Introduction

Phosphorus atom is a unique element in its ability to create chiral architectures because a trivalent phosphorus atom can adopt a conformationally stable tetrahedral structure arising from its high inversion energy.1 In fact, several such P-stereogenic phosphines have been prepared, and in particular, P-stereogenic bisphosphines have been widely employed as chelate ligands for transition metal-catalyzed asymmetric reactions.2, 3, 4, 5, 6 However, despite the widespread use of P-stereogenic phosphines, polymers containing chiral phosphorus atoms in their backbones have rarely been prepared.7 Recently, we synthesized P-stereogenic optically active polymers8, 9, 10, 11, 12, 13 and oligomers14, 15, 16 using P-stereogenic bisphosphines as chiral building blocks. These molecules formed higher-ordered chiral structures derived from P-stereogenic centers, and their conformations could be controlled through metal coordination with bisphosphine moieties.11, 12, 13 On the other hand, well-defined oligophosphines could potentially be used as platforms for binding transition metals in an orderly fashion. Several types of optically active P-stereogenic oligophosphines have already been prepared (Figure 1). Wild and co-workers reported the syntheses and coordination behaviors of optically active P-stereogenic tetraphosphines and hexaphosphines, which were obtained by separating a mixture of stereoisomers by column chromatography and successive complexation with chiral palladium complexes.17, 18, 19, 20, 21, 22, 23 Our group14, 15, 16 and Imamoto’s group24, 25, 26, 27, 28, 29 independently succeeded in synthesizing optically active P-stereogenic tetraphosphines (Figure 1) through different synthetic routes. Imamoto et al.28 prepared their transition metal complexes, and the obtained complexes were used as chiral catalysts for transition metal-catalyzed asymmetric hydrogenations.

Figure 1
figure 1

P-stereogenic oligophosphines reported thus far.

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To the best of our knowledge, all of the previously synthesized oligophosphines possess only one type of substitution group on the phosphorus atoms. For example, P-stereogenic hexaphosphine synthesized by Wild and co-workers contains phenyl substituents whereas the P-stereogenic dodecaphosphine that we synthesized contains twelve tert-butyl substituents on the phosphorus atoms. The coordination ability of a phosphorus atom is significantly affected by the substituents, and therefore, oligophosphines possessing more than one type of substituent on the phosphorus atoms could be promising as ligands specific to heteromultimetallic complexes. Herein, we report the synthesis of unsymmetric P-stereogenic oligophosphines with phosphorus atoms that are bound to different substitution groups. Namely, the tetraphosphine and hexaphosphine obtained have two types of phosphorus atoms possessing tert-butyl and phenyl substituents (Figure 2), and these phosphorus atoms have different coordination abilities. The tert-butylphosphine unit has a stronger basicity than the phenylphosphine unit, leading to the different coordination sites within a single oligophosphine chain. The synthetic procedures and coordination behaviors with boranes were investigated in detail as the first step toward the successful use for heteromultimetallics.

Figure 2
figure 2

Unsymmetrical P-stereogenic oligophosphines (S,R,R,S)-8–BH3 and (S,R,S,S,R,S)-10–BH3.

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Experimental Procedures

General methods

1H (399.2 MHz) and 13C (100.3 MHz) NMR spectra were recorded on an EX 400 spectrometer (JEOL, Tokyo, Japan), and samples were analyzed in CDCl3 using Me4Si as an internal standard. 31P (161.5 MHz) NMR spectra were also recorded on an EX 400 spectrometer (JEOL), and samples were analyzed in CDCl3 using H3PO4 as an external standard. The following abbreviations are used; s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet and br, broad. High-resolution mass spectra (HRMS) were obtained on a JMS-SX102A spectrometer (JEOL). Optical rotations were measured on an AUTOPOL IV instrument (Rudolph Research Analytical, Hackettstown, USA) using CHCl3 as a solvent. Analytical thin-layer chromatography was performed with silica gel 60 Merck F254 plates (Merck, Whitehouse, USA). Column chromatography was performed with Wako gel C-300 SiO2 (Wako Pure Chemical Industries, Osaka, Japan).

