Phosphine Complexes of an Enantiomerically Pure, Atropisomeric

Nov 21, 2011 - Research School of Chemistry, Australian National University, Canberra, Australian Capital Territory 0200, Australia. •S Supporting I...
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Phosphine Complexes of an Enantiomerically Pure, Atropisomeric Arsenium Ion† Nathan L. Kilah and S. Bruce Wild* Research School of Chemistry, Australian National University, Canberra, Australian Capital Territory 0200, Australia S Supporting Information *

ABSTRACT: The first enantiomerically pure secondary iodoarsine has been prepared by ring closure of (aR)-[{Li(TMEDA)}2{2,2′-bis(methylene)-1,1′-binaphthylyl}] with dichlorophenylarsine, followed by disproportionation of the resulting seven-membered (aR)-phenylarsepine with triiodoarsine in boiling toluene. The corresponding (aR)chloroarsepine was prepared from the (aR)-iodoarsepine by halide metathesis with silver chloride in dichloromethane. The crystal structures of the two air-stable (aR)haloarsepines have been determined. A series of phosphine-stabilized arsenium hexafluorophosphates has been prepared from the (aR)-iodoarsepine by reactions with trimethylphosphine, dimethylphenylphosphine, and [2-(methoxymethyl)phenyl]dimethylphosphine in dichloromethane in the presence of aqueous potassium hexafluorophosphate. The crystal structures of the three complexes have been determined. The structure in each case revealed the significant twist in the (aR)binaphthyl framework of the seven-membered arsepinenium ion and the coordination of the phosphine approximately orthogonal to the trigonal AsC2 plane of the arsepinenium ion. The 1H{31P} NMR spectrum of the dimethylphenylphosphine complex in dichloromethane-d2 at 25 °C contains broadened singlets for the diastereotopic PMe groups and a multiplet for the methylene protons of the arsepinenium ring because of phosphine exchange. At the slow exchange limit (ca. −50 °C), the complex cation is devoid of symmetry, as indicated in the 1H NMR spectrum by the sharp resonances for the PMe groups and separate AB and A′B′ spin systems for the two pairs of methylene groups in the arsepinenium ring. The 1 H{31P} NMR spectrum of the closely related [2-(methoxymethyl)phenyl]dimethylphosphine−arsenium complex in dichloromethane-d2 at 25 °C is sharp, however, which is in agreement with additional stabilization of the complex by coordination of the 2-methoxymethyl group to the phosphorus and arsenic atoms, as indicated in the crystal structure. The resolution of (±)-[2-(methoxymethyl)phenyl]methyl(2-naphthyl)phosphine was achieved by complexation to the enantiomerically pure (aR)-arsenium hexafluorophosphate auxiliary. The crystal structure of the less soluble aR,RP diastereomer of the phosphine−arsenium complex confirmed the absolute configuration of the resolved P-chiral phosphine.

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INTRODUCTION Phosphine-stabilized arsenium salts of the type [R3P→ AsMePh]PF6 are generally air- and water-stable solids that have chiral structures based on the trigonal pyramid: the phosphine in the complex cation is axially coordinated to the two-coordinate methylphenylarsenium ion, which has a bent structure due to the presence of a stereochemically active lone pair of electrons in the trigonal plane containing the arsenic and the ipso carbon atoms of the two organic substituents.1 The complexes undergo rapid phosphine exchange in solution between the enantiotopic faces of the prochiral methylphenylarsenium ion and attack at arsenic by anionic carbon nucleophiles to give tertiary arsines with liberation of the phosphine. Thus, complexes of the type [R3P→AsMePh]PF6 appear to be suited to the asymmetric synthesis of tertiary arsines chiral at arsenic by nucleophilic carbanion addition at arsenic in the presence of coordinated, enantiopure, tertiary phosphine auxiliaries. We have shown in earlier work that the auxiliary (aR)-1, which is readily derived from the lithiated (aR)-2,2′-dimethyl-1,1′-binaphthalene and dichloro[2(methoxymethyl)phenyl]phosphine, furnishes (S)-(+)-(n© 2011 American Chemical Society

butyl)methylphenylarsine in 85% enantioselectivity, with displacement of the (aR)-phosphepine, when treated with nbutyllithium at −95 °C.2 A feature of the design of the phosphepine auxiliary is the incorporation of the 2(methoxymethyl)phenyl substituent on phosphorus, which interacts with the arsenic of the arsenium group, in solution and in the solid state, and facilitates stereodifferentiation by the chiral phosphepine of the enantiotopic faces of the methylphenylarsenium ion. We have also shown, with use of the same chiral auxiliary, that related chelating bis(tertiary arsines) can be synthesized with high diastereoselectivities and enantioselectivities under similar conditions.3 We have now turned our attention to the resolution and asymmetric synthesis of P-chiral tertiary phosphines with use of a similar stategy, but with a reversal of the roles of the two group 15 elements (umpolung). Thus, the axially atropisomeric (aR)-iodoarsepine (aR)-2 has been synthesized and a series of Special Issue: F. Gordon A. Stone Commemorative Issue Received: September 11, 2011 Published: November 21, 2011 2658

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The chloroarsepine (aR)-3 was prepared by halide metathesis of the iodide with excess silver(I) chloride over 12 h in dichloromethane (Scheme 2). After separation of the silver

phosphine-stabilized arsenium complexes of the type (aR)-[L→ AsR2]PF6 (L = R3P) prepared, a number of which have been structurally characterized in solution and the solid state. We have then shown that (aR)-2 is a potentially useful resolving agent for P-chiral tertiary phosphines in the form of phosphinestabilized arsenium hexafluorophosphates.

