B(C6F5)3-catalyzed formation of B-P bonds by dehydrocoupling of phosphine-boranes

Article abstract of DOI:10.1039/b206559b

Tris(pentafluorophenyl)borane was used as a new catalyst in the formation of P-B bonds by dehydrocoupling of phosphine-boranes.

Full text of DOI:10.1039/b206559b

B(C6F5)3-catalyzed formation of B–P bonds by dehydrocoupling of
phosphine–boranes†
Jean-Marc Denis,*a Henrietta Forintos,a Helga Szelke,a Loic Toupet,c Thi-Nhàn Pham,b Pierre-Jean
Madecb and Annie-Claude Gaumont*b
a Université de Rennes I, CNRS-UMR 6510, Campus de Beaulieu, F35042 Rennes, France.
E-mail: jean-marc.denis@univ-rennes1.fr
b Université de Caen-ISMRA, LCLT, CNRS-UMR 6507, 6 Bd. du Mal Juin, F14050 Caen, France
c
Université de Rennes, CNRS-UMR 6625, Campus de Beaulieu, F35042 Rennes, France
Received (in Cambridge, UK) 9th July 2002, Accepted 7th November 2002
First published as an Advance Article on the web 25th November 2002
Tris(pentafluorophenyl)borane was used as a new catalyst in
the formation of P–B bonds by dehydrocoupling of phos-
phine–boranes.
with the formation of poly(phenylphosphino)boranes 3 recently
described in the literature.2 The poorly resolved 31P signals
from d 252 to 256,2 in the PH region suggested the presence of
low molecular-weight oligomeric or cyclic structures. The
coexistence of two main fractions corresponding roughly to Mw
= 3900 and 830 were observed by size exclusion chromatog-
raphy measurements (according to polystyrene standards) with
a polydispersity index of 2.3 and 1.9, respectively. Differential
scanning calorimetry confimed these results and indicated a
crystalline structure for 3 (Tm = 215 and 194 °C, respectively).
Poly(phenylphosphinoboranes) 3 are air- and moisture-stable in
the solid state.
Catalytic heterodehydrocoupling of covalent hydrides repre-
sents an interesting route to the formation of element–element
bonds. In this way, formation of Si–N and Si–C bonds has been
achieved some years ago using group 4 metallocenes as
catalysts.1 The Manners group has recently developed a new
route to B–P2 and B–N3 bond formation involving catalytic
dehydrocoupling of the corresponding phosphine– and amine–
boranes with Rh( ) as catalyst. In this preliminary communica-
I
tion we report two selected examples of the use of tris(penta-
fluorophenyl)borane,4‡ B(C6F5)3 (BX3) as a catalyst for the
formation of B–P bonds under mild conditions by dehydrocou-
pling of phosphine–borane adducts. Our results will be
compared with those obtained with the transition-metal cata-
lyzed reactions.2 A mechanism will be proposed.
The main problem encountered in extending the dehydrocou-
pling to the complex H3B·PH3 2 was due the weakness of the P–
B bond and its fast dissociation at temperatures as low as 230
°C under atmospheric pressure.5 We used for this reaction
Dehydrogenative coupling of H3B·PPhH2 1 has recently been
performed by refluxing a toluene solution overnight in the
presence of 0.5 mol% Rh(
) as catalyst.2 High molecular weight
