Conjugated compounds with anions

Pentalene (135) and acepentalene (137) are antiaromatic cross-conjugated compounds with two or three fulvene subunits. According to theoretical calculations [142-147], 135 and 137 are unstable species [147, 148] but can be ligands in transition metal complexes. The respective anions, the planar 136 [149, 150] and the slightly bowl-shaped 138 [151, 152], can be generated by deprotonation of a dihydropentalene or by treatment of4,7-bis(trimethylstannyl)-4,7-dihydroacepentalene with methyllithium at -60 °C (Scheme 10.48). 

Most heterocyclic anions may be considered to be derived by loss of a proton from a parent compound, which is therefore the conjugate acid. Such anions have at least one unshared pair of electrons at the anionic site. They are named by appending the suffix -ide , with elision of a terminal e (lUPAC recommendation RC-83.1.1), as in (190)-(193). The site may be specified by a locant placed immediately before the suffix, and so chosen as to be as low as possible consistent with the numbering of the skeleton of the parent compound. The locant may be omitted in order to designate an equilibrating mixture of positionally isomeric anions, which is what one usually obtains in practice. The anion of piperidine is often informally referred to as piperidide . 

Attachment of a basic amino group to the side chain leads to a compound with antiparkinsonian activity. Alkylation of the carbanion from phenylacetonitrile with 2-chlorotriethylamine affords the product, 36. Conjugate addition of the anion from this to acrylonitrile gives the glutarodinitrile (37). Partial hydrolysis of this in a mixture of sulfuric and acetic acid leads to phenglutarimide (38). ... 

Numerous colorless organic compounds with extended jt-electron systems can be converted to colored cations or anions with polymethyne chromophore by protonation or deprotonation. The intense coloration of the corresponding salts is usually attributable to the fact that the lone pairs of electrons of the heteroatoms participate in the mesomerism of the conjugated n-electron systems [4]. 

A 2-Methylpyridine can be deprotonated by a base such as sodium methoxide and the resultant anion can be reacted with ben-zaldehyde to form a hydroxylated adduct (Scheme 2.21). This product can then be dehydrated by acid, or base, to form the conjugated compound stilbazole. Alternatively 2-methylpyridine can be A -acetylated by reaction with acetic anhydride and the initial product deprotonated to give an A -acetylenamine intermediate that traps benzaldehyde, A similar reaction to that of the first procedure occurs, but under the reaction conditions the zwitterionic (dipolar) product eventually loses acetic acid, perhaps by an interna shift of an acetyl group from nitrogen to oxygen. 

Pentamethylcyclopentadiene is a useful aromatic building block for the preparation of other compounds. It can be converted to many salts of its conjugate base with alkali metals or strong bases such as butyl lithium.4 These pentamethylcyclopentadienyl anion salts as well as the diene itself can be transformed into n -pentamethylcyclopentadienyl ligands of organotransition metal complexes by many known methods.4... 

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