P(III)-cyclic oligomers via catalytic hydrophosphination

Article abstract of DOI:10.1039/b817960c

A bulky secondary phosphine with an alkynyl substituent was prepared and shown to undergo catalytic hydrophosphination to give cyclic oligomers. The Royal Society of Chemistry.

Full text of DOI:10.1039/b817960c

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P(III)-cyclic oligomers via catalytic hydrophosphinationw
Sharonna Greenberg, Gregory L. Gibson and Douglas W. Stephan*
Received (in Cambridge, UK) 13th October 2008, Accepted 7th November 2008
First published as an Advance Article on the web 24th November 2008
DOI: 10.1039/b817960c
A bulky secondary phosphine with an alkynyl substituent was
prepared and shown to undergo catalytic hydrophosphination to
give cyclic oligomers.
the synthesis of P(III)-based oligomers and polymers via this
method.
In envisioning P(III)-based polymers derived from hydrophos-
phination, the possibility of a step-growth process would neces-
sitate a precise stoichiometry of the two functional groups.
Thus, a monomer containing both PH and alkyne functionalities
was targeted. To this end, the Pd(II)/Cu-catalyzed coupling of
Br(CHMe2)2I(C6H2) and HCCPh was used to prepare
Br(CHMe2)2(C6H2)(CCPh) (1). This species was converted to a
Li reagent, and reacted with ClP(NEt2)2 in the presence of CuCl
to generate [Cu(m-Br)(NEt2)2P(CHMe2)2(C6H2)(CCPh)]2 (2).
Subsequent conversion using HCl(g) to the corresponding ArPCl2
species 3 and reduction with LiAlH4 afforded the primary
phosphine H2P(C6H2)(CHMe2)2(CCPh) (4). Lithiation and
alkylation with BrCH2CH(CH3)2 gave the secondary phosphine
(Me2CHCH2)PH(C6H2)(CHMe2)2(CCPh) (5) (Scheme 1).z Each
of these compounds was characterized by multinuclear NMR
and IR spectroscopy, mass spectrometry, and elemental analysis.
In the case of 4, X-ray crystallography (Fig. 1) showed
refined P–H bond lengths of 1.23(5) and 1.33(5) A, typical for
primary phosphines.y36–38 The remaining metric parameters were
unexceptional but served to confirm the formulation.
Macromolecules that incorporate phosphorus atoms have
attracted attention as a result of their flame retardant proper-
ties, thermal and oxidative stability, and potential uses as
catalyst supports and p-conjugated materials.1,2 While P(V)-
containing polymers, particularly polyphosphazenes and poly-
heterophosphazenes,3 are well developed and commercialized,
synthetic routes to P(III)-containing polymers are less deve-
loped. Manners and co-workers have developed thermal4 and
living anionic5 polymerization routes to polyferrocenyl-
phosphines, as well as a Rh-catalyzed dehydrocoupling of
primary phosphine–borane adducts to give polymeric pro-
ducts.6–8 Polymers that contain only P(III) and carbon in the
main chain have been synthesized by the anionic ring-opening
polymerization of strained cyclic phosphirenes,9 while metal-
catalyzed dehydrocoupling reactions of P–H bonds have
yielded linear and cyclic oligomers.10,11 Gates and co-workers
have prepared poly(methylenephosphine)s via the addition
polymerization of phosphaalkenes under thermal,12 radical12,13
and living anionic14,15 polymerization conditions. Oligo-
meric or polymeric p-conjugated poly(p-phenylenephospha-
alkene)s16–18 have been developed via the condensation of
bifunctional phosphines with acyl chlorides or aldehydes.
Related conjugated poly(arylphosphine)s have been prepared
by Jin and Lucht19 by employing Pd- or Ni-catalyzed cross-
coupling reactions of primary phosphines and dihaloarenes.
