Pyrido-annulated 1,3-azaphospholes: Synthesis of 1,3-azaphospholo[5,4-b] pyridines and preliminary reactivity studies

Article abstract of DOI:10.1002/ejic.201000253

Pyrido-annulated σ²-phosphorus heterocycles, 1,3-azaphospholo[5,4-b]pyridines 4 and 5, were synthesized by reduction of diethyl 2-aminopyridme-3-phosphonates 1 with LiAlH₄ and cyclocondensation of the resulting 2-amino-3-phosphanylpyridines 2 with dimethylformamide and dimethylacetamide dimethyl acetal, respectively, via intermediate phosphaalkenes 3. The P=C-N heterocycles are stable in the presence of OH and NH compounds but add tBuLi at the P=C bond. Reaction with one equivalent of M(CO)₅(thf) leads to η¹-P-coordinated (azaphospholo[5,4-b]pyridine)M(CO)₅ complexes (M = Cr, Mo, W). Spectroscopic data are in accordance with the dominance of π-acceptor properties. X-ray crystal structure analyses reveal "base-pairing" of 2-amino3-phosphanylpyridine (2a) and NH-functional azaphospholopyridine 5a by N-H...N hydrogen bonds, and the competing formation of 1,3-diphosphetane 6c from the phosphaalkene intermediate.

Full text of DOI:10.1002/ejic.201000253

FULL PAPER
DOI: 10.1002/ejic.201000253
Pyrido-Annulated 1,3-Azaphospholes: Synthesis of
1,3-Azaphospholo[5,4-b]pyridines and Preliminary Reactivity Studies
Mohamed Shaker S. Adam,[a] Peter G. Jones,[b] and Joachim W. Heinicke*[a]
Dedicated to Professor Gerd Becker on the occasion of his 70th birthday
Keywords: Phosphanes / Phosphaalkenes / P ligands / P,N heterocycles / Pyridine annulation
Pyrido-annulated σ2-phosphorus heterocycles, 1,3-azaphos- to η1-P-coordinated (azaphospholo[5,4-b]pyridine)M(CO)5
pholo[5,4-b]pyridines 4 and 5, were synthesized by reduction
of diethyl 2-aminopyridine-3-phosphonates 1 with LiAlH4
and cyclocondensation of the resulting 2-amino-3-phosphan-
complexes (M = Cr, Mo, W). Spectroscopic data are in ac-
cordance with the dominance of π-acceptor properties. X-ray
crystal structure analyses reveal “base-pairing” of 2-amino-
ylpyridines 2 with dimethylformamide and dimethylacet- 3-phosphanylpyridine (2a) and NH-functional azaphospholo-
amide dimethyl acetal, respectively, via intermediate phos-
phaalkenes 3. The P=C–N heterocycles are stable in the pres-
ence of OH and NH compounds but add tBuLi at the P=C
bond. Reaction with one equivalent of M(CO)5(thf) leads
pyridine 5a by N–H···N hydrogen bonds, and the competing
formation of 1,3-diphosphetane 6c from the phosphaalkene
intermediate.
reported.[8,9] Coordination of RhI, PdII, or NiII at the σ2-
phosphorus atom of these P=C–N heterocycles is disfa-
vored, and preliminary attempts to generate in situ (benz-
azaphosphole)Pd catalysts for amination of 2-bromopyr-
idine or benzazaphosphol-2-carboxylate NiII catalysts for
ethylene oligomerization both failed.[9] This inspired us to
explore whether electron-withdrawing annulation changes
the reactivity and coordination properties of azaphospholes
towards those of phosphabenzenes. Phosphaindolizines and
related N-bridgehead annulated 1,3-azaphospholes indeed
show much stronger deshielding of the phosphorus atom in
31P NMR spectroscopy, are less stable towards addition at
the P=C bond,[10] and display π-acidic properties in
LM(CO)5 complexes (L = annulated azaphosphole).[11] The
first pyridine-annulated 1,3-azaphospholes with a C–C
bridge, 1,3-azaphospholo[4,5-b]pyridines, were recently de-
scribed, but their purification proved difficult.[12] Here we
report on the more easily accessible 1,3-azaphospholo[5,4-b]-
pyridines, their positional isomers with both nitrogen atoms
bonded to the same carbon atom, and preliminary reacti-
vity studies.
Introduction
Trivalent phosphorus ligands are widely tunable in their
stereoelectronic properties and play a key role in many late-
transition-metal-catalyzed reactions.[1] Highly basic and
bulky phosphane ligands[2] represent an important focus of
current research in this field, but the range of less basic and
even π-acidic phosphorus ligands is also being extended by
increasing research activities on the catalytic use of phos-
phite ligands[3] and phosphabenzene[4] and phosphaalkene[5]
derivatives with neutral σ2-phosphorus. Our interest is di-
rected at another type of dicoordinated phosphorus com-
pounds with a P=C–N substructure. This is stabilized by σ-
electron withdrawal towards nitrogen, which in turn favors
π-backbonding within the P=C–N unit or stronger stabili-
zation within π-cyclodelocalized 1,3-azaphospholes[6] or an-
nulated heterocycles thereof.[7] The π-donation of nitrogen
changes the reactivity of the P=C group, studied so far
mainly for 1,3-benzazaphospholes. The NH and CH lithi-
ation of P=CR–NH or P=CH–NR by tBuLi and introduc-
tion of functional groups, alkylation at phosphorus, and
also modification of the coordination properties have been
[a] Institut für Biochemie – Anorganische Chemie, Ernst-Moritz-
Arndt-Universität Greifswald,
Results and Discussion
Felix-Hausdorff-Str. 4, 17487 Greifswald, Germany
Fax: +49-3834-864377
Synthesis
E-mail: heinicke@uni-greifswald.de
[b] Institut für Anorganische und Analytische Chemie, Technische
Universität Braunschweig,
Annulated 1,3-azaphospholes are accessible by four syn-
thetic routes: generation and orthoformate cyclization of
the corresponding 2-phosphanylarylamines,[7,9,12] reductive
cyclization of 2-phosphonoarylamines,[13] flash-vacuum py-
Hagenring 30, 38106 Braunschweig, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000253.
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M. S. S. Adam, P. G. Jones, J. W. Heinicke
FULL PAPER
rolysis of P-alkyldihydroazaphospholes,[14] or cycloconden- tuted benzazaphospholes[9b] and azaphospholo[4,5-b]pyr-
sation of suitable N-ylides with PCl3 in the presence of idines.[12] For purification, the volatile byproducts (MeOH
base.[10] However, the latter route is applicable only for N- and dimethylamine) and excess starting material were re-
bridgehead annulated systems, and the pyrolysis functions moved in vacuo, and the residues were distilled in high vac-
only for compounds of sufficient thermal stability. There- uum to give oils that rapidly crystallized. In an attempt to
fore, the synthesis according to the first two routes was crystallize crude 5c by overlayering the concentrated thf
studied. The starting materials, primary and secondary 2- solution with n-hexane, the single-crystalline phosphaalk-
amino- and 2-acylamido-3-diethylphosphonopyridines, ene dimer 6c with 1,3-diphosphetane structure was ob-
respectively, were obtained by Pd-catalyzed phosphonyl- tained. The dimerization of 3c to 6c represents an alterna-
ation of the corresponding 2-amino- and 2-acylamido-3- tive reaction of the intermediates 3 known for acyclic phos-
bromo-pyridines with triethyl phosphite.[15]
phaalkenes[17] but detected here for the first time for 1,3-
For the first route, 2-amino-3-phosphonopyridines 1a–d azaphosphole precursors (Scheme 2).
were reduced with LiAlH4 to provide the corresponding 2-
amino-3-phosphanylpyridines 2a–d in reasonable yields.
Small amounts of 2-aminopyridines were formed as side
products by P–C bond cleavage (Scheme 1).
Scheme 1. Reduction of 2-amino-3-phosphonopyridines.
Scheme 2. Cyclization of 2-amino-3-phosphanylpyridines with di-
methylformamide dimethyl acetal or dimethylacetamide dimethyl
acetal.
