Synthesis, structure, and reactivity of a stabilized phosphiranylium salt

Article abstract of DOI:10.1002/anie.201000901

Chargedl Combining ^MeBABAR-Phos and methyl triflate affords an amino-stabilized phosphiranylium ion (see picture; C. gray, N blue, P orange), which undergoes various nucleophilic addition reactions to give P-substituted phosphiranes and an N-heterocyclic carbene stabilized phosphiranylium cation. (Figure Presented)

Full text of DOI:10.1002/anie.201000901

Angewandte
Chemie
DOI: 10.1002/anie.201000901
Charged Phosphiranes
Synthesis, Structure, and Reactivity of a Stabilized Phosphiranylium
Salt**
Helen Jansen, Florian B. Lꢀng, J. Chris Slootweg, Andreas W. Ehlers, Martin Lutz,
Koop Lammertsma,* and Hansjꢁrg Grꢂtzmacher*
In memory of Pascal Le Floch
Phosphiranes with a three-membered PC2 ring have unique
electronic properties.[1] Although their ring strain is smaller
than that in cyclopropanes, they are highly reactive and
display remarkable chemistry, such as [2+1] cycloreversions
to olefins and phosphinidenes [R P],[2] cationic ring-opening
ꢀ
polymerization to poly(ethylenephosphine),[3] and special
properties as ligands in transition-metal complexes for
catalysis.[4] Positively charged species are even more reactive.
Species such as A (phosphirenylium ion) or B (phosphirany-
lium ion), which may be viewed as p-complexes of P+ to
alkynes or alkenes,[5] respectively (Scheme 1), have never
been isolated.[6] A notable exception is the crystalline
Scheme 1. Cationic three-membered phosphorus heterocycles (A–C)
and BABAR-Phos derivatives D–G.
diphosphirenium ion C.[7]
The dibenzo[a,d]cycloheptatrienyl (trop) platform was
used for the synthesis of the highly strained dibenzophospha-
semibullvalene D,[8] very robust aminophosphiranes
(BABAR-Phos) E,[9] and the phosphiranium ion F (R = iPr,
R1 = tBu; Scheme 1).[10] Isolated examples of the latter are
likewise rare.[11]
transition-metal ions (PtII, RhI) bind to the phosphorus
center, albeit very weakly.[4b]
We now report on the synthesis of a sterically unshielded
BABAR-Phos derivative that reacts with methyl triflate, a
“hard” alkylation reagent, at the “hard” nitrogen center, to
give the cation G, which can be described as an intramolecular
amine complex of the elusive phosphiranylium ion B.
The synthesis of the MeBABAR-Phos 2 is straightforward,
as is shown in Scheme 2 (see the Supporting Information for
details). As previously reported,[4a] the reduction of the
amino(dichloro)trop phosphane 1 with magnesium turnings
(room temperature, 3 h) results in the rather clean formation
of MeBABAR-Phos 2 (73%; d31P = ꢀ149.0 ppm) as a result of
the spatial proximity of the PCl2 unit and the olefin. The
Because the lone pair at phosphorus has a very high s-
orbital character,[10] phosphiranes are generally reluctant to
be alkylated, oxygenated, or sulfurated, and with bulky
substituents R in E these reactions are suppressed. However,
[*] H. Jansen, Dr. J. C. Slootweg, Dr. A. W. Ehlers,
Prof. Dr. K. Lammertsma
Department of Chemistry and Pharmaceutical Sciences, VU Uni-
versity Amsterdam
De Boelelaan 1083, 1081 HV, Amsterdam (The Netherlands)
Fax: (+31)20-598-7488
1
characteristic low-frequency shifts of the H and 13C reso-
E-mail: k.lammertsma@few.vu.nl
nances of the HCCHtrop unit and the J(H,P) and J(C,P)
Dr. F. B. Lꢀng, Prof. Dr. H. Grꢁtzmacher
Department of Chemistry and Applied Biosciences, ETH-Hꢂngger-
berg
couplings in 2 (d1H = 2.55 ppm (2J(H,P) = 18.3 Hz); d
