans-(η1-dippp)2Pd(C6H4X)Cl as the minor one (X = 4-OMe, 4-Me, H, 3-OMe, 4-COMe, 4-CHO, 4-NO2). In contrast, the dippb complex 1e oxidatively adds chlorobenzene to yield only the trans complex (η1-dippb)2Pd(Ph)Cl. Reaction monitoring reveals that the cis and trans complexes are formed in parallel pathways. Cis/trans equilibrium is on the cis side for dippp and on the trans side for dippb. Reactivity toward chlorobenzene follows the trend Pd(dippp)2 > Pd(PiPr2nBu)3 ? Pd(dippe)2 ? Pd(dppp)2. These results are interpreted in terms of chelate stability, ligand basicity, concentration of the active 14e species and effect of the P-Pd-P angle on its reactivity. The dippp ligand is unique in that it is the only one of those studied which results in Pd(0) complexes which (a) exhibit high reactivity in oxidative addition and (b) form cis complexes preferentially.
- Portnoy, Moshe,Milstein, David
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- A binuclear palladium(I) hydride. Formation, reactions, and catalysis
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(dippp)Pd(Ph)Cl (3) reacts with methanol to yield the novel hydrido Pd(I) dimer {[(dippp)-Pd]2(μ-H)(μ-CO)}+Cl- (1), (dippp)PdCl2 (4), H2, benzene, and formaldehyde. In the presence of NEt3, HNEt3+Cl- is formed instead of 4. 1 can also be formed in a reaction of Pd(dippp)2, HCl, and CO. Labeling studies and modeling reactions indicate that the novel transformation of 3 into 1 involves methanolysis of 3 followed by a β-H elimination from a methoxo intermediate to yield formaldehyde, benzene, and the 14e transient (dippp)Pd (7). Formaldehyde decarbonylation, coupling of the palladium carbonyl complex with 7, and protonation lead to 1. Alternatively, 1 can be formed by electrophilic attack of protonated 7, on the carbonyl complex (dippp)Pd(CO). A number of reactivity modes have been identified for 1. Reaction with acetylenes results in bridge-splitting to form (dippp)Pd(η2-acetylene) and in hydropalladation of the acetylene to form a vinyl complex. The hydropalladation process exhibits high regio- and stereoselectivity, resulting in cis addition and attachment of the Pd atom to the more hindered carbon, indicating electronic control. 1 undergoes exchange of the hydride for deuteride in CD3COCD3, most likely via an enol insertion into Pd-H. In the presence of an olefin, such as cyclooctene or ethyl vinyl ether, catalytic transfer deuteration takes place. α-Deuteration of the latter is preferred, indicating anti-Markovnikov Pd-H addition. The integrity of 1 is maintained during this process. With norbornene, bridge-splitting to form (dippp)Pd(norbornyl) (17) and its CO-insertion product 18 takes place. No H/D exchange catalysis is observed in this case with acetone-d6. 1 behaves as a Pd(O) complex and exhibits oxidative addition reactivity with chlorobenzene or benzyl chloride, yielding (dippp)Pd(R)Cl. The relevance of this reactivity to Pd-catalyzed reactions is discussed.
- Portnoy, Moshe,Milstein, David
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p. 600 - 609
(2008/10/08)
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