Investigating the phospholylcarbene to phosphinine conversion

Article abstract of DOI:10.1002/ejic.201100098

A DFT study suggests that the conversion of arsolylcarbenes into arsenines as described by Maerkl proceeds via arsabenzvalenes. A similar conversion does not work with phospholylcarbenes. However, we have found that, in some cases, 1-diazoalkylphosphole s

Full text of DOI:10.1002/ejic.201100098

SHORT COMMUNICATION
DOI: 10.1002/ejic.201100098
Investigating the Phospholylcarbene to Phosphinine Conversion
Hui Chen,[a] Jin Li,[a] Huaiqiu Wang,[a] Hui Liu,[a] Zheng Duan,*[a] and
François Mathey*[a,b]
Keywords: Heterocycles / Phosphorus / Arsenic / Carbenes / Density functional calculations
A DFT study suggests that the conversion of arsolylcarbenes
into arsenines as described by Märkl proceeds via arsa-
benzvalenes. A similar conversion does not work with phos-
pholylcarbenes. However, we have found that, in some cases,
1-diazoalkylphosphole sulfides are converted into phosphin-
ine sulfides whose in situ reduction by triphenylphosphite
affords the dicoordinate phosphinines.
The discovery that phosphinines can play a significant benzvalene and the transition state are shown in Figure 1.
role in homogeneous catalysis[1] has reignited the interest in Our data on arsabenzvalene (structure and energy) are close
their synthesis and chemistry.[2] In this context, our atten- to those computed by Sastry.[7]
tion was drawn to an early work of Märkl describing the
conversion of arsolylcarbenes into arsenines.[3] We were
puzzled by the fact that this chemistry has no reported
equivalent for nitrogen and phosphorus. As a first step, we
decided to study this transformation by DFT computations
at the B3LYP/6-311+G(d,p) level[4] in order to get a better
understanding of the reaction pathway. The singlet arsolyl-
carbene has a planar structure with a As=C double bond
(1.773 Å) and is closely related to the structure of phos-
phanylcarbenes.[5] Its structure forbids any interaction be-
tween the carbene and the diene orbitals. The triplet arso-
lylcarbene has a pyramidal arsenic atom (sum of the angles
293.5º) and a single As–C bond (1.888 Å). The plane of the
carbene is orthogonal to the plane of the arsole. This triplet
state lies only 3.7 kcalmol–1 above the singlet. This struc-
Figure 1. Computed structure of arsabenzvalene I and the transi-
ture is similar to the structure of cyclopentadienylcarbene
that yields benzvalene by intramolecular [1+4] carbene +
diene cycloaddition.[6] Thus, we suggest that this easily ac-
cessible triplet is readily converted into arsabenzvalene.
Computations show that this arsabenzvalene is a genuine
local minimum (no negative frequency), more stable than
the triplet arsolylcarbene by 55 kcalmol–1. It is transformed
into arsenine via a transition state (one negative frequency)
that is 35.3 kcalmol–1 higher in energy. Both the arsa-
tion state II between I and arsenine. Significant distances [Å] and
angles [º]: I: As–C1 2.168, As–C9 1.972, C1–C9 1.473, C4–C3
1.482, C3–C2 1.348; C1–As–C4 62.69, C1–As–C9 41.36. II: As–C1
2.023, As–C2 1.861, As–C3 2.836, C1–C2 2.082, C2–C3 1.483, C3–
C4 1.353, C4–C5 1.434, C5–C1 1.367; C1–As–C2 64.67, C1–As–
C3 60.85, C2–As–C3 28.16.
The case of nitrogen is different. The triplet pyrrolylcar-
bene is higher in energy than the singlet by 21.8 kcalmol–1
and, thus, difficult to reach. Besides, its structure is dif-
ferent. The plane of the carbene is orthogonal to the plane
of the ring, as in the arsenic case, but nitrogen is planar
(sum of angles at N 359.8º), therefore blocking its transfor-
mation into azabenzvalene.
As expected, the situation for phosphorus lies halfway
between those for nitrogen and arsenic. The triplet phos-
pholylcarbene is pyramidal at P (sum of angles 313.2º) and
lies higher in energy than the singlet by 12.8 kcalmol–1. It
is, of course, difficult to predict whether its transformation
into phosphabenzvalene is possible. Our structural data are
[a] Chemistry Department, International Phosphorus Laboratory,
Zhengzhou University,
Zhengzhou 450052, P. R. China
[b] Division of Chemistry & Biological Chemistry, School of
Physical & Mathematical Sciences,
Nanyang Technological University,
21 Nanyang Link, 637371 Singapore
E-mail: duanzheng@zzu.edu.cn
fmathey@ntu.edu.sg
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100098.
