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

Abstract α-Diimine dianion ligated dialumane [L(THF)Al–Al(THF)L] (1), in cooperation with Na metal, readily mediates dearomatizing C–C coupling of pyridines with high regioselectivity. Treatment of 1 with pyridine (Py) and Na in a 1:1:1 molar ratio in toluene at room temperature afforded coupled complex [Na(THF)6]2[LAl–Al(L)(μ-C10H10N2)Al(L)–AlL] (2), which features a bridging 4,4′-dihydro-4,4′-bipyridine-1,1′-diyl (C10H10N22–) unit while retaining the Al(II)–Al(II) bond. However, when 4 equiv of Py and 2 equiv of Na were used, the Al(II) center in 1 and Na both participated in promoting reductive coupling of Py, yielding Al(III) complex [Na(Py)3(Et2O)][Na⊂{LAl(μ-C10H10N2)2AlL}] (3a), which in a THF solution rearranged to a unique molecular square [Na(THF)4]4[{LAl(μ-C10H10N2)}4] (3b). 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)n]2[LAl–Al(L)(μ-C10H8N2R2)Al(L)–AlL] (R = Me (4) or Bn (5)). In contrast, when the para position is equipped with a bulky group (tBu) or is incorporated within a ring (tetrahydroisoquinoline), the thermodynamically less favored 2,2′-coupling was observed to give mononuclear complexes [Na(DME)3][LAl(NC5H4tBu)2] (6) and [Na(DME)][LAl(NC9H11)2] (7) 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.