Pierre G. Potvin

Synthesis of ring A of ambruticin and proof of its absolute stereochemistry

Appropriately derivatized ambruticin was converted by reductive ozonolysis to tetrahydropyran 9c and cyclopropane 12a, which were synthesized from L-arabinose and (R)-Feists acid in an unambiguous...

On the structure of the Kagan–Modena catalysts for asymmetric oxidation of sulfides

Abstract The reaction of Ti(O i Pr) 4 , diisopropyl ( R , R )-tartrate and N , N ′-di(trifluoromethylsulfonyl)ethane-1,2-diamine in 1:2:1 ratio provided a symmetric, dinuclear complex of formula [Ti(η 2 - dipt )(η 2 -H dipt )(O i Pr)] 2 (where H 2 dipt is diisopropyl tartrate), in which each metal bears one O i Pr ligand, a chelating tartrate diolate bridging the two metals, and a second, univalent tartrate unit in a novel binding mode, attached through alkoxy and ester carbonyl oxygens. This species appears to be stabilized through hydrogen bonding with the disulfonamide. The configuration at Ti deduced by NMR spectral information coincides with that calculated to be the most stable. In the absence of the disulfonamide, the data are consistent with an equilibrium between this complex, free tartrate and a condensation product of formula Ti 4 ( dipt ) 5 (H dipt ) 2 (O i Pr) 4 which features an η 2 ,η 2 -tartrate linking two Ti 2 (η 2 - dipt ) 2 (η 2 -H dipt )(O i Pr) 2 units. The relationship between these species and the Kagan and Modena catalysts for the asymmetric oxidation of sulfides is discussed.

Strong cooperative protonation of a small tris-urea cryptand

A small bicyclic cryptand 3 bearing urea groups along the 7-atom bridges was synthesized in 22% yield by a novel tripod-tripod coupling procedure. Protonation and complexation studies are reported and compared with results with polyoxa-[2]-cryptands and [3]-cryptand 6. These show that 3 is a moderate base, a weak metal ion binder, but a two-proton binder exhibiting strong positive cooperativity (pKa2 > pKa1) and fast proton exchange kinetics. As demonstrated by molecular mechanics calculations, an inward rotation of the carbonyl groups to form hydrogen bonds to cryptated protons was invoked to explain the cooperativity.

STRUCTURAL STUDIES ON PYRAZOLYLPYRIDINE LIGANDS AND COMPLEXES. 2. COMPARISONS BETWEEN BISPYRAZOLYLPYRIDINE LINKAGE ISOMERS AND WITH 2,2′,6′,2″-TERPYRIDINE JERZY ZADYKOWICZ

Abstract Crystal structures were obtained for the 3(C),2′;6′,3″(C)-linked bispyrazolylpyridines 2,6-di(2H-4,5,6,7-tetrahydroindazol-3-yl)pyridine (1), 2,6-di(l-methyl-4,5,6,7-tetrahydroindazol-3-yl)pyridine (2), 2,6-di(1 -(4-ethoxycarbonylphenyl)-4,5,6,7-tetrahydroindazol-3-yl)pyridine (3) and for the homoleptic RuII complex of 2, [Ru(2)2]Cl2, which crystallized with 7 molecules of CHCl3. Ligand 1 adopts the inter-and intramolecularly hydrogen-bonded syn,syn rotameric conformation, while 2 and 3 were in the anti,anti forms. Relative to the latter, iigand distortions were assessed in 1 (considered as a H+ complex) and [Ru(2)2]Cl2. Comparisons were drawn with other tridentate ligands containing a pyridine nucleus, specifically the 1(N),2′;6′,1″(N″) linkage isomers and 2,2′;6′,2″-terpyridine, in both free and RuII complexed forms, as well as with their bidentate analogues. Unlike with bidentate ligands, the bonds to the pyridine moiety are shortest, the outer heterocyclic rings are drawn inward and, overall,...

A rapid screening of Ru(II) photosensitizers

When samples containing a Ru(II)-based photosensitizer such as Ru(bpy)32+ (bpy is 2,2′-bipyridine), methyl viologen (MV2+) and the sacrificial reductant triethanolamine (TEOA) are exposed to white light, the blue colour of the methyl viologen cation radical (MV•+) develops by oxidative quenching of the sensitizers excited state. This occurs even if the samples are not degassed, that is in spite of the known quenching of MV•+ by O2. In degassed media, however, a second pathway of MV•+ formation occurs, known to involve the reduction of MV2+ by the oxidized and deprotonated form of TEOA (H−1TEOA•) that arises from its function as a sacrificial reductant. This second pathway masks the effect of the photosensitization and disallows comparative analyses, but is suppressed by O2. The second pathway also interferes in aqueous media, as the O2 content becomes diffusion-limited. In order to comparatively assess the utility of Ru(II) photosensitizers, a method is described that uses non-degassed samples in CH3CN under continuous irradiation with spectrophotometric monitoring of MV•+. This requires little sample preparation and simple equipment, and is applicable to sensitizers with widely varying excited state lifetimes. A kinetic model that takes account of the aerobic quenching furnishes rate constants relating to MV•+ formation and (non-aerobic) quenching that are reproducible, consistent with mechanistic expectations and concordant with the sensitizer excited state lifetime. The rate constants can then be used to compare sensitizers subject to the same oxidative quenching.

