Dipole repulsion between

Several structural factors have been considered as possible causes of the anomeric effect. In localized valence bond terminology, it can be recognized that there will be a dipole-dipole repulsion between the polar bonds at the anomeric carbon in the equatorial conformation. This dipole-dipole interaction is reduced in the axial conformation, and this factor probably contributes to the solvent dependence of the anomeric effect. 

Chloroketone (33) prefers to adopt the axial conformation (33a) because of dipole repulsion between C-Cl and C=0, The less hindered side of the carbonyl group is therefore trana to the chlorine giving (35) in the right conformation (35a) - trans diaxlal - for rearrangement. 

It was shown (801) that the diastereoselectivity of a-fluoroalkyl nitrones is reversed to that of the corresponding a-alkyl nitrones. This fact supports the conclusion that the conformation, due to the relief of the dipole repulsion between the fluorine atom and the oxygen atom of the nitrone is preferred in a-fluoroalkyl nitrones. 

The results obtained clearly demonstrated that the initially formed rhodium carbenoid prefers to cyclize onto the adjacent imide carbonyl group to form an isomunchnone rather than undergo C-H insertion. The explanation proposed to rationalize this result is that the preferred rhodium carbenoid conformer 79 is the one which avoids unfavorable dipole repulsion between the two carbonyl groups of the imide (i.e. 80). The conformational... 

The initial disclosure of a [6 + 4] cycloaddition between tropone (1) and a 1,3-dipole involved diphe-nylnitrilimine as depicted in Scheme In this instance the [6 + 4] pathway competed rather poorly with various alternative [4 + 2] pathways and only a small quantity of the adduct in which the dipole added across the 2- and 7-positions of (1) was recovered. The three [4 + 2] adducts (45)-(d7) that were isolated from the reaction mixture presumably arose from a base-catdyzed hydrogen shift that occurred subsequent to the initial cycloaddition. Efforts to account for the divergent behavior of dienes and 1,3-dipolar species in their reactions with tropone have included invoking a dipole repulsion between the large positive charge located on the central atom of the 1,3-dipole and the partial positive charge on tropone, which must come into close proximity in the transition state of a concerted [6 + 4] cycloaddition. A ca-... 

We developed a novel method for a highly stereoselective differentiation between two identical groups in prochiral ir-symmetric dicarboxylic acids based on a completely new idea employing (4/ )-MCTT(1). The dipole-dipole repulsion between the carbonyl and the thiocarbonyl groups in the (4/ )-MCTT amide system was utilized in order to regulate the free rotatory molecule in the transition state for chiral induction (see Fig. 2). 

Fig. 2. Dipole-dipole repulsion between carbonyl and thiocarbonyl groups.

Two explanations have been advanced to account for the anomeric effect. The first involves the stabilizing effect of bonding interactions between n electrons on one oxygen atom and the a orbital of the bond connecting the other oxygen atom and the central anomeric carbon atom [58]. The second involves destabilizing dipole-dipole repulsion between the two oxygen atoms and their lone pairs [54]. The relative importance of these factors has been difficult to establish. 

Chloral Hydrate, USP. Chloral hydrate, trichloroacetal-dehyde monohydrate. CCIjCH(OH)2 (Noctec). is an aldehyde hydrate stable enough to be isolated. The relative stability of this gem-dioi is largely due to an unfavorable dipole-dipole repulsion between the trichloromethyl carbon and the carbonyl carbon present in the parent carbonyl compound. ... 

The tendency to form lamellar structures can be understood as the result of dipole-dipole repulsions between the uniaxially ordered amphiphile molecules theoretical analyses of the pattern formation based on this principle have been carried out. It appears that the lamellar thickness is defined by the equilibrium thermodynamic conditions (Fig. 23) but that the shape and density of the patterns are determined by the dynamics of the process by which they are formed. 

From the NMR work of Karahatsos and coworkers on substituted acetaldehydes (39., +0 ), and from the infrared studies of Brown (]+l ) and Laato and Isotalo b2) with a-substituted carboxylic acid esters, it can be concluded that in solution the equilibrium between the rotamers in Figure 2 where R is CH3 and X is Cl favours rotamer II in solvents of low dielectric with rotamer III least favoured, and favors rotamer I in solvents of high dielectric. The solvents with high dielectric attenuate the dipole-dipole repulsion between the C-Cl and C=0 groups, which is maximized in rotamer I, and hence shift the equilibrium towards rotamer I. If the temperature of the solution is decreased, the associated increase in dielectric would tend to shift the equilibrium towards rotamer I. 

