Synthesis and structure of 2,2'-boryl-, germyl-, silyl-, and stannyl-substituted 1,1'-binaphthyl systems

A number of Lewis acid binaphthyl systems, substituted in 2 or 2,2'-position, are synthesized by lithiation of 2,2'-dibromo-1,1'-binaphthyl (5), followed by addition of various electrophiles. Stepwise lithiation and subsequent borylation with trimethyl berate leads to the bromoboronic acid 6, which can be stabilized by esterification with pinacol giving 7. By increasing the reaction mixture to 2 equiv. of n-butyllithium and 2 equiv. of methyl berate the path to the binaphthylmonoboronic acid ester 9 is opened up. A further increase in the quantity of electrophile also leads to the binaphthylmonoboronic acid ester 12. The 2,2'-disubstituted silyl, germyl, and stannyl derivatives 13a-15a are accessible in good yields. Treatment with boron halides leads exclusively to methyl/ halogen exchange, giving the bidentate Lewis acids 13b-15b, the former of which can be bridged by oxygen. Only in case of the bis(tributylstannyl)binaphthyl 16a does ipso substitution occur in the presence of boron trichloride, giving the bis(dichloroboryl)-substituted binaphthyl 16b which can then be hydrolysed to 11. The structures of the majority of the compounds were investigated by X-ray diffraction. In case of the 2,2'-disubstituted compounds 13a-15a the naphthyl groups are orientated perpendicular to each other. The intra- and intermolecular interactions are dominated by this binaphthyl system. In case of the oxygen-bridged compounds 13c and 14c the angle between the naphthyl planes decreases to about 70 degrees. This also has an effect on the packing of the molecule; in this instance the orientation of two naphthyl in neighbouring molecules is nearly parallel. The structure of the diboronic acid is dominated by intra- and intermolecular hydrogen bonding.