Page 6 - MetalLigand Cooperation by AromatizationDearomatization: A New Paradigm in Bond Activation and Green Catalysis

MetalLigand Cooperation Gunanathan and Milstein

probably forming a Ru(0) intermediate 30, which converts to occurs along the T1 pathway, as a result of the S1 to T1
complex 11 by proton migration from the methylene group of intersystem crossing being very efficient. 18c
the phosphine arm to the ruthenium center. The liberated b. NH Activation. NH activation by metalligand co-
hydrogen peroxide is then rapidly catalytically decomposed operation involving ligand aromatization was demonstrated
(possibly by 11, which is, as we have observed in a separate with the dearomatized complex 8.When 8 was reacted with
experiment, an excellent catalyst for H 2 O 2 disproportionation) electron-poor anilines, NH activation took place with proton
into dioxygen and water. This is a new approach toward a transfer to the unsaturated ligand arm, leading to aromatization
complete cycle for the generation of dihydrogen and dioxygen of the central pyridine ring (Scheme 10). 19 4-Nitroaniline and
from water promoted by a soluble metal complex. In addition, 2-chloro-4-nitroaniline reacted at room temperature with 8 to
17
18
isotopic labeling experiments using H 2 Oand H 2 Odemon- provide the complexes 32a and 32b, respectively. Reactions of
strate unequivocally that the process of oxygenoxygen bond 2-bromoaniline and 3,4-dichloroaniline resulted in equilibria
formation is intramolecular, establishing a fundamentally involving the activated aromatic complexes 32c and 32d,
new mode of OO bond formation, by photolytic coupling and the starting 8, even in the presence of an excess of the
of OH groups. DFT calculations by Hall show that the halogenated anilines. The reversibility of NH bond acti-
photolytic reductive elimination of H 2 O 2 can take place vation at room temperature in these complexes suggests
from a dissociative triplet state via a singlettriplet that the barrier for this process is low and that potential
crossing. 18b Another DFT study by Fang concludes that a catalytic cycles based on such systems could readily elim-
lower energy pathway for triplet O 2 formation, consistent inate product amines.
with all experimental findings, involves a nonadiabatic Thermodynamically, coordinated amine complexes of
two-step process in which concerted hydrogen transfer type 31 are the preferred forms for electron rich amines
and dehydration involving two molecules of complex 29 and ammonia. When ND 3 was reacted with complex 8,
take place. This OO bond formation is intramolecular, and formation of the deuterated complex 33 was observed after
5 min at room temperature, indicating ND activation
(Scheme 11). The reaction is highly stereospecific, and only
SCHEME 10 1
one of the two CH 2 arm hydrogen signals in the HNMR
spectrum disappeared. No exchange of the vinylic hydrogen
took place. Such dramatic selectivity suggests that the activa-
tion process occurs on only one face of the ligand in an
intramolecular manner with one coordinated molecule of
ND 3 . The reverse reaction would affect only the (endo)
hydrogen on the same face as the coordinated ND 3 moiety.
This result also suggests that NH activation, and perhaps
OH activation (see below) of other substrates, might occur in
a stereoselective fashion.
Upon heating complex 8 with an excess of isopropyla-
mine at 80 °C in a closed NMR tube, formation of the trans-
dihydride 9 was observed. Presumably, after NHactiva-
tion, H 2 elimination involving the hydride ligand and
an arm proton takes place, as suggested for complex 28

SCHEME 11

Vol. 44, No. 8 ’ 2011 ’ 588–602 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 593