C O M M U N I C A T I O N S
Table 2. Manganese-Catalyzed Homocoupling of Grignard
Scheme 3. Tentative Mechanism for the Mn-Catalyzed Reaction
Reagents with Atmospheric Oxygen as an Oxidanta
tion reactions.9 Recently, such a complex has also been proposed
as an intermediate in the palladium-catalyzed homocoupling of
organoboranes.10 It should be underlined that, in the case of the
present reaction, it is necessary to have a very rapid catalytic process
to avoid the direct oxidation of the Grignard reagent by oxygen.
For this reason, the reductive elimination from a R2Mn(II) species
such as 1 cannot be involved since diaryl-, dialkenyl-, and
dialkynylmanganese(II) compounds are generally stable at room
temperature.8,11 In fact, the best way to favor the reductive
elimination is to increase the oxidation state of the metal. Thus, it
is very reasonable to think that the formation of the unstable Mn-
(IV) species 2 is required to achieve a very quick reductive
elimination that gives 4.11
In conclusion, we have developed two very efficient iron- and
manganese-catalyzed procedures to couple aryl, alkenyl, and alkynyl
Grignard reagents under mild conditions by using atmospheric
oxygen as an oxidant. It should be noted that the reactions are
chemo- and stereoselective. To the best of our knowledge, it is the
first time that air has been used as an oxidant to perform such
reactions with Grignard reagents. Sustainable development is now
a real challenge for the chemical industry, and these economic and
eco-friendly procedures constitute an interesting contribution in this
field.
Supporting Information Available: Experimental procedures and
spectral data for all compounds are provided. This material is available
References
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7, 1943.
a The reaction was carried out on a 20 mmol scale. b Isolated yield. c The
reaction was performed at -20 °C. d The stereoselectivity was determined
by GC. e The reaction was performed at 10 °C. f The reaction was performed
at -40 °C. g 15% of catalyst was used.
(3) Nagano, T.; Hayashi, T. Org. Lett. 2005, 7, 491. For leading references
about Fe-catalyzed cross-coupling reactions, see: (a) Cahiez, G.; Ave-
dissian, H. Synthesis 1998, 1199. (b) Fu¨rstner, A.; Leitner, A.; Me´ndez,
M.; Krause, H. J. Am. Chem. Soc. 2002, 124, 13856. (c) Duplais, C.;
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E. J. Am. Chem. Soc. 2004, 126, 3686. (e) Nagano, T.; Hayashi, T. Org.
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Duplais, C.; Moyeux, A. Org. Lett. 2007, 9, 3253.
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(5) Walling, C.; Buckler, S. A. J. Am. Chem. Soc. 1955, 77, 6032. See also
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(6) See for instance: Mulzer, J.; O¨ hler, E. Topics in Organometallic
Chemistry; Springer: Berlin, 2004; Vol. 13, pp 269-366.
(7) Prepared from piperonal (see Supporting Information).
(8) With iron, it is more difficult to propose a mechanism since, contrary to
arylmanganese(II), aryliron(II) is reputed to be less stable. Nevertheless,
it is interesting to note that Ph2Fe(Et3P)2 only decomposes above 0 °C:
Maruyama, K.; Ito, T.; Yamamoto, A. Transition Met. Chem. 1980, 5,
14.
Scheme 2. Preparation of N-Methylcrinasiadine
cyclization was then achieved under the coupling conditions
previously reported. The N-methylcrinasiadine 20 was isolated in
46% yield.
A tentative mechanism is depicted in Scheme 3 for the Mn-
catalyzed reaction.8 The key step of this catalytic cycle is the
conversion of the stable diorganomanganese(II) 1 to a manganese-
(IV) peroxo complex 2. This one would undergo a rapid reductive
elimination to give the homocoupling product and a manganese-
(II) peroxo complex 3 which would react with the Grignard reagent
to give again the organomanganese 1. With manganese and iron,
the formation of peroxo complexes as catalytic intermediates is very
well established for various manganese- and iron-catalyzed oxida-
(9) Lane, B. S.; Burgess, K. Chem. ReV. 2003, 103, 2457.
(10) Adamo, C.; Amatore, C.; Ciofini, I.; Jutand, A.; Lakmini, H. J. Am. Chem.
Soc. 2006, 128, 6829.
(11) (a) Beermann, C.; Clauss, K. Angew. Chem. 1959, 71, 627. (b) Cahiez,
G. An. Quim. 1995, 91, 561. See also Supporting Information, S2.
JA075417K
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