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Harry Adams

Publications and source records attributed to Harry Adams.

50 records · Page 3Linked to original sources
PubMed2004

Promotion of iridium-catalyzed methanol carbonylation: mechanistic studies of the cativa process.

The iridium/iodide-catalyzed carbonylation of methanol to acetic acid is promoted by carbonyl complexes of W, Re, Ru, and Os and simple iodides of Zn, Cd, Hg, Ga, and In. Iodide salts (LiI and Bu(4)NI) are catalyst poisons. In situ IR spectroscopy shows that the catalyst resting state (at H(2)O levels > or = 5% w/w) is fac,cis-[Ir(CO)(2)I(3)Me](-), 2. The stoichiometric carbonylation of 2 into [Ir(CO)(2)I(3)(COMe)](-), 6, is accelerated by substoichiometric amounts of neutral promoter species (e.g., [Ru(CO)(3)I(2)](2), [Ru(CO)(2)I(2)](n), InI(3), GaI(3), and ZnI(2)). The rate increase is approximately proportional to promoter concentration for promoter:Ir ratios of 0-0.2. By contrast anionic Ru complexes (e.g., [Ru(CO)(3)I(3)](-), [Ru(CO)(2)I(4)](2)(-)) do not promote carbonylation of 2 and Bu(4)NI is an inhibitor. Mechanistic studies indicate that the promoters accelerate carbonylation of 2 by abstracting an iodide ligand from the Ir center, allowing coordination of CO to give [Ir(CO)(3)I(2)Me], 4, identified by high-pressure IR and NMR spectroscopy. Migratory CO insertion is ca. 700 times faster for 4 than for 2 (85 degrees C, PhCl), representing a lowering of Delta G(++) by 20 kJ mol(-1). Ab initio calculations support a more facile methyl migration in 4, the principal factor being decreased pi-back-donation to the carbonyl ligands compared to 2. The fac,cis isomer of [Ir(CO)(2)I(3)(COMe)](-), 6a (as its Ph(4)As(+) salt), was characterized by X-ray crystallography. A catalytic mechanism is proposed in which the promoter [M(CO)(m)I(n)] (M = Ru, In; m = 3, 0; n = 2, 3) binds I(-) to form [M(CO)(m)I(n+1)](-)H(3)O(+) and catalyzes the reaction HI(aq) + MeOAc --> MeI + HOAc. This moderates the concentration of HI(aq) and so facilitates catalytic turnover via neutral 4.

Journal Article↗
PubMed2003

Synthesis, solid state structure and polymerisation of a fully planar cyclopentadithiophene.

The new fully planar cyclopentadithiophene, 4-n-dodecylidene-4H-cyclopenta(2,1-b;3,4-b')dithiophene, shows extensive pi-stacking in the solid state with short intermolecular distances (ca. 3.5 A) between adjacent molecules. Polymerisation of this monomer by two different protocols gave solution processable alkenyl-bridged cyclopentadithiophene polymers with extended pi-conjugation in the main chain.

Journal Article↗
PubMed2003

Highly diastereoselective 1,3-dipolar cycloaddition reactions of trans-2-methylene-1,3-dithiolane 1,3-dioxide with 3-oxidopyridinium and 3-oxidopyrylium betaines: a route to the tropane skeleton.

The C2-symmetric vinyl sulfoxide, trans-2-methylene-1,3-dithiolane 1,3-dioxide, was found to react with a range of 3-oxidopyridinium betaines (bearing different substituents on nitrogen) in high yield and with total diastereoselectivity. A 2.3:1 mixture of regioisomers was formed with all of the 3-oxidopyridinium betaines but the ratio was found to change over prolonged periods of time due to reversibility of the minor regioisomer. 3-Oxidopyridinium betaines bearing methyl substituents at either the 2- or 6-position were also tested in the cycloaddition process. Improved regioselectivity (8:1) and again high diastereoselectivity were observed with the betaine having an additional substituent at the 2-position, but with betaines having a substituent in the 6-position although high regioselectivity was observed (9.9:1), the major isomer was formed with low diastereoselectivity (5.5:4.4). The origin of the regio- and diastereo-selectivity with all the betaines is discussed. Finally, the C2-symmetric vinyl sulfoxide, trans-2-methylene-1,3-dithiolane 1,3-dioxide was reacted with an oxidopyrylium betaine in moderate yield. Good regioselectivity and moderate diastereoselectivity were observed.

Betaine↗
PubMed2003

Investigation of a stereoselective co-mediated rearrangement reaction.

A stereocontrolled approach to alpha-alkyl beta-alkynyl cyclohexanones is reported through a Lewis acid mediated rearrangement reaction of enol ethers bearing an Co-alkyne moiety. The reaction proceeds with high levels of stereoselectivity in the presence of Ti- and B-Lewis acids to provide a range of alpha,beta-disubstituted cyclohexanones in high yield although the products are prone to epimerization at the alpha-position in the presence of the B-promoter system. The potential for an enantioselective variant of this process is outlined, and a rationale for the observed stereochemical trends and detailed structural analyses of the ketone products are described.

