Hypercoordinate carbon

The discovery of a significant number of hypercoordinate carboca-tions ( nonclassical ions), initially based on solvolytic studies and subsequently as observable, stable ions in superacidic media as well as on theoretical calculations, showed that carbon hypercoordination is a general phenomenon in electron-deficient hydrocarbon systems. Some characteristic nonclassical carbocations are the following. 

Bridging and hypercoordination in transition-metal complexes Hydrido-bridged carbocations 118 Carbon hypercoordinated carbocations 123 Hyperconjugation in carbocations 146 References 158... 

Because carbon is a first-row element unable to extend its valence shell, hypervalence cannot exist in carbon compounds, only hypercoordination. 

Hypercarbon compounds contain one or more hypercoordinated carbon atoms bound not only by 2e-2c but also 2e-3c (or >3c) bonds. 

Carbon is known with all coordination numbers from 0 to 8 though compounds in which it is 3- or 4-coordinate are the most numerous. Some typical examples are summarized in the Panel (p. 291). Particular mention should also be made of hypercoordinate non-classical carbo-nium ions such as 5-coordinate CHj", square pyramidal CsHs (cf. the isoelectronic cluster B3H9, p. 154), pentagonal pyramidal C6Me6 " (cf. iso-electronic Bf,Hio, p. 154) and the bicyclic cation 2-norbomyl, C7H] 1... 

Schmidbaur, H., Gabba, F.P., Schier, A. and Riede, J. (1995) Hypercoordinate Carbon in Protonated Tetraauriomethane Molecules. Organometallics, 14, 4969 971. 

This is an important parallel to the analogous process first discovered for phosphorus ylides in the late 1980s,166 and extended to simple hydrocarbons in the mid-1990s.15 Note that carbon is again found to become hypercoordinate in a square-pyramidal geometry (Scheme 37). 

The CH cation 1, protonated methane, is the parent of hypercoordinated carbocations containing a five coordinated carbon atom. It is elusive in solution and has not been observed by NMR spectroscopy but gas-phase infrared investigations have shown its fluxional structure which has been proven by ab initio molecular dynamic simulation.18... 

For the CH with a Cs symmetrical structure 1 a 13C NMR chemical shift of 11.5 ppm is calculated. This is 7.1 ppm more shielded than that calculated for the hydrocarbon CH4 at the GIAO-MP2 level.19 The shielding effect in 1 is as expected for a hypercoordinated carbocation carbon.20 The calculated minimum energy structure for diprotonated methane (CHi+) 2 has C2v symmetry with two 3c-2e... 

The results indicate that the formation of long-lived trimethyl substituted silyl cations, in the presence of aromatic solvents, as claimed by Lambert et al.95 is not feasible under these conditions. Persistent silicenium ions require sterically more shielding substituents at silicon or hypercoordinative stabilization.96 98 13C and 29Si NMR chemical shifts were calculated for a series of disilylated arenium ions 85 using density functional theory (DFT). The calculations predict consistently the unsaturated carbon atoms to be too deshielded by 8-15 ppm. Applying an empirical correction, the deviation between experiment and theory was reduced to -0.4 to 9 ppm, and the 13C NMR chemical shift of the highly diagnostic cipso is reproduced by the calculations (Ad = -3.8 to 2.7 ppm).99... 

A5 C(C1) = —29.8, (A5 C(C1) = —54.5) are observed and a relatively large /(C2H) coupling constant of 165.9 Hz is detected. This counter-intuitive low-frequency shift of the C NMR resonance of Cl and C2 as well as the large scalar CH coupling constant was rationalized for similar bishomoaromatic carbon cations like the 7-norbornenyl cation, 79, by the hypercoordinated nature of the vinylic C atoms and was put forward as spectroscopic evidence for bishomoaromaticity. " ... 

Decamethylsilicocene (82) can be regarded as an electron-rich silicon(II) compound containing a hypercoordinated silicon atom. The chemistry of 82 is determined by (a) the nucleophilicity of the silicon lone-pair (cr-donor function towards electrophiles, oxidative-addition processes) and (b) the weakness of the silicon-(cyclopentadienyl)carbon jr-bond rearrangement, Si—C bond cleavage). In the following section, the chem-... 

