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Cite this: DOI: 10.1039/c5dt00667h

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Cyclodextrin and phosphorus(III): a versatile combination for coordination chemistry and catalysis† Matthieu Jouffroy, Dominique Armspach* and Dominique Matt* With the advent of efficient methods for functionalising cyclodextrins, the synthesis of a variety of cyclodextrin-based P(III) ligands has been made possible. Capable of acting both as first and second coordi-

Received 13th February 2015, Accepted 27th May 2015 DOI: 10.1039/c5dt00667h www.rsc.org/dalton

nation sphere ligands towards various transition metals, these compounds have found many applications in homogeneous catalysis. This perspective article describes the different approaches that have been used to covalently associate the ubiquitous P(III) donor atom with a cyclodextrin cavity. In addition, special emphasis is placed on the influence the cyclodextrin receptor has on the coordination and catalytic properties of these cavity-shaped ligands.

1.

Laboratoire de Chimie Inorganique Moléculaire et Catalyse, Institut de Chimie UMR 7177 CNRS, Université de Strasbourg, 4, rue Blaise Pascal, 67070 Strasbourg cedex, France. E-mail: [email protected], [email protected] † Dedicated to Dr Loïc Toupet on the occasion of his retirement. With our warmest wishes.

Matthieu Jouffroy (1988) received his Master’s degree in Molecular and Supramolecular Chemistry from the University of Strasbourg in 2011. The same year he joined the Laboratoire de Chimie Inorganique Moléculaire et Catalyse at the University of Strasbourg to study for a Ph.D. under the joint supervision of Dr D. Matt and Prof. D. Armspach, which he completed in 2014. He is currently working as a JSPS Matthieu Jouffroy postdoctoral fellow at Tokyo Tech with Professor K. Osakada and Dr D. Takeuchi. In 2015, he will become a postdoctoral researcher at the University of Pennsylvania within the laboratory of Prof. G. Molander. His current research interests are focused on the synthesis of confining phosphines built on cyclodextrin scaffolds and their use in homogenous catalysis.

This journal is © The Royal Society of Chemistry 2015

Introduction

Since their discovery by Villiers at the end of the 19th century,1 cyclodextrins (CDs) have gathered huge interest among the scientific community.2 The considerable number of publications related to CDs (no less than 72 000 since their discovery) reflects the many applications these naturally occurring and non-toxic cyclic oligosaccharides have found in fields as diverse as drug formulation,3 analytical chemistry,4,5 bio-

Dominique Armspach studied chemistry at the Ecole Nationale Supérieure de Chimie de Mulhouse (France). He completed his Ph.D. at the University of Birmingham (UK) in 1994 on the synthesis of catenated cyclodextrins under the supervision of Professor Sir. J. F. Stoddart. The same year he joined the University of Basel (Switzerland) to carry out research as a postdoctoral fellow within the group of Dominique Armspach Professors E. C. Constable and C. Housecroft. In 1996, he was appointed lecturer in Organic Chemistry at the Robert Schuman University in Strasbourg (France) before being promoted to full professor eleven years later at the University of Strasbourg (France). His present research interests focus on synthetic methodology, supramolecular chemistry, organometallic chemistry and catalysis associated with molecular receptors.

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Fig. 1 Two different representations of native CDs. In the left one, the CDs are depicted as seen from the secondary face, with the successive glucose units being designated by letters ranged counterclockwise in alphabetical order.

mimetic chemistry,6,7 inorganic chemistry,8 catalysis,9–14 polymers,15 artificial sensors,16 and food industry.17,18 The commercially available CDs, which are the only ones with well-defined and rigid cavities, are made of six, seven or eight glucopyranose units (α-, β- and γ-CD, respectively).19 With their α-1,4-linked 4 D-(+)-glucopyranose units rigidly held in the standard C1 chair conformation, these CDs, whether native or chemically modified, adopt a stable conical shape.20,21 Native CDs comprise primary hydroxyl groups located at the narrow end of the cone called primary hydroxyl face or primary face, and secondary hydroxyl groups at the wider rim called secondary hydroxyl face or secondary face (Fig. 1). Unlike most synthetic cavitands, native CDs behave as water-soluble and chiral host molecules for many lipophilic compounds in aqueous media.22,23 Because of the possibility of introducing various donor groups at given locations on the macrocyclic structure, the CD skeleton (native or chemically modified) constitutes a versatile