I
6
safety equipment which allowed to bubble at 250 °C PH3(g)
7
poly(phosphinoboranes) [PhPH–BH2]n were thus obtained
(Table 1, entry 1). We observed the dehydro-condensation of 1
at 20 °C in a toluene solution containing 0.5 mol% of B(C6F5)3§
(entry 2).¶ Evolution of gas could be observed during the first 2
h. The reaction was completed after 3 days (yield 63% in
isolated products). 31P- and 11B-NMR displayed two separated
groups of signals ( ≈ 1+1 ratio). The 31P NMR (toluene)
presented a broad signal at d 248.9 (d, JPH ≈ 348 Hz) and
poorly resolved peaks from d 252 to 256, respectively. The
corresponding 11B NMR spectra revealed a braod peak at d
235.5 and a shoulder at d 233.2. All these data are
characteristic of four-coordinated boron centres attached to two
phosphorus.2 Similar results were obtained by performing the
reaction at 90 °C for 3 h (entry 2). The 31P and 11B values
observed at d 248.9 and 235.8, respectively, are consistent
and B2H6(g) into a CH2Cl2 solution of B(C6F5)3 ( ≈ 5 mol%
with regard to BH3) and thereafter to heat the solution to the
desired temperature. Oligomerisation started around +20 °C and
was completed by heating at 70 °C overnight (entry 3). The
oligomeric structure H3P(BH2PH2)nBH3 was proposed from the
31P and 11B spectra [disappearance of the peak at dP 2119
corresponding to the complex 2 and formation of three broad
peaks at d 2104 (t, JPH 362 Hz), 2109 (t, JPH 342) (PH2 groups)
and a small peak at d 2115 (q, JPH 356) (terminal PH3 group)].
The 11B NMR showed complex resonances (main peaks at d
232 and 235 in good agreement with such a structure).2 The
material obtained by heating the solution at 90 °C for one day
(entry 4) was assigned to 4 on the basis of the 31P NMR (very
broad peak from d 295 to 2120) and 11B NMR (very broad
peak centred at d 232). The white solid material thus obtained
after evacuation of the solvent was very sensitive to air and
moisture. Very fast oxidation prevented reliable elemental and
HRMS analyses.
† Electronic supplementary information (ESI) available: experimental
section. See http://www.rsc.org/suppdata/cc/b2/b206559b/
Table 1 B(C6F5)3-catalysed formation of B–P bonds by dehydrocoupling of the phosphine–boranes 1 and 2
Starting
material
Catalyst
(reagent)
Conv.a
(%)
31P NMR
d (JPH
11B NMR
d
Entry
Conditions
Product(s)
)
1
2
3
1b
1
RhI
110 °C, 15 h
(toluene)
100
100
100
3
3
248.9 (360)
234.7
(0.5 mol%)
(C6F5)3B
(0.5 mol%)
(C6F5)3B
(5 mol%)
20 °C, 3 days (toluene)
or 90 °C, 3 h (toluene)
Bubbling PH3 and BH3 into
CH2Cl2/(C6F5)3B
then 70 °C overnight
Bubbling PH3 and BH3 into
CH2Cl2/(C6F5)3B
248.9 (348)
252 to 256
2104 (t, 362)
2109 (t, 342)
2115 (q, 356)
Broad peak
235.8
233c
2
H3P(BH2PH2)nBH3
oligomers
Main peaks at
232 and 235
4
2
(C6F5)3B
(5 mol%)
100
[PH2BH2]n 4
Broad peak
centred at
232
centred at 2107
then 90 °C for 24 h
a Progress of the reaction was monitored by 31P NMR. b See ref. 2. c Shoulder.
54
CHEM. COMMUN., 2003, 54–55
This journal is © The Royal Society of Chemistry 2003

