Ph2P(BH3)Li: From ditopicity to dual reactivity

Article abstract of DOI:10.1021/ja201760c

A multinuclear NMR study shows that the deprotonation of diphenylphosphine-borane by n-BuLi in THF leads to a disolvated lithium phosphido-borane Ph₂P(BH₃)Li of which Li⁺ is connected to the hydrides on the boron and two THF molecules rather than to the phosphorus. This entity behaves as both a phosphination and a reducing agent, depending on the kinetic or thermodynamic control imposed to the reaction medium. Density functional theory computations show that H₂P(BH ₃)Li exhibits a ditopic character (the lithium cation can be in the vicinity of the hydride or of the phosphorus). It explains its dual reactivity (H- or P-addition), both routes going through somewhat similar six-membered transition states with low activation barriers.

Full text of DOI:10.1021/ja201760c

ARTICLE
pubs.acs.org/JACS
Ph2P(BH3)Li: From Ditopicity to Dual Reactivity
Gabriella Barozzino Consiglio,†,‡ Pierre Queval,§ Anne Harrison-Marchand,† Alessandro Mordini,‡
Jean-Franc-ois Lohier,§ Olivier Delacroix,§ Annie-Claude Gaumont,*,§ Hꢀelꢁene Gꢀerard,*,||
Jacques Maddaluno,*,† and Hassan Oulyadi†
†CNRS UMR 6014 & FR 3038, Universitꢀe de Rouen and INSA de Rouen, 76821 Mont St Aignan Cedex, France
‡ICCOM, Dipartimento di Chimica “U. Schiff”, Universita’ di Firenze, Via della Lastruccia 13, 50019 Sesto Fiorentino, Firenze, Italy
§Laboratoire de Chimie Molꢀeculaire et Thio-organique, CNRS UMR 6507 & FR 3038, ENSICAEN and Universitꢀe de Caen,
14050 Caen, France
CNRS UMR 7616, LCT, UPMC Universitꢀe Paris 6, 4 place Jussieu, 75252 Paris Cedex 05, France
S Supporting Information
b
ABSTRACT: A multinuclear NMR study shows that the deproto-
nation of diphenylphosphine-borane by n-BuLi in THF leads to a
disolvated lithium phosphido-borane Ph2P(BH3)Li of which Liþ is
connected to the hydrides on the boron and two THF molecules
rather than to the phosphorus. This entity behaves as both a
phosphination and a reducing agent, depending on the kinetic or
thermodynamic control imposed to the reaction medium. Density
functional theory computations show that H2P(BH3)Li exhibits a ditopic character (the lithium cation can be in the vicinity of the
hydride or of the phosphorus). It explains its dual reactivity (H- or P-addition), both routes going through somewhat similar six-
membered transition states with low activation barriers.
’ INTRODUCTION
The counterion being likely to play a significant role in their
reactivity, we also resorted on lithium labeling and prepared the
6Li-diphenylphosphido-borane 2 (Figure 1, left). This was done
Phosphine- and amine-boranes1 are the object of a renewed interest
due to their versatile physicochemical properties. Indeed, a flurry of
convincing new applications in the fields of PꢀB polymers,2 transition
metal complexes,3 and activation of stable molecules (H2, CO2)4 has
recently appeared in the literature. Another major application of
phosphine-boranes is for the synthesis of sophisticated phosphines,
the borane being employed as a temporary protecting group of the
phosphorus atom.5 This application often requires a preliminary
deprotonation of the BH3-protected secondary phosphines, such
as 1, which leads to the corresponding metal phosphido-borane
M(R2PBH3) 2 where M is generally an alkali (Figure 1, left). These
reagents are employed as nucleophilic entities for the synthesis of
phosphines ligands by P-reprotonation, alkylation, bromination, or
oxidation.6 Because they have been mostly considered as “in situ”
intermediates, these deprotonated derivatives have been rarely studied,
and most aspects of their structure and chemical properties are
unknown.7 However, their characteristics could trigger an interest
lying far beyond the mere academic curiosity, as they are putative
reactants for asymmetric nucleophilic phosphination or precursors for
the synthesis of polyphosphinoboranes. We report herein the first in-
depth structural investigation of a phosphido-borane in solution as well
as unprecedented chemical properties observed with this compound.
6
by adding, at ꢀ78 °C, a THF-d8 solution of Li-labeled n-
butyllithium8 (1 equiv) to diphenylphosphine-borane 1 in the
same solvent, directly in an NMR tube. The completion of the
deprotonation was confirmed by the absence of signals char-
acteristic of the PꢀH hydrogen (Figure 1, right).
6
The Li (I = 1), 11B (I = 3/2), and 31P (I = 1/2) NMR
experiments afforded, at 195 K, one singlet for each nucleus,
despite the multiple couplings expected between heteronuclei.
A progressive rising of the temperature up to 313 K (40 °C,
Figure 2) did not alter the 6Li-dimension, except for a slight shift
at lower field, but revealed the 1J multiplicities on the 11B and 31
P
6
spectra. The absence of PꢀLi coupling on the Li and 31P
spectra, whatever the temperature, remained puzzling, especially
as the LiꢀP coupling constant computed for model compounds
exhibiting a PꢀLi interaction such as those discussed below
(Figure 5) is significant (46.5 and 35.3 Hz for disolvated and
trisolvated Liþ, respectively).
The key information concerning the location of this nucleus
was brought by a complementary bidimensional 6Li,1H-HOESY
NMR experiment,9 which revealed a correlation between the
signal of the lithium cation and the broad quartet associated with
the hydrogen atoms of BH3 (Figure 3 and S14 in the Supporting
’ RESULTS AND DISCUSSION
NMR spectroscopy looked like the perfect tool to characterize
a species bearing four NMR-active nuclei (1H, 11B, 13C, 31P).
