Direct Conversion of Alcohols to Acetals and H₂ Catalyzed by an Acridine-Based Ruthenium Pincer Complex

Abstract

The crystallographically characterized ruthenium complex RuHCl(A-(i)Pr-PNP)(CO) (1) [A-(i)Pr-PNP = 4,5-bis-(diisopropylphosphinomethyl)acridine], which bears a nonplanar acridine moiety, catalyzes in a neutral medium the transformation of primary alcohols to the corresponding acetals with the liberation of H(2). In the presence of base, complex 1 catalyzes the dehydrogenative coupling of alcohols to form esters. Acetal formation may involve hemiacetal dehydration to form an enol ether followed by alcohol addition to the double bond.

Cleavage of unreactive bonds with pincer metal complexes

“…41 This procedure regenerates the benzyl complex 30. Similarly, 15 SnMe 4 in the presence of KO t Bu has been shown to induce methylene insertion into the Rh-C aryl bond. 44 Catalytic CH 2 transfer is thus possible and has indeed been demonstrated.…”

“…Cooperative participation of 10 the ligand in the C-H bond activation of acetone has been postulated, involving the addition of the C-H bond across the metal-ligand unit. Deuterium labeling studies support a stepwise process including initial oxidative C-H activation at iridium and subsequent transfer of the hydrogen to the 15 benzylic position of the ligand with concomitant aromatization. Theoretical analyses (DFT) support the proposed mechanism and predict the tricoordinate PNP iridium complex A as a critical intermediate.…”

“…Ligand cooperativity as evidenced in PNP and PNN pincer complexes is an attractive concept, and it is tempting to consider similar reaction patterns in the PNP iridium system used by Grubbs and coworkers for the formation of carbene complexes from methyl ethers and amines. In PCP iridium- 15 catalyzed alkane dehydrogenation, however, all mechanistic evidence obtained thus far suggests a different reaction pathway that does not invoke de-and re-aromatization of the aryl ligand. 7 In addition, a number of ligand scaffolds that lack the possibility to reversibly de-aromatize are known to 20 activate C-H bonds.…”

“…52 Elegant competition experiments have uncovered a pre-equilibrium involving iridium coordination either to RX or to HSiEt 3 . This 15 equilibrium is responsible for the unusual preference of catalyst 40 to convert pure RBr faster than pure RCl and much faster than RI. In mixtures, however, the expected reactivity sequence RI > RBr > RCl is restored, as the pre-equilibrium is shifted to HSiEt 3 coordination when the better coordinating 20 iodoalkane is present only in minor quantities.…”

One‐Pot Synthesis of Carbamoyl Azides Directly from Primary Alcohols and Oxidation of Secondary Alcohols to Ketones Using Iodobenzene Dichloride in Combination with Sodium Azide

“…[5] More interesting, primary alcohols can also be directly converted to primary amines using a ruthenium pincer complex as the catalyst in the presence of ammonia. [6] Alcohols can also be directly converted to lactams, [7] acetals, [8] benzazoles. [9] Carbamoyl azides are valuable synthetic intermediates in organic synthesis.…”