Tetramethylurea complexes

Malonic acid serves as a ketene precursor in solid phase N-acetylation of peptides catalyzed by diisopropylethylamine by reaction with the catalyst HBTU (O-benzotriazole-iV, N, N, JV -tetramethyluronium hexafluoro-phosphate) in dg-DMF forming a malonate/tetramethylurea complex that gives ketene and tetramethylurea, as shown by anisidine acetylation (Scheme 4.7). Computational methods favored the pathway shown for ketene generation. This method was used in the acetylation of a variety of resin bound peptides. 

Lincoln SF, White A. Ligand exchange on hexakis(l,l,3,3-tetramethylurea) complexes of trivalent lanthanides. Proton nuclear magnetic resonance study. Inorg Chim Acta. 1990 168 265-270. 

NMR has been used to study ligand exchange in non-coordinating solvents for a series of Be(C104)2-4L (L = trimethyl phosphate, DMSO, DMA, DMF, NMA, 1,1,3,3-tetramethylurea, dimethyl methylphosphonate and dimethyl phenylphosphonate) complexes. 3H NMR studies show that the mode of activation for exchange on beryllium varies from dissociative to associative depending upon L and the nature of the non-coordinating solvent.127 128... 

Interest in the interpretation of the spectral and magnetic properties of oxo-vanadium(iv) complexes has grown as it has become apparent that the general assumption of overall C4 symmetry for these complexes is unjustified. Bis-(2-methyl-8-quinolato)oxovanadium(iv), VO(quin)2,395 and bis(tetramethylurea)dichloro-oxo-vanadium(iv), VO(tmu)2Cl2,396 for example, have been found to be five-co-ordinate with a trigonal-bipyramidal co-ordination polyhedron about the vanadium atom. A crystal-field model has been developed which gives a good account of the electronic and e.p.r. spectra of VO(quin)2. 

Table 14 shows the formation of complexes between PMAA and proton-accepting polymers via hydrogen bonds. Tetramethylurea (TMU), N,N-... 

N-bridging cyanate in low yields (17-23% after heating for 24 h Scheme 11). Conversion was found to proceed at comparable rates in ethanol or acetonitrile, and it was thus concluded that hydrolytic processes by traces of water do not play a role. Cyanate formation also was observed with Af-methylurea or 7/,7/-dimethylurea, but not with Af,A -dimethylurea or tetramethylurea, which shows that at least one NH2 group is essential for the elimination reaction to occur. A possible interpretation is that bridging of urea over the binuclear core through its O atom and one amino N atom is a key step for the conversion (58). This finding is in line with the discovery of urea-to-cyanate transformation for several pyrazolate-based dinickel complexes with urea bound in the N,0-bridging mode (see below). 

FIG. 8. (a) Plot of log,0 1 against the 8-coordinate ionic radius of the lanthanide ion for Lnffod-dg) complexes. Ic, is the first-order rate constant for substrate exchange HMP = 1,2-hexamethylphosphoramide TMU = tetramethylurea. (b) Suggested energy profile for substrate exchange. (481)... 

Langer (13) has also disclosed the use of alkyllithium and dialkyl-magnesium tertiary diamine complexes as catalysts for copolymerization of ethylene and other monomers such as butadiene, styrene, and acrylonitrile to form block polymers. Examples are given in which polybuta-dienyllithium initiates a polyethylene block, as well as vice-versa. Random copolymers of these two were also prepared, and other investigators have used not only tertiary diamines but hexamethylphosphoramide (14) and tetramethylurea (15) as nitrogenous base cocatalysts in such polymerizations. Antkowiak and co-workers (11) showed the similarity of action of diglyme and TMEDA in copolymerizations of styrene and... 

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