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preorganisation platform for the synthesis of ligands that can act both as first and second coordination spheres for transition metals. This particular feature is of prime importance when the stabilisation of unusual metal organic species, notably catalytic ones, is sought.24–28 Among all coordinating atoms that have been anchored to a CD,29–31 phosphorus(III) is certainly the one that has generated the most interest in recent years because of its widespread use in catalysis.32 Thus, numerous CD derivatives bearing appended P(III) ligands have been designed to combine the exceptional host–guest properties of the chiral CDs with the versatile coordination and catalytic properties of the P(III) atom. This perspective article focuses on the synthesis and applications of these hybrid ligands, most notably in coordination and catalytic chemistry. Aqueous organometallic catalysis as well as supramolecular assemblies involving non-covalently bonded CDs and P(III) ligands will not be discussed here as this has already been recently reviewed by Monflier et al.33,34 The present article consists of two parts, the first dealing with CDs substituted with a single P(III)-containing unit (Scheme 1, left part), the second concerning multisubstituted CDs (Scheme 1, right part). Special emphasis will be placed on the ability of the CD-P(III) hybrids to stabilise unusual organometallic species, to behave as encapsulating units and to act as supramolecular catalysts. Where possible, comparison between different systems will be made in order to clarify the influence of the CD cavity on the coordination properties of the P(III) atom(s) and the catalytic outcome of reactions. Also, a rapid overview of the various functionalisation methods enabling introduction of the phosphorus moiety on the torus will be given. In schemes and figures fat arrows designate molecular structures recovered from the Cambridge Crystallographic Data Centre (CCDC). For clarity, these do not include counterions and solvents nor hydrogen atoms. Carbon atoms are depicted in beige, whilst oxygen, nitrogen, phosphorus, and sulfur atoms are in red, blue, orange and yellow, respectively.

2. Monosubstituted CDs Dominique Matt obtained his Doctorat d’Etat under Dr Pierre Braunstein at the University Louis Pasteur (ULP). After postdoctoral studies with Professor L. M. Venanzi at ETH Zurich, he got a research position at BASF’s Central Laboratories in Ludwigshafen (Germany). He then joined the CNRS in Strasbourg, becoming Directeur de Recherche in 1991. His research interests are centred on the interface of Dominique Matt coordination, organometallic and supramolecular chemistry, with an emphasis on the study of metallocavitands.

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Several CD platforms have been equipped with coordinating atoms in order to take advantage of the remarkable host– guest properties of CDs for stabilising unusual transition metal complexes or delivering a substrate with a given shape and size to a catalytic centre. Both features could lead to the development of new transition metal catalysts with improved selectivities and activities. Because of the ease with which they can be obtained, both native and modified CDs substituted with a single coordinating fragment, whether monodentate or multidentate, were the first to be studied. This section will start with a brief overview of the methods giving access to monofunctionalised CDs that can be used for grafting a P(III)-containing fragment. Monodentate P(III) ligands will be considered first, before focusing on systems having a dangling chelating fragment, whether homo- or heteromultidentate.


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Perspective

Scheme 1 CD receptors with covalently appended phosphorus(III) units and their metal complexes. Left frame: CDs in which a single anchoring point has been substituted; right frame: multisubstituted CDs.

2.1

CD monofunctionalisation

CDs monosubstituted at either the primary or secondary face have become available since the 1990’s and a number of these derivatives are now commercially available.35 In fact, CD monofunctionalisation has been covered as far back as 1998 by D’Souza et al. in a special issue of Chemical Reviews dedicated to CDs.21 A good method for synthesising CDs monosubstituted by a P(III) fragment consists in performing SN2 reactions on permethylated mono-6-O-tosyl-CDs with various P(III) containing nucleophiles, notably phosphides36 or PR2substituted phenolates.37 Permethylated mono-6-O-tosyl-CDs themselves can be obtained straightforwardly by methylating a (non-alkylated) mono-6-O-tosyl-CD with methyl iodide in the presence of NaH as base ( preferred solvent: DMF).38–40 Note that large quantities of pure mono-6-O-tosyl-CD can only be obtained from β-CD,41 and unsurprisingly, most monosubstituted, P(III)-containing CDs are β-CD derivatives. In some cases, the P(III) fragment was introduced after the tosyl group had been reacted with a nucleophile suitable for further functionalisation (typically sodium azide42 or amines43). Permethylated mono-6-O-tosyl-CDs offer more possibilities than their polyhydroxylated counterparts for the introduction of phosphine groups because these derivatives are compatible with strongly basic conditions and are easier to purify. Nevertheless, with weakly basic nucleophiles, the substitution can be performed directly on β-mono-6-O-tosyl-CD.44 On rare occasions, native or partially methylated CDs were reacted with P(III) containing electrophiles (e.g. R2PCl) under basic conditions to generate CDs monosubstituted at either the primary45 or the secondary face.46 Scheme 2 summarises the various approaches that have been used for introducing a single P(III)-containing fragment.


