Diphenylethylene dilithium

Korneev and Kaufmann successfully lithiated 2-bromo-l,l-diphenylethylene (46) by bromide-lithium exchange to form 2-lithio-l,l-diphenylethylene (47). A second lithia-tion could be effected in four hours at room temperature by deprotonation of the aromatic ring with w-butyllithium in the presence of TMEDA (Scheme 17). Like in the synthesis of compound 23, the first lithiation activates the ortho-hydrogen atom of the Z-phenyl substituent to give 1,4-dilithium compound 48. In total, three equivalents of the alkyl-lithium base are required the third equivalent is consumed in the trapping reaction of w-bromobutane with generation of octane. 

The bromide-lithium exchange of l,l-dibromo-2,2-diphenylethylene (88) was thoroughly examined by Maercker and coworkers. It could be shown that the number of side-products drastically decreases when LiDBB instead of metallic lithium is used as lithiation agent. The reaction was performed in THF at low temperatures by addition of the solution of the geminal dibrominated aUcene to the solution of LiDBB (Scheme 32). By this method, l,l-dilithio-2,2-diphenylethylene (89) could be obtained in 36% yield together with the 1,4-dilithium compound 48 and monolithiated 47 (51 and 2%, respectively). The yields were determined after trapping the reaction mixture with dimethyl sulphate. 

Similarly, Tung and co-workers (92a) reported dilithium anionic initiators based upon double 1,1-diphenylethylene compounds, such as bis-[4-(l-phenylethenyl)phenyl] either (21a). 

Dilithium reagents derived by reductive coupling of alkenes such as styrene and 1,1-diphenylethylene are important for the preparation of ring compounds (equation 7). 

A useful, hydrocarbon-soluble, dilithium initiator has been prepared by the dimerization of 1,1-diphenylethylene with lithium in cyclohexane in the presence of anisole (15 vol%) as shown in equations 14 and 15 (43). Although the initiator was soluble in this mixture, it precipitated from solution when added to the polymerization solvent (cyclohexane or benzene). Therefore, the dilithium initiator was chain extended with approximately 30 units of isoprene to generate the corresponding soluble oligomer. This initiator was used to prepare well-defined polystyrene-6ZocA-polyisoprene-6/oc -polystyrene and poly(a -methylstyrene)-6Zoc -polyisoprene-6ZocA-poly(o -methylstyrene) triblock copolymers with >90% 1,4-microstructure by sequential monomer addition. 

The addition of two equivalents of butyl-lithium to one of l,4-bis-(l-phenyl-ethenyl) benzene (and to three other double diphenylethylenes ) in benzene solution produces a dispersion of dilithium initiator." Following solubilization by the addition of a little butadiene, the initiator was capable of polymerizing a second, and much larger quantity of butadiene to polymer of dispersity of 1.2. 

The use of bis (1,1-diphenylethylenes) and tris( 1,1-diphenylethylenes) as precursors for hydrocarbon-soluble dilithium and trilithium initiators, respectively... 

The first publication on the use of 1,1-diphenylalkyllithiums as initiators for diene or styrene polymerization is the pioneering study of Morton and Fetters [49] on the preparation of poly(a-methylstyrene-btock-isoprene-f lock-a-methylstyrene) using the dilithium dimer of 1,1-diphenylethylene, 1,4-di-lithium-1,1,4,4-tetraphenylbutane (6) (see Scheme 4). The dilithium initiator... 

It is important to consider the possible reasons for the association effects which lead to bimodal molecular weight distributions for polymers formed using 90 as initiator in the absence of added Lewis base or lithium alkoxide. Leitz and Hocker [199] proposed that double diphenylethylene-based dilithium initiators form dimeric dianion aggregates (93) and that is why they are soluble in hydrocarbon solutions compared to other dilithium species. This type of dimeric structure is consistent also with the dimeric association... 

By analogy with the structure of the mcfa-substituted double diphenylethy-lene, 73, which forms a useful dilithium initiator upon addition of 2 moles of sec-butyllithium, the trifunctional diphenylethylene, 94, has been investigated as a precursor for a hydrocarbon-soluble, trilithium initiator, 95, as shown in Eq.(49) [239] ... 

Thus, multifunctional 1,1-diphenylethylenes, such as 73 and 94, are precursors for useful, hydrocarbon-soluble, multifunctional organolithium initiators such as 90 and 95, respectively. Their hydrocarbon solubility appears to be a consequence of a specific type of intermolecular association (e.g., 93) which is favored over the more usual type of 3-dimensional association which leads to insolubility for most dilithium initiators [89, 220]. However, perhaps because of their unique association, these initiators require the addition of either a Lewis base, such as tetrahydrofuran, or a lithium alkoxide salt to initiate rapidly (relative to propagation) and quantitatively. 

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