Relative to the rather smooth reduction of 2-phos-
phonoanilines to 2-phosphanylanilines,[9] the competing re-
ductive P–C bond cleavage is only slightly faster for 2-
To test the applicability of the reductive cyclization of 2-
amino-3-phosphonopyridines 1a–d, whereas introduction acylamidopyridine-3-phosphonates to access 1,3-azaphos-
of a further N atom into the heterocycle strongly enhances pholo[5,4-b]pyridines, in a manner corresponding to the
this side reaction. Attempts to reduce 2-amino-3-phos- above-mentioned second route to benzazaphospholes, 2-
phonoquinoxalines[15] to 2-amino-3-phosphanyl-quinoxalines acylamidopyridine 3-phosphonate 7[15,18] was reduced in di-
resulted in quantitative dephosphanylation. This prevented ethyl ether with excess LiAlH4. However, the reaction took
the extension of our studies to quinoxaline-annulated aza- a different course from that in the case of phosphono-
phospholes. Similar P–C bond cleavage reactions by LiAlH4 anilides. Instead of the pyrido-annulated azaphospholes,
were observed for sterically hindered o-phosphonophenols which should form by primary reduction of the phosphono
and quantitatively for 1-phosphono- and 1-(phenylphos- group and subsequent cyclocondensation with the imidate
phinoyl)naphth-2-ols.[16] In combination with the detection group, N-secondary 2-amino-3-phosphanylpyridine 8 was
of a bicyclic oxadiphosphole, these findings imply the facili- formed as the main product (Scheme 3) contaminated by
tation of phosphanylidene extrusion if PHR-groups are unidentified products. The main product was formed by re-
sited in the o-position to NHR or OH groups and subjected duction of the amide group prior to the phosphono group,
to electron withdrawal or to the sterically enforced proxim- controlled by the electron-withdrawing influence of the pyr-
ity of the PHR and OH groups (in 6-tert-butyl-2-phosphan- idine ring (cf. ref.[12]). Variation of R did not change the
ylphenols[16a]).
reaction course. For benzoyl, naphthoyl, 2-furoyl, and 2-
The envisaged pyrido-annulated P=C–N heterocycles thenoyl derivatives, the results are analogous. The crude
were synthesized by cyclocondensation of 2-amino-3-phos- aminophosphanylpyridine 8 was converted into the corre-
phanylpyridines with dimethylformamide dimethyl acetal sponding pyrido[b]-azaphosphole 9 by heating with excess
(DMFA) or dimethylacetamide dimethyl acetal (DMAA), dimethylformamide dimethyl acetal. The heterocycle could
respectively. The reactions proceed slowly and need heating not be obtained in pure form by high-vacuum distillation
at 50 °C for approximately 5 d. The cyclocondensation reac- but was unambiguously identified by spectroscopic meth-
tion proceeds via phosphaalkene intermediates 3, which un- ods.
dergo cyclization to 1,3-azaphospholo[5,4-b]pyridines 4a–d
The structure elucidation of compounds 2–9 is based on
and 5a,c, respectively. This was detected by NMR spectro- conclusive HRMS and fully assigned NMR spectroscopic
scopic monitoring of the conversion of 2a with DMAA, data, together with X-ray structure investigations of 2a, 5a,
initially revealing signals characteristic for 3a (R3 = Me) and 6c. Typical features of primary phosphanes 2 and 8 are
1
that disappear with the progress of the reaction, in a similar the proton doublets of the PH2 group with JPH = 201–
manner to that observed in the synthesis of bulky N-substi- 222 Hz at δ = 3.2–3.7 ppm and upfield phosphorus signals
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Pyrido-Annulated 1,3-Azaphospholes
tances are C1···C5 2.485(2) and P1···P2 2.6859(6) Å, with
fold angles about these axes of 38.6(1) and 41.5(1)°, respec-
tively. Two intramolecular hydrogen bonds of the form N–
H···N are observed.
Scheme 3. Attempts at reductive cyclization of 2-acylamido-3-
phosphonopyridines and cyclocondensation of the reduction prod-
ucts.
(δ = –147 to –151 ppm). Further information was obtained
by the X-ray structure analysis of single crystals of 2a
grown by slow evaporation of the solvent from a concen-
trated solution of 2a in diethyl ether. As depicted in Fig-
ure 1, inversion-symmetric pairs of molecules are formed by
intermolecular hydrogen bonds between the N–H group
and pyridine nitrogen atom of a neighboring molecule. The
phosphorus lone electron pair is directed away from the
amino group and thus cannot interact with the second pro-
ton at nitrogen. There are no other short intermolecular
contacts. Bond lengths and bond angles of 2a exhibit usual
values.
Figure 2. Molecular structure of 6c in the crystal (ellipsoids with
50% probability). Two disordered CDCl3 molecules were omitted
for clarity. Selected bond lengths [Å] and angles [°]: P1–C1
1.8868(17), P1–C5 1.8995(17), P1–C11 1.8220(16), P2–C1
1.8862(17), P2–C5 1.8838(16), P2–C31 1.8207(18), P1···P2
2.6859(6), H20···N2 2.20(3), H40···N6 2.25(3), N20···N2 3.003(2),
N40···N6 3.043(2); C1–P1–C5 82.03(7), C5–P2–C1 82.46(7), P2–
C1–P1 90.77(7), P2–C5–P1 90.46(7), N2–C1–P2 121.30(11), N2–
C1–P1 115.32(12), N6–C5–P1 122.32(12), N6–C5–P2 114.01(11),
N20–H20···N2 173(2), N40–H40···N6 173(2).
The structures of all the 1,3-azaphospholo[4,5-b]pyr-
idines are proved by unambiguous NMR spectroscopic
data. Characteristic are the aforementioned phosphorus
chemical shifts, although they vary with substituents at the
nitrogen atom and C-2. For 1,3-azaphospholo[5,4-b]pyr-
idines without substituent at C-2, a marked upfield shift is
observed in the series 4a (R2 = H), 4d (R2 = 2-Pyr), 4c
(R2 = Ph), 4b (R2 = Et) (δ = 78.3, 76.0, 71.5, 65.9 ppm,
respectively), which reflects the influence of the N-substitu-
ents on the conjugative shift of π-density from the N lone
electron pair to phosphorus. Compared to 1,3-benzaza-
phospholes and the N-position-isomeric 1,3-azaphos-
pholo[4,5-b]pyridines, the phosphorus chemical shifts are
Figure 1. Dimeric aggregate of 2a in the crystal (ellipsoids with
50% probability). Selected bond lengths [Å] and angles [°]: P–C3
1.8223(19), N1–C2 1.338(2), N1–C6 1.342(3), N2–C2 1.359(2); N2–
C2–C3 121.70(17), C4–C3–P 118.52(14), C2–C3–P 123.95(14); N2–
H04 0.85(2), H04···N1#1 2.19(2), N2···N1#1 3.029(2), N2–
H04···N1#1 172(2), (symmetry transformation: #1 –x, –y, –z + 1).
Phosphaalkene intermediate 3a was detected in the reac- changed only slightly, whereas pyrido[a]-annulated 1,3-aza-
tion of 2a with DMAA by the upfield phosphorus signal phospholes with an N-bridgehead experience strong down-
relative to that of 5a (δ = 49.5 vs. 66.6 ppm) and a slightly field shifts.[10] This shows that the electron-withdrawing ef-
2
larger JPC coupling (27.2 vs. 24.9 Hz) of the 2-methyl 13C fect of the pyridine nitrogen atom in annulated pyrido-aza-
resonance, indicative of the Me–C=P structural unit. The phospholes is strong only if this atom also acts as the N-
strongly upfield position of the signal of 3a relative to the donor site in the azaphosphole ring. The chemical shifts of
typical phosphaalkene range (δ = 200–350 ppm)[19] is attrib- C-2 and the P–C coupling constants within the five-mem-
utable to the +M effect of the amino group and is slightly bered ring are only marginally influenced by the N-substitu-
diminished in pyrido-azaphospholes 4 and 5 (δ = 65– ent. In the proton NMR spectra, the most characteristic
78 ppm) by cyclodelocalization. The nature of 6c, formed in features are the large downfield shifts (δ = 8.8–9.6 ppm) and
competition with 4c from the corresponding phosphaalkene 2JPH coupling constants (37–39 Hz) of the hydrogen atom
intermediate as a byproduct, was proved by X-crystal struc- in the 2-position. The 2-methyl signals of 5a,c appear in the
ture analysis (Figure 2). The molecule possesses no crystal- region of aromatic methyl compounds. Detailed structural
lographic symmetry. In the four-membered ring, the substit- information on 5a is given by its crystal structure analysis
uents at the carbon atoms are mutually cis, as are the sub- (Figure 3). The heterocycles are planar, and the two distinct
stituents at the phosphorus atoms. The transannular dis- σ2-P–C bond lengths display similar values to those of the
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M. S. S. Adam, P. G. Jones, J. W. Heinicke
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1,3-benzazaphospholes, as do the other bond lengths and ter quenching with CH3OD, by the phosphorus triplet
angles within the azaphosphole ring. The molecules are slightly upfield from the singlet of 4b (δ31P = 64.7 ppm, 2JPD
connected to inversion-symmetric pairs via N···H–N hydro- = 5.5 Hz), but the major reaction remains addition with for-
gen bonds in the crystal, as found for 2a. This facilitates mation of 11b after deuteriolysis (δ31P = –20.1 ppm).
crystallization and purification relative to the N-substituted
derivatives 9 or to the 1,3-azaphospholo[4,5-b]pyridines
with the pyridine N atom on the P side.[12]
Scheme 4. Reactivity of 4c.
