Transition-metal-free boron-carbon bond activation: Insertion of an NNP fragment into a boron-carbon bond

Article abstract of DOI:10.1002/ejic.200700895

The influence and reactivity of fluorine-containing Lewis acids such as neat BF₃, BF₃·OEt₂, and B(C ₆F₅)₃ on Me₃SiCl elimination in (Me₃Si)₂N-N(SiMe₃)-PCl₂ was investigated. The reaction with B(C₆F₅)₃ resulted in the formation of a novel diazadiphosphetidine with a phosphorus(III) atom attached to a pentafluorophenyl group, which can be regarded as a formal NNP fragment insertion into a B-C bond. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700895

SHORT COMMUNICATION
DOI: 10.1002/ejic.200700895
Transition-Metal-Free Boron–Carbon Bond Activation: Insertion of an NNP
Fragment into a Boron–Carbon Bond
Markus Kowalewski,[a] Burkhard Krumm,[a] Peter Mayer,[a] Axel Schulz,*[b,c] and
Alexander Villinger[b]
Dedicated to the memory of Nils Wiberg
Keywords: Azaphosphole / Diazadiphosphetidine / Insertion / Lewis acids / Boranes / X-ray elucidation
The influence and reactivity of fluorine-containing Lewis ac-
ids such as neat BF3, BF3·OEt2, and B(C6F5)3 on Me3SiCl eli-
mination in (Me3Si)2N–N(SiMe3)–PCl2 was investigated. The
reaction with B(C6F5)3 resulted in the formation of a novel
diazadiphosphetidine with a phosphorus(III) atom attached
to a pentafluorophenyl group, which can be regarded as a
formal NNP fragment insertion into a B–C bond.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
etc.), this new reaction type was named a GaCl3-assisted
[3+2] cycloaddition. The ambivalent (Me3Si)2N–N(SiMe3)–
PCl2 displays both dipolarophile and/or 1,3-dipole charac-
teristics. GaCl3 is required to release the hidden dipolaro-
phile and/or 1,3-dipole by decreasing the activation barrier
to Me3SiCl elimination. In this context, we studied the in-
fluence of fluorine-containing Lewis acids such as neat BF3,
BF3·OEt2, and B(C6F5)3 on the Me3SiCl elimination in
Introduction
The GaCl3-assisted elimination in silylated dichloro(hy-
drazino)phosphane,[1–3] (Me3Si)2N–N(SiMe3)–PCl2, fol-
lowed by a formal [3+2] cycloaddition[4] represents an ele-
gant approach to the formation of triazadiphospholes and
tetrazaphospholes, which was introduced only recently
(Scheme 1). As this reaction sequence only occurs when
GaCl3 is added into a solution of (Me3Si)2N–N(SiMe3)–
PCl2 (1) in common organic solvents (e.g. CH2Cl2, benzene,
Scheme 1. Lewis acid assisted [3+2] cycloaddition with ambivalent
dichloro(hydrazino)phosphane (1) to yield a triazadiphosphole
[ER3 = GaCl3, B(C6F5)3].
[a] LMU München, Department Chemie und Biochemie
Butenandt-Str. 5–13, 81377 München, Germany
[b] Universität Rostock, Institut für Chemie
Albert-Einstein-Str. 3a, 18059 Rostock, Germany
Fax: +49-381-4986382
E-mail: axel.schulz@uni-rostock.de
Homepage: www.chemie.uni-rostock.de/ac/schulz
[c] Leibniz-Institut für Katalyse e.V. an der Universität Rostock,
Albert-Einstein-Str. 29a, 18059 Rostock, Germany
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Scheme 2. Proposed mechanism of the B(C6F5)3-assisted 1,2-elimi-
nation of Me3SiCl in 1 followed by an unusual insertion of a NNP
fragment into a B–C bond, which finally results in the formation
of 3.
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M. Kowalewski, B. Krumm, P. Mayer, A. Schulz, A. Villinger
SHORT COMMUNICATION
(Me3Si)2N–N(SiMe3)–PCl2. Herein, we detail the reactivity
of these fluorine-containing Lewis acids towards (Me3Si)2-
N–N(SiMe3)–PCl2, which proceeds in the case of B(C6F5)3,
by the first reported example of an unusual NNP fragment
insertion into a boron–carbon bond (Scheme 2).
