Stereoselective synthesis, structural aspects, and NMR properties of stable C-fluorinated phosphaalkenes

Article abstract of DOI:10.1039/c3cc45237a

We report a stereoselective route to synthesize (E)-2-bromo-2-fluoro-1-(2, 4,6-tri-t-butylphenyl)-2-phosphaethene. Configurationally and conformationally controlled 2-fluoro-1-phosphaethene [Mes*PC(F)H; Mes* = 2,4,6-tBu₃C₆H₂] and 2-fluoro-1,3- diphosphapropene [Mes*PC(F)-PPh₂] were synthesized and the NMR data, including relaxation parameters, were collected.

Full text of DOI:10.1039/c3cc45237a

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Stereoselective synthesis, structural aspects, and NMR
properties of stable C-fluorinated phosphaalkenes†
Cite this: Chem. Commun., 2013,
49, 9221
Shigekazu Ito,* Takeshi Nakagawa and Koichi Mikami
Received 11th July 2013,
Accepted 8th August 2013
DOI: 10.1039/c3cc45237a
www.rsc.org/chemcomm
We report a stereoselective route to synthesize (E)-2-bromo-2-fluoro-1- fluorinated phosphaalkenes may yield useful compounds for
(2,4,6-tri-t-butylphenyl)-2-phosphaethene. Configurationally and many different applications.
Q
conformationally controlled 2-fluoro-1-phosphaethene [Mes*P C(F)H;
In this study we report the stereospecific synthesis of
Q
Mes* = 2,4,6-tBu3C6H2] and 2-fluoro-1,3-diphosphapropene [Mes*P
sterically encumbered 2-bromo-2-fluoro-1-(2,4,6-tri-t-butylphenyl)-
C(F)-PPh2] were synthesized and the NMR data, including relaxation 1-phosphaethene (1) using dibromofluoromethyllithium14 as
parameters, were collected.
the methylene source. We discuss the stereoselectivity based on
density functional theory (DFT) calculation of a possible inter-
The utility of low-coordinate phosphorus compounds has been mediate structure. We used 1 to install a diphenylphosphino
widely recognized since the pioneering studies of the kinetic group and produce the 2-fluoro-1,3-diphosphapropene skeleton
stabilization of multiple phosphorus bonds in compounds such as and for the stereoselective construction of a 2-fluoro-1-phos-
phosphaalkene (methylenephosphine)1 and diphosphene (phos- phaethene.11 The low coordination of phosphorus and the
phobenzene) were reported.2 For example, most phosphaalkenes3,4 combination of sp2- and sp3-phosphorus are useful for catalysis,15
possess strong p-accepting characteristics, which enable them to be whereas NMR resonance and spin–spin coupling properties of
used for transition-metal catalysis1,5,6 and as organoelectronic 31P and 19F nuclei are interesting for applications such as
materials.7,8 To develop these low-coordinated phosphines, the quantum information processing.16 Therefore, we also present
installation of various elements and functional groups into the a study of the NMR properties of the fluorinated phosphaalkenes
kinetically stabilized phosphaalkene moiety has proved to be a and a conformational analysis of the 2-fluoro-1,3-diphosphapro-
promising approach. We and others have previously shown that pene skeleton.15b
sterically encumbered 2-halo- and 2,2-dihalo-1-phosphaethenes
Two portions of butyllithium were added slowly to 2,4,6-tri-t-
[Mes*PQCXX0; Mes* = 2,4,6-tBu3C6H2, X, X0 = Cl, Br, I] are useful butylphenylphosphonous dichloride (Mes*PCl2) and tribromo-
materials for synthesizing stable low co-ordinated phosphorus fluoromethane in THF–Et2O (2:1) at ꢀ130 1C (1st: 0.067 mL minꢀ1
,
compounds via lithiation9–11 and cross-couplings.12 However, 2nd: 0.056 mL minꢀ1) (Scheme 1, see ESI† for details) to generate
2-fluorinated bulky phosphaalkenes, which were first reported highly reactive dibromofluoromethyllithium,14 which produced
by Appel and co-workers,11 have hardly been studied in terms of the chloro(dibromofluoromethyl)phosphine intermediate via
reactivity and utilization as building blocks for functional nucleophilic substitution. Subsequent halogen–metal exchange
phosphaalkenes. Installing fluorine atoms into any organic and beta-elimination of LiX (X = Br, Cl) finally yielded (E)-2-bromo-
structure is an attractive strategy for developing useful molecular 2-fluoro-1-(2,4,6-tri-t-butylphenyl)-2-phosphaethene (1) stereo-
entities for biologically active compounds, medicines, and materials selectively. The structure of 1 was unambiguously characterized
for electronics because unique steric and electronic properties are by various spectroscopic measurements and X-ray crystallography
often obtained and because fluorine is NMR active (100% natural (Fig. 1, see ESI†). When THF was used as the solvent and
abundance of the I = 1/2 isotope).13 Therefore, investigating the reaction temperature was relatively raised (ꢀ100 1C), the
Department of Applied Chemistry, Graduate School of Science and Engineering,
Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8552, Japan.
