CAMPO PArA lA vAlIDACIóN DEl ANálIsIs 2000-2005-2009
Anexo 2. MArCO NOrMATIvO
Chapter 3
Synthesis and reactivity of novel
ligands derived from glycidyl
Introduction
The high ring strain inherent in epoxides make them highly reactive electrophiles (Scheme 1) and has given rise to their widespread use in organic synthesis.1 They are useful motifs for
introducing further functionality into a molecule of interest. Furthermore, stereochemical information is retained during the course of these reactions and thus chiral epoxides are often used in the asymmetric synthesis of natural products, pharmaceuticals and ligands for homogeneous catalysis.
Scheme 1: SN2 reaction of an epoxide with a nucleophile
A wide range of nucleophiles have been shown to react with epoxides, most commonly, amines, alcohols, water and thiols. The analogous reactivity with phosphorus based nucleophiles is relatively unexplored and may represent a valuable methodology for the synthesis of chiral phosphine ligands. Despite recent progress in the design and synthesis of phosphine ligands (See Chapter 1), novel routes to these structures remain of interest due to the demand for ever more effective asymmetric catalysts for organic transformations. Numerous groups have utilised this reactivity to synthesise phosphino-alcohol structures,2–7
however, these examples are mostly trivial variations and derivatives lacking any further functionality which may allow for construction of more complex multidentate ligands and/or systematic variation of their steric and electronic properties. Ciardi et al. have synthesised a series of enantiomerically enriched β – hydroxyl - γ – amino phosphines derived from amino acids (Scheme 2).8 The naturally occurring amino acids were first converted to epoxides
which were subsequently ring-opened with lithium diphenylphosphido borane. Following deprotection, these phosphines were used as ligands in complexes of Rh(II), Ir(II) and Ru(II)
Chapter 3 –Synthesis and reactivity of novel ligands derived from glycidyl phosphine where they act as monodentate P donor ligands despite their potential to chelate via the other heteroatoms.
Scheme 2: Synthesis of chiral β – hydroxyl - γ – amino phosphines8
Müller and Feustel investigated a symmetrical ambidentate tripodal ligand derived from the exhaustive substitution of 1,1 – bis(chloromethyl)oxirane with LiPMe2 (Scheme 3).9 This
methodology is of particular interest in the development of novel ligand structures as initial attack of the phosphide at the epoxide results in formation of a halohydrin, which under the reaction conditions collapses again to reform an epoxide. The symmetrical tripod ligand is built up through successive substitution and ring closure reactions. The proposed intermediary species in this scheme are themselves attractive synthetic targets. If such species could be isolated, it may allow for the selective formation of asymmetrically substituted ligands.
Scheme 3: Müller and Feustel’s synthesis of a tripodal ligand9
A similar methodology has been applied in the synthesis of carbohydrate based 1,3 – bisphosphines (Scheme 4).10Furthermore, in this case the intermediary epoxy phosphine
species could be isolated facilitating the controlled synthesis of an asymmetrically substituted species bearing both P and S donors. The Rh and Ru complexes of these were found to be active catalysts for the asymmetric hydrogenation of olefins and ketones, although ee’s were found to be poor. Nonetheless, this illustrates the potential utility of the glycidylphosphine motif in the synthesis of chiral multidentate ligands.
Scheme 4: Stepwise synthesis of chiral carbohydrate based diphosphines10
An early report from Issleib and Rockstroh demonstrates the reaction of potassium diphenylphosphide with epichlorohydrin in order to generate the archetypal glycidyl phosphine species in 61 % yield.11 More recently, the same procedure was used by the group
of Huttner to synthesise a series of chiral diphosphine ligands (Figure 1, a).12 The phosphide
was found to attack epichlorohydrin selectively at the terminal position of the epoxide rather than at the halide position and thus racemisation of the chiral centre was not observed. The scope of this reaction was also successfully extended to encompass amine and thiol nucleophiles (Figure 1, b). The Rh complexes of these ligands were active catalysts in the hydrogenation of (Z)-2-acetamidocinnamic acid although reported ee’s were low. These ligands were further developed to convert the alcohols to various phosphite donor groups (Scheme 5).13,14 Reaction of these ligands with [Rh(COD)Cl]
2 in the presence of KPF6 furnishes
[(tripod)Rh(COD)]PF6 in good yield.
