J = 51.2 Hz), 30.3 (d, 1P, J = 51.2 Hz) [lit.,3 dP (81 MHz, CDCl3):
ꢀ26.5 (d, 1P, J = 50.5 Hz), 29.8 (d, 1P, J = 50.5 Hz)]. Bis(diphenyl-
phosphinyl)methane was also isolated (110 mg, 39%).
1,3-Bis(diphenylphosphino)propane monoxide: (217 mg, 71%). Mp
105–107 1C (lit.,16 mp 106–108 1C). dP (162 MHz, CDCl3): ꢀ17.3 (s,
1P), 33.2 (s, 1P) [lit.,3 dP (81 MHz, CDCl3): ꢀ17.2 (s, 1P), 32.0 (s, 1P)].
1,3-Bis(diphenylphosphinyl)propane was also isolated (82 mg, 26%).
1,4-Bis(diphenylphosphino)butane monoxide: (170 mg, 71%). Mp
189–191 1C (lit.,17 mp 190–191 1C). dP (162 MHz, CDCl3): ꢀ15.8 (s,
1P), 32.6 (s, 1P) [lit.,3 dP (81 MHz, CDCl3): ꢀ16.5 (s, 1P), 32.2 (s, 1P)].
Bis(diphenylphosphinyl)butane was also isolated (54 mg, 22%).
(ꢁ)-2,20-Bis(diphenylphosphino)-1,10-binaphthyl monoxide: (299 mg,
70%). Mp 266–267 1C (lit.,3,18). dP (162 MHz, CDCl3): ꢀ14.1 (s,
1P), 28.7 (s, 1P) [lit.,3 dP (81 MHz, CDCl3): ꢀ14.5 (s, 1P), 27.9 (s, 1P)].
(ꢁ)-1,10-(1,10-binaphthalene)-2,20-diylbis(1,1-diphenyl)
phosphine
oxide was also isolated (118 mg, 27%). The 31P NMR spectrum is
shown in Fig. 1.
Scheme 2 Mechanism of reduction of bis-phosphine oxides.
(S)-2,20-Bis[di(p-tolyl)phosphino]-1,10-binaphthyl monoxide: (107 mg,
73%). Mp 267–268 1C (lit.,19). dP (162 MHz, CDCl3): ꢀ15.7 (s, 1P),
28.8 (s, 1P). (S)-[1,10-binaphthalene]-2,20-diylbis[bis(4-methyl-phenyl)-
phosphine oxide] was also isolated (31 mg, 22%).
Triphenylphosphine oxide reduction: triflic anhydride (121 mL, 0.72
mmol), triphenylphosphine oxide (400 mg, 1.44 mmol), pentane-1-
thiol (355 mL, 2.87 mmol) and N,N-diisopropylethylamine (249 mL,
1.44 mmol) were reacted in dry CH2Cl2 (10 mL) according to the
procedure above. After workup and purification by silica column
chromatography (ethyl acetate–hexane, gradient from 0 : 100 to
100 : 0), triphenylphosphine was obtained as a white solid (177 mg,
47%). Mp 79–80 1C (lit.,16 mp 79–81 1C). dP (162 MHz, CDCl3): ꢀ5.1
(s, 1P). Triphenylphosphine oxide was also isolated (204 mg, 51%).
the phosphorane 5. Reductive elimination of the disulfide from 5
would give the phosphine 2. The phosphorane mechanism is
consistent with the mechanism proposed by Zard et al.12 for the
reduction of secondary aliphatic nitro compounds to imines by
tributylphosphine–diphenyl disulfide. Alternatively, attack by
the second thiolate could occur directly on sulfur (as in 4b) to
give the phosphine and the disulfide. Work by Omelanczuk and
Mikolajczyk,13 in which they obtained a phosphine with reten-
tion of configuration upon treatment of a chiral alkylthiophos-
phonium salt with tert-butylthiolate, suggests that the second
mechanism (4b) is the more plausible. Presumably the driving
force for the reduction is the activation of the bis-phosphine
oxide by the triflic anhydride, and the subsequent transfor-
mation of 3 to 4a, where the thiolate displaces an excellent
leaving group (the diphenylphosphinyl group), with the con-
comitant formation of the very stable PQO bond.
1 V. V. Grushin, Chem. Rev., 2004, 104, 1629–1662.
´
2 See for example, A. Cote, V. N. G. Lindsay and A. B. Charette,
Org. Lett., 2007, 9, 85–87.
3 V. V. Grushin, Organometallics, 2001, 20, 3950–3961.
4 Pentanethiol was available in the laboratory and is less smelly than
thiophenol (it has a smell rather like silver polish). Other thiols
such as the non-volatile 2-diethylaminoethanethiol hydrochloride
could almost certainly be used.
5 S. Ramos and W. Rosen, Tetrahedron Lett., 1981, 22, 35–38.
6 H.-C. Wu, J.-Q. Yu and J. B. Spencer, Org. Lett., 2004, 6,
4675–4678.
