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RSC Advances
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DOI: 10.1039/C5RA16826K
COMMUNICATION
Journal Name
Based on the above investigated results and literature key step for achieving the anti-Markovnikov selectivity in the
reports, plausible pathway for the iodide salt/oxone acetalization of vinylarenes under present catalytic conditions.
a
mediated catalytic nucleophilic addition of methanol to
vinylarenes is proposed and is shown in Scheme 3. The
oxidation of iodide ion by oxone in methanol generates
Acknowledgements
transient HOI species (
A
). The
A
reacts competitively with the
stable
We thank the CSIR Network project CSC-0125 for financial
support. P.S. and K.S. acknowledge the UGC, India and M.N.,
M.M.R. and C.D. acknowledge the CSIR, India for financial
support in the form of fellowships.
double bond of an aromatic alkene, to form
a
intermediate 3 (via a short-lived cyclic iodonium ion (B)
intermediate), and an unreacted I- (to form triiodide ion). The
-
oxone converts I3 back into
A
in situ to participate in the
by oxone leads to the
formation of hypervalent iodine(III) species . The species
may forms adduct
with methanol17b and then undergoes
reductive elimination to generate a transient phenonium ion
and . At this stage, the nucleophilic attack of methanol can
catalytic cycle. The oxidation of
3
4
4
Notes and references
C
1
For selected reviews, see: (a) C. J. R. Bataille and T. J.
Donohoe, Chem. Soc. Rev., 2011, 40, 114; (b) H. C. Kolb, M.
S. VanNieuwenhze and K. B. Sharpless, Chem. Rev., 1994, 94
2483; For selected recent examples, see: (c) Y. Li, D. Song
and V. M. Dong, J. Am. Chem. Soc., 2008, 130, 2962; (d) A.
D
,
A
takes place in two distinct pathways (path a and path b). The
acidic nature of the reaction medium, due to the presence of
oxone, probably makes methanol molecules as strong
nucleophiles. The nucleophilic attack of methanol molecule at
Wang, H. Jiang and H. Chen, J. Am. Chem. Soc., 2009, 131
,
3846; (e) L. V. Desai and M. S. Sanford, Angew. Chem. Int.
Ed., 2007, 46, 5737; Angew. Chem., 2007, 119, 5839; (f) H.
Zhu, P. Chen and G. Liu, J. Am. Chem. Soc., 2014, 136, 1766;
(g) T. de Haro and C. Nevado, Angew. Chem. Int. Ed., 2011,
the carbon atom, containing methoxide group, of species
followed by migration of aryl group led to desired anti-
Markovnikov methyl acetal (path a), while the methanol
addition to following path affords the vicinal
dimethoxylation product 2’. The species regenerated in the
D
50, 906; (h) W. Wei and J. Ji, Angew. Chem. Int. Ed., 2011, 50
9097.
,
2
D
b
2
3
(a) J. Muzart, Tetrahedron, 2005, 61, 5955; (b) W. G. Lloyd
and B. J. Luberoff, J. Org. Chem., 1969, 34, 3949; (c) T.
Hosokawa, T. Ohta and S.-I. Murahashi, J. Chem. Soc., Chem.
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Hosokawa, Y. Ataka and S.-I. Murahashi, Bull. Chem. Soc.
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T. Hosokawa, S. Aoki and S.-I. Murahashi, Synthesis, 1992,
558.
A
first cycle continues the catalytic cycle until the complete
conversion of starting material into the product.
The de-iodination, via oxidation of
3 by oxone to 4
followed by reductive elimination, induced semipinacol
rearrangement (path a) is the key step for achieving the anti-
Markovnikov regioselectivity in the hypoiodous acid-catalyzed
addition of methanol to vinylarenes.
4
5
6
F. Alonso, D. Sánchez, T. Soler and M. Yus, Adv. Synth. Catal.,
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A. M. Balija, K. J. Stowers, M. J. Schultz and M. S. Sigman,
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In conclusion, we have developed a novel methodology for
the catalytic nucleophilic addition of methanol to vinylarenes
for the synthesis of anti-Markovnikov acetals using in situ
generated hypoiodous acid from NH4I and oxone. This metal-
free procedure featuring the step economic approach,
compared to conventional methods which involve the pre-
synthesized aldehydes, for the preparation of terminal acetals
and the employment of facilely and commercially available
reagents under mild conditions. The other advantages of this
method are the exclusion of the need for the use of
stoichiometric or over stoichiometric hypervalent iodine
compounds, harsh reaction conditions and transition metals.
In addition, the scope and limitations of this process are
demonstrated with various terminal and internal alkenes.
Moreover, the mechanistic investigations provided the solid
evidence for the in situ generation of transient HOI as active
catalytic species from the oxidation of iodide ion by oxone and
the de-iodination of co-iodo intermediate via its oxidation to
hypervalent iodine species rather than a common iodide
abstraction by electrophiles. Furthermore, the isotope labeling
experiment unambiguously confirming that the reaction
proceeds through a semipinacol rearrangement, which is the
7
8
A. D. Chowdhury and G. K. Lahiri, Chem. Commun., 2012, 48
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9
(a) T. W. Greene and P. G. M. Wuts, Protecting Groups in
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4 |RSC Adv., 2015, 00, 1-5
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