James F. Costello

Synthesis and structure of organoantimony (V) cyclometallates: transannular interactions and the barrier to cyclisation

A variety of alpha-hydroxy carboxylic acid salts [AgO2CC(OH)(RR2)-R-1] react with SbPh3Cl2 1 to afford the cyclometalled complexes 4b-g. A single crystal X-ray analysis of(+/-)-4g reveals the presence of conformational diastereoisomers arising from the helical chirality of the SbPh3 propeller unit. Benzilic acid (R-1 = R-2 = Ph) however, reacts with 1 to afford the di-ester 3. To rationalise the steric impediment to cyclisation, the solid-state geometries of all known five coordinate dioxo cyclometallates possessing an integral XPh3 unit are presented, along with the accessible conformations for each structural alternative. Computer modelling and crystallographic correlations identify severe 1,3-transannular interactions within the fluxional species associated with the nascent cyclometallate of benzilic acid.

Conformational diastereoisomers of PPh3 coordinated to stereogenic metal centres as molecular optical switches

Abstract The specific rotation of (RFe,R,M)-4 switches sign upon epimerisation to (RFe,S,P)-5. X-Ray crystallographic studies suggest that inversion of the propeller configuration of the coordinated PPh3 ligand is a major contributor to the switch of specific rotation. A simple model for predicting the conformational diastereoisomeric forms of PPh3 is presented, suggesting future routes towards the design of molecular optical switching devices.

Kinetic resolution of the chiral iron acetyl complexes [Fe(CO)(η5-C5H5)(L)COCH3][L = PPh3, P(p-tolyl)3]via aldol reactions with camphor

The chiral iron acetyl complexes [Fe(CO)(η5-C5H5)(L)COCH3][L = PPh3, P(p-tolyl)3] have been kinetically resolved via aldol reactions involving their derived lithium enolates and camphor in the presence of lithium chloride. With (1R)-(+)-camphor the faster reacting iron acetyl enantiomer had the (S)-configuration allowing the (R)-iron acetyls to be obtained after recovery of starting material and crystallisation. The corresponding (S)-iron acetyls can be obtained by a base-promoted reverse aldol reaction of the isolated products. A mechanistic rationale for the high enantiomeric discrimination is proposed which involves a chelation-controlled chair transition state with a disfavourable 1-methyl to cyclopentadienyl interaction in the mismatched pairing.

A structural and mechanistic investigation of the mono-O-phenylation of diols with BiPh3(OAc)2

The mono-O-phenylation of enantiomerically pure pinanediol 2 using BiPh3(OAc)2 1 and the biphenyl-2,2′-ylenephenylbismuth analogue 9 has been investigated. It is postulated that reductive elimination at the trigonal bipyramidal bismuth (V) centre of 1 upon exposure to ambient light affects the transfer of an apical phenyl ligand to the least sterically encumbered hydroxyl group of the diol, affording the monophenyl ether in good yield. SbPh3(OAc)2 fails to undergo reductive elimination, affording the stable diolate 6 instead. The X-ray crystal structure of 6 provides a reasonable model for the intermediate of the bismuth mono-O-phenylation, and suggests further studies with bismuth complexes such as 9 possessing intramolecularly tethered ligands incapable of facilitating the mono-O-phenylation reaction. The discussions are supported by X-ray crystallographic correlations, and calculations indicate that (M)-(-)-6 adopts the lowest energy conformational diastereoisomer.

Kinetic resolution of the chiral iron acetyl [Fe(η5-C5H5)(CO)(PPh3)COMe]

The chiral discrimination displayed between the enolate derived from (RS)-[Fe(η5-C5H5)(CO)(PPh3)COMe] and (1R)-(+)-camphor furnishes a practical route to homochiral (R)-[Fe(η5-C5H5)(CO)(PPh3)COMe]via a kinetic resolution protocol, while a retro-aldol reaction of the diastereoisomerically pure aldol addition product of this kinetic resolution sequence provides a complementary route to the opposite enantiomer (S)-[Fe(η5-C5H5)(CO)(PPh3)COMe].

