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 lithium di-iso-propyl amide in a mixture of hexane-THF at low temperature. Preference 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)] structures in a 7:1 ratio. Ab initio calculations predict that the 5'(W) compound is more favorable than the 5'(U) by 4.7 kcal mol(-1). We have been examining the regio- and stereoselective azapentadienyl compounds using organometallic electrophiles such as EMe3+ (E = Si, Ge, Sn). The reactions of the preferential EE isomers 1'-3' with the electrophiles generally give the addition at the terminal (C5) carbon of the azapentadienyl moiety, affording the 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 corresponding 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)CH-SiMe3 (15), but diminished regioselectivity is observed with the corresponding Ge and Sn analogues. In contrast, attack at the central (C3) carbon is 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(Me)CH-SnMe3 (20'), showing the addition of -SnMe3 fragment at the terminal carbon. The kinetic and thermodynamic products were established through H-1-, C-13- and Sn-119-NMR data, along with thermal isomerization studies. Based on the computed values of density-functional theory, condensed Fukui functions derived from ab initio electronic structure calculations were computed. These are shown to be useful reactivity indexes (better than conventional atomic charges) to rationalize the observed 1'-5' chemical derivation products with electrophiles. (C) 2000 Elsevier Science S.A. All rights reserved.