Scheme 2

halides from the reaction mixture by filtration, the colorless filtrate was evaporated to dryness and the residue was recrystallized from a small quantity of boiling ethyl acetate to give colorless needles (76% yield) of the pure (aR)-3: mp 137 °C dec; [α]D19 = −237.0 deg cm2 g−1 (c 1.0, C6H6). (The attempted synthesis of the chloroarsepine by disproportionation of the phenylarsepine (aR)-6 with trichloroarsine in boiling toluene was unsuccessful.) Syntheses of the Phosphine Complexes (aR)-[L→ AsR2]PF6 (L = Me3P, Me2PhP, Me2[2-(MeOCH2)C6H4]P; (aR)-7−9). The phosphine-stabilized arsenium complexes (aR)-[L→AsR2]PF6 (L = Me3P, Me2PhP, Me2[2-(MeOCH2)C6H4]P) (aR)-7−9 were prepared from the iodoarsepine (aR)2 and the appropriate tertiary phosphine by the two-phase method described elsewhere (Scheme 3).1 Thus, a dichloro-

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RESULTS AND DISCUSSION Syntheses of (aR)-Haloarsepines. The enantiomerically pure iodoarsine (aR)-2 was prepared in 27% overall yield from (aR)-2,2′-dimethyl-1,1′-binaphthalene ((aR)-4), as shown in Scheme 1. Deprotonation of (aR)-4 with n-butyllithium in the Scheme 1

Scheme 3

methane solution of (aR)-2 containing an equimolar quantity of the tertiary phosphine was exposed for ca. 30 min to an excess of potassium hexafluorophosphate in water. The yellow color of the iodoarsine solution was almost dissipated upon the addition of the phosphine, but the solution became colorless soon after exposure to the aqueous hexafluorophosphate. The organic phase in each case was separated, dried, and filtered, and the filtrate was evaporated to dryness. The colorless solids that remained were recrystallized from dichloromethane by the addition of diethyl ether, giving the pure tertiary phosphine− arsenium complexes in high yields as air-stable, crystalline solids. NMR Spectroscopy. The 1H NMR spectrum of (aR)-2 in benzene-d6 at 25 °C revealed AB and A′B′ spin systems for the two pairs of non-equivalent methylene protons of the iodoarsepine ring of the asymmetric molecule (C1) at the slow exchange limit for intermolecular iodine exchange. In dichloromethane-d2, the four methylene protons resonate as a single AB spin system because of rapid iodine exchange. For (aR)-3, however, chlorine exchange is slow on the NMR time scale in both solvents, as evidenced in the NMR spectrum by well-resolved AB and A′B′ spin systems for the two nonequivalent pairs of methylene protons in the arsepinenium ring. The 1H NMR spectrum of (aR)-7·CH2Cl2 in dichloromethane-d2 at 25 °C contains a doublet for the PMe groups

presence of N,N,N′,N′-tetramethyl-1,2-ethylenediamine (TMEDA) furnished, by the literature procedure,4 deep red crystals of the lithiated derivative (aR)-5, which, when stirred with dichlorophenylarsine in n-hexane at 0 °C, resulted in the isolation of the phenylarsepine (aR)-6 in 57% yield after recrystallization of the crude product from toluene−methanol. The phenylarsine (aR)-6 was characterized by X-ray crystallography (Supporting Information). The aS enantiomer of (aR)-6 had been prepared previously for use in asymmetric Wittig reactions between arsonium ylides and 4-substituted cyclohexanones.5 When equimolar quantities of (aR)-6 and triiodoarsine were reacted together in boiling toluene over 24 h, the secondary iodoarsepine (aR)-2 was formed in quantitative yield, as determined by recording the 1H NMR spectrum of a benzene-d6 solution of the dark orange oil that remained after evaporation of the solvent. This oil, when it was dissolved in dichloromethane and the solution was allowed to evaporate under a stream of dry nitrogen, afforded orange crystals of the iodoarsepine (aR)-2 coated with the byproduct of the reaction, diiodophenylarsine. The impurity was extracted from the iodoarsepine by trituration and sonication in n-hexane. The resulting product was then recrystallized from dichloromethane by the addition of methanol, which provided yellow needles of pure (aR)-2 in 75% yield from (aR)-6: mp 150 °C dec; [α]D19 = +233.8 deg cm2 g−1 (c 1.0, CH2Cl2). 2659

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(2JHP = 13.8 Hz) and a complex multiplet for the methylene groups of the arsepinenium ring. The 1H{31P} spectrum of the complex, however, exhibited AB and A′B′ spin systems for these groups. The 1H NMR spectrum of (aR)-8·CH2Cl2 at 25 °C showed a pair of overlapping doublets for the diastereotopic PMe groups and a broad complex multiplet for the four methylene protons corresponding to the AB and A′B′ spin systems coupled to the 31P nuclei (Figure 1a). Decoupling of

PMe groups and sharpened the methylene resonances into a single AB spin system, which was consistent with rapid exchange of the phosphine at that temperature (Figure 1d). The coalescence temperature for the diastereotopic PMe2 resonances of (aR)-8·CH2Cl2 is >70 °C (Figure 1e). The approximate rate constant for phosphine exchange (kC) was determined from these NMR data by substitution into the expression kC = 0.5πΔν/√2, where Δν is the frequency difference between the baseline-separated resonances in Hz at the slow exchange limit. The free energy of activation for phosphine dissociation in the complex (ΔG⧧C) was then calculated from kC by substitution in the expression ΔG⧧C = −RTC ln(kCh/kBTC), where R, h, and kB are the gas, Plank, and Boltzmann constants, respectively, giving ΔG⧧C > 80 kJ mol−1. This value for the barrier to phosphine exchange in (aR)8·CH2Cl2 is consistent with the results of crossover experiments involving phosphine exchange between closely related pairs of related phosphine-stabilized arsenium complexes.1 The 1 H NMR spectrum of (aR)-9 in dichloromethane-d2 at 25 °C showed baseline separation of the resonances for the diastereotopic PMe groups and coupling of the diastereotopic pairs of methylene protons to the 31P nuclei, as shown in Figure

