Organometallics 2010, 29, 1873–1874 DOI: 10.1021/om100053d
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7-Phosphanorbornenium Borohydrides: A Powerful Route to Functional Secondary Phosphine-Borane Complexes Rongqiang Tian†,‡ and Franc-ois Mathey*,† †
Division of Chemistry & Biological Chemistry, School of Physical & Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, and ‡Chemistry Department, International Phosphorus Laboratory, Zhengzhou University, Zhengzhou 450052, People’s Republic of China Received January 22, 2010 Summary: The reaction of phosphanorbornenium triflates with sodium borohydride gives the secondary phosphineborane complexes by reductive cleavage of the two P-C bonds of the bridge. This route is compatible with functionalities such as Cl, CN, CO2Et, and Th. Due to their ease of handling and the versatility of their chemistry, secondary phosphine-borane complexes have now found widespread use in the synthesis of phosphines for homogeneous and asymmetric catalysis.1 In most cases, these complexes are simply made by reacting a source of borane with a secondary phosphine or by reductioncomplexation of chlorophosphines,2 hence implying a severe limitation on the types of functionality that can be introduced. We wish to report here on a route that does not proceed via the free secondary phosphines or by reduction-complexation of chlorophosphines and that tolerates a huge variety of functional groups. Our starting point was an observation made during the study of the hydrolysis of 7-phosphanorbornenium salts: under mildly basic conditions, the two P-C bridge bonds are cleaved to give secondary phosphine oxides.3 This led us to wonder what would be the behavior of 7-phosphanorbornenium borohydrides. Several phosphonium borohydrides have been made and used as reducing agents in organic synthesis,4 but in our case we could expect a nucleophilic attack of [BH4]- onto the positively charged phosphorus, leading to decomposition products. The comparison between the experimental X-ray structure of 1a and the computed structure of 2 is quite revealing (Figures 1 and 2). Whereas the structure of 1a is normal with a well-separated triflate counterion and two P-C bridge bonds showing no special lengthening at ca. 1.82 A˚, the structure of 2 displays a hydrogen bridge between the borohydride and *To whom correspondence should be addressed. E-mail: fmathey@ ntu.edu.sg. (1) Reviews: Ohff, M.; Holz, J.; Quirmbach, M.; Borner, A. Synthesis 1998, 1391. Carboni, B.; Monnier, L. Tetrahedron 1999, 55, 1197. Yamanoi, Y.; Imamoto, T. Rev. Heteroat. Chem. 1999, 20, 227. Darcel, C.; Kaloun, E. B.; Merdes, R.; Moulin, D.; Riegel, N.; Thorimbert, S.; Genet, J. P.; Juge, S. J. Organomet. Chem. 2001, 624, 333. Crepy, K. V. L.; Imamoto, T. Top. Curr. Chem. 2003, 229, 1. Holz, J.; Gensow, M. N.; Zayas, O.; Borner, A. Curr. Org. Chem. 2007, 11, 61. (2) Imamoto, T.; Oshiki, T.; Onosawa, T.; Kusumoto, T.; Sato, K. J. Am. Chem. Soc. 1990, 112, 5244. (3) Tian, R.; Liu, H.; Duan, Z.; Mathey, F. J. Am. Chem. Soc. 2009, 131, 16008. (4) Driscoll, J. S.; Matthews, C. N. Chem. Ind. 1963, 31, 1282. Firouzabadi, H.; Adibi, M. Phosphorus Sulfur Silicon Relat. Elem. 1998, 142, 125. r 2010 American Chemical Society
the phosphanorbornenium ions (P-H = 1.68 A˚ and B-H = 1.32 A˚). The pentacoordination of phosphorus induces a drastic lengthening of the P-C bridge bonds at 1.90 and 1.94 A˚. On this basis, we were not surprised to find that sodium borohydride readily reacts with 1a in THF at room temperature to give the secondary phosphine-borane 3a (eq 1).