Materials

Tetrahydrofuran (THF) was purchased and purified by passage through purification column under Ar pressure.30 Dehydrated grade solvents of toluene and CHCl3 were purchased and used without further purification. N,N,N′,N′-tetramethylethylenediamine was purchased and distilled from KOH under Ar atmosphere. sec-BuLi (1.0 M in cyclohexane and hexane), BuLi (1.6 M in hexane), BH3·THF (1.0 M in THF), RuCl3·nH2O, K2S2O8, KOH, PPh3, CBr4, 1,4-diazabicyclo[2.2.2]octane (DABCO), morpholine, CuCl2 and aqueous NH3 (28%) were purchased and used without purification. Bisphosphines (S,S)-1−BH3,31 (S,S)-4−BH3,32 (R,R)-9−BH3,13 and (S,S)-11−BH314, 15, 16 were prepared by the procedure of the literature. All reactions were performed under Ar atmosphere using standard Schlenk techniques.

(S,S)-2−BH3

A THF solution (30 ml) of (S,S)-1−BH3 (786.0 mg, 3.0 mmol) and N,N,N′,N′-tetramethylethylenediamine (0.58 ml, 3.9 mmol) was cooled to −78 °C under Ar atmosphere. sec-BuLi (1.0 M in cyclohexane and hexane, 3.9 ml, 3.9 mmol) was added by a syringe. After stirring for 3 h, dry O2 was bubbled into the reaction mixture. The reaction mixture was allowed to warm to room temperature and stirred overnight. The reaction was quenched by addition of 2 N HCl (100 ml). The organic layer was extracted with ethyl acetate (100 ml × 3). The combined organic layers were washed with brine and dried over MgSO4. After filtration, the solvent was removed in vacuo. The residue was subjected to column chromatography on SiO2, with ethyl acetate and hexane (v/v=1/3) as an eluent (Rf=0.20, ethyl acetate/hexane v/v=1/2). The solvent was evaporated to obtain (S,S)-2−BH3 (349.0 mg, 1.26 mmol, 42%) as a colorless solid. [α]D22+1.1 (c 0.5 in CHCl3); hydrogen nuclear magnetic resonance (1H NMR) (399.2 MHz, CDCl3) δ 0.38 (br, 6H, P-BH3), 1.20 (m, 21H, P-t-Bu and P-Me), 1.70–2.07 (m, 5H, P-CH2CH2-P and OH), 4.06 (s, 2H, P-CH2-O) p.p.m.; carbon nuclear magnetic resonance (13C NMR) (CDCl3, 100.3 MHz) δ 5.4 (d, JC–P=34.7 Hz), 11.9 (d, JC–P=29.8 Hz), 15.8 (d, JC–P=31.4 Hz), 25.1, 25.8, 27.4, 27.8, 28.2, 28.5, 56.1 (d, JC–P=35.6 Hz) p.p.m.; phosphorus nuclear magnetic resonance (31P NMR) (CDCl3, 161.5 MHz) δ +28.8 (m), +35.3 (m) p.p.m.; HRMS (electrospray ionization (ESI)) calcd. for [M+Na]+ 301.2163, found 301.2163.

