Amines hyperconjugation

The interaction of the lone-pair electrons on an amine nitrogen with adjacent C—H bonds is another example of a hyperconjugative effect that can be described in MO language. The lone-pair electrons, when properly aligned with the C—H bond, lead to a... 

In the triplet excited state, an electron is formally removed from the oxygen n0 orbital (the in-plane pv-type nonbonding orbital) of the (3 manifold and added to the 7tco antibond of the at manifold (formally breaking half the pi bond). From the viewpoint of the amine nN donor, this excitation makes available a low-lying half-filled n0 acceptor NBO suitable for strong hyperconjugative stabilization. The nN—nQ interaction... 

In piperidine the electron lone-pair can occupy either an axial or an equatorial position in 1-methylpiperidine the axial orientation (lb) is favoured by 99 1 over the equatorial (la). PE spectra and ab initio calculations on methylpiperidines indicate that axial 2-methyl substituents lower the amine lone-pair ionization potential by about 0.26 eV, while equatorial 2-methyl substituents as well as methyl groups on carbon atoms 3 and 4 lower the lone-pair IP by less than 0.1 eV63. This establishes the mechanism of stabilization of the amine radical cation as hyperconjugative electron release, which is larger for CC bonds than for CH bonds. The anti-periplanar orientation of the nitrogen lone-pair and the vicinal C—Me bond (lc) is much more favourable for this type of interaction than the synclinal orientation (Id). 

The weakening of a bonds by negative hyperconjugation with lone electron pairs also reveals itself in IR and NMR spectra. Thus, C-H, N-H, or O-H bonds oriented trans or antiperiplanar to an unshared, vicinal electron pair are weakened and have therefore a significantly reduced IR vibrational frequency [17]. The C-H vibrational frequency in aldehydes is, for example, lower than that in alkenes (Scheme 2.7). Polycyclic amines with at least two hydrogen atoms antiperiplanar to the lone pair on nitrogen have characteristic absorption bands at 2800-2700 cm-1 which have been used to infer the relative configuration of such amines [18]. 

Some 2-alkoxytetrahydropyrans show a reactivity toward oxidants which parallels the reactivity of polycyclic amines discussed above, and which is in line with the hypothesis that weakening of C-H bonds by hyperconjugation should also increase the rate of C-H bond cleavage. For instance, of the two epimeric pyrans sketched in Scheme 2.15 only that with an axial 2-H is oxidized by ozone [51]. The same selectivity has been observed in the oxidation of methyl a- and /3-glucopyranoside with ozone [52], and in homolytic C-H bond cleavage in cyclic ethers [53],... 

Adcock, W. Abeywickrema, A. N. Substituent effects in the bicyclo[2.2.2]octane ring system. A carbon-13 and fluorine-19 nuclear magnetic resonance study of 4-substituted bicyclo[2.2.2]oct-l-yl fluorides, / Org. Chem 1982,47,2957-2966. Laube, T Ha, T. K. Detection of hyperconjugative effects in expaimentaUy dete-mined structures of neutral molecules, J. Am. Chem. Soc. 1988,110, 5511-5517. Rozeboom, M. D. Houk, K. N. Stereospecific alkyl group effects on amine lone-pair ionization potentials Photoelectron spectra of alkylpiperidines, / Am Chem Soc. 1982,104,1189-1191. 

The better known hyperconjugative interaction that relates to all C-0 torsions, including C-2-0-2 is the Bohlmann effect first identified from characteristic con-formationally dependent shifts in the IR spectra of amines.168 169 This effect is a result of electron donation from lone pairs on N or O to the a orbital of adjacent C-H bonds. This effect is maximal for an anti relationship between the relevant lone pair and the C-H bond. One such interaction is always present in gauche H-C-O-C conformations and is one of the factors that stabilize such conformations. 

Me2CgH3 > Ph is accompanied by an increase of hyperconjugation from equatorial ligands (amine, F, pyrrole, Ph), resulting in an overall upheld shift. 

We should conclude by emphasizing that the stabilization of these compounds does not mean that they are unreactive. In fact, a-halo ethers and a-halo amines are highly reactive toward solvolysis. The hyperconjugation that is manifested in net stabilization weakens the carbon-halogen bond and the molecules dissociate readily in polar solvents. Methoxymethyl chloride is at least 10 times as reactive a methyl... 

However, the same interaction— hyperconjugation— is present in the parent amine, as revealed by spectroscopic features of the C—H bond (see Section 1.1.8). Recent direct measurements have been made for (CH3)3N and (CH3CH2)3N, as well as the pyrrolidine and piperidine, among others. The results obtained are shown below. The recommended value for the RSE for the aminomethyl radical is 13 1 kcal/mol. Similar results were obtained using G2(MP2) calculations with isodesmic reactions. Pyrrolidine, the five-membered cyclic amine, has a slightly larger RSE because there is some relief of eclipsing interactions in the radical. 

The mechanism in Scheme 8 was proposed for the oxidation reaction. In the first step, the Cu(II) salt, which is formed in the autooxidation of cuprous chloride, forms a complex with the amine. This is followed by a rate-determining electron transfer from the amine to the Cu(II) species giving Cu(I) and an aminium radical. The subsequent steps were considered to be fast. The authors accounted for the secondary hydrogen-deuterium kinetic isotope effect by suggesting that there was hyperconjugative electron release to the aminium ion nitrogen that forms in the slow step of the reaction. 

Ph2P(S)SePPh2=NSiMe3. The double Kirsanov reaction of (C 3PNMe>2 with primary aromatic amines gives the bis phosphinimine hydrochloride (RNH)2P(=NR)NP(NHR)2NHMe C . The structure observed (Section 7) has been rationalized on the basis of hyperconjugation of the NMe lone pair and the adjacent PN bond. 

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