The yields are overall yields from the starting 1-R-3,4dimethylphosphole5 after Diels-Alder reaction with N-phenylmaleimide,6 quaternization by methyl triflate, and reaction with sodium borohydride. The complete sequence is depicted in eq 2.
Each step was monitored by 31P NMR, and none of the intermediate products were isolated. The final products were purified by chromatography. Full experimental details are described in the Supporting Information. It must be stressed that it is possible to replace methyl triflate by benzyl bromide and to get the benzylphenylphosphine-borane in 74% yield. Only 3a has been reported in the literature so far; thus, some spectral data are given here for 3b-e.7 Since an almost endless variety of substituents can be grafted on the phospholyl ring,8 it is clear that this route has a huge synthetic potential. In order to illustrate the new possibilities offered by these functional secondary phosphine-borane (5) Breque, A.; Mathey, F.; Savignac, P. Synthesis 1981, 983. Mathey, F. Sci. Synth. 2002, 9, 553. Tran Huy, N. H.; Donnadieu, B.; Mathey, F.; Muller, A.; Colby, K.; Bardeen, C. J. Organometallics 2008, 27, 5521. (6) Mathey, F.; Mercier, F. Tetrahedron Lett. 1981, 22, 319. Published on Web 03/18/2010
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Tian and Mathey
The conversion of 3b into 1-methylphosphirane is quantitative, according to the 31P NMR spectrum of the reaction mixture. The phosphirane is identified by its high-field shift at -251.9 ppm and fully characterized as its P-W(CO)5 complex.9 It must be stressed that 1-methylphosphirane has been described only once and prepared from the environmentally unfriendly methylphosphine.10 Its use as a potential monomer for the synthesis of phosphorus polymers has been studied from a theoretical standpoint.11 Finally, it must be underlined that this chemistry is another illustration of the synthetic equivalency between 7-phosphanorbornenium and phosphenium cations.
Acknowledgment. We thank the China Scholarship Council for a fellowship to R.T., the Nanyang Technological University in Singapore for the financial support of this work, and Dr. Li Yong Xin for the X-ray crystal structure analysis. Figure 1. X-ray crystal structure of 7-phosphanorbornenium triflate (1a). Main bond lengths (A˚) and angles (deg): P1-C1 = 1.7810(19), P1-C2 = 1.7762(17), P1-C8 = 1.8183(16), P1-C13 = 1.8226(17); C8-P1-C13 = 84.84(7).
Figure 2. Computed structure (B3LYP/6-311þG(d,p) level) of 7-dimethyl-7-phosphanorbornenium borohydride (2). Main bond lengths (A˚) and angles (deg): P15-H25 = 1.6808, B24H25 = 1.3197, B24-(H26-H28) = 1.2084-1.2090, P15-C2 = 1.9427, P15-C3 = 1.8982, P15-C16 = 1.8321, P15-C20 = 1.8375; C2-P15-C3 = 78.69.
complexes, we have studied the reaction of 3b with sodium hydride (eq 3).