(S,S)-3−BH3

An H2O solution (10 ml) of KOH (729.3 mg, 13.0 mmol) and K2S2O8 (1.054 g, 3.9 mmol) was cooled to 0 °C. After addition of RuCl3·3H2O (34.0 mg, 0.13 mmol) to the H2O solution, an acetone solution (5 ml) of (S,S)-2−BH3 (361.4 mg, 1.3 mmol) was added dropwise. The reaction mixture was allowed to warm to room temperature. After stirring for 22 h, the reaction was quenched by addition of 2 N HCl (100 ml). The organic layer was extracted with ethyl acetate (100 ml × 3). The combined organic layers were washed with brine and dried over MgSO4. After filtration, the solvent was removed in vacuo. The residue was subjected to column chromatography on SiO2, with ethyl acetate and hexane (v/v=1/1) as an eluent (Rf=0.50, ethyl acetate/hexane v/v=1/2). The solvent was evaporated to obtain (S,S)-3−BH3 (234.0 mg, 0.94 mmol, 72%) as a colorless solid. [α]D22 +30.8 (c 0.5 in CHCl3); 1H NMR (399.2 MHz, CDCl3) δ 0.43 (br q, JH–B=106.7 Hz, 6H, P-BH3), 1.21 (m, 21H, P-t-Bu and P-Me), 1.58–2.12 (m, 4H, P-CH2CH2-P), 4.36 (d, J=352.5 Hz, 1H, P-H) p.p.m.; 13C NMR (CDCl3, 100.3 MHz) δ 5.8 (d, JC–P=33.9 Hz), 11.8 (d, JC–P=30.6 Hz), 16.9 (d, JC–P=30.6 Hz), 25.1, 26.8, 27.1, 27.5, 27.6, 27.9 p.p.m.; 31P NMR (CDCl3, 161.5 MHz) δ+23.9 (m), +28.1 (m) p.p.m.; HRMS (ESI) calcd. for [M+Na]+ 271.2058, found 271.2055.

(S,S)-6−BH3

A THF solution (40 ml) of (S,S)-4−BH3 (1.208 g, 4.0 mmol) and N,N,N′,N′-tetramethylethylenediamine (0.66 ml, 4.4 mmol) was cooled to −78 °C under Ar atmosphere. sec-BuLi (1.0 M in cyclohexane and hexane, 4.4 ml, 4.4 mmol) was added by a syringe. After stirring for 3 h, dry CO2 was bubbled into the reaction mixture. The reaction mixture was allowed to warm to room temperature and stirred for 2 h. After the addition of 2 N HCl (100 ml) to the reaction mixture, the organic layer was extracted with ethyl acetate (100 ml × 3). The combined organic layers were washed with brine and dried over MgSO4. After filtration, the solvent was removed in vacuo to yield crude (S,S)-5−BH3. Without purification, BH3·THF (1.0 M in THF solution, 10.0 ml, 10.0 mmol) was added to the residue at 0 °C. After stirring for 15 h at room temperature, 2 N HCl (100 ml) was slowly added. The organic layer was extracted with ethyl acetate (100 ml × 3). The combined organic layers were washed with brine and dried over MgSO4. After filtration, the solvent was removed in vacuo. The residue was subjected to column chromatography on SiO2, with ethyl acetate and hexane (v/v=1/2) as an eluent (Rf=0.40, ethyl acetate/hexane v/v=1/1). The solvent was evaporated to obtain (S,S)-6−BH3 (629.0 mg, 1.89 mmol, 47%) as a colorless solid. [α]D22 +22.7 (c 0.5 in CHCl3); 1H NMR (399.2 MHz, CDCl3) δ 0.70 (br q, JH–B=112.3 Hz, 6H, P-BH3), 1.55 (d, J=10.2 Hz, 3H, P-Me), 1.66–2.24 (m, 7H, P-CH2CH2-P, P-CH2-, and -OH), 3.82 (m, 2H, CH2-O), 7.40–7.65 (m, 10H, P-Ph) p.p.m.; 13C NMR (CDCl3, 100.3 MHz) δ 11.0 (d, JC–P=38.9 Hz), 19.6 (d, JC–P=35.5 Hz), 20.7 (d, JC–P=35.5 Hz), 29.3 (d, JC–P=34.7 Hz), 57.4, 126.6 (d, JC–P=52.9 Hz), 128.2 (d, JC–P=53.7 Hz), 129.0 (m), 131.7 (m) p.p.m.; 31P NMR (CDCl3, 161.5 MHz) δ+11.4 (m), +14.7 (m) p.p.m.; HRMS (ESI) calcd. for [M+Na]+355.1694, found 355.1697.

