Other Dipoles

Nitrile imines are related to azomethine imines, in the same manner as nitrile oxides are related to nitrones. In a single and recent report, the reactions of D-galactose derived chiral nitrile imines have been described (104). However, in reactions with nonchiral alkenes, no diastereoselection was obtained. 

3-dipolar cycloadditions of 1,3-dipoles with chiral alkenes has been extensively reviewed and thus only selected examples will be highlighted here. We have chosen to divide this section on the basis of the different types of alkenes rather than on the basis of the type of 1,3-dipole. For 1,3-dipolar cycloadditions, as well as for other reactions, it is important that the chiral center intended to control the stereoselectivity of the reaction is located as close as possible to the functional group of the molecule at which the reaction takes place. Hence, alkenes bearing the chiral center vicinal to the double bond are most frequently apphed in asymmetric 1,3-dipolar cycloadditions. Examples of the application of alkenes with the chiral center localized two or more bonds apart from the alkene will also be mentioned. Application of chiral auxiliaries for alkenes is very common and will be described separately in Section 12.3. 

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In condensed phases, the separation between molecules is of the order of the molecular dimensions and the result is that each molecule is polarized not by just the ordinary applied electric field E but by a local field F consisting of E plus the fields of all the other dipoles. Once the local field is known, we can use equation 15.8 to find the polarization, simply by substituting aF for aE. The calculation of F is difficult because the dipoles that contribute to F are themselves determined by F and a self-consistent treatment is necessary. This is achieved by relating F to P, ultimately giving an equation for Xe-... 

Intermolecular forces may be caused by a solute molecule having a dipole moment, when it can interact selectively with other dipoles. If a molecule is a good proton donor or acceptor it can interact with other such molecules by hydrogen bonding. Molecules can also interact via much weaker dispersion forces which rely on a given molecule being polarised by another molecule. 

Hydrogen bond forces. The hydrogen bond is a special case of dipole-dipole interaction but is often spoken of separately because most hydrogen bonds are more energetic than other dipole interactions. 

Similarly, nitrile oxides react with methyl acrylate 2.42 to give the adduct 2.43 with the substituent on C-5 and terminal alkenes also react in this way to place the alkyl group on C-5. Many dipoles react well with electron-rich dipolarophiles, but not with electron-poor dipolarophiles. Other dipoles are the other way round. To make matters even more complex, the presence of substituents on the dipole can change these patterns and impart their own regioselectivity. Thus the carbonyl ylid reaction 2.45 has a well defined regiochemistry determined only by the substituents, since the core dipole is symmetrical. This reaction also illustrates the point that dipolarophiles do not have to be alkenes or alkynes—they can also have heteroatoms. 

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