SolutionsAmines and anilines
Exercise 1
Propose two syntheses of 2-phenylethylamine, PhCH2CH2NH2: one from benzyl bromide and one from 2-phenylethanol. Why is it not advisable to treat 2-phenylethyl bromide with ammonia?
From benzyl bromide: NaCN gives phenylacetonitrile, PhCH2CN, which LiAlH4 (or H2 and Raney nickel) reduces to the amine, with one carbon more. From 2-phenylethanol: it is converted into the tosylate or the bromide, displaced with NaN3, and the azide is reduced with H2/Pd or with PPh3 and water (Staudinger). Ammonia would give mixtures of primary, secondary and tertiary amines, because the amine formed is more nucleophilic than ammonia.
Exercise 2
Which amine is obtained by reductive amination of acetone with (a) ammonia and (b) benzylamine? Why is NaBH(OAc)3 used rather than NaBH4?
(a) Isopropylamine, (CH3)2CHNH2. (b) N-Isopropylbenzylamine, PhCH2NHCH(CH3)2. NaBH4 would rapidly reduce acetone to propan-2-ol before the imine had formed. The electron-withdrawing acetoxy groups make triacetoxyborohydride a much weaker reducing agent, which reduces the more electrophilic iminium ion but hardly touches the ketone.