α‐Metalated Enamines

Abstract

on their response to thermal excitation the azabarbaralanes described here are divisible into two distinct categories: the pair (7) and (8) readily rearranges at ambient temperature"' whereas (5)13, 6, '1 and ( 6 ) L 3 3 '1 effectively resist change even at elevated temperatures. The difference between the two classes is perhaps best exemplified by direct comparison between (7) whose half-life was found (NMR; [D6]-acetone) to be a mere 18 min (AG' ~2 2 kcal/mol) at 32°C and ( 5 ) which may be recovered unchanged (NMR) after being heated at 140°C for 30min! Since of the four 9-azabarbaralanes described here the two that are thermally unstable, uiz. (7) and ( 8 ) , are also the only members that incorporate lone pairs which are well accessible electronically for interaction with the homotropilidine moiety of the molecule, one must conclude that the nitrogen lone pair exerts a destabilizing effect on the 9-azabarbaralane system.

-Heteroatom-Substituted 1-Alkenyllithium Reagents: Carbanions and Carbenoids for C–C Bond Formation

“…[71a] On the other hand, the selective bromine ± lithium exchange in the dibromoalkene 43 derived from isopropylidene glyceraldehyde is remarkable and leads predominantly (95:5) to the E-configured carbenoid 44, as proven after protonation and carboxylation. [75] Halogen ± lithium exchange is also a method for producing 1-lithioalkenes with oxygen, sulfur, or nitrogen [57,76] as the a heteroelements. However, it is in general only of preparative importance in special cases, such as with 1,2-dimethoxy-1lithioethene.…”

1-Alkenyllithium-Reagentien mit einem Heteroatomsubstituenten in derα-Position: Carbanionen und Carbenoide zur C-C-Verknüpfung

“…[71 a] Dagegen entsteht bemerkenswerterweise beim Brom/Lithium-Austausch in dem von Isopropylidenglyceraldehyd abgeleiteten Dibromalken 43 überwiegend (95:5) das E-konfigurierte Carbenoid 44, wie durch Protonierung und Carboxylierung gezeigt wurde. [75] Auch 1-Lithioalkene mit Sauerstoff, Schwefel oder Stickstoff [57,76] als a-Heteroelement können durch Halogen/Lithium-Austausch erzeugt werden; das Verfahren hat allerdings nur in speziellen Fällen, z. B. bei 1,2-Dimethoxy-1-lithioethen, präparative Bedeutung.…”