Page 9 - MetalLigand Cooperation by AromatizationDearomatization: A New Paradigm in Bond Activation and Green Catalysis

MetalLigand Cooperation Gunanathan and Milstein

SCHEME 17 SCHEME 19

SCHEME 18

alkoxy parts of the reactant ester are incorporated in the
product, presenting an atom economical and environmen-
tally benign method for transesterification reactions.
these conditions. Reactions with diamines give bis-amides
Next, we investigated the reaction of primary alcohols
(Scheme 18). Diethylenetriamine is chemoselectively acy-
with amines. In principle, three reactions can occur (Figure 5).
lated at the primary amine groups, requiring no protection of
Reaction of the intermediate aldehyde with the amine can
the secondary amine functionality. We have also used this
form an intermediate hemiaminal, which can sponta- 32
reaction for the preparation of polyamides. Subsequently,
neously eliminate water to form an imine, which can under-
polymerization using complex 11 as catalyst was very
go hydrogenation with the liberated H 2 to yield a secondary 33
recently reported.
30
amine. Competing hemiacetal formation can provide an
Efficient synthesis of amides by reaction of esters with
ester if nucleophilicity of the amine is low. We aimed at H 2
amines with liberation of H 2 under neutral conditions catalyzed
liberation from the presumed hemiaminal intermediate to
by complex 11 was reported very recently (Scheme 19). 34
form an amide. Indeed, amide formation occurs exclusively
Similar to the alcohol acylation process (Scheme 16), both acyl
with liberation of hydrogen, with 0.1 mol % 11 as catalyst in
and alkoxo parts of the symmetrical esters are incorporated in
the reaction of equivalent amounts of alcohols and amines
the product. These reactions are possibly initiated by NH
in refluxing toluene (Scheme 17). This fundamentally new
activation of the amine by metalligand cooperation involving
chemical reaction proceeds in high yields and high turnover
numbers. 31 11. Ester coordination followed by intramolecular nucleophilic
While the yields are good to excellent in most cases, use attack by the amido ligand at the acyl functionality is thought to
of sterically hindered substrates results in lower yields of be a key step. 34 Overall, in one catalytic cycle, two molecules of
amides, as observed with 2-methyl-1-butanol and 2-methyl- amide and of H 2 are formed.
hexamine. Also, as a result of the lower nucleophilicity of Surprisingly, when the PNP complex 5 or 8 is employed in
aniline, ester formation becomes competitive. The reactions the coupling of alcohols and amines, a different course of
are selective to the primary amine functionality, and sec- reaction takes place, providing an efficient method for the
ondary amines such as dibenzylamine do not react under synthesis of imines with liberation of dihydrogen, 35 with no

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