1‐Bromo‐1‐lithioethene

[569650-49-3] C2H2BrLi (MW 112.88) InChI = 1S/C2H2Br.Li/c1-2-3;/h1H2; InChIKey = IICIEGVXHMYNQL-UHFFFAOYSA-N (functionally dense reagent used as a two-carbon nucleophile in a variety of reactions) Physical Data: not available due to thermal instability above ca. −112 °C. Form Supplied in: not commercially available. Preparation, Solubility, Handling, Storage, and Precautions: 1-bromo-1-lithioethene is prepared at low temperature (−112 to −115 °C) via the reaction of n-BuLi with vinyl bromide in the Trapp solvent mixture (THF, ether, and pentane—4:1:1 by volume) using a specialized but readily constructed glass reactor and is utilized in situ (eq 1).1-3 (1) This solution must be prepared and maintained under an argon atmosphere to avoid adventitious water and air. The choice of solvent and careful control of reaction parameters are critical for the successful preparation and trapping of 1-bromo-1-lithioethene. Its formation via the deprotonation of vinyl bromide slows down significantly when the concentration of THF drops below 50% (by volume) and no deprotonation ensues at −110 °C in pure ether or ether–pentane mixtures. Increasing THF content above ca. 70% causes complete freezing of the reaction mixture. Although soluble in the Trapp solvent even at −115 °C, 1-bromo-1-lithioethene is not very stable in solution—about 50% decomposition occurs in solution over 1 h at −115 °C. Hence, during its preparation, care is taken to induce crystallization of 1-bromo-1-lithioethene via localized supercooling, which is triggered by a frozen coolant bridge formed to a site on the reactor.1, 3 The resulting white slurry can then be kept for 4 h at −112 to −115 °C without noticeable degradation. Care must be taken to avoid warming this slurry—decomposition is indicated by a sharp phase transition which takes place at −105 to −104 °C to afford a clear solution that no longer contains any residual 1-bromo-1-lithioethene. The stability of 1-bromo-1-lithioethene is enhanced in the presence of added LiBr (0.20–0.25 equiv). Lower loading of added LiBr results in decreased yields of trapped adducts, whereas using more than 0.5 equiv LiBr significantly retards the formation of 1-bromo-1-lithioethene via the deprotonation of vinyl bromide. The 1-halo-1-lithioethenes (Hal=Cl, Br, I) vary somewhat in their ease of preparation via the deprotonation of the corresponding vinyl halides.4 The precursor required for the preparation of 1-bromo-1-lithioethene (vinyl bromide) is an inexpensive and easily handled commercially available gas (bp 16 °C, ca. $10/mole in 2006). It is also available as a convenient but more expensive 1 M solution in THF (ca. $200/mole in 2006) that can be readily handled with common syringe techniques. Use of n-BuLi as base achieves clean deprotonation of vinyl bromide, whereas substantial competition from halogen-metal exchange is seen with tert-BuLi.1, 3 1-Chloro-1-lithioethene was originally prepared from vinyl chloride5, 6 (gas, bp −13 °C), which is quite expensive ($170/mole in 2006) and is a regulated substance. However, more recently Whitby has showed that 1-chloro-1-lithioethene can be prepared from the inexpensive liquid 1,2-dichloroethane (ethylene chloride, bp 83 °C, ca. $3/mole in 2006).7 The preparation of 1-iodo-1-lithioethene requires use of iodoethene, an unstable liquid that is not commercially available and that must be prepared immediately prior to use.8 The sequential preparation and trapping of the 1-halo-1-lithioethenes using a range of electrophiles has been most broadly developed in the case of 1-bromo-1-lithioethene (see below). The bromovinyl moiety present in the resulting products offers more scope for further synthetic manipulation than would be possible with the chlorovinyl group available from analogous reactions involving 1-chloro-1-lithioethene. The reported propensity for 1-iodo-1-lithioethene to undergo decomposition to acetylene even at low temperature8 may limit its synthetic utility. Nevertheless, it should be noted that many of the reactions described herein for 1-bromo-1-lithioethene may also be possible with other 1-halo-1-lithioethenes, although in most cases, these reactions have not yet been investigated. The readily constructed low temperature glass reactor designed by Novikov1-3 is well suited to the preparation of 1-bromo-1-lithioethene via the n-BuLi-mediated deprotonation of vinyl bromide on scales ranging from 1–3 mmol (30 ml reactor) to 15–35 mmol (250 ml reactor). Other designs for low temperature reactors have also been reported;9 however, the need for efficient (overhead) stirring and careful monitoring of the internal reaction temperature make the Novikov reactor particularly attractive for performing this chemistry. As described in the Solubility, Handling, Storage, and Precautions section (above), significant care must be exercised in controlling the reaction conditions in order to achieve the successful preparation and subsequent electrophilic trapping of 1-bromo-1-lithioethene. Purification: 1-bromo-1-lithioethene is not purified prior to use—it is prepared and trapped in situ at low temperature (see Preparation).