Efficient Mono- and Dilithiation of 2-Bromo-1,1-diphenylethene with n-Butyllithium/Tetramethylethylenediamine

Abstract

Lithiation of 2-bromo-1,1-diphenylethene (2) with n-butyllithium or tert-butyllithium/tetramethylethylenediamine (TME-DA) in pentane at -100°C effects a halogen-lithium exchange to give 2-lithio-1,1-diphenylethene (3) exclusively, which reacts with electrophiles to provide 2-substituted-1,1-diphenylethenes 5-8 in high yields. Further lithiation of the monolithium derivative 3 with n-butyllithium/TMEDA results in the direct ortho-lithiation of Z-located phenyl ring to give dilithium derivative 9, which forms disubstituted ethenes 11-13 or heterocycles 15-17 on treatment with electrophiles. tert-Butyllithium/TMEDA is ineffective for the second lithiation step.Two different reaction pathways are known for the lithiation of 2-halo-1,1-diphenylethenes. 2,3 The first path includes proton removal (Scheme 1, route a) under the action of a strong base with the formation of 1-halo-1-lithioethene A. In the case of the chloro derivative (X = Cl) this intermediate is sufficiently stable at low temperature (-100 to -85°C) to react with electrophiles, like CO 2 or Br 2 , resulting in the formation of 1-substituted 1-halo-2,2-diphenylethenes in moderate yields. 4-6 At higher temperature or in the cases of bromo and iodo derivatives (X = Br, I), which are significantly more reactive, A readily rearranges into diarylacetylene B. 5-9 This classical example of the anionic rearrangement was discovered in 1894 10 and recognised as a very suitable way for the synthesis of diarylacetylenes. 7,8 Scheme 1 X = Cl, Br, I.The second possibility is halogen-metal exchange (Scheme 1, route b) to give the vinyllithium C. 5,11,12 This way is more distinctive for bromo-and iodo-derivatives (X = Br, I) and provides a versatile synthetic tool for the preparation of 2-substituted 1,1-diarylethene D. Unfortunately, this halogen-metal exchange is accompanied usually by the formation of acetylenes that reduce the synthetic scope of the reaction. 12,13 We wish to report here improved conditions for selective and efficient halogen-lithium exchange in 2-bromo-1,1-diphenylethene (2) in the presence of TMEDA, which result in the formation of 1,1-diphenylethylene derivatives on treatment of the intermediate vinyllithium compound 3 with electrophiles. Alternatively, the vinyllithium compound 3 can undergo further lithiation of the phenyl ring.Lithiation of 2-bromo-1,1-diphenylethene (2) with 3 equivalents of n-BuLi or t-BuLi in pentane in the presence of 3 equivalents of TMEDA results in complete halogenlithium exchange at -100°C already within 20 minutes (Scheme 2). Product distribution at different temperatures were investigated by treatment of the reaction mixture with iodine (Table 1). At -100°C the iodide 5 together with a small amount of ethene 1 were identified as the only products along with traces of the starting bromide 2 (<2%). When the reaction was allowed to warm to 0°C during 1 h, further deactivation of the lithium derivative 3 to ethene 1 was observed. No rearranged tolane 4 was detected not only after instant warming up, but also on keeping th...

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Novel distyrylcarbazole derivatives as hole-transporting blue emitters for electroluminescent devices

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Organodilithium Intermediates as Useful Dianionic Synthons: Recent Advances

“…The reaction of the intermediate 315 with electrophiles yielded either disubstituted ethenes 316 or heterocycles 317, depending on the type of electrophile used (Scheme 66). Surprisingly, the t-BuLi/TMEDA combination was ineffective for the second lithiation step [139].…”