One pot synthesis of a chiral N-phosphine substituted iminophosphorane: X-ray structure and in situ NMR study

Article abstract of DOI:10.1039/b005816p

An X-ray crystal structure reveals that the attempted deprotonation of cyclopropyl(triphenyl)phosphonium bromide with sodium amide affords a chiral N-phosphine substituted iminophosphorane via an unusual mechanism that has been investigated by multi-nucle

Full text of DOI:10.1039/b005816p

View Article Online / Journal Homepage / Table of Contents for this issue
One pot synthesis of a chiral N-phosphine substituted imino-
phosphorane: X-ray structure and in situ NMR study
Andrei S. Batsanov,a Matthew G. Davidson,*†b Ignacio Fernández,c Judith A. K. Howard,a
Fernando López-Ortiz‡c and Richard D. Price†b
1
a Department of Chemistry, Science Laboratories, South Road, Durham, UK DH1 3LE
b Department of Chemistry, University of Bath, UK BA2 7AY
c Área de Química Orgánica, La Cañada de San Urbano, Universidad de Almería, 04120,
Almería, España
Received (in Cambridge, UK) 19th July 2000, Accepted 25th October 2000
First published as an Advance Article on the web 28th November 2000
An X-ray crystal structure reveals that the attempted
deprotonation of cyclopropyl(triphenyl)phosphonium
bromide with sodium amide affords a chiral N-phosphine
substituted iminophosphorane via an unusual mechanism
that has been investigated by multi-nuclear and variable
temperature in situ NMR spectroscopy.
During our recent investigations into the interactions of s-block
and main group metals with phosphonium ylides1 and imino-
phosphoranes,2 we attempted to synthesise triphenylphos-
phonium cyclopropylide by the deprotonation of its parent
phosphonium salt 1. Using sodium amide as base in THF,
a standard method of ylide formation but not one that has
been previously reported using 1,3,4 we isolated a crystalline
compound whose 31P NMR spectrum precluded the simple
cyclopropylide as the product and instead suggested that a
compound containing two phosphorus centres had been
isolated.§ A single crystal X-ray diffraction study¶ revealed the
product to be a chiral N-phosphine substituted iminophos-
phorane 11 (Fig. 1). Further NMR, including in situ studies||
of the reaction mechanism, and chemical analysis confirmed 11
as the bulk product which was isolated in good yield.
Whilst 11 is not the first example of an N-phosphine-
substituted iminophosphorane,5–8 previous examples have been
synthesised via markedly different routes, often involving
coupling reactions using reagents such as Li[N(PPh2)2].6 The
serendipitous synthesis reported here poses several interesting
mechanistic questions which we have addressed by in situ
variable temperature NMR studies. Scheme 1 represents one
possible mechanism which is consistent both with our NMR
results and with previous work.
Scheme 1
at 60 ЊC. The iminophosphorane–aminoylide tautomerism is
precedented10 and recently it has been claimed to be involved
in the synthesis of enaminecyclopentenones mediated by alkyl-
diphenyliminophosphoranes.2c
Initially, nucleophilic attack at phosphorus of 1 [δ(31P) 31.22]
by nitrogen gives the aminophosphonium salt 2 and PhNa. 2
and the PhNa thus formed react rapidly, eliminating benzene
(analogous to loss of hydrocarbon which occurs during the
hydrolysis of phosphorus ylides)9 to yield the cyclopropyl-
(diphenyl)iminophosphorane 3 as a first identifiable inter-
mediate [δ(31P) 27.84]. The transformation is very slow below
0 ЊC, but is completed after 30 min at ambient temperature. The
presence of the cyclopropyl moiety is deduced from the 13C
NMR data: δ 9.09 (CH, 1JPC 103.8 Hz), 4.3 (CH2, 2JPC 3.6 Hz).
By allowing the sample to stand at 25 ЊC, the iminophos-
phorane 3 equilibrates with the aminophosphonium cyclo-
During the equilibration process between 3 and 4 at ambient
temperature the 31P spectrum shows the appearance of
two phosphorus doublets, corresponding to the final product 11
2
(δ 23.2 and 55.7, JPP 94 Hz) together with a broad signal at
38.7 ppm (this broad signal shows no correlation to either
31P nucleus of 11 and its intensity significantly increases with
ageing, suggesting it corresponds to decomposition or hydro-
lysed products). The intensity of both groups of signals
(assigned to 11) increases with the concomitant decrease
in the intensity of the signals of 3/4. On heating the sample
at 60 ЊC for 5 hours 3 completely disappears. Further reac-
tion at ambient temperature is slow in a sealed NMR
tube, but the aminoylide 4 completely transforms to 11 after
five days.
