A truly multifunctional heterocycle: Iminophosphorane, N,P chelate, and dihydropyridine

Article abstract of DOI:10.1002/anie.200803373

(Chemical Equation Presented) Three in one: The anellated σ⁴-1λ⁵-[1,3,2]diazaphosphole 2 exists in tautomeric equilibrium with isomer 1 (see scheme) and undergoes three completely different types of reaction: trimethyl aluminum binds at the N terminus of the P=N bond with ring retention, the dihydropyridine fragment reacts with an activated acetylene in a pseudo-[2+2] cycloaddition, and P-N bond cleavage is possible with Rh^I.

Full text of DOI:10.1002/anie.200803373

Communications
DOI: 10.1002/anie.200803373
Phosphorus Heterocycles
A Truly Multifunctional Heterocycle: Iminophosphorane, N,P Chelate,
and Dihydropyridine**
Dan A. Smith, Andrei S. Batsanov, Karinne Miqueu, Jean-Marc Sotiropoulos,
David C. Apperley, Judith A. K. Howard, and Philip W. Dyer*
Bidentate ligands occupy a central position in coordination
chemistry.[1] Their electronic and steric asymmetry can be
harnessed to control reactivity at the chelated metal centre,
often as a result of the differing trans influences exerted by
the two donor components. Among such heteroditopic
ligands, bidentate N,Pchelates [2,3] are some of the most
frequently used and have been shown to be beneficial in a
number of applications such as allylic substitution,[4,5] olefin
oligomerization,[6,7] and asymmetric hydrogenation.[8]
In this context, we have previously reported the synthesis
and coordination chemistry of the k2-N,P-chelating pyridyl-N-
phosphinoimines I (Scheme 1). In combination with sources
of PdII, scaffold I (R = Ph) provides highly chemo- and
regioselective initiators for direct alkene
Reaction of N-lithiopyridylimines 1[9] and 2 with equimo-
lar quantities of bis(diisopropylamino)phosphine chloride[11]
in 1,2-dimethoxyethane (DME), followed by extraction with
hexane, afforded 3c (59% yield) and 4o (46% yield),
respectively (Scheme 2). The 31PNMR spectrum of com-
hydrocarboxylation in alcoholic media,
which affords products that result from
the coupling of two alkenes and two
equivalents of carbon monoxide.[9] While
exploring the stereoelectronic properties
of ligands I, it was of interest to probe the
Scheme 1. Pyridyl-
effect of replacing the aryl substituents of
phosphorus by s-electron-withdrawing, p-
basic amino groups.[10] Herein, we report
the synthesis of such compounds and
N-phosphino-
imines I.
Scheme 2. Synthesis of closed cyclic iminophosphorane 3c,its acyclic
(open) valence tautomer 3o,and open derivative 4o.
explore their unexpected, reversible intramolecular cycliza-
tion chemistry in solution. This unprecedented dynamic
behavior has been probed using a combination of experiment
and computation.
pound 4o shows a single resonance at d = + 71.3 ppm, a
chemical shift consistent with that for an N-bis(dialkylami-
no)phosphinoimine.[12] In contrast, the 31P{1H} NMR spec-
trum of 3 exhibits two singlet resonances: d = + 71.0 ppm
(3o), comparable to that for 4o, and d = + 42.3 ppm (3c),
present in a ratio of 3o/3c of 5:95, at 298 K.[13,14]
[*] D. A. Smith,Dr. A. S. Batsanov,Dr. D. C. Apperley,
Prof. J. A. K. Howard,Dr. P. W. Dyer
Department of Chemistry,Durham University
South Road,Durham,DH1 3LE (UK)
Fax: (+44)191-378-4737
Dark red single crystals of 3c were grown from a hexane
solution of 3, and their bulk composition verified by solid-
state 31PNMR spectroscopy ( d31P = + 42.1 ppm).[15] The
structure of the unexpected product 3c was elucidated by
an X-ray diffraction study, which revealed it to be a closed,
cyclic iminophosphorane (or, more formally, an anellated s4-
1l5-[1,3,2]diazaphosphole) derivative (Figure 1).[16] The fused
heterocycles A and B are both planar and form a dihedral
E-mail: p.w.dyer@durham.ac.uk
Dr. K. Miqueu,Dr. J.-M. Sotiropoulos
IPREM–UMR 5254–Equipe de Chimie-Physique
UniversitØ de Pau et des Pays de l’Adour
HØlioparc,2 avenue du PrØsident Angot,64053 Pau Cedex 09
(France)
=
=
angle of 4.78. The rings comprise localized double (P N2, C5
ꢀ
ꢀ
ꢀ
C6) and single (P N1, C6 N1, C4 C5) bonds, with a
[**] Durham University,the EPSRC,and Sasol Technology UK Ltd. are
warmly acknowledged for financial support of this work and
Johnson Matthey is acknowledged for the loan of rhodium salts. We
also thank Dr. A. M. Kenwright and I. McKeag for assistance with
NMR studies and Prof. R. RØau (Rennes) for invaluable discussions.