Reaction of 4c with a slight excess to one equivalent of
W(CO)5(thf) or the analogous molybdenum and chromium
compounds provided the corresponding η1-P-(pyrido-aza-
phosphole)M(CO)5 complexes 13c (M = Cr), 14c (M =
Mo), and 15c (M = W). These complexes are yellow solids,
air-stable in the solid state, sparingly soluble in hydro-
carbons such as n-hexane or benzene but soluble in thf or
CHCl3. As shown by the NMR spectroscopic data, coordi-
nation through the σ2-phosphorus is preferred to coordina-
tion via the pyridine nitrogen [pyridine M(CO)5 deriva-
tives][23], as observed for 2-pyridylphosphorines.[24] The up-
field phosphorus chemical shift of tungsten complex 15c
relative to ligand 4c (∆δ = –26.3 ppm) is comparable to the
upfield coordination shift of (triphenylphosphorine)W-
(CO)5 (∆δ = –21.9 ppm)[25] but forms a strong contrast to
Figure 3. Dimeric aggregate of 5a in the crystal (ellipsoids with
50% probability). Selected bond lengths [Å] and angles [°]: C2–
P3 1.7321(15), P3–C3A 1.7878(15), N1–C2 1.3608(19), N1–C7A
1.3735(19), C6–N7 1.3378(19), N7–C7A 1.3411(19); C2–N1–C7A
113.81(13), C2–P3–C3A 88.86(7), C4–C3A–P3 133.37(11), C7A–
C3A–P3 110.75(11), C6–N7–C7A 115.33(12); N1–H01 0.81(2),
H01···N7#1 2.09(2), N1···N7#1 2.8934(18), N1–H01···N7#1 171(2)
(symmetry transformation: #1 – x, –y + 1, –z + 1).
Reactivity
The pyrido[b]-annulated 1,3-azaphospholes 4 and 5 are the downfield shift of tungsten complex 12c vs. pyridoaza-
slowly attacked by air in the solid state, but rapidly in solu- phospholine complex 11c (∆δ = 37.9 ppm) and illustrates
tion. They are basic because of the pyridine nitrogen atom the different coordination properties of σ2- vs. σ3-phospho-
and are stable towards cold water, dilute aqueous sulfuric rus ligands. For N-ethyl derivative 15b, synthesized analo-
acid, or sodium hydroxide solution; in this respect they re- gously to 15c, the upfield coordination shift from 4b is even
semble the 1,3-benzazaphospholes more than the hydrolyti- larger (∆δ = –34.4 ppm) and similar to the value observed
cally sensitive N-bridgehead pyrido[a]-annulated 1,3-aza- for the W(CO)5 complex of 2-methyl-1,3-benzazaphosphole
phospholes. Treatment of 4c with tert-butyllithium in diethyl (∆δ = –36.3 ppm).[8c,8d] It can be attributed to π-backbond-
ether at low temperature (–80 °C) resulted, however, in ad- ing from tungsten to phosphorus, which is involved in the
dition at the P=C bond, as known for phosphabenzenes[20] cyclodelocalized π-system.[26] For the analogous Mo(CO)5
or benzoxaphospholes,[21] even in the presence of KOtBu, and Cr(CO)5 complexes with 2-methylbenzazaphosphole
which favors CH-lithiation in the case of benzazaphos- and 4c as ligands, the upfield coordination shifts are re-
pholes.[9] Quenching of the addition product 10c with duced or changed to small downfield shifts (∆δ = –8.3,
CH3OD afforded the pyridine-annulated 3-tert-butyl-2- 6.8;[8b] 1.5, 21.0 ppm) but are far away from the strong
deuterio-azaphospholine 11c (Scheme 4), whereas treatment downfield shifts of phosphane M(CO)5 complexes (Mo ∆δ
of 10c with chlorotrimethylsilane led to a mixture of the ≈ 40–44, Cr ∆δ ≈ 55–63 ppm)[27] so that the results can be
expected
2-trimethylsilyl-3-tert-butylbenzazaphospholine interpreted in the same way and the σ2-P atoms of pyr-
with unidentified byproducts. If the reaction with tBuLi is idoazaphospholes can be classified as weak donors but
carried out in thf (–80 °C), which is more easily deproton- rather strong π-acceptors. The larger upfield shifts for N-
ated by strong metalating agents,[22] the primarily formed ethyl derivative 15b and benzazaphosphole complexes can
10c is rapidly protonated by the solvent. Subsequent ad- be attributed to the π-donor properties of the tricoordinate
dition of W(CO)6 did not afford the heterocyclic Fischer nitrogen atom in the 1,3-azaphosphole ring resulting in in-
carbene W(CO)5CROLi but the η1-P-(pyrido-azaphos- creased π density at phosphorus. The carbon chemical shifts
pholine)W(CO)5 complex 12c, characterized by conclusive of 13c and 14c vs. those of 4c show a strong upfield coordi-
spectroscopic data. With the N-ethyl-pyridoazaphosphole nation shift for C-2 and a rather weak effect for C3a (∆δ
1
4b, minor CH lithiation (Ͻ20%) by tBuLi was detected, af- = –19.8, –2.6 ppm) while both JPC coupling constants are
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Pyrido-Annulated 1,3-Azaphospholes
strongly reduced (∆1J = –31.9, –28.6 Hz). For (2-methyl-
benzazaphosphole)Cr(CO)5 the coordination chemical
shifts for C-2 and C3a are more similar (∆δ = –4.8,
mercial suppliers. Compounds 1a–d were prepared as reported re-
cently.[15] NMR spectra were measured with
multinuclear
a
FTNMR spectrometer (Bruker ARX300). The 1H, 13C, and 31P
chemical shifts are δ values and are given in ppm relative to Me4Si
and H3PO4 (85%), respectively. Coupling constants refer to JHH in
1H and to JPC in 13C NMR spectra unless indicated otherwise.
HRMS measurements were carried out at the Institut für Organi-
sche und Biomolekulare Chemie, University of Göttingen with a
double focusing sector-field Finnigan MAT 95 instrument with EI
(70 eV, PFK as reference substances) or with ESI in MeOH,
–5.3 ppm) and indicate thus, in addition to the stronger 31
P
coordination shift, a clear difference from pyridoazaphos-
1
phole ligands. The effects on the JPC coupling constants
(∆1J = –29.9, –28.3 Hz) are comparable with those in 13c
and indicative for the common σ2-P nature of the two P
ligands. The coordination chemical shift for C-5 and C-6
(∆δ = 0.0, 2.0 ppm) is low and indicates absence of N-coor- MeOH/NH4OAc or MeCN with a 7T Fourier transform ion APEX
IV (Bruker Daltonics) cyclotron resonance mass spectrometer. LR
Mass spectra were measured with a single focusing AMD40 sector-
field mass spectrometer. Elemental analyses were carried out with
a CHNS-932 analyzer from LECO under standard conditions. IR
spectra were recorded as KBr pellets or capillaries or in Nujol with
a Nicolet® Magna550 instrument. Absorption bands are given as
wavenumbers in cm–1.
dinated Cr(CO)5, which should cause downfield shifts of ∆δ
= 4–6 ppm.[23b] Coordination at the nitrogen atom allows a
further modification of the character of pyrido-annulated
1,3-azaphosphole ligands, which has still to be investigated.
Conclusions
General Procedure for the Reduction of 3a–d with LiAlH4: A solu-
tion of 3a–d in diethyl ether (10–20 mL) was added dropwise at
0 °C to a solution of LiAlH4 pellets in diethyl ether (10–20 mL).
The reaction mixture was stirred for 2 d at room temperature.
Then, on cooling with ice-water, degassed water was added drop-
wise until the evolution of hydrogen ceased. Solids were filtered off
and washed with diethyl ether. The combined filtrates were dried
with Na2SO4. Compounds 4a–d were worked up by concentration
and crystallization or by vacuum distillation or used directly in
cyclocondensations.
2-Amino-3-phosphanyl-pyridines are accessible by LiAlH4
reduction of 2-amino-3-phosphonopyridines. P–C bond
cleavage may occur as a minor side reaction. [4+1]-
Cyclocondensation with dimethylformamide dimethyl ace-
tal or dimethylacetamide dimethyl acetal provides pyrido-
[b]-annulated 1,3-azaphospholes with both nitrogen atoms
on the same side, that is, 1,3-azaphospholo[5,4-b]pyridines.
2-Amino-3-phosphanyl-pyridines and the heterocycles with
NH functions tend to form dimers by intermolecular
N···H–N bridges, which favors crystallization and raises the
question whether pairing with other types of 2-amino or 2-
amidopyridines is possible. The stability of the P=C bond
towards usual nucleophiles such as water, alcohols, or
amines facilitates work with these compounds. Addition of
tBuLi at the P=C bond prevents access to functionally sub-
stituted derivatives at the 2-position. However, the addition
will allow access to functionally substituted dihydropyrido-
azaphospholes if solvents such as hexane are used, which
are not deprotonated by the strongly basic heterocyclic Li
adducts. With respect to the easier addition of RLi at the
P=C bond, the heterocycles are somewhat closer to phos-
phabenzenes, but the coordination behavior remains more
similar to that of 1,3-benzazaphospholes. In contrast to the
latter, the 1,3-azaphospholo[5,4-b]pyridines are ambident-
ate σ2 P,N ligands. Electron-rich, zero-valent group 6 metal
carbonyls coordinate preferably at the σ2 phosphorus atom.
The NMR spectra indicate dominant π-acceptor properties.
Coordination at both coordination sites, P and N, the influ-
ence of N-coordination on the coordination properties at
phosphorus, and the potential use in catalysis or for synthe-
sis of new ligands by addition reactions at the P=C bond
are challenges for further investigations of the new hetero-
cycles.
