gested,4a-d or the mechanistic proposal incorrectly em-
ployed monocationic intermediates.4e For example, the
acid-catalyzed cyclizations of propargyl-substituted ben-
zylamines have been reported (eqs 1 and 2) and dicationic
intermediates (3 and 4) are likely involved.4b,c
Reactive Dications: The
Superacid-Catalyzed Reactions of Alkynes
Bearing Adjacent N-Heterocycles or Amine
Groups
Douglas A. Klumpp,*,† Rendy Rendy, Yun Zhang,
Aaron McElrea, Alma Gomez, and Han Dang
Olah was the first to recognize the importance of
reactive, dicationic electrophiles. Based on results from
superacid-catalyzed reactions, superelectrophilic inter-
mediates were proposed, like the protionitronium ion
(NO2H2+) and the protioacetyl ion (CH3COH2+).5 Super-
electrophilic intermediates have since been proposed in
a variety of acid-catalyzed conversions and studied by
experimental and theoretical methods.6 Superelectro-
philic intermediates have been proposed in the conver-
sions involving nitriles,7 hydrogen cyanide,7 diazonium
salts,8 and carbon monoxide.9 However, there has been
no systematic study of alkynes and their possible forma-
tion of dicationic electrophiles (or superelectrophiles). In
the following report, we describe our studies of the
superacid-catalyzed chemistry of alkynes bearing an
adjacent N-heterocycle or amine functional groups. We
propose a general mechanism that invokes reactive
dicationic intermediates composed of vinylic cations and
adjacent protonated N-base sites.
Alkynes with an adjacent N-heterocycle group are
found to give the addition products from TfOH and C6H6.
For example, the ethynyl-substituted imidazole (5) gives
product 6, while the propargyl-substituted benzotriazole
(7) gives the addition product (8) in high yield (Figure
1). Both reactions give the gem-diphenyl group on the
products suggesting that two reactive dications are
generated. For the benzotriazole, the vinylic dication 10
and the benzylic dication 12 are the electrophilic inter-
mediates leading the product 8. Dication 10 is likely in
equilibrium with the corresponding vinyl triflate (11).4a
Although the arylation of alkynes with strong mineral
acids has been known for many years,10 these reports
generally note the formation of significant amounts of
polymeric “char.” Dications such as 10 and 12 are
probably more electrophilic than the analogous vinyl
cations from protonation of simple alkynes (i.e., mono-
Department of Chemistry and Biochemistry, Northern
Illinois University, DeKalb, Illinois, 60115, and Department
of Chemistry, California State Polytechnic University,
3801 West Temple Avenue, Pomona, California 91768
Received June 17, 2004
Abstract: A variety of aminoalkynes and related hetero-
cycles are reacted in the Bronsted superacid CF3SO3H (triflic
acid), and products are obtained in generally good yields
(69-99%) from Friedel-Crafts-type reactions. The reactions
are consistent with the formation of novel dicationic inter-
mediates having a vinyl cationic site and an adjacent
protonated N-heterocycle or ammonium cation.
Vinyl cations have been of general interest since the
first report of these reactive intermediates and their
chemistry has been extensively reviewed.1 We recently
described the reactions of propargyl-substituted phos-
phonium salts with superacidic triflic acid (CF3SO3H,
TfOH) in benzene.2 The results were consistent with the
formation of a dicationic intermediate (1) composed of a
vinylic cation center and an adjacent phosphonium cation
center. In another recent paper, we proposed a dicationic
intermediate (2) composed of a vinyl cation and proto-
nated carboxylic acid group.3 To our knowledge, these
(4) (a) Olah, G. A.; Spear, R. J. J. Am. Chem. Soc. 1975, 97, 1845.
(b) Brooks, J. R.; Harcourt, D. N.; Waigh, R. D. J. Chem. Soc., Perkin
Trans. 1 1973, 2588. (c) Takayama, H.; Suzuki, T.; Nomoto, T. Chem.
Lett. 1978, 865. (d) Gee, K. R.; Barmettler, P.; Rhodes, M. R.;
McBurney, R. N.; Reddy, N. L.; Hu, L.-Y.; Cotter, R. E.; Hamilton, P.
N.; Weber, E.; Keana, J. F. W. J. Med. Chem. 1993, 36, 1938. (e)
Brooks, D. N.; Harcourt, D. N. J. Chem. Soc. C 1969, 626.
(5) Olah, G. A.; Germain, A.; Lin, H. C.; Forsyth, D. J. Am. Chem.
Soc. 1975, 97, 2928.
(6) (a) Olah, G. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 767. (b)
Shudo, K.; Ohwada, T. In Stable Carbocation Chemistry; Prakash, G.
K. S., Schleyer, P. v. R., Eds.; Wiley: New York, 1997; pp 525-548.
(c) Nenajdenko, V. G.; Shevchenko, N. E.; Balenkova, E. S.; Alabugin,
I. V. Chem. Rev. 2003, 103, 229. (d) Olah, G. A.; Klumpp, D. A. Acct.
Chem. Res. 2004, 37, 211. (e) Klumpp, D. A. Recent Res. Dev. Org.
Chem. 2001, (5), 193-205, Part I.
were the first reports to propose reactive, dicationic
intermediates involving vinyl cation centers. Several
earlier studies clearly involved these types of dicationic
intermediates,4 but either no mechanism was sug-
(7) Sato, Y.; Yato, M.; Ohwada, T.; Saito, S.; Shudo, K. J. Am. Chem.
Soc. 1995, 117, 3037.
† Northern Illinois University.
(8) Rasul, G.; Prakash, G. K. S.; Olah, G. A. J. Am. Chem. Soc. 1994,
116, 8985.
(1) Dicoordinated Carbocations; Rappoport, Z., Stang, P. J., Eds;
Wiley: New York, 1997.
(9) Olah, G. A.; Prakash, G. K. S.; Mathew, T.; Marinez, E. Angew.
Chem., Int. Ed. 2000, 39, 2547.
(2) Zhang, Y.; Aguirre, S. A.; Klumpp, D. A. Tetrahedron Lett. 2002,
43, 6837.
(10) (a) Reilly, J. A.; Nieuwland, J. A. J. Am. Chem. Soc. 1928, 50,
2564. (b) Reichert, J. S.; Nieuwland, J. A. J. Am. Chem. Soc. 1923, 45,
3090.
(3) Rendy, R.; Zhang, Y.; McElrea, A.; Gomez, A.; Klumpp, D. A. J.
Org. Chem. 2004, 69, 2340.
10.1021/jo040218u CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/19/2004
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J. Org. Chem. 2004, 69, 8108-8110