Received: March 9, 2011
Published: April 05, 2011
r
2011 American Chemical Society
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Figure 1. 1H NMR spectra of diphenylphosphine-borane 1 (a) and lithium diphenylphosphido-borane 2 (b) in THF-d8 at 195 K. The large peaks in the
0.8ꢀ1.3 ppm region of (b) are due to butane.
Figure 2. NMR spectra of Ph2P(BH3)Li 2 in THF-d8 at 195ꢀ313 K.
Information), suggesting a spatial proximity between these
nuclei. This result, in perfect accord with a solid-state structure
proposed by M€uller and Brand on the basis of X-ray data
obtained in a similar case,7 justifies the absence of PꢀLi coupling.
It also fits nicely observations reported about the hydrideꢀmetal
interaction in other PꢀBꢀHꢀM systems involving lithium or
transition metals.10
left). Run in the presence of three internal references in THF-d8
at 195 K, this experiment led to the estimation of the molecular
weight of the main species in solution to MW = 370 (Figure 4,
right), pointing the finger at a disolvated lithium phosphido-
borane monomer (MW = 363) rather than a dimer, even
unsolvated (MW = 438).
Computations were carried out to get additional information
on the competition between the possible coordination modes at
lithium. The structure of [H2P(BH3)]Li, taken as a model
The degree of oligomerization and solvation of 211 was also
evaluated thanks to a 1H-DOSY NMR experiment12 (Figure 4,
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phosphido-borane, solvated by one to three Me2O molecules13
(used as a model for THF) was first optimized. Isomers locating
the lithium around the hydrides, the phosphorus, or both were
considered (Figure 5). Overall, the structures exhibiting the
lowest Gibbs free energies are in line with those deduced from
the NMR data as all structures lying within 3 kcal molꢀ1 from the
global minimum exhibit HꢀLi interactions. Nevertheless, two
structures exhibiting only a PꢀLi interaction are found close
above (between 3.8 and 5.6 kcal molꢀ1).
A closer look at the above data suggests that a dual reactivity
based on both P and H centers can be expected and 2 should
behave either as a classical phosphorus nucleophile (Scheme 1,
route 1) or as a source of hydrides (parallel to NaBH3CN, route 2),
the latter reactivity being unknown for phosphido-boranes. In
both cases, the Li cation would behave as the activating Lewis
acid toward the carbonyl, and the counterion (P or H) occupying
an adequate position in the carbonyl surrounding will be the
natural nucleophile to engage in the transition state en route to
the 1,2-addition. The following results demonstrated the validity
of this hypothesis.
to the exclusive formation of the reduction products 4a and 4b
(entries 2 and 4) in high yields. The less reactive ketones,
cyclohexanone and 2-heptanone, showed a similar behavior:
formation of the phosphination products 3c and 3d, respectively,
at low temperature (entries 5 and 7) and of the reduction
products 4c and 4d upon heating (entries 6 and 8).
All of the new compounds were fully characterized by NMR
and HRMS. In the case of 3b, single crystals were grown by slow
diffusion of pentane into an ethyl acetate solution at room
temperature. Its structure was unambiguously confirmed by
X-ray diffraction analysis (Figure 6 and Supporting Information).
Such a dramatic sensitivity to temperature hints that 3 could
be a kinetic product and 4 could be the thermodynamic one. To
probe the reversibility of the phosphination reaction,15 a cross-
over experiment was designed that consisted of adding 1 equiv of
the highly electrophilic p-cyanobenzaldehyde to an equimolar
solution of phosphination adduct 3a deprotonated by n-butyl-
lithium in THF at ꢀ68 °C (Scheme 2). The medium was
warmed to þ5 °C, then cooled to ꢀ68 °C before quenching
by 1 equiv of HCl in diethylether. The NMR analysis of the crude
product showed the formation of benzaldehyde and compound
3b (isolated in 63% yield), which results from the addition of 2
to p-cyanobenzaldehyde. This result supports a reversible P-
nucleophilic addition of the phosphide.
The dual reactivity of 2 was tested toward carbonyl derivatives
(Table 1). Addition of aldehydes, benzaldehyde or p-cyanoben-
zaldehyde, to 2 in THF led, upon quenching at ꢀ78 °C with 1
equiv of 2 M HCl in diethylether, to the exclusive formation of
the phosphine-borane adducts 3a and 3b, respectively (entries 1
and 3). In sharp contrast, the same experiment run at þ60 °C led
The details of the mechanism of the phosphination and the
hydride transfer were examined in a second DFT study using
[H2P(BH3)Li][OMe2]2 as a model of solvated phosphido-
borane and formaldehyde as a canonic electrophile (Figure 7
and Table 2 for geometrical data).
The phosphination path (Figure 7A, left part) is straightfor-
ward: it takes place with no activation barrier (0.5 kcal/mol)
through a very early transition state (where dPꢀC = 3.52 Å,
Figure 7B) and with a minor decoordination of the Liþ from
the hydrides (dHꢀLi = 1.85 in SM1 and TS1 to 2.07 Å in P1).16
Scheme 1. Possible Reactivities of Lithium Phosphido-
borane 2
Figure 3. Blow-up of the 6Li,1H-HOESY spectrum of lithium diphenyl-
phosphidoborane 2, in THF-d8 at 250 K (ꢀ23 °C). The full spectrum is
available in the Supporting Information.
Figure 4. 1H-DOSY spectrum of 2 in THF-d8 at 195 K in the presence of three internal references (left), deduced correlation function (middle), and
proposed structure (right).