2-Amino-5-methyl-3-phosphanylpyridine (2a): Reduction of 1a
(0.59 g, 2.40 mmol) by LiAlH4 pellets (0.30 g, 7.30 mmol) provided
0.20 g (59%) of 2a as white crystals from the concentrated solution
in diethyl ether. The X-ray crystal data are compiled in Table 1, the
1
molecular structure is shown in Figure 1. H NMR (CDCl3): δ =
1
2.17 (s, 3 H, 5-CH3), 3.61 (d, JPH = 202.2 Hz, 2 H, PH2), 4.71
3
4
(vbr. s, 2 H, NH), 7.52 (ddd, JPH = 10.8, J = 2.2 Hz, 1 H, 4-H),
7.91 (s, 1 H, 6-H) ppm. 13C{1H} NMR (CDCl3): δ = 17.21 (CH3-
5), 105.41 (d, 1J = 12.8 Hz, Cq-3), 123.28 (d, 3J = 8.6, Cq-5), 147.00
(d, 2J = 28.5 Hz, CH-4), 149.43 (CH-6), 158.49 (d, 2J = 1.3 Hz, Cq-
2) ppm. 31P{1H} NMR (CDCl3): δ = –147.0 ppm. MS (EI 70 eV,
25 °C): m/z (%) = 141 (5), 140 (100) [M]+, 139 (8), 123 (26), 107
(35), 106 (11), 93 (17), 80 (11). HRMS (EI): calcd. for C6H9N2P+
140.0503; found 140.0504.
2-(Ethylamino)-5-methyl-3-phosphanylpyridine (2b): Reduction of
1b (2.07 g, 7.60 mmol) by LiAlH4 pellets (0.87 g, 22.8 mmol)
yielded 0.96 g (75%) of colorless oily 2b, slightly contaminated with
1
the P–C cleavage product 2bЈ. For 2b: H NMR (CDCl3): δ = 1.25
(t, 3J = 7.2 Hz, 3 H, CH3), 2.13 (s, 3 H, 5-CH3), 3.48 (qd, 3J = 7.2,
5.3 Hz, 2 H, NCH2), 3.51 (d, 1JPH = 201.5 Hz, 2 H, PH2), 4.52 (br.
3
4
s, 1 H, NH), 7.48 (dd, JPH = 11.9, J = 2.1 Hz, 1 H, 4-H), 7.98
(br. s, 1 H, 6-H) ppm. 13C{1H} (and DEPT135) NMR (CDCl3): δ
= 14.97 (CH3), 17.16 (CH3-5), 36.71 (CH2), 105.18 (d, 1J = 13.4 Hz,
3
2
Cq-3), 120.80 (d, J = 9.8 Hz, Cq-5), 146.77 (d, J = 31.8 Hz, CH-
4), 148.99 (CH-6), 157.92 (Cq-2) ppm. 31P{1H} NMR (CDCl3): δ
= –150.4 ppm. MS (EI 70 eV, 150 °C): m/z (%) = 168 (76) [M]+,
167 (81), 108 (100). HRMS (ESI in MeOH, NH4OAc): calcd. for
C8H14N2P+ [M + H]+ 169.08892; found 169.08891. 2-Ethylamino-
5-methylpyridine (2bЈ): 1H NMR (CDCl3): δ = 1.22 (t, 3J = 7.2 Hz,
CH3), 2.16 (s, 5-CH3), 3.26 (qd, 3J = 7.2 Hz, CH2), 4.87 (s br, NH),
Experimental Section
3
3
4
General: All preparations were carried out in dried and deoxy-
genated solvents. Reactions with air- or moisture-sensitive com-
pounds were conducted under an argon atmosphere by using
Schlenk techniques. All reagents were used as received from com-
6.32 (d, J = 8.3 Hz, 3-H), 7.24 (dd, J = 8.2, J = 2–3 Hz, 4-H),
7.83 (d br, 6-H) ppm. 13C{1H} NMR (CDCl3): δ = 14.63 (CH3),
16.98 (5-CH3), 36.96 (NCH2), 106.32 (CH-3), 124.17 (Cq-5), 138.97
(CH-4), 140.79 (CH-6), 153.40 (Cq-2) ppm.
Eur. J. Inorg. Chem. 2010, 3307–3316
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3311

M. S. S. Adam, P. G. Jones, J. W. Heinicke
FULL PAPER
5-Methyl-2-(phenylamino)-3-phosphanylpyridine (2c): Reduction of 43 Hz, Cq-3a), 137.97 (d, 2J = 19.0 Hz, CH-4), 146.78 (d, 2J =
1c (5.0 g, 15.6 mmol) by LiAlH4 pellets (1.8 g, 47.4 mmol) provided
1.8 g (53%) of colorless crude viscous oily 2c, which was used with-
6.6 Hz, Cq-7a), 146.81 (d, 4J = 3.0 Hz, CH-6), 161.11 (d, 1J =
54.5 Hz, Cq-2) ppm. 31P{1H} NMR (CDCl3): δ = 65.9 ppm. MS
(EI 70 eV, 285 °C): m/z (%) = 179 (10), 178 (86) [M]+, 163 (48), 150
1
out further purification for cyclocondensations. H NMR (C6D6):
1
δ = 1.82 (s, 3 H, 5-CH3), 3.24 (d, JPH = 222.0 Hz, 2 H, PH2), 6.52 (100), 149 (22), 122 (18), 92 (17), 69 (18). HRMS (ESI in MeOH,
(br. s, 1 H, NH), 6.98 (t, 3J = 7.3 Hz, 1 H, p-H), 7.24–7.32 (m, 3
NH4OAc): calcd. for C9H12N2P+ [M + H]+ 179.07329; found
179.07326.
3
H, 4-H, m-H), 7.74 (d br, J = 7.9 Hz, 2 H, o-H), 8.08 (br. s, 1 H,
6-H) ppm. 13C{1H} NMR (C6D6): δ = 17.72 (CH3-5), 108.30 (d, 1J
5-Methyl-1-phenyl-1,3-azaphospholo[5,4-b]pyridine (4c): Compound
2c (1.50 g, 6.94 mmol) and N,N-dimethylformamide dimethyl ace-
tal (0.95 mL, 7.14 mmol) gave 0.48 g (31%) 4c, which, after high
vacuum distillation at 10–5 mbar/56–60 °C (bath temperature),
3
= 15.2 Hz, Cq-3), 120.45 (2 CH-o), 122.81 (CH-p), 124.53 (d, J =
9.2 Hz, Cq-5), 129.70 (2 CH-m), 142.19 (Cq-i), 147.84 (d, 2J =
32.0 Hz, CH-4), 150.01 (CH-6), 156.40 (Cq-2) ppm. 31P{1H} NMR
(C6D6): δ = –150.6 ppm. MS (EI 70 eV, 340 °C): m/z (%) = 217
(11), 216 (100) [M]+, 215 (41), 182 (40), 138 (24), 93 (32). HRMS
(EI): calcd. for C12H13N2P+ 216.0816; found 216.0812.
1
formed white crystals, m.p. 78 °C. H NMR (CDCl3, ref. 7.25): δ
5
= 2.45 (d, JPH = 0.6 Hz, 3 H, 5-CH3), 7.43 (tt, 1 H, p-H), 7.50–
3
4
5
7.63 (m, 4 H, 2 m-H, 2 o-H), 8.23 (ddd, JPH = 3.0, J = 2.2, J =
2
3-Phosphanyl-2-[(2Ј-pyridyl)amino]pyridine (2d): Reduction of 1d
0.8 Hz, 1 H, 4-H), 8.34 (s br, 1 H, 6-H), 8.79 (d, JPH = 37.6 Hz, 1
(0.69 g, 2.25 mmol) by LiAlH4 pellets (0.26 g, 6.85 mmol) furnished H, 2-H) ppm. 13C{1H} NMR (CDCl3): δ = 18.20 (CH3-5), 125.62
0.25 g (55%) of pale yellow oily 2d, contaminated by ca. 8 mol-% of
the P–C cleavage product 2dЈ. The crude product was used without
(superimposed, Cq-5), 125.76 (2 CH-o), 127.86 (CH-p), 129.28 (2
2
CH-m), 133.72 (d, 1J = 43.2 Hz, Cq-3a), 138.07 (d, J = 19.1 Hz,
1
3
4
further purification. 2d: 1H NMR (CDCl3): δ = 3.73 (d, JPH
=
CH-4), 140.21 (d, J = 2.4 Hz, Cq-i), 147.56 (d, J = 2.8 Hz, CH-
203.3 Hz, 2 H, PH2), 6.72 (dd, J = 7.3, 4.8 Hz, 1 H, 5Ј-H), 6.83 6), 151.67 (d, 2J = 6.5 Hz, Cq-7a), 162.69 (d, 1J = 54.3 Hz, CH-
3
3
4
(ddd, J = 7.3, 5.0, J = 0.9 Hz, 1 H, 5-H), 7.50 (br. s, 1 H, NH),
2) ppm. 31P{1H} NMR (CDCl3): δ = 71.5 ppm. MS (EI 70 eV,
25 °C): m/z (%) = 227 (13), 226 (100) [M]+, 225 (61), 184 (30), 183
(64), 84 (14), 77 (18), 51 (14). HRMS (EI): calcd. for C13H11N2P+
226.0660; found 226.0656.
