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Dendritic Carbosilanes Containing Silicon-Bonded 1-[C6H2(CH2NMe2)2-3,5-Li-4] or 1-[C6H3(CH2NMe2)-4-Li-3] Mono- and Bis(amino)aryllithium End Groups:  Structure of {[CH2SiMe2C6H3(CH2NMe2)-4-Li-3]2}2

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journal contribution
posted on 29.12.1998, 00:00 by Arjan W. Kleij, Henk Kleijn, Johann T. B. H. Jastrzebski, Wilberth J. J. Smeets, Anthony L. Spek, Gerard van Koten
A useful synthetic procedure for the incorporation of the potentially multidentate monoanionic 1-[C6H2(CH2NMe2)2-3,5]- (NCN) and 1-[C6H3(CH2NMe2)-4]- (CN) ligands via the para-position on the periphery of carbosilane (CS) dendrimers has been developed. Lithiation of suitable brominated precursors [C6H3(CH2NMe2)2-3,5-Br-1], 1, and [C6H4(CH2NMe2)-4-Br-1], 2, in Et2O at −78 °C with 2 equiv of t-BuLi gives the monolithiated aminoaryl compounds Li[C6H3(CH2NMe2)2-3,5], 1-Li, and Li[C6H4(CH2NMe2)-4], 2-Li, which can be subsequently treated in situ with the silicon chlorides ClSiMe2(CH2)nSiMe2Cl (n = 2 or 6) to yield the functionalized CS-ligand systems [CH2SiMe2C6H3(CH2NMe2)2-3,5]2, 3, and [(CH2)nSiMe2C6H4(CH2NMe2)-4]2 (for 4, n = 1; for 5, n = 3). The ligand systems 35 react with 2 equiv of t-BuLi to give the dilithiated derivatized carbosilane molecules [CH2SiMe2C6H2(CH2NMe2)2-3,5-Li-4]2 and [(CH2)nSiMe2C6H4(CH2NMe2)-4-Li-3]2 (7, n = 1) in good chemical yields. Quench reactions of the in situ prepared lithiated derivatives with either D2O, Me3SiCl/Me3SiOTf, or MeSSMe clearly showed that the CS-ligand systems have been quantitatively converted into their lithiated derivatives. In addition, the functionalized carbosilane dendrimers G0−SiMe2−(N)CN and G1−SiMe2−(N)CN (1316), which can be prepared in high yield by using the same synthetic approach used for the model compounds, can also be quantitatively converted into highly aggregated, polylithium derivatives. An X-ray molecular structure determination of 7 revealed this compound to be a unique dimeric aggregate {[CH2SiMe2C6H3(CH2NMe2)-4-Li-3]2}2 with their respective 2-electron−4-center (4c-2e) anionic Cipso centers each bonded to a Li3 face. As a result the (Me)2SiCH2CH2Si(Me)2 CS backbone exhibits a nonideal trans conformation with a vicinal dihedral angle of −161°.