Control of reactivity of phosphine imides by intramolecular coordination with an organoboryl group

Article abstract of DOI:10.1002/hc.21033

Phosphine imides with a boryl substituent 3-5 were synthesized. Their structures were revealed to have a tetracoordinated boron and a phosphorus atom, featuring the N-B coordination by X-ray crystallographic analysis and NMR spectroscopy. The phosphine imide moiety was persistent to hydrolysis, methylation, and the aza-Wittig reaction. The N-B coordination remained intact upon treating with pyridine or fluoride ion. Copyright

Full text of DOI:10.1002/hc.21033

Heteroatom Chemistry
Volume 00, Number 0, 2012
Control of Reactivity of Phosphine Imides by
Intramolecular Coordination with an
Organoboryl Group
Naokazu Kano, Kazuhide Yanaizumi, Xiangtai Meng, and
Takayuki Kawashima
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan
Received 29 March 2012; revised 11 May 2012
pends on the degree of polarization of the P+–N−
ABSTRACT: Phosphine imides with a boryl sub-
bond, which sometimes causes instability against
stituent 3–5 were synthesized. Their structures were
hydrolysis [4]. Stabilization of the phosphine imide
revealed to have a tetracoordinated boron and a
is necessary for its utilization for the reactions un-
phosphorus atom, featuring the N–B coordination by
der various conditions. One stabilization method
X-ray crystallographic analysis and NMR spec-
to avoid undesired reactions is to use bulky sub-
troscopy. The phosphine imide moiety was persistent
stituents or electronically perturbing substituents on
to hydrolysis, methylation, and the aza-Wittig reac-
the phosphorus and/or the nitrogen atoms, and an-
tion. The N–B coordination remained intact upon
other method is to lower reactivities by coordination
ꢀ
C
treating with pyridine or fluoride ion.
2012 Wi-
of the lone pair of the nitrogen atom to another atom
such as a transition metal [5]. Considering that some
reactions such as the hydrolysis of the phosphine
imide are triggered by an interaction of the nitrogen
atom with a reagent, strong N–B coordination of the
nitrogen to a Lewis acid such as an organoborane,
which shows high Lewis acidity due to its vacant
2p-orbital, would inhibit both interactions with a
reagent and subsequent undesired reactions such as
hydrolysis. Although there have been only a few ex-
amples of phosphine imides with the N–B coordina-
tion [6], perturbation of their stability and reactivity
caused by the coordination has not been revealed in
detail. We previously reported that boryl-substituted
azobenzenes formed an intramolecular N–B coordi-
nation [7], which is strong enough to inhibit pho-
toisomerization in some cases. Triphenylphosphine
imides, Ph3P NR, which also have a potentially
coordinating NR moiety like azobenzenes, are ex-
pected to form N–B coordination, which will some-
what change reactivities, if a boryl group is intro-
duced into the ortho position of the phenyl group.
As there have been several examples of reactions
ley Periodicals, Inc. Heteroatom Chem 00:1–6, 2012;
View this article online at wileyonlinelibrary.com. DOI
10.1002/hc.21033
INTRODUCTION
A phosphine imide, a nitrogen analogue of a phos-
phorus ylide, is important as a reagent for the aza-
Wittig reaction [1] and as an intermediate of the
Staudinger reaction [2] and the Staudinger ligation
[3]. Reactivity of the phosphine imide heavily de-
Correspondence to: Naokazu Kano; e-mail: kano@chem.s.u
-tokyo.ac.jp. Takayuki Kawashima; e-mail: Takayuki.Kawashima
@gakushuin.ac.jp.
Present address of Takayuki Kawashima: Faculty of Science,
Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-
8588, Japan.
Contract grant sponsor: Grants-in-Aid for the Global COE Pro-
gram for Chemistry Innovation and for Scientific Research from
the Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan.
Contract grant number: 21350025 and 22108508.
c
ꢀ
2012 Wiley Periodicals, Inc.
1