7.59 (td, 3J = 8.5, 3J = 7.3, 4J = 1.9 Hz, 1 H, 4Ј-H), 7.75 (ddd, 3JPH
3
4
3
4
= 10.8, J = 7.4, J = 1.9 Hz, 1 H, 4-H), 8.18 (ddd, J = 5.0, J =
1.9, 5J = 0.8 Hz, 1 H, 6-H), 8.21 (d br, superimposed, J = 4–5 Hz,
3
1 H, 6Ј-H), 8.30 (d br, 3J = 8.5 Hz, 1 H, 3Ј-H) ppm. 13C{1H} NMR
(CDCl3): δ = 109.38 (d, 1J = 16.5 Hz, Cq-3), 112.45 (CH-3Ј), 116.01
The attempt to isolate 4c before distillation by overlayering of a
saturated solution of the crude product in thf by n-hexane led to
single crystals of 6c, the dimer of the phosphaalkene precursor. The
structure of 6c is depicted in Figure 3, and the crystal data are
given in Table 1. Crude 4c also contains 20 mol-% of 5-methyl-2-
phenylaminopyridine (2dЈ) as impurity. 1H NMR (CDCl3): δ = 2.20
(s, 3 H, 5-CH3), 6.82 (br. d, 3J = 8.5 Hz, 3-H), 7.00 (m, p-CH),
7.27–7.32 (m, 4 H, o-H, m-H), 7.32 (dd, superimposed, 1 H, 4-H),
8.03 (dd, 4J = 1.5, 5J = 0.7 Hz, 1 H, 6-H) ppm. 13C{1H} NMR
(CDCl3): δ = 17.50 (CH3-5), 108.23 (CH-3), 119.56 (2 CH-o),
122.20 (CH-p), 124.08 (Cq-5), 129.21 (2 CH-m), 138.54 (CH-4),
140.93 (Cq-i), 147.91 (CH-6), 153.77 (s, Cq-2) ppm.
3
(d, J = 9.0 Hz, CH-5), 117.43 (CH-5Ј), 137.79 (CH-4Ј), 146.51 (d,
2J = 30.2 Hz, CH-4), 147.81 (CH-6Ј), 148.83 (CH-6), 153.20 (Cq-
2Ј), 155.85 (noise level, Cq-2) ppm. 31P{1H} NMR (CDCl3): δ =
–147.5 ppm. MS (EI 70 eV, 50 °C): m/z (%) = 204 (12), 203 (100)
[M]+, 202 (81), 187 (21), 171 (24) [M – PH]+, 170 (80), 123 (36),
109 (61), 78 (84). HRMS (ESI in MeOH, NH4OAc): calcd. for:
C10H11N3P+ [M + H]+ 204.06851; found 204.06862. 2dЈ: 13C NMR:
δ = 111.53 (CH-3), 116.31 (CH-5), 137.66 (CH-4), 147.69 (CH-6),
ca. 154 (Cq-2) ppm.
General Procedure for the Synthesis of 4 and 5: A mixture of N,N-
dimethylformamide or N,N-dimethylacetamide dimethyl acetal and
1a–d was heated with stirring for about 5 d at 40–50 °C. The prod-
uct mixture was separated by distillation at ca. 10–5 mbar.
1-(2Ј-Pyridyl)-1,3-azaphospholo[5,4-b]pyridine (4d): Compound 2d
(0.43 g, 2.12 mmol) and N,N-dimethylformamide dimethyl acetal
(0.32 mL, 2.41 mmol) gave oily 4d, which crystallized after distil-
5-Methyl-1H-1,3-azaphospholo[5,4-b]pyridine (4a): Compound 2a lation at 10–5 mbar/58–62 °C (bath temperature), yield 0.21 g
(0.20 g, 1.43 mmol) and N,N-dimethylformamide dimethyl acetal
(47%), m.p. 79 °C. 1H NMR (CDCl3 ref. solv. 7.25): δ = 7.23 (ddd,
(0.20 mL, 1.50 mmol) gave 0.12 g (56%) 4a, which crystallized after 3J = 7.8, JPH ≈ 4J ≈ 1.9 Hz, 1 H, 5-H), 7.33 (ddd, 3J = 7.3, 4.9, J
4
4
distillation at 10–5 mbar/48–50 °C (bath temperature). 1H NMR = 1.0 Hz, 1 H, 5Ј-H), 7.93 (ddd, 3J = 8.3, 7.3, 4J = 1.9 Hz, 1 H,
3
4
3
4
(CDCl3): δ = 2.48 (s, 3 H, 5-CH3), 8.26 (t br, JPH ≈ J = 2–3 Hz, 4Ј-H), 8.46 (ddd, 3J = 7.8, JPH = 3.2, J = 1.8 Hz, 1 H, 4-H), 8.51
2
3
4
5
3
4
1 H, 4-H), 8.36 (s br, 1 H, 6-H), 8.82 (d br, JPH = 37.8 Hz, 1 H, (dt, J = 8.3, J ≈ JPH ≈ 1 Hz, 1 H, 3Ј-H), 8.55 (dt, J = 4.6, J ≈
2-H), 11.22 (br. s, 1 H, NH) ppm. 13C{1H} NMR (CDCl3): δ = 5JPH = 1.5 Hz, 1 H, 6-H), 8.61 (ddd, J = 4.9, J = 1.9, J = 1 Hz,
3
4
5
18.26 (CH3-5), 125.29 (d, 3J = 10.4 Hz, Cq-5), 133.29 (d, 1J = 1 H, 6Ј-H), 9.62 (d br, JPH = 38.4 Hz, 1 H, CH-2) ppm. 13C{1H}
2
42.7 Hz, Cq-3a), 139.12 (d, 2J = 18.0 Hz, CH-4), 145.94 (CH-6), NMR (CDCl3): δ = 117.09 (d, 3J = 9.9 Hz, CH-5), 118.57 (CH-5Ј),
1
151.63 (d, 2J = 5.5 Hz, Cq-7a), 160.01 (d, JPC = 55.4 Hz, Cq- 122.25 (CH-3Ј), 134.77 (d, 1J = 43.1 Hz, Cq-3a), 138.19 (CH-4Ј),
2) ppm. 31P{1H} NMR (CDCl3): δ = 78.3 ppm. MS (EI 70 eV,
260 °C): m/z (%) = 150 (6) [M]+, 149 (100), 148 (7), 98 (16), 85
(15), 84 (14), 43 (29). HRMS (EI): calcd. for C7H7N2P+ 150.0347;
found 150.0349.
138.78 (d, 2J = 19.6 Hz, CH-4), 145.95 (d, 4J = 2.9 Hz, CH-6),
2
148.86 (CH-6Ј), 158.92 (low intensity d, J = 7 Hz, Cq-7a), 161.95
1
(d, J = 52.2 Hz, CH-2) ppm, Cq-2Ј at noise level. 31P{1H} NMR
(CDCl3): δ = 76.0 ppm. MS (EI 70 eV, 250 °C): m/z (%) = 213 (8)
[M]+, 186 (21), 178 (93), 177 (24), 163 (53), 150 (100), 149 (24).
HRMS (ESI in MeOH, NH4OAc): calcd. for C11H9N3P+ [M +
H]+ 214.05286; found 214.05299.
1-Ethyl-5-methyl-1,3-azaphospholo[5,4-b]pyridine (4b): Compound
2b (0.36 g, 2.14 mmol) and N,N-dimethylformamide dimethyl ace-
tal (0.30 mL, 2.26 mmol) gave 0.23 g (60%) of 4b, forming a white
powder after distillation at 10–5 mbar/40 °C (bath temperature). 1H
2,5-Dimethyl-1H-1,3-azaphospholo[5,4-b]pyridine (5a): Compound
1a (0.34 g, 2.43 mmol) and N,N-dimethylacetamide dimethyl acetal
3
NMR (CDCl3): δ = 1.51 (t, J = 7.2 Hz, 3 H, CH3), 2.45 (s, 3 H,
3
5-CH3), 4.51 (t, J = 7.2 Hz, 2 H, CH2), 8.19 (unresolvd. m, 1 H, (0.39 mL, 2.67 mmol) were heated for 1 d at 50 °C. 31P NMR spec-
2
4-H), 8.34 (br. s, 1 H, 6-H), 8.67 (d br, JPH = 37.7 Hz, 1 H, 2-
troscopic monitoring showed formation of a mixture of an interme-
H) ppm. 13C{1H} NMR (CDCl3): δ = 15.32 (CH3), 18.22 (CH3-5),
diate phosphaalkene 3a (R3 = Me) and product 5a (in CDCl3 δ =
43.67 (NCH2), 124.93 (d, 3J = 10.4 Hz, Cq-5), ca. 134.0 (d, 1J = 49.5, 66.6 ppm; intensity ca. 1:1). Heating was continued for 4 d
3312
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 3307–3316

Pyrido-Annulated 1,3-Azaphospholes
to complete the reaction. Distillation at 10–5 mbar/54–56 °C (bath
temperature) furnished 0.22 g oil, rapidly crystallizing to a white
solid. The main component was 5a (60 mol-%, corrected yield
35%), contaminated by 2-amino5-methylpyridine. Crystallization
by slow evaporation of CDCl3 provided pure single crystalline 5a,
m.p. 74 °C. 1H NMR (CDCl3). The crystal data are compiled in
Table 1, selected bond lengths and angles are given in Figure 2. 1H
= 9.1 Hz, C-5), 147.65 (d, 2J = 33.1 Hz, C-4), 150.62 (d, 4J =
1.0 Hz, C-6), 159.71 (C-2) ppm. 31P{1H} NMR (CDCl3): δ =
–153.55 ppm. HRMS (EI): calcd. for C11H19N2P+ 210.1286; found
210.1284. 8aЈ: 1H NMR (C6D6): δ = 0.83 (CMe3), 1.93 (5-CH3),
3
3
3.06 (d, J = 6.5 Hz, NCH2), 4.45 (s br, NH), 6.08 (d, J = 8.4 Hz,
3
4
1 H, 3-H), 6.68 (s br, 1 H, 6-H), 6.96 (dd, J = 8.4, J = 2.4 Hz, 1
H, 4-H) ppm; 6-H superimposed at δ = 8.07 ppm. 13C{1H} NMR
(C6D6): δ = 18.02 (CH3-5), 28.41 (CMe3), 32.51 (CMe3), 54.33
3
NMR (CDCl3): δ = 2.38 (s, 3 H, 5-CH3), 2.79 (d, JPH = 12.3 Hz,
3 H, 2-CH3), 8.05 (br. s, 1 H, 4-H), 8.16 (br. s, 1 H, 6-H), 11.87 (NCH2), 107.11 (CH-3), 121.50 (Cq-5), 138.62 (CH-4), 149.00 (CH-
(br. s, 1 H, NH) ppm. 13C{1H} NMR (CDCl3): δ = 18.30 (d, J =
22.7 Hz, 2-CH3), 18.32 (5-CH3), 125.21 (d, 3J = 9.2 Hz, Cq-5),
134.15 (d, 1J = 45.0 Hz, Cq-3a), 137.60 (d, 2J = 18.2 Hz, CH-4),
145.25 (d, 4J = 2.4 Hz, CH-6), 152.64 (d, 2J = 5.0 Hz, Cq-7a),
6), 159.00 (Cq-2) ppm.