We suggest that deprotonation of 3/4 by a second equivalent
of sodium amide affords the N-sodiated iminophosphorane 5
and aminoylide 6. The existence of 5 (analogous to R3PNLi)11
has previously been proposed by Schmidbaur and Fuller, as
an intermediate in the formation of a phosphonium ylide-
substituted iminophosphorane.12 In the absence of any starting
1
propylide 4 [characterised through the H, 13C, and 31P NMR
3
spectra: δ (1H) 0.88 (CH2, JPH 27.2 Hz), δ (13C) 8.72 (CH2,
2JPC 8.3 Hz), δ (31P) 16.7 ppm].** After approximately 2 h at
25 ЊC, a 1:1 equilibrium is established between both species.
At Ϫ20 ЊC, the iminophosphorane 3 predominates over the
aminoylide tautomer (60:40), whereas this ratio is reversed
† E-mail: m.g.davidson@bath.ac.uk; r.d.price@bath.ac.uk
‡ E-mail: flortiz@ualm.es
DOI: 10.1039/b005816p
J. Chem. Soc., Perkin Trans. 1, 2000, 4237–4239
This journal is © The Royal Society of Chemistry 2000
4237

View Article Online
Acknowledgements
The EPSRC are acknowledged for a Quota studentship (RDP)
and the Leverhulme Trust for a Visiting Fellowship (ASB).
The British Council and Spanish Ministry of Education and
Culture are also acknowledged for the provision of a travel
grant (MGD, F L-O).
Notes and references
§ Synthesis of 11: A suspension of (triphenyl)cyclopropylphosphonium
bromide, 1 (10.1 g, 26.4 mmol) and sodium amide (1.1 g, 28 mmol) in 50
mL dry THF was stirred at ambient temperature under Ar for 24 hours.
Removal of a white precipitate from the orange solution by filtration
was followed by removal of approximately 40 mL solvent in vacuo and
addition of dry hexane (10 mL) which precipitated 11 as a yellow solid
(2.8 g, 46%). For the X-ray study, 11 was recrystallised from a hexane–
toluene mixture at Ϫ30 ЊC. Mp 144–145 ЊC (Found: C 75.2, H 6.3,
N 2.5, P 14.0; Calc. for C30H29NP2: C 77.4, H 6.3, N 3.0, P 13.3%);
1H NMR (300 MHz, C6D6): δH = 0.58 (1H, m, H6), 0.62 (2H, over-
lapping m, H5), 0.74 (1H, m, H6), 1.02 (1H, m, H4), 1.06 (2H, over-
lapping m, H2, 3β), 2.24 (3H, overlapping m, H2, 3α, H4), 6.8–7.25
(11HAr, overlapping m), 7.75 (2H, m, H12, 16), 7.82 (2H, m, H22, 26),
7.32 (1H, m, H44), 7.48 (2H, m, H43, 45), 8.20 (2H, m, H42, 46);
Fig. 1 Molecular structure of 11, showing intermolecular H ؒ ؒ ؒ P
contacts. Atoms related via inversion centre, are primed. For clarity,
H atoms other than those involved in hydrogen bonding are omitted.
Selected bond lengths (Å): P(1)–N 1.572(3), P(2)–N 1.689(3), P(1)–C(1)
1.819(3), P(1)–C(11) 1.816(4), P(1)–C(21) 1.822(3), P(2)–C(4) 1.828(4),
P(2)–C(41) 1.839(4), C(5) ؒ ؒ ؒ P(2Ј) 4.103(4).
material available, the formation of 11 may then proceed by
nucleophilic attack of 5/6 on the neutral intermediates 3/4,
yielding the metallated iminophosphorane 7. Intramolecular
abstraction of the methine proton of 7 affords the imino-
phosphorane-substituted ylide 8, which is deprotonated in the
basic medium to give the anion 9. This can rearrange to 10 via
a process resembling the Sommelet–Hauser rearrangement.13
Subsequent neutralisation of 10 leads to the final N-phosphino
iminophosphorane 11.