2
Kano et al.
SCHEME 1
of ortholithiated triphenylphosphine imides with
transition metal complexes and main group
element compounds to introduce the elements [8],
this method is useful to introduce a boryl group
to the phosphine imide moiety. In this paper, we
report the synthesis of triphenylphosphine N-tert-
butylimides bearing a boryl substituent with an in-
tramolecular N–B coordination and perturbation of
reactivities.
to that of [Ph3PNHBut]+Br− (δP 32.0) [12], showed
downfield shifts from that of 1 (δP −8.1 ppm), sug-
gesting strong coordination of nitrogen to boron in
the solution state and contribution of phosphonium-
borate structure in 3–5 to some extent as represented
by 4ꢁ.
The crystal structures of 3–5 were character-
ized by the X-ray crystallographic analysis (Fig. 1).
The N1 atom is directed to the B1 atom to form
the N–B coordination in all 3–5. The N–B dis-
˚
˚
tances (3: 1.666(3) A, 4: 1.584(4), 5: 1.613(2) A)
RESULTS AND DISCUSSION
are considerably shorter than the sum of the van
der Waals radii (3.35 A) [13], showing intramolec-
˚
Phosphine imide, Ph3P NBut (1) [9] was lithiated
with an Et2O solution of methyllithium in Et2O at
room temperature to give a dimeric-lithiated deriva-
tive 2 solvated with an Et2O molecule in 93% yield
(Scheme 1) [10]. The reaction of the lithiated deriva-
tive 2 with trichloroborane and successive addition
of methyllithium gave dimethylborane 3 in 28%
yield. The reaction of 2 with chlorocatecholborane
gave the corresponding catecholborane 4 in 65%
yield. Reduction of 4 with LiAlH4 gave dihydrobo-
rane 5 in 41% yield. Although the almost quantitative
conversion of 4 to 5 was confirmed by the 31P NMR
spectrum of the crude product, low crystallinity of
5 in recrystallization resulted in the low yield. The
broad 11B NMR signals of 3 and 4 at δB 2.2 and 13.7
ppm, respectively, in CDCl3 were shifted fairly up-
field from those of dimethylphenylborane (δB 77.6
ppm) [11] and phenylcatecholborane (δB 31.9 ppm)
[7b]. The N–B interaction in 3 and 4 is indicated by
the 11B NMR spectroscopy. The 31P NMR spectra of
3–5 (3: δP 35.2, 4: δP 34.1, 5: δP 37.1 ppm), similar
ular N–B coordination. As a result, in all cases, the
B1 atom has a tetracoordinated tetrahedral geom-
etry. The N–B distances of 3–5 differ, and they re-
flect the Lewis acidity of each boryl group. This
distance of 4 is a little shorter than observed in o-
˚
[(pyrrolidinylmethyl)phenyl]catecholborane (1.69 A
(average of two isomers)), suggesting a better
donor property of the phosphine imide moiety
˚
[14]. The P–N bond lengths (3: 1.6090(18) A, 4:
˚
1.622(2), 5: 1.6100(14) A) are similar to those
of the N-tert-butylaminotriphenylphosphonium bro-
˚
mide (1.621(3) A) [12] and a gold complex bearing
˚
the same phosphine imide moiety (1.6145(14) A) [15]
˚
rather than that of 1 (1.543(2) A), although they are
much shorter than the sum of the covalent radii
˚
(1.78 A) [16], supporting the contribution of the be-
taine structures such as 4ꢁ.
Compounds 3–5 are thermally stable at room
temperature, and they can be handled in the air.
Phosphine imide 1 is known to react with some
Heteroatom Chemistry DOI 10.1002/hc