2
5-Methyl-1-neopentyl-1,3-azaphospholo[5,4-b]pyridine (9a): A mix-
ture of N,N-dimethylformamide dimethyl acetal (0.13 mL,
0.95 mmol) and crude 8a (200 mg, 0.95 mmol) was heated whilst
stirring for almost 4 d at 50 °C. Volatiles were removed in vacuo,
and 110 mg (Ͻ48%) of colorless oil was separated by distillation
at 10–5 mbar/60–65 °C (bath temperature). NMR spectroscopic and
1
175.84 (d, JPC = 52.8 Hz, Cq-2) ppm. 31P{1H} NMR (CDCl3): δ
= 67.6 ppm. MS (EI 70 eV, 125 °C): m/z (%) = 165 (9), 164 (100)
[M]+, 163 (75), 149 (6), 136 (10), 69 (14). HRMS (EI): calcd. for
C8H9N2P+ 164.0503; found 164.0497.
1
HRMS data indicated impure 9a. H NMR (CDCl3): δ = 1.00 (s,
Characteristic NMR spectroscopic data of intermediate 3a (R3
=
=
9 H, CH3), 2.44 (s, 3 H, 5-CH3), 4.33 (s, 2 H, NCH2), 8.17 (td,
1
3
4
5
3JPH ≈ J = 2–3, J = 0.8 Hz, 1 H, 4-H), 8.31 (s, 1 H, 6-H), 8.61
Me) in a mixture with 5a. H NMR (CDCl3): δ = 2.05 (d, JPH
4
(d, JPH = 38.0 Hz, 1 H, 2-H) ppm. 13C{1H} NMR (CDCl3): δ =
18.20 (CH3-5), 27.60 (CMe3), 31.99 (CMe3), 59.01 (d, J = 2.9 Hz,
2
5.3 Hz, 3 H, 2-CH3), 2.38 (s, 3 H, 5-CH3), 3.10 (d, JPH = 4.0 Hz,
3
6 H, NMe2), 4.87 (br. s, 2 H, NH2), 7.30 (dd, JPH = 6.4, 4J =
3
2.4 Hz, 1 H, 4-H), 7.76 (dd, 4J = 2.4, 5JPH = 0.6 Hz, 1 H, 6-H) ppm.
13C{1H} NMR (CDCl3): δ = 15.06 (d, 2J = 27.2 Hz, P=CCH3),
18.13 (CH3-5), 37.15 (NMe2), 105.40 (Cq-3a), 121.39 (Cq-5), 138.60
(d, 2J = 21.1 Hz, CH-4), 147.65 (CH-6), 148.37 (Cq-7a), 157.17 (Cq-
2) ppm. 31P{1H} NMR (CDCl3): δ = 49.5 ppm.
NCH2), 124.78 (d, 3J = 10.4 Hz, Cq-5), 133.21 (d, 1J = 42.3 Hz,
Cq-3a), 137.72 (d, 2J = 19.3 Hz, CH-4), 146.62 (d, 4J = 3.0 Hz, CH-
6), 147.1 (br., Cq-7a), 163.46 (d, 1J = 54.0 Hz, CH-2) ppm. 31P{1H}
NMR (CDCl3): δ = 62.22 ppm. MS (EI 70 eV, 340 °C): m/z (%) =
220 (9) [M]+, 121 (100). HRMS (EI): calcd. for C12H17N2P+
220.1129; found 220.1126.
2,5-Dimethyl-1-phenyl-1,3-azaphospholo[5,4-b]pyridine (5c): Com-
pound 2c (0.50 g, 2.31 mmol) and N,N-dimethylacetamide dimethyl
acetal (0.38 mL, 2.60 mmol) gave oily 5c, which crystallized after
distillation at 10–5 mbar/65–68 °C (bath temperature), yield 0.31 g
(56%), m.p. 84 °C. 1H NMR (C6D6): δ = 1.96 (s, 3 H, 5-CH3), 2.28
3-tert-Butyl-2-deuterio-5-methyl-1-phenyl-1,3-azaphospholino-
[5,4-b]pyridine (11c): A solution of tBuLi in pentane (0.27 mL,
1.7  in pentane, 0.46 mmol) was added to a solution of 4c
(100 mg, 0.44 mmol) in diethyl ether (15 mL), cooled to –80 °C.
After warming up slowly to room temperature, the solution was
cooled again to –80 °C, and CH3OD (10 µL, 0.63 mmol) was
added. The solvent was removed in vacuo, the residue was washed
with degassed water, and the product was extracted with diethyl
ether and dried with Na2SO4. Removing of ether afforded 61 mg
(48%) of white solid 11c, slightly contaminated by 4c. 1H NMR
3
3
(d, JPH = 13.6 Hz, 3 H, 2-CH3), 6.90 (m, J = 7.8 Hz, 2 H, o-H),
7.00–7.10 (m, 3 H, p-H, m-H), 7.82 (unresolved t, 1 H, 4-H), 8.20
(s br, 1 H, 6-H) ppm. 13C{1H} (DEPT) NMR (C6D6): δ = 18.56
(CH3-5), 18.91 (d, 2J = 24.9 Hz, CH3-2), 126.63 (d, 3J = 9.8 Hz,
Cq-5), 129.02 (2 CH-o), 129.52 (CH-p), 129.88 (2 CH-m), 133.94
(d, 1J = 42.4 Hz, Cq-3a), 137.66 (d, 2J = 18.7 Hz, CH-4), 139.35
4
(Cq-i), 147.39 (d, J = 2.8 Hz, CH-6), 155.4 (unresolved d, Cq-7a),
3
(C6D6): δ = 0.80 (d, JPH = 12.4 Hz, 9 H, CMe3), 1.86 (s, 3 H, 5-
1
177.17 (d, J = 52.1 Hz, Cq-2) ppm. 31P{1H} NMR (CDCl3): δ =
2
2
CH3), 3.49 (d, JPH = 3.9 Hz, 0.7 H, 2-Hanti to lp), 3.65 (d, JPH
=
66.8 ppm. MS (EI 70 eV, 100 °C): m/z (%) = 241 (21), 240 (100)
[M]+, 239 (88), 198 (33), 183 (78), 170 (44), 152 (47), 138 (90), 136
(69), 84 (73), 57 (86). HRMS (ESI in MeOH, NH4OAc): calcd. for
C14H14N2P+ [M + H]+ 240.08891; found 240.08896.
3
4
23.9 Hz, 0.3 H, 2-Hsyn to lp), 6.96 (tt, J = 7.3, J = 1 Hz, 1 H, p-
3
4
H), 7.27 (tm, 3 H, 2 m-H, 4-H), 7.78 (tm, J = 7.9, J = 1 Hz, 2
H, o-H), 8.03 (dd, J, JPH = 1.6, 0.8 Hz, 1 H, 6-H) ppm. 13C{1H}
4
5
2
(DEPT) NMR (C6D6): δ = 18.11 (CH3-5), 26.60 (d, J = 15.0 Hz,
CMe3), 30.80 (d, 1J = 18.4 Hz, CMe3), 46.01 (q, JPC
≈ =
1JCD
1
Attempts to Synthesize Benzazaphospholes by Reductive Cyclization
23 Hz, CH-2), 120.34 (d, 1J = 14.9 Hz, Cq-3a), 120.97 (d, 4J =
5-Methyl-2-(neopentylamino)-3-phosphanylpyridine (8a): A solution
of 7a (0.40 g, 1.22 mmol) in diethyl ether (20 mL) was added drop-
wise at 0 °C to LiAlH4 pellets (0.14 g, 3.69 mmol) stirred in diethyl
ether (10 mL) for almost 1 h. The reaction mixture was stirred for
2 d at room temperature. Then, on cooling with ice-water, degassed
water was added dropwise until the evolution of hydrogen ceased.