Alternatively, a direct route from 8 to 11 could be achieved
by invoking a Stevens’ rearrangement. The thermal phospha-
Stevens’ rearrangements of phosphonium ylides is known to
proceed under forcing conditions.14 However, the conjugation
2
13C NMR (75.5 MHz, C6D6): δC2= 2.69 (d, JPC 13.9 Hz, C6), 4.04
2
2
(d, JPC 11.6 Hz, C5), 10.95 (d, JPC 1.1 Hz, C2/3), 11.35 (t, JPC
=
4JPC 1.1 Hz, C2/3), 17.58 (dd, JPC 17.6, JPC 9.7 Hz, C4), 24.82 (d,
1
3
1JPC 103.1, C1), 126.74 (d, JPC 2.8 Hz, C44), 129.75 (d, JPC 18.9 Hz,
4
2
1
1
C41), 130.86 (d, JPC 92.0 Hz, C11/C21), 131.09 (d, JPC 97.6 Hz,
C21/C11), 133.17 (dd, JPC 9.0, JPC 2.3 Hz, C12, 16/22, 26), 133.30
2
4
2
4
(dd, JPC 6.9, JPC 0.9 Hz, C12, 16/22, 26), 127.4–132.74 (13 CAr),
139.42 (dd, 2JPC 4.6, 4JPC 0.9 Hz, C31), 149.96 (dd, JPC 16.6, 3JPC 14.8
1
Hz, C41); 31P NMR (121.49 MHz, C6D6): δP = 22.5 (d, 2JPP 89 Hz, P1),
56.0 (d, 2JPP 89 Hz, P2). All assignments refer to Fig. 1 (α and β refer to
protons above and below the plane of the cyclopropyl rings as depicted
in Fig. 1).
¶ Crystal data for 11: C30H29NP2, M = 465.5, monoclinic, space
group P21/c (No. 14), at T = 150 K, a = 9.555(1), b = 10.182(1), c =
25.711(2) Å, β = 98.75(1)Њ, U = 2472.3(4) Å3, Z = 4, Dx = 1.25 g cmϪ3
,
of the P᎐C linkage in 8 with the iminophosphorane moiety may
᎐
—
λ (Mo-Kα) = 0.71073 Å, µ = 2.0 cmϪ1. 14385 data (4252 unique) with
2θ ≤ 50Њ were measured with a SMART CCD area detector; least
squares refinement of 334 variables on F2 (ref. 23) gave R = 0.062
[for 3030 data with F2 > 2σ(F2)] and wR(F2) = 0.134.
allow the rearrangement to occur at ambient temperature.15
Unambiguous characterisation of 11 was achieved via a
single crystal X-ray structure.¶ The molecular geometry of 11
is unremarkable: the P–N distances and the P–N–P angle
are similar to those found in other substituted iminophos-
phoranes,4–7 and the conformations of the cyclopropyl groups
are usual.16 The crystal packing of 11 is characterised by
H ؒ ؒ ؒ P() contacts between inversion-related molecules (see
Fig. 1). The P ؒ ؒ ؒ H distance, 3.13 Å for the observed H
position (C–H 0.98(4) Å) or 3.01 Å for the idealised one, is
longer than in (Ph3PMe)ϩ {[C6H2(CF3)3-2,4,6]2P}Ϫ (2.79 Å)17
and close to the sum of the van der Waals radii (3.07 Å from
crystallographic data,18 3.24 Å from ab initio calculations19).
Thus the contact cannot be regarded as a hydrogen bond
CCDC reference number 207/492. See http://www.rsc.org/suppdata/
p1/b0/b005816p/ for crystallographic files in .cif format.
|| In situ NMR studies were performed on a 0.1 mmol scale in a d8-
toluene solution (a relatively dilute sample was used because of the
reduced solubility of the starting materials). The temperature range
covered by the study was Ϫ90 ЊC to ϩ60 ЊC, in 20 ЊC incremental steps
from Ϫ90 ЊC to Ϫ30 ЊC, then 10 ЊC steps thereafter once progress
of the reaction was evident. 1H and 31P{1H} were recorded for all
temperatures, while 13C{1H} APT, DEPT, 2D 1H, 13C gHMQC and
gHMBC were measured for selected temperatures. A 1H, 15N gHMQC
spectrum acquired at Ϫ20 ЊC showed only the 15N signal corresponding
to the NaNH2 (δ Ϫ387 ppm).
proper, as in Ph PC(Ph)᎐CBunB(OH)CBun NButCMe (with
᎐
2
2
** The ylidic carbon of 4 could not be identified by APT or HMBC
NMR experiments. This may be due to very slow relaxation and broad-
ening due to solution dynamics.
the OH ؒ ؒ ؒ P distance of 2.30 Å),20 but neither is it likely to be
merely incidental, given the acidity of the cyclopropyl hydrogen
atom and its pointing almost exactly towards the lone electron
pair of the P(2) atom. The existence of an H ؒ ؒ ؒ P contact,
rather than an H ؒ ؒ ؒ N interaction such as was previously
found21 in Ph P᎐NH, can be explained by steric overcrowding
1 D. R. Armstrong, M. G. Davidson and D. Moncrieff, Angew. Chem.,
Int. Ed. Engl., 1995, 34, 478.
