1,1,4,7,7-Pentamethyldiethylenetriamine, ligand structure

In the presence of N,N,N, N",N"-pentamethyldiethylenetriamine ( = PMDETA), monomeric lithium complexes of bulky formamidinate ligands can be isolated. The compounds (Scheme 12) comprise a Li(PMDETA) center coordinated by a bulky formamidinate in either the E-syn- or E-anti-isomeric form. Two of the structures display coordination of the pendant amidinate imine, and can therefore be considered the first examples of if. r -C = N,N metal amidinate coordination. ... 

Another system that has been investigated by C CP/MAS NMR spectroscopy as a function of different ligands is a-(dimethylamino)benzyllithium (2, Scheme 1) . The DEE complex was proven to exist in the solid state as an rf coordinated dimer . All the studied complexes are of an tf type according to comparison to solution NMR data. However, the actual structure varies as reflected by the shift difference between the two orf/zo-carbons. This difference ranges from 4.4 ppm for the N, N, N, N, N"-pentamethyldiethylenetriamine (PMDTA) complex to 20.3 ppm for the TMEDA complex. 

The ligand effects in the formation of tertiary carbanions from substituted tertiary aromatic amides have been investigated." Reaction of 2-isopropyl-(iVA -diisopropyl)-benzamide (10) with f-BuLi in ether resulted in ortho deprotonation and the formation of a hemisolvate based on a tetranuclear dimer of (10-Li )2 Et2O (Scheme 7). Reaction of (10) with f-BuLi in the presence of the tridentate Lewis base NjNjN pentamethyldiethylenetriamine (PMDTA) or tridentate diglyme (DOME) takes a different course (Scheme 7). The soUd-state structures revealed a dimer core in which the amide oxygen atoms fail to stabilize the metal ions for the former case and a remarkable benzylic deprotonation giving the tertiary benzyllithium (lO-Li )-PMDTA for the latter. 

Monomeric lithium enolates were rarely characterized by crystal structure analysis. It seems that prerequisite to monomer formation is an optimal solvation of the counterion lithium only if the metal is satisfied by a threefold, strong coordination by an external cosolvent, a monomeric enolate was detectable in the crystal state. This is illustrated by a crystal structure of the lithium enolate of dibenzyl ketone grown from a THF/diethyl ether solution containing the tri-dentate amine ligand A/ M M M" M"-pentamethyldiethylenetriamine (PMDTA), shown in Figure 3.4 [16a]. 

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