Carine Maaliki

Carbeniophosphanes and their carbon → phosphorus → metal ternary complexes

The discovery, recent developments and prospects of the carbeniophosphines, and in particular of imidazoliophosphines, are related. The P-conjugated positive charge brought by the stabilized carbenium center gives stable (Im+)R2P → metal dative bonds, while the ligands themselves contain a relatively stable C → P+R2 dative bond, making imidazoliophosphines, more accurately described as NHC–phosphenium adducts. The preparation methods, structural features, electronic and coordination properties, reactivity, and catalytic properties of carbeniophosphines are reviewed.

Carbene-Stabilized Phosphenium Oxides and Sulfides

Carbene→chalcogenophosphenium adducts, which correspond to an intermolecular stabilization mode of the so far elusive, free oxo- and thiooxophosphenium species [R(2)P(+) = X] (X = O, S) by imidazolylidene (NHC) and diaminocyclopropenylidene (BAC) donors, have been isolated and fully characterized. The dative character of the R(2)C:→P(+)(X)Ph(2) bond was confirmed experimentally by nucleophilic displacement of the carbene donor with a chloride ion and by an exchange reaction of the NHC ligand of the NHC:→P(+)(O)Ph(2) adduct with an independently prepared BAC ligand, thereby giving the BAC:→P(+)(O)Ph(2) adduct. This dative character was further characterized by the DFT-calculated preference of carbene→chalcogenophosphenium systems for a heterolytic dissociation mode over a homolytic one.

P-oxidation of gem-dicationic phosphines

In spite of the general oxophilic character and λ5 valence state accessibility of phosphorus centers, the oxidation of phosphanes bearing proximal positive charges remains challenging, in particular for dicationic representatives. Previously described diimidazolo- and diimidazolio-phosphane substrates were first resumed in the P-phenylated series, where the reported absence of coordinating ability of the N,N-dimethylated dication towards a neutral Rh(I) center is here confirmed towards a promoted oxygen atom. By contrast, the P-tert-butylated homologue is shown to undergo P-oxidation to give a stable gem-dicationic phosphine oxide (in 91% yield with m-CPBA). The latter could also be obtained by N,N-dimethylation of the neutral diimidazolo-phosphine oxide precursor. The strict covalent vs. dative character of the N2C–P bond in dicationic systems (N2C+)2P(X)R (R = Ph, tBu; X = lone pair, :O) has been investigated by DFT calculations on a model series where (N2C+) = 1,3-dimethyl-1H-imidazolio-2-yl. The relative magnitude of Gibbs free energies of the homolytic and heterolytic dissociation modes shows that diamidinio-phosphines and corresponding P-oxides are not N2C → P donor–acceptor adducts (as mono(amidinio)phosphines are), but rather true gem-dicationic phosphines and phosphine oxides in the strict sense, respectively.

Chapter 20:Conformationally restricted glycoside derivatives as mechanistic probes and/or inhibitors of sugar processing enzymes and receptors

X-ray crystallography and kinetic isotopic effects applied to glycosyl-processing enzymes in complex with their substrates or inhibitors allowed deciphering of the sugar ring conformation at the atomic level. This information, crucial for the development of potent and selective inhibitors of these enzymes, has been extensively exploited to design conformationally restricted sugar analogues. This chapter gives an overview of the main classes of conformationally restricted sugar analogues aimed at interfering with hydrolytic and non-hydrolytic glycosyl-processing enzymes that have been reported in the literature.