(S,R)-7−BH3

A CH2Cl2 solution (10 ml) of (S,S)-6−BH3 (498.0 mg, 1.5 mmol) and PPh3 (524.6 mg, 2.0 mmol) was cooled to 0 °C. To this solution, CBr4 (663.2 mg, 2.0 mmol) was added in one portion. After 5 min, the reaction mixture was allowed to warm to room temperature. After stirring for 2 h, the reaction mixture was evaporated. The residue was subjected to column chromatography on SiO2, with ethyl acetate and hexane (v/v=1/2) as an eluent (Rf=0.50, ethyl acetate/hexane v/v=1/2). The solvent was evaporated to obtain (S,R)-7−BH3 (363.8 mg, 0.92 mmol, 61%) as a colorless solid. [α]D22 +39.1 (c 0.5 in CHCl3); 1H NMR (399.2 MHz, CDCl3) δ 0.68 (br q, JH–B=111.7 Hz, 6H, P-BH3), 1.56 (d, J=10.3 Hz, 3H, P-Me), 1.68–2.18 (m, 4H, P-CH2CH2-P), 2.47 (m, J=9.0 Hz, 2H, P-CH2-), 3.16–3.56 (m, 2H, -CH2-Br), 7.42–7.64 (m, 10H, P-Ph) p.p.m.; 13C NMR (CDCl3, 100.3 MHz) δ 11.1 (d, JC–P=38.3 Hz), 19.3 (d, JC–P=34.7 Hz), 20.7 (d, JC–P=34.7 Hz), 24.3, 30.5 (d, JC–P=29.8 Hz), 125.4 (d, JC–P=51.2 Hz), 128.1 (d, JC–P=53.7 Hz), 129.1 (d, JC–P=9.9 Hz), 129.3 (d, JC–P=9.9 Hz), 131.5 (d, JC–P=9.1 Hz), 131.8, 132.0 (d, JC–P=9.1 Hz), 132.3 p.p.m.; 31P NMR (CDCl3, 161.5 MHz) δ+11.6 (m), +17.9 (m) p.p.m.; HRMS (ESI) calcd. for [M+Na]+ 417.0850, found 417.0847.

(S,R,R,S)-8−BH3

A THF solution (15 ml) of (S,S)-3−BH3(198.3 mg, 0.80 mmol) was cooled to −78 °C. BuLi (1.6 M in hexane, 0.50 ml, 0.80 mmol) was added by a syringe. After stirring for 2 h, a THF solution (10 ml) of (S,R)-7−BH3 (315.9 mg, 0.80 mmol) was added dropwise. The reaction mixture was allowed to warm to room temperature. After stirring for 21 h, the reaction was quenched by addition of 2 N HCl (100 ml). The organic layer was extracted with ethyl acetate (100 ml × 3). The combined organic layers were washed with brine and dried over MgSO4. After filtration, the solvent was removed in vacuo. The residue was subjected to column chromatography on SiO2, with ethyl acetate and hexane (v/v=1/4) as an eluent (Rf=0.35, ethyl acetate/hexane v/v=1/2). The solvent was evaporated to obtain (S,R,R,S)-8−BH3 (310.9 mg, 0.55 mmol, 69%) as a colorless solid. [α]D22 +8.1 (c 0.5 in CHCl3); 1H NMR (399.2 MHz, CDCl3) δ 0.51 (br, 12H, P-BH3), 1.06 (d, J=13.7 Hz, 9H, P-t-Bu), 1.18 (d, J=13.9 Hz, 9H, P-t-Bu), 1.24 (d, J=9.5 Hz, 3H, P-Me), 1.30–2.25 (br, 12H, P-CH2CH2-P), 1.48 (d, J=10.2 Hz, 3H, P-Me), 7.40–7.67 (m, 10H, P-Ph) p.p.m.; 13C NMR (CDCl3, 100.3 MHz) δ 5.3 (d, JC–P=33.9 Hz), 11.1 (d, JC–P=38.0 Hz), 13.9 (d, JC–P=28.9 Hz), 14.7 (d, JC–P=28.9 Hz), 16.0 (d, JC–P=30.6 Hz), 19.5 (d, JC–P=34.7 Hz), 20.1 (d, JC–P=33.9 Hz), 20.8 (d, JC–P=34.7 Hz), 25.1, 25.4, 27.6 (d, JC–P=33.0 Hz), 29.0 (d, JC–P=31.4 Hz), 125.3 (d, JC–P=50.4 Hz), 128.1 (d, JC–P=53.7 Hz), 129.1 (d, JC–P=9.9 Hz), 129.3 (d, JC–P=9.9 Hz), 131.5 (d, JC–P=9.1 Hz), 131.8, 132.2 (d, JC–P=9.1 Hz), 132.3 p.p.m.; 31P NMR (CDCl3, 161.5 MHz) δ+11.6 (m), +21.6 (m), +29.0 (m), +36.9 (m) p.p.m.; HRMS (ESI) calcd. for [M+NH4]+ 580.4199, found 580.4208.

