New N-[2-chloropropyl]-heterocyclohexanes. NMR long range shielding effects of chlorine substituent. Use of BH₃ as freezing conformational agent

The syntheses of the new compounds: 5-(2-chloropropyl)-[1,3,5]-dithiazinane (1), 3,5-di-(2-chloropropyl)-[1,3,5]-thiadiazinane (2) and 1,3,5-tri-(2-chloropropyl)-[1,3,5]-triazinane (3) and their N-borane adducts 1(BH₃), 2(BH₃)₂ and 3(BH₃)₂ are reported. The heterocyclohexanes are derived from the enantiomerically pure (S)-1-chloro-propylamine hydrochloride. NMR analyses in CD₂Cl₂ or TDF at rt indicate that heterocycles 1-3 are fluxional molecules whereas their corresponding borane adducts recorded in the same NMR conditions showed anchored conformations. The ¹H spectrum of the N-borane adducts gave evidence that chlorine atoms produce γ and ε shielding electronic effects on protons. These effects have been rarely reported and only observed in rigid cyclic molecules. The ¹³C spectra showed another uncommon long range effect, a chlorine δ shielding effect. The preferred conformations that produced these chlorine chemical shift effects were determined with the aid of optimized conformers by using B3LYP/6-311 + G∗density functional calculations and from data of X-ray diffraction analysis of compound 2. Compounds 1(BH₃), 2(BH₃)₂ and 3(BH₃)₂ at room temperature in solution are slowly transformed into reduced products which are described.