Fluorenyllithium complexes

In a first apphcation of general interest, fluorenyllithium complexes (1, Scheme 1) were studied by solid state NMR spectroscopy. One reason for the choice of this system was that the results from the X-ray investigation presented at that time and solution NMR investigations were in conflict. The bis-quinuchdine complexes investigated in the solid state by X-ray analysis show that the lithium cation is asymmetrically positioned relative to the carbon framework of the anion, mainly interacting in a fashion with carbons C-1, C-9a and C-9 (Figure 9) . 

From these investigations it is clear that the Li chemical shift gives a clear indication of the lithium cation position when there are direct effects from ring currents in delocalized anions. However, as shown for the quinuclidine CIP and THF SSIP fluorenyllithium complexes, the correct assignment cannot be reached solely based on the chemical shifts. Furthermore, there is no clear-cut information about solvation to be gained from the chemical shifts. As we discuss in the following Section, the quadrupolar interaction is much more sensitive to these effects. In order to obtain maximal structural information from Li NMR spectroscopy, the chemical shift should be determined and used in combination with the quadrupolar coupling constant. 

In his classical review, Stucky (1) has already mentioned that in many n complexes the position of the lithium-base fragment is determined by the interaction of the frontier orbitals in the n fragment with the relevant orbitals at the lithium atom. This is nicely demonstrated by a series of cyclopentadienyl-, indenyl-, and fluorenyllithium complexes containing two further nitrogen atoms coordinated to the lithium as portrayed in Figs. 1 and 2. 

The crystal structure of the fluorcnylpotassium-TMEDA complex (compound XlXa in Fig. 3) has been solved by Stucky et al. (45). The coordination sphere of the potassium atom is made up of two tertiary amines and two unsaturated groups instead of two tertiary amines and one unsaturated organic group as found for the lithium atom in the fluorenyllithium complex VI (12). 

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). 

The THF complex of fluorenyllithium is according to the small x value an SSIP-type structure (entry 13). The polymorphism of fluorenyllithium is in accordance with theoretical investigations which show that the global minimum of monomeric unsolvated fluorenyllithium is a structure where the lithium cation is interacting in an rf fashion with the central five-membered ring However, the potential-energy surface above... 

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. 

The same type of addition—as shown by X-ray analysis—occurs in the cationic polymerization of alkenyl ethers R—CH=CH—OR and of 8-chlorovinyl ethers (395). However, NMR analysis showed the presence of some configurational disorder (396). The stereochemistry of acrylate polymerization, determined by the use of deuterated monomers, was found to be strongly dependent on the reaction environment and, in particular, on the solvation of the growing-chain-catalyst system at both the a and jS carbon atoms (390, 397-399). Non-solvated contact ion pairs such as those existing in the presence of lithium catalysts in toluene at low temperature, are responsible for the formation of threo isotactic sequences from cis monomers and, therefore, involve a trans addition in contrast, solvent separated ion pairs (fluorenyllithium in THF) give rise to a predominantly syndiotactic polymer. Finally, in mixed ether-hydrocarbon solvents where there are probably peripherally solvated ion pairs, a predominantly isotactic polymer with nonconstant stereochemistry in the jS position is obtained. It seems evident fiom this complexity of situations that the micro-tacticity of anionic poly(methyl methacrylate) cannot be interpreted by a simple Bernoulli distribution, as has already been discussed in Sect. III-A. 

Racemic 1-phenylethyl methacrylate is resolved efficiently by a cy-clohexylmagnesium chloride-(—)-sparteine complex to give, at 70% conversion, optically active polymer and the unreacted monomer in greater than 90% ee (181). Similarly, reaction of racemic phenyl-2-pyridyl-o-tolylmethyl methacrylate in the presence of 4-fluorenyllithium and (+)- or (—)-2,3-dimethoxy-l,4-bis(dimethylamino)butane proceeds with a high degree of kinetic resolution (182). 

With diphenylhexyllithium 121) (the product of addition of butyl-lithium to 1,1-diphenylethylene) kinetic results are the same as found for fluorenyllithium initiation in the presence of moderate amounts of ether. Even in pure toluene, the rates are first order with respect to initiator concentration and monomer concentration. This simple behaviour is caused by a constant fraction of the initiator forming low molecular weight polymer. If butyllithium is used as initiator, the kinetic behaviour is too complex for analysis. 