Results and Discussion
Absolutely no reaction was observed when a large excess
of neat BF3 (b.p. –100 °C, ca. 15 mL) was added to 1 at
–110 °C. Also, after further stirring of this reaction mixture
for 5 h at –100 °C no Me3SiCl elimination could be ob-
served; only the starting material was isolated after removal
of BF3. Obviously, thermal activation is needed to trigger
Me3SiCl elimination. Hence, 1 was treated with BF3 in di-
ethyl ether at ambient temperature. This reaction led to
complete decomposition. Compounds [N2H5]+[BF4]–,[5]
Me3SiF, and PFCl2 could be identified amongst the decom-
position products.
These failures prompted us to utilize the sterically de-
manding fluorine-containing Lewis acid B(C6F5)3. Again,
no reaction was observed when 1 was added to a solution
of B(C6F5)3 in CH2Cl2 at ambient temperature. Only upon
heating of this solution in a sealed tube up to 70 °C did
slow Me3SiCl elimination occur, which was completed after
3 d of stirring as shown by 31P NMR spectroscopic studies
(1: 31P NMR: δ = 166.6 ppm disappeared).
Contrary to the reaction of 1 with GaCl3, the reaction
with B(C6F5)3 yielded two products: (1) a highly soluble
B(C6F5)3 adduct of the expected trizadiphosphole (2)
(NMR yield 24%, Scheme 1, multiplets in the 31P NMR
spectrum at δ = 317.4 and 286.4 ppm; cf. GaCl3 adduct:
317.2 and 292.1 ppm)[1] and (2) a new species (NMR yield
42%, 31P NMR: δ = 253 ppm). Surprisingly, upon cooling
of the above reaction mixture down to 5 °C, a large amount
of beautiful orange crystals of the new species was obtained
(isolated yield 29%). Single-crystal X-ray studies revealed a
surprising 2,4-bis(pentafluorophenyl)-1,3-bis[N-trimethyl-
silyl-N-bis(pentafluorophenyl)boranyl]amino[1,3,2,4]diaza-
diphosphetidine (3) as the product (Scheme 2, Figure 1).
Pure dry 3 is stable at temperature up to 400 °C (m.p.
225 °C), is neither heat nor shock sensitive, and decomposes
even only slowly in water and on air indicating a good ki-
netic protection by the bulky C6F5 groups. It should be
mentioned that 3 is almost insoluble in any common sol-
vent at ambient temperature.
Figure 1. ORTEP drawing of the molecular structure of 3 (con-
former A) in the crystal. Thermal ellipsoids with 50% probability
at 200 K (hydrogen atoms omitted for clarity). Selected bond
lengths [Å] and angles [°]: P2–N2 1.664(2), P2–N2Ј 1.793(2), P2–
C13 1.913(3), P2–P2Ј 2.649(2), Si1–N1 1.818(2), N1–B1 1.400(3),
N1–N2 1.442(3); N2–P2–N2Ј 80.0(1), N2–P2–C13 105.2(2), N2–
P2–P2Ј 41.78(7), B1–N1–N2 117.4(2), B1–N1–Si1 125.4(2), N2–
N1–Si1 115.4(1), N1–N2–P2 131.8(2), P2–N2–P2Ј 100.0(1), N1–
B1–C7 122.2(2), C1–B1–C7 114.3(2).
membered ring. Moreover, this molecule is found to be dis-
ordered. Two different positions, which displays the exis-
tence of two conformers, are found for both P–C6F5 moie-
ties with an occupation of 61.5 (A) and 38.5% (B). Both
positions correspond to a rotation about the N–N axis with
an angle of 109.2° between both rings. In both conformers
the P2N2 ring is ideally planar [Є P2–N2–P2Ј–N2Ј = 0.0°
(A), Є P1–N2–P1Ј–N2Ј = 0.0° (B)].