Chapter 3 –Synthesis and reactivity of novel ligands derived from glycidyl phosphine
Scheme 5: Synthetic route to chiral tripodal phosphines13
The glycidylphosphine motif is attractive as a potential synthon for the construction of novel ligand structures but has remained relatively unexplored. To date, diphosphine ligands have received most attention; complexes of these ligands and their derivatives have been used in Negishi couplings,15 catalytic hydrogenation,10,12,14,16,17 solid supported catalysis18–22 and
myocardial imaging.23 The ligands typically chelate via the two P donors, however alkoxide
binding has been observed in Li and Na aggregates9,24 and Ti and Ta alkylidene complexes.25
Aims of chapter 3
This chapter will assess the scope and utility of the glycidyl phosphine motif in the synthesis of a number of chiral heterodonor ligands. Epichlorohydrin is a cheap and readily available precursor and via well-known hydrolytic kinetic resolution techniques can be readily enantiomerically enriched to a high degree.26 The reaction of epichlorohydrin with lithium
diphenylphosphide occurs cleanly in a stereospecific and regioselective manner (vide supra) allowing rapid access to glycidyldiphenylphosphine. The subsequent reaction of this key intermediary species with various nucleophiles will result in ring opening to produce a series
of heterodonor ligands (Scheme 6). The presence of a pendant alcohol in such structures may result in these species acting as hemilabile ligands which have proven useful in the stabilisation of many interesting structures.27 Incorporation of a chiral alcohol is also of
interest synthetically as it may allow for functionalisation of the ligand backbones via formation of various ether/ester derivatives. Derivatives such as these will allow for rational catalyst development by tuning properties such as steric bulk or solubility.
Scheme 6: Design of chiral heterodonor ligands
Synthesis and stability of glycidyl phosphines
Glycidyldiphenylphosphine, 1, is unstable and cannot be stored for prolonged periods. The initial colourless oil becomes a viscous orange resin-like substance within hours. Similar observations have previously been made with the analogous glycidyl amines.28 These
observations suggest that the mechanism of decomposition is not a simple matter of oxidation at phosphorus and is likely due to a polymerisation reaction. This may potentially occur via two pathways (Scheme 7), either through nucleophilic attack of the phosphine lone pair at the epoxide ring generating phosphonium alkoxides or via ring opening of the epoxide to give a polyether bearing pendant phosphines. Additionally, it is likely that some degree of oxidation to P(v) species does indeed occur and in principle a combination of all three degradation routes is possible.
Chapter 3 –Synthesis and reactivity of novel ligands derived from glycidyl phosphine
Scheme 7: Potential polymerisation products of 1
The pendant phosphine material may possess some utility in applications such as immobilised heterogeneous catalysis or heavy metal scavenging materials, thus it is of interest to further explore the nature of this material. The MALDI mass spectrum shows peaks attributable to di-, tri- and tetrameric species suggesting that although oligomerisation is occurring large polymeric chains are not formed under these conditions. The 31P{1H} NMR
spectrum (Figure 2) of this material displays a number of peaks between δP = –25 and –12
ppm while a second grouping of resonances are observed between δP = 10 and 40 ppm.
1 appears at δP = -24.8 ppm,13 thus the primary grouping are likely those of similar tertiary
phosphine environments. The downfield grouping correlates with resonances of phosphine oxide and/or phosphonium species.
The spectroscopic evidence suggests that the decomposition route proceeds largely via nucleophilic attack at the epoxide yielding phosphonium alkoxide species. It has been shown that phosphonium salts are unstable in the presence of alkoxides and tend to degrade to phosphine oxides and ethers.29,30 The presence of the internal alkoxide moiety may further
promote this pathway. Many of the potential intermediary species in this process have the potential for further side reactions thus providing a source for the observed complexity in the 31P{1H} NMR spectrum.
In order to fully investigate the reactivity of these species an alternative system was sought which would provide air stability and prevent nucleophilic substitution reactions. It was envisaged that donation of the phosphine lone pair to a suitable Lewis acid would achieve both of these goals and thus the borane adduct, 2 (Scheme 8), was prepared and characterised by 1H, 13C and 31P NMR spectroscopy. Due to the complexity of its 1H NMR
spectrum, 2 can be conveniently identified by its resonance (δ = 22.2 ppm) in the 31P NMR
spectrum.
