Copper(I) Complexes of Amide Functionalized Bisphosphine: Proximity Enhanced Metal–Ligand Cooperativity and Its Catalytic Advantage in C(<i>sp</i><sup>3</sup>)–H Bond Activation of Unactivated Cycloalkanes in Dehydrogenative Carboxylation Reactions
Harish S. Kunchur, Sachin C. Sonawane, Prateek Saini, Srinivasan Ramakrishnan, Maravanji S. Balakrishna
Abstract
The reactions of amide functionalized bisphosphine, o -Ph 2 PC 6 H 4 C-(O)N(H)C 6 H 4 PPh 2 - o ( 1 ) (BalaHariPhos), with copper salts is described. Treatment of 1 with CuX in a 1:1 molar ratio yielded chelate complexes of the type [CuX{( o -Ph 2 PC 6 H 4 C(O)N(H)C 6 H 4 PPh 2 - o )}-κ 2 -P,P ] (X = Cl, 2; Br, 3; and I, 4 ), which on subsequent treatment with KO t Bu resulted in a dimeric complex [Cu( o -Ph 2 PC 6 H 4 C(O)(N)C 6 H 4 PPh 2 - o )] 2 ( 5 ). Interestingly, complexes 2 – 4 showed weak N–H···Cu interactions. These weak H-bonding interactions are studied in detail both experimentally and computationally. Also, Cu I complexes 2 – 5 were employed in the oxidative dehydrogenative carboxylation (ODC) of unactivated cycloalkanes in the presence of carboxylic acids to form the corresponding allylic esters. Among complexes 2 – 5, halide-free dimeric Cu I complex 5 showed excellent metal–ligand cooperativity in the oxidative dehydrogenative carboxylation (ODC) in the presence of carboxylic acids to form the corresponding allylic esters through C( sp 3 )–H bond activation of unactivated cycloalkanes. Mechanistic details of the catalytic process were established by isolating the precatalyst [Cu{( o -Ph 2 PC 6 H 4 C(O)(NH)C 6 H 4 PPh 2 - o )-κ 2 -P,P }(OOCPh)] ( 6 ) and fully characterized by mass spectrometry, NMR data, and single-crystal X-ray analysis. Density functional theory-based calculations further provided a quantitative understanding of the energetics of a series of H atom transfer steps occurring in the catalytic cycle.