(S,R,S,S,R,S)-10−BH3

A THF solution (15 ml) of (S,S)-3−BH3(247.9 mg, 1.0 mmol) was cooled to −78 °C. BuLi (1.6 M in hexane, 0.61 ml, 1.0 mmol) was added by a syringe. After stirring for 2 h, a THF solution (10 ml) of (R,R)-9−BH3 (243.9 mg, 0.50 mmol) was added dropwise. The reaction mixture was allowed to warm to room temperature. After stirring for 21 h, the reaction was quenched by addition of 2 N HCl (100 ml). The organic layer was extracted with ethyl acetate (100 ml × 3). The combined organic layers were washed with brine and dried over MgSO4. After filtration, the solvent was removed in vacuo. The residue was subjected to column chromatography on SiO2, with ethyl acetate as an eluent (Rf=0.30, ethyl acetate/hexane v/v=1/2). The solvent was evaporated to obtain (S,R,S,S,R,S)-10−BH3 (280.2 mg, 0.34 mmol, 68%) as a colorless solid. 1H NMR (399.2 MHz, CDCl3) δ 0.52 (br, 18H, P-BH3), 1.06 (d, J=13.6 Hz, 18H, P-t-Bu), 1.18 (d, J=13.6 Hz, 18H, P-t-Bu), 1.25 (d, J=9.7 Hz, 6H, P-Me), 1.36–2.22 (m, 20H, P-CH2CH2-P), 7.44–7.61 (m, 10H, P-Ph) p.p.m.; 13C NMR (CDCl3, 100.3 MHz) δ 5.4 (d, JC–P=34.7 Hz), 13.9 (d, JC–P=28.9 Hz), 14.6 (d, JC–P=28.9 Hz), 16.0 (d, JC–P=30.6 Hz), 19.4 (d, JC–P=34.7 Hz), 20.0 (d, JC–P=34.7 Hz), 25.1, 25.4, 27.7 (d, JC–P=33.1 Hz), 29.0 (d, JC–P=31.4 Hz), 128.6 (d, JC–P=81.0 Hz), 129.4 (t, JC–P=5.0 Hz), 132.2 (t, JC–P=4.5 Hz), 132.4 p.p.m.; 31P NMR (CDCl3, 161.5 MHz) δ+21.7 (s), +29.2 (m), +37.0 (s) p.p.m.; HRMS (FAB) calcd. for C40H90B6P6 822.6027, found 822.6041.

