Site-Selective Coupling of Pyridines Mediated by Low-Valent Aluminum Compounds: 4,4′- versus 2,2′-Coupling

With a relatively small 4-substituent on pyridine (4-Me-Py or 4-benzyl-Py), 4,4′-coupling still occurred to form products [Na(Solv)_<i>n</i>]₂[LAl–Al(L)(μ-C₁₀H₈N₂R₂)Al(L)–AlL] (R = Me (<b>4</b>) or Bn (<b>5</b>)). In contrast, when the <i>para</i> position is equipped with a bulky group (<i>t</i>Bu) or is incorporated within a ring (tetrahydroisoquinoline), the thermodynamically less favored 2,2′-coupling was observed to give mononuclear complexes [Na(DME)₃][LAl(NC₅H₄<i>t</i>Bu)₂] (<b>6</b>) and [Na(DME)][LAl(NC₉H₁₁)₂] (<b>7</b>) with a chelating 2,2′-dihydro-2,2′-bipyridine-1,1′-diyl ligand.

Such selective C–C couplings of pyridines have not been reported for Al compounds before, and the regioselectivity of 4,4′- versus 2,2′-coupling is verified by DFT computations.