Barbara Odell

Unusual inverse temperature dependence on reaction rate in the asymmetric autocatalytic alkylation of pyrimidyl aldehydes.

Observations of an intriguing inverse temperature dependence on reaction rate and a profound induction period in the Soai autocatalytic reaction are reported along with detailed kinetic and NMR investigations of the product alkoxide at low temperatures, leading to the suggestion that the active catalyst is derived in situ from the tetrameric ground state.

All‐or‐Nothing Cooperative Self‐Assembly of an Annulene Sandwich

Large p-conjugated macrocycles and annulenes are intriguing compounds for testing theories of electronic structure because of their high symmetry, lack of end-group effects, strain, and distorted p overlap. Some of these features lead to extraordinary linear and nonlinear optical properties as well as unusual host–guest chemistry based on interactions of curved p surfaces. With increasing ring size, these unique properties disappear because the rings become so flexible that they adopt polymer-like random-coil conformations. We recently introduced Vernier templating as a strategy for the synthesis of large monodisperse p-conjugated macrocycles, such as the [12]porphyrin nanoring c-P12 (Figure 1 a). Structural rigidification of such macrocycles by coordination to radial templates becomes challenging because of the inaccessibility of large rigid templates. Linear conjugated porphyrin oligomers bind bidentate ligands, such as 1,4diazabicyclo[2.2.2]octane (DABCO) and 4,4’-bipyridine, to form rigid, self-supported double strands. Planarization of the p systems in these ladder complexes enhances the twophoton absorption and the charge-carrier mobilities. Here we show that double-strand formation can also be used to lock the conformations of large p-conjugated macrocycles, as illustrated for c-P12 in Figure 1. The two simplest geometries providing complete p conjugation around an annulene are those in which the p orbitals are radial (as in carbon nanotubes) or axial (as in classical aromatic molecules). In principle, the formation of a double-strand complex offers a unique opportunity to switch between these two geometries, without altering the covalent structure. Recent STM imaging experiments indicated that porphyrin nanoring c-P12 lies flat on a gold(111) surface, with the porphyrin macrocycles parallel to the substrate, in the correct conformation to assemble a double-strand sandwich. Encouraged by this result, we tested the formation of (cP12)2·(DABCO)12 by adding DABCO to c-P12. When following this process by H NMR spectroscopy, we observed the appearance of a new species of lower symmetry which is in

Hydrogen Bonding to Hexafluoroisopropanol Controls the Oxidative Strength of Hypervalent Iodine Reagents

Hexafluoroisopropan-2-ol (HFIP) has been found to be an unusually beneficial solvent for undertaking hypervalent iodine-initiated [2+2] cycloaddition of styrenes. For the initiator phenyliodine(III) diacetate (PIDA), voltammetric data demonstrate that the enhanced reactivity in HFIP is due to its greater oxidizing abilities in this fluorinated solvent such that in HFIP the reactivity of PIDA is comparable if not superior to its fluorinated analog phenyliodine(III) bis(trifluoroacetate). These results contrast with the often reported view that the role of the fluoroalcohol is to stabilize a radical cation formed by single electron transfer. Moreover, combined NMR and HRMS results reveal the formation of a strong H-bonded adduct between the solvent and oxidizing reagent which is the physical origin of the observed altered synthetic reactivity.

Self-Assembly of Russian Doll Concentric Porphyrin Nanorings.

Electronic communication between concentric macrocycles with wave functions that extend around their circumferences can lead to remarkable behavior, as illustrated by multiwalled carbon nanotubes and photosynthetic chlorophyll arrays. However, it is difficult to hold one π-conjugated molecular ring inside another. Here, we show that ring-in-ring complexes, consisting of a 6-porphyrin ring locked inside a 12-porphyrin ring, can be assembled by placing different metals in the two rings (zinc and aluminum). A bridging ligand with carboxylate and imidazole binding sites forms spokes between the two rings, resulting in a highly cooperative supramolecular self-assembly process. Excitation is transferred from the inner 6-ring to the outer 12-ring of this Russian doll complex within 40 ps. These complexes lead to a form of template-directed synthesis in which one nanoring promotes formation of a larger concentric homologous ring; here, the effective template is an eight-component noncovalent assembly. Russian doll templating provides a new approach to amplifying the size of a covalent nanostructure.

