Rapid intramolecular heterolytic dihydrogen activation by a four-membered heterocyclic phosphane-borane adduct

Article abstract of DOI:10.1039/b710475h

A four-membered cyclic intramolecular phosphane-borane adduct activates dihydrogen to yield the respective ethylene-bridged zwitterionic phosphonium-hydridoborate system, which reduces benzaldehyde. The Royal Society of Chemistry.

Full text of DOI:10.1039/b710475h

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Rapid intramolecular heterolytic dihydrogen activation by a four-
membered heterocyclic phosphane–borane adduct{
Patrick Spies,a Gerhard Erker,*a Gerald Kehr,a Klaus Bergander,a Roland Fro¨hlich,{a Stefan Grimme§a and
Douglas W. Stephanb
Received (in Cambridge, UK) 10th July 2007, Accepted 13th September 2007
First published as an Advance Article on the web 2nd October 2007
DOI: 10.1039/b710475h
A four-membered cyclic intramolecular phosphane–borane
adduct activates dihydrogen to yield the respective ethylene-
bridged zwitterionic phosphonium–hydridoborate system,
which reduces benzaldehyde.
Heterolytic dihydrogen cleavage has been shown to be of
importance in enzymatic reaction cycles involving metal-contain-
ing hydrogenases.1 Related heterolytic dihydrogen cleavage and
usage has been observed in a great variety of molecular transition
metal complexes2 and catalysts.3 In contrast, very few examples are
known where the H2 molecule is split into a proton and a hydridic
component in a completely metal-free environment, solely by
means of reagents containing main group elements.4,5 Stephan
et al. have recently described such a hydrogen activation, occurring
by means of phosphane/borane combinations that were specially
designed to avoid efficient Lewis acid/Lewis base self-quenching.4
Scheme 1
The R3P/BR93 examples tested reacted rapidly (and reversibly)
This was supported by the results of a DFT calculation (Fig. 1).9
with dihydrogen to give the corresponding [R3PH+][HBR93
]
2
salts.5 We have now prepared the first cyclic alkylene-bridged
system of this type and observed its rapid hydrogen uptake to give
a zwitterionic salt, which is shown to exhibit hydridic reactivity
towards an organic carbonyl compound.6 Some of the details of
this pertinent study will be described in this account.
Compound 4 features a calculated gas phase structure that
contains a close to planar four-membered ring with a calculated
P–B distance of 2.21 s.10 A very prominent structural feature of 4
is the parallel arrangement of one P–mesityl ring with one B–C6F5
group. The resulting p–p donor–acceptor interaction between these
spatially close aryl p-systems seems to contribute to the stability of
the unusual four-membered heterocyclic structure of 4.11 The DFT
calculation localizes two isomeric open chain local minima of 4
(gauche-49, trans-49), which are both only ca. 7 kcal mol21 above
the cyclic (p-stacking stabilized) ground state of 4. We assume that
the open isomers are responsible for the observed H2-activation
(see below).
Chlorodimesitylphosphane (1)7 was reacted with vinyl magne-
sium bromide to yield vinyldimesitylphosphane [2; 48% isolated
after chromatographic purification; 31P NMR (d6-benzene):
d 222.0; 1H NMR: d 6.98 (1H) and 5.35/5.29 (LCH2); 13C
NMR: d 129.4 and 121.8 (–CHLCH2)] (Scheme 1). Subsequent
treatment with HB(C6F5)2 (3)8 (1 equiv.) in pentane resulted in a
clean anti-Markovnikov addition to yield 4 (75%). The spectro-
scopic data of 4 indicate the formation of a four-membered
boraphospha heterocycle, exhibiting a P–B interaction." 1H NMR
A solution of compound 4 in pentane was stirred in a
dihydrogen atmosphere (1.5 bar) at room temperature. Within
15 min, a large amount of a white precipitate had formed, which
was isolated (60% yield) and subsequently identified as the inner
phosphonium-borate salt Mes2PH+CH2CH2BH2(C6F5)2 (5).
resonances of the [P]–CH2–CH2–[B] moiety occur at d 2.87 (2JPH
=
8 Hz) and 2.29 (3JPH = 42 Hz), respectively. The compound’s
spectroscopic studies also included 19F [d 2128.8 (o-), 2157.0 (p-)
and 2163.6 (m-C6F5)], 11B (d 8.5) and 31P NMR data (d + 20.6),
which indicate internal adduct formation.
aOrganisch-Chemisches Institut, Universita¨t Mu¨nster, Corrensstraße 40,
D-48149 Mu¨nster, Germany. E-mail: erker@uni-muenster.de;
Fax: +49 251-83 36503
bDepartment of Chemistry and Biochemistry, University of Windsor,
Windsor, Ontario, Canada N9B 3P4
{ Electronic supplementary information (ESI) available: Experimental
section and details of the quantum chemical calculations. See DOI:
10.1039/b710475h
{ X-Ray crystal structure analyses.
§ Quantum chemical calculations.