Scheme 8: Preparation of borane adduct 2
Initial stability tests of 2 proved favourable as no obvious degradation occurred of the white solid when exposed to air. However, after 2 hours it was apparent that 2 was hygroscopic as the friable solid gradually became oily. The 31P NMR spectrum of the oil shows a small
quantity of 1 present in this sample. The presence of water appears to slowly displace the borane, liberating the free phosphine which is then free to undergo various degradation processes (vide supra). Dissolution of the oil in chloroform gives a white solid, presumably borane hydrolysis products, suspended in a pale yellow solution. Subsequently, for synthetic
Chapter 3 –Synthesis and reactivity of novel ligands derived from glycidyl phosphine ease and integrity of the products, 1 was not routinely isolated, but formed immediately prior to reaction with various nucleophilic substrates.
Reactions with phosphides
Given the prevalence of diphosphino ligands in the literature (vide supra), the reaction of 1 with dialkylphosphines was studied. Thus, reaction of (S)-1 with lithium diisopropylphosphide at -78 °C affords (S)-1-(diisopropylphosphanyl)-3- (diphenylphosphanyl)propan-2-ol, 3, in good yield (Scheme 9).
Scheme 9: Synthesis of chiral diphosphine. Reagents and conditions; i) Ph2PH, BuLi,
THF, -78 °C; ii)iPr
2PH, , BuLi, THF, -78 °C
Diphosphine 3 was isolated as a colourless oil and displayed two peaks in the 31P NMR
spectrum typical of diarylalkyl and trialkyl phosphines. Preliminary studies of the coordination chemistry of 3 were carried out. Hence, a solution of 3 in toluene was added to a suspension of CoCl2·6H2O in toluene, whereupon the pink solid dissolved to give a
green/blue solution. Following workup, the paramagnetic product was identified as the bis- diphosphine complex, 4, on the basis of mass spectrometry and elemental analysis. Likewise, following treatment of 3 with excess NiCl2·6H2O a dark red crystalline material, 5, was
collected. Upon coordination the two 31P NMR peaks are significantly broadened and shifted
downfield. The material has been formulated as the neutral bisphosphino complex (Scheme 10) on the basis of X-ray diffraction data (vide infra). Contrarily, the mass spectrum of this material shows no evidence for the 1:1 adduct, instead displaying a strong signal associated with the cationic 2:1 complex. This unexpected observation can be rationalised as a result of fragmentation under the conditions within the spectrometer. All evidence suggests a
chelating κ2-P,P’ coordination mode of the diphosphine rather than any of the potential
alternative bridging, mono- or tridentate coordination modes.
Scheme 10: Formation of Ni diphosphine complex
The Ni complex, 5, crystallises readily from saturated MeOH solution giving crystals suitable for X-ray diffraction. There are 4 independent molecules in the unit cell, one of which is shown below (Figure 3). The complex adopts a square planar geometry around Ni as might be expected of such a complex. The average bond lengths (P1-Ni = 2.156 Å, P2-Ni = 2.176 Å, Cl1-Ni = 2.195 Å, Cl2-Ni = 2.195 Å) are typical of such complexes and are in accord with other known structures. The slight increase in bond length of the P2-Ni bond compared to that of P1-Ni bond is reflective of the increased steric bulk provided by the isopropyl substituents. The average P1-Ni-P2 bite angle of 95.4 ° is slightly in excess of the ideal bond angle for a square planar geometry due to the need to accommodate a six membered chelate ring at Ni.
Figure 3: ORTEP representation of the structure of 5. H atoms omitted for clarity. Asymmetric unit contains 4 independent molecules. Selected bond lengths (Å) and angles
(°): P1-Ni1 = 2.164(2), P2-Ni1 = 2.177(2), Cl1-Ni = 2.196(2), Cl2-Ni = 2.211(2), P1-Ni-P2 = 94.04(9), P1-Ni-Cl2 = 165.96(11), P2-Ni-Cl1 = 173.97(11)
Chapter 3 –Synthesis and reactivity of novel ligands derived from glycidyl phosphine
Reactions with amines
Ligand synthesis
Reaction of 1 with dialkylamines allows ready access to a series of chiral γ-amino-β- hydroxyphosphines (Scheme 11). The dimethyl-, diethyl- and morpholinyl- substituted species, 6, 7 and 8, respectively, were formed as air stable colourless oils in moderate to good yield. The stereochemistry of the hydroxyl group can be controlled by use of chirally resolved epichlorohydrin in the preparation of 1. It was anticipated that these species would exhibit rich coordination chemistry due to the varied properties and characteristics of the three potential donor sites.