2.2

Monodentate ligands

Phosphinites 1 and 2 were synthesised by Ichikawa et al. by reacting respectively α- and β-CD with chlorodiphenylphosphine in pyridine. Both ligands were tested in the hydroformylation of mixtures of 1-decene and 4-phenyl-1-butene in DMF (Scheme 3).45 While 1 showed practically no substrate selectivity (like PPh3), 4-phenyl-1-butene was hydroformylated 1.7 times faster than 1-decene when using 2. Contrary to the authors’ suggestion, this effect cannot arise from the formation of an inclusion complex between the aromatic substrate and the β-CD cavity, as DMF is known to prevent such non-covalent interactions.34 Hydroformylation regioselectivities were determined for 1-decene, 4-phenyl-1-butene, 1-octene, and styrene only in the case of ligand 2. The linear to branched aldehyde selectivities (l/b = 2.8, 2.8, 0.4 and 1.8 for the four above substrates, respectively) are comparable to those obtained with Rh/PPh3 catalysts.47–49 From 2010 onwards, Monflier et al. developed a series of phosphines in which the P(III) coordinating unit is covalently linked to a fully methylated β-CD (4 and 5) or to the more water-soluble, randomly methylated β-CD (RAME-β-CD) platform (6). All of them were tested in metal-catalysed reactions (in water or heptane) such as hydrogenation or hydroformylation of olefins.36 Phosphines 4 and 5 were synthesised by nucleophilic substitution of the tosyl group of 3 with diphenylphosphide and 4-(diphenylphosphino)phenolate, respectively (Scheme 4). Extensive NMR experiments in D2O proved that one of the phenyl rings of 4 is included in the β-CD cavity, however this being no longer the case in organic solvents. The observed self-inclusion is strong enough to prevent the formation of a stable inclusion complex in water with 1-adamantanecarboxylate (ACNa), known for being an excellent guest

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Scheme 2

Typical approaches for anchoring a single P(III)-containing unit on a β-CD platform.

molecule for β-CDs. Note that the α-CD analogue of 4 was reported in a PhD thesis, but a possible inclusion of a PPh ring was not considered for this ligand.50 The catalytic system [Rh(COD)2]BF4/4 (COD = 1,5-cyclooctadiene) turned out to be a robust catalyst for hydrogenating the water-soluble olefin 2-methyl-3-buten-2-ol in water. Ligand 4 was also used for the rhodium-catalysed hydroformylation of methyl 4-pentenoate (S1, Table 1). Good activities were observed in water (Table 1, entries 1–3), but both chemoselectivity and regioselectivity were in the same range as those observed in heptane (Table 1, entries 4–6). These results are nearly identical to those obtained under the same conditions with the related TPPTS/ water (TPPTS = tris(3-sulfophenyl)phosphine trisodium salt) and PPh3/heptane systems indicating that the presence of the

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CD cavity has limited impact on the catalytic outcome (Table 1, entries 17 and 20). Entrapment of one of the phenyl rings of 4 makes the formation of inclusion complexes with hydrophobic substrates highly unfavourable, hence the poor selectivity observed. Ligand 5 (Scheme 4) was synthesised in order to prevent a similar self-inclusion phenomenon. In this ligand the CD platform is equipped with a longer and more rigid coordinating arm.37 Unlike 4, ligand 5 is poorly soluble in water, probably because the hydrophobic PPh3 unit is here fully exposed to the aqueous outside environment. However, rhodium complexes of 5 are noticeably more water-soluble than the free ligand. In fact hydrogenation in water of 2-methyl-3-buten-2-ol with 5 was nearly as fast as with 4.


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Scheme 3 Ichikawa’s phosphinites 1 and 2 and olefins used for substrate-selective hydroformylation studies.

Scheme 4

Synthesis of phosphines 4 and 5.

