Reactivity of Dithiazinanes towards BH3, BD3 and BF3. New Heterocycles: 5,5‐Dimethyl‐1,3‐dithia‐5‐azonia‐4‐boratacyclohexane and 6,6‐Dideuterio‐5‐methyl‐5[D1]methyl‐1,3‐dithia‐5azonia‐4‐boratacyclohexane. A Method for the Dimethylation and Monodeuteriomethylation of Primary Amines