11132
J. Am. Chem. Soc. 1997, 119, 11132-11133
and the reaction took only a few seconds (eq 1). The structure
Triple C-H/N-H Activation by O2 for Molecular
Engineering: Heterobifunctionalization of the
19-Electron Redox Catalysts FeICp(arene)
(1)
Ste´phane Rigaut, Marie-He´le`ne Delville, and Didier Astruc*
of 2 is shown by 1H and 13C NMR. In order to better understand
the reaction of 1 with O2, we performed it stepwise since we
knew that single C-H activation in FeICp(C6Me6) only
consumed 0.25 equiv of O2 at 20 °C.2 Indeed, reaction of 1
with 0.25 equiv of O2 instantaneously gave a red complex 3
(the usual color of cyclohexadienyl FeII complexes with an
exocyclic double bond); 3 is not thermally stable and rapidly
turned green at 20 °C, giving 4. Addition of 0.25 equiv of O2
to 4 instantaneously gave the red color again, forming 5; this
complex is not thermally stable at 20 °C either and gave the
green complex 6. Addition of 0.25 equiv of O2 to 6 gave the
red complex 2 which was relatively stable (thermally and toward
O2). That the green species 4 and 6 are also d7, 19-electron
Laboratoire de Chimie Organique et Organome´tallique,
UMR CNRS N°5802
UniVersite´ Bordeaux I
33405 Talence Cedex, France
Regiospecific C-H activation is one of the most challenging
problems in molecular chemistry. Fundamental recent discover-
ies have led to considerable advances in this field.1 One of the
next major steps is to adapt and use C-H activation systems to
molecular engineering en route toward applications. We know
that single benzylic C-H activation of the most acidic hydrogen
atom can be achieved by O2 in the 19-electron complexes FeI-
Cp(arene).2 Since these complexes also catalyze the cathodic
reduction of nitrate and nitrite to ammonia in water,3 we have
attempted to use this simple and powerful benzylic C-H
activation system to attach these redox catalysts onto star
molecules and dendrimers.5 We report here an unexpected but
extremely mild and very useful triple C-H/N-H activation
whose principle is derived from that of the benzylic C-H
activation by O2 indicated above. This leads to the heterobi-
functionalization of the redox catalyst at room temperature and
ultimately to both its solubilization in water and its attachment
to star molecules without loss of catalytic activity.
Scheme 1
The starting 18-electron complex 1+ was synthesized by the
complexes was indicated by their ESR spectra showing, as for
1, the classic rhombic distortion (g values are 2.002, 2.074, and
1.858 for 4 and 2.009, 2.074, and 1.904 for 6). Since the C-H
activation by O2 proceeds by electron transfer from FeI to O2
followed by deprotonation of the cationic organo-FeII intermedi-
ate by superoxide radical anion in the cage-ion pair,2 we have
also achieved the first C-H activation giving 3 starting from
1+ and KH in THF at -20 °C. This reaction also gave 3, and
at 20 °C, 3, generated in this way, also turned green and reacted
twice with 0.25 equiv of O2 exactly as above to give 2. The
color change from red 3 to green 4 occurred even at -10 °C in
pentane upon transfer by cannula and upon shaking the Schlenk
flask, which suggests a radical mechanism (Scheme 1). The
synthesis of 3 from 1+ and KH at -30 °C in THF-d8 allowed
the recording of its 1H and 13C NMR spectra, which unambigu-
ously showed its cyclohexadienyl structure. Given this informa-
tion, it is probable that formal H-atom abstraction proceeds each
time at the benzylic position to give the red cyclohexadienyl
complexes which rearrange by H-atom transfer from the amine
chain to the methylene group. This latter thermal process is at
least in part intermolecular since it became faster as concentra-
tion was increased. In the ESR spectrum of 4, the expected
line around g ) 2.00 for the nitrogen-centered radical is
presumably hidden by the unusually broad central line.
