Page 10 - MetalLigand Cooperation by AromatizationDearomatization: A New Paradigm in Bond Activation and Green Catalysis

MetalLigand Cooperation Gunanathan and Milstein

SCHEME 20

also tolerated. This new catalytic reaction works very well in
the challenging synthesis of unstable aliphatic imines. Con-
venient to practical applications, the reaction can be carried
out in air. 35
A common, simplified catalytic cycle for the direct ester-
ification, amidation, and imination reactions based on alco-
hols with liberation of hydrogen is proposed in Figure 6.
Upon reaction of dearomatized complexes (5, 8,or 11) with
alcohols, facile OH activation takes place to provide the
saturated hydrido-alkoxy complexes. This is followed by β-
hydride elimination, the mechanism of which is not clear at
this stage. It may occur by arm opening to provide a cis
coordination site, as depicted in Figure 6, but other mechan-
isms are also possible, including alkoxide dissociation from
the saturated complex accompanied by hydride abstraction. 36
This step generates an intermediate aldehyde and the
known trans-dihydride complexes (6, 9,or 12, respectively),
FIGURE 6. Proposed simplified catalytic cycle for the synthesis of esters,
amides, and imines from alcohols, catalyzed by PNP and PNN ruthe- which were demonstrated to liberate H 2 and regenerate the
nium pincer complexes. Note that mechanisms not involving amine dearomatized complexes. The generated aldehyde is in
arm dissociation are also possible (see text). equilibrium with the corresponding hemiacetal (X = O),
which undergoes a similar catalytic cycle providing the
7
imine hydrogenation taking place (Scheme 20). A range of ester. In the presence of amines, the course of reaction
t
alcohols and amines containing either electron-donating or - differs based on L 2 (i.e., P Bu 2 or NEt 2 ). With the PNN complex
withdrawing substituents react to yield the corresponding 11, the intermediate hemiaminal (X = NH, but see next
imines in very good yields. Sterically hindered amines are paragraph) undergoes dehydrogenation to provide amides, 31

Vol. 44, No. 8 ’ 2011 ’ 588–602 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 597