Journal Article↗
PubMed2003

The effect of tautomeric constant on the specificity of nucleotide incorporation during DNA replication: support for the rare tautomer hypothesis of substitution mutagenesis.

The nucleoside analogue dP (6-(2-deoxy-beta-D-ribofuranosyl)-3,4-dihydro-6H,8H-pyrimido[4,5-c][1,2]oxazin-2-one) displays ambivalent hydrogen bonding characteristics whereby the imino tautomer of P can base-pair with adenine and its amino tautomer can base-pair with guanine. Fixed imino and amino tautomers of 6-methyl-3,4-dihydro-6H,8H-pyrimido[4,5-c][1,2]oxazin-2-one (N-methyl P) have been synthesised and their structures obtained by X-ray crystallography. The tautomeric constant of N-methyl P has been calculated from pK(a) values of the fixed tautomers and the kinetic parameters for the incorporation of its 5'-triphosphate (dPTP) by exonuclease-free Klenow fragment of DNA polymerase I have been determined. A strong correlation between the tautomeric constant and the incorporation specificity of dPTP is found. These results lend support to the proposal that the minor tautomeric forms of the natural bases may play an important role in substitution mutagenesis during DNA replication. Furthermore, they imply that DNA polymerases impose specific steric requirements on the base-pair during nucleotide incorporation.

Adenine↗
PubMed2002

Steric and electronic effects on the reactivity of rh and ir complexes containing P-S, P-P, and P-O ligands. Implications for the effects of chelate ligands in catalysis.

Kinetic studies of the reactions of [M(CO)(L-L)I] [M = Rh, Ir; L-L = Ph(2)PCH(2)P(S)Ph(2) (dppms), Ph(2)PCH(2)CH(2)PPh(2) (dppe), and Ph(2)PCH(2)P(O)Ph(2) (dppmo)] with methyl iodide have been undertaken. All the chelate ligands promote oxidative addition of methyl iodide to the square planar M(I) centers, by factors of between 30 and 50 compared to the respective [M(CO)(2)I(2)](-) complexes, due to their good donor properties. Migratory CO insertion in [Rh(CO)(L-L)I(2)Me] leads to acetyl complexes [Rh(L-L)I(2)(COMe)] for which X-ray crystal structures were obtained for L-L = dppms (3a) and dppe (3b). Against the expectations of simple bonding arguments, methyl migration is faster by a factor of ca. 1500 for [Rh(CO)(dppms)I(2)Me] (2a) than for [Rh(CO)(dppe)I(2)Me] (2b). For M = Ir, alkyl iodide oxidative addition gives stable alkyl complexes [Ir(CO)(L-L)I(2)R]. Migratory insertion (induced at high temperature by CO pressure) was faster for [Ir(CO)(dppms)I(2)Me] (5a) than for its dppe analogue (5b). Reaction of methyl triflate with [Ir(CO)(dppms)I] (4a) yielded the dimer [[Ir(CO)(dppms)(mu-I)Me](2)](2+) (7), which was characterized crystallographically along with 5a and [Ir(CO)(dppms)I(2)Et] (6). Analysis of the X-ray crystal structures showed that the dppms ligand adopts a conformation which creates a sterically crowded pocket around the alkyl ligands of 5a, 6, and 7. It is proposed that this steric strain can be relieved by migratory insertion, to give a five-coordinate acetyl product in which the sterically crowded quadrants flank a vacant coordination site, exemplified by the crystal structure of 3a. Conformational analysis indicates similarity between M(dppms) and M(2)(mu-dppm) chelate structures, which have less flexibility than M(dppe) systems and therefore generate greater steric strain with the "axial" ligands in octahedral complexes. Ab initio calculations suggest an additional electronic contribution to the migratory insertion barrier, whereby a sulfur atom trans to CO stabilizes the transition state compared to systems with phosphorus trans to CO. The results represent a rare example of the quantification of ligand effects on individual steps from catalytic cycles, and are discussed in the context of catalytic methanol carbonylation. Implications for other catalytic reactions utilizing chelating diphosphines (e.g., CO/alkene copolymerization and alkene hydroformylation) are considered.

Journal Article↗
PubMed2002

A tetranuclear nickel(II) complex assembled from an asymmetric compartmental ligand and bearing an intramolecular [H3O2]-bridge.