The three olefins S1, S2 and S3 were hydroformylated in water with a Rh/5 catalyst system (Table 1, entries 7–13). Hydroformylation of S1 and S2, which show no affinity for the β-CD cavity, resulted in about the same l/b ratios and conversions as for 4 and TPPTS (Table 1, entries 1 and 17). The presence of competitive CD guests such as sodium dodecyl sulfate (SDS) or 1-adamantanecarboxylate (ACNa) did not alter the catalytic outcome (Table 1, entries 8 and 10, respectively). Conversely, substrate S3, which forms an inclusion complex with 5 in water, displayed remarkable activity (TOF up to 1980 h−1) and led to a noticeably higher l/b ratio (2.8) than for the two other olefins (Table 1, entry 11). Because the CD cavity plays an active role in the catalytic process, addition of ACNa reduced dramatically the activity but had no real impact on selectivity (Table 1, entries 12–13).


Perspective

A phosphine with guest-tuneable properties (7) was obtained by copper-catalysed azide–alkyne cycloaddition between borane-protected diphenylpropynylphosphine and 6-mono-azido-RAME-β-CD 6 followed by phosphine deprotection (Scheme 5). While in 4 one of the phenyl rings is firmly held in the CD cavity and cannot be displaced by a competitive guest in water, the buried phenyl group of phosphine 7 can be expelled from the cavity by ACNa. The influence of such a conformational change on the catalytic performance of 7 was assessed by carrying out rhodiumcatalysed hydrofomylation reactions in water with olefin S1. In the presence of competitive guests (ACNa or sodium dodecanoate (SD)), standard l/b ratios of around 2 were observed (Table 1, entries 15–16 and 17–19) whereas without, the branched product became the major product (Table 1, entry 14). The authors suggest that with a guest molecule included, 7 is bulkier than on its own, a feature which may favour the formation of the linear regioisomer (Scheme 6).49 However, steric crowding around the donor atom is probably more severe in the self-included species than in the guest-occupied one. Such a feature is known to be a key factor for promoting the formation of monophosphine complexes at the expense of bis( phosphine) ones. Because singly phosphorus-ligated complexes are known to promote the formation of branched aldehydes, the unusual regioselectivity observed here is probably a direct consequence of a higher than usual proportion of P-monoligated complexes under hydroformylation conditions. Recently, the group of Harada has described the use of the dicyclohexylphosphine complexes [RuCl2(8)(H2O)] and [RuCl2(9)(H2O)] in the ring-opening metathesis polymerisation (ROMP) of the water-soluble diol S4 in water (Scheme 7).51 The CD cavity was shown to play an active role in the catalytic reaction as conversions with these complexes (31 and 80% conversion for [RuCl2(8)(H2O)] and [RuCl2(9)(H2O)], respectively within 14 h at 45 °C) were much higher than with CD-free water-soluble analogues. Moreover, CD-guest molecules such as 3-chlorophenol or adamantane inhibit the catalytic reaction to a certain extent, suggesting that supramolecular catalysis is at work in this case. As expected, the best host-molecule for olefin S4, namely [RuCl2(9)(H2O)], was more active than its smaller analogue [RuCl2(8)(H2O)]. Note that possible selfinclusion of a P-bound cyclohexyl ring into the macrocycle was not discussed.

2.3

Multidentate ligands

The very first CDs bearing appended P(III) ligands were reported independently by the groups of Ito and Reetz in 1993. The group of Ito managed to attach a chiral ferrocenyldiphosphine to the secondary face of a 2,6-permethylated-β-CD (DMβ-CD) by reacting a mesylated derivative of hydroxyethoxy-substituted 1,1′-bis(diphenylphosphino)ferrocene with the fully deprotonated DM-β-CD.46 Although the resulting ligand (10) is practically insoluble in water, the corresponding [PdCl2(10)] complex displays high solubility in this medium (Fig. 2). Conductivity measurements showed that [PdCl2(10)] assembled in

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Table 1

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Rhodium-catalysed hydroformylation using 4, 5 or 7a

Entry

L

Substrate

Solvent

Competitive guest (equiv.)b

t [h]

Conv.c [%]

l/bd

Ref.