The M3PEC-MØsocentre (Aquitaine-Bordeaux 1,France) is thanked
for providing calculation facilities.
ꢀ
ꢀ
significant degree of allylic character across the C1 C2 C3
ꢀ
unit of ring A. The two exo-cyclic P N bonds show essentially
single bond character.[17] This structure is in good agreement
with both the 1H and 13C NMR spectroscopic data, which
3
[18]
show a typical JPH coupling for H-C6 of the 6-membered
heterocycle (5.6 Hz) and characteristic nonconjugated
alkenyl resonances for endo-cyclic carbons C5 and C4 (ca.
d = 110 ppm).[15]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803373.
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Chemie
Figure 1. X-ray molecular structure of 3c.[16] H atoms are omitted for
clarity. Thermal ellipsoids are drawn at the 50% level. Selected bond
lengths [Š] and angles [8]: P–N1 1.681(3),P–N2 1.589(3),N2–C1
1.399(4),C1–C2 1.376(4),C2–C3 1.420(4),C3–C4 1.353(4),C5–C6
1.336(4),N1-P-N2 97.41(14),P-N2-C1 109.6(2),P-N1-C2 108.0(2).
On dissolution of a crystalline sample of 3c in toluene,
both products 3o (d31P = + 71.0 ppm) and 3c (d31P =
+ 42.3 ppm) were observable by 31PNMR spectroscopy in
the same ratio (3o/3c = 5:95) as found for the crude reaction
mixture at 298 K.[13,14] This is consistent with variable temper-
ature 31PNMR spectroscopic studies, which have clearly
established that 3o and 3c may be regarded as interconvert-
ing valence tautomers in solution. On raising the temperature
of a solution of 3 in toluene to 365 K the concentration of the
open compound 3o increased (3o/3c = 25:75), whereas on
cooling to 225 K, 3o virtually disappeared (< 2%).[15,19] In
contrast, compound 4o does not undergo such a tautomeri-
zation process, with a single 31PNMR resonance at d =
+ 71.3 ppm being observed across the temperature regime
225–365 K and upon prolonged heating at 335 K. Ring closure
is presumed to be impeded in 4o by the presence of the
additional methyl substituent, which is consistent with the
computational results (see below).
Scheme 3. Summary of the reactions of compounds 3o/3c.
Figure 2. X-ray molecular structure of 6.[16] H atoms are omitted for
clarity. Thermal ellipsoids are drawn at the 50% level. Selected bond
lengths [Š] and angles [8]: Rh–P 2.2060(6),Rh–N1 2.1532(19),Rh–C0
1.811(2),P–N2 1.7085(19),N2–C1 1.272(3),N1-Rh-P 84.98(5),Rh-N1-
C2 127.45(14),Cl-Rh-C0 90.08(8).
This intriguing 3o/3c tautomerization process is without
precedent, making it of interest to explore whether it would
impact on the reactivity of 3. A solution of 3 was treated with
gray selenium, which resulted in the quantitative formation of
the s4,l5-selenide derivative 5 (Scheme 3),[15] the 31PNMR
spectrum of which exhibits a characteristic resonance at d =
1
+ 55.1 ppm (j JSeP j = 821 Hz).[10] Clearly, reaction of sele-
by the formation of the chelate. Hence, in an attempt to
observe the complexation of the closed tautomer 3c, the
reaction with a hard alkyl aluminum reagent was explored.
