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The invention provides electrophilic monohalomethylating reagents, methods for their prepn. and methods for prepn. of monohalomethylated biol. active compds. using such reagents. Typical monohalomethyl groups transferred are FH2C-, CIH2C- and others. The reagents used for transferal of the groups are described by Formulas A-D: wherein: X=F, Cl, Br, I, sulfonate esters, phosphate esters or another leaving group; R11=tetrafluoroborates, inflates, halogen, perchlorate, sulfates, phosphates or carbonates The other variables are as defined in the claims.