Supporting Information Available: Text giving experimental details and a CIF file giving the X-ray crystal structure analysis of compound 1a. This material is available free of charge via the Internet at http://pubs.acs.org. (7) 3b: 31P NMR (CDCl3) δ -24.0 (br. q, 1JB-P = 52.5 Hz); 1H NMR (CDCl3) δ 0.54 (br q, JB-H ≈ 100 Hz, 3 H, BH3), 1.46 (dd, J = 11.4 Hz, J = 5.0 Hz, 3 H, Me), 2.16-2.36 (m, 2H, PCH2), 3.75-3.85 (m, 2H, ClCH2), 4.96 (dm, 1JP-H = 373.1 Hz, J = 6.4 Hz, 1 H, PH); 13C NMR (CDCl3) δ 5.43 (d, 1JC-P = 38.5 Hz, Me), 26.43 (d, 1JC-P = 35.6 Hz, PCH2), 39.25 (s, ClCH2). 3c: 31P NMR (CDCl3) δ -20.2 (br q, 1JB-P = 52.8 Hz); 1H NMR (CDCl3) δ 0.51 (br q, JB-H ≈ 102 Hz, 3 H, BH3), 1.23 (t, JH-H = 7.2 Hz, 3H, Et), 1.43 (dd, J = 11.1 Hz, J = 5.7 Hz, 3 H, PMe), 2.63-2.87 (m, 2H, PCH2), 4.15 (d, JH-H = 7.2 Hz, 2H, OCH2), 5.01 (dm, 1JP-H = 373.2 Hz, J = 2.7 Hz, 1 H, PH); 13C NMR (CDCl3) δ 5.29 (d, 1JC-P = 37.0 Hz, PMe), 14.13 (s, Me), 29.4 (d, 1JC-P = 31.0 Hz, PCH2), 61.9 (s, OCH2), 167.4 (d, 2JC-P = 7.9 Hz, CO). 3d: 31P NMR (CDCl3) δ -19.1 (br q, 1JB-P = 52.1 Hz); 1H NMR (CDCl3) δ 0.52 (br q, JB-H ≈ 106 Hz, 3 H, BH3), 1.50 (dd, J = 11.9 Hz, J = 5.5 Hz, 3 H, Me), 1.98-2.25 (dm, 2H, PCH2), 2.63-2.79 (m, 2H, CH2CN), 4.94 (dm, 1 JP-H = 368.2 Hz, J = 6.0 Hz, 1 H, PH); 13C NMR (CDCl3) δ 5.23 (d, 1 JC-P = 38.4 Hz, Me), 12.9 (s, -CH2CN), 18.5 (d, 1JC-P = 35.5 Hz, PCH2), 118.1 (d, JC-P = 11.5 Hz, CN). 3e: 31P NMR (CDCl3) δ -28.1 (dm, 1JB-P = 61 Hz); 1H NMR (CDCl3) δ 0.89 (br q, JB-H ≈ 118 Hz, 3 H, BH3), 1.69 (dd, J = 11.0 Hz, J = 5.0 Hz, 3 H, Me), 5.80 (dm, 1JP-H = 380.1 Hz, J = 6.4 Hz, 1 H, PH), 7.03 (q, J = 3.7 Hz, J = 5.0 Hz, 1H, Th), 7.09 (d, J = 3.7 Hz, 1H, Th), 7.14 (d, J = 3.7 Hz, 1H, Th), 7.18-7.26 (m, 3H, Th), 7.48-7.51 (m, 1H); 13C NMR (CDCl3) δ 9.7 (d, 1JC-P = 40.3 Hz, Me), 123.7 (d, 1JC-P = 56.6 Hz, PTh), 124.4 (s, Th, CH), 124.7 (s, Th, CH), 124.8 (d, 2JC-P = 10.6 Hz, Th, CH), 125.3 (s, Th, CH), 126.0 (s, Th, CH), 128.2 (s, Th, CH), 134.5 (s, Th), 136.8 (s, Th), 138.2 (s, Th), 138.3 (d, 3JC-P = 9.6 Hz, Th, CH), 145.8 (s, Th). (8) Holand, S.; Mathey, F. Organometallics 1988, 7, 1796. (9) 5: 31P NMR (CDCl3) δ -200.5 (1JW-P = 258.5 Hz); 1H NMR (CDCl3) δ 1.10-1.14 (t, 2 H, CH2), 1.26-1.32 (m, 2H, CH2), 1.37 (d, 2JH-P = 7.3 Hz, 3H, Me); 13C NMR (CDCl3) δ 8.4 (d, 1JC-P = 11.6 Hz, CH2), 16.5 (d, 1JC-P = 16.4 Hz, Me), 195.7 (d, 2JC-P = 8.7 Hz, CO). (10) Chan, S.; Goldwhite, H.; Keyzer, H.; Rowsell, D. G.; Tang, R. Tetrahedron 1969, 25, 1097. (11) Hodgson, J. L.; Coote, M. L. Macromolecules 2005, 38, 8902. Coote, M. L.; Hodgson, J. L.; Krenske, E. H.; Wild, S. B. Heteroat. Chem. 2007, 18, 118.
