A study of the reactivity of secondary phosphanes with radical sources: A new dehydrocoupling reaction

Article abstract of DOI:10.1002/hlca.201000114

The reactions of secondary phosphanes with radical sources have been investigated.Astoichiometric dehydrocoupling of Ph₂PH with 1,1′-azobis[cyclohexane-1-carbonitrile] (VAZO 88) affords tetraphenyldiphosphane in good yields, whilst reduction of the nitrosyl function was observed upon using 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO). Dialkylphosphane - borane adducts also undergo a dehydrocoupling reaction in the presence of VAZO 88 to form R₄P₂.

Full text of DOI:10.1002/hlca.201000114

Helvetica Chimica Acta – Vol. 93 (2010)
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A Study of the Reactivity of Secondary Phosphanes with Radical Sources:
A New Dehydrocoupling Reaction
by Robert J. Baker* and Emtithal Hashem
School of Chemistry, University of Dublin, Trinity College, IE-Dublin 2
(phone: þ 353-1-8963501; fax: þ 353-1-6712826; e-mail: bakerrj@tcd.ie)
The reactions of secondary phosphanes with radical sources have been investigated. A stoichiometric
dehydrocoupling of Ph2PH with 1,1’-azobis[cyclohexane-1-carbonitrile] (VAZOꢀ 88) affords tetraphe-
nyldiphosphane in good yields, whilst reduction of the nitrosyl function was observed upon using 2,2,6,6-
tetramethylpiperidin-1-oxyl (TEMPO). Dialkylphosphane – borane adducts also undergo a dehydro-
coupling reaction in the presence of VAZOꢀ 88 to form R4P2.
Introduction. – The syntheses of molecules featuring PꢀP bonds are effectively
accomplished via stoichiometric and catalytic processes [1]. Of the former, the two
main synthetic methodologies are either via a Wꢀrtz type reduction of the phosphane
halides or via a dehydrohalogenation of R2PX and R2PH. The stoichiometric reduction
using alkali metals is commonly used, although PꢀC bond cleavage of aryl phosphanes
has been observed [2]. The catalytic dehydrocoupling reaction to form R2PꢀPR2,
RHPꢀPHR, or cyclo-(PR)n (n ¼ 5, 6) is gaining in prominence using a Ti- [3], Zr- [4],
and Rh-based [5] catalysis, whilst Manners and co-workers have extensively published
[6] on the use of Rh catalysis for the hetero-dehydrocoupling of phosphane – borane
adducts to form PꢀB containing rings, chains, and polymers from phenyl [7], tBu [8],
and fluorous phosphanes [9], along with mechanistic studies [10]. The use of B(C6F5)3
as a catalyst has also been reported to dehydrocouple H3P · BH3 and PhPH2 · BH3 with
low catalyst loadings [11].
We have been investigating new methods towards the preparation of phosphane-
substituted tetramethylcyclopentadienes as precursors of unusual actinide complexes.
Whilst a number of synthetic methodologies have been reported, there are difficulties
concerning low yields or long reaction times with all [12]. One method we tried was via
the radical initiated hydrophosphination [13][14] of the pendent C¼C bond in 1-allyl-
2,3,4,5-tetramethylcyclopentadiene, and surprisingly none of the expected tertiary
phosphane has been observed. Herein, we report on the unusual reaction chemistry of
dialkyl- and diarylphosphanes, and their borane adducts with radical sources.
Experimental. – All manipulations were carried out using standard Schlenk and glove-box
techniques under an atmosphere of high-purity N2. Toluene, hexane, and THF were distilled over K,
whilst Et2O was distilled over Na/K. The secondary phosphane – borane adducts were prepared
according to the procedure described in [15], whilst all other reagents were obtained from commercial
sources and used as received. IR Spectra: Perkin Elmer Spectrum One spectrometer with attenuated total
reflectance (ATR) accessory. 1H-, 11B-, 13C{1H}-, and 31P-NMR spectra: Bruker AV400 spectrometer
ꢁ 2010 Verlag Helvetica Chimica Acta AG, Zꢂrich

