7-Phosphanorbornenium borohydrides: A powerful route to functional secondary phosphine-borane complexes

Article abstract of DOI:10.1021/om100053d

The reaction of phosphanorbornenium triflates with sodium borohydride gives the secondary phosphine-borane 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.

Full text of DOI:10.1021/om100053d

Organometallics 2010, 29, 1873–1874 1873
DOI: 10.1021/om100053d
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 phosphine-
borane 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.
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).
˚
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 reduction-
complexation of chlorophosphines,2 hence implying a severe
limitation on the types of functionality that can be intro-
duced. We wish to report here on a route that does not
proceed via the free secondary phosphines or by reduc-
tion-complexation of chlorophosphines and that tolerates
a huge variety of functional groups.
The yields are overall yields from the starting 1-R-3,4-
dimethylphosphole5 after Diels-Alder reaction with N-phe-
nylmaleimide,6 quaternization by methyl triflate, and reac-
tion with sodium borohydride. The complete sequence is
depicted in eq 2.
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-phosphanorborne-
nium 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 be-
tween the experimental X-ray structure of 1a and the com-
puted structure of 2 is quite revealing (Figures 1 and 2).
Whereas the structure of 1a is normal with a well-sepa-
rated triflate counterion and two P-C bridge bonds showing
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
˚
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.
(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.
r
2010 American Chemical Society
Published on Web 03/18/2010
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1874 Organometallics, Vol. 29, No. 8, 2010
Tian and Mathey
The conversion of 3b into 1-methylphosphirane is quanti-
tative, 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 environ-
mentally 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 Techno-
logical 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
˚
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.
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).
(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, P-
Me), 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,
2
PCH2), 61.9 (s, OCH2), 167.4 (d, JC-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,
1JP-H = 368.2 Hz, J = 6.0 Hz, 1 H, PH); 13C NMR (CDCl3) δ 5.23 (d,
1
1JC-P = 38.4 Hz, Me), 12.9 (s, -CH2CN), 18.5 (d, JC-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
=
Figure 2. Computed structure (B3LYP/6-311þG(d,p) level) of
7-dimethyl-7-phosphanorbornenium borohydride (2). Main
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).
˚
bond lengths (A) and angles (deg): P15-H25 = 1.6808, B24-
H25 = 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.
(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.
complexes, we have studied the reaction of 3b with sodium
hydride (eq 3).
(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.