Amines inversion

In contrast to the amines, inversion of configuration for phosphines is generally negligibly slow at ambient temperature. This property has made it possible for chiral phosphines to be highly useful as ligands in transition metal-catalyzed asymmetric syntheses. 

If one of the four groups attached to nitrogen is a lone pair, the enantiomers cannot be separated because they interconvert rapidly at room temperature. This is called amine inversion (Section 21.2). One way to picture amine inversion is to compare it to an umbrella that turns inside out in a windstorm. 

Although the activation enthalpy of this process AG = 70 kJ mol i.e. substantially greater than with a secondary aliphatic amine, inversion occurs so rapidly at room temperature that the diastereo-isomers are not separable. However, in the case of l-chloro-2-methylaziridine, where AG = 112 kJ mol , the mixture of stereoisomers can be separated. 

The most important structural feature of amines comes from the presence of the pair of nonbonding electrons acting as the fourth substituent on the pyramid. In alkanes, where there are four substituents, there is nothing more to consider once the pyramid is described. In amines there is, because the pyramid can undergo an umbrella flip, amine inversion, forming a new, mirror-image pyramid (Rg. 6.39). 

FIGURE 6.39 Amine inversion interconverts enantiomeric amines. 

FIGURE 6.40 The transition state for amine inversion is planar. 

Amine inversion (Section 6.7) The conversion of one pyramidal form of an amine into the other through a planar, hybridized transition state. 

Amine inversion takes place through a transition state in which the sfp nitrogen becomes an nitrogen, The three groups bonded to the sp nitrogen lie in a plane and the lone pair is in a p orbital. The "inverted" and "noninverted" amine molecules are enantiomers. 

The energy required for amine inversion is approximately 6 kcal/mol (or 25 kJ/mol), about twice the amount of energy required for rotation about a carbon-carbon single bond, but still small enough to allow the enantiomers to interconvert rapidly at room temperature. As a result, the enantiomers cannot be separated. 

When hydride is transferred from ruthenium to carbon, a new lone pair is generated perpendicular to the C=N plane in one of the two orientations. The preferred orientation of the newly generated lone pair apparently results from steric effects pushing the amine substituents away from the sterically crowded dienone. The observed stereospecificity requires that amine inversion is slow compared to amine coordination. 

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