Imine compounds transient imines

Even if the imine may not be isolated, the transient species may sometimes be trapped by reaction with a suitable nucleophile. This is the basis of the reductive amination reaction in which an amine is formed from the reaction of ammonia with a carbonyl compound in the presence of a reducing agent such as sodium borohydride or formate. Use of a primary or secondary amine results in the specific formation of secondary or tertiary amines respectively (Fig. 5-45). This synthetic method allows the preparation of high yields of amines, in contrast to the unselective and uncontrollable reaction of alkylating agents with amines. A specific example involving the preparation of a-phenylethylamine from acetophenone is presented in Fig. 5-46. 

When diethyl malonate is the diketone component, the ester (135) is first formed, which then cyclizes to the expected (136). Reaction of (131) with diketene gives a mixture of (137) and (138), the latter on acid cyclization also yields (134). This rearrangement is by a 1,3-shift of the acetoacetyl group to give the transient imine followed by cyclization. Similar rearrangements are known. Compound (139), the 7-one isomer of (134), is obtained from the reaction of (131) with a-bromocrotonic acid (76JHC291). 

Light-induced intramolecular proton transfer has long been suspected to play an important role in the mechanism for photochromism in both crystals and solutions. One class of compounds that has been extensively studied is the A -salicylidene anilines. The main evidence for invoking an excited-state proton-transfer step in the photochromic mechanism comes from the observation that the colored species exists as the keto amine tautomer (in the ground state) while the bleached form exists as an enol imine (in the ground state). Replacement of the labile hydrogen with a methyl group prevents the photochemical reaction that leads to the colored species. Picosecond transient spectroscopy was used to study iV-salycylidene-o-toluidine (NST) and the related compound 2-(o-hydroxyphenyl)benzothiazole (HBT). ... 

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