Pentacoordinated carbon

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... 

Distinct evidence for the equilibration of bicyclobutonium with a minor isomer, bisected cyclopropylcarbinyl cation, comes from the ultra-low temperature CPMAS studies of Myhre, Webb and Yannoni25. They have observed a major isomer, the bicyclobutonium ion, with a l3C chemical shift of 15 ppm for the pentacoordinated carbon, and a minor bisected cyclopropylcarbinyl cation, whose cationic center s chemical shift was found to be at 235 ppm. The NMR chemical shifts of the cation are also comparable with those calculated by the IGLO method at that temperature26 27. The energies of these cations were shown to be nearly the same (AAH° = 0.05 kcalmol1). 

The preferred structure of the diprotonated C2H82+ ethane dication (452) considered theoretically786 incorporates two pentacoordinate carbons and an unprotonated C C bond (HF/6-31G level). A new calculation by Olah et al.788 (MP2/6-31G ) has... 

Protonation of methylhalonium ions to form diprotonated dications CH3XH22+ have been calculated (B3LYP/6-31-G level)348 to occur primarily on the C—H bond to form a pentacoordinated carbon with 2e-3c bond. The halogen atoms in dications, as expected, carry more charge than in the corresponding monocations and the dications are less stable than the monocations (49.5-13.Okcal mol-1). 

The MINDO/3 calculations [215] on (C4H9)+ considered the 2-butyl cation decomposing to the 1-methylallyl cation and hydrogen. The transition state contained a pentacoordinated carbon atom and the loss of H2 from the transition state formally resembled a 1,1 elimination. The large proportion of the reverse critical energy observed to be released as translation was attributed to repulsive forces and tightness of the transition state. 

Commonly accepted mechanisms of the MMO reactions are based on the concept of the activation of dioxygen or the use of shunts such as H202. Nevertheless, the new mechanism of methane oxidation via an intermediate complex containing pentacoordinated carbon has been forwarded (Shilov, 1997, Karasevich et al 1998, 1999). This suggestion is based on experiments on the multiple H-D exchange and methane oxidation catalyzed by platinum (II) complexes, ([H2PtCl6], for instance). Formation of methyl platinum (IV) chloride complex in methane oxidation was confirmed by its NMR spectrum. 

Overall, the gaseous Sn2 reaction shown in Fig. 5-4 consists of a collision step, which generates reactant ion-dipole complexes (b), followed by a chemical activation step forming activated complexes (c) that contain pentacoordinated carbon atoms. Once the transition state has been surmounted, product ion-dipole complexes (d) are formed, the dissociation of which yields the products (e). 

However, intramolecular nucleophilic participation by the conjugate base during protonolysis of a C—Hg bond is questionable. A study of the acidolysis of the carbon-mercury bond in unsymmetrical di-alkylmercurials rather suggests that the reaction proceeds via a three-center transition state.In any case, substantial kinetic and stereochemical evidence has led to the idea that reaction occurs by a concerted, front side attack with a transition state that involves a pentacoordinate carbon center. In some cases unimolecular mechanisms, SeI, also have been observed. 

The complex (iri -C5H5)2Zr(CH3)2 reacts with the highly electrophilic borane HB(CfiF5)2 to form a product having stoichiometry (CH2)[HB(C5p5)2]2L(C5H5)2Zr] the product is a rare example of pentacoordinate carbon. 

In the proton-coupled 13C NMR spectrum of the norbornyl ion no coupling was observed between the methylene hydrogens at the pentacoordinated carbon (C.6) and the cyclopropane-like carbons (C.l and C.2). This is expected from the non-classical structure since the two-electron, three-center bonds are longer and weaker than normal 3—Csp2 bonds. 

Other examples of directly observed long-lived norbomyl ions in which the tetra- and/or pentacoordinated carbons were identified by NMR spectroscopy are the 7-norbomenyl and 7-norbomadienyl cations10,204 209) 164 and 165. 

Similarly, many carbon-heteroatom bonds are also cleaved under strong acid catalysis involving pentacoordinate carbon intermediates [Eq. (6.17)]. 

Hydroboration, the addition of borane (or diborane) to n-donor substrates such as alkenes, acetylenes, carbonyl compounds, and so on, is a most useful synthetic reaction and was developed by the extensive work of Brown et al. 2 "52 In the absence of more nucleophiUc n- and n-donor sites, borane will also attack a bonds in alkanes, silanes, and aromatic compounds to yield addition, cleavage, and rearrangement products. The interaction of borane with hydrocarbons involves 3c-2e bonded pentacoordinate carbons. 

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