Fig. 1 DFT-calculated structure of 4 featuring p-stacking stabilization.
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Compound 5 shows a PH unit in the 31P NMR spectrum at d 26.5
1
as a doublet with a coupling constant JPH # 490 Hz. The
corresponding 1H NMR PH resonance was located at d 7.87 (dt,
3
1JPH = 486 Hz, JHH = 7.3 Hz, 1H). The 19F NMR spectrum
features a set of typical borate C6F5 resonances at d 2133.4 (o),
2165.8 (p) and 2168.0 (m), and the 11B NMR signal is split into a
doublet by coupling with the directly-bonded hydrogen [d 220.1
(d, 1JBH = 88 Hz)]. The analogous reaction of 4 with D2 generated
the corresponding product 5-D2, which features a 1 : 1 : 1 intensity
2
31P NMR triplet resonance at d 26.97. We located the broad H
NMR [B]-D resonance at d 2.94 and the [P]-D resonance as a
1
doublet at d 7.87 (d, JPD = 75 Hz; in THF).
Compound 5 was characterized by X-ray diffraction (single
crystals were obtained by diffusion of pentane vapor into a THF
solution of 5)." In the crystal, the zwitterion 5 features the bulky
2
Mes2PH+ and HB(C6F5)2 substituents in an antiperiplanar
Fig. 3 A view of the molecular structure of product 6.
orientation at the central –CH2–CH2– bridging unit [P1–C1
1.811(2), C1–C2 1.530(3) and C2–B1 1.631(3) s; hP1–C1–C2–B1
179.0(1)u]. The bond angles at P1 [C21–P1–C11 114.9(1), C21–P1–
C1 107.6(1) and C11–P1–C1 118.7(1)u] and B1 [C31–B1–C2
110.1(2), C31–B1–C41 108.1(2) and C41–B1–C2 114.1(2)u] indicate
pseudo-tetrahedral coordination geometries at both the phos-
phorus and boron atoms in 5 (see Fig. 2).
starting from an intramolecular phosphane–borane adduct. The
resulting borohydride moiety features the typical ability to serve as
a reducing agent, here shown by its facile reaction with
benzaldehyde. These observations are likely to represent interesting
and useful steps towards the utilization of dihydrogen in the
absence of metals, and in the development of novel active
‘‘organohydrogenation’’ systems,12 here having the useful feature
of an easily modifiable organic bridging group between phos-
phorus and boron.
The borohydride moiety in 5 transfers a hydride anion to a
suitable receptor. The reaction of 5 with benzaldehyde in THF
solution at ambient temperature resulted in hydride addition to the
carbonyl carbon, with the formation of the respective zwitterionic
[B]–OCH2Ph product 6 [11B NMR: d 20.3 (br s); 19F NMR: d
2133.7 (o), 2164.3 (p) and 2167.4 (m); 31P NMR: d 24.8 (d); 1H
NMR: d 7.86 (dt, 1JPH = 492 Hz, 3JHH = 7.6 Hz, PH) and 4.09 (s,
2H); 13C NMR: d 65.6 (OCH2Ph)]. The formation of 6 was
confirmed by an X-ray crystal structure analysis (single crystals
obtained from THF/pentane by the diffusion method)." It features
a distorted antiperiplanar arrangement of the central P1–C2–C3–
B4 core (h = 151.4(7)u), with the attachment of the newly formed
–OCH2Ph substituent at the tetravalent, pseudotetrahedral boron
center (see Fig. 3) (average C–P–C angle at P1 = 113.6u, average
angle at B1 = 107.8u).
Notes and references
" Compounds 4, 5 and 6 were isolated and characterized by elemental
analysis, and by 1H, 2H, 11B, 13C, 31P and 19F NMR spectroscopy. In
addition, compounds 5 and 6 were characterized by X-ray crystal structure
analyses.
Crystal data for 5: C32H28BF10P, M = 644.32, monoclinic, space group
P21/c (no. 14), a = 12.3680(3), b = 18.5413(5), c = 13.1716(1) s, b =
96.361(2)u, V = 3001.90(13) s3, Dc = 1.426 g cm23, m = 1.572 mm21, Z = 4,
l = 1.54178 s, T = 223(2) K, 21035 reflections collected (¡h, ¡k, ¡l),
[(sin h)/l] = 0.60 s21, 5312 independent (Rint = 0.060) and 4566 observed
reflections [I ¢ 2s(I)], 411 refined parameters, R = 0.050, wR2 = 0.144.
CCDC 650365.
¯
Crystal data for 6: C39H34BF10OP, M = 750.44, triclinic, space group P1
(no. 2), a = 12.1089(8), b = 12.7662(10), c = 12.8752(8) s, a = 104.669(3),
b = 100.577(5), c = 100.279(3)u, V = 1839.0(2) s3, Dc = 1.355 g cm23, m =
1.385 mm21, Z = 2, l = 1.54178 s, T = 223(2) K, 20114 reflections
Our work shows that heterolytic dihydrogen splitting and
activation can be readily achieved under metal-free conditions
collected (¡h, ¡k, ¡l), [(sin h)/l] = 0.60 s21, 6171 independent (Rint
=
0.078) and 4081 observed reflections [I ¢ 2s(I)], 479 refined parameters,
R = 0.077, wR2 = 0.227. CCDC 650366.
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b710475h
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Fig. 2 A view of the molecular structure of the zwitterionic dihydrogen
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Chem. Commun., 2007, 5072–5074 | 5073

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5074 | Chem. Commun., 2007, 5072–5074
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