7 L. D. Quin, A Guide to Organophosphorus Chemistry, Wiley
Interscience, New York, 2000, p. 12.
Notes and references
z This was a serendipitous discovery. In attempting to convert N-(2-
mercaptoethyl)benzamide into 2-phenyl-4,5-dihydro-1,3-thiazole
using the reagent 3, we found that bis(2-benzamidoethyl)disulfide
was a by-product of the reaction. Upon further investigation, the
bis-phosphine monoxide 2 was identified by 31P NMR.
8 S. Griffin, L. Heath and P. Wyatt, Tetrahedron Lett., 1998, 39,
4405–4406.
9 See for example, L. D. Quinn, K. C. Caster, J. C. Kisalus and
K. A. Mesch, J. Am. Chem. Soc., 1984, 106, 7021–7032.
10 K. E. Elson, I. D. Jenkins and W. A. Loughlin, Aust. J. Chem.,
2004, 57, 371–376.
11 See for example, (a) J. B. Hendrickson and M. S. Hussoin, J. Org.
Chem., 1987, 52, 4137–4139; (b) J. B. Hendrickson and M. S. Hussoin,
J. Am. Chem. Soc., 1989, 54, 1144–1149; (c) S. Caddick, J. D. Wilden
and D. B. Judd, J. Am. Chem. Soc., 2004, 126, 1024–1025.
12 D. H. R. Barton, W. B. Motherwell, E. S. Simon and S. Z. Zard,
J. Chem. Soc., Perkin Trans. 1, 1986, 2243–2252.
13 J. Omelanczuk and M. Mikolajczyk, J. Am. Chem. Soc., 1979, 101,
7292–7295.
y Synthesis of 1,2-bis(diphenylphosphino)ethane monoxide.
Triflic anhydride (199 mL, 1.18 mmol) was added slowly to a solution of
1,2-bis(diphenylphosphinyl)ethane (509 mg, 1.18 mmol) in dry CH2Cl2
(15 mL) at 0 1C under a nitrogen atmosphere. A thick white slurry was
formed almost immediately which was stirred at 0 1C for 30 min.
Consecutive addition of pentane-1-thiol (585 mg, 4.73 mmol) and N,N-
diisopropylethylamine (409 mL, 2.37 mmol) to the slurry gave a pale
yellow solution which was allowed to reach room temperature and then
left stirring for 16 h. The reaction solution was washed with sodium
hydrogen carbonate (5% aqueous solution, 2 ꢃ 30 mL), dried (Na2SO4)
and filtered. The solvent was removed under reduced pressure and the
residue purified by silica column chromatography (CH3OH–ethyl acet-
ate–hexane, gradient from 0 : 0 : 100 to 10 : 90 : 0). 1,2-Bis(diphenylphos-
phino)ethane monoxide (2) was obtained as a white solid (422 mg,
86%). Mp 193–196 1C (lit.,14 mp 193–194 1C). dP (162 MHz, CDCl3):
ꢀ11.6 (d, 1P, J = 48.9 Hz), 33.2 (d, 1P, J = 48.9 Hz) [lit.,3 dP (81
MHz, CDCl3): ꢀ11.5 (d, 1P, J = 48 Hz), 32.3 (d, 1P, J = 48 Hz)].
1,2-Bis(diphenylphosphinyl)ethane (1) was also isolated (50 mg, 10%).
Analogous results were obtained with the corresponding 1,1-, 1,3-, and
1,4-bis-phosphine oxides, 2,20-bis(diphenylphosphino)-1,10-binaphthyl
monooxide and 2,20-bis[di(p-tolyl)phosphino]-1,10-binaphthyl monox-
ide to give, respectively:
14 N. A. Bondarenko, M. V. Rudomino and E. N. Tsvetkov, Izv.
Akad. Nauk SSSR, Ser. Khim., 1990, 2180–2181.
15 S. O. Grim, L. C. Satek, C. A. Tolman and J. P. Jesson, Inorganic
Chemistry, 1975, 14, 656–660.
16 Sigma-Aldrich catalogue, 2008.
17 M. L. Williams, P. C. Healy, N. K. Loh, S. P. C. Dunstan and G.
Smith, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2003, E59,
o596–o598.
18 S. Gladiali, S. Pulacchini, D. Fabbri, M. Manassero and M.
Sansoni, Tetrahedron: Asymmetry, 1998, 9, 391–395.
19 J. W. Faller, B. J. Grimmond and D. G. D’Alliessi, J. Am. Chem.
Soc., 2001, 123, 2525–2529.
Bis(diphenylphosphino)methane monoxide: (142 mg, 52%). Mp 188–190 1C
(lit.,15 mp 191–192 1C). dP (162 MHz, CDCl3): ꢀ27.3 (d, 1P,
ꢂc
This journal is The Royal Society of Chemistry 2008
4494 | Chem. Commun., 2008, 4493–4494