The hydrolysis of geminal ethers: a kinetic appraisal of orthoesters and ketals.

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 (1)H NMR, labelling and computational studies was used to assess the effects that may govern the observed relative rates of hydrolyses.

Conformational analysis of triphenylphosphine ligands in stereogenic monometallic complexes: tools for predicting the preferred configuration of the triphenylphosphine rotor.

The extension of our simple model for predicting the propeller configuration of a triphenylphosphine ligand co-ordinated to achiral metal centres to include stereogenic metal systems is described. By considering nadir energy planes (NEP's) and a series of rigid-body calculations, a model has been developed to reliably predict the configuration of the triphenylphosphine rotor of stereogenic metal complexes. For complexes of the form [M(η(5)-C5H5)(PPh3)(L(1))(L(2))], where it is assumed that L(1) is larger than L(2), the configuration of the triphenylphosphine rotor may be predicted by viewing a Newman projection along the L(1)-M bond. In the orientation where the PPh3 unit is pointing vertically downwards and the orthogonal L(2) ligand is pointing to the right [i.e., an (RM)-configured complex, assuming that L(2) is ranked higher priority than L(1)], the conformation of L(1) can be expected to place the most sterically demanding substituent in the top-right quadrant. In cases where ligand L(1) still presents a steric incursion towards the PPh3 ligand (any part of L(1) other than H proximal to the PPh3 in the approximate zone -30° to +60° from the M-P bond) an (M)-configured rotor is expected, and when this interaction is not present a (P)-configured propeller is predicted. Without exception, these rules are consistent with all empirical data (>140 known crystal structures).

Upon the intriguing stereoselective formation of organobismuth(V) complexes.

The preparation of triphenylbismuth(V) 3a-k and antimony(V) 4e-k bis-carboxy ester complexes is described. A range of studies in solution suggest that the diastereoselective formation of (RR,SS)-3a-j is governed by the thermodynamic stability of rapidly interconverting epimeric species. Diastereoselectivity is absent in the case of the corresponding Sb complexes, leading to the conclusion that a combination of both ligand-ligand (steric) and metal-ligand (hyperconjugative) interactions govern stereoselectivity. The formation of homochiral complexes (RR,SS)-3a-j is rationalised using a simple model, invoking for the first time a palindromic BiPh3 propeller moiety, which correlates the chirality of the trans axial carboxy-ester ligands. The X-ray crystal structures of both hetero- and homochiral diastereoisomeric antimony complexes (4h and 4i, respectively) are presented in support of this model.