Catalytic hydrophosphination is a powerful tool for the
synthesis of new phosphorus-containing compounds.20,21 This
atom-economical methodology involves the addition of a P–H
bond to a CQX (X = C, N, O) or CRC multiple bond, and
can be catalyzed by late transition metals,21–31 lanthanides32
and heavier group II elements.33 Radical initiators34 or bases,
including potassium t-butoxide34 and n-butyllithium,31 can
also serve as catalysts. Additionally, the reaction can be
effected by thermolysis or microwave irradiation.28,35 Herein,
we describe the synthesis of bifunctional monomers containing
PH and alkyne units, and their subsequent catalytic hydrophos-
phination/polymerization. While bisphosphoroyl addition to a
bisalkyne has been reported to give P(V)-containing polymers,
this report is, to the best of our knowledge, the first to describe
The polymerization of 5 was achieved by treatment with
0.2 equiv. of n-BuLi in THF at ambient temperature overnight
(Scheme 2). While preliminary preparations were undertaken
on an NMR scale, the synthesis was also scaled up using the
same ratio of monomer : initiator, and product 6 was isolated
Department of Chemistry, University of Toronto, 80 St George Street,
Toronto, Ontario, Canada M5S 3H6.
E-mail: dstephan@chem.utoronto.ca
w Electronic supplementary information (ESI) available: Experimental
and spectroscopic details, and crystal data for compound 4. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b817960c
Scheme 1 The synthetic route to 1–5.
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Fig. 1 A POV-ray drawing of 4.
Fig. 2 The MALDI-TOF mass spectrum of 6 (note: only a portion of
the vertical scale is shown).
permeation chromatography (GPC) using polystyrene
standards indicated a Mw of 9200 and Mn of 3600, corres-
ponding to a number-average degree of polymerization of ten
repeat units. Gates and co-workers have previously demon-
strated that the molecular weights of some phosphorus-
containing oligomers may be underestimated when using
polystyrene standards.12 For polymer 6, light scattering GPC
revealed significantly higher molecular weights (Mw = 25 000,
Mn = 21 000). Collectively, these data are consistent with
the formulation of 6 as a mixture of cyclic oligomers with
8–70 repeat units.
Sulfurization of polymer 6 was achieved by treating a THF
solution of 6 with elemental sulfur (Scheme 2).8 The resulting
product, 7, gave rise to a broad 31P NMR resonance at d 46.2.
The corresponding MALDI-TOF mass spectrum showed
five repeat units spaced by 382 m/z units, consistent with
repeated loss of the sulfurized monomers. GPC vs. polystyrene
standards revealed Mw = 9600 and Mn = 3000, corres-
ponding to a number-average degree of polymerization of
eight repeat units. This indicates that there is minimal chain
degradation upon treatment with sulfur.
Scheme 2 The synthesis and sulfurization of one regio/stereoisomer
of polymer 6.
In conclusion, we have demonstrated that hydrophosphina-
tion employing a bifunctional phosphine is a viable method for
the preparation of P(III)-containing oligomers/polymers.
Studies exploring the impact of other catalysts on the nature
of the polymeric products, as well as the chemistry of these
phosphorus-containing materials, are ongoing.
by the repetitive precipitation from THF into hexanes to give a
brown residue.z 31P NMR spectroscopy of the reaction
mixture was consistent with complete consumption of the
monomer, as evidenced by the disappearance of the monomer
peak at d ꢁ99. Similarly, the CRC stretch at 2320 cmꢁ1 in the
IR spectrum of 5 was notably absent in the spectrum of 6. In
addition, in the 31P NMR spectrum, the emergence of a broad
signal at d ꢁ20 was consistent with the formation of a new
oligomeric or polymeric product, 6. Moreover, the 31P NMR
spectrum showed no evidence of resonances corresponding to
phosphorus-containing end groups, suggesting the formation
of cyclic products. In the 1H NMR spectrum of 6, the
resonances attributable to the alkene fragment were observed
to be very broad, presumably reflecting variations in the
regiochemistry of addition and stereoisomerism at the
phosphorus atom.