Scheme 11: Synthesis of chiral aminophosphines
Ru complexes
Scheme 12: Synthesis of RuCl2(cymene)(phosphine) complexes. Identical procedure used for
the synthesis of (S)- enantiomers.
Reaction of the chiral phosphino amines with [RuCl2(cymene)]2 in methylene chloride gives
the κ1-P Ru piano stool complexes, 9-11 (Scheme 12). Coordination to Ru is accompanied by
spectra resemble those of the free ligands with additional resonances associated with the cymene moiety. A large number of Ru(η6-arene) complexes are now known and such
complexes have found extensive use as catalysts for various organic transformations,31,32
including oxidation,33 epoxidation,34 hydroformylation,35 olefin metathesis,36
hydrogenation37,38and hydrogen transfer.39 It is of interest therefore to assess the catalytic
competency of Ru complexes 9-11. The pendant chiral hydroxyl can be anticipated to impart only a small steric influence but may affect the stereochemistry of the product through hydrogen bonding interactions with polar substrates.40The systems developed by Noyori et
al. affect the catalytic reduction of benzoin and benzils to the 1,2-diol species in high yield
and enantiomeric excess under relatively mild conditions (Scheme 13).41 The catalyst
structure in this reaction has proven to be remarkably variable; a number of chiral co-ligands are able to be incorporated in order to tune the properties of catalyst.42
Scheme 13: Noyori’s asymmetric reduction of benzil41
Under identical conditions to those used by Noyori41 using 9-11 in place of
RuCl[(R,R)-Tsdpen](cymene) no conversion was seen and benzil was recovered quantitatively. The presence of the anionic sulfonamide is critical for operation of Noyori’s catalyst by facilitating efficient hydrogen transfer to the substrate.43 Given the absence of
this functionality in the present systems it is unsurprising that similar reactivity is not observed. Beyond this, the most striking dissimilarity between the two catalyst structures is the differing coordination modes of supporting chiral ligands, 8-10 possess monodentate phosphine ligands whereas most other systems incorporate a bidentate chelating ligand at
Chapter 3 –Synthesis and reactivity of novel ligands derived from glycidyl phosphine the Ru centre.42This will have a marked influence on the electronic properties of the Ru
centre in addition to influencing the stability and longevity of the complex under the reaction conditions. Furthermore, a more rigidly bound ligand can be expected to more precisely define the chiral space around the metal centre resulting in a greater degree of chiral discrimination. The amine and/or alcohol functional groups present in ligands 6-8 may potentially facilitate such a chelating geometry. Halide abstraction is envisioned to open up a vacant site around the metal centre and allow for coordination of the pendant donor atoms.
Described below is the reactivity and spectroscopic data associated with complex (S)-9, closely similar results were observed in the case of (R)-9, (S)-10, (R)-10, (S)-11 and (R)-11. Following addition of 1 equivalent of AgPF6 to a solution of (S)-9 in methylene chloride, the
solution turned a paler shade of orange and a cloudy, off white precipitate was formed. After filtration and removal of the solvent the product was collected as a red solid. High resolution mass spectra indicated the presence of mono-chloro cations, [Ru(p-cymene)((S)-9)Cl]+, in the
sample. 31P NMR spectrum (Figure 4) shows a number of species with resonances at δ P =
19.9, 20.4 and 49.9 ppm alongside three small signals appearing at δP = 51.1, 53.6 and 56.8
ppm. The peak at δ = 20.4 ppm corresponds to (S)-9 and thus the peak at δP = 19.9 ppm can
be assumed to be a similarly monodentate ligated complex, potentially a dimeric structure formed from the putative coordinatively unsaturated species. A downfield shift of the 31P
NMR signal is often associated with chelate ring formation.44,45 Furthermore, the similar
complexes [(η6-C
6Me6)Ru(P^O)Cl]BPh4 and [(η6-p-cymene)Ru(P^N)Cl]BF4 (where P^O =
PPh2(CH2CH2OMe),46 and P^N = [η6-(R,R)-o-{(NMe2)CHMe}C6H4PPh2]Cr(CO)3)47 display 31P
NMR chemical shifts in this region (δP = 51.2 and 42.7 ppm respectively); thus the peaks
