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27.8.1.15.2 Variation 2: Of Aromatic Compounds

DOI: 10.1055/sos-SD-027-00291

Abbaspour Tehrani, K.; De Kimpe, N.Science of Synthesis, (200427340.

Despite the importance of the Vilsmeier reaction in the formylation of 1,1-dialkylanilines, in only one case has the intermediate iminium salt been isolated, and then only in 8% yield.[‌155‌] Vilsmeier formylation of fluorenes with equimolar amounts of phosphoryl chloride and dimethylformamide also leads to isolable iminium salts in high yields.[‌156‌] Pyrrole and 1-methylpyrrole undergo reaction with N-chloromethylene-N,N-dimethylammonium chloride in the α-position [the former to give iminium chloride 101 (Scheme 32)], while N-(triisopropylsilyl)-protected pyrroles can be formylated in the β-position.[‌157‌] N,N-diisopropylformamide can be used in combination with oxalyl chloride to formylate pyrrole.[‌158‌] The Vilsmeier complex of phosgenedimethylformamide does not attack 4-methylpyridine (102) at a ring carbon, but instead reacts selectively at the 4-methyl group to give compound 103 (Scheme 32).[‌159‌]

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