Organoboranes with a Phosphine Imide
3
FIGURE 1 ORTEP drawings of 3 (left), 4 (center), and 5 (right) with thermal ellipsoid plot (50% probability). Selected bond
◦
˚
lengths (A) and bond angles ( ) of 3: N1–B1 1.666(3), N1–P1 1.6090(18), B1–C1 1.617(3), B1–C23 1.630(3), B1–C24 1.624(3),
C1–B1–C23 109.15(19), C1–B1–C24 108.38(18), C1–B1–N1 101.53(16), C23–B1–C24 113.11(19), N1–B1–C23 111.60(18),
N1–B1–C24 112.35(18). 4: N1–B1 1.584(4), N1–P1 1.622(2), B1–C1 1.601(4), B1–O1 1.519(4), B1–O2 1.499(4), C1–B1–O1
111.1(2), C1–B1–O2 113.1(2), C1–B1–N1 104.2(2), O1–B1–O2 103.7(2), N1–B1–O1 110.4(2), N1–B1–O2 114.5(2). 5: N1–B1
1.613(2), N1–P1 1.6100(14), B1–C1 1.604(2), B1–H1 1.131(17), B1–H2 1.167(19), C1–B1–H1 111.0(9), C1–B1–H2 110.2(9),
C1–B1–N1 103.23(11), H1–B1–H2 112.1(12), N1–B1–H1 109.9(9), N1–B1–H2 110.1(9).
lithium occurred at the boron center. In addition,
treatment of 4 with excess lithium aluminum hy-
dride gave the corresponding dihydroborane 5 with
the iminophosphorane moiety intact. In contrast,
treatment of 3 with neither excess tetrabutylammo-
nium fluoride in CDCl3 nor pyridine as the solvent
at room temperature caused any change in NMR
spectra after 12 h, although formation of a fluo-
roborate or a pyridine–borane complex was antic-
ipated. Similar attempts using 4 instead of 3 also
failed. These results can be reasonably understood
taking into consideration that the reactions with
lithium aluminum hydride can keep the N–B bond,
whereas the reaction with pyridine or tetrabutylam-
monium fluoride must accompany with the cleav-
age of the bond. It is good contrast with the reactiv-
ity of [2-(phenylazo)phenyl]catecholborane, which
is in equilibrium between an intramolecularly azo-
coordinated form and a pyridine-coordinated com-
plex form in the presence of pyridine [7a]. These
results indicate the intramolecular coordination of
the iminophosphorane moiety is stronger than that
of the azobenzene moiety.
SCHEME 2
carbonyl compounds [17], and the aza-Wittig re-
action readily proceeded by treatment of 1 with
benzaldehyde to give N-benzylidene-tert-butylamine
and triphenylphosphine oxide. However, borylated
phosphine imides 3–5 were unreactive to benzalde-
hyde. For example, heating a toluene solution of 4
with 6 equiv of benzaldehyde at 120◦C for 36 h re-
sulted in no reaction and quantitative recovery of
4 (Scheme 2). Treatment of 4 with methyl iodide
also resulted in the recovery of 4. The depressed
reactivity of the iminophosphorane moiety is as-
cribed to the steric congestion around the nitrogen
atom and/or decreased nucleophilicity due to the in-
tramolecular N–B coordination. In these cases, only
the boron moiety in the molecule can be converted
by a reagent that can react with both boron and
iminophosphorane moieties. In the preparation re-
action of 3 from the in situ generated dichloroborane
derivative, the substitution reaction with methyl-
In summary, we synthesized and elucidated
the crystal structure of the boron-substituted
iminophosphoranes. The boron atom was revealed
to be tetracoordinated by the intramolecular co-
ordination from the nitrogen atom. The N–B
Heteroatom Chemistry DOI 10.1002/hc