2 (a) A. S. Batsanov, M. G. Davidson, J. A. K. Howard, S. Lamb,
C. Lustig and R. D. Price, Chem. Commun., 1997, 1211; (b)
F. López-Ortiz, E. Pelaez-Arango, B. Tejerina, E. Perez-Carreno
and S. Garcia-Granda, J. Am. Chem. Soc., 1995, 117, 9972;
(c) J. M. Álvarez-Gutiérrez and F. López-Ortiz, Chem. Commun.,
1996, 1583.
3 A. W. Johnson, with special contributions by W. C. Kaska, K. A. O.
Statzewski and D. A. Dixon, Ylides and Imines of Phosphorus,
John Wiley & Sons, Inc., New York, 1993, ch. 4.
4 H. Schmidbaur, A. Schier, B. Milewski-Mahrla and U. Schubert,
Chem. Ber., 1982, 115, 722.
5 H. G. Ang, Y. M. Yai, L. L. Koh and W. L. Kwik, J. Chem. Soc.,
Chem. Commun., 1991, 850.
6 P. Braunstein, R. Hasselbring, A. Tiripicchio and F. Ugozzoli,
J. Chem. Soc., Chem. Commun., 1995, 37.
7 A. Schmidpeter, K.-H. Zirzow, G. Burget, G. Huttner and I. Jibril,
Chem. Ber., 1984, 117, 1695.
8 K. V. Katti, B. D. Santarsiero, A. A. Pinkerton and R. G. Cavell,
Inorg. Chem., 1993, 32, 5919.
᎐
3
of 11, wherein the N atom lone pair is almost entirely masked
by the H atoms at C(2) and C(42), which lie at 2.69 and 2.57 Å
from the N atom and close to its sp2 plane. This is a feature of
11 that we also expect to be significant in its coordination
chemistry.
Although the mechanism described above is speculative,
the first intermediates involved have been identified by NMR
and overall this route emphasises the strong involvement of
metallic base interactions in organic/inorganic transformations
involving organophosphorus species, even under such mild
conditions as these. This is, for example, known to be a factor
in the stereochemical outcome of Wittig reactions, but is still
little understood.22
We hope to extend our knowledge in this area through
further studies on these and related systems, including their
potential use as ligands to main group and transition metals.
4238
J. Chem. Soc., Perkin Trans. 1, 2000, 4237–4239

View Article Online
9 A. W. Johnson, with special contributions by W. C. Kaska, K. A. O.
Statzewski and D. A. Dixon, Ylides and Imines of Phosphorus,
John Wiley & Sons, Inc., New York, 1993, ch. 5 and references
therein.
10 T. A. Mastryukova and M. Y. Kabachnik, Russ. Chem. Rev., 1983,
52, 1012.
11 H. Schmidbaur and H. Jonas, Chem. Ber., 1967, 100, 1120.
12 H. Schmidbaur and H.-J. Fuller, Angew. Chem., 1976, 15, 501.
13 G. M. Brooke and J. A. K. J. Ferguson, J. Chem. Soc., Perkin Trans.
1, 1988, 2023.
16 F. H. Allen, J. P. M. Lommerse, V. J. Hoy, J. A. K. Howard and
G. R. Desiraju, Acta Crystallogr., Sect. B, 1996, 52, 734.
17 M. G. Davidson, K. B. Dillon, J. A. K. Howard, S. Lamb and
M. D. Roden, J. Organomet. Chem., 1998, 550, 481.
18 M. Yu. V. Zefirov and P. M. Zorkii, Russ. Chem. Rev., 1989, 58, 421.
19 M. Francl, R. F. Hout, Jr. and W. J. Hehre, J. Am. Chem. Soc., 1984,
106, 563.
20 I. A. Litvinov and V. A. Naumov, Zh. Strukt. Khim., 1993, 34(3), 91;
see discussion in Yu. V. Zefirov and P. M. Zorkii, Russ. Chem. Rev.,
1995, 64, 415.
14 (a) D. G. Gilheany, D. A. Kennedy, J. F. Malone and J. B. Walker,
J. Chem. Soc., Chem. Commun., 1984, 1217; (b) A. Maercker,
A. Bsata and R. Jung, Main Group Met. Chem., 1987, 10, 11.
15 K. Makita, J. Koketsu, F. Ando, Y. Ninomiya and N. Koga, J. Am.
Chem. Soc., 1998, 120, 5764.
21 M. G. Davidson, A. E. Goeta, J. A. K. Howard, C. W. Lehmann,
G. M. McIntyre and R. D. Price, J. Organomet. Chem., 1998, 550,
449.
22 B. E. Maryanoff and A. B. Reitz, Chem. Rev., 1989, 89, 863.
23 G. M. Sheldrick, SHELXL93, University of Göttingen, 1993.
J. Chem. Soc., Perkin Trans. 1, 2000, 4237–4239
4239