1,3-dipoles, resonating structures

In acetovinyl derivative 54 (Scheme 56), the bond-length alternation (as it is known from parent azulenothiophene 45a) almost disappears this fact indicates a considerable contribution of the dipole resonance structure 233 (99TL6609). 

The 1,3-dipolar molecules are isoelectronic with the allyl anion and have four electrons in a n system encompassing the 1,3-dipole. Some typical 1,3-dipolar species are shown in Scheme 11.4. It should be noted that all have one or more resonance structures showing the characteristic 1,3-dipole. The dipolarophiles are typically alkenes or alkynes, but all that is essential is a tc bond. The reactivity of dipolarophiles depends both on the substituents present on the n bond and on the nature of the 1,3-dipole involved in the reaction. Because of the wide range of structures that can serve either as a 1,3-dipole or as a dipolarophile, the 1,3-dipolar cycloaddition is a very useful reaction for the construction of five-membered heterocyclic rings. 

Azulene, an isomer of naphthalene, lias a remarkably large dipole moment for a hydrocarbon (/i = 1.0 D). Explain, using resonance structures. 

Hydrogen abstraction reactions potential surfaces for, 25-26,26,41 resonance structures for, 24 Hydrogen atom, 2 Hydrogen bonds, 169,184 Hydrogen fluoride, 19-20, 20,22-23 Hydrogen molecules, 15-18 energy of, 11,16,17 Hamiltonian for, 4,15-16 induced dipoles, 75,125 lithium ion effect on, 12... 

In Chapter 10 of Part A, the mechanistic classification of 1,3-dipolar cycloadditions as concerted cycloadditions was developed. Dipolar cycloaddition reactions are useful both for syntheses of heterocyclic compounds and for carbon-carbon bond formation. Table 6.2 lists some of the types of molecules that are capable of dipolar cycloaddition. These molecules, which are called 1,3-dipoles, have it electron systems that are isoelectronic with allyl or propargyl anions, consisting of two filled and one empty orbital. Each molecule has at least one charge-separated resonance structure with opposite charges in a 1,3-relationship, and it is this structural feature that leads to the name 1,3-dipolar cycloadditions for this class of reactions.136... 

Structure I is the most important of the three. A covalent azide such as HN3 (dipole moment = 1.70 D) can be represented by the resonance structures... 

The first nitrile ylide stable enough to be isolated (i.e., 1) has been prepared by the carbene/nitrile method (1). For this dipole, the anionic component is stabilized by electron delocalization and the nitrilium component by the steric bulk of the adamantyl group to such an effect that it has a melting point of 230 °C. The X-ray structure showed that the nitrile ylide moiety is close to linear and much like the resonance structure shown below. 

Azulene can be written as fused cyclopentadiene and cycloheptatriene rings, neither of which alone is aromatic. However, some of its resonance structures have a fused cyclopentadienyl anion and cycloheptatrienyl cation, which accounts for its aromaticity and its dipole moment of 1.0 D. 

Problem 16.4 Write resonance structures for the COOH group and show how these and orbital hybridization account for (a) polarity and dipole moments (1.7-1.9D) of carboxylic acids (b) their low reactivity toward nucleophilic additions, as compared to carbonyl compounds. ... 

Van der Waals forces can be grouped into three major classes dipole-dipole, dipole-induced dipole, and dispersion or London forces. Complexes stabilized by van der Waals forces should be characterized by the juxtaposition of dipoles. In caffeine, resonance structures such as ... 

Thi3 resonating structure of the molecule would lead to the observed value of the electric dipole moment if each bond (including the bent bonds) had 10 percent partial ionic character. This amount of ionic character is somewhat less than the amount given in Table 3-10 as that applying to a carbon-oxygen bond (22 percent). We conclude that the structure derived from the observed bond length is compatible with the observed electric dipole moment, to within the uncertainty of the calculation. 

The contribution of structure III would be expected to amount to a few percent for BrNO and C1NO, and much more for FNO, because of the greater stability of the fluorine-nitrogen double covalent bond. A resonating structure with 50 percent contribution of II, 25 percent of I, and 25 percent of III is compatible with both the observed F—N distance in FNO and the observed value18 1.81 D of the electric dipole moment. 

Mesoionic compounds have been known for many years and have been extensively utilized as substrates in 1,3-dipolar cycloadditions.158-160 Of the known mesoionic heterocycles, munchnones and sydnones have generated the most interest in recent years. These heterocyclic dipoles contain a mesoionic aromatic system i.e. 206) which can only be depicted with polar resonance structures.158 Although sydnones were extensively investigated after their initial discoveiy in 1935,160 their 1,3-dipolar character was not recognized until the azomethine imine system was spotted in the middle structure of (206). C-Methyl-N-phenylsydnone (206) combines with ethyl phenylpropiolate to give the tetrasub-... 

The classical structure of a pseudoazulene of the [h]-series (91) is inconsistent with its aromatic character and important dipole moments. A set of resonance structures involving dipolar forms such as 91a and 91b as contri-... 

FIGURE 2. Analysis of the experimental dipole moment (E) of A. A -di methyl lomiamide and/or N,N-dimethylbenzamide in terms of the resonance structures 78a and 78b (A and B) and of the mesomeric dipole moment (pm) according to various approximations... 

The extreme forms of resonance structures in ozone molecules can be represented as shown in Figure 4.7. This structure illustrates that the ozone molecule has two types of dipoles, which can serve as electrophilic and nucleophilic agents, respectively. In organic solvents, the following three reactions can be observed ... 

The unique spectral absorption of the Rhodonines contains two visual band components, an isotropic absorption associated with the conjugated dipole molecular structure of the molecule, and a anisotropic absorption associated with an additional resonant slow-wave stmcture intimately associated with the triplet electrons of the oxygen atoms of the molecule. The unusual relaxation properties of these molecules are also associated with these triplet state electrons. The Rhodonines do not fluoresce or phosphoresce significantly while in a dilute liquid solution. 

IR spectra show that the C-0 band has a very low ketonic character. These results tend to show the significance of l-oxa-6,6a IV-dithia-pentalene resonance structures. However, compounds containing one oxygen atom have generally larger dipole moments than the com-pouuds containing only sulfur. 

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