Lone pairs electron groups with

Four Electron Groups with Lone Pairs... 

Lewis acid catalysis enormously enriches the scope of Diels-Alder reactions, but it is limited to reagents containing Lewis basic sites, i.e. functional groups with lone pairs such as carbonyl, amino, ether or nitro close to the reaction centre. As we have seen in the discussion about the FMO aspects of Lewis acids, the major reason for catalysis is the reduction of the HOMO-LUMO gap. In case of Diels-Alder reactions with normal electron demand, it follows that the coordination of the Lewis acid lowers the LUMO energy of the dienophile. Such interactions are only possible if there is a spatial proximity or an electronic conjugation between the coordinated Lewis basic site and the reaction centre. Fortunately, in nearly every Diels-Alder reaction one of the reagents, mostly the dienophile, meets this requirement. 

Substituted dialkylalanes of the type R2AIX, where X is an atom or a group with lone-pair electrons like NR2, OR or F, associate through the formation of N, 0, or F bridges. 

Effective electron donors include atoms or functional groups with lone pairs on their attachment points and functional groups containing atoms with low electronegativities. 

A nucleophile is a molecule that donates electrons (to an electrophile) in a chemical reaction. These are typically functional groups with lone pairs of electrons, but can also be n bonds, or, in some rare cases, o bonds. 

In the electron-dot formula of water, H2O, there are also four electron groups, which have minimal repulsion when the electron-group geometry is tetrahedral. However, in H2O, two of the electron groups are lone pairs of electrons. Because the shape of H2O is determined by the two H atoms bonded to the central O atom, the H2O molecule has a bent shape with a bond angle of 109°. Table 10.3 gives the molecular shapes for molecules with two, three, and four bonded atoms. 

Some compounds in which a nitrogen atom bonds direcdy to electronegative groups with lone pair electrons form stable mine derivatives. These compounds include hydroxylamine, hydrazine, and substituted amine such as semicarbazide and 2,4-dinitrophenylhydrazine. 

The high electrophilicity of the positively charged element can be modified by intramolecular donation from remote donor substituents. This interaction leads to solvent-free cations with coordination numbers for the positively charged element > 3 and to a considerable electron transfer from the donor group to the element. Frequently used donor substituents utilize heteroatoms with lone pairs (e.g. amino, hydrazino, methoxy, carboxy, phosphino, etc.), in many cases in combination with pincer-type topology of the ligand, for the stabilization of the cationic center. These strongly stabilized cations are beyond the scope of this review and instead we will concentrate on few examples where we have weak donors such as CC multiple bonds, which stabilize the electron-deficient element atom. 

Groups that are particularly strong electron releasers do not achieve this by an inductive effect, but they have heteroatoms with lone pair electrons that are able to stabilize resonance structures by transferring the charge to the heteroatom, i.e. an electron-releasing resonance effect. An amino group is typical of this type of substituent. 

Little is known about the extent to which heteroatoms can be incorporated into the conjugated chain of a homoconjugative system. In principle it should be possible to include heteroatoms with lone-pair electrons that can contribute their n-type electrons. Alternatively, replacement of the CH2+ group by BH2 or other groups with empty pir-orbitals should also lead to a retention of homoconjugation (see the next chapter3). 

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