Reactions of Iso(thio)cyanates with Dialanes: Cycloaddition, Reductive Coupling, or Cleavage of the C=S or C=O Bond

The dialanes [(dpp-Bian)Al-Al(dpp-Bian)] (1) and [(dpp-dad)Al(THF)-(THF)Al(dpp-dad)] (2) (dpp-Bian = 1,2-[(2,6-iPr2C6H3)NC]2C12H6, dpp-dad = [(2,6-iPr2C6H3)NC(CH3)]2) react with some isothiocyanates, isocyanates, and diphenylketene via [2 + 4] cycloaddition of the C=O or C=S bond across the C=C - N - Al fragment to afford complexes [L(X=C - Y)Al - Al(X=C - Y)L] with an intact Al-Al single bond (3, L = dpp-Bian, X = PhN, Y = O; 4, L = dpp-Bian, X = Ph2C, Y = O; 6, L = dpp-dad, X = BnN, Y = S; 7, L = dpp-dad, X = tBuN, Y = O; 8, L = dpp-dad, X = iPrN, Y = S; and 9, L = dpp-dad, X = CyN, Y = S). A mixed C=N and C=O mode cycloadduct, [(dpp-Bian)(TosN=C - O)Al - Al(TosN - C=O)(dpp-Bian)] 5, was obtained in the reaction of 1 with tosylisocyanate. Heating the solution of 3 resulted in a thermal transformation and a change of the cycloaddition mode from C=O to C=N to give the product [(dpp-Bian)(PhN - C=O)Al(O)Al(PhN - C=O)(dpp-Bian)] 10. The reduction of 7 and 8 with Na yielded the products [Na(THF)n]2[(dpp-dad-H)(X=C - Y)Al]2 (12, X = iPrN, Y = S, n = 2 and 13, X = tBuN, Y = O, n = 3) in which one of the methyl groups of the backbone of the initial dpp-dad ligand was dehydrogenated. When 2 was reacted with the bulky adamantyl isocyanate AdNCO, the C-C coupling of two substrates occurred to form 14 [(dpp-dad)Al(O=C - NAd)2Al(dpp-dad)] in which the coupled dianionic oxamide ligand bridged two Al atoms in a μ,ν4-N,O/N,O mode. Moreover, in the presence of 2.0 equiv of Na metal, precursor 2 reacts with tBuNCS, p-TolylNCS, or Me3SiNCO, possibly through the reduced AlI intermediate, to yield the sulfur- or oxygen-bridged dimer [Na(solv)n]2[(dpp-dad)Al(μ-E)]2 (15, E = S, solv = THF, n = 3 and 16, E = O, solv = DME, n = 2) upon C=S or C=O bond cleavage. Dialane 1 reacts with dimethylsulfone to give a Lewis adduct [(dpp-Bian)(Me2SO2)Al]2 (17), which releases dimethylsulfone upon heating. The diamagnetic compounds 3-10 and 12-17 were characterized by NMR and IR spectroscopy. The molecular structures of 3-17 were established by single-crystal X-ray diffraction analysis. Electronic structures of the compounds and possible isomers have been examined by DFT calculations.