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27.7.3.1.2 Method 2: Reaction of Imidates with Organometallic Reagents

DOI: 10.1055/sos-SD-027-00230

Abbaspour Tehrani, K.; De Kimpe, N.Science of Synthesis, (200427264.

Imidates are mostly employed for the preparation of cyclic imines.[‌175‌] The reaction of N-(ethoxymethylene)aniline with 2 equivalents of phenylmagnesium bromide gives rise to N-benzylideneaniline in almost quantitative yield. The sole formation of this imine can be explained by the stability of the initial adduct of the imidate with the Grignard reagent.[‌176‌] The same reaction performed with old refrigerated solutions of phenylmagnesium bromide, containing the more reactive diphenylmagnesium, results in double addition reactions to the imidate. If alkyllithium reagents are used in this reaction, double addition to the imidate function is always observed. The reaction of 5-methoxy- and 5-ethoxy-3,4-dihydro-2H-pyrrole with alkylmagnesium halides also leads to the 5-alkyl-3,4-dihydro-2H-pyrroles,[‌177‌] the yields increasing from methyl- to ethyl-, propyl-, and phenylmagnesium bromide or iodide. The reaction of cyclic imidates (lactim ethers) 30 with alkyllithium reagents, on the other hand, can also lead to cyclic alkylated imines 31 (Scheme 16).[‌178‌] In this case, however, yields can be lowered by the formation of cyclic α,α-dialkylated amine derivatives 32 and the presence of unreacted starting material. This method, however, fails to provide 6-methyl-2,3,4,5-tetrahydropyridine and 7-methyl-3,4,5,6-tetrahydro-2H-azepine. Similarly, cyclic S-methyl thioimidates undergo replacement of the methylsulfanyl group with an alkyl group by reaction with heptylmagnesium bromide,[‌179‌] butylmagnesium chloride, ethylmagnesium bromide,[‌180‌] or pent-4-enylmagnesium chloride.[‌181‌] The preferred solvent for this conversion is dichloromethane, since the reaction completely fails in tetrahydrofuran or diethyl ether.

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