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.