Hexamethylphosphoric triamide ligand

Nakayama, Y Shibahara, T. Fukumoto, H. Nakamura, A. Syndiospecific polymerization of methyl methacrylate catalyzed by lanthanoid thiolate complexes bearing a hexamethylphosphoric triamide ligand. Macromolecules 1996, 29, 8014—8016. 

The amount of molecular sieves 4 A largely influences the product s ee[11]. Usually 100 mg (for the CMHP oxidation) or 1 g (for the TBHP oxidation) of MS 4A for 1 mmol of substrate is enough however, in the case where chemical yield and/or optical yield are not high, use of excess MS 4A often improves them. The addition of achiral ligands such as tributylphosphine oxide, tri-o-tolyl- and tri-/)-tolylphosphine oxides, hexamethylphosphoric triamide, triphenylpho-sphate, lutidine N-oxide, and l,3-dimethyl-2-imidazolidinone were found to be less effective than that of triphenylphosphine oxide in the epoxidation of chalcone. 

Chiral enamines prepared from /f-oxo esters and the tcrt-butyl ester of (.V)-valine are lithiated by LDA (—78 °C, toluene or THF, 1 h)18 19. Both enantiomers of the alkylation product are obtained with a high degree of diastercoselectivity starting from one auxiliary when the reaction is performed under the addition of different ligands (see Table 6). Addition of one equivalent of hexamethylphosphoric triamide (1IMPA) causes coordination of the lithium atom and alkylation from the top side18. 

The solubility of rare earth hydrides in organic solvents is increased by appropriate additives, too. For this purpose the hydrides are reacted with electron-donor ligands such as alkyl benzoates, alkyl propionates, alkyl tolu-ates, dialkylethers, cyclic ethers, alkylated amines, N,N -dimclhylacelamide, AT-methyl-2-pyrrolidone, trialkyl and triaryl phosphines, trialkyl phosphates and triaryl phosphates, trialkyl phosphates, hexamethylphosphoric triamide, dimethyl sulfoxide, etc. Prior to use as a polymerization catalyst the prereacted mixture of the rare earth hydride plus the additive is prereacted with Al-alkyl-based Lewis acids in the temperature range of 60-100 °C for 10 min to 24 h [351,352]. 

When 3 equiv of hexamethylphosphoric triamide (HMPA) ligand is added to the (BNA)2—O2—Sc system, the O2 /Sc complex becomes significantly stable and the ESR spectrum of the O2 -Sc (HMPA) complex can be detected even at 60°C under irradiation with light (78). The lanthanide complexes of radical anions of aromatic ketones are stabilized by the presence of HMPA ligand (79, 80). Oxygen enriched in can provide valuable information about the inequivalency... 

D2d symmetry. The high-spin [MS ]2 complexes in solution are very sensitive to oxygen and display typical electronic ligand field spectra. On the basis of the 10 Dq transitions in the near-IR spectra of these complexes, the thiophenolate ligand is placed between Cl" and hexamethylphosphoric triamide (HMPA) in the spectrochemical series. 

The intervention of hexacoordinated intermediates in intramolecular ligand exchanges can be an alternative mechanism to pseudorotation. Hexacoordinated intermediates are involved in intermolecular ligand exchanges of pentacoordi-nated silicon compounds (286) and in the hexamethylphosphoric triamide (HMPA) catalyzed F-Cl exchange of tetracoordinated silanes (355). 

Hexamethylphosphoric triamide. 13, 142-143 14, 176 15, 165-166 16, 174-175 Solvent effects. The stereochemical outcome of the alkylation of a-alkyl /3-keto ester enamines prepared from (S)-valine t-butyl ester is subject to solvent control. It seems that the bulky HMPA coordinates to Li from the opposite face of the isopropyl group, preventing the approach of the electrophile from that side. On the other hand, a smaller and weaker ligand allows the electrophile to attack from the opposite side of the isopropyl moiety. 

The use of hexamethylphosphoric triamide and triethylamine as ligands in place of the usual phosphines in the addition of ester enolates to 1 results in the formation of cyclopropanes via the unusual initial attack at the central carhon atom (eq 6). The process is very limited with respect to nucleophile, however, as only branched ester enolates produce a cyclopropane. N,NJ 1, N -tetramethylethylenediamine and carbon monoxide as hgands also promote central carbon attack by ester enolates. ... 

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