4
Kano et al.
coordination remained intact upon treating with
pyridine or fluoride ion. The intramolecular N–B
coordination is suggested to be stronger than that
of the 2-borylazobenzene derivatives. The coordi-
nation deactivated the reactivity of the iminophos-
phorane moiety very much, and the iminophospho-
rane moiety is persistent to hydrolysis, methylation,
the aza-Wittig reaction, and lithium aluminum hy-
dride reduction, whereas dichloro- and catechol-
borane moieties can be replaced by dimethyl- and
dihydroborane moieties, respectively. The reactivity
perturbation will be useful to stabilize some reactive
species.
filtration through Celite, and evaporation and re-
crystallization from ethanol gave 3 (148 mg, 28%)
as colorless crystals.
3: colorless crystals, mp 184.8–185.3◦C. 1H NMR
(400 MHz, CDCl3) δ 0.20 (s, 6H), 1.24 (s, 9H), 6.96–
7.03 (m, 2H), 7.31–7.35 (m, 1H), 7.43–7.57 (m, 7H),
7.81–7.86 (m, 4H). 11B NMR (128 MHz, CDCl3) δ 2.2.
13C NMR (126 MHz, CDCl3) δ 14.46 (br. s), 33.18 (d,
3 JPC = 5.8 Hz), 55.08 (s), 125.11 (d, JPC = 13.9 Hz),
125.98 (d, JPC = 19.3 Hz), 128.41 (d, 1 JPC = 123.6 Hz),
128.69 (d, JPC = 12.0 Hz), 128.93 (d, JPC = 14.6 Hz),
1
130.54 (d, JPC = 85.7 Hz), 130.76 (d, JPC = 2.8 Hz),
132.12 (d, JPC = 2.6 Hz), 133.24 (d, JPC = 10.7 Hz).
One 13C signal could not be detected. 31P NMR (162
MHz, CDCl3) δ 35.2. Anal. Calcd for C24H29BNP: C,
77.22; H, 7.83; N, 3.75. Found: C, 77.05; H, 7.92; N,
3.62.
EXPERIMENTAL
Formation of {o-C6H4[PPh2N(tert-Bu)]Li}2 ·
(Et2O)
Synthesis of o-C6H4[PPh2N(tert-Bu)]
B(1,2-benzenendiolato) (4)
A 1.09 M diethyl ether solution of methyllithium
(21.0 mL, 22.9 mmol) was added to a diethyl ether so-
lution (20 mL) of iminophosphorane 1 (3.87 g, 11.6
mmol) at room temperature and stirred for 2 h. After
evaporation of the solvent, crude solids were washed
with dry hexane under argon atmosphere and dried
under reduced pressure to give [o-C6H4{PPh2N(tert-
Bu)}Li]2·(Et2O) (2) (3.51 g, 93%) as pale yellow pow-
der. The powder was used as such for the following
reaction without further purification.
To a diethyl ether solution (20 ml) of 2 (896 mg,
1.19 mmol) was added dropwise a diethyl ether solu-
tion (10 ml) of chlorocatecholborane (0.404 g, 2.62
mmol) at room temperature. The reaction mixture
was stirred at room temperature for 17 h. After
evaporation of the solvent, the residue was sepa-
rated by column chromatography (Al2O3, ethyl ac-
etate/hexane = 1:3) to give 4 (680 mg, 65%) as
colorless solids. Recrystallization from ethanol gave
analytically pure crystals.
2: pale yellow powder, mp 152.6–153.4◦C. 1H
NMR (C6D6, 400 MHz) δ 0.80–1.00 (m, 6H), 1.14 (s,
18H), 3.03–3.19 (m, 4H), 7.04 (dd, J = 13.5, 8.0 Hz,
2H), 7.07–7.13 (m, 12H), 7.29 (dd, J = 11.8, 7.8 Hz,
4H), 7.89–8.00 (m, 8H), 8.25 (d, J = 4.8 Hz, 2H).
4: colorless crystals, mp 231–231.5◦C. H NMR
1
(400 MHz, CDCl3) δ 1.21 (s, 9H), 6.66–6.69 (m, 2H),
6.78–6.81 (m, 2H), 7.06–7.11 (m, 1H), 7.15–7.19 (m,
1H), 7.37–7.42 (m, 1H), 7.54–7.67 (m, 7H), 7.93–
7.99 (m, 4H). 11B NMR (128 MHz, CDCl3) δ 13.7.
1
13C{ H} NMR (C6D6, 125 MHz) δ 15.17 (s), 35.51 (d,
J = 13.0 Hz), 52.54 (d, J = 7.3 Hz), 65.71 (s), 124.52
(d, J = 15.8 Hz), 129.71 (d, J = 27.6 Hz), 130.40
(d, J = 1.4 Hz), 132.90 (d, J = 9.4 Hz), 133.92 (d,
J = 8.8 Hz), 136.51 (d, J = 72.0 Hz), 140.11 (d, J =
26.3 Hz), 146.38 (d, J = 141.9 Hz), 190.37 (s). One
signal overlapped with a peak of deuterated benzene.
31P NMR (C6D6, 202 MHz) δ 14.3.
1
13C{ H} NMR (125 MHz, CDCl3) δ 32.33 (d, JPC = 4.9
Hz), 55.24 (s), 108.96 (s), 117.93 (s), 125.83 (d, JPC
15.7 Hz), 127.08 (d, JPC = 91.5 Hz), 128.28 (d, JPC
=
=
14.0 Hz), 128.36 (d, JPC = 113.0 Hz), 129.17 (d,
JPC = 12.4 Hz), 129.54 (d, JPC = 14.8 Hz), 129.67
(d, JPC = 13.2 Hz), 132.68 (d, JPC = 3.3 Hz), 133.17
(d, JPC = 2.5 Hz), 133.42 (d, JPC = 10.7 Hz), 153.21
1
(s). 31P{ H} NMR (162 MHz, CDCl3) δ 34.1. GC-MS
Synthesis of o-C6H4[PPh2N(tert-Bu)]BMe2 (3)
m/z = 451. Anal. Calcd for C28H27BNO2P: C, 74.52;
To a diethyl ether solution (10 mL) of 2 (535 mg, 0.75
mmol), a 1.0 M hexane solution of trichloroborane
(1.50 mL, 3.05 mmol) was added dropwise at −78◦C.
The reaction mixture was stirred overnight, whereas
the temperature was raised to room temperature.
A 1.09 M diethyl ether solution of methyllithium
(2.8 mL, 3.05 mmol) was added to the reaction mix-
ture at room temperature and was further stirred
overnight. Evaporation of the solvent, extraction
of the resulting white solid with dichloromethane,
H, 6.03; N, 3.10. Found: C, 74.41; H, 6.15; N, 3.10.
Synthesis of o-C6H4[PPh2N(tert-Bu)]BH2 (5)
To a THF solution (15 mL) of lithium aluminum hy-
dride (150 mg, 3.95 mmol), a THF solution (5 mL)
of 4 (160 mg, 0.355 mmol) was added dropwise at
room temperature and stirred for 3 h. After quench-
ing with ethyl acetate, evaporation of the solvent,
extraction of the resulting solids with chloroform,
Heteroatom Chemistry DOI 10.1002/hc

