Lewis base phosphorus oxide

Chiral phosphoramides, particularly C2-symmetric examples, are widely used in asymmetric synthesis (see section 3.2). One example is the asymmetric catalysis of Aldol reactions, where the phosphoramide catalyst is used in combination with a Lewis base. A solid state and solution study of the structure of chiral phosphoramide-tin complexes used in such reactions has now been reported. A number of chiral, non-racemic cyclic phosphoramide receptors (387) have been synthesised and their interactions with homochiral amines studied using electrospray ionisation MS. Although (387) bind the amines strongly, no evidence of chiral selectivity was found. Evidence from a combination of its X-ray structure, NMR, and ab initio calculations suggests that the cyclen phosphorus oxide (388) has an N-P transannular interaction in the solid state. A series of isomers of l,3,2-oxazaphosphorino[4,3-a]isoquinolines(389), containing a novel ring-system, have been prepared and their stereochemistry and conformation studied by H, C, and P NMR spectroscopy and X-ray crystallography... 

Desymmetrization with halogen nucleophiles was effectively demonstrated with two mechanistically-divergent chiral catalysts. Denmark disclosed a Lewis-base activated delivery of chloride that was catalyzed by the enantiopuxe phos-phoramide 16. Binding of the phosphoramide was believed to induce dissociation of SiCl4 into the chiral phosphorus/silicon cation and chloride anion, which subsequently ring-opened the activated epoxide. The best enantioselectivity was observed with cfs-stilbene oxide, which was formed in 94% yield and 87% ee (Scheme 14) [28]. 

In recent years many complexes of early transition metals in relatively high oxidation states have been prepared which contain neutral phosphorus donor ligands. Although the metal atom may prefer a harder Lewis base, in ihe absence of one. bonding lo a phosphine may occur. Fryzuk. M. D. Haddad. T. SBerg. 0. J. Coord. Chem. Rev. 1990. 99. 137-212. 

Other Lewis base derivatives are known as for example, with Et20, acetone, pyridine MeCN, BuNC. In the X-ray structure of the complex Cp2YbCl(Me2PCH2PMe2) (Tilley et al. 1983) where divalent Yb is oxidized by YbCls to the trivalent metallocene, Yb is coordinated to the two Cp ligands, to the chlorine and to one phosphorus of the dmpm ligand in a nearly tetrahedral arrangement. Yb-Cl bond length is 2.532(2), Yb-P is 2.941(3)A. 

E/ is a normalized i<30) scale based on water and tetramethylsilane as extreme values. See Ref. 15, pp. 364-371, for a compilation and discussion. AN is measured by the NMR shift values of triethylphosphine oxide phosphorus in a 1 1 complex with the test solvent measured against the 1 1 complex with SbCls test solvent using n-hexane as a neutral solvent. This is a normalized scale of Lewis acidity. Values are taken from Ref. 15, pp. 23-24. 

Phosphorus-based flame retardants are usually more suitable for engineering plastics that undergo charring than for commodity polymers. In some plastics, such as PC-ABS or poly(phenylene oxide)-HIPS blends, phosphorus-based flame retardants are more effective then halogenated flame retardants. Antimony trioxide, which is a part of halogen-containing formulations, is a Lewis acid and may destabilize some condensation polymers. Furthermore, the impact properties of engineering polymers may suffer due to the presence of powdery antimony trioxide. 

---