Intramolecular union

The oxidative coupling/cyclization reaction is the intramolecular union of two arenes with formal loss of H2 promoted by a Pd(II) species (typically Pd(OAc)2). In an early example of this transformation, treatment of diphenylamines 1 with Pd(OAc)2 in acetic acid yielded carbazoles 2 [8]. The role of acetic acid in such oxidative cyclization processes is to protonate one of the acetate ligands, which affords a more electrophilic... 

The second type of perturbation, i.e., intramolecular union, can be treated in a manner exactly similar to that for the monocentric perturbations. We are concerned here with a process in which two conjugated atoms k and /)... 

In the intramolecular union of 1,3,5-hexatriene to give (5)-(8), the expression for the change in 7r-electron energy is the same in all cases, namely, (2.13). The 2—5 and 1—4 bond orders in 1,3,5-hexatriene are negative, so 5E would be positive in the cases (5) and (1). The 1—5 bond order is zero, which accounts for the fact that fulvene (6) behaves like a strained polyene. The 1—6 bond order is positive, which accounts for the aromatic energy of benzene (8). These calculations are discussed in detail in Chapter 3. 

It can be seen at once that the same result holds for multiple intramolecular union, the total perturbation energy being given by a sum of terms like that in equation (2.13), one for each new connection. 

The procedure developed above can be used quite generally to study the stability of cyclic even AHs, and also of nonalternant even hydrocarbons in which there are two adjacent odd-numbered rings. It cannot, however, be extended to other odd systems since an odd conjugated system cannot be derived by union of two odd AHs. Here we need the third type of perturbation considered in Section 2.4, i.e., intramolecular union between two positions in an AH. 

Intramolecular union between positions r and s in an AH corresponds to a change in the corresponding resonance integral from zero to some final value prs- The corresponding change in n energy [equation (2.13)] is 2p Xs where p s is the rs bond order in the original AH. We can represent the inter-molecular union in AH as follows ... 

This analysis can also be extended to hydrocarbons containing a single odd-numbered ring in an otherwise alternant system. Opening the odd-numbered ring then gives rise to an AH from which the parent compound can be derived by intramolecular union, i.e.. 

The ready conversion of heptafulvene (15) to methyltropylium (16) by acid is another example of an equilibrium involving nonalternant HCs. Heptafulvene is easily seen to be nonaromatic, being derived by intramolecular union between positions of like parity in octatetraene (17). The conversion of (15) to (16) is therefore assisted by the aromatic energy of the product (16). 

The easiest way to examine the stabilities of open-chain and cyclic excited states using the PMO method is to compare the two by intramolecular union. 

The change in 7r-energy on intramolecular union between two positions i and / in an alternant hydrocarbon (AH) (e.g. Fig. 2.4(a)) might be expected to depend on the coefficients of the occupied orbitals at the point of union. These coefficients are reflected in the calculated rr-bond order between atoms / and / in the original structure. The energy change is given by rule I. 

The cyclic odd non-AH may be regarded as being formed by intramolecular union of the corresponding open chain odd AH (as the cation, anion or radical). Rule I gives the energy as... 

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