Unusual metalation and halogen-metal exchange reactions between 1,3-xylyl crown ethers and organomagnesium reagents. X-ray structure of 2-[(p-tert-butylphenyl)magnesio]-1,3-xylylene-18-crown-5

Article abstract of DOI:10.1021/om00057a011

The scope of the surprising exchange reactions between 1,3-xylyl crown ethers and organomagnesium reagents was investigated. Halogen-metal exchange was found for both 2-bromo-1,3-xylylene-15-crown-4 (4) and 2-bromo-1,3-xylylene-18-crown-5 (14) as substrates, while metalation occurred with 1,3-xylylene-15-crown-4 (1). Attempts with other substrates failed, which demonstrates the highly specific structural requirements of these reactions. They are proposed to proceed after initial formation of a 1:1 complex of crown ether and organometallic reagent. Support was obtained by the metalation of 5-bromo-1,3-xylylene-15-crown-4 (17) at the intraannular 2-position. Product formation was investigated by ¹H NMR spectroscopy and by derivatization. An X-ray structure determination of the metalation product 2-[(p-tert-butylphenyl)magnesio]-1,3-xylylene-18-crown-5 (25) was carried out: space group P1? (triclinic), a = 10.297 (1) ?, b = 10.825 (1) ?, c = 12.476 (2) ?, α = 109.27 (1)°, β = 94.43 (1)°, γ = 106.74 (1)°, V = 1234.0 (3) ?³, Z = 2, R = 0.052. Reactions of 2-(phenylmagnesio)-1,3-xylylene-15-crown-4 (2) with 9-phenylfluorene or with diphenylzinc suggest the formation of a stable organomagnesium cation of the type RMg⁺ (32, 33, 34; R = 1,3-xylylene-15-crown-4-2-yl). Metal-halogen exchange is not restricted to organomagnesiums as illustrated by the reaction of diphenylcalcium with 14, which gave 2-(phenylcalcio)-1,3-xylylene-18-crown-5 (35).