Lewis acid mediated P-P bond hydrogenation and hydrosilylation

Article abstract of DOI:10.1039/b925126j

The reaction of H₂, R₃SiH or R₂SiH ₂ with P₅Ph₅ in the presence of stoichiometric B(C₆F₅)₃ results in excellent yields of the phosphine-borane adducts (PhPH

Full text of DOI:10.1039/b925126j

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COMMUNICATION
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Lewis acid mediated P–P bond hydrogenation and hydrosilylationw
Stephen J. Geier and Douglas W. Stephan*
Received (in Cambridge, UK) 1st December 2009, Accepted 22nd December 2009
First published as an Advance Article on the web 13th January 2010
DOI: 10.1039/b925126j
The reaction of H2, R3SiH or R2SiH2 with P5Ph5 in the
presence of stoichiometric B(C6F5)3 results in excellent yields
liquors of these reactions were observed to contain residual
P5Ph5. Altering the stoichiometry to employ 5 equivalents of
B(C6F5)3 per equivalent of P5Ph5 resulted in the quantitative
conversion to (1) which was subsequently isolated in 91%
yield as a crystalline solid [eqn (1)].
of
the
phosphine–borane
adducts
(PhPH2)B(C6F5)3,
((R3Si)PhPH)B(C6F5)3 and ((R2SiH)PhPH)B(C6F5)3, respectively.
The recent renaissance in the chemistry of the main group
elements has nowhere been shown more clearly than in the
chemistry of phosphorus.1 For example, the interception of
unprecedented new P-based chains2 and clusters3 stabilized by
carbene ligands has been described by the groups of
Robinson4 and Bertrand.5 Shaffer et al.6 have reported the
characterization of nanostructures derived from elemental P.
In addition, Cummins et al.7–9 have developed a highly
innovative and elegant, metal-mediated synthesis of P3As.
Collectively, these findings illustrate remarkable and artful
methods to assemble new elemental allotropes and binary
materials. In a related vein, the functionalization of polyalkyl-
or polyaryl-phosphines has been extensively explored by
Burford and co-workers.10–23 Very recently Weigand et al.24
have integrated elemental P with phosphenium cations affording
the discovery of unprecedented cationic P-clusters [Ph2P5]+,
ð1Þ
In a similar procedure, a mixture of P5Ph5 and five equivalents
of B(C6F5)3 was treated with excess Et3SiH. Over the course of
12 hours, this resulted in the quantitative conversion to the
silylphosphine–borane adduct ((Et3Si)PhPH)B(C6F5)3 (2)33 as
evidenced by the observation of the single 31P NMR signal
at ꢀ46.6 ppm. Quaternization of B is consistent with the 19F
NMR signals at ꢀ129.8, ꢀ156.5 and ꢀ163.7 ppm and the 11
B
NMR resonance at ꢀ12.3 ppm. Formation of the secondary
phosphine is confirmed by the 1H NMR resonance attributable
[Ph4P6]2+, and [Ph6P7]3+
.
1
to the P–H proton at 4.72 ppm, with JP–H = 345 Hz. This
Targeting applications of the reactivity of P–P bonds, we
have reported a Rh-catalyst for the hydrogenation and
silylation of P–P bonds providing secondary phosphines and
silylphosphines.25 Concurrent with these studies, we have also
demonstrated that bulky polyphosphines do not form classical
Lewis acid–base adducts with B(C6F5)3 and thus these
combinations of reagents behave as ‘‘frustrated Lewis pairs’’
(FLPs).26,27 Exploiting the reactivity of such systems, we have
recently shown that such FLPs react with alkynes to effect the
addition of P and B to the alkyne affording zwitterionic
polyphosphino-phosphonium-alkenyl-borates.28 This finding
of FLP reactivity prompted us to probe the possibility of
reduction of P–P bonds via FLP-hydrogenation.29–31 Thus, in
this communication, we report the high yielding, stoichio-
metric hydrogenation and hydrosilylation of the P–P bonds
of P5Ph5 mediated by the Lewis acid B(C6F5)3.