31P MAS NMR studies proved the existence of two con-
formers in the solid state (Figure S1),[6] which is indicated
by two resonances at δ = 264.0 (isomer A, Figures 1 and 2)
and 240.7 ppm (isomer B, Figure 2). Interestingly, we were
able to assign these two resonances on the basis of the excel-
lent agreement between theoretically and experimentally
obtained values of the 31P NMR chemical shift tensor (see
Supporting Information).[6a]
X-ray elucidation of the orange crystals revealed a four-
membered P–N heterocatenated ring (3) with two P–C6F5
and two N–R [R = N(SiMe3)(B(C6F5)2)] centers, and its
formation can be described as a dimerization of monomeric
R–N–N=P–C6F5, which is formed by Me3SiCl elimination
(Figure 1 and Scheme 2) followed by an intriguing transfer
of one pentafluorophenyl group (see below). Compound 3
crystallizes in cubes in the triclinic space group P1 with one arrangement of the two C6F5 groups attached to the P
unit per cell. The asymmetric unit of 3 consists of only half atoms, which is also found in conformer B. As expected for
of the molecule. The entire molecule can be generated by tricoordinated phosphorus(III) species, two rather long and
an inversion center sited on the centroid of the P2N2 four- two short P–N distances are found [d(P2–N2) = 1.664(2) Å,
Figure 2. Illustration of the disorder problem in 3. Conformer A
corresponds to the N2–P2–N2Ј–P2Ј plane, B to the N2–P1–N2Ј–
P1Ј. All H, C, and F atoms are omitted for clarity.
Conformer A is shown in Figure 1, and it displays a trans
¯
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Transition-Metal-Free Boron–Carbon Bond Activation
d(P2–N2Ј) = 1.793(2) Å (A); d(P1–N2) = 1.631(2) Å, d(P1–
To estimate the thermodynamics of the formation of 2
N2Ј) = 1.838(3) Å (B)]. Comparison with the sum of the and 3 and to obtain insight into the bond-formation pro-
covalent radii [dcov(N–P) = 1.8 Å, dcov(N=P) = 1.6 Å] indi- cess, computations were carried out at the B3LYP/6-
cates partial π bonding for the shorter P–N distance in both 31G(d,p) level of theory.[6d,6e,14] The calculated energy for
conformers [1.664(2) and 1.631(2) Å]. A large and a small the formation of triazadiphosphole (2) is an exothermic and
angle are found for the P2N2 ring [Є N2–P2–N2Ј = an exergonic process for all of the Lewis acids studied
80.0(1)°, Є P2–N2–P2Ј = 100.0(1)° (A); Є N2–P1–N2Ј = [Scheme 1;
∆
298G0
=
–317.3, –331.2, –348.6, and
79.5(1)°, Є P1–N2–P1Ј = 100.5(1)° (B)]. Selected structural –304.2 kJmol–1 for BF3, BCl3, BBr3, and B(C6F5)3, respec-
data of both conformers are summarized in Tables S2 and tively]. The latter value can be compared with the formation
S3.[6b]
of 3 for which a smaller energy release was calculated
The complete mechanism for the formation of a triazadi- (∆298G0 = –258.0 kJmol–1). The formation of the mono-
phosphole (2) according to Scheme 1 is still unknown al- meric species [according to 1 + B(C6F5)3 Ǟ ½3 + 2Me3-
though 1,2-elimination of Me3SiCl was experimentally SiCl] is associated with an energy gain of –135.8 kJmol–1,
proven as the initial step.[3] When B(C6F5)3 is used as the whereas dimerization is slightly endergonic with
Lewis acid two things are worthy to be discussed: (1) a large 13.6 kJmol–1. It is interesting to note that monomeric 3 is
thermal activation (3 d at 70 °C) is needed to trigger Me3- observed with a large intensity in the mass spectrum besides
SiCl elimination (cf. GaCl3 reaction: 1 h at 20 °C) and (2) the dimer.