For further synthetic use, 2 could be carboxylated in the
benzylic position by reaction with CO2 (1 atm) at 20 °C and
then acidification with aqueous HPF6 gave the aldehyde 7. From
1+, the one-pot reaction of eq 2 proceeded in a 60% overall
yield of 7.
3
reaction of FeII(C5H4CO2H)(C6Me6)PF6 with SOCl2 (reflux),
then with propylamine (0-20 °C), and finally with BH3 in THF
(reflux) in 65% overall yield. Single-electron reduction to the
19-electron isostructural complex 1 was achieved by reaction
with Na/Hg in THF in 20 °C.4a The ESR spectrum of 1 shows
the characteristic 3-line pattern for the Jahn-Teller active FeI
state with rhombic distortion:4b gx ) 1.978, gy ) 2.068, and gz
) 1.850. Evidence that the structure is unchanged at this point
was obtained by quantitative ferricinium oxidation back to 1+.
Much to our surprise, the reaction of the forest-green complex
1 with O2 in pentane at 20 °C consumed 0.75 equiv of O2 and
gave the red complex 2 resulting from triple H-atom abstraction.
On a preparative scale, excess O2 was bubbled into the solution,
(1) (a) Bergman, R. G.; Mobley, J. A.; Peterson, T. H. Acc. Chem. Res.
1995, 28, 154. (b) Arndten, B. A.; Bergman, R. G. Science 1995, 270,
1970. (c) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983, 250,
395. (d) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51. (e)
Thompson, M. E.; Bercaw, J. E. Pure Appl. Chem. 1984, 56, 1. Stahl, S.
S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 5961. (f)
Brown, S. H.; Crabtree, R. H. J. Am. Chem. Soc. 1989, 111, 2935, 2946.
Crabtree, R. H. Chem. ReV. 1995, 95, 987. (g) Barton, D. H. R.; Doller,
D. Acc. Chem. Res. 1992, 25, 504. (h) Saillard, J.-Y.; Hoffmann, R. J.
Am. Chem. Soc. 1984, 106, 2006. (i) Lin, M.; Hogan, T.; Sen, A. J. Am.
Chem. Soc. 1997, 119, 6048. (j) ActiVation and Functionalization of
Alkanes; Hill, C. L., Ed.; Wiley: New York, 1989. (k) Shilov, A. E.
ActiVation of Saturated Hydrocarbons by Transition Metal Complexes;
Reidel: Dordrecht, The Netherlands, 1984. (l) SelectiVe Hydrocarbon
ActiVation; Davies, J. A., Watson, P. L., Liebman, J. F., Greenberg, A.,
Eds.; VCH: New York, 1990.
(2) (a) Astruc, D.; Hamon, J.-R.; Roma´n, E.; Michaud, P. J. Am. Chem.
Soc. 1981, 103, 7502. (b) Astruc, D. Electron Transfer and Radical
Processes in Transition-Metal Chemistry; VCH: New York, 1995. (c) See,
for instance, the first step in Scheme 1. This reaction produces 0.5 equiv
of H2O.
(3) Buet, A.; Darchen, A.; Moinet, C. J. Chem. Soc., Chem. Commun.
1979, 447.
(2)
(4) (a) Hamon, J.-R.; Astruc, D.; Michaud, P. J. Am. Chem. Soc. 1981,
103, 758-766. (b) Rajasekharan, M. V.; Giezynski, S.; Ammeter, J. H.;
Hamon, J.-R.; Michaud, P.; Astruc, D. J. Am. Chem. Soc. 1982, 104, 2400.
(5) (a) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic
Molecules: Concepts, Syntheses and PerspectiVes; VCH: New York, 1996.
(b) Tomalia, D. A.; Dupont Durst, H. Top. Curr. Chem. 1993, 165, 193.
(c) Ardoin, N.; Astruc, D. Bull. Soc. Chim. Fr. 1995, 132, 875.
The heterobifunctional complex 7 could be attached to the
extremities of the branches of the star-shaped molecule 8 to
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