Treatment of 3 with a stoichiometric quantity of AlMe3 in
hexanes afforded 7 in 65% yield (Scheme 3).[15] The 31PNMR
spectrum of complex 7 exhibits a single resonance at
+ 46.6 ppm (cf. 3c: d31P = + 42.3 ppm), together with obser-
nium has occurred with the minor, open, valence tautomer 3o.
It was then tempting to probe reactions with a transition
metal Lewis acid that could potentially react with both
isomers 3o/3c. Since both iminophosphoranes[20,21] and N-
phosphinoimines[9] are established ligands for Rh(I), 3 was
treated with 0.5 mole equivalents of [{RhCl(CO)2}2]
(Scheme 3). This gave rise to the Rh complex
[RhCl(CO)(k2-N,P-3o)] (6) in 91% yield (d31P =
+ 117.8 ppm {1JRh,P = 209 Hz}) as a single isomer.[15] An X-
ray diffraction study confirmed the structure of 6 (Figure 2),
which is similar to that of its diphenylphosphino-substituted
ligand analogue, [RhCl(CO)(k2-N,P-I)].[9] The Rh centre of 6
adopts a slightly distorted square planar geometry with k2-
N,P-coordinated 3o.
3
6
1
ꢀ
vation of a JPH coupling of 6.4 Hz for H C . The H NMR
spectrum shows a single sharp resonance corresponding to an
AlMe3 fragment (d = ꢀ1.18 ppm). These data are clearly
supportive of a closed heterocyclic structure for 7, which was
confirmed by a molecular structure determination (Figure 3).
As for 3c, 7 comprises an anellated s4-1l5-diazaphosphole
ring, but with an AlMe3 fragment bound through the endo-
cyclic sp2 N atom (Al-N2 1.9883(9) Š). This causes an
=
It is likely that the formation of complex 6, which bears
the open (3o) rather than the closed (3c) form of 3, is driven
elongation of the P N2 bond by 0.04 Š, but otherwise little
change in structure compared to 3c is observed. Such k1-N
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pyridyl nitrogen of 3o at phosphorus, or through a concerted
1,5-electrocyclization. Although a number of examples of
intramolecular dative N!PIII interactions have been
reported,[26–28] which can be regarded as “arrested” inter-
mediates of a stepwise cyclization process, it was of interest to
probe the course of ring closure for 3 in more detail.[15]
A computational study of compounds 3o,c and 4o,c was
undertaken at the B3LYP/6-31G** level of theory.[15] The
natural bond orbital (NBO) analysis and computed molecular
orbitals and structure of 3c are in good agreement with the
experimentally observed iminophosphorane tautomer.[15] For
the methylpyridine system 4, N-phosphinoimine 4o[29] and
iminophosphorane 4c are both minima on the hypersurface;
however, 4c is 20.2 kcalmolꢀ1 (at 258C) higher in energy. This
is consistent with only the open form 4o being observed
experimentally because of steric constraints imposed by the
methyl substituent; this is supported by the calculated
repulsion energies for 4o/4c.[15] In contrast, the minima
located for the closed 3c and open 3o[29] tautomers are at
almost identical energies (DE = 3.0 kcalmolꢀ1 at 258C). These
calculations support the existence of the 3o/3c equilibrium in
solution. No transition state was found, thus showing there to
be no kinetic barrier to this thermodynamically allowed
summarization process, which indicated that it is very likely
that cyclization of 3o takes place in a concerted manner.
In summary, it has been established both experimentally
and theoretically that the pyridyl-N-phosphinoimine bearing
diisopropylamino substituents at phosphorus, 3o, exists in
valence tautomeric equilibrium in solution with the closed
anellated s4-1l5-[1,3,2]diazaphosphole derivative 3c. The
small energy difference between 3o and 3c means that 3
may react as an iminophosphorane, an N,Pchelate, and a
dihydropyridine depending on the reagents added. This study
highlights that slight modifications of the Psubstituents may
dramatically affect the chemical behavior of organophospho-
rus compounds and that incorporation of the bis(diisopropy-
lamino)phosphino moiety is a powerful methodology for the
stabilization of unusual, reactive species.[30,31]
Figure 3. X-ray molecular structure of 7.[16] H atoms are omitted for
clarity. Thermal ellipsoids are drawn at the 50% probability level.