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Table 1. Reactivity of 2 with Carbonyl Derivatives at Differ-
ent Temperatures (°C)
entry
substrate
T (°C) t (min)
3/4a
yield (%)
1
2
3
4
5
6
7
8
benzaldehyde
ꢀ78
60
1
10
1
100/0
0/100
100/0
0/100
100/0
0/100
100/0
0/100
91 (3a)
79 (4a)
90 (3b)
87 (4b)
89 (3c)
81 (4c)
91 (3d)
82 (4d)
benzaldehyde
p-cyanobenzaldehyde
p-cyanobenzaldehyde
cyclohexanone
cyclohexanone
2-heptanone
ꢀ78
60
45
1
ꢀ78
60
60
1
ꢀ78
60
2-heptanone
a Determined from 1H NMR ratio.
90
Figure 6. X-ray structure of 3b.
Scheme 2. Reaction of the Phosphination Adduct 3a and
p-Cyanobenzaldehyde in the Presence of n-BuLi
energy barrier of the overall sequence is þ9.2 kcal/mol as the (i)
and (ii) steps are reversible.
This part of the DFT computations also helps to understand
through which mechanism(s) 3 is related to 4 (Scheme 3). Two
hypotheses can be proposed: (i) a back-elimination of the
phosphide (mechanism A, step 1) followed by an independent
and irreversible hydride transfer (step 2); and (ii) a direct
intramolecular hydride transfer simultaneous to, or immediately
followed by, the departure of the phosphorus appendage
(mechanism B). The above theoretical data suggest that the
nonactivated phosphination should be the only reaction occur-
ring at low temperature. Its product lies high enough in energy
for the reaction to be reversible, allowing the irreversible reduc-
tion pathway to become competitive upon rising of the tempera-
ture. Thus, while the computed figures are in good agreement
with mechanism A, a TS corresponding to the hydride transfer of
mechanism B could not be located for an intramolecular version
of the reaction.
Figure 5. Gibbs free energies (kcal/mol, relative to the lowest structure)
and interactions energies14 (kcal/mol, in parentheses) of [H2P(BH3)Li]
solvated by n molecules of Me2O [n = 1 (left), 2 (middle), and 3 (right)]. P
(pink), Li (orange), B (blue), C (green), H (white), O (red).
This addition product lies only 8.3 kcal/mol below the starting
material.
By contrast, the reduction process (Figure 7A, right) follows a
multistep path, requiring (i) a rearrangement within the original
aldehyde complex (SM1) favoring its interaction with the
“hydride side” of the lithium, such that the πCO bond can interact
with the lithium cation17 (hydrideꢀcarbonyl distance is shor-
tened: 2.21 Å in SM2); (ii) a hydride transfer to the aldehyde to
afford an alkoxide exhibiting an elongated CꢀH bond (1.28 Å),
the same proton being involved in an agostic bond with the
boron atom (dBꢀH = 1.40 Å in IR2);16 and (iii) a rearrangement
of this primary product to form a very stable heterodimer (ꢀ31.5
kcal/mol with respect to the starting docking complex) between the
lithium alkoxide and the PH2BH2. This complex is organized
around a four-membered PꢀBꢀOꢀLi quadrilateral (P2) in which
both the Lewis base and the Lewis acid characters of ambiphilic
PH2BH2 are expressed (dPꢀLi = 2.45 Å and dBꢀO= 1.54 Å).18 The
’ CONCLUDING REMARKS
6
A multinuclear (1H, Li, 11B, 13C, 31P) NMR spectroscopic
study showed that the deprotonation of diphenylphosphine-
borane 1 by butyllithium in THF leads quantitatively to a lithium
phosphido-borane of which Liþ is directly connected to the
hydrides borne by the boron, in line with data obtained in the
solid phase.7 This result suggested that Ph2P(BH3)Li could act as
a dual phosphination and reducing agent in THF, depending on
the control (kinetic or thermodynamic) imposed by the reaction
conditions. We have shown that these two pathways are indeed
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Figure 7. (A) Gibbs free energies (kcal/mol) with respect to SM1 for phosphination versus reduction. Full lines stand for steps of which connectivity
was confirmed, and curled lines stand for a postulated easy interconversion. (B) Optimized structures of stationary points and transition states of the
phosphination (top) and reduction (bottom) pathways. The atom color codes are featured on TS1. Distances are in angstroms.
Table 2. Main Distances (in Å) along the Phosphination and
Reduction Pathways
Scheme 3. Two Possible Routes Connecting Phosphination
Product 5 to Reduction Product 6
P1
TS1
M1
M2
TS2 IR2 TS2P
P
LiꢀP
LiꢀH
3.067 3.308 3.181 2.491 2.451 2.537 2.688 2.454
1.971 1.845 1.848 1.922 2.543
2.155 1.837 1.847 2.649 2.909
LiꢀO(dC) 1.772 1.937 1.959 1.956 1.866 1.755 1.666 1.795
PꢀB
CdO
PꢀC
CꢀH
BꢀH
1.965 2.003 2.005 2.015 2.003 1.964 1.9
2.031
1.347 1.224 1.223 1.227 1.238 1.303 1.374 1.413
1.924 3.517 3.617
2.212 1.909 1.275 1.118 1.099
1.221 1.238 1.238 1.235 1.251 1.199 1.189 1.215
1.219 1.237 1.238 1.238 1.224 1.199 1.189 1.215
1.202 1.206 1.205 1.204 1.206 1.398 2.658 2.731
3.287 3.522 1.538
’ EXPERIMENTAL SECTION
All of the reactions were carried out under argon or nitrogen atmo-
sphere. Argon was dried and deoxygenated by bubbling through a
commercial solution of butyllithium in hexane. Tetrahydrofuran-d8
was distilled over sodium and benzophenone. Tetrahydrofuran and
toluene were purified by an Innovative Pure Solvent Device (activated
alumina column containing a copper catalyst and molecular sieves).