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Investigating the Phospholylcarbene to Phosphinine Conversion
Figure 2. Computed structures of singlet and triplet heterolylcarbenes. Significant distances [Å] and angles [º]: Singlet As: As–C4 1.899,
As–C1 1.931, As–C9 1.773; C1–As–C4 89.81, As–C9–H10 107.91. Singlet P: P–C1 1.778, P–C4 1.799, P–C10 1.642; C1–P–C4 94.11, P–
C10–H11 114.40. Singlet N: N–C4 1.419, N–C1 1.426, N–C9 1.332; C1–N–C4 106.98, N–C9–H10 105.65. Triplet As: As–C1=As–C4
1.954, As–C9 1.888; C1–As–C4 86.03, C1–As–C9=C4–As–C9 103.74, As–C9–H11 131.30. Triplet P: P–C1=P–C4 1.808, P–C10 1.741;
C1–P–C4 91.36, C1–P–C10=C4–P–C10 110.90, P–C10–H11 132.43; Triplet N: N–C1=N–C4 1.399, N–C9 1.365; C1–N–C4 108.08, C1–
N–C9=C4–N–C9 125.86, N–C9–H10 127.30.
summarized in Figure 2. The structures of the triplets fol- of Divisia[8] reporting the thermal conversion of Ph2P(S)–
low the same trend as those of the corresponding heteroles C(N2)–C(O)Ph into PhP(S)=C(Ph)–C(O)Ph by migration
(exocyclic CH replaced by CH3), while the increasing pyr- of Ph from phosphorus to the carbene center. DFT compu-
amidal inversion barrier of the heteroatom from N to As tations showed us that the decomposition of a phosphole
reduces the singlet–triplet gap. The high tendency of As to sulfide, such as 2, cannot yield the corresponding carbene,
be pyramidal logically destabilizes the planar singlet state. which does not correspond to a local minimum on the po-
In order to check the fate of phospholylcarbenes, we pre- tential energy surface. The product is, in fact, the corre-
pared a diazo precursor 1, as shown in Scheme 1. This pre- sponding phosphinine sulfide 3 [Equation (1)], whose struc-
cursor proved to be unstable upon standing at room tem- ture is shown in Figure 3.
perature, but its decomposition does not yield the corre-
sponding phosphinine. Only a mixture of ill-defined decom-
position products was observed. On the contrary, the corre-
(1)
sponding P-sulfide is stable and has been fully charac-
terized. At this point, our attention was drawn to a paper
Scheme 1.
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H. Chen, J. Li, H. Wang, H. Liu, Z. Duan, F. Mathey
SHORT COMMUNICATION
Figure 3. Computed structure of phosphinine sulfide 3. Significant
distances [Å] and angles [º]: P–S 1.936, C4–P 1.723, C3–C4 1.384,
C2–C3 1.401, C1–C2 1.386, C1–C9 1.405, C9–P 1.741, C9–C10
1.487; C4–P–C9 105.84.
Figure 4. X-ray crystal structure of dihydrophosphinine sulfide 6.
Significant distances [Å] and angles [º]: C1–P1 1.798(3), C1–C2
1.497(4), C2–C4 1.340(4), C4–C6 1.448(4), C6–C7 1.347(4), C7–P1
1.790(3), P1–O1 1.587(2), P1–S1 1.9321(12); C1–P–C7 101.48(14).
Experimental Section
Phosphinine sulfides are highly reactive species that can-
not be isolated except when stabilized by bulky substitu-
ents.[9] The practical use of this transformation needed the
addition of a mild reducing agent whose role would be to
reduce only the phosphinine sulfide without altering the
phosphole sulfide precursor. We were happy to find that
Synthesis of 2-Ethoxycarbonyl-4,5-dimethylphosphinine (5a):
A
solution containing ethyl diazoacetate (105 μL) in dry Et2O (6 mL)
and dry THF (2 mL) was cooled to –110 °C, and nBuLi (1 mmol,
0.35 mL of a 2.8 m solution) was added dropwise to the mixture.
After stirring for 30 min, a solution containing 1-cyanol-3,4-di-
methyl phosphole (108 mg, 1 mmol)
[12]
in dry THF (2 mL) was
triphenyl phosphite is able to perform this task (but triethyl added to the mixture at –78 °C, which was then warmed to 0 °C
(phosphole 1 is not stable at room temperature, even under argon).
Sulfurization was performed overnight by addition of sulfur pow-
der (38.4 mg, 1.2 mmol) at 0 °C. After elimination of the solvent
under vacuum, the residue was chromatographed with petroleum
ether/CH3CO2Et (5:1) as the eluent to yield a yellow solid, 4a
(192 mg, 75%) [δ31P (CDCl3) = 36.0 ppm].
phosphite is not effective) (Scheme 1).