Solids were filtered off and washed with diethyl ether. The filtrate
was dried with Na2SO4, and the solvent was removed in vacuo.
Vacuum distillation at 10–5 mbar/40–44 °C (bath temperature) gave
0.19 g of viscous liquid 8a (yield 55%), contaminated by ca.
15 mol-% 5-methyl-2-(neopentylamino)-pyridine (8aЈ), based on 1H
NMR integration, and three unidentified phosphanes (each 5 mol-
3
1.8 Hz, 2 CH-o), 123.07 (s, CH-p), 123.51 (d, J = 5.6 Hz, Cq-5),
129.41 (s, 2 CH-m), 141.74 (d, J = 20.9 Hz, CH-4), 144.33 (s, Cq-
2
i), 149.69 (s, CH-6), 161.57 (s, Cq-7a) ppm. 31P{1H} NMR (C6D6):
δ = –26.49 ppm. MS (EI 70 eV, 60 °C): m/z (%) = 286 (6), 285 (27)
[M]+, 229 (35), 228 (100), 227 (62), 226 (50), 183 (33), 77 (36), 51
(32). HRMS (ESI in MeOH, NH4OAc): calcd. for C17H21N2PD+
[M + H]+ 286.15779; found 286.15783.
3-η1-(3-tert-Butyl-5-methyl-1-phenyl-1,3-azaphospholino[5,4-b]pyr-
idine)pentacarbonyltungsten (12c): A solution of tBuLi in pentane
(0.27 mL, 1.7 , 0.46 mmol) was added to a solution of 4c (102 mg,
0.45 mmol) in thf (10 mL), cooled to –80 °C. After warming up
%). Compound 8a: 1H NMR (CDCl3): δ = 0.90 (s, 9 H, CMe3), slowly to 0 °C, the solution was cooled again to –80 °C, and tung-
1
1.83 (s, 3 H, 5-CH3), 3.29 (d, JPH = 199.0 Hz, 2 H, PH2), 3.49 (d, sten hexacarbonyl (159 mg, 0.45 mmol) in thf (5 mL) was added.
3
3J = 6.0 Hz, 2 H, NCH2), 4.58 (br. s, 1 H, NH), 7.25 (dd, JPH
=
The reaction mixture was stirred for 1 h at –80 °C and for 3 d at
12.3, 4J = 2.4 Hz, 1 H, 4-H), 8.07 (s, 1 H, 6-H) ppm. 13C{1H} room temperature. The solvent and volatiles were removed in
NMR (CDCl3): δ 17.71 (s, 5-CH3), 28.32 (s, CMe3), 32.94 (s, vacuo. The product was extracted with CDCl3, leaving 196 mg
1
3
1
CMe3), 53.61 (NCH2), 105.49 (d, J = 13.4 Hz, C-3), 121.57 (d, J (72%) of pale brownish solid after evaporation of the solvent. H
Eur. J. Inorg. Chem. 2010, 3307–3316
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3313

M. S. S. Adam, P. G. Jones, J. W. Heinicke
FULL PAPER
3
3
NMR (CDCl3): δ = 1.15 (d, JPH = 15.6 Hz, 9 H, CH3), 2.28 (s, 3
7.42–7.65 (m, 5 H, phenyl), 8.21 (d br, JPH = 7.2 Hz, 1 H, 4-H),
2
2
4
5
2
H, 5-CH3), 4.33 (dd, J = 13.1, JPH = 7.1 Hz, 1 H, NCHA), 4.44 8.42 (t, J ≈ JPH ≈ 1.9 Hz, 1 H, 6-H), 8.69 (d, JPH = 31.3 Hz, 1
(dd, J = 13.1, JPH = 5.9 Hz, 1 H, NCHB), 7.14 (t, J = 7.4, J = H, 2-H) ppm. 13C{1H} NMR (CDCl3): δ = 18.39 (CH3-5), 125.85
2
2
3
4
1–2 Hz, 1 H, p-H), 7.36–7.42 (m, 3 H, 2 m-H, 7-H), 7.51 (dd, J ≈ (d, 4J = 0.9 Hz, 2 CH-o), 126.62 (d, 3J = 12.4 Hz, Cq-5), 128.46
3
4
3
4
1
8.5, J ≈ 1 Hz, 2 H, o-H), 7.61 (dd, J = 7.8, J = 2.1 Hz, 1 H, 6- (CH-p), 129.48 (2 CH-m), 131.31 (d, J = 13.8 Hz, Cq-3a), 135.73
H), 8.03 (br. s, 1 H, 4-H) ppm. 13C{1H} NMR (CDCl3): δ = 17.81 (d, 2J = 11.4 Hz, CH-4), 139.58 (Cq-i), 149.35 (d, 4J = 4.3 Hz, CH-
1
2
(CH3-5), 25.09 (d, 2JPC = 6.6 Hz, CMe3), 34.78 (d, 1JPC = 15.9 Hz,
6), 151.57 (Cq-7a), 157.38 (d, J = 20.2 Hz, CH-2), 214.87 (d, J =
1
CMe3), 53.20 (d, J = 21.9 Hz, PCH2), 121.39 (2 C, CH-o), 123.88
16.6 Hz, 4 cis CO), 221.13 (d, 2J = 4.5 Hz, trans CO) ppm. 31P{1H}
3
(d, J = 7.1 Hz, Cq-5), 124.21 (CH-p), 129.15 (2 C, CH-m), 140.80 NMR (CDCl ): δ = 87.84 ppm. IR (KBr): ν
= 2068 (w), 1935
˜
3
(CO)
(d, 2J = 10.6 Hz, CH-4), 151.19 (br., CH-6), 196.46 (d, 2J = 7.0 Hz,
(vs) cm–1. MS (EI 70 eV, 40 °C): m/z (%) = 419 (1) [M]+, 418 (5),
362 (4) [M+ – 2CO], 334 (1) [M+ – 3CO], 306 (6) [M+ – 4CO], 278
(63) [M+ – 5CO], 227 (4) [L]+, 226 (20). C18H11CrN2O5P (418.26):
calcd. C 51.69, H 2.65, N 6.70; found C 51.35, H 2.83, N 6.53.
4 COcis) ppm; Cq-3a, Cq-7a, Cq-i and COtrans at noise level. 31P{1H}
1
NMR (CDCl3): δ = 11.4 ppm (satellites, JPW = 239.5 Hz). IR
(KBr): ν
= 2071 (w), 1980 (sh), 1934 (vst) cm–1. HRMS (ESI
˜
CO
in CH3CN): calcd. for the three most intensive isotopic peaks of
C22H21N2O5PW+ 607.07422, 609.07743, 611.08054; found
607.07445, 609.07761, 611.08073.
η1-(5-Methyl-1-phenyl-1,3-azaphospholo[5,4-b]pyridine-P)pentacarb- Mo(CO)6 (0.150 g, 0.568 mmol) in thf (with replacement of CO by
Detection of η1-(5-Methyl-1-phenyl-1,3-azaphospholo[5,4-b]pyr-
idine-P)pentacarbonylmolybdenum (14c): Reaction of 4c (0.107 g,
0.473 mmol) with Mo(CO)5(thf), prepared by UV irradiation of
onylchromium (13c): A solution of Cr(CO)5(thf), prepared by UV
N2), and workup as described for 13c gave 0.189 g of yellow solid
irradiation of Cr(CO)6 (0.191 g, 0.868 mmol) in thf (30 mL), was containing residual Mo(CO)6. 1H NMR (CDCl3): δ = 2.51 (s, 3 H,
3
added to a solution of 4c (0.140 g, 0.619 mmol) in thf (5 mL) at
–10 °C. The solution was stirred for two days at room temperature.
Then thf was removed in vacuo. The residual dark yellow powder,
which was sparingly soluble in C6D6, was separated from the solu-
5-CH3), 7.42–7.63 (m, 5 H, phenyl), 8.16 (ddq, JPH = 7.4, 4J =
4
4
5
2.0, J = 0.9 Hz, 1 H, 4-H), 8.42 (t, J ≈ JPH ≈ 2.1 Hz, 1 H, 6-H),
8.68 (d, JPH = 31.8 Hz, 1 H, 2-H) ppm. 13C{1H} NMR (CDCl3):
δ = 18.38 (CH3-5), 125.0 (d, J = 12 Hz, Cq-5), 125.83 (2 CH-o),
2
3
tion, dried in vacuo and high vacuum, and extracted with CDCl3
(yield 230 mg, 89%). H NMR (CDCl3): δ = 2.51 (s, 3 H, 5-CH3), 138.4 (Cq-i), 136.0 (d, J = 12 Hz, CH-4), 149.2 (br., CH-6), 151.2
128.47 (CH-p), 129.47 (2 CH-m), ca. 130.8 (noise level, Cq-3a),
1
2
Table 1. Crystal data and structure refinement of the X-ray crystallographic analyses.
