Use of Suzuki cross-coupling as a route to 2-phenoxy-6-iminopyridines and chiral 2-phenoxy-6-(methanamino)pyridines

The anisyl boronic acids, 2-OMe-3-R2-5-R1-C6H2B(OH)2 (R1=R2=H (a); R1=H, R2=Ph (b); R1=Me, R2=H (c); R1=Cl, R2=H (d); R1=t-Bu, R2=H (e)), have been employed in Suzuki cross-coupling reactions with either 2-bromo-6-formylpyridine (I) or 2-bromo-6-acetylpyridine (II) generating, following a facile deprotection step, the 2-phenoxy-6-carbonylpyridines, 2-(2′-OH-3′-R2-5′-R1-C6H2)-6-(CHdouble bond; length as m-dashO)C5H3N (R1=R2=H (1a); R1=Me, R2=H (1c); R1=Cl, R2=H (1d); R1=t-Bu, R2=H (1e)) and 2-(2′-OH-3′-R2-5′-R1-C6H2)-6-(CMedouble bond; length as m-dashO)C5H3N (R1=R2=H (2a); R1=H, R2=Ph (2b)). Condensation reactions of 1 and 2 with 2,6-diisopropylaniline proceed smoothly to give the 2-phenoxy-6-iminopyridines, 2-(2′-OH-3′-R2-5′-R1-C6H2)-6-{CHdouble bond; length as m-dashN(2,6-i-Pr2C6H3)}C5H3N (R1=R2=H (3a); R1=Me, R2=H (3c); R1=Cl, R2=H (3d); R1=t-Bu, R2=H (3e)) and 2-(2′-OH-3′-R2-5′-R2-C6H2)-6-{CMedouble bond; length as m-dashN(2,6-i-Pr2C6H3)}C5H3N (R1=H, R2=Ph (4a), R1=H, R2=Ph (4b)). Reduction of the imino unit (and concomitant C–C bond formation) in 3 can be achieved by treatment with trimethylaluminium or methyllithium which, following hydrolysis, furnishes the racemic chiral 2-phenoxy-6-(methanamino)pyridines, 2-(2′-OH-3′-R2-5′-R1-C6H2)-6-{CHMe-NH(2,6-i-Pr2C6H3)}C5H3N (R1=R2=H (5a); R1=Me, R2=H (5c); R1=Cl, R2=H (5d); R1=t-Bu, R2=H (5e)). This work represents a straightforward and rapid synthetic route to libraries of sterically and electronically variable phenoxy-substituted imino- and methanamino-pyridines, which are expected to act as useful ligands or proligands for late and early transition metal-mediated alkene polymerisation catalysis.