Pentane, 1-bromobutane, benzaldehyde, cyclohexanone, and 2-hepta-
none were dried by stirring over CaH2 and then distilled. Commercial
6Li (95%) purchased from Aldrich was washed in freshly distilled
pentane. Structural NMR experiments were carried out by using Bruker
AVIII 400 and DMX 500 spectrometers (Bruker, Wissembourg, France).
The Bruker AVIII 400 was equipped with a 10 A gradient amplifier and a 5
mm BBFO probe including shielded z-gradients. The Bruker Avance
DMX 500 was equipped with a 10 A gradient amplifier and a 5 mm
BꢀO
competitive, one being reversible (the phosphination) in con-
trast to the other (reduction). The DFT results are fully
consistent with these observations. Theory indeed suggests that
when Ph2P(BH3)Li is reacted with a carbonyl derivative
(aldehyde or ketone), the reaction will expectedly start with a
docking of the oxygen of the electrophile on the Liþ. The
unforeseen ditopic character of this reagent then explains that
two pathways compete in the following, both H- and P- 1,2-
additions being possible. Their respective occurrence corre-
sponds to a simple switch between the two opposite head-to-
tail arrangements of the PꢀBꢀH and CꢀOꢀLi triads. At low
temperature, the lithium is mainly located toward the hydrides,
and the phosphination is favored. At higher temperature, the
lithium moves toward the phosphorus and give its chance to the
reduction. The parallel between these two reactions explains that
they finally proceed through similar six-membered transition
states with a low activation barrier (no dimer or rearrangement
needed, Figure 8). Underlying all of these observations is the
exceptional strength of the donorꢀacceptor PꢀB bond,19 which
survives all of the transformations undergone by this system.
6
{1HꢀX} BBI or 5 mm {1H, Li, 13C, and 15N} quadruple-resonance
probe. Measuring frequencies were 400 or 500 MHz for 1H, 160 MHz
for 11B, 162 or 202 MHz for 31P, 100 or 125 MHz for 13C, and 73 MHz
for 6Li. 1H and 13C NMR chemical shifts are reported in ppm using the
residual peak of chloroform-d (7.26 and 77.16 ppm) or THF-d8
(1.73 and 25.37 ppm) as internal standards. 11B and 31P NMR spectra
were referenced (δ = 0.0 ppm) to calculated boron and phosphorus
frequencies in BF3 Et2O and 85% H3PO4 references compounds,
3
respectively.20 6Li spectra were referenced to the external 0.30 M 6LiCl
solution in THF-d8 (δ = 0.0 ppm). Coupling constants are reported in
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were placed diphenylphosphine-borane (20 mg, 0.10 mmol) and freshly
distilled tetrahydrofuran-d8 (0.5 mL). After the mixture was cooled
to ꢀ78 °C (acetone/dry ice bath), freshly prepared [6Li] n-butyllithium
(67 μL, 0.10 mmol, 1.5 M in tetrahydrofuran-d8) was added under dry
argon leading to 2 (complete conversion) (CAS no. 145130-18-3).
Representative Procedure for the Addition to Carbonyl
Derivatives. Diphenylphosphine-borane (1.0 equiv) and tetrahydro-
furan were placed in a dry Schlenk tube equipped with a thermometer.
The reactor was cooled at ꢀ78 °C, and n-butyllithium (1.0 equiv) was
added under nitrogen. The aldehyde or ketone (1.0 equiv) was added
dropwise at the same temperature followed by precooled hydrochloric
acid (1.0 equiv) so as to avoid any increase of the temperature. The
reaction mixture was warmed to room temperature and concentrated
under reduced pressure. The resulting viscous oil was dissolved in ethyl
acetate and water and extracted with ethyl acetate. The combined
organic phases were washed with brine, dried over magnesium sulfate,
filtered, and concentrated under vacuum.
(1-Hydroxy-1-phenyl)methyldiphenylphosphine-borane
3a. The title compound was prepared from diphenylphosphine-borane
1 (100 mg, 0.50 mmol), n-butyllithium (217 μL, 0.50 mmol, 2.3 M
in hexane), benzaldehyde (50 μL, 0.50 mmol), and hydrochloric acid (250
μL, 0.50 mmol, 2.0 M in diethylether) in tetrahydrofuran (2.5 mL). The
desired product 3a (153 mg, 91%) was obtained as a colorless oil that
solidified on standing (mp = 93 °C) after flash chromatography (ethyl
acetate/cyclohexane 15/85) purification. 1H NMR (400 MHz, CDCl3):
δ 7.86ꢀ7.79 (m, 2H), 7.60ꢀ7.56 (m, 3H), 7.51ꢀ7.47 (m, 3H), 7.41ꢀ
7.37 (m, 2H), 7.29ꢀ7.25 (m, 1H), 7.23ꢀ7.20 (m, 2H), 7.07ꢀ7.05
(m, 2H), 5.69 (d, 2JPꢀH = 2.0 Hz, 1H), 2.93 (br s, 1H), 1.39ꢀ0.53 (br m,
3H). 13C{1H} NMR (101 MHz, CDCl3): δ 136.4 (d, 2JCꢀP = 1.3 Hz),
Figure 8. Schematic (top) and optimized (bottom) TS’s for phosphi-
nation (left) versus reduction (right) of HCHO by [H2P(BH3)Li]ꢀ
(OMe2)2.
hertz (Hz). Abbreviations are used as follows: s = singlet, d = doublet, q =
quadruplet, m = multiplet, br = broad. IR spectra were recorded on a
Spectrum One Perkin-Elmer spectrometer, and only the strongest or
structurally most important peaks are listed. Melting points were
measured on a Gallenkamp Melting Point Apparatus. High resolution
mass spectroscopy was performed on a Q-TOF Micro Waters spectro-
meter or Waters LPC Premier.