Selected bond lengths [Š] and angles [8]: P–N1 1.6745(9),P1–N2
1.6308(9),N1–C2 1.429(1),N2–C1 1.436(1),C1–C2 1.360(1),C2–C3
1.438(1),C3–C4 1.358(2),C5–C6 1.3433(15),Al–N2 1.9883(9),N1-P-
N2 95.70(4),P-N2-C1 109.46(7),P-N1-C2 109.97(7).
coordination of the AlMe3 fragment is consistent with the
=
localized endo-cyclic P N bond of 3c, and is entirely
analogous to the reactions of classical iminophosphoranes in
which hard Lewis acids bind to the N terminus of the P–N
linkages.[22,23]
Since 3c demonstrates reactivity analogous to iminophos-
phoranes, it was of interest to explore other reactions typical
of this type of functional group, for example cycloaddition
[23]
=
across the P N bond. Thus, a solution of 3c in dichloro-
methane was treated with dimethyl acetylene dicarboxylate
(DMAD) at room temperature. Over a period of 12 h, a new
product 8 slowly formed and was isolated in 95% yield
(Scheme 3).[15] However, the 31PNMR spectrum of 8 clearly
highlights that cycloaddition has not occurred at the P–N
multiple bond, as a single resonance is observable at a
chemical shift similar to that for 3c (8: d = + 46.6; 3c: d =
+ 42.3 ppm). Instead, a pseudo-[2+2] cycloaddition occurred
at the localized C3–C4 double bond of the six-membered
heterocycle, which is readily confirmed by a combination of
1H, 13C, and 2D NMR spectroscopy and mass spectrometry
(m/z = 554). This reactivity perfectly mirrors the established
chemistry of N-substituted 1,2-dihydropyridines, which
behave as enamines in their reactions with DMAD to
afford cyclobuta[b]pyridines.[24] It is reasonable to suggest
that 3c is prevented from undergoing cycloaddition across the
Experimental Section
All reactions/manipulations were carried out under an atmosphere of
dry nitrogen by using standard Schlenk and glovebox techniques.3c:
Phenyllithium (1.8m, 5.3 mL, 9.6 mmol) was added to a cooled
(ꢀ788C) solution of 2-cyanopyridine (1.00 g, 9.6 mmol) in DME
(50 mL). The mixture was stirred for 1 h before (iPr2N)2PCl (2.56 g,
9.6 mmol) in DME (70 mL) was added. After stirring for 22 h at room
temperature, volatile components were removed in vacuo and the
product extracted with hexane. On cooling (ꢀ308C) red crystals of 3c
formed (2.33 g, 59%). 31P{1H} NMR (161.91 MHz, C6D6): d =
=
P N bond as a result of steric constraints imposed by the
bulky diisopropylamino substituents.
As would be expected, the preference of pyridyl-N-
phosphinoimines towards intramolecular cyclization is very
sensitive to both the nature of the substituents of the
phosphorus atoms and of the pyridyl moiety (see above).
Indeed, reaction of 1 with alkyl or aryl chlorophosphines
R2PCl (R = Ph,[9] iPr, cyclohexyl, and tBu[25]) affords exclu-
sively acyclic, open products. These observations are consis-
tent with the diisopropylamino substituents rendering the
phosphorus centre of 3o more electrophilic and hence more
susceptible to cyclization by reaction with the nucleophilic
pyridine moiety, compared with its P-dialkyl and -diaryl
analogues. However, the mechanism for the formation of the
closed heterocycle 3c could be envisaged as occurring by
either a stepwise intramolecular nucleophilic attack of the
+ 42.3 ppm
(s);
MS
(FAB+)
m/z:
412
[M+].
