Acridine Pincer complexes

An air-stable acridine pincer complex 194 was quantitatively prepared on treatment of the tridentate PNP ligand 193 with a ruthenium complex and was applied as a catalyst for the direct synthesis of primary amines from alcohols and ammonia, Scheme 76 (2008AGE8661, 2011ACR588). 

Using the acridine-derived PNP pincer complex 13, primary alcohols can be directly transformed into acetals under neutral conditions [16, 36]. When the reactions were carried out in the presence of 1 equivalent of base (relative to complex 13, 0.1 mol%), the corresponding esters were obtained in very good yields (Scheme 1.7). Mechanistic studies revealed that a p-hydrogen is essential for the acetal formation thus, alcohols such as benzyl alcohol provided benzyl benzoate in 99.5% yield, rather than acetal, even under neutral conditions [16]. 

Ye X, Plessow PN, Hofmann P (2014) Alcohol amination with ammonia catalyzed by an acridine-based ruthenium pincer complex a mechanistic study. J Am Chem Soc 136(16) 5923-5929... 

Recently, we have discovered a new mode of bond activation by metal—Ugand cooperation based on aromatization-dearomatization of pyridine- and acridine-based heteroaromatic pincer complexes [50-60]. Deprotonation of a pyridinyl-methylenic proton of a pyridine-based pincer complex can lead to dearomatization. The dearomatized complex can then activate a chemical bond (H-Y, Y = H, OR, NR2, C) by cooperation between the metal and the ligand, thereby regaining aromatization (Fig. 1). The overall process does not involve a change in the metal oxidation state. In this chapter, we describe the novel, environmentally benign catalytic transformations that operate via this new metal-ligand cooperation based on aromatization-dearomatization processes. 

Interestingly, unlike the pyridine-based PNP and PNN pincer complexes 1 and 4 which did not react with alcohols in neutral media, the acridine-derived air-stable PNP complex 16 catalyzes the conversion of primary alcohols to acetals [75] under neutral conditions (Scheme 7). Small amounts of esters and aldehydes were also formed. Murahashi et al. reported the first acetal formation directly from the alcohols [62]. Efficient direct catalytic transformations of alcohols to acetals are of interest as this circumvents the need for aldehydes or aldehyde derivatives. 

Scheme 6 Synthesis of new PNP ligand and ruthenium(II) pincer complex based on acridine...

Scheme 7 Synthesis of acetals directly from alcohols catalyzed by acridine-based pincer complex 16...

Scheme 8 Esterification of alcohols catalyzed by the acridine-based pincer complex 16 in the presence of base...

The acridine-based PNP pincer complex 16 catalyzes the selective formation of primary amines [73] directly from alcohols and ammonia (Table 6). Among the amines, the terminal primary amines are the most useful, but in general then-selective synthesis is challenging. Primary amines are more nucleophilic than ammonia and compete with it in reaction with electrophiles such as alkyl halides or aldehydes, producing secondary amines, which can also react, leading to the formation of mixtures of products. 

New iron pincer complexes were also prepared. 4,5-Bis(diphenylphos-phino) acridine 195 with iron(II)bromide in tetrahydrofuran gave the... 

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