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Helvetica Chimica Acta – Vol. 93 (2010)
operating at 400.13 (1H), 128.40 (11B), 100.65 (13C), and 162.01 MHz (31P), resp., and were referenced to
the residual 1H or 13C resonances of the solvent used, or external BF3(OEt2) (11B) or H3PO4 (31P).
Reaction of Ph2PH with VAZOꢀ 88. To a soln. of Ph2PH (0.20 g, 1.07 mmol) in toluene (5 cm3) was
added 1,1’-azobis[cyclohexane-1-carbonitrile] (VAZOꢀ 88; 0.52 g, 2.17 mmol), and the mixture was
heated to 1108 for 36 h. The solvent was removed in vacuo, and the residue was extracted with hexane
(3 ꢁ 10 cm3). Placement at ꢀ 308 overnight yielded a white microcrystalline solid (0.34 g, 88%).
Spectroscopic data are consistent with literature data [16].
Reaction of Ph2PH with TEMPO. To a soln. of Ph2PH (0.20 g, 1.07 mmol) in toluene (5 cm3) was
added TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl; 0.33 g, 2.15 mmol), and the mixture was heated to
1108 for 36 h. The solvent was removed in vacuo, and the residue was extracted with CH2Cl2 (10 cm3).
Addition of hexane precipitated a white solid, which was recrystallized from MeOH to give white needles
(0.07 g, 37%). M.p. 249 – 2518. IR (ATR): 2934 (NH), 2580, 1625, 1434 (P¼O), 1368, 1168, 1118, 1044,
1023, 755, 711. 1H-NMR (CD3OD): 1.43 (s, 4 Me); 1.67 (m, 2 CH2); 1.81 (m, CH2); 7.37 (6 arom. H); 7.78
(4 arom. H). 13C-NMR (CD3OD): 15.7 (CH2); 26.3 (Me); 34.7 (CH2); 56.3 (C); 127.3 (d, 3J(P,C) ¼ 11),
129.4 (d, 4J(P,C) ¼ 2); 130.9 (d, 2J(P,C) ¼ 8); 138.84 (d, 1J(P,C) ¼ 134). 31P{1H}-NMR (CD3OD): 20.7.
ESI-MS: 360 (100, [M þ H]þ). Anal. calc. for C21H30NO2P (359.20): C 70.17, H 8.41, N 3.89; found: C
69.84, H 8.49, N 3.74.
Reaction of R2PH · BH3 with VAZOꢀ 88. To a soln. of R2PH · BH3 (0.75 mmol) in toluene (5 cm3) was
added VAZOꢀ 88 (0.33 g, 1.51 mmol), and the mixture was heated to 1108 for 36 h. The solvent was
removed in vacuo, and the residue was extracted with hexane (3 ꢁ 10 cm3). Removal of the solvent in
vacuo afforded the diphosphane as a solid or oil. Yields are given in the Table. All spectroscopic data for
t
iPr4P2 [17], Bu4P2 [18], and (cyclopentyl)4P2 [5a] are consistent with literature data.
Table. Yields of the Reaction of R2PH · BH3 with VAZOꢀ 88
R
Yield [%]
R
Yield [%]
Ph
iPr
92
84
tBu
Cyclopentyl
62a)
86
a) From NMR analysis.
Results and Discussion. – Our initial attempts at hydrophosphination of the
pendent alkene 1-allyl-2,3,4,5-tetramethylcyclopentadiene using Ph2PH and the radical
source VAZOꢀ 88 did not yield the expected tertiary phosphane, rather we were able to
identify tetraphenyldiphosphane, Ph2PꢀPPh2, as one of the products in the 31P{1H}-
NMR spectrum. When the reaction of Ph2PH and VAZOꢀ 88 (1,1’-azobis[cyclohexane-
1-carbonitrile]) was examined in toluene, the only P-containing product observed by
NMR spectroscopy was Ph2PꢀPPh2 (Scheme 1). No H2 evolution was observed, and
the by-product cyclohexanecarbonitrile was identified in the 1H- and 13C-NMR
spectrum. Using deuterated Ph2PꢀD, the presence of (1-D)cyclohexanecarbonitrile is
clearly evidenced (d(C) ¼ 27.15 ppm, 1J(C,D) ¼ 20.3 Hz). Isolation of the diphosphane
was readily achieved by evaporation of the solvent and extraction into hexane to afford
Ph4P2 as a pure white solid after recrystallisation. The spectroscopic data are identical
to those of authentic diphosphane. We have examined the scope of the reaction with
dialkylphosphanes but no reaction was observed, even after prolonged reflux. This can
be explained by the more effective inductive effect from the alkyl group which reduces
the polarity of the PꢀH bond and thus reducing the reactivity. Notably, using AIBN as
the radical source, a number of peaks in the 31P-NMR spectrum was observed, and no
pure products could be isolated.