A second major rationalization of the anomeric effect is based on anticipated dipole-dipole repulsion between the two electronegative atoms (one part of the ring, the other exocyclic) and their associated lone electron pairs in the equatorial isomer, with the consequent preference for the axial conformation with the smallest dipole moment (Scheme 3) (55C1L1102, 93M14,95M15,95M17, 99M18). 

The a,P-unsaturated sulfoxides (174a), (174b), and (174c) (Table 5.5), in which the s-trans conformers are the most stable owing to dipole-dipole repulsion between the S=0 and C=0 bonds and to steric repulsion, exhibited high diastereoselectivities, and the relative configurations of the cycloadducts could be explained by the addition of diene to the less hindered face of the dienophilic double bond, containing the lone-pair electrons on sulfur. 

The dienophile (175) was considered to exist mainly in the s-cis conformation owing to dipole-dipole repulsion between the S=0 and C=0 bonds and also exhibited high diastereoselectivities in the Diels-Alder reaction, the major cycloadducts arising by attack of the diene from the sulfur lone-pair side of the double bond. On the other hand, dienophiles (176a) and (176b), which could exist in both s-trans and s-cis conformations because of the absence of a- or (Z)-P substituents, exhibited low diastereoselectivities upon cycloaddition. 

It will be of interest to note at this point some facts regarding the cause or basis of the anomeric effect. There has been considerable discussion regarding this point. It is evident from Table 7.5 (and see also Table 7.6) that the value of the anomeric effect in the gas phase is substantial, with an energy difference between the most favorable axial and equatorial conformations of 1.42 kcal/mol. But also note that the value is reduced to about half (0.76 kcal/mol) at a dielectric constant of 20. This indicates that the anomeric effect contains a substantial contribution from electrostatics (i.e., dipole-dipole repulsions between the C-O bonds, and preferential solvation of the equatorial conformation). But they also show that a sizable portion of the effect is from something other than simple electrostatics. If the cause were simply electrostatics, the 1.42 kcal/mol number would be reduced to a sizable negative number, and while there is substantial energy reduction, it does not go nearly that far. 

Tu et al. reported a Macmillan s catalyst 172-catalyzed asymmetric a-alkylation of tetrahydrofuran 170 containing an a,p-unsaturated aldehyde, via which chiral spiroether 171 could be prepared (Scheme 64) [129]. The sequential [l,5]-hydride transfer/cyclization was facilitated via cascade iminium/enamine activation. The presence of strong acid was indispensable to ensure sufficient electrophilicity of the iminium intermediate. Theoretically, substrate 170 reacts with 172 to give iminium intermediate I. Owing to the steric interaction of the bulky ferf-butyl group, the E enamine II is formed preferentially upon [1,5]-HT, which exists in two possible conformers III and IV. Because of dipole repulsion between the cyclic-oxocarbe-nium and enamine moieties in conformer III, IV is the more favored conformer, which undergoes intramolecular C-C bond formation to afford the final product 171. 

An alkyl radical is produced by the reaction of tributylstannyl radical and iodoalkanes to trigger the reaction. An alkyl radical reacts with stannyl enol ethers via Sh2 mechanism to give an fl -alkyl ketone and regenerate a stannyl radical. In the presence of an electron-deficient alkene like fumarate and maleate, an alkyl radical adds to the alkene to generate a radical whose conformation is governed by a dipole-dipole repulsion between the two ester carbonyl groups. Subsequent reaction with the stannyl enol ether occurs preferentially in the opposite side to R to give an anti (erythro) adduct preferentially (Scheme 3-198). 

The balance point in the equilibrium between (Z)-l and (E)-l appears to depend on both steric and electrostatic factors (Table 6.1) [2a,b]. (E)-l has steric repulsion between R and the ester group, whereas the (Z)-isomer suffers from dipole-dipole repulsion between the bond and ester... 

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