Diphenylphosphine-borane 1. This compound was prepared
following the procedure described in the literature21 (CAS no. 41593-
58-2).
134.0 (d, 2JCꢀP = 7.6 Hz), 133.5 (d, 2JCꢀP = 7.6 Hz), 131.9 (d, 4JCꢀP
=
2.5 Hz), 131.6 (d, 4JCꢀP = 2.5 Hz), 128.8 (d, 3JCꢀP = 10.1 Hz), 128.7 (d,
3JCꢀP = 10.1 Hz), 128.5 (d, 5JCꢀP = 2.5 Hz), 128.0 (d, 4JCꢀP = 2.5 Hz),
127.4 (d, 3JCꢀP = 3.8 Hz), 126.7 (d, 1JCꢀP = 54.2 Hz), 125.6 (d, 1JCꢀP
=
52.9 Hz), 73.4 (d, 1JCꢀP = 36.6 Hz). 31P NMR (162 MHz, CDCl3): δ
28.3ꢀ26.5 (m). 11B NMR (160 MHz, CDCl3): δ ꢀ39.1 to ꢀ42.6 (m).
IR (neat): 3458, 3059, 2378, 1436. CAS no. 954422-32-3.
(1-Hydroxy-1-(4-cyanophenyl))methyldiphenylphosphine-
borane 3b. This compound was prepared from diphenylphosphine-
borane 1 (100 mg, 0.50 mmol), n-butyllithium (217 μL, 0.50 mmol,
2.3 M in hexane), p-cyanobenzaldehyde (0.40 mL, 0.50 mmol, 1.25 M in
tetrahydrofuran, precooled solution), and hydrochloric acid (250 μL,
0.50 mmol, 2.0 M in diethylether) in tetrahydrofuran (2.1 mL). The
desired product 3b (166 mg, 90%) was obtained as a colorless oil that
solidified on standing (mp = 117 °C) after flash chromatography (ethyl
acetate/cyclohexane 20/80) purification. Single crystals suitable for
X-ray diffraction were grown by slow diffusion of pentane into an ethyl
acetate solution of 3b at room temperature. 1H NMR (400 MHz,
CDCl3): δ 7.81ꢀ7.76 (m, 2H), 7.60ꢀ7.55 (m, 3H), 7.54ꢀ7.39
Preparation of [6Li] n-Butyllithium Salt-Free Solution in
Pentane8,22. Finely cut 6-lithium metal ribbon (0.3 g, 50 mmol) was
introduced in a two-necked round bottomed flask under dry argon. The
metallic cuttings were covered with freshly distilled pentane (10 mL).
After intensive stirring, the pentane was removed, and the metal was
washed twice with the same solvent. A new amount of 10 mL of pentane
was introduced, and freshly distilled 1-bromobutane (2.14 mL, 20
mmol) was syringed at room temperature over a period of 4 h. The
resulting reaction mixture was stirred for 20 h at room temperature. The
hydrocarbon solution was then pumped off the flask with a syringe and
directly inserted into centrifugation tubes placed under dry argon. The
residual traces of salt were centrifugated, and the clear final solution was
collected in a dry flask flushed under dry argon, and then titrated23 and
kept until further use.
3
2
(m, 7H), 7.17ꢀ7.15 (m, 2H), 5.72 (dd, JHꢀH = 5.2 Hz, JPꢀH
=
2.9 Hz, 1H), 3.02 (dd, 3JHꢀP = 7.2 Hz, 3JHꢀH = 5.2 Hz, 1H), 1.38ꢀ0.43
(br m, 3H). 13C{1H} NMR (101 MHz, CDCl3): δ 141.8 (d, JCꢀP
=
2
1.2 Hz), 134.0 (d, 2JCꢀP = 8.7 Hz), 133.3 (d, 2JCꢀP = 8.7 Hz), 132.3 (d,
4JCꢀP = 2.5 Hz), 132.0 (d, 4JCꢀP = 2.5 Hz), 131.6 (d, 4JCꢀP = 2.2 Hz),
Preparation of [6Li] n-Butyllithium Salt-Free Solution in
8
6
Tetrahydrofuran-d8 . A solution of Li n-butyllithium in pentane
prepared as above (2.5 mL) was syringed in a tube fitted with a septum
and flushed under dry argon. The tube was then placed under vacuum
(20 mmHg) for 1 h to remove the main part of the pentane. Freshly
distilled tetrahydrofuran-d8 was then added at ꢀ78 °C to the resulting
concentrated solution, and the residual pentane was evaporated at room
temperature under vacuum for 1 h. After the mixture was cooled
to ꢀ78 °C, tetrahydrofuran-d8 (3ꢀ3.5 mL) was added, and the resulting
solution was titrated.23 The solution was kept at ꢀ78 °C.
129.0 (d, 3JCꢀP = 10.0 Hz), 129.0 (d, 3JCꢀP = 10.0 Hz), 128.0 (d, 3JCꢀP
=
1
1
2.2 Hz),126.1 (d, JCꢀP = 54 Hz), 124.6 (d, JCꢀP = 54 Hz), 118.6
(d, 6JCꢀP = 1.6 Hz), 112.1 (d, 5JCꢀP = 2.8 Hz), 73.0 (d, 1JCꢀP = 35 Hz).
31P NMR (162 MHz, CDCl3): δ 29.7ꢀ28.1 (m). 11B NMR (160 MHz,
CDCl3): δ ꢀ39.0 to ꢀ43.3 (m). IR (neat): 3427, 3059, 2384, 2231,
1437. HRMS (ESI) calcd for [M þ Na]þ: 354.1195, found 354.1202.
(1-Hydroxy)cyclohexyldiphenylphosphine-borane 3c.
