Regio- and stereoselectivity studies
If NHCs to be recommended as an efficient catalyst for alkyne hydrochalcogenation, it should provide better selectivity along with lowered energy barriers. Thus NHC catalyzed alkyne hydro-thiolation and selenation reaction of propyne leading to Markovnikov as well as (E/Z)-anti -Markovnikov products (Scheme 2) has been computationally explored. Characterized TS2 of IH catalyzed Markovnikov and (E/Z)-anti -Markovnikov is given in Figure 4. Explored reaction mechanism indicates that the reaction regio- and stereoselectively leads to the formation of Z-anti -Markovnikov product over Markovnikov and E-anti -Markovnikov products by 7.86 and 13.64 kcal/mol respectively. Similar selectivity preferences are computed for hydroselenation reactions as well (Table 5). Selectivity studies on N,N-disubstituted analogues of IH showed that electron- donating substituents favor the formation of Z-anti -Markovnikov product (except INH2 in hydrothiolation). For IOH and IOMe catalysts either small energy difference between Markovnikov and Z-anti -Markovnikov product formation, or exclusive Markovnikov selectivity is observed. ISH and ISMe catalysts provide even better Z selectivity for both reactions than IH. BH and BMe catalysts also offer good Z- selectivity. Six-membered TH catalyzed S/Se-H addition prefers Markovnikov product formation while PH catalyzed S/Se-H addition offers Z-anti -Markovnikov selectivity. Substituted TMe and PMe catalyzed hydrothiolation favours Z-anti -Markovnikov selectivity and the corresponding hydroselenation prefers Markovnikov product formation.