Non-stereospecificity in neutral 1,1-addition to isocyanides. An ab initio study of the reactions of HNC with water, ammonia, water dimer, ammonia dimer, and the water–ammonia complex

Abstract

Ab initio calculations showed that the insertion reaction into the N–H bond of ammonia and the O–H bond of water with HNC as substrate is not stereospecific. Both reactions are mildly exothermic, but the enrgy barrier is high (ca. 67 kcal mol^–1 for H₂O at MP4SDQ/6–31G**). Addition of polarization functionss to the basis set increase and correlation energies decrease the barrier. These effects essentially cancel so that the simple basis set (3–21G) gives a good estimate of the transition state enrgy barrier. Such large energy barriers render these additions unlikely. However when a second molecule of H₂O or NH₃ is present to act as a catalyst then addition takes place via a compact five-membered ring. For the addition of NH_3′ H₂O is the favoured catalyst (rather than a second NH₃ molecule). With (H₂O)₂ addition to HNC to give formimidic acid, the proton transfers are synchronous while formamidine formation with ammonia is asynchronous [e.g. for (NH₃)_2′ the second NH₃ acts as a basic catalyst while for NH₃·H₂O addition, H₂O acts as an acid]. In all reactions considered, the transition structures can have either Z- or E-configuration with respect to the C–N bond, but the corresponding C–N–H bending mode (responsible for the nitrogen inversion) is quasi-free. Both isomers can be formed when the transition state is passed on the reaction pathway, implying that the relevant addition is not stereospecific, as experimentally observed.