Five-coordinate carbon

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... 

In the case of cobalt substituted Zn-fingers [102], the differences between the chemical shifts for corresponding resonances in the Co(II) and Zn(II) complexes allow the determination of the orientation and anisotropy of the magnetic susceptibility tensor [103]. Similar studies are available for pseudotetrahedral Co(II) in the zinc site of superoxide dismutase [104] and five coordinated carbonic anhydrase derivatives [105]. 

NMR spectroscopy has been used to give weight to the suggestion of five-coordinate carbon in (I). The metallated carbon is at 168.1 ppm... 

A common misconception is that carbon can, at most, be four-coordinate. Although carbon is bonded to four or fewer atoms in the vast majority of its compounds, many examples are now known in which carbon has coordination numbers of 5, 6, or higher. Five-coordinate carbon is actually rather common, with methyl and other groups frequently forming bridges between two metal atoms, as in Al2(CH3)g (see Figure 8-15). There is even considerable evidence for the five-coordinate ion Many organometallic cluster... 

No compound with five-coordinate carbon as the central atom has yet been crystallized. Burgi, however, has studied a series of trigonal bipyramidal complexes of divalent cadmium with three equatorial sulfur ligands and with axial ligands iodide, hydroxyl, or sulfur groups ( Y and X). 

A carbocation, real or hypothetical, that contains at least one five-coordinate carbon atom. 

The leaving group is forced out by the nucleophile, as introduced in Section 4.2.7. The transition state is a five-coordinate carbon. The tetrahedral configuration at the carbon atom is inverted. The transition state has a carbon p orbital partially bonded to the nucleophile on one side and the leaving group on the other (back side attack, see Fig. 7.5). The 2 in the name Sn2 indicates that there are two reactant molecules involved in the rate-determining step (the nucleophile and the molecule attacked). 

Figure 1.10. MOs mvolving the five-coordinate carbon atom in the transition state in an Sn2 reaction.

A new type of benzo[fo]furan was constructed by a bimolecular nucleophilic substitution reaction at the sp carbon atom, which was envisaged to proceed via a five-coordinate carbon compound as a transition state <05OL2739>. 

Among the attempts to synthesize five-coordinate carbon species with a linear XCR3X structure only those by Martin and his coworkers will be mentioned here [38-40]. Eight molecules of type 44 with different counter ions and in different... 

Analysis of the results described here as well as those published in the literature for monooxygenases and their chemical analogues shows that, besides the traditional radical mechanism of C-H hydroxylation in the presence of metal complexes, another, five-coordinate, carbon mechanism exists which involves addition of an oxygen atom to a C-H bond, followed by insertion into the C-H bond to form an alcohol. However, the exact nature of the latter mechanism remains to be elucidated and may be somewhat different for different systems. 

Alkanes may be activated by superacids or complexes of the transition metals. The superacid reactions lead to the formation of cations possessing a five-coordinate carbon atom ... 

LC is associated with H-atom cleavage mechanism and CC with ozone incorporation along the C-H bonds. LC is characterized by the ozone interaction with the C-H bond via one of its terminal atom and thus there is a possibility for free rotation around H-0 and 0-0 while in the CC it forms five member cycle with participation of five coordinated carbon. In Table 5, are summarized the alculated values of A for the two AC forms. 

Because the E2 reaction is a single-step process, with precise stereochemical requirements, we can do more to control the outcome. As far as the substrate is concerned, branching at a- and/ or p-positions slows substitution more than it does elimination. Because in substitution, there is a requirement for a five-coordinate carbon at the transition state, the steric demand of this process is much higher than for elimination. Also, branching at the a- or p-positions stabilizes an alkene-like transition state. As we have previously noted, stereochemistry is critically important, especially to E2 reactions. Elimination will be inhibited if the tram-anti arrangement of the atoms or groups to be eliminated is not available or is disfavored. Substitution will be unaffected. If we compare the reactions of 10.11a and 10.11b using ethoxide (which can be either a... 

Aurophilic interactions also play an important role in stabilizing the remarkable hypervalent compounds [(Ph3PAu)gC] + with a six-coordinate carbon atom [30], [(Ph3PAu)5C]+ with a five-coordinate carbon atom [31], and [(Ph3PAu)5N] + with a five-coordinate nitrogen atom [32] as seen in Figure 3.7. 

---