Phosphates apical, equatorial

Results from H and 13C NMR studies of cyclic phosphate (8) indicate that the 1,3,2-dioxaphos-phorinane ring is attached apical/equatorial to phosphorus and in a twist rather than a chair conformation <90JOC3444>. Cyclic pentacovalent phosphorus(V) adducts are proposed as inter-... 

Carbon-13 NMR spectra for the cyclic phosphate (8) over a temperature range of 25 °C to — 113°C show that a slow exchange limit is reached at which the low-field signals due to equatorial methoxy carbons are twice as intense as the resonances at higher field due to the apical methoxy group. This means the 1,3,2-dioxaphosphorinane ring is attached apical/equatorial and not diequa-torial. 

Two groups have reported the synthesis of compounds of type (139) as potential models for ring-opening reactions of cyclic AMP. These studies included cases where the dioxaphosphorinane ring was both apical-equatorial and diequatorial, and conformations were studied by n.m.r.i98,i99 xhe predominant formation of 3 -monophosphates in the base-catalysed hydrolysis of nucleoside 3, 5 -cyclic phosphates has been interpreted in terms of the lone pair orientation effect, which may decrease the energy of the transition state for P-O bond-breaking.200 Conformational studies have been carried out on the diastereomers of adenosine cyclic 3 ,5 -phosphorothioate, where chair conformations predominate, and for the / p-isomer of deoxyadenosine cyclic phosphoranilidate, where a chair-twist equilibrium... 

The chemical reactivities of groups in the apical and equatorial positions of pentacovalent intermediates are different.664 In particular, elimination of a nucleophilic group to form a tetrahedral phosphate is easier from an apical position than from an equatorial position. For the in-line displacement of Eq. 12-27 elimination of RO should be easy. However an adjacent attack would leave - OR in an equatorial position. Before it could be eliminated, the intermediate would probably have to undergo a permutational rearrangement by which -OR would be transferred from an equatorial to an apical position. 

The molecular constraint which precludes ligand exchange unambiguously ensures that the tricyclic compounds undergo nucleophilic attack exclusively at the pseudo-equatorial carbon atom, irrespective of the nature of the carbon atom situated in the apical ligand of the TBP. The compounds 1-4 were obtained by alkylation of the chalcogen atom of the corresponding bicyclic (thio)phosphate, 6 and 7, with trialkyloxonium tetrafluoroborate at -78 °c (2). 

Substitution for uranyl ions can take place within the uranophane-sheet anion topology. In the framework structure of Na3,5Ho.5(U02)[Na2(U02)2(P04)4], NaOs polyhedra occupy half of the pentagonal positions of the sheet (Fig. 45). Double sheets are formed by sharing of apical vertices between the uranyl pentagonal bipyramids and the NaOs polyhedra. Uranyl square bipyramids in the interlayer share two equatorial vertices with phosphate tetrahedra from each adjacent sheet, and their apical vertices with the NaOs polyhedra to form a framework. The remaining Na atoms occupy the cavities of the structure [176],... 

A nucleophile adds, forming a trigonal bipyramidal pentacovaient intermediate that then ejects the leaving group in the microscopic reverse of the nucleophilic addition reaction. The pentacovaient intermediate is often short-lived, and this is shown by enclosing it in brackets. The pentacovaient intermediate bonds that are colinear are called axial (or apical) bonds, whereas the three bonds that lie in the plane perpendicular are called equatorial. Three common elements that can react by this pathway are silicon, phosphorus, and sulfur. Hydrolysis of phosphate esters like RNA occurs by this path. 

Studies of hypervalent phosphorus compounds in biological phosphoryl transfer reactions include the preparation of novel anti-apicophilic penta-coordinated phosphoranes with frozen stereomutation using bulky biden-tate ligands. Kinetic studies have enabled the activation enthalpy of the stereomutation of an 0-equatorial phosphorane to its O-apical stereoisomer to be calculated. The involvement of hexacoordinated phosphoranes in phosphate transfer reactions has been clarified by theoretical calculations as well as in vitro studies. Applications of hexacoordinated compounds as catalysts have also been described. 

---