Nitrogen lone-pair effects

Nitrogen lone pair effects on /HC values indicate aldimine and oxime configuration (see Section 4.1.1.2.). 

The same situation is observed in the series of alkyl-substituted derivatives. Electron-donating alkyl substituents induce an activating effect on the basicity and the nucleophilicity of the nitrogen lone pair that can be counterbalanced by a deactivating and decelerating effect resulting from the steric interaction of ortho substituents. This aspect of the reactivity of thiazole derivatives has been well investigated (198, 215, 446, 452-456) and is discussed in Chapter HI. 

This is due to a resonance effect. Aniline is stabilized by sharing its nitrogen lone-pair electrons with the aromatic ring. In the anilinium ion, the resonance stabilization is dismpted by the proton bound to the lone pair. 

Another example of a stereoelectronic effect is observed in amines. Amines in which a C—H bond is oriented antiperiplanar to the nitrogen lone pair show a shift in the C—H bond stretching frequency that corresponds to a weakening of the bond by about... 

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... 

The six-membered rings 8.12a and 8.12b adopt chair conformations with all three halogen atoms in axial positions. This arrangement is stabilized by the delocalization of the nitrogen lone pair into an S-X a bond (the anomeric effect) All the S-N distances are equal within experimental error [ld(S-N)l = 1.60 (8.12a)/ 1.59 A (8.12b) ]. 

De la Mare and Hassan254 obtained second-order rate coefficients (in parenthesis) for the following 4-methylacetanilide (1.53), 2-methylacetanilide (0.193), 2,6-dimethylacetanilide (0.0118), acetanilide (0.93), 4-acetamidodiphenyl (0.248) and 1,4-diacetamidobenzene (0.231) the results for the acetanilides demonstrated the effect of steric hindrance to coplanarity thereby inhibiting resonance of the nitrogen lone pair with the aromatic ring. The rate coefficients for chlorination of 3-chloroacetanilide (0.215), 4-chloroacetanilide (0.010) 3-nitroacetanilide (6.7 x 10 5) and phenyl benzoate (3.2 x 10-6) have also been measured258,261. 

Imide-terminated telechelics are also synthesized by metathesis depolymerization, and it is found that phthalimide-substituted olefins allow for productive depolymerization when only one methylene spacer separates the nitrogen atom and the olefin (Fig. 8.21). This combination of steric hindrance around the nitrogen lone pair and decreased electron donation from resonance prevents the negative neighboring group effect. However, secondary acyclic amines are unable to produce telechelics through metathesis depolymerization because of unfavorable catalyst-amine interactions. 

In triaziridine 23, the o-relaxation is appreciable due to the lone pair effect (Sect. 2.1.4) as is the case with triphosphirane 14 which has almost the same SE as tetra-phosphetane 15. Triaziridine 23 has lower SE (31.8 kcal moF ) than tetraazetidine 24 (SE = 33.4 kcal moF ) (Scheme 12) [12]. Lone pairs on nitrogen atoms have high s-character. The hybrid orbitals for the N-N bonds have lower s-character (sp ) in 23 than that (sp ) in triazane 25 [12]. The geminal a-a interaction is less antibonding in 23 (IBP, = -0.004) than in 25 (IBP. = -0.014). The geminal... 

P3H314 and PjH 77 molecules are less strained than the nitrogen congeners. As mentioned earlier (Sect. 2.1.4), the lone pair on phosphorus atoms is richer in s-character. The lone pair effect on o-relaxation is more remarkable. 

This observation is consistent with the assumptions of the authors predicting microwave effects when the polarity is enhanced in a dipolar TS. The kinetic rate-determining step consists of an intramolecular attack of the nitrogen lone pair on the carbon atom of the carbonyl moiety (Scheme 3.15). 

The main aim was, on the one hand, to ascertain the presence of a through-space interaction between the two nitrogen lone pairs and the role played by the Si—C and Sn—C bonds in the mechanism of this interaction, and on the other hand, to evaluate the effect of the change in tin oxidation state (+2 in 27, +4 in 30) on the electronic structure of these molecules. The He I and He II spectra were interpreted by comparison... 

The trans configuration of the nitrogen lone pair and the Nu fragment in nitroso acetals can be attributed to the anomeric effect of this pair (see Chart 3.23, b). 

Another such instance of anticooperativity can be seen by comparing 5 with 13. In the latter case, the two n— -nco interactions make competitive use of the same busy nitrogen lone pair, and each such interaction (47.9 kcal mol-1) is thereby weakened relative to the value (59.8 kcal mol-1) in 5. Similarly, the comparison of vinylborane 6 with divinylborane 14 reveals an anticooperative effect involving competition for the ns pi acceptor, with each 7tcc—>-113 interaction in 14 (24.2 kcal mol-1) being slightly weakened compared with its value in 6 (26.2 kcal mol-1). 

A particularly dramatic effect of hyperconjugation on the form of the ethylenic double bond NBOs is shown in Fig. 3.50. This figure depicts one of the two equivalent C=C bond and antibond NBOs of vinylamine, twisted slightly out of coplanarity with the nitrogen lone pair with dihedral

[Pg.219]

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