Pentacoordinate

Lower alkanes such as methane and ethane have been polycondensed ia superacid solutions at 50°C, yielding higher Hquid alkanes (73). The proposed mechanism for the oligocondensation of methane requires the involvement of protonated alkanes (pentacoordinated carbonium ions) and oxidative removal of hydrogen by the superacid system. 

Tri- and pentacoordinate phosphoms compounds often react by electron-pair mechanisms as demonstrated by the nucleophilic reactivity of the lone pair electrons in trivalent compounds, and the electrophilicity of the phosphoms atom in the pentavalent compounds. Some compounds also react by free-radical mechanisms. The theoretical and synthetic aspects of the chemistry of phosphoms compounds have been described (6—9). 

Fig. 1. Berry pseudorotation about pentacoordinate ( ) phosphorus, where (Q) represent fluorine atoms, (a) Original trigonal bipyramid (b) square...

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

It is believed that this process involves migration through a pentacoordinate protonated cyclopropane in which an alkyl group acts as a bridge in an electron-deficient carbocation structure. The cyclohexyl- methylcyclopentyl rearrangement is postulated to occur by rearrangement between two such structures. 

Carbon-6, which bears two hydrogens, is pentacoordinate and serves as the bridging atom in the cation. 

ATP is activated by coordination to magnesium ion, and nucleophilic addition of a fatty acid caiboxylate to phosphorus then yields a pentacoordinate intermediate. . ... 

Allylic transposition is observed in fluoride-induced reactions of allyl(trifluoro)silanes50 and allyl(trichloro)- and allyl(trialkoxy)silanes in the presence of hydroxylic promoters, e.g., 1,2-benzenediol51,52. Pentacoordinated silicates are believed to be involved53. 

Of particular interest are reactions between a-hydroxy ketones and allyl(trifluoro)silanes which proceed with participation of the hydroxy group via a pentacoordinated allylsilane, with intramolecular delivery of the allyl group to the ketone. Excellent stereoselectivity is obtained at up to three contiguous stereogenic centers66. 

Treatment of allyl bromides with the complex obtained from tin(II) chloride and the disodium salt of diethyl 2,3-dihydroxybutanedioate gives an intermediate which reacts with aldehydes to provide homoallylic alcohols with 50-65% ee. Lower enantiomeric excesses were obtained with bulky aldehydes and for allylstannanes with y-substituents. Pentacoordinated allyltin complexes may be involved97. 

The mechanism of the reaction was postulated as being either synchronous attack of the RO" ion (R = Me or H) at tin and breaking of the aryl-tin bond, or rapid formation of a pentacoordinate tin intermediate [ROMe3SnPh-] followed by rate-determining breaking of the aryl-tin bond. In either case the phenyl-carbanion could separate as such and then react rapidly with solvent, or it could react with solvent as it separated. 

Ligand field interpretation of some cases of pentacoordination, C. Furlani, Coord. Chem. Rev., 1968,3,141-167 (58). 

Bimolecular substitution and oxidation reactions of 17-electron pentacoordinate metal carbonyl radicals. A. Poe, Transition Met. Chem. (Weinheim, Ger.), 1982,7, 65-69 (41). 

Aspects of pentacoordinate tin compounds. A. Tzschach and K. Jurkschat, Comments Inorg. Chem., 1983,3,35-50 (55). 

The three different functions correspond to the three possible O-P-O angles around the pentacoordinated phosphate. 

The solid-state structure of tetrabutylbis(2,3 4,6-diisopropylidene-2-keto-L-gulonato)distannoxane has been determined. The [Bu2Sn(IV)] complex of D-lactobionic acid also proved to be oligomeric, containing octa- (within the chain) and pentacoordinated (at the end of the chain) Sn centers in a ratio of 2 4. ... 

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