Lone pairs conjugation with

The amide (a) is the least basic (lone pair conjugated with the carbonyl group). The pyridine (c) nitrogen, with the lone pair in an sp- hybridized orbital, is less basic than the piperidine (d). The most basic is the quinuclidine (b), which is a tertiary amine in which the ring system ties the alkyl groups back, exposing the lone pair. 

However, on bending the carbene or nitrene 10° out of the molecular plane -as required for rearrangement — both methods give identical answers the carbene or nitrene lone-pair conjugates with the ring. The HOMO and LUMO orbitals in phenylnitrene are shown below. The more electrophilic the ring, the more pronounced... 

Groups with lone pairs conjugated to the ring... 

It seems that 15N chemical shifts of amide nitrogen are much less sensitive to electron withdrawal and donation than 15N shifts of amines because of the strong conjugation of the amide nitrogen lone-pair electrons with the carbonyl groups, which compete for conjugation with the 7r-systems of the benzene rings. 

With transition-metal free cationic phosphenium, an amino substituent is known to stabilize the cation. Theoretical studies46-50 support the existence of conjugation involving the nitrogen lone pairs and the vacant phosphorus p orbital, and estimate that the N-P+-N lone pair conjugation in [P(NH2)2]+ contributes 68.1 kcal/mol.47... 

With RAMSES, the conjugation between the C=0 rr-system and the lone pair of the nitrogen atom in the amide group is taken into account (see Figure 2-51b). 

Typical auxochromes are hydroxyl, alkoxyl and aroxyl, amino, alkyl-amino and arylamino, all of which promote conjugation with lone pairs on oxygen or nitrogen atoms. 

The results obtained for 1-phenylpyrazole (32) and its conjugate acid (34) are consistent with those of Minkin. The bond order between thd two rings decreases by protonation (from 0.341 to 0.241) and this is in agreement with the expected effect of the N-2 positive charge on the delocalization of the lone pair on N-1 over the phenyl ring. 

An interesting and useful property of enamines of 2-alkylcyclohexanones is the fact that there is a substantial preference for the less substituted isomer to be formed. This tendency is especially pronounced for enamines derived from cyclic secondaiy amines such as pyrrolidine. This preference can be traced to a strain effect called A or allylic strain (see Section 3.3). In order to accommodate conjugation between the nitrogen lone pair and the carbon-carbon double bond, the nitrogen substituent must be coplanar with the double bond. This creates a steric repulsion when the enamine bears a p substituent and leads to a... 

CaveU and Chapman made the interesting observation that a difference exists between the orbital involved in the quatemization of aromatic nitrogen heterocycles and aromatic amines, which appears not to have been considered by later workers. The lone pair which exists in an sp orbital of the aniline nitrogen must conjugate, as shown by so many properties, with the aromatic ring and on protonation or quatemization sp hybridization occurs with a presumed loss of mesomerism, whereas in pyridine the nitrogen atom remains sp hybridized in the base whether it is protonated or quaternized. Similarly, in a saturated compound, the nitrogen atom is sp hybridized in the base and salt forms. 

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