Fluoromethylbenzoic acids, metallation, 9, 26-27 Fluoro(phenyl) complexes, with platinum(II), 8, 482 Fluorosilanes, elimination in fluorinated alkene activation, 1, 732 in fluorinated aromatic activation, 1, 731 and hydrodefluorination, 1, 748 Fluorosilicate anions, hypercoordinated anions, 3, 484 Fluorotoluenes, metallation, 9, 21 Fluorous alkylstannanes, preparation, 3, 820 Fluorous biphasic system, as green solvent, 12, 844 Fluorous ligands, with supercritical carbon dioxide, 1, 82 Fluorous media... 

Silylium ions R3Si+ are known to be more stable in the gas phase than their carbon analogues, which is due to the fact that silicon is more electropositive than carbon. It is easy to observe them by mass spectrometry or ion cyclotron resonance spectroscopy of triorganosilyl compounds. [5] However, their existence in solvent phases is difficult to prove and, therefore, is still a matter of dispute. Early attempts to synthesize silylium ions in solution with methods successful in the preparation of carbocations were not successful. [78] The lack of stability of R3Si+ in solution was attributed to the strong tendency of Si to form hypercoordinated compounds, in which Si has five or even six bonding partners. [79]... 

The much more highly charged silicon atom can interact far more readily with nucleophiles. Silyl cations may even be complexed simultaneously and symmetrically by two electron pair donors (hypercoordination), in contrast to carbocations. With ammonia, the methyl cation gives the very stable protonated methyl amine, H3C-NH3 a second ammonia molecule is only weakly bound to this complex. If both NH3 groups are forced to be equidistant from carbon, a Sn2 transition state results, 20 kcal mol" higher in energy than the minimum. 

Tetragoldmethane complexes are obtained from tetra-(boryl)methane compounds upon reaction with gold-halide complexes in the presence of an ionic fluoride (equation 35)." Tetragoldmethane species, such as (1), can only be isolated with bulky tertiary phosphines, L, which shield the molecule from further attack by [TAu]+ nucleophiles. With smaller ligands T, penta- and hexaauration occurs, leading to hypercoordinate carbon compounds. " " " ... 

Tithiated dicarbaborane clusters yield (organo)gold-substituted clusters on reaction with T AuX or R2AUX compounds, also with hypercoordinate carbon atoms (equations 36-38)." " ... 

Compound (15) also exhibits a 2-centre, le bond between P atoms and again the bond is long (about 2.76 A), while the unusual biradicaloid (16) is thermally stable up to 150°C. The constraints placed upon the phosphorus atoms in compound (17) result in hypercoordination of the phosphorus adjacent to the oxygen, while (18) constitutes the first cationic phosphorus-carbon cluster, namely nido-[3,5- Bu2-1,2,4-C2P3]. All such observations show great promise for future development. 

Distorted Square-pyramidal Hypercoordinated Carbon Compounds... 

The first observation of penta- and hexaeoordinate silicon compounds was reported at the beginning of the 19th century by Gay-Lussac [87] and Davy [88], Subsequent investigation of hypercoordination in silicon compounds stimulated widespread use of nucleophilic activation and catalysis in the application of organosilicon compounds as reactive species in organic synthesis. Synthetic application for silicon-fluorine bond formation can be found in several reviews over the last two decades, and this section focuses on recent advances in the use of hypervalent organosilicon compounds in selective organic synthesis, in particular, selective carbon-carbon bond formation [89]. 

GIAO-MP2/SCF/DFT calculated NMR hemical shift relationships in isostructural onium ions containing hypercoordinate boron, carbon, aluminum, and silicon atoms91... 

The hydrogen-bridged carbocation (B) contains a two-coordinate hydrogen atom. Hypercoordination— which includes two-coordination for hydrogen and at least five-coordination for carbon—is generally... 

To illustrate the wide and developing scope of hypercarbon chemistry by illustrating the variety of compounds now known to contain hypercarbon atoms (carbocations,organometallics, carboranes, metal-carbon cluster compounds," and metal carbides ).They include bridged metal alkyls such as alkyl-lithium reagents (LiR) in which the hypercoordinated nature of the metal-attached carbon atoms, and the roles that the metal atoms play in their chemistry, are often overlooked. 

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