The phosphinine 5a was purified by chromatography and
identified by comparison of its spectroscopic data with
those reported in the literature.[10] Our attempts to general-
ize this reaction met with a mixed success. The replacement
of the CO2Et substituent by SiMe3 proved to be a failure.
The trimethylsilyl analogue of 1 displays a better stability
but its sulfide does not yield the corresponding phosphinine
when following the procedure in Scheme 1. On the contrary,
we can modify the substitution pattern of the ring carbons.
The necessary precursors are synthesized as shown in
Scheme 1. The [1,5] shift of R is a classical reaction of phos-
phole chemistry.[11] We also wanted to establish the interme-
diacy of the phosphinine sulfides as proposed in Equa-
tion (1). This was achieved by heating 4a in the presence of
methanol as shown in Equation (2).
A mixture of 4a (192 mg, 0.75 mmol) and triphenyl phosphite
(263 μL, 0.75 mmol) in xylene was heated for 5 h at 120 °C. After
elimination of the solvent under vacuum, the residue was chro-
matographed with hexane/CH2Cl2 (3:1) as the eluent to give a col-
orless solid, 5a (45 mg, 30%).
Spectral Characterization of Phosphinines (5b,c)
5b: 31P NMR (CDCl3): δ = 205.2 ppm. 1H NMR (CDCl3): δ = 1.40
3
5
(t, JHH = 7.1 Hz, 3 H, Me), 2.26 (d, JHP = 1.8 Hz, 3 H, C4-Me),
2.50 (d, JHP = 3.3 Hz, 3 H, C5-Me), 4.41 (q, JHH = 7.2 Hz, 2 H,
4
3
OCH2), 7.13–7.33 (m, 5 H, Ph), 8.42 (d, JHP = 4.2 Hz, 1 H, C3-
3
H) ppm. 13C NMR (CDCl3): δ = 13.04 (s, Me), 17.86 (s, Me), 22.13
(s, Me), 60.09 (s, OCH2), 125.94 (CH Ph), 126.75 (CH Ph), 128.22
(d, JCP = 9.0 Hz, CH Ph), 136.80 (d, JCP = 13.3 Hz), 139.35 (d,
JCP = 13.7 Hz), 141.49 (d, JCP = 26.6 Hz), 143.53 (d, JCP
=
10.9 Hz), 152.48 (d, 1JCP = 51.0 Hz, Cα), 166.96 (d, 2JCP = 24.2 Hz,
1
(2)
CO2), 170.30 (d, JCP = 49.1 Hz, Cα) ppm. MS (ESI): m/z = 272.8
(M+).
5c: 31P NMR (CDCl3): δ = 209.3 ppm. 1H NMR (CDCl3): δ = 1.41
(t, 3JHH = 7.1 Hz, 3 H, Me), 2.48 (m, 6 H), 4.42 (q, 3JHH = 7.1 Hz,
3
2 H, OCH2), 6.90–7.20 (m, 5 H, Th), 8.40 (d, JHH = 4.2 Hz, 1 H,
The structure of 6 was established by X-ray analysis (Fig-
ure 4).
Although the phosphinine yields are modest, interesting
substitution patterns can be obtained and the procedure is
simple. This approach is thus a valuable addition to the
phosphinine synthetic toolbox.
C3-H) ppm. 13C NMR (CDCl3): δ = 13.28 (s, Me), 18.37 (s, Me),
22.44 (d, JCP = 2.0 Hz, Me), 60.43 (s, OCH2), 122.71 (CH-Th),
122.86 (CH-Th), 123.48 (CH-Th), 126.83 (CH-Th), 127.33 (d, JCP
2
= 10.1 Hz, CH-Th), 136.00 (s), 137.51 (d, JCP = 13.1 Hz, C3),
137.68 (s), 139.77 (d, JCP = 13.1 Hz), 141.85 (d, JCP = 31.4 Hz),
1
145.41 (d, JCP = 10.7 Hz), 153.10 (d, JCP = 51.5 Hz, Cα), 161.09
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Investigating the Phospholylcarbene to Phosphinine Conversion
1
(d, JCP = 49.9 Hz, Cα), 166.86 (d, 2JCP = 23.9 Hz, CO2) ppm. MS
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CCDC-800546 (compound 6) contains the supplementary crystal-
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charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): Complete experimental section.
Acknowledgments
The authors thank the National Natural Science Foundation of
China (No. 20702050, 21072179), the Scientific Research Founda-
tion for the Returned Overseas Chinese, Zhengzhou University and
Nanyang Technological University in Singapore for the financial
support of this work.
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Received: January 28, 2011
Published Online: February 22, 2011
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