Financial support from the NSERC of Canada is gratefully
acknowledged. S. G. is grateful for the award of a Canada
Graduate scholarship. We also thank Prof. Mark Nitz and Dr
Richard Jagt for their help with MALDI-TOF mass spectro-
metry, and Prof. Derek P. Gates and Dr Kevin J. T. Noonan
for their assistance with GPC measurements.
Notes and references
z Synthesis of 5. Compound 4 (190 mg, 0.645 mmol) and hexanes
(6 mL) were placed in a 20 mL scintillation vial inside a brass plate
designed to surround the bottom and walls of the vial, and the entire
assembly was cooled to ꢁ35 1C. Freshly titrated n-BuLi in hexanes
(0.41 mL, 1.578 mol Lꢁ1, 0.647 mmol, 1.00 equiv.) was added to the
stirred solution to generate an orange opaque mixture and the entire
assembly warmed to room temperature over 4 h. BrCH2CH(CH3)2
(88 mg, 0.64 mmol, 1.0 equiv.) was then added dropwise and the
reaction mixture stirred overnight. The orange-brown mixture was
filtered through Celite and all volatiles were removed in vacuo to give a
The matrix-assisted laser desorption/ionization-time of
flight (MALDI-TOF) mass spectrum (Fig. 2) of product 6
exhibited a pattern of repeat peaks up to 2800 m/z units,
spaced by 350 m/z units, corresponding to a monomer
fragment. These data support the formulation of 6 as
an oligomeric species with up to eight repeat units. Gel
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1
brown oil; yield: 179 mg (79%). H NMR (C6D6): d 7.62–7.57 (m, 4H,
o-C6H5 and C6H2), 7.03–6.97 (m, 3H, m- and p-C6H5), 4.36 (ddd, 1H,
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1
3
3
PH, JP–H = 212 Hz, JH–H = 9 Hz, JH–H = 6 Hz), 3.78–3.67
(m, 2H, ArCHMe2), 1.86–1.76 (m, 1H, PCHaHb), 1.74–1.64
(m, 1H, PCH2CHMe2), 1.49–1.37 (m, 1H, PCHaHb), 1.20 (d, 6H,
3
ArCH(CH3)a(CH3)b, JH–H = 7 Hz), 1.13 (d, 6H, ArCH(CH3)a(CH3)b,
3JH–H = 7 Hz), 0.97 (d, 3H, PCH2CH(CH3)a(CH3)b, JH–H = 4 Hz),
3
0.94 (d, 3H, PCH2CH(CH3)a(CH3)b, 3JH–H = 4 Hz). 31P NMR (C6D6):
d ꢁ99.0 (d, 1JP–H = 212 Hz). 13C{1H} NMR (C6D6): d 153.5 (d, ipso-C,
1JP–C = 11 Hz), 133.9 (s, Ar), 133.6 (s, Ar), 132.0 (s, Ar), 128.7
(s, Ar), 126.9 (s, Ar), 124.6 (s, Ar), 124.1 (s, Ar), 90.9 (s, CRC), 90.4
1
(s, CRC), 34.2 (d, PCH2CHMe2, JP–C = 13 Hz), 33.0 (d, ArCHMe2,
3JP–C = 13 Hz), 28.4 (d, PCH2CHMe2, JP–C = 12 Hz), 24.7
2
(s, ArCH(CH3)a(CH3)b), 24.3 (s, ArCH(CH3)a(CH3)b), 23.94
(s, PCH2CH(CH3)a(CH3)b), 23.86 (s, PCH2CH(CH3)a(CH3)b). EI-MS:
m/z = 350.2 (37%) [M]+, 293.1 (100%) [M ꢁ CH2CHMe2]+. HRMS:
calc. for C24H31P: 350.2163, found: 350.2164. FT-IR (nujol): n(CRC) =
2320 cmꢁ1 (br).