Figure 1. Variable-temperature NMR spectra of (aR)-8·CH2Cl2 (CD2Cl2): (a) 1H at 25 °C; (b) 1H{31P} at 25 °C; (c) 1H{31P} at 40 °C; (d) 1H{31P} at 55 °C; (e) 1H{31P} at 70 °C.

the 31P nuclei in the 1H{31P} spectrum revealed the AB and A′B′ spin systems (Figure 1b). Heating the sample of (aR)8·CH2Cl2 in dichloromethane-d2 to 40 °C resulted in the loss of baseline separation of the methylene resonances in the 1 H{31P} NMR spectra (Figure 1c). Further heating to 55 °C reduced the peak separation between the resonances for the

Figure 2. Variable-temperature NMR spectra of (aR)-9 (CD2Cl2): (a) 1 H at 25 °C; (b) 1H{31P} at 25 °C; (c) 1H{31P} at 80 °C.

2a. The spectrum indicated that phosphine exchange in the complex at this temperature is considerably slower than that observed for the complex (aR)-8·CH2Cl2, where the phosphine 2660

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lacks the 2-(methoxymethyl)phenyl substituent. The 1H{31P} spectrum of (aR)-9 in dichloromethane-d2 at 25 °C exhibits AB and A′B′ spin systems for the protons of the methylene groups, as expected for an asymmetric (C1) cation at the slow exchange limit (Figure 2b). The 2-(methoxymethyl)phenyl substituent of the phosphine has been shown elsewhere to facilitate prochiral face discrimination of the prochiral methylphenylarsenium ion.1,2 Heating of the NMR sample of (aR)-9 in the spectrometer to 80 °C led to a broadening of all of the signals (Figure 2c). Crystal Structures. The iodoarsepine (aR)-2 crystallizes in the hexagonal space group P65 with six molecules in the unit cell. An ORTEP diagram of the compound is shown in Figure

the Flack parameter, which was 0.008(12). The geometry around the arsenic center is trigonal pyramidal, with As1−C1 and As1−C2 distances of 1.983(4) and 1.992(4) Å, respectively. The chloroarsepine (aR)-3 is isomorphous with (aR)-2. An ORTEP diagram of (aR)-3 has been included in the Supporting Information. The phosphine-stabilized arsenium complex (aR)-7 crystallizes from concentrated dichloromethane solution as the dichloromethane solvate in the orthorhombic space group P212121 with four formula units in the unit cell (Table 1). The structure of the complex cation of (aR)-7 is shown in Figure 4.

Figure 4. ORTEP diagram of the cation of (aR)-7·CH2Cl2 with selected atoms labeled (30% ellipsoid probability shown). Hydrogen atoms have been omitted for clarity. Figure 3. ORTEP diagram of (aR)-2 with selected atoms labeled (30% ellipsoid probability level shown). Hydrogen atoms have been omitted for clarity. Principal bond lengths (Å) and angles (deg) are as follows: As1−I1, 2.5751(5); As1−C1, 1.983(4); As1−C2, 1.992(4); I1−As1− C1, 99.38(12); I1−As1−C2, 96.51(13); C1−As1−C2, 94.82(16).

The dichloromethane of crystallization is disordered and was modeled over two sites of refined occupancy. The absolute configuration of the (aR)-[2,2′-bis(methylene)-1,1′binaphthyl]arsepinenium group was assigned on the basis of the known absolute configuration of the starting material (aR)4 and was confirmed by refinement of the Flack parameter (0.007(14)). The As(1)−P(1) distance of 2.3183(11) Å in the cation is longer than the sum of the covalent radii of the two elements, viz. 2.22 Å,6 and is similar in length to the arsenic−

3, and principal bond lengths and angles are given in the figure caption. The absolute configuration of the 1,1′-binaphthyl framework was assigned on the basis of the known configuration of the starting material, (aR)-2,2′-dimethyl-1,1′binaphthalene ((aR)-4), and was confirmed by refinement of

Table 1. Crystallographic Data and Experimental Parameters for X-ray Structural Analyses (aR)-2 empirical formula formula wt cryst syst space group a, Å b, Å c, Å β, deg V, Å3 Z Dcalcd, g cm−3 cryst size, mm μ, mm−1 no. of unique rflns no. of rflns obsd Flack param final R1, wR2 a

(aR)-3

C22H16AsI 482.20 hexagonal P65 16.8728(5)

C22H16AsCl 390.74 hexagonal P65 16.7760(4)

11.2567(2) 2775.34(13) 6 1.731 0.23 × 0.07 × 0.05 3.505 4121 2929 a 0.008(12) 0.0285, 0.0301

(aR)-7·CH2Cl2

(aR)-8·C4H8O

11.0926(2)

C26H27AsCl2F6P2 661.26 orthorhombic P212121 8.8101(1) 11.5091(2) 28.4709(4)

C34H35AsF6OP2 710.51 orthorhombic P212121 8.2770(1) 13.4613(2) 29.0898(4)

2703.59(10) 6 1.440 0.18 × 0.08 × 0.08 2.033 4113 2972 a 0.007(12) 0.0382, 0.0423