=
13C
Wolfgang-Pauli-Strasse, 8093 Zurich (Switzerland)
E-mail: gruetz@inorg.chem.ethz.ch
Dr. M. Lutz
Bijvoet Center for Biomolecular Research
Crystal and Structural Chemistry, Utrecht University (The Nether-
lands)
[**] This work was supported by the Council for Chemical Sciences of
the Netherlands Organization for Scientific Research (NWO/CW),
the ETH Zꢁrich, and the Swiss National Science Foundation. We
acknowledge the National Center for Computing Facilities (SARA)
and NWO/NCF for computer time and the VU University Amster-
dam for a travel grant (H.J.). We thank Dr. M. Smoluch for the exact
mass determination.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000901.
Scheme 2. Synthesis of 2 and 3.
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Communications
[a]
24.1 ppm (1J(C,P) = 41.7 Hz)) clearly indicate its formation
(vs. d1H = 6.83 ppm and d13C = 130.9 ppm in 1).
ꢀ
Table 1: Characterization of the P N bond with AIM.
H(rc)[b]
Q(P)
Q(N)
ꢀ
P N Bond Length
Reaction of MeBABAR-Phos 2 with methyl triflate in
toluene (room temperature, 2 h) resulted exclusively in N-
alkylation (91%; d31P = ꢀ83.6 ppm; Scheme 2). The structure
of the novel salt 3 was established by X-ray crystal structure
determination (Figure 1)[12] and displays a remarkable long
2
1.727 ꢃ
1.885 ꢃ
1.660 ꢃ
ꢀ0.912
ꢀ0.670
ꢀ1.102
1.34
1.09
2.47
ꢀ1.33
ꢀ1.11
ꢀ1.41
3
FMe
[a] The B3PW91/6-311+G(d,p) calculated structures were used; [b] the
bond critical point rc corresponds to a saddle point of the electron
density in the bonding region.
ꢀ
P N bond (1.845(1) ꢀ) compared with that in the neutral
analogues E (1.73–1.74 ꢀ)[4a,13] and phosphiranium ion F
(1.628(4) ꢀ).[10]
respect to 2, N-methylation diminishes both the negative
charge at N and the positive charge on P. That is, 3 contains
ꢀ
the longest, weakest, and least-polar P N bond (DQ(N,P) =
2.20 e [vs. 2.67 e (2) and 3.88 e (FMe)].
From the obtained energy densities (H(rc); Table 1), it can
Me
ꢀ
be derived that the P N bond in 2 and F has a strong
covalent character.[16] However, the decreased energy density
ꢀ3
ꢀ
(H(rc) = ꢀ0.670 hartreeꢀ ) of the P N bond in 3 indicates a
significant contribution of resonance structure 3B to the
electronic ground state of 3, which may be viewed as an
intramolecular donor–acceptor N!P complex[17] of an
amino-stabilized phosphiranylium ion (B).
Figure 1. Displacement ellipsoid plot (50% probability) of 3. The OTfꢀ
anion is omitted for clarity. Selected bond lengths [ꢃ] and angles [8]:
P1–N1 1.8449(14), P1–C4 1.8532(18), P1–C5 1.8593(17), N1–C1
1.541(2), N1–C16 1.509(2), N1–C17 1.510(2), C4–C5 1.523(2); N1-P1-
C4 97.68(7), N1-P1-C5 97.95(7), C5-P1-C4 48.44(7); S(P): 244.18.
The reactivity of the amino phosphiranylium salt 3 was
investigated as well. Addition of methyl lithium at ꢀ788C
ꢀ
resulted in P-alkylation under breaking of the P N bond
yielding the P-methyl phosphiranes endo-4 (d31P =
DFT calculations at the B3PW91/6-311 + G(d,p) level of ꢀ173.3 ppm) and exo-4 (d31P = ꢀ205.7 ppm; 4:1 ratio;
theory were performed to understand and compare the Scheme 3) quantitatively.[18] The reaction of 3 with thiolate
electronic structure and reactivity of 2, 3, and the non- [Li(SPh)] gives exclusively the sterically less encumbered
observed P-methylated product FMe [14]
.
The HOMO of 2 isomer exo-5 (d31P = ꢀ132.8 ppm), which is stable in solution
(ꢀ5.81 eV) is mainly located on the nitrogen atom, while the and in the solid state.
HOMOꢀ1 (ꢀ6.35 eV; Figure 2) can be attributed to the “lone
