Lone pairs on heteroatoms

Our definition does not take account of lone pairs on heteroatoms in the ring which do not contribute to the -system we are normally concerned here with pyridinoid nitrogen atoms, and their influence on aromaticity is discussed in Section V,B. It is also noteworthy that for molecules possessing an unsubstituted pyrrole-type nitrogen atom, the aromaticity of the anion produced on deprotonation can be significantly greater than that of the protonated form. This has been found in a number of cases with aromatic annulenes,15-18 and we will meet similar results with certain mesoionic azapentalenes (Section IV,C,l,a). 

The term is not just whimsy on the author s part certain stereoelectronic phenomena arising from the presence of lone pairs on heteroatoms in a 1,3-relationship were once called the rabbit-ear effect , and a photograph of the eponymous creature even appeared on the cover of the Swedish journal Kemisk Tidskrift. History of the term, photograph Eliel EL (1990) From Cologne to Chapel Hill. American Chemical Society, Washington, DC, pp 62-64... 

Net hydride transfer may also occur in a stepwise fashion without radical intermediates. There may be a-bond formation between donor and acceptor, particularly when resonance stabilized cationic acceptors react with a donor containing nucleophilic lone pairs on heteroatoms (9). Plausible fragmentations then lead to the products of transfer to the cation of hydride 0- to the heteroatom. 

While not technically the reaction of a substituent, lone pairs on heteroatoms can act as Lewis bases toward Lewis acids. Two examples among several are the coordination of phosphasiliranes and phosphasilirenes to W(CO)s, employed both to stabilize the ring thermally and to allow formation of crystalline materials <1987AGE776, 2000AGE4127>. 

Skeletal structures often leave out lone pairs on heteroatoms, but don t forget about them. 

Lone Pairs as Bronsted Bases Lone Pairs on Heteroatoms as Nucleophiles Softness, Solvation, Size, Basicity Easily Oxidizable Metals... 

Competition for orbital of corbene from Lone pair on heteroatom X... 

Chemists do not always draw all the lone pairs on heteroatoms, and it is important for the reader to realize that the lone pairs are there. When the lone pair is used as a source we always draw it, and when a lone pair is created we will draw it. This is the convention used in this textbook. Very often, however, chemists stop drawing all lone pairs, even when they are sources, and you should recognize what is meant by the arrows. 

Sheridan et al. ° use similar screens, except that they distinguish atoms based on aromaticity, hybridization, connectivity, and charge at neutral pH. They also calculate screens based on the positions of the lone pairs on heteroatoms and of the center of mass of rings. Thus, they have many more screens. To accommodate this, they use an inverted bit map, that is, one in which each row contains the information on which molecules possess a particular screen. This facilitates searching for example, screening 223,000 structures typically requires about 1 min CPU time on a VAX 8650. However, updating an inverted bit map when a new compound is added is much more demanding of computer resources. 

Such interactions are even better when one metal cation can interact with several lone pairs on heteroatoms in the same molecule. Species that can do this are known as bidentate, tridentate. 

Deprotonated I H- 1,2,4]diazaphospholes, exhibit aromatic delocalization by virtue of 6ir electrons like cyclopentadienyl ligand in the anionic ring, which, in addition to the donor lone pairs on N or/and P heteroatoms, makes them promising ligands with varied coordination modes. However, till the last decade, they had... 

Heteroatomic Substituents. The effect of attaching a heteroatomic substituent to the carbyne carbon, for example, NMe2, is to remove the degeneracy of the 7r-type LUMO s (28). There is extensive mixing (w-donation) from the lone pair on nitrogen into one of the p orbitals on carbon. The ir -orbital for the N - C -interaction is moved to higher energy, and the other p orbital becomes the sole LUMO. 

Molecular modelling (AMI) indicated that the bridge hetero-atom separations in 137 were as follows N-N = 8A, N-0 = 4.6A, 0-0 = 2.7 A. In addition, the invertomer preference of the N-benzyl groups position them over the aromatic rings thereby ensuring that the lone pairs on the heteroatoms are concentrated within the cavity section of the molecule. 

In this section the preparation and uses of heteroatom-substituted carbene complexes L M=C(Xn)R,2 n) (n = 1, 2 X NRj, OR, SR) will be discussed. In these complexes the electron deficit at the carbene carbon atom is compensated both by electron-donation from the lone pairs on the heteroatom and by d-electron backbonding from the metal (Figure 2.1). 

Oxidation of iodoalkanes involves removal of an electron from the halogen nonbonding orbital. The radical-cations of primary and secondary alkyl iodides can be identified in aqueous solution by their absorption spectra and have half-lives of microseconds [1]. They are formed during pulse radiolysis of the iodoalkane in aqueous solution in the presence of nitrous oxide. This system generates hydroxyl radicals, which remove an electron from the iodine atom lone pair. Iodoalkane radical-anions complex with the lone-pair on other heteroatoms to form a lollo three-electron bond. In aqueous solution, the radical-cation of iodomethane is involved in an equlibrium indicated by Equation 2.1. 

By definition, a generalized anomeric effect is observed at carbon of an XCY system when a molecule preferentially adopts a conformation that optimizes a secondary, stabilizing electronic interaction involving overlap between the lone pair on one heteroatom with the a orbital of the bond between the central carbon atom and the second heteroatom . Figure 5a illustrates that in XNY systems, as with anomeric carbon centres, two anomeric interactions are possible and involve either an ny-CT x ° nx-o NY overlap where nx and ny represent the p-type lone pairs on X and Y and NX and NY represent the N—X and N—Y a orbitals. In either case, the result is a net stabilization of the lone pair of electrons (Figure 5b). Except where the nitrogen is symmetrically substituted, one of these interactions will be strongest. 

For molecules containing heteroatoms (O, N, Cl, Br, etc., a very common fragmentation is the cleavage of the a,P-bond (often referred to as cleavage (3 to the heteroatom). In such molecules, the lone pairs on the heteroatom will be the least tightly bound in the molecule, and it will be one of these electrons which is lost upon electron impact (Scheme 5.9), leading to cleavage P to the heteroatom. 

The analogous reactions of pyridines with these electrophilic reagents at the lone pair on the nitrogen atom are well known. All neutral azoles contain a pyridine-like nitrogen atom and therefore similar reactions with electrophiles at this nitrogen would be expected. However, the tendency for such reactions varies considerably in particular, successive heteroatom substitutions markedly decrease the ease of reaction. One convenient quantitative measure of the tendency for such reactions to occur is found in the basicity of these compounds this is treated in Section 3.4.1.3.5 and 3.4.1.3.7. 

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