the formation of a different species, a diazadiphosphetidine
(3), as the main product beside 2 is observed (Schemes 1 orbital)[6d,6e,15] the NP bonds within the P2N2 ring of 3 are
and 2). purely σ in character and are strongly polarized (N: 77%,
As expected, according to NBO analysis (natural bond
Presumably, after the initial elimination of Me3SiCl, free P: 23%) with a partial charge of +1.28e at the phosphorus
borane can attack the second chlorine atom attached to the atom and –0.95e at the nitrogen atom. We calculate a
phosphorus atom of the iminophosphane chloride, which charge of –0.86e at the exocyclic nitrogen atom and a posi-
leads to chlorine/C6F5 exchange (Scheme 2). The intermedi- tive charge of +0.87e and +1.93e at the boron and silicon
ate ClB(C6F5)2 reacts under Me3SiCl elimination to give atoms, respectively.
monomeric 3. Reactions of ClB(C6F5)2 with Me3Si-substi-
tuted amines are known,[7] for example, ClB(C6F5)2 treated
Conclusion
with HN(SiMe3)2 yields HN(SiMe3)[B(C6F5)2] or HN[B-
(C6F5)2]2 depending on the stoichiometry. Moreover, the
The formation of 3 represents the first example of the
insertion of a NNP fragment into a B–C bond. In this con-
text, our results offer a new perspective on the insertion and
activation of boron–carbon bonds. Moreover, 3 is the first
example of a structurally characterized diazadiphosphetid-
ine with phosphorus(III) attached to a perfluoro group.
–
ClB(C6F5)3 anion is well known.[8] Finally, dimerization
results in the formation of diazadiphosphetidine 3. For ki-
netic reasons, the formation of 3 is favored when B(C6F5)3
is used, whereas only 2 is observed when GaCl3 is used as
the Lewis acid. The overall reaction of 1 with B(C6F5)3 can
be regarded as the insertion of Me3Si–NNP (covalent azide
analogue containing one phosphorus atom) into one B–C
bond, followed by a final dimerization process or a formal
[3+2] addition of the borane to Me3SiNNP, which is gener-
ated in situ after elimination of two molecules Me3SiCl. It
should be emphasized that both reactions according to
Schemes 1 and 2 can be reproduced.
Experimental Section
3: To a stirred solution of dichloro[N,NЈ,NЈ-tris(trimethylsilyl)hy-
drazino]phosphane (1.75 g, 5 mmol) in dichloromethane (10 mL)
was slowly added a solution of tris(pentafluorophenyl)borane
(2.82 g, 5.5 mmol) in dichloromethane (10 mL) by syringe at ambi-
ent temperature. The yellowish solution was heated in a sealed tube
under reflux for 3 d at 70 °C, which resulted in an orange suspen-
sion. After cooling to room temperature, the solution was concen-
trated to a total volume of 10 mL and stored at 0 °C overnight to
afford an orange, microcrystalline powder. Dichloromethane
(10 mL) was added, and the orange residue was filtered (F4) and
Transfer reactions involving the C6F5 group are well
known in transition-metal chemistry,[9] and they are often
responsible for catalyst poisoning.[10] Such transition-metal-
induced C6F5 transfer reactions often follow after the for-
mation of intermediates, and they are stabilized by F···M
agostic interactions with a cationic transition-metal center
(M).[11] The latter interaction often leads to C–F activation, washed several times by repeated back distillations of the solvent.