These results prove the efficiency of B(C6F5)3 as a new
catalyst for the preparation of poly(phosphinoboranes) by
dehydrocoupling of phosphine–boranes (Table 1). In order to
get a better understanding of the mechanism, a stoechiometric
experiment between 5∑ ** and 1 was performed. Attempts to
isolate the primary product 7 resulting from dehydrocoupling
reaction were unsuccessful, even at 210 °C. The poly-
(phosphinoborane) 3 was in all the cases the main observed
product.
B(C6F5)3 as catalyst. The ease of the dehydrocoupling of 1 and
2 is attributed to the strong acidic character of the P–H bond of
complexes 5 and 6. The polymerisation presumably followed a
process involving iterative dehydrocoupling reactions and BX3/
BH3 exchanges. This work addresses mechanistic questions.
Extension of this dehydrocoupling route to other element–
element bond formation is in progress.
Notes and references
‡ B(C6F5)3 is a Lewis acid of comparable strength to BF3. Application of
this water-tolerant reagent as a catalyst in organic synthesis is rapidly
growing.4
§ The use of anhydrous grade B(C6F5)3 was critical. For each experiment,
the product was sublimed by plunging the flask maintained under vacuum
(0.02 mbar) into an oil-bath previously heated at 105 °C. All the
manipulations should be carried out under neutral gas in dry solvents and
reagents.
¶ Typical experiment: 2.5 3 1024 mol of the borane complex PhPH2·BH3 in
toluene (400 mL) was slowly added into a toluene solution (100 mL) of the
freshly sublimated BX3 (6.0 3 1024 g; 1.2 3 1026 mol, 0.5 mol%) and the
solution was maintained at the considered temperature. Progress of the
reaction was monitored by 31P NMR. Traces of free phosphine or phosphine
oxide were sometimes detected by 31P.
We supppose from this experiment that complex 5 (Fig. 1)††
formed by ligand exchange was probably the reactive inter-
mediate in the catalytic dehydrocoupling. The acceptor strength
of Lewis-acidic perfluororinated triarylborane compounds is
well established.8,9 This effect should contribute to activate the
P–H bond by withdrawing electron density from the phospho-
rus, making dehydrocoupling easier. Polymerisation presum-
ably followed a process involving iterative dehydrocoupling
reactions and BX3/BH3 exchanges.
∑ An authentic sample of 5 was easily prepared and fully characterised by
11B and 1H NMR, HRMS, and single crystal X-ray diffraction.
** An authentic sample of 10 was easily prepared and characterised by
NMR and X-ray diffraction. CCDC 189752. (ESI†.
†† Crystal data for 5: C24H7BF15P, M = 622.8, T = 293(2) K, l =
¯
0.71069, triclinic, space group P1, unit cell dimensions: a = 8.070(5), b =
11.337(9), c = 12.992(9) Å, a = 86.66(9) b = 77.22(7), g = 87.560(8)°,
V = 1156.7(14) Å3, Z = 2, Dc = 1.786 g cm23, m = 0.254 mm21, F(000)
= 612, crystal size: 0.32 3 0.24 3 0.16 mm, q Range for data collection
1.61–24.97°, index range, 0 < h < 9, 213 < k < 13, 215 < 1 < 15,
reflections collected: 4389, independent reflections: 4070 [Rint = 0.0156],
reflections observed ( > 2s): 2495, refinement method, full-matrix least-
squares on F2, data/restraints/parameters, 4070/0/377, goodness-on-fit on
F2 = 1.009, final R indices [(I > 2s(I)]: R1 = 0.0422, wR2 = 0.0788, R
indices (all data): R = 0.0989, wR = 0.0918, largest diff. peak and hole,
0.196 and 20.209 e Å23. CCDC 189751.
Fig. 1 Molecular structure of 5. Selected bond length (Å): P(1)–B (1)
2.039.
Crystal data for 9: C20H10B2F15PS, M = 619.93, T = 293(2) K, l =
Poly(phosphinoboranes) 3, 4 were also formed by another
route involving in the first step the formation of the complexes
8, 9 (Fig. 2)†† respectively and slow decomposition of these
intermediates (20 °C for 8 and 110 °C, 3 h for 9) To explain the
formation of the complex BX3–SMe2 10, a transient three-
coordinate complex R(H)P–BH2 was suggested as inter-
mediate.10
¯
0.71069, triclinic, space group P1, unit cell dimensions: a = 9.6926(4), b =
10.6562(5), c = 12.3789(7) Å, a = 64.353(2) b = 86.081(2), g =
84.137(3)°, V = 1146.16(10) Å3, Z = 2, Dc = 1.796 g cm23, m = 0.343
mm21, F(000) = 612, crystal size: 0.12 3 0.10 3 0.03 mm, q Range for
data collection 1.83–27.61°, index range, 0 < h < 12, 2 13 < k < 13, 215
< 1 < 16, reflections collected: 5270, reflections observed: 5270, goodness
of fit: 1.029, final R indices: R = 0.0478, wR = 0.1320. CCDC 189752.
See http://www.rsc.org/suppdata/cc/b2/b206559b/ for crystallographic
data in CIF or other electronic format.
In conclusion, we have presented a new and efficient route to
poly(phosphinoboranes) by using the strong Lewis acid
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