3o: 31P{1H} NMR (161.91 MHz, C6D6): d = + 71.0 ppm (s).
4o: An identical procedure to that used for the preparation of 3c
was employed, using 2-cyano-6-methyl-pyridine (0.50 g, 4.2 mmol),
phenyllithium (1.8m, 2.4 mL, 4.2 mmol), and (iPr2N)2PCl (1.13 g,
4.2 mmol). 4o was isolated as a brown solid (830 mg, 46%). 31P{1H}
NMR (80.96 MHz, C6D6): d = + 71.3 ppm; MS (EI+) m/z: 426 [M+].
5: A C6D6 solution of 3 (30 mg, 0.073 mmol) was treated with grey Se
(58 mg, 0.73 mmol) and heated at 508C for 80 h. 31P{1H} NMR
(80.96 MHz, C6D6): d = + 55.5 ppm (s + satellites, 1JSe,P = 821 Hz).
6: A solution of 3 (198 mg, 0.5 mmol) in toluene (10 mL)was added to
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[{RhCl(CO)2}2] (92 mg, 0.25 mmol) in toluene (10 mL). The mixture
was degassed and stirred for 24 h. Volatile components were removed
in vacuo to give 6 as a red-brown solid (252 mg, 91%). 31P{1H}
(161.91 MHz, CDCl3): d = + 117.8 ppm (d, 1JP,Rh= 209 Hz); MS
(FAB+) m/z: 550 [MꢀCO+]; IR (KBr, CDCl3/CHCl3): n(CO) =
spectroscopy and gave rise to two resonances at d = + 70.3 and
+ 42.0 ppm in a 1:99 ratio, respectively. This is consistent with
the 3o/3c equilibrium that exists in solution.
[15] See the Supporting Information for details.
[16] CCDC 686754 (3c), 686755 (6), and 686756 (7) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[17] F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G.
Orpen, R. Taylor, J. Chem. Soc. Perkin Trans. 2 1987, Supple-
ment, S1 – S18.
1997 cmꢀ1
.
7: A solution of AlMe3 (25 mg, 0.35 mmol) in hexane (10 mL) was
added to a solution of 3 (157 mg, 0.38 mmol, 1.1 equiv) in hexane
(30 mL). After 1 h the precipitate was isolated by filtration, washed
with hexane (3 ” 4 mL) and dried in vacuo (116 mg, 65%). 31P{1H}
NMR
(80.95 MHz,
CD2Cl2):
d = + 46.6 ppm
(s).
8: DMAD (13 mg, 0.09 mmol) was added to a solution of 3 (38 mg,
0.09 mmol) in CD2Cl2 (0.8 mL). After 24 h, volatile components were
removed in vacuo to afford 8 (95%). 31P{1H} NMR (161.91 MHz,
CD2Cl2): d = + 46.6 (s); MS (EI+) m/z: 554 [M+].
[18] S. Berger, S. Braun, H.-O. Kalinowski, NMR-Spektroskopie von
Nichtmetallen, Georg Thieme, Stuttgart, 1993.
[19] The values of the equilibrium constant (K = 16.5), and the
thermodynamic parameters DH8 (ꢀ22.4 ꢁ 0.05 kJmolꢀ1), DG8
(ꢀ7.0 ꢁ 0.3 kJmolꢀ1) and DS8(298K) (ꢀ51.8 ꢁ 2.6 Jmolꢀ1 Kꢀ1) were
determined and are consistent with facile cyclization/ring-open-
ing processes that favor the closed heterocycle 3c at 298 K.
[20] K. T. K. Chan, L. P. Spencer, J. D. Masuda, J. S. J. McCahill, P.
Wei, D. W. Stephan, Organometallics 2004, 23, 381 – 390.
[21] C. Freund, N. Barros, H. Gornitzka, B. Martin-Vaca, L. Maron,
D. Bourissou, Organometallics 2006, 25, 4927 – 4930.
[22] N. Burford, R. E. v. H. Spence, J. F. Richardson, J. Chem. Soc.
Dalton Trans. 1991, 1615 – 1619.