(S,R,R,S)-12−BH3

A THF solution (40 ml) of (S,S)-4−BH3 and N,N,N′,N′-tetramethylethylenediamine (0.72 ml, 4.8 mmol) was cooled to −78 °C under Ar atmosphere. sec-BuLi (1.0 M in cyclohexane and hexane, 4.8 ml, 4.8 mmol) was added slowly by a syringe. After stirring for 3 h, CuCl2 (807.0 mg, 6.0 mmol) was added in one portion. The reaction mixture was allowed to warm to room temperature and stirred overnight. The reaction was quenched by 28% aqueous NH3. The organic layer was extracted with ethyl acetate (100 ml × 3). The combined organic layers were washed with brine and dried over MgSO4. After filtration, the solvent was removed in vacuo. The residue was subjected to column chromatography on SiO2, with hexane and dichloromethane (v/v=2/3 to 0/10) as an eluent (Rf=0.30, ethyl acetate/hexane v/v=1/2). The solvent was evaporated to obtain (S,R,R,S)-12−BH3(455.5 mg, 0.76 mmol, 63%) as a colorless solid. 1H NMR (399.2 MHz, CDCl3) δ 0.65 (br d, JH–B=107.7 Hz, 12H, P-BH3), 1.54 (d, J=10.2 Hz, 6H, P-Me), 1.63–1.85 (m, 6H, P-CH2CH2-P), 1.93–2.06 (m, 6H, P-CH2CH2-P), 7.37–7.62 (m, 20H, P-Ph) p.p.m.; 13C NMR (CDCl3, 100.3 MHz) δ 11.1 (d, JC–P=38.0 Hz), 18.8, 19.0, 19.1, 19.4, 20.7 (d, JC–P=34.7 Hz), 125.2 (d, JC–P=51.3 Hz), 128.0 (d, JC–P=53.7 Hz), 129.0, 129.1, 129.2, 131.5, 131.5, 131.8, 132.0, 132.2 p.p.m.; 31P NMR (CDCl3, 161.5 MHz) δ 11.5, 21.1 p.p.m. HRMS (ESI) calcd. for [M+NH4]+ 620.3573, found 620.3553.

1H, 13C, and 31P NMR spectra of all new compounds are shown in Supplementary Figures S1–S21 in Supplementary Information.

Deboranation by morpholine

Tetraphosphine (S,R,R,S)-8−BH3 (5.6 mg, 0.01 mmol) was dissolved in degassed morpholine (1.0 ml). After stirring for 48 h at 50 °C, morpholine was removed in vacuo. The residue was dissolved in CDCl3 and subjected to NMR spectroscopy.

Deboranation by DABCO

Oligophosphine (S,R,R,S)-8−BH3 (5.6 mg, 0.01 mmol) or (S,R,S,S,R,S)-10−BH3 (8.2 mg, 0.01 mmol) was dissolved in CDCl3, together with DABCO. After stirring for 18 h at 55 °C, the 31P NMR spectrum was measured.

Results and Discussion

The synthetic routes for the production of enantiomerically pure unsymmetrical P-stereogenic tetraphosphine (S,R,R,S)-8−BH3 and hexaphosphine (S,R,S,S,R,S)-10−BH3 are shown in Scheme 1. Bisphosphine precursors (S,S)-1−BH331 and (S,S)-4−BH332 were synthesized as described previously. Bisphosphine (S,S)-1−BH3 was lithiated with 1.3 equivalent sec-butyllithium, and after O2 babbling, (S,S)-2−BH3 was obtained in 42% yield.33 The ruthenium-catalyzed oxidation and base-promoted decarboxylation26 of (S,S)-2−BH3 provided (S,S)-3−BH3 in 72% yield. The treatment of bisphosphine (S,S)-4−BH3 with 1.1 equivalent sec-butyllithium and CO2 gas gave (S,S)-5−BH3.34, 35 Without purification of (S,S)-5−BH3, its reduction with BH3·THF was carried out to obtain (S,S)-6−BH3 in 47% yield, which was followed by the Appel reaction36 of (S,S)-6−BH3 to afford (S,R)-7−BH3 in 61% yield. After the lithiation of (S,S)-3−BH3, the addition of 1.0 equivalent (S,R)-7−BH3 provided the target tetraphosphine (S,R,R,S)-8−BH3 in 68% yield, whereas the addition of 0.5 equivalent (R,R)-9−BH313 gave hexaphosphine (S,R,S,S,R,S)-10−BH3 in 69% yield.