1 2e 3f 4 5e 6f 7 8 9 10 11 12 13 14 15 16 17 18 19 20

4 4 4 4 4 4 5 5 5 5 5 5 5 7 7 7 TPPTS TPPTS TPPTS PPh3

S1

Water

— — — — — — — ACNag (0.3) — ACNa (0.3) — ACNa (1) ACNa (9) — ACNa (0.3) SDg (0.3) — ACNa (0.3) SD (0.3) —

2

96 97 89 83 82 88 82 82 87 70 99 80 32 99 99 98 98 98 98 98

1.8 1.8 1.7 1.2 1.2 1.2 1.1 1.1 0.1 0.1 2.8 3.3 3.2 0.7 1.8 1.9 1.8 1.8 1.8 1.8

36

Heptane S1

Water

S2

Water

S3

Water

S1

Water

S1

Water

S1

Heptane

2 2 2 0.25 6 2 2

37

42 36 42 42 36

a

Substrate/rhodium = 500, ligand/rhodium = 4, P(CO/H2) = 50 bar, (CO/H2) = 1/1, T = 80 °C. b Equivalent of competitive guest added with respect to substrate. c Determined by GC using decane as internal standard. d l/b aldehyde ratio. e Run carried out with ligand/rhodium = 8. f Run carried out with P(CO/H2) = 25 bar. g ACNa and SD stand for sodium adamantylcarboxylate and sodium dodecanoate, respectively.

Scheme 5

Synthesis of 7 via click-chemistry.

micelle-like aggregates above a given concentration. So far, no catalytic studies involving ligand 10 have been reported. Around the same time, Reetz et al. reported the heterobidentate phosphine–thioether ligand 11 (Fig. 2), which was synthesised by nucleophilic substitution of β-mono-6-O-tosyl-CD by 2-(diphenylphosphino)ethanethiol. Phosphine 11 readily forms the complexes [Rh(11)(NBD)]BF4 44 (NBD = norbornadiene) and [AuCl(11)]52 (Fig. 2). Surprisingly, unlike the related cavityfree complex [Rh(Ph2PCH2CH2SMe)(NBD)]BF4, [Rh(11)(NBD)]-

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Scheme 6 Guest tuneable conformation of 7 and main hydroformylation products for each case.

BF4 turned out to be rather ineffective as a hydrogenation catalyst, but no explanation was given for this lack of reactivity.53,54 A crystal structure reported eight years later showed that the


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Scheme 7 ROMP of olefin S4 using [RuCl2(8)(H2O)] and [RuCl2(9)(H2O)]. MeO groups of the primary face are probably coordinated to the ruthenium centres.

Fig. 2

Complexes of phosphine-functionalised CDs 10 and 11.

cavity of 11 is able to entrap specifically one of the two phenyl groups by forming a self-inclusion complex in the solid state as for previously mentioned phosphines 4 and 7 (Fig. 3). However, there is no clear evidence for such diastereotopic group recognition in either organic solution or water.55 The first ligands combining a chelating diphosphine unit with a CD cavity, namely 17–21, were reported by Reetz et al. These authors proved that metal complexes of these ligands

Fig. 3 Molecular structure of phosphine–thioether 11 showing one of the two phenyl groups included in the CD cavity (side view).


Perspective

may operate as supramolecular catalysts when used in waterpolar organic solvent mixtures.56,57 Ligands 17–21 were obtained in good yields from 6-monoamino-CDs 12–16, respectively, in a Mannich-like reaction with [Ph2P(CH2OH)2]Cl (Scheme 8). Competitive hydrogenation experiments were carried with catalysts prepared in situ from 17–21 and [Rh(COD)2]BF4.58,59 For solubility reasons, the reactions were performed in a 30% DMF–water mixture. The lipophilic olefin 4-phenyl-1-butene was converted up to 6.7 times faster than 1-decene when using 18, whereas the same reaction with a related CD-free catalyst led to no substrate selectivity. For comparison, a 4-phenyl-1-butene (not drawn) : 1-decene substrate selectivity of only 55 : 45 was observed with the standard Wilkinson catalyst. Preferential substrate inclusion by the CD cavity, as demonstrated by the decrease in selectivity upon addition of the competitive guest p-xylene to the reaction mixture, was invoked to explain this remarkable product distribution. The CD cavity of diphosphine 17 is also responsible for the high chemoselectivity observed in the reduction of halo-nitro aromatic compounds under organic solvent-free biphasic conditions using either [PtCl2(COD)] or [Rh(COD)2]BF4 (Scheme 9).60 Thus, hardly any dehalogenation products were observed with these metal catalysts, which also behave as phase transfer agents (dehalogenation product

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