product (2) was easily isolated in nearly quantitative yield and
subsequently characterized by X-ray crystallography (Fig. 1).z
The structural data confirm the formation of the secondary
silylphenylphosphine adduct of B(C6F5)3 with P–B and P–Si
bond distances of 2.093(6) A and 2.333(2) A, respectively. The
former is similar to the B–P bond lengths in the adducts
(R2PH)B(C6F5)3 (R = Ph 2.098(3) A, tBu 2.094(7) A)34,35
while the P–Si distance is typical.36–40
A 1 : 1 mixture of P5Ph5–B(C6F5)3 was exposed to 4 atm of
H2. This resulted in the formation of a new species (1) which
was shown spectroscopically to be the phenylphosphine–borane
adduct, (PhPH2)B(C6F5)3.32,33 In addition, the corresponding
reaction with D2 gave rise to a 31P NMR signal at ꢀ43.6 ppm
2
and a D signal at 5.09 ppm with JP–D of 64 Hz. The mother
Department of Chemistry, University of Toronto, 80 St George St,
Toronto, Ontario, Canada M5S3H6.
E-mail: dstephan@chem.utoronto.ca
w Crystal data: CCDC 756444. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b925126j
Fig. 1 POV-ray drawing of (2).
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phosphonium center to yield the secondary silylphosphine
adduct of B(C6F5)3.
In conclusion, combination of P5Ph5 and B(C6F5)3 produces
a frustrated Lewis pair, that can activate H2 or secondary or
tertiary silanes to yield the phosphine adducts (PhPH2)B(C6F5)3,
((R3Si)PhPH)B(C6F5)3
and
((R2SiH)PhPH)B(C6F5)3,
respectively. While the present reductions proceed stoichio-
metrically, these observations represent rare examples of
metal-free reductions of P–P bonds. Moreover, these findings
point to the possibility of metal-free catalysts for the synthesis
of primary and secondary phosphines from P–P bonded
species. Such developments could also provide avenues to
asymmetric catalysis offering unique routes to phosphines that
are chiral at P. Efforts to develop such metal-free catalysts are
on-going.
Scheme 1 Synthesis of (2)–(7).
DWS gratefully acknowledges the financial support of
NSERC of Canada and the award of a Canada Research
Chair and a Killam Research Fellowship. SJG is grateful for
the support of a NSERC postgraduate fellowship from
NSERC of Canada and the assistance of Dr Alan J. Lough.
In a similar fashion a series of silanes was employed to reduce
P5Ph5 to the analogous adducts, ((RR0R00Si)PhꢂPH)B(C6F5)3
(3)–(7)33 (Scheme 1). Isolated yields ranged between 82–96%.
The 31P{1H} NMR signals for these products were broad
presumably due to P coupling to B, however P–H coupling
was evident in 1H NMR spectrum of each product. Of
particular interest is the diastereomeric product derived from
the P5Ph5 reduction with Ph(Me)SiH2, i.e. ((PhMeSiH)-
PhPH)B(C6F5)3 (6). The 31P{1H} NMR spectrum of this
product shows two resonances at ꢀ40.9 and ꢀ41.6 ppm in a
4 : 5 ratio indicating a slight preference for the formation of
one of the diastereomers under these conditions. In addition,
use of 2.5 equivalents of the disilane C6H4(SiHMe2)2 affords
the bis-phosphine–borane adduct (7) in high yield.
Notes and references
z X-Ray data: 2: C30H21BF15PSi, a = 9.5286(7) A, b = 9.8618(7) A,
c = 17.2444(13) A, a = 95.866(3)1, b = 99.447(3)1, g = 105.895(2)1,
3
ꢀ
V = 1518.74(19) A , space group: P1, total data: 7604, data (>2s):
5223, Rint: 0.0541, variables: 438, R (>2s) = 0.0675, Rw (>2s) =
0.1697, GOF = 1.077.