The title compound was prepared from diphenylphosphine-borane 1
(100 mg, 0.50 mmol), n-butyllithium (217 μL, 0.50 mmol, 2.3 M in
hexane), cyclohexanone (52 μL, 0.50 mmol), and hydrochloric acid
(250 μL, 0.50 mmol, 2.0 M in diethylether) in tetrahydrofuran (2.5 mL).
[6Li] Lithium Diphenylphosphido-borane 2 in Tetrahydro-
furan-d8 solution. In a dry NMR tube equipped with a rubber septum
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The desired product 3c (132 mg, 89%) was obtained as a white solid
(mp = 120 °C) after flash chromatography (toluene). H NMR (400
in tetrahydrofuran (7.5 mL) at 60 °C for 1 h. The desired product 4c
(122 mg, 81%) was obtained as a colorless oil after flash chromatography
(ethyl acetate/cyclohexane 1/9) and recrystallization in pentane/
2-propanol (9/1). H NMR (400 MHz, CDCl3): δ 3.59ꢀ3.48 (m,
1
MHz, CDCl3): δ 8.02ꢀ7.95 (m, 4H), 7.53ꢀ7.49 (m, 2H), 7.49ꢀ7.43
(m, 4H), 1.92ꢀ1.82 (m, 2H), 1.80ꢀ1.72 (m, 3H), 1.70ꢀ1.49 (m, 6H),
1.46ꢀ0.57 (br m, 3H). 13C{1H} NMR (101 MHz, CDCl3): δ 134.4 (d,
2JCꢀP = 8.1 Hz), 131.4 (d, 4JCꢀP = 2.5 Hz), 128.7 (d, 3JCꢀP = 9.6 Hz),
1
1H), 2.38 (br s, 1H), 1.91ꢀ1.79 (m, 2H), 1.75ꢀ1.62 (m, 2H),
1.55ꢀ1.45 (m, 1H), 1.29ꢀ1.18 (m, 4H), 1.18ꢀ1.05 (m, 1H). 13C{1H}
NMR (101 MHz, CDCl3): δ 70.2, 35.4, 25.4, 24.3. CAS no. 108-93-0.
2-Heptanol 4d. The title compound was prepared from diphenyl-
phosphine-borane 1 (800 mg, 4.00 mmol), n-butyllithium (2.22 mL,
4.00 mmol, 1.8 M in hexane), and 2-heptanone (0.56 mL, 4.00 mmol) in
tetrahydrofuran (10 mL) at 60 °C for 1.5 h. The desired product 4d (380
mg, 82%) was obtained as a colorless oil after distillation on ball tube
followed by filtration on silica with diethyl ether. 1H NMR (400 MHz,
CDCl3): δ 3.80ꢀ3.75 (m, 1H), 1.53 (br s, 1H), 1.46ꢀ1.36 (m, 3H),
1.34ꢀ1.23 (m, 5H), 1.17 (d, 3JHꢀH = 6.2 Hz, 3H), 0.88 (t, 3JHꢀH = 6.9
Hz, 3H). 13C{1H} NMR (101 MHz, CDCl3): δ 67.9, 39.3, 31.9, 25.5,
23.3, 22.6, 14.0. CAS no. 543-49-7.
1
1
2
126.5 (d, JCꢀP = 52 Hz), 74.5 (d, JCꢀP = 38 Hz), 32.7 (d, JCꢀP
=
7.5 Hz), 25.2 (d, 4JCꢀP = 1.1 Hz), 20.6 (d, 3JCꢀP = 9.4 Hz). 31P NMR
(162 MHz, CDCl3): δ 30.8ꢀ28.7 (m). 11B NMR (160 MHz, CDCl3):
δ ꢀ38.4 to ꢀ42.1 (m). IR (neat): 3565, 2944, 2389, 1436. HRMS (ESI)
calcd for [M þ Na]þ: 321.1556, found 321.1568.
(1-Hydroxy-1-methyl)hexyldiphenylphosphine-borane 3d.
The title compound was prepared from diphenylphosphine-borane 1
(100 mg, 0.50 mmol), n-butyllithium (294 μL, 0.50 mmol, 1.7 M in
hexane), 2-heptanone (70 μL, 0.50 mmol), and hydrochloric acid (250 μL,
0.50 mmol, 2.0 M in diethylether) in tetrahydrofuran (2.5 mL). The desired
product 3d (157 mg, 91%) was obtained as a colorless oil after flash
chromatography (ethyl acetate/cyclohexane 5/95). 1H NMR (500 MHz,
CDCl3): δ 8.02ꢀ7.95 (m, 4H), 7.54ꢀ7.48 (m, 2H), 7.48ꢀ7.43 (m, 4H),
1.95 (d, 3JHꢀP = 2.4 Hz, 1H), 1.83ꢀ1.68 (m, 2H), 1.44 (d, 3JHꢀP = 14.2 Hz,
3H), 1.49ꢀ1.40 (m, 1H), 1.39ꢀ1.29 (m, 1H), 1.39ꢀ1.29 (m, 1H),
Reversibility of the Phosphination (from 3a to 3b).
(1-Hydroxy-1-phenyl)methyldiphenylphosphine-borane 3a (207 mg,
0.68 mmol) and tetrahydrofuran (4.0 mL) were placed in a dry Schlenk
tube equipped with a thermometer. The reactor was cooled at ꢀ68 °C
(internal temperature), and n-butyllithium (453 μL, 0.68 mmol, 1.5 M
in hexane) was added under nitrogen followed by a solution of
p-cyanobenzaldehyde (89 mg, 0.68 mmol, 0.68 M in tetrahydrofuran).
