Page 12 - MetalLigand Cooperation by AromatizationDearomatization: A New Paradigm in Bond Activation and Green Catalysis

MetalLigand Cooperation Gunanathan and Milstein

FIGURE 8

SCHEME 21 SCHEME 22

This process is the most efficient of the very few ketone SCHEME 23
hydrogenation reactions catalyzed by well-defined iron com-
plexes. 42b,c

Direct Conversion of Alcohols to Acetals and H 2
Unlike the pyridine-based complexes 4 and 10, which do not
7
react with alcohols in neutral media, air-stable acridine-
based complex 34 catalyzes the conversion of primary
alcohols to acetals (Scheme 23). 21 Small amounts of esters
and aldehydes are also formed. Acetal formation from
alcohols was reported by Murahashi et al. 26
Our studies indicate that the reaction proceeds via obser- Selective Synthesis of Primary Amines
vable enolether intermediates, resulting from hemiacetals by The acridine-based 34 catalyzes the selective formation of
water elimination. Thus, formation of the acetal likely takes primary amines directly from alcohols and ammonia
place by addition of the alcohol to the CdCbondofthe (Scheme 25), 43 with no derivatives of alcohols (such as alkyl
enolether (route (ii), Figure 9). halides) being required. Selective formation of primary amines
In the presence of 1 equiv of base (relative to Ru), complex is a challenging task, since being more nucleophilic than
34catalyzes alcohol dehydrogenation to esters (Scheme 24), ammonia they compete with it in reaction with electrophiles. 44
as observed for pyridine based pincer complexes 4 and 10.A Minor amounts of secondary amines are observed in reactions
Ru(0) intermediate, possibly formed by deprotonation of 34, of aliphatic alcohols. Our studies indicate that these might be
maybethe actual catalyst. Further studies suggest that the formed in two pathways: (i) reaction of the product primary
reactions proceed via the hemiacetal intermediate 26 rather amine with an intermediate aldehyde (Figure 10, route a), or (ii)
than by a Tischenko disproportionation 25 of an aldehyde NH activation of the initially formed primary amine followed
intermediate. by β-H elimination and nucleophilic attack by another

Vol. 44, No. 8 ’ 2011 ’ 588–602 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 599