Page 8 - MetalLigand Cooperation by AromatizationDearomatization: A New Paradigm in Bond Activation and Green Catalysis

MetalLigand Cooperation Gunanathan and Milstein

SCHEME 14

FIGURE 5

SCHEME 15
SCHEME 13

SCHEME 16

Interestingly, dearomatization of the central acridine ring
was also observed upon reaction of 34 with ammonia,
leading to complex 37 (Scheme 12). The X-ray structure of
37 shows an unusual fac configuration of the PNP ligand.
Refluxing a toluene solution of primary alcohols with
Presumably, decoordination of the acridine nitrogen fol-
complex 11 (0.1 mol %) as catalyst resulted in formation
lowed by NH 3 coordination places the bent acridine ring in
of the corresponding esters in excellent yields with liberation
a favorable position for hydride transfer to C9. The flexibility 7
of H 2 (Scheme 14). Ester yields of over 90% (TON > 900)
of the acridine PNP ligand enables formation of both mer
were obtained under mild, neutral conditions. Only traces of
(i.e., 3436) and fac (i.e., 37) PNP complexes.
aldehydes were formed. As opposed to the normal ester-
ification of an acid and alcohol, in which an equilibrium
Facile Transformation of Alcohols into Esters,
mixture is generated, the evolved hydrogen shifts the equi-
Amides, and Imines with Liberation of H 2
librium toward completion.
Esters, amides, imines, and amines are important funda- Mechanistically ester formation could occur by a Tischen-
mental building blocks in the chemical industry. Conven- ko type condensation, 25 or hemiacetal formation followed
tional syntheses of these compounds involve carboxylic by its dehydrogenation. 26 Our studies establish that the
acids and their derivatives, often using promoters or cou- latter pathway is operative. Thus, no ester is formed upon
7
pling reagents and leading to much waste. 23,24 Green reaction of benzaldehyde with a catalytic amount of 11,
processes for their production are highly desirable. Guided while reaction of equivalent amounts of benzaldehyde and
by our metalligand cooperation studies, we have devel- benzyl alcohol results in quantitative formation of benzyl
oped catalytic processes for the syntheses of these products benzoate (Scheme 15). 7
directly from alcohols with liberation of molecular H 2 (or H 2 O) The dehydrogenative coupling of alcohols can be carried
as the only byproduct, using no toxic reagents and producing out using the air stable saturated complexes 4 or 10 as
no waste. precatalysts in the presence of one equivalent of base
Reaction of the dearomatized complex 11 with alcohols (relative to these complexes). 7,27 Similarly, 7 catalyzes the
results in OH activation to provide the aromatic coordina- acceptorless dehydrogenation of secondary alcohols to
tively saturated hydridoalkoxy complex 38 (Scheme 13). This ketones. 28
observation, together with the fact that dearomatized pincer Furthermore, complex 11 catalyzes the acylation of sec-
complexes react with H 2 reversibly to form trans-dihydrides ondary alcohols by nonactivated esters, such as ethyl acet-
(Scheme 2), suggested to us the possibility of new catalytic ate, with liberation of H 2 (Scheme 16). 29 When symmetrical
7
reactionsbasedonthedehydrogenationofalcohols (Figure5). esters are used as acylating substrates, both the acyl and

Vol. 44, No. 8 ’ 2011 ’ 588–602 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 595