1-Bromo-1-lithioethene:  A Practical Reagent in Organic Synthesis

Abstract

[reaction: see text] A reliable preparative scale synthesis of 1-bromo-1-lithioethene is reported. This reagent undergoes clean 1,2-addition with a range of aldehydes and ketones at -110 degrees C to afford the corresponding 2-bromo-1-alken-3-ols in moderate to excellent yield. Trapping with other electrophiles (acylsilanes, chlorosilanes, tributyltin chloride, iodine) cleanly provides practically useful yields of various 1-substituted 1-bromoethene products. Unexpectedly high diastereoselectivities were observed during the addition of 1-bromo-1-lithioethene to alpha-siloxy aldehydes (typically 10:1, Felkin-Ahn control) and protected ketopyranose and ketofuranose sugars (> or = 10:1, addition from the less-hindered face). The title organolithium reagent possesses relatively low basicity at low temperature, and is compatible with a variety of common protecting groups. We believe that these unusual properties of 1-bromo-1-lithioethene may originate from the specific crystalline structure of the reagent in which lithium is coordinatively saturated and thus unavailable for chelation.

Synthesis of a C(sp2)‐bridged Phosphine‐Borane by Ionic Coupling

“…Synthesis of 2: 1‐Bromo‐1‐lithioetene was prepared as described by lithiation of vinylbromide (1 mL, 14.2 mmol) in THF:ether:pentane solution (4:1:1v, 120 mL) at –110 °C with n BuLi (1.6 m in hexanes, 3.35 mL, 5.4 mmol). The chlorodiisopropylphosphine (0.74 mL, 4.6 mmol) was added dropwise over 5 minutes.…”

“…Our synthetic route involves 1‐bromo‐1‐lithioethene as key precursor. This vinyl carbenoid has rarely been used in synthesis, except its reaction with aldehydes and ketones as reported by Novikov and Sampson , . Starting from bromoethene, H 2 C=C(Br)Li can in fact be easily generated upon treatment with n BuLi at –110 °C.…”

Enantiospecific Alkynylation of Alkylboronic Esters

“…However, small amounts of insertion product 4 a , a compound formed by 1,2‐migration of vinylboronate 5 , were also detected (entries 2 and 3, Table ) . The significant amounts of 1 a recovered (likely because of the instability of 1,1‐lithiobromoethene),, and competing rearrangement of 5 into 4 a , prompted us to conduct the transformation at a lower temperature (−95 °C) and increase the relative amounts of vinyl bromide and LDA used (2.0 equiv). Under these conditions, conversion into 3 a was improved and 4 a was not detected (entry 4, Table ).…”

1-Bromo-1-lithioethene

“…LiBr (0.20-0.25 equiv) THF, ether, pentane (4:1:1) (Trapp solvent), -112 to -115 °C (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.…”