The 31P NMR spectrum of tetraphosphine (S,R,R,S)-8−BH3 showed four signals at +11.6, +21.6, +29.0 and +36.9 p.p.m., as expected. It is known that bisphosphines (S,S)-1−BH331 and (S,S)-4−BH3,32 consisting of tert-butyl- and phenylphosphines exhibit 31P NMR signals at +28.2 and +11.1 p.p.m., respectively. In addition, the signals of tetraphosphine (S,R,R,S)-11−BH314, 15, 16, 28 with tert-butyl substituents appear at +31.5 and +39.9 p.p.m.; these signals are assigned to outer and inner phosphorus atoms, respectively. Thus, the peaks representing the outer tert-butylphosphines are observed at around 30 p.p.m., and representing the inner tert-butylphosphines appear at magnetic fields ∼8 p.p.m. below those of the outer ones. These results lead to an assignment of the tetraphosphine (S,R,R,S)-8−BH3 signals, as summarized in Figure 3.

Figure 3
figure 3

31P NMR spectrum of (S,R,R,S)-8–BH3 and comparison of chemical shifts.

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The 31P NMR signals of hexaphosphine (S,R,S,S,R,S)-10−BH3 were assigned, as shown in Figure 4. The signals were confirmed in the same manner as those of (S,R,R,S)-8−BH3. The spectrum exhibited two peaks of the tert-butylphosphine units at +29.2 and +37.0 p.p.m., and one peak representing the phenylphosphine units at +21.7 p.p.m.

Figure 4
figure 4

31P NMR spectrum of (S,R,S,S,R,S)-10–BH3.

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We attempted to remove the boranes coordinated with the phosphorus atoms in tetraphosphine (S,R,R,S)-8−BH3 by using organic bases such as morpholine23 (For deboranation with morpholine; for example, see: Jarroux N, Keller P, Mingotaud A-F, Mingotaud C, & Skyes C. Shape-tunable Polymer nodules Grown from Liposomes via Ring-opening Metathesis Polymerisation J. Am. Soc. 126, 15958–15959 (2004)) and DABCO. (For deboranation with DABCO, for example, see: Morisaki et al.11, 12, 13). When (S,R,R,S)-8−BH3 was stirred in morpholine for 48 h, all the boranes were readily removed. As shown in Figure 5 (31P NMR spectra of (S,S)-1 and (S,S)-4 after deboranation appeared at −10.0 and −32.0 p.p.m., respectively), the 31P NMR spectrum of (S,R,R,S)-8 exhibited four signals at −32.6, −17.4, −10.9 and +7.5 p.p.m., which were observed at higher magnetic fields than the signals of (S,R,R,S)-8−BH3 because the electron densities of phosphorus atoms had increased after deboranation.

Figure 5
figure 5

Removal of BH3 from (S,R,R,S)-8–BH3 with morpholine, and 31P NMR spectra.

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On the other hand, we found that DABCO removed boranes selectively from the phenylphosphine moieties; in other words, the boranes on the tert-butylphosphine units remained. The coordinate bond between borane and tert-butylphosphine is stronger than that between borane and phenylphosphine, which allowed for the chemoselective removal of boranes from (S,R,R,S)-8−BH3. The molar ratio of DABCO to phosphine was varied, and the 31P NMR measurement for 6.0 equivalent DABCO is shown in Figure 6. (The trace of 31P NMR spectra of (S,R,R,S)-8−BH3 with DABCO is shown in Figure S22 in Supplementary Information). This spectrum suggests that the borane on the inner phenylphosphine unit (before deboranation: δ+21.6 p.p.m., after deboranation: δ−17.4 p.p.m.) was more easily removed. The deboranation of the external phenylphosphine unit (before deboranation: δ+11.6 p.p.m., after deboranation: δ−32.6 p.p.m.) competed with that of the inner tert-butylphosphine unit (before deboranation: δ+29.0 p.p.m., after deboranation: δ−10.9 p.p.m.). The excess amount of DABCO removed boranes on tert-butylphosphine units, and small peaks derived from the bare tert-butylphosphine appeared at around δ+8 and −10 p.p.m. Thus, the addition of 4–6 equivalent DABCO was the optimized condition for the removal of boranes from (S,R,R,S)-8−BH3. (The trace of 31P NMR spectra of (S,R,R,S)-8−BH3 with DABCO is shown in Figure S22 in Supplementary Information.)

