Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue 3 Pd-h -C6H9 complexes of the Trost modular ligand: high nuclearity columnar aggregation controlled Cite this: Chem. Sci.,2015,6,5793 by concentration, solvent and counterion† Daugirdas Tomas Racys,‡a Julian Eastoe,a Per-Ola Norrby,bc Isabelle Grillo,d Sarah E. Rogerse and Guy C. Lloyd-Jones*f Under optimised conditions, the Trost modular ligand (TML) series induces high levels of asymmetric induction in an extraordinarily wide range of reactions involving palladium p-allyl intermediates. Prior 3 mechanistic investigations into reactions involving Pd-h -C6H9 intermediates have focussed on the monomeric 13-membered ring formed via P,P-chelation of the ligand to Pd. However, it is also 3 recognised that ring-opening oligomerisation provides a pool of high nuclearity Pd-h -C6H9 species that, by affording a low level, or even the opposite sense, of asymmetric induction relative to the mononuclear species, are responsible for a reduction in selectivity under non-optimised conditions. Creative Commons Attribution 3.0 Unported Licence. Herein we describe an investigation by NMR spectroscopy, molecular mechanics, molecular dynamics, 3 and small-angle neutron scattering (SANS), of a Pd-h -C6H9 cation bearing the 1,2-diaminocyclohexane Received 2nd April 2015 TML ligand (2). Using both nondeuterated and perdeuterated (D ) isotopologues of the resulting Accepted 12th June 2015 47 complexes ([1]+), we show that a two-stage oligomerisation-aggregation process forms self assembled DOI: 10.1039/c5sc01181g cylindrical aggregates of very high nuclearity (up to 56 Pd centres). We also investigate how www.rsc.org/chemicalscience concentration, solvent and counter-anion all modulate the extent of oligomerisation. high-enantiopurity chiral building blocks. However, reactions Introduction 8,9 This article is licensed under a involving the TML can exhibit memory effects, and a high The Trost modular ligand (TML) series1 has been applied to an sensitivity of the enantioselectivity to reaction temperature, extraordinarily wide range of allylic alkylation (Tsuji-Trost) catalyst concentration, solvent and nucleophile counter-ion. 2 Open Access Article. Published on 15 July 2015. Downloaded 9/28/2021 7:38:12 AM. reactions. Under carefully optimised conditions, these ligands frequently provide very high enantioselectivity in reactions that have proven challenging with other chiral ligands, particularly those involving cyclic allylic substrates.3–6 These features have led to broad use of the TML in the synthesis of natural prod- ucts,7 as well as industrial application for the construction of aSchool of Chemistry, University of Bristol, Office S314, Cantock's Close, Clion, Bristol BS8 1TS, UK. E-mail: [email protected] Our previous mechanistic studies of this system focussed on b + 3 AstraZeneca Pharmaceutical Development, Global Medicines Development, the monomeric cationic complex [1] , in which a Pd(h -C6H9) Pepparedsleden 1, SE-431 83 Molndal,¨ Sweden. E-mail: per-ola.norrby@ unit is chelated by the 1,2-diaminocyclohexane-derived TML astrazeneca.com ligand (2).10 The monomeric cation [1]+ was identied as an cDepartment of Chemistry and Molecular Biology, University of Gothenburg, Kemigarden˚ 4, #8076, SE-412 96 Goteborg,¨ Sweden intermediate capable of leading to high asymmetric induction dILL, CS20156, 38042 Grenoble Cedex 9, France. E-mail: [email protected] on attack of, for example, a malonate anion nucleophile, eISIS-STFC, Rutherford Appleton Laboratory, Chilton, Oxon, OX11 0QX, UK. E-mail: Scheme 1. Detailed NMR studies facilitated by isotopic labelling [email protected] – in conjunction with MM-DFT simulations – led to a model10 in fSchool of Chemistry, University of Edinburgh, Joseph Black Building, West Mains which the amide units in the catalyst facilitate enantioselective Road, Edinburgh, Scotland EH9 3JJ, UK. E-mail: [email protected] ligand-accelerated catalysis.11 † Electronic supplementary information (ESI) available: Synthesis and For the ligand-accelerated catalysis to function efficiently, Characterisation of compounds, and full details of aggregation studies by NMR, + SANS, and MM3. See DOI: 10.1039/c5sc01181g cation [1] requires a degree of exibility. This exibility is ‡ Current address: School of Chemistry, University of Glasgow, Joseph Black provided by the 13-membered chelate ring, but at a cost: + Building, University Avenue, Glasgow, Lanarkshire G12 8QQ, UK. E-mail: complex [1] can readily undergo ring-opening oligomerisation [email protected]. This journal is © The Royal Society of Chemistry 2015 Chem. Sci.,2015,6,5793–5801 | 5793 View Article Online Chemical Science Edge Article Scheme 1 Asymmetric allylic alkylation of racemic 2-cyclohexenyl acetate; 2 ¼ 1,2-diaminocyclohexane TML ligand, THAB ¼ tetrahexyl- ammonium bromide. Fig. 1 Speciation of [(R,R)-1][BArF] (BArF ¼ B(C6F5)4) determined by 31 1 P{ H} NMR (CD2Cl2,25 C). Solid lines through data are solely a guide