Axial lone pair

The Aae criterion was applied to the problem of the piperidine equilibrium (10 11) by examining the H-NMR spectrum of the deutero derivatives 12, 13, and 14 at — 85°C when ring inversion is slow on the NMR time scale.37 The Aae values are recorded in Table I. The preferred conformation for the N-substituted piperidines 13 and 14 was assigned as lone-pair axial because for these compounds Aae values were similar to those in quinolizidine. However, in piperidine (12), where the Aae value is similar to that in cyclohexane, the lone pair was assumed to occupy the equatorial position in the preferred conformation. In support of this contention, protonation of... 

Plots of Aae against the percentage of lone-pair axial conformer (estimated by dipole-moment measurements) for some monocyclic heterocyclic compounds with differing N-alkyl substitution showed no correlation.44 This is to be expected because the nature of the N-alkyl substituent also affects A,e40 Table III). 

Some of the objections raised44 against Jgcm/6 relationships may no longer be valid, as some of the earlier estimates of lone-pair axial conformer... 

Arbuzov s group showed by NMR analysis the presence of a chair conformation with P—Ph equatorial and the nitrogen lone pair axial. Use of Ni(acac)2 (acac = acetylacetone) simplified the spectra by decoupling hydrogen and phosphorus. At low concentrations, coordination of the nickel is with phosphorus (80IZV1571). 

For trigonal bipyramidal geometry, there are two possible locations of lone pairs, axial and equatorial. If there is a single lone pair, for example in SF4, the lone pair occupies an equatorial position. This position provides the lone pair with the most space and minimizes the interactions between the lone pair and bonding pairs. If the lone pair were axial, it would have three 90° interactions with bonding pairs in an equatorial position it has only two such interactions, as shown in Figure 3-11. The actual structure is distorted by the lone pair as it spreads out in space and effectively squeezes the rest of the molecule together. 

The nitrogen lone pair is sterically undemanding, and so usually predominantly occupies an axial site. Solvation can, however, considerably alter this picture. 

The isoxazolidine ring exists primarily as an envelope (77AHQ2l)207) and the nitrogen lone pair can occupy an axial or equatorial position. Photoelectronic spectroscopy is a useful tool to determine conformational analysis of molecules possessing vicinal electron lone-pairs. Rademiacher and Frickmann (78TL841) studied isoxazolidine and 2-methyl- and 2-t-butyl-isoxazolidine and found mixtures of equatorial and axial (e/a) compounds. The ratios of H, Me and Bu in the efa position were 1 3, 4 1 and 10 1, respectively. 

The axial C—H bonds are weaker flian the equatorial C—H bonds as can be demonstrated by a strongly shifted C—H stretching frequency in the IR spectrum. Axial C-2 and C-6 methyl groins lower the ionization potential of the lone-pair electrons on nitrogen substantially more than do equatorial C-2 or C-6 methyl groups. Ehscuss the relationship between these observations and provide a rationalization in terms of qualitative MO theory. 

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) ]. 

For oxathiane 1, lone pair selectivity is controlled by steric interactions of the gem-dimethyl group and an anomeric effect, which renders the equatorial lone pair less nucleophilic than the axial lone pair. Of the resulting ylide conformations, 25a will be strongly preferred and will react on the more open Re face, since the Si face is blocked by the gem-dimethyl group (Scheme 1.9) [3, 15]. 

The most direct evidence that stereoelectronic effects are also important in these reactions follows from the specificity observed in hydrogen atom abstraction from conformationally constrained compounds,18 60 C-H bonds adjacent to oxygen113"118 or nitrogen110 and which subtend a small dihedral angle with a lone pair orbital (<30°) are considerably activated in relation to those where the dihedral angle is or approaches 90°. Thus, the equatorial H in 20 is reported to be 12 times more reactive towards /-butoxy radicals than the axial 11 in 21.115... 

The substitution of a heteroatom for an a-sulfoxy methylene group substantially increases the preference for an axial orientation of the sulfoxide oxygen320, despite the smaller space requirement of the sulfur with its lone pairs, compared to that of a methylene group321, at least in the case of 1,3-dithiolane oxides. The substituting heteroatom, therefore, should decrease the conformation stability (i.e. lower the barrier to chair-chair interconversion). 

FIGURE 3.4 (.0 A lone pair in an axial position is close to three equatorial atoms, (bi In an equatorial position, a lone pair is close to only two atoms, a more favorable arrangement. 

Avogadro s constant The number of objects per mole of objects (Na = 6.022 14 X 102 mol ). Avogadro s number is the number of objects in one mole of objects (that is, the dimensionless number 6.022 14 XlO2 ). Avogadro s principle The volume of a sample of gas at a given temperature and pressure is proportional to the amount of gas molecules in the sample V n. axial bond A bond that is perpendicular to the molecular plane in a bipyramidal molecule, axial lone pair A lone pair lying on the axis of a bipyramidal molecule. 

Second-row heteroatoms are known to show a substantial anomeric effect. There appears to be evidence for a reverse anomeric effect in 2-aminotetrahydro-pyrans. ° It has been called into question whether a reverse anomeric effect exists at all. ° In 94, the lone-pair electrons assume an axial conformation and there is an anomeric effect. In 95, however, the lone-pair electron orbitals are oriented gauche to both the axial and equatorial oc-CH bond and there is no anomeric effect. ... 

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