The mixture was allowed to warm to 5 °C (internal temperature), and
precooled hydrochloric acid (0.34 mL, 0.68 mmol, 2 M in diethyl ether)
was added dropwise at ꢀ68 °C (internal temperature) so as to avoid any
increase of the temperature. The reaction mixture was warmed to room
temperature and concentrated under reduced pressure. The resulting
viscous oil was dissolved in ethyl acetate (5 mL) and water (5 mL) and
further extracted with ethyl acetate (2 ꢁ 5 mL). The combined organic
phases were washed with brine (10 mL), dried over magnesium sulfate,
filtered, and concentrated under reduced pressure to give a crude
mixture of (1-hydroxy-1-(4-cyanophenyl))methyldiphenylphosphine-
borane 3b and benzaldehyde. (1-Hydroxy-1-(4-cyanophenyl))methyl-
diphenylphosphine-borane 3b (142 mg, 63% yield) was obtained as a
white solid after flash chromatography (ethyl acetate/cyclohexane
20/80).
Computational Details. The full geometry optimizations were
systematically conducted with no symmetry restraints using the Gaussian
03 program24 within the framework of the density functional theory
(DFT) using the hybrid B3LYP exchange-correlation functional25 and
the 6-31þþG** basis set for all atoms as implemented in the Gaussian
program, to reproduce both the PꢀC bond formation and the hydride
transfer in a balanced way. This functional and basis set have been shown
to properly reproduce structural properties on closely related lithium
amidoboranes.26 Solvation at the lithium cation is ensured via an explicit
mode by including one, two, or three dimethylether molecules (as a
model for THF)27,28 coordinated to the lithium. This is essential as it
leads to a stronger competition between bi- and tridentate structures, in
contrast to the use of nonsolvated lithium.29 Frequencies were evaluated
within the harmonic approximation and used unscaled to compute free
enthalpies at either 195 or 298.15 K using the standard protocol.
Concerning the relative Gibbs free energies of the isomers, it can be
shown that the interaction energy14 is driving the stability. Thus, for a
given degree of solvation, the energies and free enthalpies follow the
same ordering. The nature of the transition states was ensured by
confirming the presence of a single imaginary frequency. The connec-
tion between transition states and minima was ensured by carrying out
small displacements of all atoms in the two directions along the im-
aginary frequency mode and carrying out geometry optimization using
these geometries as starting points.
3
1.29ꢀ1.17 (m, 4H), 0.84 (d, JHꢀH = 7.1 Hz, 3H), 1.40ꢀ0.60 (br m,
3H). 13C{1H} NMR (126 MHz, CDCl3): δ 134.4 (d, 2JCꢀP = 8.0 Hz),
134.3 (d, 2JCꢀP = 8.0 Hz), 131.5 (d, 4JCꢀP = 2.5 Hz), 131.4 (d, 4JCꢀP
=
2.5 Hz), 127.0 (d, 1JCꢀP = 51.8 Hz), 126.9 (d, 1JCꢀP = 51.5 Hz), 75.0 (d,
1JCꢀP = 37.0 Hz), 37.4 (d, 3JCꢀP = 9.5 Hz), 32.1 (s), 22.9 (d, 2JCꢀP = 8.9
2
H), 22.8 (s), 21.9 (d, JCꢀP = 8.7 Hz), 14.1 (s). 31P NMR (202 MHz,
CDCl3): δ 31.9ꢀ30.2 (m). 11B NMR (160 MHz, CDCl3): δ ꢀ38.5
to ꢀ41.7 (m). IR (neat): 3501, 2954, 2383, 2331, 1436. HRMS (API þ)
calcd for [M þ H ꢀ BH3 ꢀ H2O]þ: 283.1616, found 283.1611.
Representative Procedure for the Reduction of Carbonyl
Compounds. Diphenylphosphine-borane 1 (1.0 equiv) and freshly
distilled tetrahydrofuran were placed in a dry Schlenk tube. After the
mixture was cooled to ꢀ78 °C, n-butyllithium (1.0 equiv) was added
dropwise followed by the aldehyde or ketone (1.0 equiv) under nitrogen.
The reaction mixture was warmed to room temperature, then heated to
60 °C for x minutes. After the mixture was cooled to room temperature,
water (5.0 equiv) was added, and the solution was concentrated under
reduced pressure. The resulting viscous oil was dissolved in ethyl acetate
and water and extracted with ethyl acetate. The combined organic
phases were washed with brine, dried over magnesium sulfate, filtered,
and concentrated under reduced pressure.
Benzyl Alcohol 4a. The title compound was prepared from
diphenylphosphine-borane 1 (200 mg, 1.00 mmol), n-butyllithium
(0.56 mL, 1.00 mmol, 1.8 M in hexane), and benzaldehyde (101 μL,
1.00 mmol) in tetrahydrofuran (5.0 mL) at 60 °C for 10 min. The
desired product 4a (85 mg, 79%) was obtained as a colorless oil after
flash chromatography (ethyl acetate/cyclohexane 5/95). 1H NMR (400
MHz, CDCl3): δ 7.42ꢀ7.35 (m, 5H), 4.66 (s, 1H), 3.03 (br s, 1H).
13C{1H} NMR (101 MHz, CDCl3): δ 141.0, 128.6, 127.6, 127.0, 65.1.
CAS no. 100-51-6.
