DOI: 10.1002/adsc.201300091 Calixarene-Derived Mono-Iminophosphoranes: Highly Efficient Ligands for Palladium- and Nickel-Catalysed Cross-Coupling Laure Monnereau, a David SØmeril, a, * and Dominique Matt a, * a Laboratoire de Chimie Inorganique MolØculaire et Catalyse, Institut de Chimie UMR – CNRS 7177, UniversitØ de Strasbourg, 67008 Strasbourg Cedex, France Fax: (+ 33)-3-6885-1637; phone: (+ 33)-3-6885-1719; e-mail: dsemeril@unistra.fr or dmatt@unistra.fr Received: January 30, 2013; Published online: April 30, 2013 Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300091. Abstract: The first mono-iminophosphoranes based on a calix[4]arene skeleton have been synthesised and tested in the arylation of aryl bromides and aryl chlorides. Combining these ligands with [PdACHTUNGTRENNUNG(OAc) 2 ] or [NiACHTUNGTRENNUNG(cod) 2 ] resulted in highly active Suzuki– Miyaura and Kumada–Tamao–Corriu cross-coupling catalysts, respectively. TOFs up to ca. 410 5 mol- ACHTUNGTRENNUNG(ArBr)·mol(M) À1 ·h À1 were obtained in each case. The remarkable activities observed probably arise from the ligands ability to form complexes with cavity-entrapped “MArX” moieties (endo-com- plexes), their highly crowded metal environment fa- vouring formation of mono-ligated intermediates over that of less reactive bis-ligated ones. Possible supramolecular interactions within the cavity involv- ing the receptor wall and the aromatic substrate may also significantly influence the reaction rates, notably by increasing the proportion of endo-complexes. Keywords: calix[4]arenes; cross-coupling; iminophos- phoranes; Kumada–Tamao–Corriu reaction; nickel; palladium; supramolecular catalysis; Suzuki–Miyaura reaction Introduction Iminophosphoranes, also termed phosphazenes or phosphinimines, are analogues of Wittig compounds (R 3 P = CR’ 2 ) in which the alkylidene group has been substituted by an NR unit. They are generally de- scribed in terms of two mesomeric forms, one contain- ing a P =N double bond (ylene), the other having a zwitterionic ylidic (P + À N À ) structure (Scheme 1). The phosphorus substituents strongly influence the P À N bond length and polarisation, and therefore play a major role in controlling the relative weight of the two mesomeric forms. Iminophosphoranes are usually represented as possessing a phosphorus-nitrogen double bond, this accounting for its short length. However, this representation is only used for the sake of simplicity, theoretical analysis showing that the P À N bond is in fact a single bond reinforced by negative hyperconjugation of the lone pair of the nitrogen atom onto other orbitals centred on the phosphorus atom. [1,2] Iminophosphoranes can bind transition metals via their nitrogen atom, [3–12] but their binding strength is rather weak in comparison with that of imines. To in- crease their capacity to form metal complexes, they are often covalently associated with another, stronger donor that enables them to bind as chelate species. It is noteworthy that this class of ligands has only spar- ingly been investigated in homogeneous catalysis. A few studies have focussed on their use in cross-cou- pling reactions, in which the iminophosphoranes em- ployed were all potential chelators, particular exam- ples being bis-iminophosphoranes [13] or iminophos- phoranes containing another coordinating func- tion. [14–16] In the present study, we describe the synthe- sis of the first mono-iminophophoranes based on a calix[4]arene unit and their use in Suzuki–Miyaura and Kumada–Tamao–Corriu (KTC) cross-coupling. Calixarenes are valuable synthons for the construction of receptor ligands and/or of sterically demanding li- gands. [17–19] Scheme 1. The two low-energy mesomeric forms of an imi- nophosphorane. Adv. Synth. Catal. 2013, 355, 1351 – 1360  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1351 FULL PAPERS