to the eye. Inset shows proportion (%) of [Pd]tot that is in oligomeric form. 16 h3 allyl complexes of ligand 2 are -C3H5 complexes with tri ate counter anions: one a racemic tetranuclear cyclo-oligomer,12 the 17 Creative Commons Attribution 3.0 Unported Licence. other an acyclic dinuclear bis-P,O-chelate. The extent of solution-phase oligomerisation of cationic + 31 1 Scheme 2 Oligomerisation of [1]+ erodes enantioselectivity during complexes of type [1] can be conveniently estimated by P{ H} 10,12 asymmetric Pd-catalysed allylic alkylation mediated by 2 (as Scheme NMR spectroscopy. Analysis of [(R,R)-1][BAr4] complexes in 1). Nu ¼ nucleophile. CH2Cl2, where Ar ¼ C6Cl5, 3,5-(CF3)2C6H3,orC6F5 (“BArF”), indicates a maximum monomer concentration ([1]+) of about + 12 4 mM, Fig. 1. In THF, the monomer maximum is lower (approx. to generate polynuclear species ([1] )n, Scheme 2. Competing 1.6 mM) and decreases as [Pd] is raised above 10 mM. With nucleophilic attack on the oligomer, rather than the monomer tot smaller, less charge-diffuse, counter-anions such as chloride or [1]+ is, in part, responsible for a reduction in overall enantio- This article is licensed under a triate, the maximum monomer concentrations are lower still. selectivity under non-optimised conditions.13,14 We were unable to t simple analytical solutions18 for mono- To date, the structure and origin of the formation of these mer-oligomer distributions to any of the 31P NMR data, indic- oligomeric species has not been studied in detail. Herein, we Open Access Article. Published on 15 July 2015. Downloaded 9/28/2021 7:38:12 AM. ative that physicochemical effects dominate over simple describe an investigation of the oligomerisation of D0 and D47 solution-phase equilibria, even at low [Pd] . isotopologues of [1]+, employing NMR spectroscopy, molecular tot We thus elected to study the oligomeric species by SANS – a mechanics (MM), molecular dynamics (MD), and contrast technique that can be used for characterising the shape and variation small-angle neutron scattering (SANS). The data dimensions of self-assembly structure and colloids.19 We obtained indicate that the impact of a rst-stage of depletion of beganwith[(R,R)-1][BAr ],20 and, to aid the studies, also syn- the monomeric species [1]+ from the catalyst pool, via cyclic F thesised the perdeuterated enantiomeric complex [(S,S)-[D ]- oligimerisation, is amplied by a second-stage process 47 1][BAr ].NotonlydoesthisfacilitateSANSinanon-deuter- involving columnar aggregation of the oligomers, leading to F ated solvent, thus providing greater neutron scattering species with very high nuclearity (up to 56 Pd centres). The contrast,19 it also allows pseudo racemic and pseudo scalemic effects of solvent, ligand enantiopurity and counter-ion on the mixtures to be prepared by mixing [(S,S)-[D ]-1][BAr ]with degree of aggregation are explored in detail, and it is concluded 47 F [(R,R)-1][BAr ]. The perdeuterated complex was synthesised that a relatively small and restricted set of conditions facilitate F from benzoic acid ([D ]-3), chlorobenzene ([D ]-4)and dissolution of the complexes in a low aggregation state, 5 5 cyclohexene ([D ]-5), Scheme 3. A major hurdle was the consistent with the extensive optimisation frequently required 10 ortho-metallation of ester [D ]-6 with (TMP) Mg$LiCl,21 which for these catalyst systems. 5 2 proceeded with an unexpectedly large net kinetic isotope 22 effect (kH/kD z 30). This required an excess of base to be Results and discussion employed, and interfered with a planned direct phosphina- tion of the metallated intermediate. Instead, the intermediate Preliminary NMR studies and synthesis of [D47]-[1][B(C6F5)4] was trapped with I2. The iodide [D4]-7 was then converted to a Despite extensive efforts,15 we have been unable to crystallise any more conventional Grignard reagent,23 before reaction with 3 10,24 Pd(h -C6H9) complexes of 2, in either oligomeric or monomeric chlorophosphine [D10]-8 to give phosphine [D14]-9,and 25 forms. Indeed, to date, the only the X-ray crystal structures of Pd- thus acid [D14]-10. 5794 | Chem. Sci.,2015,6,5793–5801 This journal is © The Royal Society of Chemistry 2015 View Article Online Edge Article Chemical Science 26 Cyclohexene [D10]-5, was epoxidised, to give [D10]-11, and 27 28 this ring-opened to give aminoalcohol [D10]-12. Aziridine [D10]-13, obtained under Mitsunobu conditions, was converted 29 to azide [D10]-14. A er diastereoisomer separation, hydro- 30 genolysis gave (S,S)-diamine [D10]-15, which was coupled with acid [D14]-10 to afford Trost ligand [D38]-2. The chloro-bridged 31 dimer [D18]-16, prepared from [D10]-5, was converted to cationic complex [D9]-17, and then reacted with [D38]-2 to generate [(S,S)-[D47]-1][BArF] in good yield. SANS analysis of aggregation of [1][BArF] in THF 32 We began with SANS analysis of enantiopure [(R,R)-1][BArF]in THF-D8 at 25 C, with [Pd]tot concentrations 16 to 64 mM, well above the oligomerisation threshold.