Solution Studies of the Dynamic Stereochemistry of Palladium(II) Carboxylate Complexes

1975 ◽  
Vol 53 (17) ◽  
pp. 2558-2574 ◽  
Author(s):  
Thomas R. Jack ◽  
John Powell
Keyword(s):  
Ring Opening ◽  
Variable Temperature ◽  
Equilibrium Constants ◽  
Quantitative Measure ◽  
Intermediate Species ◽  
Tertiary Phosphine ◽  
Crystalline Complex ◽  
Carboxylate Complexes ◽  
Solution Studies ◽  
Dynamic Stereochemistry

A series of dimeric tertiary phosphine carboxylate complexes of palladium(II) of the general types [(PR3)Pd(O2CR)2]2 and [(PR3)Pd(O2CR)(O2CR)]2 have been synthesized and struc-turally characterized. The dynamic stereochemistry of the complexes where PR3 = Me2PhP has been investigated using variable temperature n.m.r. techniques. Exchange of the stereochemically nonequivalent methyl groups of the coordinated Me2PhP ligand occurs via a solvent assisted process involving ring opening of the "Pd2(OCO)2" bridged structure. For the trifluoroacetate complexes solution equilibria between monomer and dimer are readily apparent at low temperatures in the 19F n.m.r. The observed equilibrium constants are a function of the solvent and the size of the PR3 ligand. The reaction of [(PMe2Ph)Pd(O2CCF3)2]2 with excess AgO2CCF3 gives an unusual crystalline complex of stoichiometry Ag[(PMe2Ph)Pd(O2CCF3)3]. A quantitative measure of the ability of a coordinated carboxylate to stabilize intermediate species via chelation has been obtained from a variable temperature n.m.r. study of [(Π-2-methylallyl)Pd(O2CR)(Me2PhP)] (R=CH3, CH2Cl, CHCl2, CCl3, CF3, CMe3, CPh3, CH2Ph).

Further N.M.R. Studies of the Dynamic Stereochemistry of π-Allylic Palladium(II) – Tertiary Phosphine Systems

1973 ◽  
Vol 51 (2) ◽  
pp. 274-287 ◽  
Author(s):  
M. Oslinger ◽  
J. Powell
Keyword(s):  
Variable Temperature ◽  
Chloride Ion ◽  
Ligand Substitution ◽  
Phosphine Ligands ◽  
Plane Rotation ◽  
Substitution Reactions ◽  
Tertiary Phosphine ◽  
Original Proposal ◽  
Dynamic Stereochemistry

A series of cations of the type 1,2-bis(diphenylphosphino)ethane(π-all)palladium(II) (all = allyl, 2-methylallyl, 2-tert-butylallyl, 1-methylallyl, 1,1-dimethylallyl, syn-1-tert-butyl-2-methylallyl) have been isolated as PF6− and/or BF4− salts. Variable temperature n.m.r. data fully substantiate the proposal that pyridine promoted exchange of syn and anti proton sites in such complexes occurs via a π → σ → π mechanism that involves consecutive SN2 ligand substitutions. Comparison of this variable temperature n.m.r. data with that obtained for the complexes 1,2-bis(diphenylphosphino)ethane(π-all)palladium(II) chloride (prepared in situ) has shown that the chloride ion, functioning as a nucleophile, is also effective in promoting π → σ → π reactions. Studies of bis(dimethylphenylphosphine) (π-1,1-dimethylallyl)palladium(II) chloride in CDCl3 below −30° have shown that rapid consecutive SN2 substitution reactions, in which chloride acts as a nucleophile, are also occurring in systems containing monodentate tertiary phosphine ligands. As such the PR3 promoted syn–syn and anti–anti proton site interchange observed in complexes of the type [(π-2-methylallyl)PdCl(PR3)] has been reinterpreted in terms of consecutive ligand substitution reactions involving PR3 and Cl ligands instead of the original proposal which involved an in plane rotation of the π-allylic ligand.

scholarly journals Syntheses, Solid-state Structures and Catalytic Activity of Zinc Carboxylate Complexes in Lactide Polymerization

2014 ◽  
Vol 69 (11-12) ◽  
pp. 1365-1374 ◽  
Author(s):  
Christoph Scheiper ◽  
Christoph Wölper ◽  
Dieter Bläser ◽  
Joachim Roll ◽  
Stephan Schulz
Keyword(s):  
Ring Opening ◽  
Zinc Complex ◽  
X Ray Diffraction ◽  
Polymerization Catalysts ◽  
Co Catalyst ◽  
Carboxylate Complexes ◽  
X Ray ◽  
Carboxylate Groups ◽  
Solid State Structures ◽  
Lactide Polymerization

