Pyramidal carbon atom

JA4155). However, when the 6-31G basis set is employed, the Cs structure with the pyramidalized carbon atom turns out to be a local minimum on the PES (90JA4155). 

The energetically unfavorable situation with pyramidal carbon atoms in double bonds was early realized by Bredt. His famous rule12,13, which states that a carbon atom in a C=C bond cannot be a bridgehead in a bicyclic system, is based on studies of camphenes and pinenes and is thus intended to be valid only for five- and six-membered rings. 

Five-Center Four-Electron Bonding Structures. The potential for the existence of 5-center 4-electron (5c-Ae) bonding structures 558 have recently been surveyed by Tantillo and Hoffmann980 [calculations at the B3LYP/6-31G(d) level], A cation with three anthracenyl units joined around the C -H-C -H--C core with two approximately trigonal pyramidal carbon atoms and one five-coordinate trigonal bipyramidal carbon was found to have 5c-Ae bonding. The anticipated existence of... 

Fig. 5.48 The hydrocarbon pyramidane, C5H4, evidently (pyramidane has not been synthesized) has a lone pair of electrons on its pyramidal carbon atom, like carbene (methylene), CH2. While the lone pair on CH2 is no surprise (draw the Lewis structure for the singlet), a cycloalkane with an unshared electron pair is remarkable...

Whereas the structural assignment by one-dimensional 13C NMR spectroscopy is unambiguous in the case of D2-Cj(,2,51 Achiba and co-workers were able to determine the carbon atom connectivity by 2D 13C NMR INADEQUATE (incredible natural abundance double-quantum transfer experiment) analysis performed on an isotopically enriched sample (20% 13C).58 In particular, they found that the observed chemical shifts correlate well with the curvature of the spheroid, the more strongly pyramidalized carbon atoms being shifted toward lower magnetic field. 

Bridgehead Double Bonds. Typical bridgehead olefins represented by 44 (cf. Table 4) have been extensively discussed by Bredt (3). Inspection of molecular models, which do not allow for rehybridization at the carbon atoms of the double bond, suggest that structure 68 should prefer extensive torsional deformations. Extended Hiickel calculations showed, however, that the structure of a bridgehead double bond is best described by the overlap of a spx hybridized orbital at the bridgehead with an orbital at the adjacent carbon atom of high p character. This type of n bond is favored over a structure with two strongly pyramidalized carbon atoms (79). 

Bowlane (43) and some of its homologues 44-49 have been the first molecules possessing a pyramidal carbon atom in the ground state to be proposed as plausible synthetic targets on the basis of MM calculations (35). Work is in progress in this laboratory on other hypothetical molecules that should have pyramidal carbon atoms in the ground state. 

Molecule 5, C5H4 Removal of an appropriate pair of hydrogens from 3 gives, as an alternative to 4, molecule 5, tetracyclo[2.1.0.0 . 0 ]pentane, pyramidane. This was still unknown as of early 2007, despite having sparked a fair amount of interest (19 references in Chemical Abstracts) because it is the canonical molecule with a pyramidal carbon atom. Pyramidane was discussed in Chapter 2. Noteworthy are the facts that it has a lone pair at the apex (prominent electrostatic potential region), and that computations provide good evidence that it will prove to be reasonably stable, perhaps even isolable at room temperature. 

Recent theoretical calculations (DFT-NL) on M( 7 -C6o)(PH3)2 (M = Ni, Pd, Pt) reveal that the metal-fullerene bond-dissociation energies (Pd < Ni < Pt) are higher than the corresponding metal-ethylene complexes but lower than those of substituted ethylene (C2X4, X = F, CN). Similar studies on a series of complexes (7/ -C,) Pt(PH3)2 ( = 60, 70, 84 n = 1-6) indicate that the binding energy between the Pt(PH3)2 unit is almost independent of the size of the cage and the number of metals coordinated on the fullerene surface. In the C70 species, the most reactive site corresponds to the bond with the most pyramidalized carbon atoms in the free fullerene. ... 

Scheme 1-220. More "hot species" having in-line pyramidalized carbon atoms.

More important, tire surface curvature of tire carbon network exerts a profound impact on tire reactivity of tire fullerene core [6, 7]. In tliis context, tire most striking consequence emerges from tire pyramidalization of tire individual carbon atoms. Influenced by tire curvature, tire sp hybrids which exist in tmly two-dimensional planar... 

JOC1537). The mechanisms of these transformations may involve homolytic or heterolytic C —S bond fission. A sulfur-walk mechanism has been proposed to account for isomerization or automerization of Dewar thiophenes and their 5-oxides e.g. 31 in Scheme 17) (76JA4325). Calculations show that a symmetrical pyramidal intermediate with the sulfur atom centered over the plane of the four carbon atoms is unlikely <79JOU140l). Reactions which may be mechanistically similar to that shown in Scheme 18 are the thermal isomerization of thiirane (32 Scheme 19) (70CB949) and the rearrangement of (6) to a benzothio-phene (80JOC4366). 

There are a number of important kinds of stereogenic centers besides asymmetric carbon atoms. One example is furnished by sulfoxides with nonidentical substituents on sulfur. Sulfoxides are pyramidal and maintain dieir configuration at room temperature. Unsymmetrical sulfoxides are therefore chiral and exist as enantiomers. Sulfonium salts with three nonidentical ligands are also chiral as a result of their pyramidal shape. Some examples of chiral derivatives of sulfur are given in Scheme 2.1. 

Cp(CO)2Re(THF) forms the complex 105 upon reaction with thiophene (89JA8753, 910M2436). Similar species are known for 2- and 3-methyl-, 2,5-dimethyl, and tetramethylthiophene (91IC1417). Thiophene in 105 is S-coordi-nated, and the sulfur atom is pyramidal. Treatment of 105 with Fc2(CO)9 produces 106, where the thiophene ligand is bridge-coordinated via the sulfur atom to rhenium and four carbon atoms of the dienic system with iron (the coordination mode). The pyramidal nature of the sulfur atom is preserved. The -coordination of thiophene separates the dienic and sulfur counterparts of the ligand and decreases the TT-electron delocalization, which leads to the enhanced basicity of the sulfur atom. 

Although the ion pairs of a-substituted benzyl anions and the corresponding cations are chiral species, which, in addition, often bear a pyramidal and hence stereogenic carbon atom, in most cases rapid racemization of the alkali and alkaline earth metal derivatives occurs in solution... 

H/D exchange of H and Hg protons of sulfone 86 and estimated the difference in the free energies of activation for 79a and 79b to be < 1.2 kcal mol , based on the kjk value of 3 0.5. In the base-catalyzed H/D exchange of 87, kjk = 1.6, where k and k are the rate constants of H/D exchange of H, and H, respectively. Based on the small kjk value. Brown and colleagues suggested that if the carbanion is pyramidal, the steric stabilities of 79a and 79b are almost identical. Meanwhile, based on their C-NMR study Chassaing and Marquet proposed that the hybridization of the carbon atom of the sulfonyl carbanion, PhSOjCHj , would be between sp and sp . 

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