Organoboranes with a Phosphine Imide
5
TABLE 1 Crystal Data for the Phosphine Imides 3–5.
3
4
5
Empirical formula
Formula weight
Crystal system
Space group
C24H29BNP
373.26
Monoclinic
P21/c
C28H27BNO2P
451.29
Monoclinic
P21/c
C22H25BNP
345.21
Monoclinic
P21/n
˚
a (A)
8.834(5)
9.364(5)
9.137(5)
˚
b (A)
14.353(7)
13.892(8)
12.526(6)
˚
c (A)
17.793(9)
103.1815(18)
18.024(10)
99.189(3)
2315(2)
4
1.295
0.0427
0.0859
0.879
16.541(9)
98.320(3)
1873.2(17)
4
1.224
0.0383
0.1107
1.048
β(deg)
3
˚
V (A )
2197(2)
4
1.129
0.0468
0.1394
1.095
Z
Dcalc (g/cm3)
R1 (I > 2σ(I ))
wR2 (all data)
GOF
drying with anhydrous magnesium sulfate, and fur-
ther evaporation gave white solids. The residue was
purified by column chromatography (Al2O3, ethyl ac-
etate/hexane = 1:4) to give 5 (50.1 mg, 41%) as color-
less solids. Recrystallization from ethanol gave ana-
lytically pure colorless crystals.
posited with the Cambridge Crystallographic Data
Centre (CCDC), U.K. with CCDC-698641 (3), CCDC-
698642 (4), and CCDC-698643 (5).
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5: colorless crystals, mp 179.0–180.0◦C. 1H NMR
(400 MHz, CDCl3) δ 1.29 (s, 9H), 3.26 (br, 2H),
7.03–7.06 (m, 2H), 7.32–7.35 (m, 1H), 7.48–7.52 (m,
4H), 7.55–7.58 (m, 2H), 7.64–7.67 (m, 1H), 7.78–
7.84 (m, 4H). 11B NMR (128 MHz, CDCl3) δ −9.0.
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13C NMR (75 MHz, CDCl3) δ 31.73 (d, JPC = 4.9
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Heteroatom Chemistry DOI 10.1002/hc

6
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Heteroatom Chemistry DOI 10.1002/hc