E-mail: ito.s.ao@m.titech.ac.jp; Fax: +81 3 5734 2143
† Electronic supplementary information (ESI) available: Details of the experi-
mental procedure, spectral data, DFT calculation data, copies of NMR spectra and
relaxation analysis. CCDC 948834 and 948835. For ESI and crystallographic data
Scheme 1 Stereoselective synthesis of 1.
in CIF or other electronic format see DOI: 10.1039/c3cc45237a
c
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Fig. 1 Molecular structure of 1 (50% probability ellipsoids). Hydrogen atoms are
omitted for clarity. Bond lengths (Å) and angles (1): P–C1 1.674(3), P–C2 1.846(3),
C1–Br 1.893(3), C1–F 1.332(3), C1–P–C2 98.5(1), P–C1–F 128.9(2), P–C1–Br
121.0(2), Br–C1–F 110.1(2).
Fig. 3 Molecular structure of 4 (50% probability ellipsoids). Hydrogen atoms
are omitted for clarity. The p-t-butyl group is disordered, and the atoms of
predominant occupancy factor (0.77) are shown. Bond lengths (Å) and angles (1):
P1–C1 1.673(1), P2–C1 1.821(2), C1–F 1.370(2), P1–C2 1.847(2), P2–C20 1.833(2),
P2–C26 1.833(2), C1–P1–C2 100.81(7), P1–C1–F 123.2(1), P1–C1–P2 119.19(9),
P2–C1–F 117.6(1), C1–P2–C20 102.60(7), C1–P2–C26 99.25(7), C20–P2–C26
103.31(7).
stereoselectivity was reduced and a 1 : 1 mixture of 1 and its
geometrical isomer (10) was obtained.
To examine the stereoselective formation of 1 under the
carefully controlled conditions, we performed DFT calculations
for the unobservable reaction intermediates at the CAM-B3LYP/
6-31G(d) level (see ESI†).17,18 Fig. 2 shows the relative energy
of the three possible conformers of the putative intermediate
(Int-1–3). The most stable conformer, Int-1, would afford the
E-isomer (1) via butyllithium-promoted anti-elimination.
Unstable conformer Int-2 also would provide the E-isomer via
rotation along the P–C(sp3) single bond. The conformer, Int-3,
which has a slightly higher energy than Int-1, would yield the
Z-isomer (10) (Fig. 2). The small energetic difference estimated
by DFT would explain the experimental observation that higher
temperatures reduced the stereoselectivity.
2-Bromo-2-fluoro-1-phosphaethene (1) was then used for a
further transformation involving regioselective halogen–metal
exchange. A THF solution of 1 was allowed to react with
2 equivalents of butyllithium at ꢀ78 1C, and subsequently the
lithium intermediate 2 was treated with methanol to yield 311
exclusively (Scheme 2a). The reaction using 1 equivalent of
butyllithium produced lower yields because of undesirable side
reactions such as alkylation of 2. The dominant reactivity of the
bromine in 1 and the configurational stability of 2 at low
temperatures enabled selective formation of 3. Furthermore,
reaction of 2 with chlorodiphenylphosphine yielded the corre-
sponding 1,3-diphosphapropene 4 in a moderate yield. In the
phosphanation of 2, the use of tetramethylethylenediamine as
an additive improved the yield (Scheme 2b) (see ESI†).
The structure of 4 was unambiguously determined by X-ray
crystallography and Fig. 3 shows an ORTEP drawing of 4 (see
ESI†). The typical metric parameters were comparable to most
kinetically stabilized phosphaalkenes.3,9 The Mes* aryl group was
almost perpendicular to the PQC plane and the conformation of
the diphenylphosphino group produced the Cs-type diphospha-
propene skeleton. This conformation was very similar to that of
the chloro derivative (5).15e, f No substantial intermolecular inter-
action was observed in the crystalline state.
