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

• New N-[2-chloropropyl]-heterocyclohexanes were synthesised.
• NMR long range shielding effects of chlorine substituents have been observed.
• The use of BH3 as freezing conformational agent is described.

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.