2886.84(7) 4 1.521 0.23 × 0.10 × 0.08 1.527 6873 4149 a 0.0007(14) 0.029, 0.031

3241.16(8) 4 1.456 0.35 × 0.19 × 0.09 1.209 7730 4937 a 0.032(11) 0.0287, 0.0367

(aR)-9 C32H31AsF6OP2 682.46 monoclinic C2 20.4881(3) 7.9518(1) 19.5861(3) 110.2897(9) 2992.92(7) 4 1.514 0.25 × 0.15 × 0.09 1.514 7079 4555 a 0.009(7) 0.0262, 0.0300

(aR,RP)10·0.5C4H8O C43H39AsF6O1.5P2 830.64 tetragonal P43212 18.7230(4) 18.7230(4) 23.0827(4) 8091.7(3) 8 1.363 0.22 × 0.08 × 0.07 0.98 9636 3553 b 0.012(10) 0.0389, 0.0412

I > 3σ(I). bI > 2σ(I). 2661

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phosphorus distance in [(Ph3P)AsMePh]PF6 (2.3480 Å).1 The coordination geometry around the arsenic center is best described as a distorted trigonal pyramid in which the sixelectron AsC2 group of atoms and the lone pair of electrons of the arsepinenium group occupy the base and the phosphorus atom is situated at the apex. The angles P(1)−As(1)−C(1) = 101.11(10)° and P(1)−As(1)−C(2) = 98.36(9)° in the cation are notably larger than those in the previously characterized complex [(Ph3P)AsMePh]PF6, viz. 92.31(8) and 97.04(6)°.1 Interactions between the cation of the complex and the hexafluorophosphate were observed at distances of As(1)···F(5) = 3.321(3) and As(1)···F(6) = 3.314(3) Å. Crystals of the butanone solvate (aR)-8·C4H8O were obtained by the addition of diethyl ether to a concentrated butanone solution of the complex; the solvate crystallized in the orthorhombic space group P212121 with four molecules in the unit cell. The structure of the cation of the complex is shown in Figure 5. The butanone of crystallization is disordered in the

viz. 2.22 Å,6 and the angles P(1)−As(1)−C(1) = 102.08(11)° and P(1)−As(1)−C(2) = 99.46(10)° align the phosphine approximately orthogonal to the AsC 2 plane of the arsepinenium ion. Other features of interest within the crystal structure of (aR)-8·C4H8O are the interaction between As(1) of the cation and F(5) of the hexafluorophosphate ion at 3.347(3) Å and the embrace between C(12)−H(121) of a naphthalenyl group and the phenyl ring of the dimethylphenylphosphine group at 2.7228(3) Å (Figure 5). The phosphine-stabilized arsenium salt (aR)-9 crystallizes in the monoclinic space group C2 with four formula units in the unit cell. The asymmetric unit contains one cation and one anion. The fluorine atoms of the hexafluorophosphate ion are disordered and were modeled as two sets of six fluorine atoms by refinement of their relative occupancies. The 2(methoxymethyl)phenyl group on phosphorus is also disordered and was modeled as two components having occupancies of 64.2% and 35.8%. The ORTEP diagram of the major conformer of the molecule is shown in Figure 6a, and the minor conformer is shown in Figure 6b. The two conformers are related by rotation about the phosphorus−carbon bond of the [2-(methoxymethyl)phenyl]phosphine group. The two conformers of the cation exhibit oxygen−arsenic and oxygen− phosphorus interactions. In the major conformer, As(1)···O(11) = 3.111(4) Å and P(1)···O(11) = 2.944(4) Å; in the minor component, As(1)···O(21) = 3.721(8) Å and P(1)···O(21) = 3.039(8) Å. A phenyl−phenyl embrace is evident between the 2-(methoxymethyl)phenyl groups of symmetry-related molecules within the crystal lattice. Previously determined crystal structures of phosphine-stabilized arsenium salts containing the [2-(methoxymethyl)phenyl]phosphine group have indicated a weakening of the arsenic− phosphorus bond by a destabilizing chelate effect,1,7 but this effect was not observed in the present structure, where the As(1)−P(1) distance of 2.3358(8) Å is very close to the corresponding distance in the parent complex (aR)-8·C4H10O (viz. 2.3382(9) Å). Resolution of a P-Chiral Phosphine. Chiral tertiary phosphines (and to a lesser extent chiral arsines) play an important role in asymmetric synthesis. The field is dominated by phosphine ligands that possess stereogenic backbones, such as in BINAP, DIOP, Chiraphos, and Duphos.8 Far less

Figure 5. ORTEP diagram of the cation of (aR)-8·C4H8O with selected atoms labeled (30% ellipsoid probability level shown). Hydrogen atoms have been omitted for clarity.

lattice, as indicated by the high thermal parameters of the refined atoms. Attempts to model the disorder did not provide a satisfactory model. Refinement of the Flack parameter gave a value of 0.032(11). The As(1)−P(1) distance of 2.3382(9) Å is longer than the sum of the covalent radii of the two elements,

Figure 6. ORTEP diagram of the major (a) and the minor (b) conformers of the cation of (aR)-9 with selected atoms labeled (30% ellipsoid probability level shown). Hydrogen atoms have been omitted for clarity. 2662