The asymmetric di-aminic compartmental ligand HL5 forms a tetranuclear nickel(II) complex in which the core is assembled from two confacial bioctahedra [Ni...Ni, approximately 2.90 A]; the open faces of the bioctahedra are joined at the O atoms of two mu-cresolato bridges [Ni...Ni, 3.72 A], and the shared faces of the bioctahedra are linked by a tetradentate (mu 4, eta 2)-perchlorate anion and by an unusual tetradentate (mu 4, eta 2)-[H3O2]-bridge.

Hydrogen Peroxide↗
PubMed1999

Origins of Enantioselectivity with Nitrogen-Sulfur Chelate Ligands in Palladium-Catalyzed Allylic Substitution.

The reaction of 1,3-diphenyl-2-propenyl acetate (9) with dimethylmalonate to give the substitution product 10 is effectively catalyzed by Pd complexes containing chiral imine-sulfide chelate ligands derived from amino acids. The ligand of choice, (S)-N-2'-chlorobenzylidene-2-amino-3-methyl-1-thiophenylbutane (6e), prepared in only two steps from (S)-valinol, gave an ee of 94%. Because the explanation of selectivity with the majority of other nitrogen-sulfur chelate ligands in this reaction assumes the selectivity to be controlled by an electronic bias, which contradicts our results, we characterized the Pd-allyl intermediate 14 by X-ray diffraction and solution NMR. The possible mechanism of chirality transfer is discussed. The site of nucleophilic attack on the allyl ligand is not trans to the perceived better pi-acceptor ligand (sulfur), which would be analogous to chiral nitrogen-phosphorus systems. This reaction occurs trans to the imine donor, and the enantioselectivity is ultimately controlled by the subtle steric environment of the chiral imine-sulfur chelate ligand, which predisposes the allyl unit of the reaction intermediate to a preferred reaction trajectory. In light of results that emphasize the power of electronic desymmetrization for chiral recognition, these results suggest that electronically dissimilar ligands may not give rise to chiral recognition through electronic dissimilarity.

Journal Article↗
PubMed1999

Barium Templating Schiff-Base Lateral Macrobicycles.

Schiff-base lateral macrobicycles containing two different binding units, a rigid and unsaturated N(2)X set (X: N, O) and a flexible and cyclic N(2)O(n)() set, linked by two aromatic bridges, have been prepared by reaction of the appropriate bibracchial diamines N,N'-bis(aminobenzyl)-diaza-crown and diformyl precursors in the presence of Ba(II) as templating agent. The expected cryptands do not form in the absence of the cation; the presence of this metal ion is necessary to orient the diamine precursor in a syn conformation. Comparison of the X-ray crystal structures of the barium complex of the bibracchial diamine N,N'-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 and the barium complex of the cryptand derived from it incorporating a pyridine unit indicates that the encapsulation of the metal ion is clearly more effective in the case of the cryptand. The coordination of Ba(II) ion to the pyridine nitrogen atom promotes the displacement of the metal into the cavity of the cryptand as well as important changes in the conformation of the crown unit, although the fold of the pendant arms remains practically unchanged. The cryptands reported constitute the first example of a novel family of macrobicycles.

Journal Article↗
PubMed1998

Intramolecular Ligand Hydroxylation: Mechanistic Studies on the Reaction of a Copper(I) Schiff Base Complex with Dioxygen.

The dinuclear copper(I) complex of 1,3-bis[N-(2-pyridylethyl)formimidoyl]benzene, [Cu(2)(H-BPB-H)(CH(3)CN)(2)](BF(4))(2), as well as the 5-nitro derivative, [Cu(2)(NO(2)-BPB-H)(CH(3)CN)(2)](BF(4))(2), react with dioxygen to form phenolate-bridged complexes as products. In a detailed kinetic study activation parameters of DeltaH() = 47 +/- 9 kJ/mol, DeltaS() = -53 +/- 11 J/(mol K), and DeltaV() = -9.5 +/- 0.5 cm(3)/mol for the reaction of [Cu(2)(H-BPB-H)(CH(3)CN)(2)](BF(4))(2) with dioxygen were obtained which account, together with further kinetic findings, for the occurrence of an intermediate peroxo complex that cannot be observed spectroscopically. The crystal structures of the products of the reaction were determined. Crystal data: complex [Cu(2)(H-BPB-O)-OH(H(2)O)](BF(4))(2), formula C(22)H(24)B(2)Cu(2)F(8)N(4)O(3), monoclinic space group P2(1)/c, Z = 4, a = 10.122(2) Å, b = 28.711(6) Å, c = 9.283(2) Å, alpha = 90 degrees, beta = 100.78(3) degrees, and gamma = 90 degrees; complex [Cu(2)(NO(2)-BPB-O)-OH(H(2)O)(3)](BF(4))(2), formula C(22)H(27)B(2)Cu(2)F(8)N(5)O(7), triclinic space group, P&onemacr;, Z = 2, a = 10.144(2) Å, b = 10.7612(2) Å, and c = 16.000(4) Å.

Journal Article↗