1.4- Dianions heterocyclics

EPR studies of S-N radicals were reviewed in 1990. Many radicals containing the S-N linkage are persistent for more than several hours in solution at room temperature. Perhaps the best known example is the nitrosodisulfonate dianion [0N(S03)] , named as Fremy s salt. In the solid state this radical dianion dimerizes through weak N 0 interactions, but it forms a paramagnetic blue-violet monomer in solution. Although most chalcogen-nitrogen radicals dimerize in the solid state, a few heterocyclic C-S-N systems can be isolated as monomers (Section 11.3). 

Fully unsaturated azocines are 7r-equivalent heterocyclic analogs of cy-clooctatetraene. Addition of two electrons to the completely unsaturated azocine (34) can lead to a dianion 35 and removal of a proton from a dihy-droazocine (36) to the monoanion 37. Both the mono- and the dianions are lOTT-electron systems, corresponding to 7r-equivalent and 7r-excess analogs of cyclooctatetraenide [84CHEC-I(7)653], Aromatic dianions related to 35 have been fully characterized by and NMR (87TL2517). 

This sequence has been proposed as a novel approach to olefin separation and purification. Ffowever, there is disagreement about the reversibility of the reaction because it has been shown that irreversible reduction to the dianionic complex occurs through an ECE process.1082 A convenient new route to 1,2-enedithiolate complexes of Ni has been reported, which starts from the bisfhydrosulfldo) complex [Ni(dppe)(SFI)2] and various a-bromoketoues(heterocycle-C(0)CH2Br). 

Diboratacarbazole heterocycles 137 are obtained in 60% isolated yield by heating the phosphine-stabilized 2,2 -diborabiphenyl derivative 138 with primary amines in toluene for 20h (Scheme 55). Further double deprotonation of the heterocycle 137 (Ar = Ph) with a lithium amide leads to the dianionic 9,11-diboratacarbazole derivative 139 (98%, S nB 31.71 ppm). Structures 137 (Ar = Ph) and 139 were characterized by X-ray crystallography <20040M3085>. 

Aromaticity of the products is only one of the factors accounting for the efficiency of these cyclizations as evidenced by the discovery of a dianionic synthesis of nonaromatic 5-membered heterocycles by Tamao and coworkers who found that reduction of bis(phenylethynyl)dialkylsilanes with lithium naphthalenide resulted in formation of a cyclized product by endo-endo cyclization15 (Scheme 7). 

This section is devoted to heterocyclic molecules isoelectronic with dications and dianions of the molecules that are central to the aromaticity concept, i.e., cyclobutadiene and benzene (Scheme 5). 1,3-Diboretene... 

It of interest to note that the isobutyl group may also be replaced by a heterocyclic ring. The route to this compound, pirprofen (51-6), starts with the direct methylation of unesterihed 4-nitrophenylacetic acid (51-1). The observed selectivity for monoalkylation in this case may reside in the structure of the dianion, whose most stable form is presumably that depicted in (51-2). Catalytic reduction of the product (51-3) gives the corresponding aniline this is then converted to its acetanilide (51-4) with acetic anhydride. Treatment with chlorine followed by hydrolysis gives the chloroaniline (51-5). Double alkylation of this last intermediate with 1,4-dichloro-but-2-ene (depicted as the cis isomer for aesthetic reasons) forms the dihydropyrrole ring. There is thus obtained the NSAID pirprofen (51-6) [52]. 

A,A -Dianionic fused six-membered heterocyclic ligands derived from phenazine and quinoxaline have been found in Ln complexes including [(SmCp )2(p-ri ri -Ci2HgN2)]... 

There is much current interest in four-membered ring compounds containing heteroatoms that are isoelectronic with a cyclobuta-l, 3-dienediide. The first thermally robust parent homocyclic dianion [ CfSiMe,), " was only reported in 2000 the first neutral heterocyclic analogue [P(C6H2Bu 3-2,4,6) p-C( Cl) ]2, containing a Group 14 element (C) dates from 1995 Extensions to the 4" and 5" quantum group elements Ge and Sn respectively, were published in 2004. 

VI. Heterocyclic Betaines Isoelectronic with Even Alternant Hydrocarbon Dianions... 

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