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attention has been applied to the use of P-chiral phosphines (and As-chiral arsines) in asymmetric synthesis, although there are examples of asymmetric syntheses where ligands of this type have outperformed those with stereogenic backbones.9 Tertiary phosphines and arsines of the type (±)-R1R2R3E (E = P, As) can be resolved and isolated under ambient conditions because of their resistance to unimolecular pyramidal inversion;10 resolutions of such compounds have been achieved by a number of methods, but the most convenient route to configurationally pure tertiary phosphines and arsines is by bridge splitting of enantiomerically pure ortho-metalated [N′,N′-dimethyl(α-methyl)benzyl- and naphthylamine]palladium(II) complexes followed by fractional crystallization of the resulting diastereomers.11 The resolution of a P-chiral tertiary phosphine by the fractional crystallization of the diastereomers of a phosphine-stabilized arsenium complex is another appealing strategy because of the avoidance of palladium and the ready displacement of a resolved phosphine by the addition of phenylithium to a configurationally pure diastereomer of the complex, with concomitant regeneration of the arsine auxiliary. The reaction of (±)-[2-(methoxymethyl)phenyl]methyl(2naphthyl)phosphine with (aR)-2 furnished the phosphinestabilized arsenium complex (aR,RP)-/(aR,SP)-10 (Scheme 3). The mixture of diastereomers was precipitated by the addition of diethyl ether to a concentrated dichloromethane solution of the crude reaction product; the resulting white powder was collected and dried in vacuo. The 1H and 31P{1H} NMR spectra of the powder indicated an unequal mixture of two diastereomers. Integration of the PMe resonances in the 1H NMR spectrum of the complex gave the ratio of 79% (δ 1.96) to 21% (δ 2.37); integration of the 31P NMR resonances for the complex gave an identical ratio of 79% (δ 10.14) to 21% (δ 12.73). Recrystallization of the 79/21 mixture of (aR,RP)-/ (aR,SP)-10 from dichloromethane by the addition of diethyl ether afforded the major diastereomer in 22% yield as colorless needles in pure form, but the crystals were unsuitable for X-ray crystallography. A further recrystallization of the needles from butanone by the addition of diethyl ether provided crystals of the hemibutanone solvate (aR,RP)-10·0.5C4H8O, suitable for an X-ray crystal structure determination. The configurationally pure diastereomer crystallizes in the tetragonal space group P43212, with eight units in the unit cell; the asymmetric unit contains the cation of (aR,RP)-10 and the hexafluorophosphate ion distributed over two sites of symmetry, as well as a region of disordered solvent. Attempted modeling of the disordered solvent molecule was unsuccessful. Analysis through the plane of the peripheral atoms of the modeled solvent by a Slant Fourier map in CRYSTALS indicated the presence of electron density, but no obvious atomic locations could be identified. Analysis with Squeeze in PLATON revealed a solventaccessible void of 1404.6 Å3 containing 64 electrons, which corresponded to 0.5 equiv of butanone per asymmetric unit. The absolute configuration of the cation was assigned on the basis of the known configuration of the starting material (aR)-4 and was confirmed by refinement of the Flack parameter, which was −0.012(11). The structure of the cation of the complex is shown in Figure 7. The bond between As(1) and P(1) in (aR,RP)-10 of 2.3433(17) Å is similar to those observed in the complexes (aR)-7·CH2Cl2, (aR)-8·C4H8O, and (aR)-9. Approximately orthogonal coordination of the phosphine to the planar arsenium ion is evident from the angles P(1)−As(1)− C(1) and P(1)−As(1)−C(2), which are 98.19(19) and

Figure 7. ORTEP diagram of the cation of (aR,RP)-10·0.5C4H8O with selected atoms labeled (30% ellipsoid probability level shown). Hydrogen atoms have been omitted for clarity.

98.20(17)°, respectively. The oxygen atom of the 2(methoxymethyl)phenyl group interacts with the arsenic and phosphorus atoms at distances of 2.931(5) and 2.955(5) Å, respectively. Additional bond lengths and angles of note are given in Table 2. Contrary to expectation, the naphthyl group of the phosphine does not interact with the binaphthyl group in an aryl−aryl embrace.

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CONCLUSIONS

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EXPERIMENTAL SECTION

Enantiomerically pure chloro- and iodoarsines based on the (aR)-2,2′-bis(methylene)-1,1′-binaphthyl group have been prepared and structurally characterized. The iodoarsepine (aR)-2 can be used for the two-phase synthesis of air-stable, phosphine-stabilized arsenium salts containing tertiary phosphines. The first resolution of a P-chiral tertiary phosphine as a phosphine-stabilized arsenium salt is reported, and the crystal structure of a configurationally pure diastereomer of the salt has been determined.

Safety. Chloroarsines are known vesicants. Appropriate personal protective equipment should be used to prevent skin contact. Syntheses. Reactions involving air-sensitive compounds were performed under a positive pressure of nitrogen using Schlenk techniques. Dry, degassed solvents were obtained by distillation over appropriate drying agents and stored under nitrogen.12 (aR)-2,2Dimethyl-1,1′-binaphthalene ((aR)-4),13 (aR)-[{Li(TMEDA)}2{2,2′bis(methylene)-1,1′-binaphthylyl}] ((aR)-5),4 dichlorophenylarsine,14 triiodoarsine,15 trimethylphosphine,16 dimethylphenylphosphine,17 and [2-(methoxymethyl)phenyl]dimethylphosphine7 were synthesized as indicated. Routine NMR spectra were measured on a Varian Mercury spectrometer operating at 300 MHz (1H), 75 MHz (13C{1H}), and 120 MHz (31P{1H}); routine and variable-temperature NMR were measured on a Varian Inova spectrometer operating at 300 MHz (1H), 75 MHz (13C{1H}), and 120 MHz (31P{1H}). Variabletemperature NMR experiments were performed with the use of an NMR tube fitted with a Young tap, which was pressure tested to ca. 100 °C prior to insertion into the NMR spectrometer. Chemical shifts (δ) are reported in parts per million (ppm) relative to the residual solvent peaks for 1H and 13C{1H} spectra and external aqueous H3PO4 (85%) for 31P{1H} spectra. Melting points were measured with use of a Stanford Research Systems OptiMelt melting point apparatus in sealed glass tubes. EI mass spectra were recorded on a VG AutoSpec M series sector instrument and ESI mass spectra on a VG Quattro II 2663

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Table 2. Selected Bond Distances (Å) and Angles (deg) (aR)-7·CH2Cl2

