Transannular bond

It should be noted, on the other hand, that a symmetry reduction is predicted even in molecules II, VIII and XIII whose peripheral skeletons correspond to 4n-l-2 cyclic polyenes. The transannular bonds in these molecules are different in nature from those mentioned above. For example, the introduction of the transannular bonds between atoms 2 and 8 and between 3 and 7 of cyclododecapentaene to form bowtiene (II) (Fig. 3) brings about the splitting of the top filled degenerate orbitals of the unperturbed system into two levels, one with its energy raised and... 

Azulene (XI) possesses a transannular bond which has the same effect as those of bowtiene (Fig. 3). The splittings of the top filled and bottom empty degenerate orbitals of cyclododecapentaene in this case are half the corresponding splitting in the case of bowtiene, and are not large enough to produce an effective vibronic interaction between the ground and lowest excited states of the resultant azulene molecule. 

Though these substances are not monocyclic like benzene—and Hiickel s rule should not, strictly, apply to them—the introduction of the transannular bond, that makes them bi- and tricyclic, respectively, seems to cause relatively little perturbation, so far as delocalisation of the n electrons over the cyclic group of ten or fourteen carbon atoms is concerned. 

Crystalline trithia dication 151 contains a 3-ccntcrA-clcctron bond with a central hypervalent sulfur atom. It was shown to be sufficiently stable to be isolated and studied spectroscopically. One important feature is its boat-boat conformation resulting from the formation of the transannular bond between the three sulfur atoms <1988JA1280>. 

Optimizations at various levels either gave pronounced bicyclic structures or a completely delocalized structure with no transannular bonding. 

The unusually facile formation of a disulfonium dication from sulfide 10 is the result of stereochemical features of the eight-membered ring, which favor the formation of a transannular bond.31 According to X-ray data (see in Chapter 7.1 Table 1), the distance between the two sulfur atoms in 1,5-dithiacyclooctane 10 is smaller than the sum of their van der Waals radii (3.75 A), which results in a strong non-bonded interaction between the atoms confirmed by photoelectron spectroscopy and mass spectrometry.32 33 This interaction and the sulfur-sulfur distance can be decreased as a result of bond formation with an electronegative substituent as in sulfoxide 13 or sulfoximine 14.34,35... 

The transannular bond formation between Te-Te and Te-S bonds giving the dications E and E is afforded by treatment of compounds A and B with the oxidizing agents nitrosyl tetrafluoroborate and hexafluorophosphate or with D2SO4. 

Fulleroids Bridged Adducts with Open Transannular Bonds... 

Formation of cluster opened methano- and imino[60[fullerenes (fulleroids and azafUlleroids) Thermal [3+2]-cycloadditions ofdiazo compounds or azides leads to the formation of fulleropyrazolines or fullerotriazolines. Thermolysis of such adducts after extrusion of N2 affords as kinetic products the corresponding [5,6]-bridged methano and iminofullerenes with an intact 60 re-electron system and an open transannular bond (see Chapter 4) [88-91]. The corresponding [6,6]-bridged structures with 58 re-electrons and a closed transannular bond are formed only in traces. 

The radical cation (19 " ) of the strained bicyclo[2.1.0]pentane also has a puckered conformation, supported by one strongly coupled (flagpole) proton (flsyn = 4.49 mT). Ab initio calculations indicate that the transannular bond retains some bonding and that the bridgehead carbons remain pyramidal. ... 

The main aspects of the chemical reactivity of helicenes (e.g. electrophilic substitution) equally not deviate from those of planar aromatic compounds, and remarkable reactions of helicenes, which are incidentally found (e.g. the transannular bond formation between a C(l)-substituent and a part of the inner helix) can ultimately be reduced to known principles of aromatic reactivity. 

Monomeric, diamagnetic 78 has a planar centrosymmetric (SnN)2 ring with the trans-Si atoms only 6° out of this plane the truns-Cl ligands are disposed orthogonally. The Sn- Sn separation of 3.398 A shows there is no transannular bonding [likewise, the... 

The carbon-carbon bonds of the cyclopentapyridine moiety of molecule 26c and the length of the transannular bond both coincide with the situation in azulene.223... 

This third type of effect is, of course, absent in the 15N case, so one can expect that interpretation of chemical shift data in terms of the transannular bond should be more... 

Mutual weakening of the axial bonds in silatranes predetermines the unprecedented difference between the Si-e- N bond lengths in the crystal and in the gas phase. Electron diffraction studies of 1-methylsilatrane (2)169 and 1-fluorosilatrane (37)170 found that in the vapors their transannular bond is 0.28 A longer and the X—Si bonds are somewhat shorter than those in their crystals. 

Recent ab initio calculations of a wide series of Si-substituted 2,8,9-tricarbasilatranes (X = H, Me, H2N, HO, F, 113 S i, H2P, HS and Cl) found a single minimum corresponding to the endo form on the potential energy coordinate of the Si<—N bond deformation194. The transannular bond was shown to be very soft and sensitive to the medium. The substituent effect on this bond length and mutual weakness of the axial bonds of the TBP silicon are well described by the 3c4e bonding model. 

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