TMEDA complex

Crystal structure determination has also been done with -butyllithium. A 4 1 n-BuLi TMEDA complex is a tetramer accommodating two TMEDA molecules, which, rather than chelating a lithium, link the tetrameric units. The 2 2 -BuLi TMEDA complex has a structure similar to that of [PhLi]2 [TMEDA]2. Both 1 1 -BuLi THF and 1 1 -BuLi DME complexes are tetrameric with ether molecules coordinated at each lithium (Fig. 7.2). These and many other organolithium structures have been compared in a review of this topic. ... 

Fig. 9.5. Structure of TMEDA complex of Uthium bicyclo[3.2.1]octa-2,6-dienide. (Reproduced from Ref. 121 by permission of Wiley-VCH.)...

Table 1. l-(Diisopropylaminocarbonyloxy)-2-alkenyl-lithium-TMEDA Complexes by Deprotonation of Achiral or Racemic 2-Alkenyl Diisopropylcarbamates with Butyllithium (Selected Examples)... 

I-(Diisopropylaminocarbonyloxy)-2-alkenyllithium-TMEDA Complexes (Enantiomerically Enriched or Racemic) General Procedure19,6S. 

The lithium-TMEDA complex 1, obtained by deprotonation of (S)-(E)-1 -methyl-2-butenyl diisopropylcarbamate (84% ee), affords, after metal exchange by tetraisopropoxytitanium and addition to 2-methylpropanal, the homoaldol adduct ( + )-4 with 73% ee, whereas (-)-4 (53 % ee) is obtained when chlorotris(diethylamino)titanium is used104. 

The reagents prepared by lithiation (see Section 1.3.3.3.1.2.) and titanium exchange of (S)-(Z)-l-methyl-2-butenyl diisopropylcarbamate106 show a diminished reactivity when compared with those derived from the ( -isomer, indicating that in both metalation steps the doublebond geometry is retained16. After treatment of the lithium -TMEDA complex with chlorotris-(diethylamino)titanium and 2-methylpropanal, the homoaldol adduct (3S,47f)-(Z)-4-hydroxy-1,3,5-trimethyl-l-hexenyl diisopropylcarbamate [( + )-4], is formed with 88% ee16. 

Figure 40 The structure of the exo-(isodicyclopentadienyl)butylmagnesium tmeda complex 74.

Alkynyl enals cyclize on treatment with a stoichiometric amount of Ni(COD)2/TMEDA complex to give nickel enolates such as 193,436>436a These metallacycles react with electrophiles including methyl iodide and benzaldehyde to yield cyclopentenol derivatives (Scheme 91). 

FIGURE 7. (a) Experimental and (b) WINFIT simulated static Li solid state NMR spectmm of the TMEDA complex of trimethylsHylcyclopentadienyllithium. The x and r values are 156 kHz and 0.1, respectively. The small effect of CSA was neglected in the simulation. In (b), the singularities are indicated together with the relation of these to the x and r] values ... 

TMEDA complexes using solid state and variable temperature (VT) CP/MAS NMR spectroscopy (see Section II.E) . 

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. 

FIGURE 11. CP/MAS NMR spectra of fluorenyUithium complexes (a) Quinuclidine complex, (b) DEE complex, (c) TMEDA complex and (d) TFIF complex. Reprinted with permission from Reference 128. Copyright 1990 American Chemical Society... 

In the solid state NMR study, uncomplexed phenyllithium, assumed to be a tetramer, as well as the TMEDA complexed dimer and the PMDTA complexed monomer were investigated. Both Li and Li isotopes were used in the preparations. The C spectra of the complexes are presented in Figure 12. It is evident that the substitution of Li with Li has profound effects on the Unewidths, especially of the ipso-carbon at ca 180 ppm in the aggregated uncomplexed system (Figure 12a and 12b, respectively). This is in accordance with the previously mentioned study of methyllithium. However, even the other positions are affected by the dipolar couplings to the four quadrupolar lithium cations, but to a lesser extent due to the larger C-Li distances. 

In a study of 2,2 - and l,2 -biindenyl dilithium (6, 7, Scheme 1), C solid state NMR investigations were used, in combination with solution NMR studies and calculations, to obtain information about the existence of so-called triplet structures . Triplet structures are alkali metal complexes of dianions that arrange in a structure akin to dimers in Figure lb and were found by X-ray crystallography for the TMEDA complex of the dilithium salt of 9,9 -bifluorene (Figure 13) . This kind of structure has been proposed to account for the unexpected small difference in the first and second pXj value in some systems, where ApXa is found to be less than 1 pXa unit. ... 

