ipso-and para-Functionalization of meta-terphenyl ligands with substituted methyl groups: Unusual head-to-tail coupling of terphenyl moieties

The synthesis and characterization of several ipso  -functionalized derivatives of the bulky terphenyl group C6H3-2,6(C6H3-2′,6′-Pr2i)2(Ar′) are described. These include the primary alcohol Ar′CH2OH (1), the bromo derivative Ar′CH2Br (2), and the terphenyl formate Ar′CH2OC(O)H (3). The alcohol 1 was obtained by treatment of LiAr′ with formaldehyde, and 1 was readily converted to the bromo derivative 2 using HBr. The reaction of 1 with formic acid afforded 3 in good yield. Attempts to form the Grignard derivative of 1, i.e., Ar′CH2MgBr, resulted in a head-to-tail reaction of the terphenyl benzyl units to yield an unusual coupled product 4. An approach to the avoidance of this coupling involved the synthesis of the terphenyl derivatives 1-I-C6H2-2,6(C6H3-2′,6′-Pr2i)2-4-Me(5) and 1-I-C6H2-2,6(C6H2-2′,4′,6′-Pr3i)2-4-Me(6), bearing methyl groups in the para positions of the central aryl ring, which could be prepared in good yield, and converted to their respective lithium salts 7 and 8 without complication . The compounds were characterized by 1H and 13C NMR spectroscopy, IR spectroscopy (1) and X-ray crystallography (2, 4, 5 and 6).

Graphical abstractAttempted synthesis of the Grignard reagent Ar′CH2MgBr(Ar′=C6H3-2,6(C6H3-2,6-Pr2i)2) results in the isolation of the illustrated coupled product probably by a radical mechanism.Figure optionsDownload full-size imageDownload as PowerPoint slide