Received: July 11, 2008
Published online: October 1, 2008
Keywords: coordination modes · cycloaddition ·
.
diazaphospholes · iminophosphoranes · N,P ligands
[1] P. Braunstein, F. Naud, Angew. Chem. 2001, 113, 702 – 722;
Angew. Chem. Int. Ed. 2001, 40, 680 – 699.
[23] Y. G. Gololobov, L. F. Kasukhin, Tetrahedron 1992, 48, 1353 –
1406.
[2] P. J. Guiry, C. P. Saunders,Adv. Synth. Catal. 2004, 346, 497 – 537.
[3] C. Bianchini, A. Meli, Coord. Chem. Rev. 2002, 225, 35 – 66.
[4] C. Markert, A. Pfaltz, Angew. Chem. 2004, 116, 2552 – 2554;
Angew. Chem. Int. Ed. 2004, 43, 2498 – 2500.
[5] G. Helmchen, A. Pfaltz, Acc. Chem. Res. 2000, 33, 336 – 345.
[6] C. E. Anderson, A. S. Batsanov, P. W. Dyer, J. Fawcett, J. A. K.
Howard, Dalton Trans. 2006, 5362 – 5378.
[7] F. Speiser, P. Braunstein, L. Saussine, Acc. Chem. Res. 2005, 38,
784 – 793.
[8] G. Xu, S. R. Gilbertson, Tetrahedron Lett. 2003, 44, 953 – 955.
[9] P. W. Dyer, J. Fawcett, M. J. Hanton,J. Organomet. Chem. 2005,
690, 5264 – 5281.
[10] P. W. Dyer, J. Fawcett, M. J. Hanton, R. D. W. Kemmitt, R.
Padda, N. Singh, Dalton Trans. 2003, 104 – 113.
[11] R. B. King, N. D. Sadanani, Synth. React. Inorg. Met.-Org. Chem.
1985, 15, 149 – 153.
[12] A. Maraval, K. Owsianik, D. Arquier, A. Igau, Y. Coppel, B.
Donnadieu, M. Zablocka, J.-P. Majoral, Eur. J. Inorg. Chem.
2003, 960 – 968.
[13] Notably, similar ratios of 3o/3c were obtained upon dissolving
crystals of 3c in a variety of chlorinated, ethereal, or hydro-
carbon solvents.
[24] R. M. Acheson, G. Paglietti, P. A. Tasker, J. Chem. Soc. Perkin
Trans. 1 1974, 2496 – 2500.
[25] D. A. Smith, P. W. Dyer, unpublished results.
[26] S. Green, C. Jones, G. Jin, A. Stasch, Inorg. Chem. 2007, 46, 8 –
10.
[27] T. R. van den Ancker, P. C. Andrews, S. J. King, J. E. McGrady,
C. L. Raston, B. A. Roberts, B. W. Skelton, A. H. White, J.
Organomet. Chem. 2000, 607, 213 – 221.
[28] L. Baiget, A. S. Batsanov, P. W. Dyer, M. A. Fox, M. J. Hanton,
J. A. K. Howard, P. K. Lane, S. A. Solomon,Dalton Trans. 2008,
1043 – 1054.
[29] Z configuration about the N C bond.
[15]
=
[30] R. W. Reed, G. Bertrand in Phosphorus-31 NMR Spectral
Properties in Compound Characterization and Structural Anal-
ysis (Eds.: L. D. Quin, J. D. Verkade), VCH, New York, 1994,
chap. 15, pp. 189 – 200.
[31] See, for example: C. Buron, H. Gornitzka, V. Romanenko, G.
Bertrand, Science 2000, 288, 834 – 836; S. Marrot, T. Kato, F. P.
Cossio, H. Gornitzka, A. Baceiredo, Angew. Chem. 2006, 118,
7607 – 7610; Angew. Chem. Int. Ed. 2006, 45, 7447 – 7450; P. W.
Dyer, J. Fawcett, M. J. Hanton, D. M. Mingos, A.-M. Williamson,
Dalton Trans. 2004, 2400 – 2401.
[14] The solid resulting from removal of solvent from a solution of
crystals of 3c was also analyzed by solid-state CPMAS 31PNMR
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