The product was dried in vacuo to yield 0.94 g (29%) of 3 as an
which may even result in M–F bond formation by C–F
bond cleavage. Metal-free C–F bond activation is also
known, for example, when secondary phosphanes such as
(Me3C6H2)2PH are treated with B(C6F5)3 to yield
(Me3C6H2)2PH(C6F4)BF(C6F5)2.[12] Shifts of the C6F5
group are known to possess large activation barriers, for
example, a large activation barrier for a 1,2-shift of a C6F5
group has already been observed by Metzler and Denk in a
silylene–borane adduct, which slowly rearranges to a stable
silylborane with a half-life of about one month.[13]
orange solid. M.p. 225 °C. FTIR (KBr): ν = 2992 (vw), 2964 (vw),
˜
2908 (vw), 1648 (m), 1519 (s), 1480 (s), 1397 (m), 1366 (m), 1312
(m), 1285 (m), 1259 (m), 1150 (m), 1089 (m), 1043 (w), 1017 (w),
976 (s), 928 (w), 854 (s), 778 (m), 722 (w), 669 (w), 629 (w), 503
(w) cm–1. FTIR (ATR): ν = 2992 (vw), 2966 (vw), 2912 (vw), 1647
˜
(m), 1516 (s), 1466 (s), 1395 (m), 1365 (m), 1311 (m), 1284 (m),
1257 (m), 1204, w, 1147 (m), 1139 (m), 1084 (m), 1044 (w), 1016
(w), 970 (s), 928 (w), 910 (w), 838 (s), 776 (m), 761 (w), 748 (w),
721 (w), 670 (w), 658, w, 628 (w), 618 (w) cm–1. Raman (200 mW,
25 °C): 2994 (2), 2967 (2), 2911 (6), 2546 (1), 1649 (10), 1520 (2),
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M. Kowalewski, B. Krumm, P. Mayer, A. Schulz, A. Villinger
SHORT COMMUNICATION
[6] See the Supporting Information: a) chemical shifts (31P{1H},
31P MAS) and assignment was carried out on the basis of the
excellent agreement between theoretically (utilizing the two
structures obtained from X-ray elucidation) and experimentally
obtained shifts (δ[31P]); b) crystal data; c) full description of
the experimental data, technique and details; d) computational
details [B3LYP/6-31G(d,p)]; e) summary of the NBO analysis
for 3: (i) partial charges, (ii) hybridization effects, and (iii) po-
larization.
1401 (8), 1363 (7), 1309 (2), 1309 (2), 1288 (2), 1141 (2), 1118 (3),
1094 (2), 1021 (2), 982 (1), 848 (2), 819 (2), 785 (2), 771 (2), 658
(2), 621 (2), 596 (6), 527 (8), 501 (7), 446 (8), 397 (8), 363 (4) cm–1.
31P{1H} NMR (121.5 MHz, CD2Cl2, 25 °C): δ = 253 (br.) ppm.
31P MAS NMR (25 °C, ωrot = 20 kHz): δ = 240.7 (s), 264.0 (s)
ppm. MS (EI, 70 eV, Ͼ5%): m/z (%) = 168 (26.2) [C6F5], 277 (26.8),
345 (19.5) [B(C6F5)2], 365 (28.0), 629 (58.4) [½ 3Me], 644 (100) [½
3], 1288 (30.6) [3]. C42H18N4B2F30P2Si2 (1288.3): calcd. C 39.16, H
1.41, N 4.35; found C 40.62, H 1.00, N 4.22.
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Crystals suitable for single-crystal X-ray studies were obtained by
cooling the reaction solution slowly to 5 °C. An X-ray quality crys-
tal of 3 was selected in silicon oil at ambient temperature and was
cooled to 200(2) K during measurement. The data for compound
3 was collected with a Nonius Kappa CCD diffractometer by using
graphite-monochromated Mo-Kα radiation (λ = 0.71073). The
structure was solved by direct methods and refined by full-matrix
least-squares procedures (SHELXL-97).[16] All non-hydrogen
atoms were refined anisotropically; hydrogen atoms were included
in the refinement at calculated positions by using a riding model.
The P(C6F5) unit was split over two positions and the occupancy
of each part (A and B) refined. For the carbon atoms of the split
C6F5 rings, rigid group refinement was performed. A second X-
ray study from a different experiment gave the same result (see
Supporting Information). CCDC-653980 contains the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from the Cambridge Crystallographic Data
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cle): Full experimental details and characterization data, X-ray
crystallographic refinement data, selected bond lengths and angles,
and computational details of 3.
Acknowledgments
Financial support by the Deutsche Forschungsgemeinschaft
(SCHU 1170/4-1) is gratefully acknowledged. We thank Dr. Jörn
Schmedt auf der Günne for recording the MAS NMR spectra.
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Received: August 7, 2007
Published Online: October 25, 2007
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Eur. J. Inorg. Chem. 2007, 5319–5322