Tetrameric Iridium Hydride-Rich Clusters Formed under Hydrogenation Conditions**

Asymmetric hydrogenation using iridium catalysts has been recognized as an important alternative to rhodium catalysis in recent years, because it enables the creation of a new stereogenic center without reliance on polar functionality in the substrate. Previous study of Ir hydrogenations provided significant insights into the mechanism, and underscored that an important deactivation process involves the formation of catalytically inactive trimers. The formation of such species may be limited by a judicious choice of nitrogen and phosphorus ligands, which leads to a new family of active catalysts. Herein, we highlight a quite different aspect of these studies. Hydrogenation of a specific iridium procatalyst under substrate free conditions leads not only to trimers but also higher nuclearity hydride-rich cluster compounds of iridium. Weller and McIndoe demonstrated that hydride-rich cluster compounds of rhodium have interesting structural and chemical features, but the corresponding compounds of iridium have not previously been studied. Herein, we describe the unique structures of two tetrameric hydriderich iridium clusters which are formed irreversibly under substrate-free conditions, in addition to a conventional trimer, as previously described (Scheme 1). The research suggests an important general route to this class of cluster compounds, because the nuclearities of the clusters formed under these conditions may in future be fine-tuned by varying the steric and electronic properties of the ligands. Hydrogenation of complex 1 ([Ir(PCy3)(cod)(C9H11N)] PF6) in CD2Cl2 was monitored by H NMR spectroscopy, until the reduction of 1,5-cyclooctadiene (cod) was complete. At this stage, the H NMR spectrum showed a complex iridium hydride region, in which the species 2 (an analogue of the dicationic Crabtree trimer) was detected, although it provided 40 % of the total signal intensity. The high resolution electrospray (ES) mass spectrum of solutions analyzed after complete reduction also revealed the presence of the standard trimer [Ir3H7(PCy3)3(C9H11N)3] 2+ (2), and an ion formed by loss of one N ligand from 2. In addition, two strong peaks corresponding to [Ir4H10(PCy3)4(C9H11N)2] 2+ (3a) and [Ir4H11(PCy3)4(C9H11N)] + (3b) were observed. Both of these gave characteristic patterns based on the stable isotope ratio (Ir/Ir = 37.3:62.7), providing unambiguous confirmation of the cation formulae (Figure 1). The structure obtained by single-crystal X-ray diffraction, after very slow crystallization of the hydrogenation product from dichloromethane/pentane, corresponds to the complex 3a, [Ir4H10(Cy3P)4(C9H11N)2] 2+ (PF6 )2. [5] The hydrogen atoms could not be located in the difference map, owing to the close proximity of heavy atoms and the associated termination Scheme 1. Hydrogenation of complex 1 under ambient substrate-free conditions.

Hydrothermal reactions of lactic acid catalysed by group: VIII Metal complexes

Abstract At 220–250°C, in the presence of [PtH(PEt 3 ) 3 ] + in aqueous solution, α-hydroxypropanoic acid (lactic acid) is converted into propanoic, propenoic, β-hydroxypropanoic, pyruvic and acetic acids. The reactions catalysed by [PtH(PEt 3 ) 3 ] + appear to be double bond hydration and hydrogenation, alcohol dehydration, transfer hydrogenation, decarbonylation and the water-gas shift reaction. Propenoic acid under similar conditions gives similar products. Attempts to catalyse the dehydrocarboxylation of lactic acid using PdCl 2 , [PtCl 2 (PPh 3 ) 2 ], [IrH(CO)(PPh 3 ) 3 ] and [Ni(CO) 2 (PPh 3 ) 2 ] were partially successful, although competing disproportionation gave a wide range of products.