1 C. A. Russell, Angew. Chem., Int. Ed., 2009, 48, 4895–4897.
2 J. D. Masuda, W. W. Schoeller, B. Donnadieu and G. Bertrand,
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T. S. Cameron and R. McDonald, Inorg. Chem., 2003, 42,
4949–4954.
Mechanistically, the hydrogenation reaction is thought to
proceed via initial heterolytic cleavage of H2 by
P5Ph5–B(C6F5)3 to give [P5Ph5H][HB(C6F5)3]. The proposition
of such an intermediate is supported by our recent report of
the formation of [tBu2P(PHtBu2)][HB(C6F5)3] via treatment of
tBu4P2 and B(C6F5)3 with H2. Moreover, Burford and
co-workers have isolated a series of alkylated cationic species
of the general formula [P5Ph5R]+, the alkylated analogs to the
proposed intermediate. Subsequent attack on the transient
phosphino-phosphonium salt by the borohydride counterion
prompts extrusion of (1) with formal liberation of ‘‘P4Ph4’’.
This proposition is consistent with the previously observed
reaction of Me3P with [P4Cy4Me]+, which results in nucleophilic
attack on the cationic P, forming the new cation [Me3PPCyMe]+
and P3Cy3.12 As P4Ph4 is not observed spectroscopically, the
nature of the P-containing intermediates remains unclear;
nonetheless, the complete conversion of P5Ph5 to primary or
secondary phosphine–borane adducts, in this case (1), results
from subsequent reaction.
The hydrosilylations of P5Ph5 to give 2–7 are thought to
proceed via a mechanism similar to B(C6F5)3 catalyzed
hydrosilations of imines and carbonyl-compounds studied by
Piers and co-workers.41–43 In those cases, interaction of borane
with the Si–H bond is followed by attack of the Lewis base at
silicon. In the present case, attack of the polyphosphine at
silicon would generate transient intermediate salts of the
form [P5Ph5SiR3][HB(C6F5)3], which are subsequently
attacked by the borohydride anion [HB(C6F5)3], at the
15 N. Burford, P. J. Ragogna, R. McDonald and M. J. Ferguson,
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16 N. Burford, P. J. Ragogna, R. McDonald and M. J. Ferguson,
Chem. Commun., 2003, 2066–2067.
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18 C. A. Dyker and N. Burford, Chem. Asian J., 2008, 3, 28–36.
19 C. A. Dyker, N. Burford, G. Menard, M. D. Lumsden and
A. Decken, Inorg. Chem., 2007, 46, 4277–4285.
20 C. A. Dyker, N. Burford, M. D. Lumsden and A. Decken, J. Am.
Chem. Soc., 2006, 128, 9632–9633.
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21 C. A. Dyker, S. D. Riegel, N. Burford, M. D. Lumsden and
A. Decken, J. Am. Chem. Soc., 2007, 129, 7464–7474.
22 J. J. Weigand, N. Burford, A. Decken and A. Schulz, Eur. J. Inorg.
Chem., 2007, 4868–4872.
23 J. J. Weigand, S. D. Riegel, N. Burford and A. Decken, J. Am.
Chem. Soc., 2007, 129, 7969–7976.
