Introduction
With the rising applications of vinyl chalcogenides as pharmaceuticals, functional materials or synthetic reagents, economic and selective methods for their production became an inevitable requirement.1–5 Two common approaches for the synthesis of these target compounds are cross coupling and addition reactions. The main problem that limits the synthetic application of the former is the by-product formation. 2 Absence of by-products and the ease of availability of alkynes over the corresponding vinyl halides, make the addition reaction attractive over cross coupling.6 Addition of thiols/selenols to alkynes can possibly lead to the formation of Markovnikov as well as (E/Z)anti -Markovnikov products. 1 Under thermal, photochemical or basic conditions, a mixture of (E/Z)anti -Markovnikov species has been reported.7-11 Transition metal mediated reaction can bring in remarkable regio- as well as stereoselectivity. 3Recently, organocatalyzed reactions have come to surpass metal catalyzed reactions in terms of cost effectiveness and environmental friendliness.12 Among organocatalysts, N-Heterocyclic carbenes (NHCs) are strong candidates with increasing reports of versatile catalyzed reactions, thus playing a pivotal role in the development of many areas such as biochemistry, medicine, and synthetic chemistry.12-18
NHCs are very strong nucleophiles and exhibit high Bronsted basicity due to its electron richness and neutral σ donor character.12, 16, 17, 19-23 The steric and electronic properties of NHCs are tunable, subject to change in N-substituents and central heterocycle. 21, 23-24 The credit for the present advancements in the domain of NHC catalyzed reactions owes to Ukai et al, for his pioneering work on in situ generated thiazol-2-ylidene catalyzed benzoin condensation. 21, 23, 25-26 Breslow proposed a mechanism for the reaction which accelerated studies utilizing the nucleophilic nature of NHCs in a variety of organic reactions including nucleophilic acylations, transesterifications, polymerizations, β-alkylations, hydroacylations, Stetter reactions, annulation reactions and so on. 27NHC catalyzed reactions are also reported for C-C, C-N and C-O bond formation. 28 The experimental results of Z. Cong et al. report the capability of substituted imidazol-2-ylidenes in catalyzing alkyne hydrothiolation and suggested a plausible mechanistic pathway. 29 While 5-membered rings are the largest studied class of NHCs, ring-expanded NHCs are also reported.21 Stable versions of six-membered perimidine-based carbenes as well as saturated tetrahydropyrimidin based ring-expanded NHCs are reported. 30, 31 However organocatalytic reactions based on these six-membered NHCs are not reported yet. Though extensive studies on NHC catalyzed reactions are reported, detailed investigation on the catalytic activity and mechanistic aspects of NHC catalyzed hydro- thiolation and selenation reactions are rare. Thus, the present work explores the catalytic mechanism and selectivity of various NHCs towards alkyne hydrothiolation and hydroselenation reaction. Electronic and steric factors that affect the catalytic efficiency of NHC toward these reactions are also investigated.