Ru(II) complexes of polyarylated terpyridines: unexpected side-chain C-metallation and photosensitization of electron transfer

Abstract The new ligands 4,4″-diphenyl-6,6″-di(4-ethoxycarbonylphenyl)-2,2′:6′,2″-terpyridine (H 4 ), its non-carboxylated 4,6,4″,6″-tetraphenyl analogue (H 5 ) and the 4,4″-diphenyl-6,6″-di(4-methoxyphenyl) analogue were prepared in high yields via double Krohnke reactions. Reactions with activated (ttpy)RuCl 3 (ttpy is 4′- p -tolyl-2,2′:6′,2″-terpyridine) provided the symmetrical N 6 -coordinated complexes, [Ru(H 4 )(ttpy)](PF 6 ) 2 and [Ru(H 5 )(ttpy)](PF 6 ) 2 , along with the novel, unsymmetrical C -metallated analogues [Ru( 4 )(ttpy)]PF 6 and [Ru( 5 )(ttpy)]PF 6 , in which an unprecedented side-chain metallation occurred in lieu of incomplete substitution. A crystallographic analysis of [Ru( 5 )(ttpy)]NO 3 confirmed the N 5 C donor set in these products and revealed a distorted binding of 5 − as well as ‘π-stacking’ between its uncoordinated pyridine and the ttpy ligand. The diester [Ru(H 4 )(ttpy)](PF 6 ) 2 was hydrolyzed to the diacid complex [Ru(H 2 3 )(ttpy)](PF 6 ) 2 (H 2 3 is 4,4″-diphenyl-6,6″-di(4-carboxyphenyl)-2,2′:6′,2″-terpyridine). Measurements of the photogeneration of methyl viologen cation radical with [Ru(H 5 )(ttpy)](PF 6 ) 2 and [Ru(H 2 3 )(ttpy)](PF 6 ) 2 as sensitizers showed that the presence of carboxyl groups in the latter provided a distinct benefit, owing to the formation of the neutral form [Ru( 3 )(ttpy)] 0 but this was insufficient to overcome the detrimental effect of inter-ligand repulsions.

STRUCTURAL STUDIES ON PYRAZOLYLPYRIDINE LIGANDS AND COMPLEXES. COMPARISONS BETWEEN LINKAGE ISOMERS AND WITH 2,2′-BIPYRIDINE

Abstract Crystal structures were obtained for the 3′(C),2-linked pyrazolylpyridines 2H-3-(pyridin-2-yl)-4,5,6,7-tetrahydroindazole (1) and 1-(4″-ethoxycarbonylphenyl)-3-(pyridin-2-yl)-4,5,6,7-tetrahydroindazole (2), and for the ZnII complex of the methyl ester analog of 2, (Zn(3)Cl2). With 2 found in the anti rotameric conformation, ligand distortion was assessed in the syn forms found in 1 (treated as a H+ complex), Zn(3)Cl2 and [Ru(bpy)(2)2](PF6)2. Several differences were noted from similar analyses on structures for representative ZnII and RuII complexes of bipyridine or 1,10-phenanthroline, for complexes of other 3′(C),2-linked 2-(pyrazol-3-yl) pyridines and for complexes of isomeric 1′(N′),2-linked 2-(pyrazol-1-yl)pyridines. A notable finding is that bpy and the N′,2-linked pyrazolylpyridines lose planarity upon complex formation due to steric congestion between the rings, whereas 1 and complexed 2 or 3 remain coplanar, a difference attributable to differences in the bond lengths and angles at the b...

Dithiocarbamate complexes of Ti(IV) alkoxides: synthesis, characterization, and electrochemistry.

Isopropoxy- and tert-butoxy-tris(dithiocarbamato)titanium(IV) complexes of five dithiocarbamate ligands were prepared and characterized by LDI-MS, (1)H NMR, (13)C NMR, and elemental analysis, as well as by crystallographic determination of two examples. Both showed strongly pi-coordinated alkoxy groups and two separate dithiocarbamate coordination environments that, in solution, were in rapid exchange. Cyclic voltammetry in CH(2)Cl(2) revealed irreversible but reproducible oxidation peaks between +1.2 and +1.6 V vs Ag/AgCl, about 1 V positive of those from the free ligands, as well as reduction peaks in the -1.9 to -2.2 V range vs Ag/AgCl assigned to Ti(IV/III) couples, and second reductions in some cases. The corresponding diisopropoxy-bis(dithiocarbamato) analogues were not isolable and slowly transformed to the more stable tris species. Indeed, these were shown to be in slow equilibrium.

Zn2+ inclusion complexes of endodentate tripodands as carbonic anhydrase-inspired artificial esterases. Part 2.1 Micellar systems

Facile syntheses of two lipophilic, endo-tridentate tris-imidazole podands (3 and 4) are reported. These were designed for micellar media, where pre-organization for metal binding was anticipated to better reproduce the active site of carbonic anhydrase (CA). pH-Metric titrations and p-nitrophenyl acetate (pNPOAc) hydrolyses were carried out in the presence of Zn2+ and the results compared with those obtained with hydrophilic tripodand varieties. The results confirm stronger metal binding with the micelle-bound ligands and show reasonably low pKa values near 8 for the deprotonation of metal-bound H2O, as well as evidence for a proton shuttling analogous to that seen in the enzyme. The second-order rate constant for pNPOAc hydrolysis by [Zn(3)OH]+ was estimated at 0.19(1) M–1 s–1, the second-highest value measured for a biomimetic CA model.

Condensation Reactions of 2-Acetylpyridine and Benzaldehydes: New Cyclohexanol Products and an Improved Synthesis of 4′-p-Tolyl-2,2′:6′,2″-terpyridine

Condensations of 2-acetylpyridine with p-tolualdehyde and 4-tert-butylbenzaldehyde furnish three new cyclohexanol 3:2 condensates (3d,e and 7), including a new diastereomer, and open a better route to 4′-p-tolyl-2,2′:6′,2″-terpyridine (1b) via the new 2:1 condensate, 1,5-di(2-pyridyl)-3(4-methylphenyl)pentane-1,5-dione (6).