Compound
2a
5a
6c·2CDCl3
Empirical formula
Formula weight
C6H9N2P
140.12
C8H9N2P
164.14
C32H36D2Cl6N6P2
783.33
Temperature [K]
Wavelength [Å]
Crystal system
Space group
133(2)
133(2)
133(2)
0.71073
0.71073
triclinic
0.71073
monoclinic
P21/n
triclinic
¯
P1
¯
P1
Unit cell dimensions
a [Å]
b [Å]
c [Å]
α [°]
β [°]
3.9982(8)
8.5561(16)
10.800(2)
77.041(4)
88.774(4)
82.332(4)
356.83(12)
2
6.0780(7)
11.6398(14)
11.5946(16)
90
105.058(4)
90
2792.11(17)
4
1.376
11.2029(9)
12.8093(10)
15.6194(12)
112.449(2)
90.904(2)
112.887(2)
1872.6(3)
2
1.390
0.58
γ [°]
Volume [Å3]
Z
Density (calculated) [Mgm–3]
1.304
Absorption coefficient [mm–1] 0.29
0.28
F(000)
Crystal habit
148
344
808
colorless lath
0.35ϫ0.20ϫ0.10
1.93 to 28.28
–5 Յ h Յ 5,
–11 Յ k Յ 11,
–14 Յ l Յ 14
4048
1726 [R(int) = 0.061]
98.1% to θ = 28°
none
colorless tablet
0.23ϫ0.20ϫ0.10
2.52 to 30.48
–8 Յ h Յ 8,
–16 Յ k Յ 16
–16 Յ l Յ 16
8839
2405 [R(int) = 0.062]
99.4% to θ = 30°
none
colorless tablet
0.4ϫ0.3ϫ0.2
1.44 to 30.51
–16 Յ h Յ 16,
–18 Յ k Յ 18,
–22 Յ l Յ 22
34749
11306 [R(int) = 0.030]
99.1% to θ = 30°
semi-empirical
from equivalents; 0.738–0.893
Crystal size [mm3]
θ range for data collection [°]
Index ranges
Reflections collected
Independent reflections
Completeness
Absorption correction;
transmissions
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices
full-matrix least-squares on F2
2405/0/106
1.04
R1 = 0.044,
wR2 = 0.095
R1 = 0.061,
1726/3/99
1.04
R1 = 0.047,
wR2 = 0.109
R1 = 0.060,
wR2 = 0.114
0.35/–0.33
11306/96/455
1.01
R1 = 0.045,
wR2 = 0. 105
R1 = 0.071,
wR2 = 0.119
0.85/–0.86
[IϾ2σ(I)]
R indices (all data)
wR2 = 0.100
0.38/–0.27
Largest diff. peak/hole [eÅ–3]
3314
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 3307–3316

Pyrido-Annulated 1,3-Azaphospholes
(Cq-7a), ca. 157 (uncertain, CH-2), 203.7 (d, 2J = 11 Hz, 4 cis-
CO) ppm; trans-CO at noise level. 31P{1H} NMR (CDCl3): δ =
Acknowledgments
68.33 ppm. IR (KBr): ν
= 2075 (w), 1943 (vs) cm–1.
˜
(CO)
Financial support by the Deutsche Forschungsgemeinschaft
(DFG) and a scholarship by the Deutsche Akademische Aus-
landsdienst (to M. S. S. A.) are gratefully acknowledged. Further-
more, we thank Dr. M. K. Kindermann and B. Witt for numerous
NMR spectra and Dr. H. Frauendorf and G. Sommer-Udvarnoki
(Georg-August-Universität Göttingen, Institut für Organische und
Biomolekulare Chemie) for HRMS measurements.
η1-P-(1-Ethyl-5-methyl-1,3-azaphospholo[5,4-b]pyridine)pentacarb-
onyltungsten (15b): Reaction of 4b (0.124 g, 0.697 mmol) with a
solution of W(CO)5(thf), prepared by UV irradiation of W(CO)6
(0.368 g, 1.045 mmol) in thf (30 mL), and work up as described for
13c afforded 0.288 g (ca. 80%) of orange solid, containing a small
1
3
amount of residual W(CO)6. H NMR (CDCl3): δ = 1.47 (t, J =
3
4
7.3 Hz, 3 H, CH3), 2.43 (s, 3 H, 5-Me), 4.50 (qd, J = 7.3, JPH
=
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3
4
4
1.9 Hz, 2 H, CH2), 8.01 (ddq, JPH = 7.4, J = 2.0, J = 0.8 Hz, 1
H, 4-H), 8.35 (t, 4J ≈ JPH ≈ 2.1 Hz, 1 H, 6-H), 8.42 (d, JPH
=
5
2
31.3 Hz, 1 H, 2-H) ppm. 13C{1H} NMR (CDCl3): δ = 15.38 (CH3),
3
18.38 (5-Me), 44.16 (CH2), 126.26 (d, J = 12.4 Hz, Cq-5), 130.06
(d, 1J = 22.7 Hz, Cq-3a), 135.82 (d, 2J = 11.8 Hz, CH-4), 148.75
3
1
(d, J = 4.3 Hz, CH-6), 150.52 (s, Cq-7a), 154.59 (d, J = 27.4 Hz,
CH-2), 194.44 (d, 2J = 9.1 Hz, 4 CO, cis-CO), 199.21 (d, 2J =
30.8 Hz, 1 CO, trans-CO) ppm. 31P{1H} NMR (CDCl3): δ = 31.5
1
(satellites, JPW = 247.6 Hz) ppm. C14H11N2O5PW (502.06): calcd.
C 33.49, H 2.21, N 5.58; found C – (incomplete combustion), H
2.43, N 5.27.
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Detection of η1-P-(5-Methyl-1-phenyl-1,3-azaphospholo[5,4-b]pyr-
idine)pentacarbonyltungsten (15c): Reaction of 4c (0.111 g,
0.49 mmol) with a solution of W(CO)5(thf), prepared by UV irradi-
ation of W(CO)6 (0.207 g, 0.59 mmol) in thf (30 mL), and workup
as described for 13c afforded 218 mg of a yellow solid containing
1
residual W(CO)6. H NMR (CDCl3): δ = 2.51 (s br, 3 H, 5-CH3),
3
7.44–7.64 (m, 5 H, phenyl), 8.13 (dq, JPH = 7.8, 4J = 2.0, 4J =
4
5
0.8 Hz, 1 H, 4-H), 8.42 (br. t, J ≈ JPH ≈ 2.2 Hz, 1 H, 6-H), 8.62
2
(br. d, JPH = 31.0 Hz, 1 H, 2-H) ppm. 13C{1H} NMR (CDCl3): δ
= 18.39 (CH3-5), 127.29 (Cq-5), 129.33 (2 CH-o), 129.49 (CH-p),
129.80 (2 CH-m), 135.92 (d, 2J = 11.3 Hz, CH-4), 139.5 (Cq-i),
149.50 (d, 4J = 3.8 Hz, CH-6), 194.55 (d, 2J = 8.8 Hz, 4 COcis
)
ppm; other signals in noise. 31P{1H} NMR (CDCl3): δ = 40.55
(satellites, 1JPW = 252.9 Hz) ppm. IR (KBr): ν(CO) = 2075 (w), 1936
˜
(vs) cm–1.
Crystal Structure Analyses: X-ray diffraction data for 2a, 5a, and
6c were recorded with a Bruker SMART CCD 1000 diffractometer
by using monochromated Mo-Kα radiation. The structures were
refined by full-matrix least-squares methods on F2 for all unique
reflections (SHELXL-97).[28] Hydrogen atoms bonded to nitrogen
or phosphorus were located directly in difference syntheses and re-
fined freely, but with restraints to bond lengths; other H atoms
were calculated by assuming idealized geometries and refined with
a riding model or rigid methyl groups. Special features: Compound
6c crystallizes as a deuteriochloroform disolvate; the solvent mole-
cules are disordered by rotation about the C–D bond. Alternative
chlorine positions were refined by using a system of similarity re-
straints. Crystallographic data are given in Table 1, and selected
bond lengths and angles are presented in Figures 1, 2, and 3.
CCDC-767984 (for 2a), -767985 (for 5a), and -767986 (for 6c) con-
tain the crystallographic data for this article. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Supporting Information (see also the footnote on the first page of
this article): 31P and 13C NMR spectra of 4a–d, 5a, 5c, and 11c,
13c, 15b; 31P NMR spectra of 12c, 14c, 15c, tables of positional
parameters, bond lengths and bond angles, dihedral angles, hydro-
gen bonds for compounds 2a, 5a, and 6c.
Eur. J. Inorg. Chem. 2010, 3307–3316
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3315

M. S. S. Adam, P. G. Jones, J. W. Heinicke
FULL PAPER
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M. S. S. Adam, O. Kühl, M. K. Kindermann, J. Heinicke, P. G.
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Received: March 3, 2010
Published Online: May 31, 2010
After publication in Early View, a dedication has been added.
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A Guide for the Synthetic Organic and Organometallic Chemist,
3316
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