Synthesis of the First C-2 Cyclopalladated Derivatives of 1,3-Bis(2-pyridyl)benzene. Crystal Structures of [Hg(N-C-N)Cl], [Pd(N-C-N)Cl], and [Pd₂(N-C-N)₂(μ-OAc)]₂[Hg₂Cl₆]. Catalytic Activity in the Heck Reaction

Abstract

The synthesis of C-2 cyclopalladated derivatives of 1,3-bis(2-pyridyl)benzene (N-CH-N) has been achieved for the first time through a transmetalation reaction. C-2 mono- and dinuclear complexes have been obtained by reaction of Pd(OAc)2 and the organomercury(II) compound [Hg(N-C-N)Cl] (1) as precursors. The new species 2−11 were characterized in solution and, in the case of [Pd(N-C-N)Cl] (2) and [Pd2(N-C-N)2(OAc)]2[Hg2Cl6] (11) as well as of [Hg(N-C-N)Cl] (1), in the solid state. The X-ray crystal structures unambiguously show the pincer behavior of the N-C-N ligand in the palladium species and the linear coordination C−Hg−Cl in the mercury precursor. Displacement of the coordinated halide gives cationic species such as [Pd(N-C-N)(H2O)][BF4] (5) or, in the absence of donor solvents or anions, the dinuclear halide-bridged species [Pd2(N-C-N)2(μ-X)][BAr‘4] (X = Cl (6), Br (7), I (8); Ar‘ = 3,5-(CF3)2-C6H3). For comparison purposes [Pd(N-C-N)(OAc)] (9) and [Pd2(N-C-N)2(μ-OAc)][BAr‘4] (10) were also prepared. The mononuclear complexes 2, 4, 5, and 9 are high-performance catalysts in the Heck reaction of iodobenzene and methyl acrylate, even at moderate temperatures. Under microwave heating a turnover frequency as high as 280 000 h-1 is observed with complex 2 at the nominal temperature of 135 °C.

Unsymmetrical, oxazolinyl-containing achiral and chiral NCN pincer ligand precursors and their complexes with palladium(II)

“…As shown in Table 4, when the reaction was conducted in DMF at 140°C for 12 h, KFÁ2H 2 O, K 2 CO 3 and Na 2 CO 3 as a base gave excellent yields (entries 1-6). When toluene was used as solvent, excellent yields were also obtained at 110°C after 12 h with KFÁ2H 2 O, K 2 CO 3 , K 3 PO 4 Á3H 2 O and NaOH as a base (entries [7][8][9][10][11][12]. Several other solvents using K 2 CO 3 as the base were also tested.…”

“…Much of the research has focused on symmetrical NCN pincer palladium complexes, which are symmetrical with two identical N donors such as amines [5][6][7], imines [8,9], pyridines [10][11][12], oxazolines [13][14][15][16][17][18][19][20] or other N-containing heterocycles [21,22] and 2 equiv. five-membered palladacycles (Scheme 1).…”

“…five-membered palladacycles (Scheme 1). Some of the complexes have been successfully applied as catalysts for stannylation of allyl [5] or propargylic substrates [6], Diels-Alder reaction [15], Heck [8,10,21,22], Suzuki and Sonogashira [22] coupling reactions. And the chiral complexes proved to be effective in asymmetric Michael reaction between a-cyanocarboxylates and methyl vinyl ketone (up to 83% ee) [8,17].…”

Three types of reactions with intramolecular five-membered ring compounds in organic synthesis

“…Pincer compounds are also used as the catalysts of the metal-catalyzed cross-coupling reactions described in the Section 4.2 [207][208][209][210][211][212][213]. For example, the Heck reactions [207][208][209][210] with the pincer palladium compounds, proceed in high yields and high TON's as shown in Eq.…”

“…For example, the Heck reactions [207][208][209][210] with the pincer palladium compounds, proceed in high yields and high TON's as shown in Eq. (52-1).…”