Abstract Three dinuclear zinc carboxylate complexes [L1-3Zn(μ,η2-O2CPh)]2 (1, 2, 4) containing either the bidentate N,N′-chelating β-diketiminate ligand RNC(Me)C(H)C(Me)NR (R = 2,6-iPr2-C6H3, L1, complex 1), the tridentate O,N,N-chelating ligand OC(Me)C(H)C(Me)NCH2CH2NMe2 (L2, complex 2) or the bis-N,N′-chelating bis-β-diketiminate ligand RNC(Me)C(H)C(Me)NNC(Me)- C(H)C(Me)NR (R = 2,6-iPr2-C6H3, L3, complex 4) were synthesized and characterized including single-crystal X-ray diffraction. Reaction of the neutral bis-β-diketimine (L3(H)2) with two equivalents of ZnMe2 leads to the expected heteroleptic dinuclear zinc complex L3(ZnMe)2 3 in 93% yield. Further reaction with benzoic acid PhCO2H leads to complex 4. Complex 2 forms a rather strong carboxylate-bridged dimer, whereas the carboxylate groups in complexes 1 and 4 act as asymmetrical bridges between both Zn atoms, pointing to the formation of a weakly bonded dimer. The zinc atoms in 1 and 4 are tetrahedrally coordinated, whereas in 2 the coordination number is increased to five due to the coordination of the pendant donor arm. The ring opening polymerization (ROP) of rac-lactide was investigated with the zinc complexes 1-4 and diazabicycloundec-7-ene (DBU) as a co-catalyst. Complexes 2 and 3 are active polymerization catalysts, which in the presence of DBU converted 200 equiv. of rac-lactide into polylactide within 10 min at ambient temperature. The analysis of the crude polymer showed that the lactide polymerization with catalyst 2 occurs via a slightly modified activated-monomer mechanism.

Studies of the activity of chymotrypsin

1970 ◽  
Vol 257 (813) ◽  
pp. 89-104 ◽  
Keyword(s):  
Equilibrium Constants ◽  
Hydrolytic Enzyme ◽  
Enzyme Activation ◽  
Ethyl Esters ◽  
Activation Mechanism ◽  
Relaxation Techniques ◽  
Kinetic Measurements ◽  
Ph Values ◽  
Solution Studies ◽  
Hydrolysis Of

Physical and chemical studies of chymotrypsin in solution and kinetic measurements of individual steps in reactions catalysed by this hydrolytic enzyme are reported. The solution studies also provide a basis for correlation of individual reaction steps with structural information obtained by Drs Blow and Hartley and discussed at this Discussion. The rate and equilibrium constants pertaining to the three-step mechanism shown below have been determined for the chymotrypsin catalysed hydrolysis of three specific ester substrates—the ethyl esters of V-acetyl-L-tyrosine, N -acetyl-L-tryptophan, and N -acetyl-L-phenylalanine—at selected pH values with use of both flow and relaxation techniques. iV-acetyl-L-phenylalanine—at selected pH values with use of both flow and relaxation techniques. / * E + S ^ 4 E S -y » EP 2 ,---► E + P 2 , * 2 3 £ 3 4 where E is enzyme, S is substrate, ES is a complex, EP 2 is an intermediate compound, and the products P 1 and P 2 are, respectively, an alcohol or amine and an acid. The hydrolysis of chymotrypsin specific substrates is shown to be considerably more complex than is indicated by this previously proposed mechanism. In addition to the steps shown, there exist a number of ionization and conformational equilibria which account for the bell-shaped pH-rate profiles observed for chymotrypsin catalysed reactions. A catalytically active enzyme conformation, which is in pH dependent equilibrium with a catalytically inactive conformation, is shown to have its origin in the enzyme activation mechanism. In the chymotrypsin catalysed hydrolysis of an amide substrate, furylacryloyltryptophanamide, there has been detected an additional intermediate which differs from the intermediate EP 2 observed in ester hydrolysis.

Quaternary Nitrogen Heterocycles. I. Equilibrium and Spectral Data for Pseudobase Formation by the N-Methyl Cations of Diazanaphthalenes

1972 ◽  
Vol 50 (6) ◽  
pp. 917-931 ◽  
Author(s):  
John W. Bunting ◽  
William G. Meathrel
Keyword(s):  
Spectral Data ◽  
Ring Opening ◽  
Lithium Aluminum Hydride ◽  
Equilibrium Constants ◽  
Nitrogen Heterocycles ◽  
Aluminum Hydride ◽  
Ionization Constants ◽  
Lithium Aluminum ◽  
Quaternary Nitrogen ◽  
Hydride Reduction

Equilibrium constants have been measured at 25° for the formation of pseudobases from the 2-methyl-phthalazinium, 1-methylquinoxalinium, 1-methyl- 1,5-naphthyridinium, 6-methyl-1,6-naphthyridinium, 7-methyl-1,7-naphthyridinium, 1-methyl-1,8-naphthyridinium, 1-methyl-3-nitroquinolinium, and 2-methyl-4-nitroisoquinolinium cations. Ionization constants have also been obtained for ionization of some of these pseudobases to alkoxide anions. For each cation the site of hydroxide attack has been determined by comparison of the u.v. and p.m.r. spectra of the pseudobases, the corresponding methoxide adducts, and the lithium aluminum hydride reduction products. In all cases, except for the 1-methylquinoxalinium cation, only one major pseudobase species is present in solution, and ring-opening does not occur to any appreciable extent. The pseudobase of the 1-methylquinoxalinium cation exists in equilibrium with a considerable amount of its covalent hydrate from addition of water across the C3—N4 bond.

High sensitivity photoacoustic spectrometer for variable temperature solution studies

1990 ◽  
Vol 61 (5) ◽  
pp. 1395-1403 ◽  
Author(s):  
James V. Beitz ◽  
M. M. Doxtader ◽  
V. A. Maroni ◽  
S. Okajima ◽  
D. T. Reed
Keyword(s):  
Variable Temperature ◽  
High Sensitivity ◽  
Photoacoustic Spectrometer ◽  
Solution Studies ◽  
Temperature Solution
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