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.