The geometry of fluorenyllithium bisquinuclidiene (31) (36), also can be visualized by means of a simple orbital interaction picture, shown along with 31. In the 8 HOMO, the two largest Huckel coefficients are at C(l) and C(9). Alternative locations over the five- and the sbc-membered rings, found for some transition metal derivatives, are not favored here. Albright et al. (53) provide a lucid analysis of the interactions in polycyclic complexes. 

Whatever the explanation, there is no doubt that polymer of molecular weight 500—800 is formed extremely rapidly at the start of polymerization and can be isolated from the final product. With fluorenyllithium [168], (toluene—ether, —60°C) a first order disappearance of monomer is observed, which extrapolates at zero time, not to the original added monomer concentration but to a concentration corresponding to the immediate loss of three molecules of monomer per initiator molecule. With 1,1-diphenylhexyllithium [174] (toluene,—30°C) this extrapolation corresponds to the rapid addition to the initiator of about five monomer units. In this case termination at various times and isolation of precipitant-soluble material confirms that polymer of molecular weight 830 is formed rapidly and does not change appreciably in amount throughout the polymerization. With butyllithium [173] (toluene, —30°C) the course of reaction is more complex in the initial stages but eventually a steady concentration of active centres is probably formed as the reaction settles down to first order decay in monomer. A second addition of monomer at the end of the reaction then produces a first order disappearance of monomer immediately. The two first order rate coefficients are identical. Evidently products are produced with butyl-lithium which disturb the reaction, and until these are removed a steady concentration of active centres is not achieved. 

Aminoboranediyl-bridged ansa- Cp-Flu) zirconocene dichloride 1195 has been prepared according to the salt metathesis approach shown in Scheme 281, which involves sequential reactions of 9-fluorenyllithium and CpNa with (Pr1)2NBCl2, followed by treatment with lithium diisopropylamide and ZrCl4.922 The molecule has ( -symmetry both solution spectroscopic and solid-state X-ray data reveal a partial double bond character between N and B atoms as a result of the B-N 7r-bonding. Upon activation with MAO, this complex polymerizes propylene to syndiotactic PP ([rr] =81%). 

Lithium [749,750,760-762] and sodium [750,760] organic compounds, lithium alcoholates [752,757,760-762], sodiomalonic diesters [755], complex bases from alkali imides and alcohols or alcoholates [756], phosphines [758,759], and others [751,753,754] have been used as initiators. It was found that with THF as solvent and fluorenyllithium or phenyllithium as initiator, molar mass is independent of initiator and monomer concentration. Relatively low masses of 2600 to 4200 were found. With DMF as solvent, the molecular mass increases with the monomer concentration at low (1.5mmol/L) initiator levels. With cyclopentadienyllithium or cyclopentadienyl sodium at high concentrations (68 mmol/L) and DMF as solvent, the molecular mass increases strongly with the monomer concentration. This is explained on the basis of a polyfunctionality of cyclopentadienyllithium and cyclopentadienyl sodium initiators. This view is supported by ozonolysis of the incorporated initiator, which leads to a decrease in the molar masses only of those polymers that were initiated by cyclopentadienyllithium or cyclopentadienyl sodium [750]. 

In contrast to these data the later investigations by Beletskaya and coworkers [13, 14] showed that ar -complexes are formed in the reactions of fluorenyllithium with LnCl3 independently of the reagent ratio ... 

A number of solid complexes of various organolithium compounds with various bases have been reported, e.g., C4H9Li-S(CH3)2 (56) [( 113)3 CLi]2 THF (55) 9-fluorenyllithium dietherate, a yellow solid (57) phenyllithium dioxanate, a colorless solid of formula 2C(HsLi 3C4Hg02 (58). The structures of none of these compounds are known, but they serve as examples to establish that the alkyllithium compounds do behave as Lewis acids toward n-type bases. 

Chiral ligands used as complexes with 9-fluorenyllithium ... 

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