Page 1 - MetalLigand Cooperation by AromatizationDearomatization: A New Paradigm in Bond Activation and Green Catalysis

MetalLigand Cooperation by
AromatizationDearomatization: A New

Paradigm in Bond Activation and “Green”
Catalysis

CHIDAMBARAM GUNANATHAN AND DAVID MILSTEIN*
Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot
76100, Israel
RECEIVED ON FEBRUARY 2, 2011

CONSPECTUS

n view of global concerns regarding the environment and sustainable
I energy resources, there is a strong need for the discovery of new,
green catalytic reactions. For this purpose, fresh approaches to
catalytic design are desirable. In recent years, complexes based on
“cooperating” ligands have exhibited remarkable catalytic activity.
These ligands cooperate with the metal center by undergoing
reversible structural changes in the processes of substrate activation
and product formation.
We have discovered a new mode of metalligand cooperation,
involving aromatizationdearomatization of ligands. Pincer-type
ligands based on pyridine or acridine exhibit such cooperation,
leading to unusual bond activation processes and to novel, envi-
ronmentally benign catalysis. Bond activation takes place with no
formal change in the metal oxidation state, and so far the activation of
3
2
HH, CH (sp and sp ), OH, and NH bonds has been demon-
strated. Using this approach, we have demonstrated a unique water
splitting process, which involves consecutive thermal liberation of H 2
and light-induced liberation of O 2 , using no sacrificial reagents,
promoted by a pyridine-based pincer ruthenium complex. An acridine
pincer complex displays unique “long-range” metalligand coopera-
tion in the activation of H 2 and in reaction with ammonia.
In this Account, we begin by providing an overview of the metal
ligand cooperation based on aromatizationdearomatization processes. We then describe a range of novel catalytic reactions that
we developed guided by these new modes of metalligand cooperation. These reactions include the following: (1) acceptorless
dehydrogenation of secondary alcohols to ketones, (2) acceptorless dehydrogenative coupling of alcohols to esters, (3) acylation of
secondary alcohols by esters with dihydrogen liberation, (4) direct coupling of alcohols and amines to form amides and polyamides
with liberation of dihydrogen, (5) coupling of esters and amines to form amides with H 2 liberation, (6) selective synthesis of imines
from alcohols and amines, (6) facile catalytic hydrogenolysis of esters to alcohols, (7) hydrogenolysis of amides to alcohols and
amines, (8) hydrogenation of ketones to secondary alcohols under mild hydrogen pressures, (9) direct conversion of alcohols to
acetals and dihydrogen, and (10) selective synthesis of primary amines directly from alcohols and ammonia. These reactions are
efficient, proceed under neutral conditions, and produce no waste, the only byproduct being molecular hydrogen and/or water,
providing a foundation for new, highly atom economical, green synthetic processes.

Introduction benign, sustainable catalytic reactions, which use no toxic
A major challenge of catalysis today is the replacement of reagents and produce no waste. In most reactions catalyzed
traditional synthetic processes with environmentally by metal complexes, the catalytic activity is metal-based,
588 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 588–602 ’ 2011 ’ Vol. 44, No. 8 Published on the Web 07/08/2011 www.pubs.acs.org/accounts
10.1021/ar2000265 & 2011 American Chemical Society