(aR)-8·C4H8O

(aR)-9

(aR,RP)-10·0.5C4H8O

As1−C1 As1−C2 As1−P1 P1−C30 P1−C40

1.985(3) 1.975(3) 2.3183(11) 1.772(5) 1.764(4)

2.000(3) 1.987(3) 2.3382(9) 1.800(4) 1.803(4)

2.003(6) 1.976(5) 2.3433(17) 1.808(6) 1.795(6) 1.798(7)

P1−C50 As1···O11 P1···O11 As1···O21 P1···O21 P1−As1−C1 P1−As1−C2 C1−As1−C2 As1−P1−C30 As1−P1−C40

1.791(6)

1.792(4)

101.11(10) 98.36(9) 97.03(12) 104.2(2) 110.60(19)

102.08(11) 99.46(10) 97.38(13) 109.98(13) 105.12(14)

As1−P1−C50 C19−C10−C20−C29

116.62(18) −68.5(4)

116.06(11) −70.0(4)

1.988(3) 1.990(3) 2.3359(8) 1.800(4) 1.791(4) 1.810(9) 1.811(15) 3.111(4) 2.943(4) 3.721(8) 3.039(8) 99.58(9) 97.94(9) 95.66(11) 113.51(13) 111.33(12) 106.8(4) 109.0(8) −67.5(4)

triple-quadrupole instrument. The notation [M] refers to the mass of the cation of the complex salt in amu. Optical rotations were recorded with a Perkin−Elmer Model 241 MC polarimeter, using 1 dm quartz cells on solutions of c g of material in 100 mL of solvent. Specific rotations are within ±0.05 deg cm2 g−1. Elemental analyses were performed by staff within the Microanalytical Unit of the Research School of Chemistry. (aR)-4-Phenyl-4,5-dihydro-3H-dinaphtho[2,1-c;1′,2′-e]arsepine ((aR)-6). A suspension of (aR)- [{Li(TMEDA)}2{2,2′bis(methylene)-1,1′-binaphthylyl}] ((aR)-5; 17.9 g, 33.9 mmol) in nhexane (150 mL) was stirred at 0 °C as dichlorophenylarsine (4.6 mL, 34.0 mmol) was added. The temperature of the reaction mixture was maintained at 0 °C for 2 h, and then it was warmed to room temperature and stirred for ca. 15 h. The reaction vessel was then fitted with a reflux condenser, and the suspension was heated under reflux for 2 h. The n-hexane was removed from the reaction mixture by evaporation in vacuo, and toluene (175 mL) and deoxygenated water (200 mL) were added to the residue. The organic layer was separated, and the aqueous phase was washed with toluene (2 × 25 mL). The combined organic fractions were dried (MgSO4), and the solution was filtered through a Schlenk frit. The collected solids were washed with toluene (2 × 50 mL), and the combined organic fractions were evaporated to dryness in vacuo. The resulting foam was dissolved in toluene and methanol was added, whereupon the pure product crystallized and was collected, washed with methanol, and dried in vacuo. Yield: 8.34 g (57%). Mp: 150 °C dec. [α]D19 = +233.8 deg cm2 g−1 (c 1.0, CH2Cl2). 1H NMR (CDCl3): δ 2.65 (d, 1H, 2JHH = 12.6 Hz, CHH), 2.69 (d, 1H, 2JHH = 12.6 Hz, CHH), 2.78 (d, 1H, 2JHH = 10.5 Hz, CHH), 2.96 (d, 1H, 2JHH = 10.5 Hz, CHH), 6.85−7.94 (m, 17H, ArH). 13C{1H} NMR (CDCl3): δ 29.9, 30.6, 124.6, 124.9, 125.7, 125.9, 126.4, 126.4, 127.0, 127.1, 128.1, 128.1, 128.2, 128.2, 128.4, 132.1, 132.1, 132.5, 132.6, 132.8, 134.9, 135. 7, 139.3. EI MS: m/z 432 ([M]+, 100), 355 amu ([M − Ph]+, 8). Anal. Calcd for C28H21As: C, 77.8; H, 4.9. Found: C, 77.5; H, 5.2. (aR)-4-Iodo-4,5-dihydro-3H-dinaphtho[2,1-c;1′,2′-e]arsepine ((aR)-2). A solution of the phenylarsepine (aR)-6 (3.47 g, 8.03 mmol) and triiodoarsine (3.70 g, 8.12 mmol) in toluene (15 mL) was heated under reflux for 24 h. The toluene was removed in vacuo, and the resulting deep orange oil was dissolved in dichloromethane. The solution was filtered, and the filtrate was evaporated under a stream of nitrogen, which resulted in the crystallization of the crude product. The orange crystals were covered with n-hexane, and the vessel was sonicated for 1 h. The crystals were collected by filtration, and the procedure was repeated until the product was free of the byproduct, diiodophenylarsine. The washed crystals were recrystallized from

2.931(5) 2.955(5)