(d, J = 4 Hz), 3.3 (d, J = 6 Hz), 7.3 (d, J = 3 Hz), 7.7 (d, J = 3 Hz),
129.7 (d, J = 10 Hz), 131.1 (d, J = 3 Hz), 133.4 (d, J = 7 Hz). (5):
yield: 147 mg (94%). Anal. calcd for C36H17BF15PSi: C, 53.75; H,
2.31%; found: C, 54.20; H, 2.81%. 1H NMR (C6D6): 5.46
2
3
(dd, JP–H = 27 Hz, JH–H = 5 Hz, 1H, SiH), 5.63 (1H, dd,
1JP–H = 363 Hz,3JH–H = 5 Hz, 1H, PH), 6.81 (td, J = 8 Hz, J = 2
Hz, 2H), 6.95 (m, 2H), 7.09 (t, J = 7 Hz, 2H),7.29 (m, 8H), 7.70
(dd, J = 8 Hz, J = 2 Hz, 2H), 7.75 (1H, dd, J = 8 Hz, J = 2 Hz,
1H), 7.82 (dd, J = 8 Hz, J = 2 Hz, 1H). 19F NMR (C6D6): ꢀ129.4
24 J. J. Weigand, M. Holthausen and R. Frohlich, Angew. Chem., Int.
Ed., 2009, 48, 295–298.
¨
25 S. J. Geier and D. W. Stephan, Chem. Commun., 2008, 99–101.
26 D. W. Stephan, Org. Biomol. Chem., 2008, 6, 1535–1539.
27 D. W. Stephan, Dalton Trans., 2009, 3129–3136.
28 S. J. Geier, M. A. Dureen, E. Y. Ouyang and D. Stephan,
Chem.–Eur. J., 2009, DOI: 10.1002/chem.200902369.
29 P. A. Chase, T. Jurca and D. W. Stephan, Chem. Commun., 2008,
1701–1703.
3
(d, JF–F = 21 Hz, o-C6F5), ꢀ156.4 (br s, p-C6F5), ꢀ163.3 (br s,
m-C6F5). 31P{1H} NMR (C6D6): ꢀ47.1 (br s); 11B NMR (C6D6):
ꢀ12.8 (br s); 13C{1H} NMR (CD2Cl2) partial: 128.9 (d, J = 10 Hz),
131.1 (d, J = 3 Hz), 133.8 (d, J = 7 Hz), 135.5 (d, J = 17 Hz),
135.5 (d, J = 17 Hz). (6): yield: 64 mg (96%). Anal. calcd for
C
31H15BF15PSi: C, 50.16; H, 2.04%; found: C, 50.18; H, 2.26%.
3
2
1H NMR (CDCl3): 0.50 (dd, JP–H = 7 Hz, JH–H = 4 Hz, 3H,
30 P. A. Chase, G. C. Welch, T. Jurca and D. W. Stephan,
Angew. Chem., Int. Ed., 2007, 46, 8050–8053.
CH3 major), 0.67 (dd, 3JP–H = 6 Hz, 2JH–H = 4 Hz, 3H, CH3 minor),
4.86 (d, JP–H = 29 Hz, 1H, Si–H), 5.03 (d, JP–H = 357 Hz, 1H,
2
1
31 V. Sumerin, F. Schulz, M. Nieger, M. Leskela, T. Repo and
B. Rieger, Angew. Chem., Int. Ed., 2008, 47, 6001–6003.
32 J.-M. Denis, H. Forintos, H. Szelke, L. Toupet, T.-N. Pham,
P.-J. Madec and A.-C. Gaumont, Chem. Commun., 2003, 54–55.
33 (1): yield: 110 mg (91%). Spectral data were identical to published
1
3
P–Hminor), 5.13 (d, JP–H = 363 Hz, P–Hmajor), 6.92 (dd, JP–H
=
11 Hz, J = 8 Hz, 1H, o-PC6H5), 7.07 (dd, 3JP–H = 11 Hz, J = 8 Hz,
1H), 7.12–7.48 (m, 8 H). 19F NMR (CDCl3): ꢀ130.0 (br s), ꢀ156.6
(t, J = 27 Hz, p-C6F5 minor), ꢀ156.7 (t, J = 27 Hz, p-C6F5 minor),
2
data.32 31P{1H} NMR ꢀ43.6 (br); D 5.09 (d, JP–D = 64 Hz). All
ꢀ163.5 (m, m-C6F5). 31P{1H} NMR (C6D6): ꢀ40.9minor, ꢀ41.6major
;
silylphosphine adducts were prepared in a similar fashion, thus
only one preparation is described: to a solution of P5Ph5 (22 mg,
0.041 mmol) in 4 mL of dichloromethane was added B(C6F5)3
(100 mg, 0.20 mmol). To this solution was added Et3SiH (50 mg,
0.43 mmol). The mixture was allowed to stir overnight whereupon
the solvent was removed. X-Ray quality crystals of (2) were grown
from hexanes at ꢀ35 1C. (2): yield: 142 mg (99%). Anal. calcd for
C30H21BF15PSi: C, 48.93; H, 2.87%; found: C, 49.07; H, 3.02%.
