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Site-Selective Coupling of Pyridines Mediated by Low-Valent Aluminum Compounds: 4,4'- versus 2,2'-Coupling
Weixing Chen1, Li Yang2, Yihu Zhang1
1Key Laboratory of Synthetic and Natural Functional Molecule of the Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi'an710127, China.
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.
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