Figure 6
figure 6

Chemoselective removal of BH3 from (S,R,R,S)-8–BH3 with diazabicyclo[2.2.2]octane (DABCO), and 31P NMR spectra.

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We also studied the chemoselective deboranation of (S,R,S,S,R,S)-10−BH3, and these results are shown in Figure 7. The signal that appeared at around −17 p.p.m. was assigned to the bare phosphorus atom of the phenylphosphine moiety. The use of 4–6 equivalent DABCO allowed for the most efficient chemoselective removal of boranes from (S,R,S,S,R,S)-10−BH3 (The trace of 31P NMR spectra of (S,R,S,S,R,S)-10−BH3 with DABCO is shown in Figure S23 in Supplementary Information), similar to the result obtained for (S,R,R,S)-8−BH3. This chemoselectivity arises from the different coordination affinities of the phosphorus atoms to boranes. Accordingly, various transition metals could potentially be added to these molecules to produce a desired sequence by utilizing unsymmetrical oligophosphines.

Figure 7
figure 7

Chemoselective removal of BH3 from (S,R,S,S,R,S)-10–BH3 with diazabicyclo[2.2.2]octane (DABCO), and 31P NMR spectra.

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The specific optical rotations and melting points of the oligophosphines were also examined, and the results are summarized in Table 1. The specific rotation of (S,R,R,S)-8−BH3 was +8.1 (c 0.5 in CHCl3), whereas that of (S,R,R,S)-11−BH3 was −3.4 (c 1.0 in CHCl3). The specific optical rotations of (S,S)-1−BH3 and (S,S)-4−BH3 were −9.1 (c 1.0 in CHCl3) and +33.6 (c 0.98 in CH2Cl2), respectively. The value of (S,R,R,S)-11−BH3 was lower than that of (S,S)-1−BH3 because the oligophosphine containing a unified substitution group resembles also-called isotactic oligomer from the structural viewpoint. Melting points (Tm) were measured using differential scanning calorimetry. The Tm of (S,R,R,S)-8−BH3was found to be 180 °C, which was lower than those of (S,R,R,S)-11−BH3 (208 °C) and (S,R,R,S)-12−BH3 (227 °C), owing to the lower crystallinity that results from its unsymmetrical structure. (DSC thermograms are shown in Figures S24–S26 in Supplementary Information.)

Table 1 Specific rotation and melting point values of P-stereogenic oligophosphines
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Conclusion

In conclusion, optically active P-stereogenic oligophosphines with different substituted groups on their phosphorus atoms were synthesized from P-stereogenic bisphosphines. Chemoselective deboranation was performed by selecting an adequate reaction condition; the boranes on the phenylphosphine unit were chemoselectively removed by DABCO. The coordination ability could thus be controlled by the substitution groups on the phosphorus atoms in a single oligomer chain. Therefore, heteromultimetallic complexes, in which transition metals are sequentially aligned, are possible to obtain. The synthesis of enantiomerically pure P-stereogenic polymers containing both phenyl and tert-butyl substituents is currently underway. Studies on the coordination behaviors and conformational changes of these P-stereogenic oligophosphines and polymers will form the next step toward the sequential alignment of metals on a single polymer chain.

scheme 1

Synthesis of unsymmetrical P-stereogenic oligophosphines (S,R,R,S)-8–BH3 and (S,R,S,S,R,S)-10–BH3.