Page 13 - MetalLigand Cooperation by AromatizationDearomatization: A New Paradigm in Bond Activation and Green Catalysis

MetalLigand Cooperation Gunanathan and Milstein

FIGURE 9

SCHEME 24

SCHEME 25 FIGURE 10

SCHEME 26

While the mechanism of this new selective amination
molecule of the primary amine on the imine. Elimination of reaction is not clear at this stage, it may involve “long-range”
ammonia from intermediates 41 (Figure 10, route b) and metalligand cooperation with C9 of acridine as a coopera-
subsequent hydrogenation delivers secondary amines. Thus, tive site, involving dearomatization of the heteroaromatic
it is important to stop the reaction as soon as the alcohol has acridine ring, as observed upon reaction of complex 34 with
been consumed in order to avoid self-coupling of primary ammonia (Scheme 12). 20
amines.
An assortment of primary alcohols was directly converted to Concluding Remarks
primary amines by reaction with ammonia (Scheme 25). The In this Account, we have reviewed our work on new modes
reaction takes place effectively also in neat alcohols, requiring of metalligand cooperation, based on aromatization
no added solvent. dearomatization of the heteroaromatic core in pyridine- and
Interestingly, the reaction can proceed in water as acridine-based pincer complexes. In the pyridine-based com-
reaction medium with excellent selectivity for primary plexes, these processes involve reversible deprotonation of the
amines (Scheme 26). Benzyl alcohols and phenethyl benzylic position, while with the acridine system “long range”
alcohol, which are insoluble in water at room tempera- metalligand cooperation takes place, involving the electro-
ture, form a homogeneous solution on heating while philic C-9 position of the acridine moiety. In addition to the
aliphatic alcohols such as 1-hexanol were not miscible higher stability associated with pincer complexes, the relatively
with water even on heating and the reaction may take low resonance energy of heteroaromatics as compared with
place “on water”. 45 arenes, and stabilization of the dearomatized ligand by the

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