Stilbenes azomethines

The use of lithium amides to metalate the a-position of the N-substituent of imines generates 2-azaallyl anions, typically stabilized by two or three aryl groups (Scheme 11.2) (48-62), a process pioneered by Kauffmann in 1970 (49). Although these reactive anionic species may be regarded as N-lithiated azomethine ylides if the lithium metal is covalently bonded to the imine nitrogen, they have consistently been discussed as 2-azaallyl anions. Their cyclization reactions are characterized by their enhanced reactivity toward relatively unactivated alkenes such as ethene, styrenes, stilbenes, acenaphtylene, 1,3-butadienes, diphenylacetylene, and related derivatives. Accordingly, these cycloaddition reactions are called anionic [3+2] cycloadditions. Reactions with the electron-poor alkenes are rare (54,57). Such reactivity makes a striking contrast with that of N-metalated azomethine ylides, which will be discussed below (Section 11.1.4). 

Azomethine imines were generated by cleavage of certain 1,3,4-oxadia-zolidines (42) and subsequently trapped with dipolarophiles such as styrene and stilbene affording pyrazolidine heterocycles (96TL4323). 

Bridging units azomethine, azo, N-oxide, stilbene, tolane, aliphatic chains, alkyl ethers,. .. and derivatives... 

B. Liquid Crystals Containing Azo-, Azomethine, Stilbene or Tolane Units... 

FIGURE 9. Structures of stilbene and tolane derived azomethine liquid crystals... 

TABLE 6. Mesomorphic properties of azomethine/stilbene/tolane liquid crystals... 

TABLE 7. Structural anisotropy of stilbene and tolane based azomethines ... 

Deprotonation of N-methylpiperidine Al-oxide with LDA in THF at — 78°C takes place regioselectively on the least crowded methyl carbon to generate azomethine ylide 122 (8SJOC2910). Ylide 122 is again highly reactive toward nonactivated olefins such as ethene, cyclopentene, styrene, (E)- and (Z)-stilbene, and ( )-)S-methoxystyrene. 

Potassium tert-butoxide/dimethylformamide Stilbenes from azomethines... 

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