To understand the conformation of the Mes*-substituted
1,3-diphosphapropene, we performed DFT calculations for 4
and derivatives 515e, f and 615b (Table 1, see ESI†). In the fluorine
(4) and chlorine (5) derivatives, the Cs symmetry conformation
(Cs-form) was the most stable structure, which corresponds to
the observed results. On the other hand, the C1 conformation
(C1-form) is the most stable for the silyl derivative (6). We
previously studied the primitive 1,3-diphosphapropene skeleton
Fig. 2 Possible structures of the intermediary chlorophosphine in Scheme 1.
Numbers in italics denote relative energies at the CAM-B3LYP/6-31G(d) level.
Table 1 Conformational properties of 1,3-diphosphapropenes
Erela/kcal molꢀ1
Compound
R
Cs-form
C1-form
4
5
6
F
Cl
SiMe3
0.00
0.00
3.38
2.50
0.96
0.00
Scheme 2 Generation of 2 via halogen–metal exchange followed by (a) protonation
a
and (b) phosphanation.
CAM-B3LYP/6-31G(d).
c
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Table 2 NMR properties of 3a
This work was supported in part by Grants-in-Aid for
Scientific Research (KAKENHI; No. 22350058 and 23655173)
from the Ministry of Education, Culture, Sports, Science, and
Technology, Japan, Sumitomo Chemical Co., Ltd., and Nissan
Chemical Industries, Ltd. The authors thank Prof. Hiroharu
Suzuki and Dr Masataka Oishi of Tokyo Institute of Technology
for their assistance with X-ray crystallographic analysis.
Nuclei
d/ppm
2JFXb/Hz
2JPH/Hz
T1/s
T2/s
31P
1H
19F
144.3
7.4c
ꢀ68.6
135
73
—
58
—
—
2.710
0.454
1.040
1.017
1.430
1.164
a
b
c
In CDCl3. X = P, H. PQC–H proton.
Table 3 NMR properties of 4a
Notes and references
Nuclei
d/ppm
2JPP/Hz
2JPF/Hz
T1/s
T2/ms
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31P(sp2)
31P(sp3)
19F
204.4
ꢀ3.9
241
—
—
145
20
—
0.863
3.880
0.449
550
943
317
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a
In CDCl3.
and concluded that the C1 conformation is predominant, although
the energetic difference is quite small (1.1–1.7 kcal molꢀ1).15b Thus,
the conformation can be easily changed by the conditions. In the
case of 4 and 5, the steric effects of the Mes* substituent and
the phenyl groups may stabilize the Cs conformation. In contrast,
the trimethylsilyl derivative maintains the C1 conformation,
because of the long C–Si bond that minimizes steric encumbrance
around the diphosphapropene skeleton.
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resonance frequencies, coupling constants, and relaxation
properties. Tables 2 and 3 summarize the 31P, 19F, and
1H NMR data for 3 and 4, respectively. The spin–lattice relaxation
(T1) and spin–spin relaxation (T2) were measured by the inversion
recovery technique19 and the Carr–Purcell–Meiboom–Gill
method,20 respectively. The 31P chemical shifts of the sp2
phosphorus atoms had higher field resonances compared with
most similar non-fluorinated phosphaalkenes3 due to the +Ip or +R
effect.11,14 In contrast, the 19F chemical shifts are comparable to the
corresponding fluorinated alkenes.14 The 2JPF parameters are com-
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is characterized by larger 31P–31P spin–spin coupling,5 the structure
of 4 is maintained even in the solution.
The T2 parameters for 3 are long; the T1 parameter of
the QCH proton of 3 is shorter than that of T2. Although the
exact reason is unknown, the characteristics of the relaxation
may relate to the structure of the Mes*PQC–H framework. The
sp2–31P and 19F nuclei of 4 exhibited smaller relaxation para-
meters than those of 3, because of the larger molecular size.
In conclusion, we have developed a stereoselective synthesis
of novel C-fluorinated phosphaalkenes by using a fluorine-
containing methyllithium reagent under optimised reaction
conditions. DFT calculations were used to explain the stereo-
selectivity and conformational characteristics of the C-fluori-
nated phosphaalkenes. The NMR properties of the fluorinated
phosphaalkenes were intensively analyzed. We are currently
investigating the coordination chemistry of the fluorinated
phosphaalkenes with the aim of storing qubit states and
exploring unique metal catalyst systems.
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Chem. Commun., 2013, 49, 9221--9223 9223