Helvetica Chimica Acta – Vol. 93 (2010)
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Scheme 1
These observations are pertinent to the mechanism of the hydrophosphination
reaction. It is known that the first step is the formation of a phosphinyl radical that
reacts with an alkene to form a C-atom-centred radical that undergoes intermolecular
hydrogen abstraction from another molecule of the phosphane. If there is no alkene
present, then presumably two phosphinyl radicals can combine to form the observed
diphosphane. The observation of cyclohexanecarbonitrile also indicates that the
VAZOꢀ 88 abstracts a hydrogen from Ph2PH. Of note is that the only other report in
the literature of a diphosphane observed under these conditions has been from the
sterically very bulky (Me3Si)2PꢀH [14].
The reactivity of the stable nitroxide radical 2,2,6,6-tetramethylpiperidin-1-oxyl
(TEMPO) has been investigated with other main group hydrides and shown to extract
a hydrogen from aluminium and gallium hydrides to form a nitroxide group-13 metal
hydride species [19], or the reduction of TEMPO to the corresponding 2,2,6,6-
tetramethylpiperidin-1-ol and formation of MꢀM bonded R3EꢀER3, where E ¼ Sn or
Ge [20]. When the reaction of TEMPO and diphenylphosphane in a 2 :1 ratio is
followed by 31P-NMR spectroscopy, three resonances are observed at ca. 30 ppm. After
workup, white alcohol-soluble crystals were obtained. These crystals were subjected to
X-ray diffraction, and, although the crystals were twinned, atom connectivity could be
established and shown to be the salt 2,2,6,6-tetramethylpiperidinium diphenylphos-
phate (Scheme 2). The spectroscopic data confirm this formulation. For instance, the
31P resonance at 20.7 ppm is similar to that found in [NH4][Ph2PO2] (d(P) 13.3 ppm)
[21], whilst the P¼O stretch is at 1434 cmꢀ1 in the IR spectrum.
Scheme 2
The formation of this species can be rationalised as shown in Scheme 3. Given the
results obtained above, TEMPO presumably reacts with Ph2PH via a hydride-
abstraction reaction to form the phosphane-centred radical which reacts with a second
equivalent of TEMPO to form an intermediate phosphinite. Hydrolysis of this
phosphinite with adventitious H2O liberates the amine and diphenylphosphonic acid,
which react together to form the observed product. The reduction of TEMPO by R3SiH
forms the amine via a similar mechanism [20], whilst EtSH has also been observed to
reduce TEMPO to the amine [22]. Diphenylphosphonic acid is well-known to be
deprotonated by amines to form the corresponding salts [23].
Given the recent interest in dehydrocoupling of phosphane – borane adducts to
form phosphinoborane rings and chains [6], and the use of these adducts in the
hydrophosphination of unactivated alkenes [24], we were interested to investigate the

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Helvetica Chimica Acta – Vol. 93 (2010)
Scheme 3
Scheme 4
reactivity with radical sources. The reaction of Ph2PH · BH3 with AIBN afforded a
number of products that could not be separated. However, using VAZOꢀ 88 as the
radical source, the formation of Ph2PꢀPPh2 was seen as the only P-containing product
(Scheme 4). As the BH3 group effectively increases the polarity of the PꢀH bond, we
reasoned that more electron-donating dialkylphosphane-boranes would react, possibly
to form the known rings or chains. However, monitoring by 31P-NMR spectroscopy
showed that for dicyclopentyl- and diisopropylphosphane – borane, the only observable
products were due to R2PH and R4P2. Upon addition of more BH3 · THF and heating
for a longer period of time, all secondary phosphane can be converted to the
tetraalkyldiphosphane (Table). This implies that the borane adduct is the reactive
species, although the mechanism is unclear; monitoring the reaction by 11B-NMR
spectroscopy shows only the phosphane – borane adduct, BH3 · THF, and an unidenti-
fied peak at ꢀ 13.5 ppm, which is a triplet in the H-coupled 11B spectrum (1J(B,H) ¼
t
t
107 Hz). The reaction with the bulky Bu2PH · BH3 does give some of the Bu4P2 (ca.
60% by NMR analysis) along with other unidentified products. The lower yield of this
compound is probably due to the extremely sterically bulky diphosphane, whose crystal
t
structure has recently been reported [25]; the dehydrocoupling of Bu2PH · BH3
requires more forcing conditions compared to the diphenyl analogue [9]. We were
unable to satisfactorily purify this compound.
In conclusion, we have found that diarylphosphanes can be readily coupled to form
tetraaryldiphosphanes in the presence of the radical source VAZOꢀ 88, whilst reduction
of the nitrosyl group of TEMPO has been observed. Dehydrocoupling of dialkylphos-
phanes can be achieved using the more reactive phosphane – borane adducts.
We thank Trinity College, Dublin, for financial support.

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Received March 22, 2010