Geminal dianions

Three examples of nitrile-stabilized enolates have been described by Boche et al. Two of these structures incorporate the anion of phenylacetonitrile. Hie TMEDA-solvated dimer (178) crystallizes out of benzene solution however, the mixed nitrile anion LDA-(TMEDA)2 complex (179) is obtained when excess LDA is present. This latter complex has often been mistaken as a geminal d anion since it frequently gives products that appear to arise from a dianion. The crystal structure of the anion l-cyano-2,2-dimethylcyclopropyllithium (180) consists of an infinite polymer (181) that is solvated by THF. Interestingly, there are C— Li contacts in this structure and the carbanionic carbon remains tetrahedral. 

Vinylsilanes as mono- and geminal disilyl-substituted C=C-double bonds like 1 [2] or 3 [3] afford, when brought to reaction with lithium metal in THE, the products of reductive dimerization, i. e., the 1,4-dilithiobutanes 2 and 4. This type of reaction is known as Schlenk dimerization [4]. Symmetrically tetrasilyl-substituted C=C-double bonds as in 5 on the other hand add lithium metal with formation of 1,2-dilithioethanes (Scheme 1) as stable intermediates in these reactions radical anions, like 7, can be observed, which are then reduced once again, here to the dianion 6 [5]. These two types of reaction are analogous to the reductions of the corresponding styrene derivatives... 

More recently, the synthesis of a geminal dianion by Sekiguchi et has attracted some attention (Eq. 13). 

The high field shift of the pyrene units protons (<5 = 0.64—2.61 ppm, for both 46b and 47b) is direct evidence for the anti-aromaticity of the species. Another strong indication of the anti-aromaticity of these dianions comes from the chemical shift of the tether hydrogens, which function as probes for the anisotropy effect in the system. The best example comes from the geminal hydrogens H14 and H14 in 46b/2K+, which appear at b = 9.93 and 3.22 ppm, respectively. 

Trost has published additional applications of a-lithiocyclopropyl phenyl sulphide, including an approach to spiro-sesquiterpenes, involving geminal alkylation, which utilizes a process that is formally the reversal of dithian-carbonyl addition(also see related studies by Marshall Synthetic applications of the highly nucleophilic methoxy-phenylthio-methyl-lithium PhSCH(Li)OMe and of the acyl anion equivalent methylthioacetic acid dianion RSCHCLOCOgLi (R = Me or Ph ) have also been described, as have synthetic procedures based on anions of a-alkylthio- or a-phenylthio-ketones and methylthiomethyl dithiocarbamates. ... 

Reactions of an aminodichloroborane with 1,4-oligosilanyl dianions gave mono-or bicyclic disilylated boranes [154], whereas reaction with the geminal dianion (f-Bu2MeSi)2SiLi2 led to a stable silaborene (Scheme 21) [194],... 

Lee VY, Sekiguchi A (2011) Novel organometallic reagents geminal dianionic derivatives of the heavy group 14 elements. Inorg Chem 50 12303... 

Stable Geminal Dianions as Precursors for Gem-Diorganometallic and Carbene Complexes... 

A question prior to the one written above is can a species featuring an alkaline earth (AE) metal-carbon formal double bond be synthesized In fact, this is no trivial matter and AE=CH2 species are likely polymeric. The first synthesis of such a molecular complex was only realized in 2006. Therefore, examples of structurally characterized alkaline earth metal complexes featuring a hypervalent phosphorus-stabilized geminal dianion as a ligand are scarce enough to be thoroughly reviewed here. Wlule a few examples are found for magnesium [24, 37, 38], calcium [39-41], and even barium [30], there is to date no example of such complexes with strontium or beryllium. The full set of complexes is represented in Chart 3. 

Table 2 List of different rare earth complexes featuring a geminal dianion as a ligand and the corresponding reference...

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