pair” located on the phosphorus atom. This finding is in
accord with the preferred alkylation at nitrogen. Further, the
N-alkylated cation 3 is 9.5 kcalmolꢀ1 more stable than the
computed P-alkylated derivative FMe
.
Figure 2. HOMO (left) and HOMOꢀ1 (right) of MeBABAR-Phos 2.
Scheme 3. Reactivity of the amino phosphiranylium salt 3.
ꢀ
The nature of the P N bond was studied by analyzing the
electron density by using the theory of atoms in molecules
(AIM)[15] to investigate the relative weight of the resonance
Solutions of 4 can be stored at ꢀ188C for days, but within
structures 3A (ammonium salt with the positive charge on N) 24 h at room temperature clean fragmentation to the cyclic
and 3B (intramolecular amino-stabilized phosphiranylium phosphinidene oligomer (PMe)5 (6; Scheme 3)[19] and (5-H-
cation with the positive charge localized on the PC2 ring; dibenzo[a,d]cyclohepten-5-yl)-dimethylamine (trop-NMe2)
Scheme 2).[5e] The atomic charges of the basins for nitrogen occurs. Addition of diphenyldisulfide to a freshly prepared
Q(N) and phosphorus Q(P) of 2, 3, and FMe are given in solution of the phosphiranes 4 or 5 resulted in the quantitative
Table 1. As expected, the positive charge at the P atom in formation of the phosphonodithious acids
7
(d =
31P
phosphonium-type ion FMe is the highest (+ 2.47 e). With 85.2 ppm)[20] and
8
(d31P = 130.8 ppm),[21] respectively
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(Scheme 3). Formally, these reactions correspond to the
In conclusion, our computational and experimental results
indicate that [N,N-dimethyl-BABAR-Phos]OTf and
[(Me2N-trop)P(IiPr2Me2)]OTf 10 may be regarded as the
first isolated examples of intra- and intermolecular amino- or
carbene-stabilized phosphiranylium salts, respectively.
[22]
ꢀ
expulsion of a phosphinidene [R P], but at this stage of
3
ꢀ
our investigations, we assume a [R P] transfer process with-
out the intermediacy of free phosphinidenes as our calcu-
lations predict that this process costs approximately 50 kcal
molꢀ1.
Received: February 12, 2010
Revised: April 26, 2010
Published online: July 2, 2010
Interestingly, 3 also reacts with neutral nucleophiles such
as N-heterocyclic carbenes (NHCs; Scheme 3). Namely,
addition of IiPr2Me2 9 resulted in the NHC-stabilized
phosphiranylium cation 10 (d31P = ꢀ214.3 ppm).[23,24]
The molecular structure of 10, established by X-ray crystal
structure analysis (Figure 3),[12] may be viewed as an olefin
carbene PI cation. The sum of bond angles at the phosphorus
Keywords: carbenes · cations · heterocycles · phosphiranes ·
.
phosphorus
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Selected bond lengths [ꢃ] and angles [8]: P1–N1 2.581(2), P1–C4
1.908(3), P1–C5 1.896(2), P1–C18 1.881(3), N1–C1 1.478(3), N1–C16
1.467(3), N1–C17 1.466(3), N2–C18 1.350(3), N2–C19 1.384(3), N3–
C18 1.351(3), N3–C20 1.391(3), C4–C5 1.500(3), C19–C20 1.354(4);
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ꢀ
sterically demanding substituents. The strength of the P
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ꢀ
in the P C bond dissociation energies (BDEs), which are
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ꢀ
ꢀ0.763 hartreeꢀ ) is comparable to that of the P N bond in
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5487

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[18] It is unclear whether exo-4 and endo-4 are in a very slow
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