The axially chiral ligands 2-(diphenylphosphanyl)-2'-methoxy-1,1'-binaphthalene (MOP; 6) and 2'-dimethylamino-2-(diphenylphosphanyl)-1,1'-binaphthalene (MAP; 7) coordinate to a cationic allylpalladium fragment in an unusual bidentate (P,C)-mode through the triarylphosphane and ipso-carbon atom (C1'). The readily prepared MAP and MOP complexes [Pd[(P,C)-(L)](n3-allyl)][OTf] (9 (L = 7) and 10 (L = 6)) have been characterised in solution (NMR), in which two diastereoisomeric rotamers are observed. The stereochemical identity of the rotamers is established by one- and two-dimensional NMR spectroscopy experiments. In both the solid state and in solution, the allyl unit is shown to coordinate in a slightly distorted n3-mode that results in a more alkene-like character at the allyl terminus trans to phosphane ligand. The opposite allyl terminus, which is trans to the ipsocarbon atom (C1'), is more strongly bound and the dominant allyl stereodynamic process involves C-C bond rotation in an n'-allyl intermediate bound through this carbon. Palladium complexes of MAP and MOP are very efficient catalysts for allylic alkylation of racemic cyclopentenyl pivalate with [NaCH(CO2Me)2] in THF. Isotopic desymmetrisation revealed that the reaction occurs with powerful stereochemical memory effects and consequently with low global ee values. The memory effect is suggested to arise through selective generation of diastereoisomeric [Pd[(P,C)-L](n3-cyclopentenyl)]+ ions (L = MAP or MOP) and subsequent capture by nucleophile before ion-pair collapse or equilibration occurs.

A versatile synthesis of 2-arylpyrroles and 2-arylindoles is described based on the use of either N-(Boc) pyrrole-2-boronic acid or N-(Boc) indole-2-boronic acid as components for Suzuki coupling.We have described the design of a series of 2,5-disubstituted pyrroles 1 as selective dopamine D 3 receptor antagonists, the synthesis of which required 2-arylpyrroles 2 as key intermediates. 1 Initial approaches to 2 involved reaction of the appropriate benzoyl chloride 3 with Grignard reagent 4 and subsequent treatment with ammonium acetate in a modification of a known procedure. 2 This method is, however, incompatible with the presence of acidic protons or basic nitrogens in 2, and each desired substitution pattern requires repetition of a lengthy reaction sequence. An alternative strategy based on palladium-catalysed aryl cross-coupling methodology offered promise as a shorter, direct and more flexible route and here we describe the successful implementation of this approach.The few literature examples of palladium-catalysed cross coupling involving metallated pyrroles have mainly described the use of magnesium, zinc and tin species. 3,4,5 The use of N-(triisopropylsilyl)pyrrole-3-boronic acid as a Suzuki coupling substrate has been reported, 5 and more recently, the coupling of N-(Boc) pyrrole-2-boronic acid 6 5 to a pyrrole-2-triflate has been described. 7 The use of N-(phenylsulfonyl) pyrrole-2-boronic acid in Suzuki couplings has also been reported. However, the synthesis of this boronic acid starting material proceeds in only 7.5% yield. 8The wide availability of aryl halides, together with the reported stability of 5 suggested a general methodology for the synthesis of 2-arylpyrroles. The N-Boc pyrrole 5 can be readily prepared on a 20 g scale according to the procedure of Martina et al. 6 and the cross coupling reactions of 5 with a range of aryl halides were carried out using the Gronowitz conditions. 9 A mixture of the appropriate aryl iodide or bromide (ArX), tetrakis-(triphenylphosphine)palladium(0) (5 mol%) and 5 (1.4 equivalents) in 1,2-dimethoxyethane, with an excess of aqueous sodium carbonate as base, was heated at reflux for 0.5-18 h to give the corresponding N-Boc aryl pyrroles 6 10 (Scheme 1). The results of the study are summarised in Table 1. For the simple monosubstituted and unsubstituted aryl halides (entries 1-7), a trend was observed towards greater reactivity with increased electron deficiency of the Ar group. In the case of Ar = Ph, iodobenzene (entry 1) gave a cleaner reaction than bromobenzene (entry 2).In all instances the major competing side reactions were deboronation of 5 to give N-(Boc) pyrrole and production of the homodimer 7. 11 The 2-methoxy substituted halides (entries 8-11) gave particularly high yields, possibly due to participation by the 2-methoxy group in the coordination of the intermediate arylpalladium species, and this methodology proved to be compatible with the presence of acidic NH residues (entries 9 and 10) and with the presence of a basic nitrogen centre (entry ...

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Reaction of the C2-symmetric "Trost modular ligand" with cationic Pd(II) allyl fragments allows isolation of air- and bench-stable pro-catalysts for the asymmetric allylic alkylation of racemic cycloalkenyl esters. In solution, three distinct complexation modes are observed. When mixed in a ligand/Pd ratio of 1/2, a binuclear bis-P,O-chelate complex is generated. This species does not induce enantioselectivity in the reaction. In contrast, with a ligand/Pd ratio of 1/1, a highly enantioselective, P,P-coordinated pro-catalyst system is generated in which there are two basic coordination modes: monomeric and oligomeric. The monomeric form is mononuclear and exists as two 13-membered chelates, isomeric through loss of C2-symmetry in the ligand. The oligomeric form is polynuclear and forms chains and rings of alternating ligand and cationic Pd(allyl) units, one of which was identified by single-crystal X-ray diffraction. In solution, the monomeric and oligomeric species are in dynamic equilibrium with populations and interconversion rates controlled by concentration, temperature, and counterion. Isotopic desymmetrization analysis suggests that the monomer-oligomer equilibrium plays a crucial role in both the selectivity and efficiency of the asymmetric allylic alkylation reaction.

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Organic chemistryOrganic chemistry Z 0200 Coordination of the Trost Modular Ligand to Palladium Allyl Fragments: Oligomers, Monomers, and Memory Effects in Catalysis-[42 refs.]. -(LLOYD-JONES, G. C.; STEPHEN, S. C.; FAIRLAMB, I. J. S.; MARTORELL, A.; DOMINGUEZ, B.; TOMLIN, P. M.; MURRAY, M.; FERNANDEZ, J. M.; JEFFERY, J. C.; RIIS-JOHANNESSEN, T.; et al.; Pure Appl. Chem. 76 (2004) 3, 589-601; Sch. Chem., Univ. Bristol, Cantock's Close, Bristol BS8 1TS, UK; Eng.) -Lindner 39-253