The Development of Novel Fuel Dehydrating Icing Inhibitors

Dissolved water is a normal component of jet fuel which is vapourised during combustion; however, free water is a contaminant that can starve engines, freeze to form ice crystals capable of blocking fuel feeds, support microbial growth, and contribute towards corrosion. Jet fuel may be protected from the potentially hazardous effects of free-water using biocides and icing/corrosion inhibitors. This investigation seeks to identify novel chemical approaches to the dual management of both water contamination and ice formation in jet fuel. The strategy of using organic molecules as dehydrating agents remains a relatively neglected approach perhaps because of the complexity of the physical organic chemistry involved in developing and refining these systems. However, organic molecules with well characterised dehydrating properties - such as ortho esters, acetals, hemiacetals, ketals, and hemiketals - present themselves as an excellent starting-point for the development and optimisation of novel Fuel Dehydrating Icing Inhibitors (FDII). This paper describes our systematic approach towards the development of jet fuel additives which are kinetically fast, selective, lipophilic water scavengers that produce, upon hydrolysis, a hydrophilic ice inhibitor. A brief human and environmental toxicological screening of candidates is described. We anticipate that this class of FDII represents a novel approach towards protecting jet fuel against the effects of water contamination. Copyright

The hydrolysis of geminal ethers: A kinetic appraisal of orthoesters and ketals

Summary A novel approach to protecting jet fuel against the effects of water contamination is predicated upon the coupling of the rapid hydrolysis reactions of lipophilic cyclic geminal ethers, with the concomitant production of a hydrophilic acyclic hydroxyester with de-icing properties (Fuel Dehydrating Icing Inhibitors - FDII). To this end, a kinetic appraisal of the hydrolysis reactions of representative geminal ethers was undertaken using a convenient surrogate for the fuel–water interface (D2O/CD3CN 1:4). We present here a library of acyclic and five/six-membered cyclic geminal ethers arranged according to their hydroxonium catalytic coefficients for hydrolysis, providing for the first time a framework for the development of FDII. A combination of 1H NMR, labelling and computational studies was used to assess the effects that may govern the observed relative rates of hydrolyses.

A novel one-pot synthesis of homochiral (R)-(–)- and (S)-(+)- Fe(CO)(η5-C5H5)(PPh3)COCH3

Treatment of the salt [Fe(CO)2(η5-C5H5)(PPh3)]Br with potassium L-mentholate followed by lithium bromide and methyllithium generates, after work-up and crystallisation, homochiral (S)-(+)-[Fe(CO)(η5-C5H5)(PPh3)COCH3] in 30% yield.

The conformational analysis of phosphine ligands in organometallic complexes. Part 2. Triphenylphosphine coordinated to achiral and prochiral octahedral metal centres 1

The novel concept of nadir energy planes has been used to illuminate the principles governing the preferred orientation of rings A–C in PPh3 1, coordinated to achiral octahedral metal complexes of the type [M(PPh3)L1–5] 2a–d, and prochiral octahedral metal complexes of the type [M(η5-C5H5)(PPh3)L2] 3, and [M(η6-C6H6)(PPh3)L2] 4. An achiral arrangement of complex-bound 1 (reminiscent of the transition state for the one-ring flip stereoisomerisation process) orients all three ring apices proximal to three of the four orthogonal nadir energy planes associated with an octahedral metal centre. From this achiral arrangement, the favoured, degenerate conformations of complex-bound 1 may be subsequently derived by applying the following principles (in descending order of priority), (i) superimpose the vertical ring A of the achiral conformer onto the least encumbered nadir plane, (ii) allowring B to tilt onto the least encumbered nadir plane orthogonal to the vertical ring A, and (iii) tilt ring B in a manner which orients the flatter ring C beneath the smallest ligand. These principles are wholly consistent with 60 X-ray crystal structures, and detailed conformational analyses. The phenomenon of PPh3 ligand tilting, consistent with intramolecular steric strain, is characterised.

The conformational analysis of phosphine ligands in organometallic complexes: Part 1: Triphenylphosphine coordinated to an achiral metal centre 1

The conformational analyses of PPh3 1 and the representative achiral metal complexes [Al(PPh3)(Me)3] 2, [Fe(PPh3)(CO)4] 7 and [Ir(PPh3)2(CO)3]+ 8 possessing tetrahedral and trigonal-bipyramidal geometries are reported, and comparisons between the calculated and solid-state structures are made. The intramolecular non-bonded interactions which govern the conformational preferences of PPh3 in both the free and complex-bound state are characterised. The equilibrium between the opposing inter ring–ring and inter ring–ligand interactions which govern the minimum energy conformations of these complexes is examined. Analysis of the conformational preferences of PPh3 ligands in metal complexes is facilitated by the introduction of the novel concept of the plane of nadir energy.