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the reaction of ketones with sulfonic anhydrides in the pres-
ence of base; consequently, a mixture of cis/trans isomers as
well as the various double-bond isomers of the vinyl sulfonate
are obtained.[32,33] Therefore, the drawbacks of the above meth-
ods demonstrate the need to develop a general and practical
process for the efficient synthesis of stereo- and regiodefined
vinyl sulfonates that is highly active and selective. The most
straightforward and atom-economical method to access vinyl
sulfonates is through transition-metal-catalyzed intermolecular
addition of sulfonic acids to alkynes. In recent years, transition-
metal-catalyzed reactions of alkynes with various nucleophilic
reagents such as water,[34–38] alcohols,[39–44] amines,[12,45–54]
thiols,[55–64] and carboxylic acids[10,65–67] have been developed to
construct CÀX (X=C, N, O, S) bonds. Among them,
Rh-catalyzed carbon–heteroatom bond formation has become
one of the most studied and widely used meth-
ods.[8,10,37,39,52,54,56,57,63,65,68–74] Despite such progress, no results
regarding the Rh-catalyzed intermolecular addition of sulfonic
acids to alkynes to produce regiodefined vinyl sulfonates have,
to the best of our knowledge, been reported to date. In this
respect, the regioselective addition of sulfonic acids to a range
of alkynes represents an underdeveloped area in hydrosulfona-
tion catalysis.
for each vinyl sulfonate ester). Additionally, we present a study
of the effect of the concentration of methanesulfonic acid on
the yield of the vinyl sulfonate ester (Table S1 in Section 3,
Supporting Information) and an examination of the effects of
different types of ligands, solvents, and Rh precursors on the
yield of the vinyl sulfonate ester (Table S2 in Section 4, Sup-
porting Information). On the basis of the results presented in
Table S1 (Supporting Information), we chose to run all experi-
ments as part of our mechanistic investigation of hydrosulfo-
nation at 0.5 mmol of methanesulfonic acid, as this amount
minimized the formation of acetophenone.
Effect of PPh3 concentration on selectivity of Rh-catalyzed
hydrosulfonation
We performed the addition of methanesulfonic acid to phenyl-
acetylene with an equimolar amount of PPh3 as a model reac-
tion in the presence of [RhCl(cod)]2 (1 mol%) in 1,2-dichloro-
ethane (DCE) at 708C for 120 min, similar to the reaction con-
ditions reported previously by Yamaguchi (Scheme 1). We ach-
We present a study of the addition of sulfonic acids to
a series of alkynes by using [RhCl(cod)]2 as the catalytic precur-
sor in the presence of varying amounts of exogenous PPh3
ligand to determine the optimal concentration of PPh3 for the
selective formation of vinyl sulfonate esters. We examine the
effect of the concentration of PPh3 and the sulfonic acid on
the product distribution of the hydrosulfonation of phenylace-
tylene with methanesulfonic acid. We then screen the catalytic
system by using a variety of alkynes, sulfonic acids, phospho-
rus ligands, solvents, and other Rh precursors to demonstrate
the robust utility of our system. Additionally, we provide
31P NMR spectroscopic analysis of the Rh species in solution to
determine the catalytically relevant structure(s) of the Rh spe-
cies. Finally, we compile kinetic and spectroscopic data and in-
troduce kinetic isotope effect (KIE) experimental data to pro-
pose a mechanism for the Rh-catalyzed hydrosulfonation reac-
tions. Our proposed mechanism consists of the formation of
vinyl sulfonate esters at low concentrations of PPh3 through
the oxidative addition of the sulfonic acid and migratory inser-
tion of the alkyne with the possibility of both syn and anti ad-
dition across the triple bond of the alkyne, whereas the exclu-
sive formation of the anti-Markovnikov vinylphosphonium salt
results at high concentrations of PPh3, which is in agreement
with Arisawa and Yamaguchi’s original work.[20]
Scheme 1. Ligand-concentration-controlled addition of methanesulfonic acid
to phenylacetylene. For amounts in excess of 50 mol% PPh3, vinylphospho-
nium salt A is the only product, whereas for amounts less than 5 mol%
PPh3, vinyl sulfonate ester B and hydration byproduct C (acetophenone)
form preferentially. The hydration product forms as a result of trace amounts
of water in methanesulfonic acid.
ieved up to 90% isolated yield of (E)-(2-phenylethenyl)triphe-
nylphosphonium methanesulfonate (A). In the absence of the
Rh precursor, or PPh3, or both, A was not observed even with
prolonged reaction times, which indicates the presence of
[RhCl(cod)]2 is required for the formation of A. However, in the
absence of the Rh precursor and PPh3, the Markovnikov 1-phe-
nylvinyl methanesulfonate (B) adduct was obtained in 14%
yield and the formation of acetophenone (C) as an unwanted
byproduct derived from the hydration of phenylacetylene in
the presence of acid was formed in up to 59.3% yield after 2 h
(methanesulfonic acid contains trace amounts of water).
Results and Discussion
We investigated the influence of the PPh3 concentration on
the Rh-catalyzed hydrosulfonation (see Scheme 1 for the spe-
cific details of all compounds involved in the general reaction),
and the results are shown in Figure 1. Addition of one equiva-
lent of PPh3 (1 mol%) with respect to [RhCl(cod)]2 led to a dra-
matic increase in the yield of B from 8.7 to 37.1%. A further in-
crease in the molar ratio of PPh3 relative to that of the Rh pre-
cursor resulted in a continuous increase in the yield of B, but
All experimental details, characterization (HRMS and 1 H NMR,
2H NMR, and 13C NMR spectroscopy), and purification of the
vinyl sulfonate esters synthesized throughout this study are
provided in the Supporting Information (see Sections 1 and 2
for information regarding the obtained materials and the ex-
perimental details of the hydrosulfonation reactions and Sec-
tions 9 and 10 for the full NMR spectroscopic and HRMS data
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