11B NMR (CDCl3): ꢀ13.1 (br s); 13C{1H} NMR (CDCl3) partial:
128.9 (d, J = 21 Hz), 129.1 (d, J = 10 Hz), 129.5 (d, J = 10 Hz),
131.3 (d, J = 3 Hz), 131.6 (d, J = 3 Hz), 131.7 (d, J = 1 Hz), 131.9
(d, J = 1 Hz), 135.0 (d, J = 10 Hz). (7): yield: 134 mg (96%). Anal.
calcd for C58H28BF15P2Si2: C, 48.56; H, 1.97%; found: C, 48.46; H,
2.18%. 1H NMR (CDCl3): 0.44 (d, J = 6 Hz, 6H), 0.50
1
(d, J = 6 Hz, 6H), 4.88 (2H, d, JP–H = 354 Hz, 2H, P–H), 6.87
(m, 4H), 7.12 (t, J = 7 Hz, 4H), 7.18 (d, J = 2 Hz, 4H), 7.30 (t, J =
7 Hz, 2H). 19F NMR (CDCl3): ꢀ130.0 (br s, o-C6F5), ꢀ156.7
1
1H NMR (C6D6): 0.54 (m, 15 H, CH2CH3), 4.72 (d, JP–H = 345
3
4
Hz, 1H, PH), 6.66 (td, JH–H = 7 Hz, JH–P = 2 Hz, 2H, o-CH),
6.77 (td, 3JH–H = 7 Hz, J = 2 Hz, 1H, p-CH), 6.85 (ddd, 3JH–H
3
3
(t, JF–F = 22 Hz, p-C6F5), ꢀ163.5 (t, JF–F = 21 Hz, m-C6F5).
31P{1H} NMR (CDCl3): ꢀ37.0 (br s); 11B NMR (CDCl3): ꢀ15.3
(br s); 13C{1H} NMR (CDCl3) partial: 0.1 (d, J = 11 Hz), 0.7 (d,
J = 10 Hz), 0.8 (d, J = 10 Hz), 125.1 (d, J = 5 Hz), 124.6 (d, J =
5 Hz), 132.4 (d, J = 10 Hz), 134.5 (d, J = 3 Hz), 136.9, 137.1 (d, J =
6 Hz), 140.3 (dm, J = 247 Hz, CF), 151.3 (dm, J = 244 Hz, CF).
34 G. C. Welch, L. Cabrera, P. A. Chase, E. Hollink, J. D. Masuda,
P. Wei and D. W. Stephan, Dalton Trans., 2007, 3407–3414.
35 G. C. Welch, R. Prieto, M. A. Dureen, A. J. Lough,
O. A. Labeodan, T. Holtrichter-Rossmann and D. W. Stephan,
Dalton Trans., 2009, 1559–1570.
36 R. T. Boere and J. D. Masuda, Can. J. Chem., 2002, 80, 1607–1617.
37 M. Okazaki, K. A. Jung and H. Tobita, Organometallics, 2005, 24,
659–664.
38 M. A. Petrie and P. P. Power, J. Chem. Soc., Dalton Trans., 1993,
1737–1745.