y X-Ray data for 4. Space group: monoclinic, P21/c, a = 12.8438(18),
b = 10.7033(15), c = 13.4456(19) A, b = 103.084(2)1, V = 1800.4(4) A3,
Z = 4. Data: 3179, variables: 198, R = 0.0523, Rw = 0.1388,
GOF = 1.032, CCDC 705289.w
z Synthesis of 6. Compound 5 (0.999 g, 2.85 mmol) and THF (3 mL)
were placed in a 20 mL scintillation vial, to which freshly titrated
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n-BuLi in hexanes (0.365 mL, 1.578 mol Lꢁ1
,
0.576 mmol,
0.200 equiv.) was added with stirring. The resultant dark brown
mixture was stirred overnight and then precipitated into a vortex of
pentane. The brown supernatant was decanted to give a dark brown
gummy residue, which was then dissolved in THF (2 mL) and
re-precipitated into hexanes. This step was repeated (three or four
precipitations in total) and the residue was then dried in vacuo to give a
dark brown solid; yield: 0.380 g (38%). 1H NMR (THF-d8): d 7.7–5.9
(br, 7H, ArH), 4.1–3.6 (br, 2H, ArCH(CH3)2), 1.6–0.5 (br, 21H,
PCH2CH(CH3)2 and ArCH(CH3)2). 31P{1H} NMR (C6D6): d ꢁ20
(br). 13C{1H} NMR (THF-d8, partial): d 156 (br, Ar), 148 (br, Ar), 143
(br, Ar), 130 (br, Ar), 129 (br, Ar), 126 (br, Ar), 37 (br, alkyl), 35.6
(s, alkyl), 33 (br, alkyl), 32.7 (s, alkyl), 30.1 (s, alkyl), 29 (br, alkyl), 25
(br, alkyl), 24.6 (s, alkyl), 19.1 (s, alkyl), 14.6 (s, alkyl), 10.4 (s, alkyl).
FT-IR (25 1C, deposited from a C6D6 solution): extremely weak peak
at 2280 cmꢁ1. GPC (triple detection): Mn = 3600 g molꢁ1, Mw
9200 g molꢁ1. GPC (RI): Mn = 3300 g molꢁ1, Mw = 13 800 g molꢁ1
GPC (LS): Mn = 21 000 g molꢁ1, Mw = 25 000 g molꢁ1
=
.
.
8 Synthesis of 7. Compound 6 (50 mg, 0.14 mmol) and THF (4 mL)
were placed in a 20 mL scintillation vial, to which elemental sulfur
(6 mg, 0.19 mmol) was added. The reaction was stirred overnight at
room temperature. The brown solution was precipitated into a vortex
of hexanes, and the resultant beige solid was isolated from the super-
natant by decanting and dried in vacuo; yield: 45 mg (82%). 1H NMR
(THF-d8): d 7.9–6.5 (br, 7H, ArH), 4.3–4.0 (br, 2H, ArCH(CH3)2),
2.3–0.3 (br, 21H, PCH2CH(CH3)2 and ArCH(CH3)2). 31P{1H} NMR
(C6D6): d 46.2 (br). 13C{1H} NMR (THF-d8, partial): d 157 (br, Ar),
139 (br, Ar), 133 (br, Ar), 131 (br, Ar), 129 (br, Ar), 128 (br, Ar), 36
(s, alkyl), 33 (s, alkyl), 31 (br, alkyl), 30 (s, alkyl), 28 (s, alkyl), 24
(s, alkyl), 21 (s, alkyl), 14 (s, alkyl), 12 (s, alkyl). FT-IR
(25 1C, deposited from a C6D6 solution): extremely weak peak at
2279 cmꢁ1. GPC (refractive index detection vs.polystyrene standards):
Mn 3000 g molꢁ1, Mw 9600 g molꢁ1
.
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