Add bond

SN = 3, sp hybridization, bent molecule. The 2p orbitals perpendicular to the plane of the molecule can be combined into a tt molecular orbital containing one pair of electrons. This orbital adds bond order 1/2 to each N—O bond, for a total bond order of 3/2 per bond. 

The implicit bonds are determined by calculating the interatomic distances as drawn on the screen, and bonding all atoms within 1.6 A of one another. A "delete bond" feature allows one to remove extra, unwanted bonds. The "add bond" feature, allows the bonds to be assigned explicitly by indicating the atom pairs to be bonded and depressing a key on the keyboard. The bonding operations automatically determine the connectivity data. 

Let us illustrate the concept of formal charge using the ozone molecule (O3). Proceeding by steps, as we did in Examples 9.3 and 9.4, we draw the skeletal structure of O3 and then add bonds and electrons to satisfy the octet rule for the two end atoms ... 

Planning reaction schemes which incorporate overbred molecular skeletons looks like an exotic undertaking. To do this, one needs a good knowledge of C-C bond breaking reactions [167, 174], which could be introduced ret-rosynthetically as an add bond operation. Add bond is used in the ret-rosynthesis of anellated and spirocyclic targets (Scheme 6.51). 

Scheme 6.51 Add bond strategy for syntheses of bicycles via an overbred skeleton...

There is a peculiarity in the examples in Scheme 6.53. The bond that is cleaved in reversal of the add bond operation is not the one that was made in the preceding bicyclization. Rather, it is one which has been intentionally introduced with one of the partners of the bicyclization. For the generation of eight-membered rings via an overbred skeleton, not only do bicyclo[4.2.0]octane systems serve well (Scheme 6.53), but so too do bicy-clo[3.3.0]octane systems, i.e., anellated 5/5-ring systems (Scheme 6.54). 

Apply the add bond strategy to twistane (add a bond between two CH2-units). Which of the linkages possible in this manner appears to be most advantageous Suggest a reaction scheme following this bond-set. What kind of regioselectivity problem would have to be solved ... 

An add-bond strategy via an overbred skeleton offers the possibilities (a) and (b) shown in Scheme 13.31. Choice (a) suggests an intramolecular photocycloaddition between a cyclobutene and a cyclohexene—not really attractive. Choice (b) indicates a well precedented cyclopropanation reaction, that was indeed used [29] to realize a quick synthesis of twistane. However, this faced the problem of the regioselective cleavage of a distinct cyclopropane bond. 

The description of molecules using molecular mechanics force fields, which add bond stretching, bond angle bending, and torsional potentials to the intermolecular potentials discussed earlier, is well established. Difficulties arise when describing adsorption in the compatibility of molecular and ionic force fields. 

Figure 22. Molecular Sophe Graphical User Interface showing the Add Bond window. Note the atom eontaining unpaired electrons occurs in the left hand combo box. In this example Fel is the atom containing unpaired electrons.

---