Allyl anion analogs

In 1989 we reported on the synthesis and structure of the first l,3-diphospha-2-sila-allylic anion 3a [4], mentioning its value as a precursor for phosphino-silaphosphenes. In analogy to 3a we obtained the anions 3b-f [5] by treatment of 4 equivalents of the lithium phosphide 1 with the adequately substituted RSiC, of which 3b and 3c were investigated by X-ray analyses. The very short P-Si bond lengths (2.11-2.13 A) of 3a-c and the almost planar arrangement of Pl-Sil-P2-Lil indicate the cr-character of the Lithium P-Si-P allyl complex. 

Germylene 185 can be reduced with substoichiometric amounts of KCg to give the cyclotrigermenyl radical 186, which has been structurally characterized (Equation (329)), while reaction of 185 with an excess (2 equiv.) of KC8 produces 187 which is the germanium analog of the allyl anion (Equation (330)).400 The structure of 187 has been obtained as well. 

An analogous mechanism was proposed for the conversion of the triflate 416 to the vinyl-, allyl- and allenyl-A2-cephems 448 in yields of 47-71% by the respective tributyltin compounds in the presence of cuprous chloride (Scheme 6.91) [176]. Accordingly, the cyclic allene 417 should be liberated from 416 in the first step. Then, the organocopper species would transfer a hydrocarbon group to the central allene carbon atom of 417, leading to an allyl anion derivative, which is protonated during the workup. These reactions of 416 and 443 indicate that the cyclic allenes 417 and 444 behave toward nucleophiles as 1,2-cyclohexadiene (6) (Schemes 6.11— 13) and its non-polar derivatives such as 215 (Scheme 6.51), 221 (Scheme 6.52), 311 (Scheme 6.67) and 333 (Schemes 6.71 and 6.73), that is, they interact with nucleophiles at the central carbon atom of the allene system exclusively. 

Pentadienyl carbanions are analogous to allyl anions with an extended delocalization of charge. Reaction of 1,3- or 1,4-pentadienes and alkali metals in THF in the presence of a base, such as NMes or TMEDA, affords crystalline pentadienylalkali metal complexes. A contact ion pair structure is predicted for these compounds by theoretical calculations and is consistent with solution structural data obtained by NMR. The pentadienyl anion usually interacts with the cation as an rj - or ) -ligand depending on the structural orientation of the backbone carbon atoms of the pentadienyl anion (W-, S-, or U-shaped skeletal structures). A contact ion pair structure having a W-shaped pentadienyl ligand is shown (16). 2,4-Disubstituted... 

The frontier orbital treatment for vinyl cation cycloadditions, such as those of ketenes, has some merits. It satisfyingly shows that the bond forming between C-l and C-l develops mainly from the interaction of the LUMO of the ketene (n of the C=0 group) and the HOMO of the alkene 6.178, and that the bond between C-2 and C-2 develops mainly from the interaction of the HOMO of the ketene (i/j2 of the 3-atom linear set of orbitals analogous to the allyl anion) and the LUMO of the alkene 6.179. 

These predictions involve some assumptions and approximations when applied in a generalised form. Thus symmetry is lacking in most reactants in cycloadditions, either because of different substitution at identical atoms (for instance vinyl derivatives instead of ethylene) or because different atoms are present as reactions centers (as in most 1.3-dipoles). In the former case the substance of the previous considerations should be unaltered , but in the latter case the selection rules (e.g. derived for the allyl anion taken as model of 1,3-dipole) may be less stringent when applied only on the basis of analogy. 

When one deprotonates propene, it is the methyl hydrogens that are the most acidic. Deprotonation creates the resonance stabilized allylic anion. When the analogous reaction is attempted with cyclopropene, a vinylic hydrogen is the one removed. Deprotonation of the CH2 group in cyclopropene (Eq. 2.19) would create an antiaromatic anion, an undesirable effect, and this reversal in acidities provided early support for the notion of destabilization due to antiaromaticity. 

This analogy enables us to see immediately that the S 2 and 5 2 mechanisms are reasonable. Union of ethylene with CH3" or CH3 to form allyl anion or cation leads of course to a decrease in n energy (see Section 3.16). The isoconjugate interactions of the filled AO of a nucleophile Y or of the... 

Addition of racemic allylic sulfoxide anions to 2(5//)-furanone gives y-1,4-addition adducts1. The simple and induced diastereoselectivities are completely analogous to that of 2-cyclopen-tenone described earlier. 

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