Adducts of t-butyl-substituted diphosphines with triflic acid: Synthesis, characterization and application to Pd-catalyzed methoxycarbonylation-aminolysis tandem reaction

The acid-base adducts (A and B) as the ionic salts in the format of [L-2H]²⁺[OTf]₂⁻, derived from the neutralization of t-butyl-substituted diphosphines [L1: 1,2-bis[(di-tert-butylphosphanyl)methyl]benzene; L2: 4,5-bis(di-tert-butylphosphino)-9,9-dimethylxanthene] with triflic acid (HOTf, CF₃SO₃H), were synthesized, characterized, and then applied in Pd-catalyzed methoxycarbonylation-aminolysis tandem reaction for the synthesis of linear primary amides. For the first time, the structural configurations of such adducts were deconvoluted by the single crystal X-ray diffraction analysis. In A, the two P-H bonds were distinct, and one of the P-H bond was hydrogen-bonded with CF₃SO₃⁻-anion whereas the two P-H bonds in B were equivalent without hydrogen-bond effect. A became more robust against moisture and oxygen than its counterpart of L1 due to the available hydrogen-bond interaction whereas B became more labile in comparison to L2. When A was applied in Pd-catalyzed methoxycarbonylation-aminolysis tandem reaction, the linear primary amides were obtained in the yields of 69–84% whereas B led to the poor result. It was found that A-Pd(OAc)₂ system not only successfully catalyzed the first-step methoxycarbonylation of alkenes to generate the carboxylic esters with L/B > 90/10 by predominantly following anti-Markovnikov's rule, but also accelerated the second-step aminolysis.