Synthesis, Characterization, and Reactivity of Tricarbastannatranes
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The synthesis of a series of tricarbastannatrane complexes is described, and the structure of ionic triptych complexes [N(CH2CH2CH2)3Sn](BF4), [N(CH2CH2CH2)3Sn](SbF6), [N(CH2CH2CH2)3Sn]4[(SbF6)3Cl], [(N(CH2CH2CH2)3Sn)2OH][MeB(C6F5)3] [[N(CH2CH2CH2)3Sn]2Cl0.2F0.8][B[3,5-(CF3)2C6H3]4], and [(N(CH2CH2CH2)3Sn][allyl(B(C6F5)3] is established by NMR spectroscopy and X-ray crystallography. After demonstrating the Lewis acidity of tricarbastannatrane complexes toward various Lewis bases by NMR studies, the reactivity of tricarbastannatranes in conjugate addition to electrophilic alkenes was studied. Using alkyl-tricarbastannatranes as nucleophiles, the first B(C6F5)3-promoted conjugate addition to benzylidene Meldrum’s acids was carried out under mild conditions. The mechanism of the addition has been investigated by deuterium labeling experiments. It was shown that unsaturated carbonyl compounds can be efficiently activated by the Lewis acidic tricarbastannatrane. Furthermore, the structure of the reaction intermediates was determined by NMR and mass spectroscopy. The reactivity of tricarbastannatranes was further investigated by the addition of iPr-tricarbastannatrane to activated double bonds. In the presence of catalytic amounts of B(C6F5)3, iPr-tricarbastannatrane acts as a hydride source to generate [HB(C6F5)3]–, and reduces olefins, namely benzylidene 1,3-dimethylbarbituric acids. Detailed mechanistic studies on the reduction reaction were performed by NMR spectroscopy and mass spectrometry. Conjugate additions of isopropyl group to the benzylidene 1,3-dimethylbarbituric acids along with the reduced products were observed. To expand the applications of tricarbastannatranes in carbon–carbon bond formation reactions, allyl-tricarbastannatrane was added to carbon–carbon double bonds that bear strongly electron-withdrawing substituents under mild reaction conditions. The tin enolate species, which is generated by the addition of allyl-tricarbastannatrane to benzylidene 1,3-dimethylbarbituric acid, is characterized by multinuclear NMR spectroscopy.