O-H activation in phosphates: oxidative addition to an iridium(I) center and reactivity of the resulting iridium(III) species

Abstract

The oxidative addition of E-H bonds to [Ir(1,5-COD)(PMe₃)₃]Cl (1,5-COD = 1,5- cyclooctadiene) has been previously studied (E = B, C, N, O, S). Some of the resulting species have shown activity as catalysts for hydrogenation and the addition of other molecules to unsaturated species. More recently, the addition of amino acids to [Ir(1,5- COD)(PMe₃)₃]Cl was studied in an attempt to create molecules with biological activity as well as species which might be active in asymmetric catalysis. Although inactive as catalysts, one of the these amino acid complexes was shown to be an anti-HIV agent. This finding prompted research into the additions of other molecules of biological importance to [Ir(1,5-COD)(PMe₃)₃]Cl.

Since phosphates are an important functional group in biochemistry, the purpose of this research was to study the possible binding modes of the phosphate group to the iridium center and then to react biologically occurring phosphates with [Ir(1,5-COD)(PMe₃)₃]Cl. The reaction of dibenzyl phosphate (DBP) with [Ir(1,5-COD)(PMe₃)₃]Cl has been studied in detail. The resulting complex, Ir(PMe₃)₃(H)(DBP)Cl, is soluble in most common laboratory solvents with the exception of ether and was characterized by infrared, ¹H, ³¹P, and ¹³C NMR spectroscopy and elemental analysis. The dibenzyl phosphate ligand is labile and is easily displaced by nucleophiles. The complex does not undergo reaction with molecular hydrogen at ambient temperatures.

Dialkyl phosphate complexes were easily made using the same synthesis as the dibenzyl phosphate. This reaction was unsuccessful for the preparation monoalkyl and diaryl phosphate complexes.

The reaction of [Ir(1,5-COD)(PMe₃)₃]Cl with the nucleotides 2'-deoxyadenosine- 5'-monophosphate (d-AMP) and (-)-adenosine-3'-5'-cyclic monophosphate (cyclic-AMP) did not result in the formation of a single product perhaps because the phosphate groups in these compounds are diacids. The reaction of [Ir(1,5-COD)(PMe₃)₃]Cl with 1,2- dihexadecanoyl-rac-glycero-3-phosphoethanolamine (PEA), a dialkyl phosphate, produced only the N-H addition product.