Synthesis, characterization and a theoretical investigation of the formation of silicon, germanium and tin azapentadiene compounds from lithium azapentadienyls

A series of lithium-1-azapentadienyl compounds Li[t-Bu-N ... CH ... C(R)... C(R')... CHR"] (1'-5') has been synthesized by metallation of imines t-Bu- N=CH-C(R)=C(R')CH2R" [R = R' = R" = H (1); R = Me, R' = R" = H (2); R = R" = H, R' = Me (3); R = R' = H, R" = Me (4); R = R" = Me, R' = H (5)] with li thium di-iso-propyl amide in a mixture of hexane-THF at low temperature. Pr eference for the exo W-shaped isomer is proposed for compounds 1'-4', based on their chemical derivation products with electrophiles, such as EMe3Cl ( E = Si, Ge, Sn). Compound 5' shows both W- [5'(W)] and U-shaped [5'(U)] str uctures in a 7:1 ratio. Ab initio calculations predict that the 5'(W) compo und is more favorable than the 5'(U) by 4.7 kcal mol(-1). We have been exam ining the regio- and stereoselective azapentadienyl compounds using organom etallic electrophiles such as EMe3+ (E = Si, Ge, Sn). The reactions of the preferential EE isomers 1'-3' with the electrophiles generally give the add ition at the terminal (C5) carbon of the azapentadienyl moiety, affording t he corresponding complexes t-Bu-N=CH-CH=CH-CH2-EMe3 [E = Si (6), Ge (7), Sn (8)]; t-Bu-N=CH-C(Me)=CHCH2-EMe3 [E = Si (9), Ge (10), Sn (11)] and the co rresponding EE and ZE isomers: t-Bu-N=CH-CH=C(Me)-CH2-EMe3 [E=Si (12), (12' ); Ge (13), (13'); Sn (14), (14')], respectively. Complex 4' reacts highly regioselectively with SiMe3Cl affording the EE isomer t-Bu-N=CH-CH=CH-(Me)C H-SiMe3 (15), but diminished regioselectivity is observed with the correspo nding Ge and Sn analogues. In contrast, attack at the central (C3) carbon i s observed for 5', giving t-Bu-N=CH-(Me)C(EMe3)-CH=CH-CH3 [E = Si, (18); Ge , (19)]; although the regioselectivity remains for Si and Ge derivatives, a thermodynamic rearrangement occurs for the Sn complex t-Bu-N=CH-C(Me)=CH(M e)CH-SnMe3 (20'), showing the addition of -SnMe3 fragment at the terminal c arbon. The kinetic and thermodynamic products were established through H-1- , C-13- and Sn-119-NMR data, along with thermal isomerization studies. Base d on the computed values of density-functional theory, condensed Fukui func tions derived from ab initio electronic structure calculations were compute d. These are shown to be useful reactivity indexes (better than conventiona l atomic charges) to rationalize the observed 1'-5' chemical derivation pro ducts with electrophiles. (C) 2000 Elsevier Science S.A. All rights reserve d.