4-(Hydroxymethyl)benzonitrile 4b. The title compound was
prepared from diphenylphosphine-borane 1 (152 mg, 0.76 mmol),
n-butyllithium (0.51 mL, 0.76 mmol, 1.5 M in hexane), and p-cyano-
benzaldehyde (100 mg, 0.76 mmol) in tetrahydrofuran (4.0 mL) at
60 °C for 45 min. The desired product 4b (88 mg, 87%) was obtained as
a white solid (mp = 40 °C;30 39ꢀ40 °C) after flash chromatography (ethyl
1
acetate/cyclohexane 3/7). H NMR (400 MHz, CDCl3): δ 7.64ꢀ7.60
(m, 2H), 7.47ꢀ7.44 (m, 2H), 4.76 (s, 2H), 2.16 (br s, 1H). 13C{1H}
NMR (101 MHz, CDCl3): δ 146.4, 132.4, 127.1, 119.0, 111.2, 64.3. CAS
no. 874-89-5.
Cyclohexanol 4c. The title compound was prepared from diphe-
nylphosphine-borane 1 (300 mg, 1.50 mmol), n-butyllithium (0.94 mL,
1.50 mmol, 1.6 M in hexane), and cyclohexanone (155 μL, 1.50 mmol)
X-ray Structural Analysis Details. The structure was solved
using direct methods and refined by full-matrix least-squares analysis on
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Journal of the American Chemical Society
ARTICLE
F2 with SHELXTL. All non-hydrogen atoms were refined with aniso-
tropic displacement parameters. Hydrogen atoms were placed in
geometrically idealized positions and included as riding atoms. CCDC
813642 contains the supplementary crystallographic data for this Article.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/
cif.
(8) Patꢀe, F.; Oulyadi, H.; Harrison-Marchand, A.; Maddaluno, J.
Organometallics 2008, 27, 3564–3569 and references therein.
(9) The following parameters were used for acquiring and proces-
sing the spectra in phase-sensitive mode: 256 experiments with 2048
data points and 16 scans each were recorded; pure phase line shapes
were obtained by using time proportional phase incrementation (TPPI)
phase cycling; mixing times of 1.4 s were used for the sample; one time
zero filling in f1; π/2 and π/3 shifted sine square window functions were
applied to f2 and f1 dimensions, respectively, before Fourier transforma-
tion. Processing of NMR data was performed on a PC computer, using
the manufacturer’s program Topspin2.1 (Bruker). For details, see:
Bauer, W.; Clark, T.; Schleyer, P. v. R. J. Am. Chem. Soc. 1987, 109,
70–977.
(10) (a) Kakizawa, T.; Kawano, Y.; Shimoi, M. Organometallics
2001, 20, 3211–3213. (b) Rudzevich, V. L.; Gornitzka, H.; Romanenko,
V. D.; Bertrand, G. Chem. Commun. 2001, 1634–1635. (c) Izod, K.;
Wills, C.; Clegg, W.; Harrington, R. W. Organometallics 2006, 25, 38–40.
(d) Kawano, Y.; Hashiva, M.; Shimoi, M. Organometallics 2006,
25, 4420–4426. (e) Kawano, Y.; Yamaguchi, K.; Miyake, S.-y.; Kakizawa,
T.; Shimoi, M. Chem.-Eur. J. 2007, 13, 6920–6931. (f) Izod, K.; Wills, C.;
Clegg, W.; Harrington, R. W. Dalton Trans. 2007, 3669–3675. (g) Izod,
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(11) Solid-state monomer and dimer of related lithium phosphino-
boranes have been identified before, and their structure in solution was
questioned; see ref 10b.
(12) The following parameters were used for acquiring the BPP-
LED (bipolar pulse pair-longitudinal eddy-current delay) spectrum: 32
experiments with 16 K data points and 8 scans each were recorded;
bipolar rectangular gradients were used with total durations of 0.6 ms,
gradient recovery delay was 0.5 ms, and diffusion times was 1 s.
Processing of DOSY data was performed with the manufacturer’s
program Topspin2.1 (Bruker), using the processing method Gifa-
MaxEnt, and the diffusion coefficient was measured by the T1/T2
relaxation module. For more details, see: Wu, D.; Chen, A.; Johnson, C. S.,
Jr. J. Magn. Reson., Ser. A 1995, 115, 260–264. For DOSY experiments
applied to organolithium derivatives, see: Kagan, G.; Li, W.; Hopson, R.;
Williard, P. G. Org. Lett. 2009, 11, 4818–4821. Review: Li, D.; Keresztes, I.;
Hobson, R.; Williard, P. G. Acc. Chem. Res. 2009, 42, 270–280.
(13) These solvation numbers are in agreement with those obtained
using ab initio molecular dynamics in periodic boundary conditions on a
lithium-containing ion pair, and especially hydrides such as LiAlH4:
Bikiel, D. E.; Di Salvo, F.; Gonzꢀalez Lebrero, M. C.; Doctorovich, F.;
Estrin, D. A. Inorg. Chem. 2005, 44, 5286–5292.
’ ASSOCIATED CONTENT
1
Supporting Information. Full H, 11B, 13C, 31P NMR
S
b
and IR spectra for 3aꢀd, 1H HOESY spectrum of 2, and crystal
details for 3b, as well as complete ref 24. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mails: annie-claude.gaumont@ensicaen.fr; helene@
lct.jussieu.fr; jmaddalu@crihan.fr.
’ ACKNOWLEDGMENT
The Agence Nationale pour la Recherche supports this work
(ANR-07-BLAN-0294-01). Funds by the CNRS, the Rꢀegion
Haute Normandie, and Rꢀegion Basse Normandie (Crunch
interregional network) as well as an ERDF endowment (ISCE-
Chem and INTERREG IVa programs) are also acknowledged.
Calculations have been performed at CRIHAN (Saint-Etienne-du-
Rouvray, France), CINES (Montpellier, France), and IDRIS
(Orsay, France). This work is dedicated to Professor Carmen
Najera on the occasion of her 60th birthday.
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