39 O. Tardif, Z. M. Hou, M. Nishiura, T. Koizumi and Y. Wakatsuki,
Organometallics, 2001, 20, 4565–4573.
40 O. Tardif, M. Nishiura and Z. M. Hou, Tetrahedron, 2003, 59,
10525–10539.
41 J. M. Blackwell, D. J. Morrison and W. E. Piers, Tetrahedron,
2002, 58, 8247–8254.
42 J. M. Blackwell, E. R. Sonmor, T. Scoccitti and W. E. Piers, Org.
Lett., 2000, 2, 3921–3923.
=
10 Hz, JH–H = 7 Hz, JP–H = 2 Hz, 2H, m-CH). 19F NMR
(C6D6): ꢀ129.8 (br s, o-C6F5), ꢀ156.5 (br s, p-C6F5), ꢀ163.7
(br s, m-C6F5). 31P{1H} NMR (C6D6): ꢀ46.6 (br s, n1/2 B400 Hz);
11B NMR (C6D6): ꢀ12.3 (br s); 13C{1H} NMR (C6D6) partial: 4.4
(d, J = 8 Hz), 6.8 (d, J = 3 Hz), 128.9 (d, J = 9 Hz), 130.7 (d, J =
3 Hz), 133.6 (d, J = 7 Hz). (3): yield: 164 mg (95%). Anal. calcd for
C42H21BF15PSi: C, 57.29; H, 2.40%; found: C, 57.34; H, 2.57%.
1H NMR (C6D6): 5.59 (d, 1JP–H = 347 Hz, P–H), 6.49 (td, J = 8 Hz,
J = 2 Hz, 2H, o-CH), 6.66 (td, J = 7 Hz, J = 2 Hz, 1H, p-CH),
3
4
3
3
4
6.78 (ddd, JH–H = 10 Hz, JH–H = 8 Hz, JP–H = 2 Hz, 2H,
m-CH), 6.94 (t, J = 7 Hz, 6H), 7.05 (t, J = 7 Hz, 3H, p-CH), 7.40
(dd, J = 8 Hz, J = 1 Hz, 6H); 19F NMR (C6D6): ꢀ129.2 (br s,
o-C6F5), ꢀ155.1 (br s, p-C6F5), ꢀ163.0 (br s, m-C6F5). 31P{1H}
NMR (C6D6): ꢀ45.3 (br s); 11B NMR (C6D6): ꢀ11.7 (br s);
13C{1H} NMR (C6D6) partial: 122.2 (d, J = 40 Hz), 128.4, 130.6
(d, J = 3 Hz), 131.3, 134.6 (d, J = 6 Hz), 136.4 (d, J = 1 Hz). (4):
yield: 57 mg (82%). Anal. calcd for C28H17BF15PSi: C, 47.48; H,
2.42%; found: C, 47.22; H, 2.90%. 1H NMR (C6D6): 0.30 (m, 4H,
CH2CH3), 0.51 (t, 3JH–H = 8 Hz, 6H, CH2CH3), 3.83 (d, 2JP–H
=
29 Hz, 1H, Si–H), 4.73 (d, JP–H = 357 Hz, 1H, P–H), 6.55
1
3
4
(td, JH–H = 8 Hz, JP–H = 2 Hz, 2H, m-C6H5), 6.69 (m, 3H,
o-C6H5, p-C6H5). 19F NMR (C6D6) d: ꢀ129.8 (br s, o-C6F5),
3
3
ꢀ156.2 (t, JF–F = 21 Hz, p-C6F5), ꢀ163.5 (td, JF–F = 21 Hz,
4JF–F = 5 Hz, m-C6F5). 31P{1H} NMR (C6D6): ꢀ53.7 (br s); 11B
NMR (C6D6): ꢀ12.8 (br s); 13C{1H} NMR (C6D6) partial: 2.5
43 D. J. Parks and W. E. Piers, J. Am. Chem. Soc., 1996, 118,
9440–9441.
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