Lone pairs conjugate acceptors

Almost all known inorganic heterocychc molecules, where N, O and S atoms with lone pair orbitals are donors while B atoms with vacant p orbitals are acceptors, are classified into discontinuous conjugation. The donors and the acceptors are alternately disposed along the cyclic chain. The thermodynamic stabilities are controlled by the non-cycUc electron delocalization or by the number of neighboring donor-acceptor pairs, but not by the number of % electrons [83]. In fact, both 4n % and 4n + 2% electron heterocycles are similarly known [84,85] (Scheme 33), contradicting the Hueckel rule. 

Cyclic conjugation is continuous in l,2-dihydro-l,2-azaborine with one N-B bond (Scheme 34). The nitrogen atom with a lone pair is donor. The B atom with a vacant p orbital is acceptor. Whether the remaining C=C bonds are donors or accepters, the donors are disposed on one side of the cychc chain while the acceptors are on the other side. The orbital phase property or the number of electrons is important. The phase continuity or the six n electrons predicts that 1,2-dihydro-l-,2-azaborine could be aromatic. 

To maximize the conjugative interaction with a specified acceptor 7ra, one could progressively polarize the donor pi moiety until its entire amplitude is on the atom adjacent to 7ta. In this limit, the two-center donor reduces to a one-center nonbon-ded orbital nb (lone pair), and the donor-acceptor interaction is of nb 7ta type. 

Method Donor substituents usually contain heteroatoms whose lone pairs can be modeled in Hiickel calculations by doubly occupied orbitals. To a first approximation, acceptor substituents (polar conjugating groups such as C=0, C=N, NOz) can be represented by a low-lying vacant orbital (jtco for C=0,7tCN for C=N, etc.). In cases where a more realistic model is required, the occupied (nco, 7tCN, etc.) orbitals are added to the calculation. 

Conjugation, due to the interaction of a lone pair of electrons on the heteroatom with an adjacent tr-orbital, diminishes the ability of the heteroatom to coordinate with an acceptor molecule. 

Classification of hyperconjugation and conjugation as positive or negative is useful when either referring to an individual interaction or to an imbalanced situation, as is the case of an interaction with a very strong donor or acceptor orbital with the rest of the molecule dominating over other delocalization effects. This imbalance often occurs when either a lone pair acts as a donor or when an empty p-orbital acts as an acceptor. Use of these terms in other situations can be uninformative or even misleading. 

Stable singlet carbenes have been known for almost 30 years yet in their ground state, the lone pair is located in the orbital of a symmetry. An innovative approach has been explored in order to stabilize a ground state where the lone pair would be located in a r orbital. Computations have shown that combinations of in-plane lone pair donations and r acceptor substituents (such as conjugated systems) can force the lone pair of the carbene to be positioned in the tt orbital leaving the [Pg.179]

Substituents with lone electron pairs, such as alkoxy, hydroxy, alkyl, and aryl-amino groups, are known as electron donors. The CH3 group, despite the absence of such free electron pairs, is also considered an electron donor. Functional groups with conjugated rr-electron systems, such as NOz, COOH, COOR, SOz, or SOzAr act as electron acceptors. 

In 5-aminouridine [AMURID], the weak three-center bond from 0(3 )H includes an intramolecular component ( ) to 0(20 and a rare example of an NH2 acceptor (Fig. 17.19). This group, however, is not conjugated as, for example, the amino group in adenine, guanine, and cytosine, but only in conjugation with the C(5)=C(6) double bond. We assume that it is still pyramidal with the lone electron pair located on the N atom, and therefore serves as hydrogen-bond acceptor. 

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