Pyramidalized double bonds

TABLE 6. Pyramidalization angles (0 and j/) for compounds containing strongly pyramidalized double-bond carbon atoms... 

Similar geometric optimization has been reported for bicyclo[3.2.2]nona-6,8-diene (BND). The double bond situated in the opposite direction to the methylene group was found to be more exo-pyramidalized than the other double bond and the electron density (qi, HOMO) of the former double bond in HOMO of the molecule higher than that of the latter double bond. The exo and endo faces of exo-pyramidalized double bonds proved not to be equal and the electron density was found to be higher on the endo faces. The endo molecular complexes with bromine have been found by the HF/321G method to be more stable than their exo congeners this was attributed to electronic and steric factors. As a result, endo-facial stereoselectivity of bromination ( ) predominates.21 A related theoretical study of facial selectivity and regioselectivity of the electrophilic addition of chlorine to exo-tricyclo[4.2.1.02,5]nona-3,7-diene (exo-TND) has also been reported.22... 

As a reversible protection measure, complexation with a Fe(CO)4 moiety appeared to be most promising. The monoene 68 was indeed found to react smoothly with Fe(CO)s at room temperature to give the tetracarbonyliron complex 73b in 75% yield after crystallization. The complex 73b proved to be stable for days in air as a solid and in solution (CHCl3, benzene), and its ligand 68 can be conveniently liberated under mild oxidative conditions [Ce(N03)4/ MeOH/THF] at room temperature (Scheme 14) [77]. Although the use of metal complexation as a protection method for such compounds with highly pyramidalized double bonds works well for the monoene 68 and the diene 70, the complexation of more highly unsatu-... 

Superphanes, Cyclophanes, and Betweenanenes. The simplest member of the superphanes, tricyclo[4.2.2.22,5]dodeca-l,5-diene (56) (cf. Table 5) has been experimentally and theoretically described by Wiberg and co-workers (57b, 85). Ab initio as well as molecular mechanics calculations reproduce the structural features and the pyramidalization in 56 as observed in X-ray analysis rather well. Due to the small distance between them, the two pyramidalized double bonds interact with each other (85). Using the CCF method, Ermer has discussed the structure of columnenes 70-72 (11). Ab initio studies gave, for these columnenes pyramidalization angles T of 18.2°, 29.3°, and 47.3°, respectively strong through-space n-n interactions were found in all three (86). 

The sulphite ion, SO3, has a pyramidal structure and the short S—O bond length suggests the presence of double bonding, i.e. 

The structure of the organometallic tin(II) compound 14, which was the first stable bivalent tin compound in non-polar organic solvents84, is changed when passing from the solution to the solid state 21 108). In the crystal discrete dimers are present (Fig. 5). Since the tin atom is pyramidal and the Sn—Sn distance quite large (276 pm), no normal ct, Jt-double bond can be responsible for this geometry. 

The transition state TS[7b 8b] that occurs at a distance of 1.9-2.1 A of the emerging C-C a-bond can adopt two different conformations. A square-planar transition state is crossed along the path for formation of all-t-CDT, in which the trans double bond is not coordinated to nickel. On the other hand, the c,c,Z-CDT and e,/,z-CDT generating paths involve a square-pyramidal transition state, in which the coordinated trans double bond is... 

For INOR1, INOR2 and SNOR a significant pyramidalization of the endocyclic double bonds can be observed by all methods. The out-of-plane deviations appear to be around 1.9° to 3.1° (see Table 14). 

Pyramidal nitrogen is favorable for slow inversion. In this case, two methyl groups in the SilV Bu fragment are nonequivalent, whereas two SiMej groups are, on the contrary, equivalent. However, any two identical substituents at the nitrogen atom become nonequivalent in the presence of the asymmetric center G attached to the C,C double bond. 

Evidently, in the crystalline state, BENA can adopt one of two conformations (A or B) presented in Fig. 3.3. Nevertheless, the nitrogen atom is characterized by a high degree of pyramidalization in both conformations, and the C-N bond is substantially elongated compared to similar bond in classical enamines (see, e.g., (417)). It should also be noted that the C,C double bond in BENA is substantially shortened compared to the analogous bond in standard enamines. 

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