Catalytic Palladium Phosphination: Modular Synthesis of C₁‐Symmetric Biaryl‐Based Diphosphines

Abstract

A new family of C(1)-symmetric bis(diphenylphosphino)biphenyls have been prepared starting from readily available ortho,ortho'-dihalobiphenyl precursors by a palladium-catalyzed C-P coupling reaction. This process does not require the use of an additional ligand. To date, the synthesis of such diphosphines, by reaction of an intermediate biphenyldiyl dianion with ClPPh(2), mainly afforded the undesired cyclic phosphafluorene derivative. So far, no synthetic pathway has been found to avoid this intramolecular reaction. Herein we report the first general and external-ligand-free palladium-catalyzed phosphination reaction that allows the synthesis of a wide variety of substituted ortho,ortho'-bis(diphenylphosphino)biphenyls. With the aim of illustrating the scope and efficiency of this methodology, we applied it to the establishment of a straightforward access to C(1)-symmetrical analogues of the most powerful ligands used in homogenous catalysis and extended it to more challenging substrates.

Preparation of phosphines through C–P bond formation

“…Since no protective groups were introduced, the method proves to be compatible with several functionalities. This methodology or in a slightly modified form has been used by several authors for the phosphination of a large variety of compounds [180–188]. Microwave-assisted procedures have also been developed [189–191].…”

Syntheses of P –aryl- and P –alkyl substituted 2,2′,6,6′–tetraphosphanyl–1,1′–biphenyls

“…The synthesis of fully aryl substituted tetraphosphanes 1 is therefore not possible. Recently, the group of Leroux reported a powerful protocol to couple diphenylphosphanes to 2,2 0 -dihalo-as well as 2,2 0 ,6-trihalo biphenyls to obtain the corresponding di-and triphosphanes [6]. Indeed this protocol allows the coupling of four diphenylphosphanes to the biphenyl skeleton of 3 and opened for us the way to the tetraphosphane 1d, however to the moment in low yield (20% isolated) but high purity.…”

“…For 4a the dialkylphosphine substituents suppress the intramolecular degradation by PC bond cleavage. The all aromatic substituted tetraphosphane 1d can be obtained by a Stelzer coupling employing Leroux's protocol [6]. Furthermore the tetraphosphanes 1a-d react with the common PdCl 2 source Pd(COD)Cl 2 to form mono or dinuclear complexes 7 and 2.…”

“…Indeed treatment of 4a with 5 equivalents of t BuLi lead to the complete conversion of 4a to 6a within 30 min at À80 C. The drawback is a significant faster formation of decomposition products that became visible after another hour stirring at À80 C. For the stability of the dilithiated species two decomposition pathways might be of importance. First, the intramolecular PeC bond cleavage leading to a phosphafluorene [6,32] liberating one equivalent of an organo lithium species. Clearly this is not favored in case of 6a because the isopropyl group is less electronegative than the aromatic system of the biphenyl core.…”

“…[4,10e18] Despite the large number of known 2,2 0 -diphosphanyl-1,1 0 -biphenyls the number of reported examples of 2,2 0 ,6-triphosphanyl biphenyls is limited to a few ones. [6,19] Only in 2012, the first example of a 2,2 0 ,6,6 0 -tetraphosphanyl biphenyl 1a was reported by our group (Scheme 1) [20] The synthetic procedure was facilitated by two consecutive double halogen/lithium exchange and electrophilic quenching reactions starting from 2,2 0 -diiodo-6,6 0 -dibromobiphenyl 3 (Scheme 2). So far we only reported the synthesis of the symmetric 2,2 0 ,6,6 0 -tetrakis(diisopropylphosphanyl)biphenyl 1a and the related dinuclear palladium complex 2a.…”