Amides Atomic orbitals

A priori borataethenes would be expected to be much more stable than the corresponding boraethenes because, as Rundle presciently noted about organometallic structures,50 all low-lying atomic orbitals are involved in the bonding. In fact, the stabilization of 52 is the driving force for the acidity of C—H bonds of proper orientation a to tricoordinate boron. Such acidity was first detected by Rathke and Kow through the treatment of B-methyl-9-borabicyclo[3.3.1] nonane (53) with hindered lithium amides, such as lithium 2,2,6,6-tetramethylpiperidide51 (Eq. 18) ... 

The formation of an amide bond in a lactam molecule occurs by overlap of px atomic orbitals of nitrogen and carbon. In addition, another bond between these two atoms can be formed by sharing of the two electrons of nitrogen penetrating into the 2p atomic orbital of carbon. 

With 3-chloro, there is the possibility of inductive electron withdrawal and, additionally, acceptance of electron density into its 3d atomic orbitals. The high reactivity of the p-lactam ring of cefaclor is linked to these electronic factors. Note that the p-lactam amide and enamine systems of 3-cephems are not coplanar. Hence there is no clear distinction between u- and ir-electron density involved in a shift from the p-lactam carbonyl to the 3 substituent. 

In contrast with amines, amides (RCONH ) are nonbasic. Amides don t undergo substantial protonation by aqueous acids, and they are poor nucleophiles. The main reason for this difference in basicity between amines and amides is that an amide is stabilized by delocalization of the nitrogen lone-pair electrons through orbital overlap with the carbonyl group. In resonance terms, amides are more stable and less reactive than amines because they are hybrids of two resonance forms. This amide resonance stabilization is lost when the nitrogen atom is protonated, so protonation is disfavored. Electrostatic potential maps show clearly the decreased electron density on the amide nitrogen. 

Heteroatoms a to a C —II insertion site can activate that site for insertion. It was shown that an acyclic amide such as 2-diazo-lV,jV-diisopropyl-3-oxobutanamide can cyclize smoothly to the /Mactam56. It may be that in this case overlap between the filled orbital on the nitrogen atom and the C-H orbital can increase the electron density in the latter, thus making it more reactive. It may be pertinent that as the starting acyclic amide becomes more product-like, amide resonance decreases, and the electron-donating ability of the nitrogen atom could increase. 

The product amido-hydride is the first structurally characterized transition metal complex that features both a terminal amide and hydride. The X-ray crystal structure shows that the nitrogen centre has planar coordination. This geometry results from 71-bonding between the nitrogen electron pair and the lowest unoccupied metal orbital which lies in the equatorial coordination plane of the atoms lr(Ci ethine)(H)(N). This is consistent with earlier calculations on related species. The possibility that the addition of... 

Furthermore, the oxygen atom of the carbonyl group in the amide function has an electron pair oriented antiperiplanar to the polar C-N bond there is therefore an electronic delocalization caused by the overlap of that oxygen electron pair orbital with the antibonding orbital of the C-N sigma bond (o ) as shown in two dimensions by structure 5 and in three dimensions by structure . This additional n-o delocalization is referred to here as a secondary electronic delocalization. Thus, amides are similar to esters because they both have the primary electronic effect and one secondary electronic effect. This is in contrast with Z esters which have two secondary electronic effects besides the primary electronic effect. 

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