Page 2 - MetalLigand Cooperation by AromatizationDearomatization: A New Paradigm in Bond Activation and Green Catalysis

MetalLigand Cooperation Gunanathan and Milstein

NR 2 , C) by cooperation between the metal and the ligand,
thereby regaining aromatization (Figure 1). Such processes
occur with no formal change in the metal oxidation state.
1
Examples are shown in Schemes 13. HNMR spectraof
complexes 2, 5, 8, 11, 14,and 15 indicate dearomatization as
the pyridine protons are shifted to lower frequency. X-ray
structure of complex 2 (Figure 2) shows that the C6C7 bond
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(1.350 Å) is significantly shorter than C1C2 (1.505 Å). In
complex 5,C1C2 (1.450 Å) is significantly shorter than
C7C6 (1.552 Å) whereas C1P1 (1.803 Å) is shorter to a
smaller extent than P2C7 (1.843 Å), indicating that the
FIGURE 1
dearomatizedresonancestructureof5contributessignificantly
more than the aromatized phosphor-ylide form (Figure 1). 9
while the ligands, which can play crucial roles in affecting the
Reaction of the dearomatized 2 with benzene results in
properties of the metal center, do not undergo themselves
CD activation with concomitant aromatization of the pyr-
bond making and breaking processes and do not actively 8
idine ring to form the IrPh complex 3 (Scheme 1). Interest-
1
participate in the catalytic cycle. However, there are catalysts
ingly, upon reaction with dihydrogen, the dearomatized
in which the ligands can cooperate with the metal center by
ruthenium complexes 5, 8,and 11 undergo aromatization to
undergoing themselves reversible structural changes in the
quantitatively yield the trans-dihydride complexes 6, 9,and 12,
processes of substrate activation and product formation, such
respectively (Scheme 2). Both complexes 6 and 12 slowly lose
as in the case of hydrogenation of polar bonds. 2
H 2 at room temperature to regenerate complexes 5 and 11,
Transition metal complexes of bulky, electron-rich “pin-
respectively, while complex 9 is stable at room temperature.
cer” ligands have important applications in synthesis, bond
Dearomatization of the cationic platinum complex 13 by
activation, and catalysis. 3,4 Electron donating, pyridine-
treatment with base forms the neutral complex 14. Interest-
based PNP and PNN pincer ligands can stabilize coordina-
ingly, reaction of 14 with an equivalent of MeLi results in attack
5
tively unsaturated metal complexes. Recently, we have
at the metal center with decoordination of the amine arm,
discovered new modes of metalligand cooperation invol-
rather than chloride substitution, to give the anionic, dearo-
ving aromatizationdearomatization processes of PNP and
matized PtMe complex 15. This complex is relatively stable,
6
PNN pyridine-based pincer complexes, leading to unusual
although it bears no stabilizing π acceptors. Upon reaction of
bond activation processes and to novel, environmentally
15 with water, selective protonationaromatization to give
benign catalysis. Recently, we have also observed unusual
complex 16 takes place, with no protonation of the methyl
“long-range” metal ligand cooperation with an acridi-
group. Reaction of 16 with MeLi results in dearomatization,
nepincer ruthenium complex, with reversible dearomatiza-
with no chloride substitution being observed (Scheme 3). 10
tion of the middle acridine ring. These processes might be
involved in catalysis by this complex. In this Account, we
2
3
sp and sp CH Activation Based on
provide an overview of the metalligand cooperation based
MetalLigand Cooperation
on aromatizationdearomatization processes. We first de-
scribe stoichiometric processes discovered in our laboratory Our studies indicate that an Ir(III) oxidative addition inter-
and then the development of new catalytic reactions involving mediate 18 is involved in the CD activation of deuterated
alcohols, esters, acetals, amines, imines, ketones, and amides. benzene by the dearomatized Ir(I) complex 2 (Schemes 1
8
and 4). Complex 18 can be generated and fully character-
MetalLigand Cooperation Based on ized spectroscopically at 70 °C by deprotonation of the
AromatizationDearomatization of cationic 17. 11,12 Upon warming up, proton transfer to the
Pyridine-Based Pincer Systems side arm takes place, with aromatization and concomitant
Pyridine-based pincer complexes can undergo deprotonation reduction of the metal center to provide complex 3.
at the pyridinylmethylenic carbon, resulting in dearomatiza- Complex 2 reacts with acetone to yield the acetonyl Ir(I)
3
tion of the pyridine moiety. 79 The dearomatized complex complex 19,bysp CH activation. Like benzene CH activa-
can then activate chemical bonds (HY; Y = H, OH, OR, NH 2 , tion, this reaction also proceeds via an Ir(III) intermediate 21.
Vol. 44, No. 8 ’ 2011 ’ 588–602 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 589