The chemical shifts obtained in solution under different experimental conditions were compared to the shifts of the TMEDA complexes in the solid state. For 6, all data indicate a triplet structure of the CIP. The most compelling evidence for the triplet structure is the polarization of charge towards the two bridging carbons under CIP conditions. However, for 7, the charge distribution differs under CIP conditions in solution and in the solid state, as reflected by the chemical shifts. In solution, the shift data indicate again a triplet structure. In the solid state, however, the cations are not located at the bridging carbons, but shifted towards the five-membered ring systems. 

FIGURE 13. X-ray structure of the TMEDA complex of the dUithium salt of 9,9 -bifluorene, where a triplet structure is evident... 

In the DEE complex a chemical shift of —5.6 indicates that the ring current has a much more profound effect in this complex. However, the solid state structure was not known at that time. In the TMEDA complex, the lithium cation is postulated to be positioned above the central five-membered ring, and the Li chemical shift is —7.5 ppm, i.e. in the range of CIPs of cyclopentadienyllithium. In the THE complex, a shift of —2.6 ppm was observed. Again, no effect from the ring current is observed. However, based on the quadrupolar interaction this system was assigned as an SSIP, as discussed below. 

Two other complexes of known solid state structure have been included the SSIP of the dianion of ethylene-bisfluorenyllithium and the CIP TMEDA complex of indenyllithium . The observed chemical shifts were in accordance with expectations i.e. —1.8 ppm for the SSIP complex and —7.9 ppm for the CIP complex. 

FIGURE 14. (a) The monomeric PMDTA complex of phenyUithium . (b) The dimeric TMEDA complex of phenylUthium. (c) The tetrameric DEE complex of phenyUithium . (d) The internally coordinated tetramer of 2-[(dimethylamino)methyl]phenyUithium ... 

Other delocalized anions have been investigated as well, such as complexes of indenyl and fluorenyllithium. These data are also included in Table 8. The sole investigated indenyllithium system was the TMEDA complex. It is known from X-ray crystallography that the lithium cation is located above the five-membered ring and that the TMEDA binds in a bidentate fashion . The x value is somewhat larger than for the corresponding cyclopentadienyllithium complexes (entry 9). 

In our context, especially C-Li distances are of interest. A first successful Li- C REDOR study was undertaken in order to establish the structure of the previously discussed TMEDA complex of fluorenyllithium °, prepared from Li-enriched w-butyllithium and fluorene with at natural abundance. The REDOR pulse sequence used is depicted in Figure 20. The number of rotor cycles is increased in a symmetric fashion about the central jr-pulse in order to increase the dephasing time. 

FIGURE 21. C MAS spectrum of the TMEDA complex of fluorenylUthium. ssb = spinning sideband. Reprinted with permission from Reference 220. Copyright 1997 American Chemical Society... 

FIGURE 23. Stereoview of the position of the lithium cation relative to the fluorenyl system in the TMEDA complex of fluorenyUithium as determined by REDOR NMR spectroscopy... 

Temperature-dependent lineshape changes were observed in an early study of the fluo-renyllithium(TMEDA) complex. A detailed study by lineshape analysis, which was also applied to the TMEDA complex of 2,3-benzofluorenyllithium(TMEDA) (Figure 29f, yielded barriers AG (298) of 44.4 and 41.9 kJmoD for the 180° ring flip in these systems, respectively . A second dynamic process, which was detected via the temperature dependence of, the spin-lattice relaxation time in the rotating frame, is characterized by barriers of 35.1 and 37.6 kJmoD, respectively, and may be ascribed to the ring inversion process. For the fluorenyl complex, a barrier AG (298) of 15.9 kJmoD for the methyl rotation in the TMEDA hgand was determined from temperature-dependent NMR spectra of the deuteriated system. 

Only few types of benzyUithium compounds being configurationally stable in solution at —78°C are known lithium-TMEDA complexes of secondary 0-benzyl Af,Af-dialkyl carbamates, such as 211 or the 2,4,6-triisopropylbenzoate 212 ° , of secondary N-aryl-Af-Boc-benzylamines (213) and the dUithio-(—)-sparteine derivative 214 . ... 

These results were recorded for the TMEDA complexes, but there is little doubt that the sparteine complexes exhibit similar reactivity, presumably with a further shift towards inversion. 

The TMEDA complex of a-lithiobenzyl iV,iV-diisopropylcarbamate was found to be configurationally stable on the microscopical scale in the Hoffmann test . The (—)-sparteine complex 222 has moderate configurational stability on the macroscopic scale, which could not been brought to useful selectivities in substitution reactions . As... 

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