98.19(19) 98.20(17) 97.0(2) 110.5(2) 110.24(19) 115.2(2) −66.0(7)

warm dichloromethane by the addition of methanol, which gave the pure product as yellow needles. Yield: 2.90 g (75%). Mp: 136 °C dec. [α]D18 = −249.0 deg cm2 g−1 (c 1.0, C6H6). 1H NMR (C6D6): δ 2.38 (d, 1H, 2JHH = 13.2 Hz, CHH), 2.68 (d, 1H, 2JHH = 13.2 Hz, CHH), 2.84 (d, 1H, 2JHH = 11.0 Hz, CHH), 3.00 (d, 1H, 2JHH = 11.0 Hz, CHH), 6.88−7.72 (m, 12H, ArH). 13C{1H} NMR (C6D6): δ 30.9, 35.0, 125.4, 125.8, 126.1, 126.4, 126.7, 126.7, 127.1, 127.7, 128.5, 128.7, 128.7, 132. 6, 133.0, 133.2, 133.5, 133.5, 133.7, 134.6, 134.1 (one resonance obscured by C6D6). EI MS: m/z 482 ([M]+, 38), 355 amu ([M − I]+, 100). Anal. Calcd for C22H16AsI: C, 54.8; H, 3.3. Found: C, 54.6; H, 3.6. (aR)-4-Chloro-4,5-dihydro-3H-dinaphtho[2,1-c;1′,2′-e]arsepine ((aR)-3). The iodoarsepine (aR)-2 (2.00 g, 4.15 mmol) and freshly prepared silver chloride (2.97 g, 20.74 mmol) were suspended in dichloromethane (150 mL), and the mixture was stirred in the absence of light for ca. 15 h. The mixture was then filtered to remove silver iodide and unreacted silver chloride, and the solvent was removed from the filtrate in vacuo. The residue was recrystallized from a small quantity of boiling ethyl acetate to give colorless prisms of the pure product. Yield: 1.23 g (76%). Mp: 137 °C dec. [α]D19 = −237.0 deg cm2 g−1 (c 1.0, C6H6). 1H NMR (CD2Cl2): δ 2.70 (d, 1H, 2JHH = 13.5 Hz, CHH), 2.76 (d, 1H, 2JHH = 11.0 Hz, CHH), 3.15 (d, 1H, 2JHH = 13.5 Hz, CHH), 3.49 (d, 1H, 2JHH = 11.0 Hz, CHH), 7.14−7.92 (m, 12H, ArH). 13C{1H} NMR (CD2Cl2): δ 37.1, 40.2, 125.5, 125.9, 126.3, 126.6, 126.6, 126.8, 127.0, 127.8, 128.6, 128.6, 128.8, 128.9, 132.4, 133.0, 133.0, 133.3, 133.5 (two signals superimposed), 133.9, 133.9. EI MS: m/z 390 ([M]+, 88), 355 amu ([M − Cl]+, 6). Anal. Calcd for C22H16AsCl: C, 67.6; H, 4.1. Found: C, 67.8; H, 4.4. General Procedure for the Preparation of Tertiary Phosphine Stabilized Arsenium Hexafluorophosphates. A solution of the iodoarsepine (aR)-2 (1.0 equiv) in dichloromethane (50 mL) was treated with the tertiary phosphine (1.05 equiv) and an aqueous solution of potassium hexafluorophosphate (50 mL) at room temperature. The two-phase mixture was stirred for 30 min, and then the two phases were separated. The organic phase was separated, dried (MgSO4), and filtered and the solvent removed in vacuo to leave the crude product, which was recrystallized from dichloromethane− diethyl ether. (aR)-(Trimethylphosphine-P)[2,2′-bis(methylene)-1,1′-binaphthylarsepinenium] Hexafluorophosphate Dichloromethane ((aR)-7·CH2Cl2). Iodoarsepine (aR)-2 (0.50 g, 1.05 mmol), trimethylphosphine (83.5 μL, 1.10 mmol), potassium hexafluorophosphate (0.97 g, 5.25 mmol). Yield: 0.48 g (69%), Mp: 175 °C dec. [α]D19 = +171.0 deg cm2 g−1 (c 1.0, CH2Cl2). 1H NMR (CD2Cl2): δ 1.82 (d, 9H, 2JHP = 13.8 Hz, PCH3), 2.91−3.34 (m, 4H, CH2), 5.33 (s, 2664

