Amines

AMINES

Amines are saturated, nitrogen-containing functional groups that are widely en-countered. Because the nitrogen atom of amines is basic, nucleophilic, and oxidizable, some constraints on the preparation of amines result. A collection of textbook amine preparations includes the following:

Amines are at the same low oxidation level as alcohols and consequently are easily prepared by reduction. Amides and nitriles are reduced efficiently by LAH to amines. Nitriles give only primary amines while amides give 1^◦ , 2^◦ , or 3^◦ amines depending on the number of carbon substituents on the amide nitrogen. The advantage of this method is that amides are easy to prepare from acid chlorides and amines while nitriles are available by displacement reactions.

One problem with this method is that the workup must be done carefully as the amine products tend to complex tenaciously with the aluminum salts formed from the LAH upon workup and thus are not recovered easily. There are standard workups which avoid these issues, but these should be followed carefully. Reduction of azides by catalytic reduction, phosphine or phosphite reagents, or Sn(II) chloride are all effective methods. The azides are also available from displacement reactions and give primary amines upon reduction.

Aromatic amines are readily prepared by the reduction of aromatic nitro groups by Fe/HCl or Sn/HCl or by catalytic hydrogenation.

Displacement reactions are rarely used for the preparation of amines as poly-alkylation reduces yields and makes product mixtures more complex. However, reaction of primary amines with primary and secondary sulfonates can provide good yields of monoalkylated product if care is taken to control the conditions and mode of addition. Benzylamine is particularly common as a primary amine nucleophile since the benzyl group can be removed by hydrogenolysis to give a primary amine.