Dipole first examples

The first example will be the separation of a ferredoxin mixture using a bonded phase that contains aromatic nuclei as well as aliphatic chains. The stationary phase will thus, exhibit polar interaction from induced dipoles if the aromatic ring comes into contact with a strong dipoles of the solute and, at the same time, exhibit dispersive interactions between the aliphatic chains and any dispersive centers of the solute molecule. An example of the separation obtained is shown in figure 16. 

Another example of a microwave-assisted 1,3-dipolar cycloaddition using azomethine ylides and a dipolarophile was the intramolecular reaction reported for the synthesis of hexahydrochromeno[4,3-fo]pyrrolidine 105 [70]. It was the first example of a solvent-free microwave-assisted intramoleciflar 1,3-dipolar cycloaddition of azomethine ylides, obtained from aromatic aldehyde 102 and IM-substituted glycinate 103 (Scheme 36). The dipole was generated in situ (independently from the presence of a base like TEA) and reacted directly with the dipolarophile present within the same molecifle. The intramolecu-... 

The First Examples of Transition Metal-Mediated 1,3-Dipole Formation... 

Mercaptophenylacetic acid, acetic anhydride, and sodium nitrite yielded the first example (169, R = Ph) of a meso-ionic 1,3,2-oxathiazol-5-one this compound has a dipole moment of 4.5 D. An X-ray crystallographic investigation of the 4-phenyl-l,3,2-oxathiazol-5-one (169, R = Ph) has been reported. This compound is incorrectly described as a thiosydnone. The bond distances support the structure with tetracovalent sulfur, and on this evidence alone the compound 169, R = Ph, could be judged not to be meso-ionic because it can, on the basis of the X-ray evidence, be more satisfactorily represented by a covalent formulation. 

The use of chiral azomethine imines in asymmetric 1,3-dipolar cycloadditions with alkenes is limited. In the first example of this reaction, chiral azomethine imines were applied for the stereoselective synthesis of C-nucleosides (100-102). Recent work by Hus son and co-workers (103) showed the application of the chiral template 66 for the formation of a new enantiopure azomethine imine (Scheme 12.23). This template is very similar to the azomethine ylide precursor 52 described in Scheme 12.19. In the presence of benzaldehyde at elevated temperature, the azomethine imine 67 is formed. 1,3-Dipole 67 was subjected to reactions with a series of electron-deficient alkenes and alkynes and the reactions proceeded in several cases with very high selectivities. Most interestingly, it was also demonstrated that the azomethine imine underwent reaction with the electronically neutral 1-octene as shown in Scheme 12.23. Although a long reaction time was required, compound 68 was obtained as the only detectable regio- and diastereomer in 50% yield. This pioneering work demonstrates that there are several opportunities for the development of new highly selective reactions of azomethine imines (103). 

The locked permanent-dipole capture system may serve as our first example for an isotropic potential where SACM and PST coincide. In this case, the adiabatic channel potential curves have the form... 

The cycloaddition of azides to multiple -ir-bonds is an old and widely used reaction. Organic azides are well known to behave as 1,3-dipoles in thermal cycloaddition reactions.178 The first example of this reaction was observed by Michael in 1893.179 Since then the addition of azide to carbon-carbon double and triple bonds has become the most important synthetic route to 1,2,3-triazoles, -triazolines and their derivatives.180-184 The cycloadditions of simple organic azides with electron-rich dipolarophiles are LUMO controlled.3 Since the larger terminal coefficients are on the unsubstituted nitrogen in the azide and unsubstituted terminus in the dipolarophiles, the 5-substituted A2-triazolines are favored, in agreement with experiment.185-187 Reactions with electron-deficient dipolarophiles are HOMO controlled, and... 

This is the first example we will encounter of selection rules for allowed transitions. Physically, the selection rules arise because the electric field needs a dipole moment in order to interact with the atom, and only these specific changes in the quantum numbers create a dipole moment. More generally, the selection rules for absorption in a hydrogen atom are... 

Nitrones were the first as well as the most widely used dipoles in asymmetric cycloadditions. The first report on the use of enantiomerically pure vinylsulf-oxides as dipolarophiles was due to Koizumi et al. [153], who described in 1982 the reaction of (-R)-vinyl p-tolyl sulfoxide 1 with acyclic nitrones 191. The reactions required 20 h in refluxing benzene to be completed, yielding a mixture of only two compounds, 192 and 193 (Scheme 91). They exhibited identical endo or exo stereochemistry (which was not unequivocally assigned), deduced from the fact that their reduction yielded enantiomeric thioethers. The major component, 192, exhibits (S) configuration at C-3, determined by chemical correlation. The authors claim this paper [153] to be the first example of 1,3-dipolar cycloaddition using chiral dipolarophiles. 

As early as 1965, the first example of the the carbon-carbon bond cleavage of an aziridine generating an azomethine ylide 1,3-dipole was reported by Heine and Peavy (65TL3123). This also offers the first generation of acyclic azomethine ylides by the aziridine route. Thus, 1,2,3-triphenylaziridine, with its stereochemistry unspecified, was heated under reflux in toluene in the presence of diethyl acetylenedicarboxylate. The azomethine ylide (1) generated was captured by the acetylene to give a quantitative yield of diethyl l,2,5-triphenyl-3-pyrroline-3,4-dicarboxylate. 

Meanwhile, let us consider a few simple examples, illustrating the spatial behavior of the migrated electromagnetic fields. The first example represents the results of migration of the magnetic component of the field generated by a local horizontal electric dipole, located at some depth, zq, in the homogeneous lower half-space of conductivity ab- The current in the dipole is described by the delta-pulse ... 

Both involve the interaction of an ion with a dipole. In the first case, the dipole is permanent (preexisting), whereas in the second, it is induced by the approach of the ion. Induced dipole forces are weaker than ion-dipole forces. Examples Na with HCl (ion-dipole) Na with CI2 (induced dipole). 

As a first example consider birefringence in an electric field by a gas of molecules with D3h symmetry and a twofold degenerate ground electronic term belonging to representation E. Using the explicit form of the matrices of the components of the operators of the dipole moment [Eq. (20)] and polarizability and by means of Eqs. (84) and (85a)-(85c), we obtain for the molecular constant in the Kerr effect in the E term case under consideration the expression... 

When f) 7 g), the matrix element in the middle will equal the one-photon transition moment between the two excited states /) and ). On the other hand, when I/) = g) the same matrix element will equal the difference between the dipole moment of the excited state /) and the ground state 0). This provides a first example of the possibility of extracting lower-order properties of molecular excited states from higher-order ground state properties. 

It is instructive to take as a first example the general expression for molecular polarizability, the response tensor that formally mediates elastic light scattering in the electric dipole approximation. The result is obtained by application of Eq. (74) with m = 2 (one photon is annihilated and another of the same frequency is created). Here there are only two time orderings, or state-sequence pathways, as illustrated in Figs. 5 and 6, respectively. Each generates a term whose numerator is a product of transition dipole moment components. For... 

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