Figure 4: Expanded 31P NMR spectrum of (S)-9 following treatment with 1 equivalent of
AgPF6. Inset: Full spectrum
It is hypothesised that abstraction of a chloride from (S)-9 generates a transient 16 e-
intermediate, which is highly reactive towards incoming ligands. One potential route by which this species may alleviate the electronic unsaturation is via dimerization giving a bis(chloro-bridged) diruthenium complex. In subsequent reactions MeCN was used as the solvent as its donor ability may mitigate the tendency towards decomposition of the coordinatively unsaturated species. Indeed, following addition of further AgPF6 in MeCN the
resonance at δP = 19.9 ppm was no longer observed. Otherwise the 31P NMR spectrum
remains unaffected. The major resonances in this case were those of (S)-9 and the peak at δP = 49.9 ppm presumably that of the P^N chelate species. Attempts to purify these materials
Chapter 3 –Synthesis and reactivity of novel ligands derived from glycidyl phosphine
Reactions with thiols
Ligand Synthesis
Reaction of 1 with p-thiocresol in the presence of catalytic triethylamine gives 1-(diphenylphosphanyl)-3-(p-tolylthio)propan-2-ol (Figure 14), 12, as an air stable pale yellow oil. Similarly, reaction of 1,2-ethanedithiol with (S)-1 gives the multidentate ligand (2R,2'R)-3,3'-(ethane-1,2-diylbis(sulfanediyl))bis(1-(diphenylphosphanyl)propan-2-ol), 13. In contrast to compound 12, this compound is sensitive to oxidation in air. The 31P NMR
spectrum of 13 shows a single resonance at δ = -22.9 ppm, whereas the oxide appears at δ = 35.1 ppm. The 1H NMR spectrum of 13 confirms the proposed structure; in particular, the
resonance associated with the ethylene bridge appears as a singlet (δ = 2.59 ppm) reflecting the symmetry of the ligand and confirming twofold substitution of the dithiol.
Scheme 14: Synthesis of P,O,S donors
To date only very few reports have been made of compounds containing this P,O,S motif,10,12,48and the coordination chemistry of these ligands is relatively unexplored.
The soft nature of thioether donors give them similar utility to phosphines as supporting moieties in homogeneous catalysis. Furthermore, S donors tend to be more labile than phosphines and are used in homogeneous catalysis to temporarily introduce vacant coordination sites during catalytic cycles.49
Coordination chemistry with transition metals
Complexes of the general form [M(13)]Cl2;where M = Ni (14), Pd (15) or Pt (16); were formed
by refluxing the ligand in EtOH or EtOH/H2O mixtures with NiCl2·6H2O, Na2[PdCl4] or K2[PtCl4]
respectively. All complexes were characterised by NMR, MS, IR and elemental analyses. Crystals suitable for X-ray diffraction of the Pd complex, 15, were grown from a saturated solution in MeOH. The structure of 15 is shown in Figure 5. Complex15 has C2 symmetry with
the ligand bound in a P,S,S,P tetradentate manner around a distorted square planar metal centre. The two alcohol groups are trans to one another and have a hydrogen bond to the chloride anions. Bond lengths and angles are in good agreement with similar complexes from the literature.50 The five membered S-Pt-S chelate ring adopts a puckered conformation,
while the two six membered P-Pd-S chelate rings adopt distorted chair conformations. The stereochemical information of the ligand backbone appears to have been transferred to the S donors, which are both of (S)- stereochemistry, upon coordination. The hydroxyls are observed in both axial and equatorial arrangements with only a slight preference for the equatorial conformation (53% equatorial).
Figure 5: ORTEP Representation of complex 15. Hydrogen atoms, chloride counter ions and disordered -OH groups (47:53%) omitted for clarity. Selected bond lengths (Å) and angles (°): Pd-S 2.3227(12), Pd-P 2.2867(10), S-Pd-S’ 90.65(6), P-Pd-S 87.34(4), P-Pd-P’
Chapter 3 –Synthesis and reactivity of novel ligands derived from glycidyl phosphine
The structure of the Ni complex, 14, appears to be far more complex than that of the heavier congeners. The 31P NMR spectrum shows the presence of at least three distinct P
environments with resonances visible at δ = 21.7, 31.4 and 34.0 ppm. Further qualitative