Synthetic control of retinal photochemistry and photophysics in solution.

Understanding how molecular structure and environment control energy flow in molecules is a requirement for the efficient design of tailor-made photochemistry. Here, we investigate the tunability of the photochemical and photophysical properties of the retinal-protonated Schiff base chromophore in solution. Replacing the n-butylamine Schiff base normally chosen to mimic the saturated linkage found in nature by aromatic amines results in the reproduction of the opsin shift and complete suppression of all isomerization channels. Modification of retinal by directed addition or removal of backbone substituents tunes the overall photoisomerization yield from 0 to 0.55 and the excited state lifetime from 0.4 to 7 ps and activates previously inaccessible reaction channels to form 7-cis and 13-cis products. We observed a clear correlation between the presence of polarizable backbone substituents and photochemical reactivity. Structural changes that increase reaction speed were found to decrease quantum yields, and vice versa, so that excited state lifetime and efficiency are inversely correlated in contrast to the trends observed when comparing retinal photochemistry in protein and solution environments. Our results suggest a simple model where backbone modifications and Schiff base substituents control barrier heights on the excited-state potential energy surface and therefore determine speed, product distribution, and overall yield of the photochemical process.

Conformational analysis of fluorinated pyrrolidines using 19F-1H scalar couplings and heteronuclear NOEs.

A series of 3-fluoropyrrolidines have been studied to investigate the influence of the stereoelectronic fluorine gauche effect on ring conformations in the solid state by single-crystal X-ray analysis and in solution phase by NMR spectroscopy. As part of these studies 1D (19)F-(1)H heteronuclear NOE (HOESY) experiments have been optimised for applications to small molecules and are described in detail. These have been employed to estimate (19)F-(1)H internuclear distances and were combined with vicinal (3)J(F,H) and (3)J(H,H) scalar coupling constants to analyse the ring conformations. Where possible the derived solution-phase structural data have been compared with those of the crystalline state. The results demonstrate the influence of the gauche effect in stabilising C(γ)-exo conformations of the fluorinated pyrrolidines. It was further shown that when steric interactions were also present, this conformational bias was diminished and the contribution of the alternative C(γ)-endo conformation was seen to increase in solution at lower sample temperatures.

A Two-Directional Approach to Enantiopure 1,4-Difluoro-cyclohexenes: Synthesis of Difluorinated Cyclitol Analogues

Enantiopure 1,4-difluoro-cyclohexenes were prepared from readily available acetonide-protected (3 S,4 S)-hexa-1,5-diene-3,4-diol. In a two-directional mode, a double cross-metathesis reaction using allyltrimethylsilane as the olefinic partner, followed by electrophilic fluorination, afforded diastereomeric acetonide-protected 3,6-difluoro-octa-1,7-diene-4,5-diols. These dienes were found to be suitable substrates for ring-closing metathesis, delivering cyclohexenes featuring fluorine atoms on the two allylic positions flanking the double bond. Upon dihydroxylation, novel difluorinated cyclitol analogues were formed.

Studies on phytanoyl-CoA 2-hydroxylase and synthesis of phytanoyl-coenzyme A

Phytanoyl-CoA 2-hydroxylase (PAHX), an iron(II) and 2-oxoglutarate-dependent oxygenase, catalyses an essential step in the mammalian metabolism of beta-methylated fatty acids. Phytanoyl-CoA was synthesised and used to develop in vitro assays for PAHX. The product of the reaction was confirmed as 2-hydroxyphytanoyl-CoA by NMR and mass spectrometric analyses. In accord with in vivo analyses, hydroxylation of both 3R and 3S epimers of the substrate was catalysed by PAHX. Both pro- and mature- forms of PAHX were fully active.