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2H, CH2Cl2), 6.98−8.06 (m, 12H, ArH). 31P{1H} NMR (CD2Cl2): δ 4.57 (s), −143.2 (sept, 1JFP = 711.5 Hz, PF6−). ES MS: m/z 431 ([M]+, 28), 355 ([M − PMe3]+, 83), 77 amu ([PMe3]+, 100). Anal. Calcd for C26H27AsCl2F6P2: C, 47.2; H, 4.1. Found: C, 47.2; H, 4.2. Crystals of (aR)-7·CH2Cl2 suitable for X-ray diffraction were obtained from a concentrated solution of the complex in dichloromethane. (aR)-(Dimethylphenylphosphine-P)[2,2′-bis(methylene)1,1′-binaphthylarsepinenium] Hexafluorophosphate Dichloromethane ((aR)-8·CH2Cl2). Iodoarsepine (aR)-2 (0.50 g, 1.05 mmol), dimethylphenylphosphine (156 μL, 1.10 mmol), potassium hexafluorophosphate (0.97 g, 5.25 mmol). Yield: 0.62 g (92%). Mp: 176 °C dec. [α]D19 = −23.7 deg cm2 g−1 (c 1.0, CH2Cl2). 1H NMR (CD2Cl2): δ 2.05 (d, 3H, 2JHP = 11.1 Hz, PCH3), 2.09 (d, 3H, 2JHP = 11.1 Hz, PCH3), 2.98 (d, 2H, 2JHH = 11.7 Hz, CHH), 3.18 (d, 2H, 2 JHH = 11.7 Hz, CHH), 5.33 (s, 2H, CH2Cl2), 7.00−7.99 (m, 17H, ArH). 31P{1H} NMR (CD2Cl2): δ 5.16 (s), −143.3 (sept, 1JFP = 711.5 Hz, PF6−). ES MS: m/z 493 ([M]+, 69), 355 ([M − PMe2Ph]+, 67), 139 amu ([PMe2Ph]+, 100). Anal. Calcd for C31H29AsCl2F6P2: C, 51.5; H, 4.0. Found: C, 52.0; H, 4.1. Crystals of (aR)-8·C4H8O suitable for X-ray crystallographic analysis were grown from a butanone solution of the complex by dilution with diethyl ether. (aR)-(Dimethyl[2-(methoxymethyl)phenyl]phosphine-P)[2,2′-bis(methylene)-1,1′-binaphthylarsepinenium] Hexafluorophosphate ((aR)-9). Iodoarsepine (aR)-2 (0.50 g, 1.05 mmol), [2(methoxymethyl)phenyl]dimethylphosphine (0.210 g, 1.15 mmol), potassium hexafluorophosphate (0.96 g, 5.20 mmol). Yield: 0.52 g (74%). Mp: 145 °C dec. [α]D19 = +147.2 deg cm2 g−1 (c 1.0, CH2Cl2). 1 H NMR (CD2Cl2): δ 1.88 (d, 3H, 2JHP = 12.5 Hz, PCH3), 2.13 (d, 3H, 2JHP = 12.5 Hz, PCH3), 2.83−3.31 (m, 4H, CH2), 3.51 (s, 3H, CH2OCH3), 4.73 (d, 1H, 2JHH = 12.5 Hz, CHHOCH3), 4.76 (d, 1H, 2 JHH = 13.0 Hz, CHHOCH3), 6.29 (d, 1H, 3JHH = 8.5 Hz, ArH), 6.96− 7.99 (m, 15H, ArH). 31P{1H} NMR (CD2Cl2): δ 4.26 (s), −143.8 (sept, 1JFP = 711.7 Hz, PF6−). ES MS: m/z 537 ([M]+, 5), 355 ([M − PMe2(MeOCH2C6H4)]+, 8), 183 amu ([HP(MeOCH2C6H4))]+, 100). Anal. Calcd for C32H31AsF6OP2: C, 56.3; H, 4.6. Found: C, 56.0; H, 4.4. (aR,R P )-[(2-(Methoxymethyl)phenyl)methyl(2-naphthyl)phosphine-O,P)-[2,2′-bis(methylene)-1,1′-binaphthylarsepinenium] Hexafluorophosphate ((aR,RP)-10). Iodoarsine (aR)-2 (0.523 g, 1.08 mmol), (±)-2-(methoxymethyl)phenyl)methyl(2naphthyl)phosphine (0.335 g, 1.14 mmol), potassium hexafluorophosphate (1.00 g, 5.4 mmol). The powder that remained was dissolved in a small quantity of dichloromethane, and diethyl ether was added to precipitate a colorless solid. Yield: 0.40 g, (47%). 1H and 31P{1H} NMR spectroscopy of the solid indicated an unequal mixture of two diastereomers (δH 1.96 (79%, 2JHP = 12.0 Hz, PCH3), δH 2.37 (21%, 2 JHP = 12.9 Hz, PCH3), δP 10.14 (79%), δP 12.73 (21%)). Recrystallization of the mixture by dissolution in dichloromethane and dilution with diethyl ether afforded colorless crystals of one diastereomer of the complex. Yield: 0.185 g (22%). Mp: 165 °C dec. [α]18D = +221.0 deg cm2 g−1 (c 1.0, acetone). 1H NMR (CD2Cl2): δ 1.95 (d, 3H, 2JHP = 11.7 Hz, PCH3), 2.89 (s, 3H, CH2OCH3), 3.02− 3.42 (m, 4H, As(CH2)2), 3.90 (d, 1H, 2JHH = 11.7 Hz, CHH), 4.39 (d, 1H, 2JHH = 12.6 Hz, CHH), 6.07 (d, 1H, 3JHH = 8.4 Hz, ArH), 6.98− 8.24 (m, 22H, ArH). 31P{1H} NMR (CD2Cl2): δ 9.61 (s), −143.9 (sept, 1JFP = 711.1 Hz, PF6−). ES MS: m/z 649 ([M]+, 28), 389 ([M − P2-(MeOCH2C6H4)Me(C10H7)]+, 4), 213 amu ([P2-(CH2C6H4)Me2Ph]+, 100). HRMS: calcd for C41H35AsOP 649.164 15, found 649.161 60. Crystals of (aR,RP)-10·0.5C4H8O suitable for X-ray crystallographic analysis were grown from a butanone solution by the addition of diethyl ether. Crystal Structures. Crystallographic data and experimental parameters for the X-ray structural analyses are given in Table 1. Single-crystal X-ray diffraction data were collected using graphitemonochromated Mo Kα radiation (λ = 0.710 73 Å) on a Nonius Kappa CCD diffractometer. The diffractometer was equipped with a Cryostream N2 open-flow cooling device,18 and the data were collected at 200(2) K. Series of ω scans were performed to a maximum resolution of 0.77 Å. Data were processed using DENZOSMN and Scalepack software19 and corrected for absorption by the

Gaussian integration method implemented in maXus.20 The structures were solved by direct methods with SIR9221 and refined by full matrix on F with use of CRYSTALS.22 All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were included at calculated positions and were allowed to ride on the atoms to which they were attached. Molecular graphics were produced with ORTEP-3.23

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ASSOCIATED CONTENT

S Supporting Information *

Text giving the synthetic procedure for (±)-[2(methoxymethyl)phenyl]methyl(2-naphthyl)phosphine and CIF files giving crystallographic data for the crystal structures in this paper. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *Tel: +61 2 6125 4236. Fax: +61 2 6125 0750. E-mail: sbw@ rsc.anu.edu.au.

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DEDICATION Dedicated to the fond memory of Gordon Stone.

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REFERENCES

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Structures; Nonius, Delft, The Netherlands, MacScience, Japan, and University of Glasgow, Glasgow, U.K., 1999. (21) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (22) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.; Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487. (23) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.

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