Syntheses of 2-unsubstituted 1h-1,3-benzazaphospholes from n-formyl-2-bromoanilides

Article abstract of DOI:10.1002/hc.21111

The phosphonylation of 2-bromo-formylanilides 1 with triethyl phosphite in the presence of preformed Pd(0)(triethyl phosphite)_n catalyst furnished 2-phosphono-formanilides 2 in good yields. Reduction with excess LiAlH₄ provided mainl

Full text of DOI:10.1002/hc.21111

Heteroatom Chemistry
Volume 24, Number 6, 2013
Syntheses of 2-Unsubstituted
1H-1,3-Benzazaphospholes from
N-Formyl-2-bromoanilides
Mohammed Ghalib,1 Basit Niaz,1 Peter G. Jones,2
and Joachim W. Heinicke1
1Institut fu¨ r Chemie und Biochemie, Ernst-Moritz-Arndt-Universita¨t Greifswald, 17487
Greifswald, Germany
2Institut fu¨ r Anorganische und Analytische Chemie der Technischen Universita¨t Braunschweig,
38023 Braunschweig, Germany
Received 12 May 2013; revised 30 June 2013
INTRODUCTION
ABSTRACT: The phosphonylation of 2-bromo-
formylanilides 1 with triethyl phosphite in the pres-
ence of preformed Pd(0)(triethyl phosphite)n catalyst
furnished 2-phosphono-formanilides 2 in good yields.
Reduction with excess LiAlH4 provided mainly N-
methyl-2-phosphinoanilines 3 and minor amounts
of 1,2-unsubstituted benzazaphospholes 4. N-Methyl-
1,3-benzazaphospholes 5 were synthesized by the
cyclocondensation of 3 with dimethylformamide
dimethylacetal (DMFA). A more convenient route to
5, avoiding the chromatographic separation of 4,
is the reduction of 1 to 2-bromo-N-methylaniline
6, followed by phosphonylation to 7, LiAlH4 reduc-
tion, and cyclization with DMFA. The coordination
properties at σ2P of benzazaphospholes are charac-
terized by structural data obtained by the crystal
1H-1,3-Benzazaphospholes present aromatically
stabilized, formal π-excess heterocycles (10 π elec-
trons for nine ring atoms) with trivalent dicoordi-
nated phosphorus. The donation of two π electrons
by the nitrogen atom just in conjugation to phos-
phorus ( P CH NR
↔
P– CH NR+ ) causes
increased π density at phosphorus and is indicated
by the strong upfield shift of the phosphorus reso-
nance compared with the 31P signals of P C com-
pounds without a (cyclo)conjugated π-donor group
[1]. The ligand properties have still been only spar-
ingly investigated in contrast to the related phos-
phinines [2], for which various studies on transition
metal complexes and catalytic applications thereof
have been reported; [3] in particular, the knowl-
edge of nonzerovalent transition metal complexes
is limited to a few complexes with d10 metal cations
[4]. The low stability of the complexes with tran-
sition metal cations might be improved by addi-
tional donor groups using the chelate effect. Apart
from specific syntheses, for example, of biaryl-type
benzazaphospholes [5], such groups can be intro-
duced via CH lithiation of 2-unsubstituted benza-
zaphospholes [6]. The synthetic access to the lat-
ter needs, however, to be improved to facilitate
such studies. A new short route to 1-methyl-1,3-
benzazaphospholes by disproportionative cyclocon-
densation of an N-primary o-phosphinoaniline with
ꢀC
structure analysis of (5b)W(CO)5. 2013 Wiley Pe-
riodicals, Inc. Heteroatom Chem. 24:452–459, 2013;
View this article online at wileyonlinelibrary.com.
DOI 10.1002/hc.21111
Correspondence to: Joachim W. Heinicke; e-mail: heinicke@uni-
greifswald.de.
ꢀC
2013 Wiley Periodicals, Inc.
452

Syntheses of 2-Unsubstituted 1H-1,3-Benzazaphospholes from N-Formyl-2-bromoanilides 453
excess paraformaldehyde was recently presented by
us [7]. Here we report alternative routes using N-
formyl-2-bromoanilides as the starting material. Fi-
nally, the first crystal structure of a 2-unsubstituted
benzazaphosphole W(CO)5 complex is presented.
RESULTS AND DISCUSSION
N-Acyl-2-bromoanilides of aliphatic and aromatic
carboxylic acids allowed a convenient two-step
synthesis of various 2-alkyl- and 2-aryl-1H-1,3-
benzazaphospholes by NiCl2- or, better, NiBr2-
catalyzed phosphonylation with triethyl phosphite
and the subsequent reduction with LiAlH4 [8]. How-
ever, attempts at analogous phosphonylation of N-
formyl-2-bromoanilide with triethyl phosphite and
∼10 mol% of NiCl2 by heating at 200◦C (10–15 min)
failed and furnished a violet-brown polymeric con-
densation product [8a]. Reinvestigation of the cou-
pling by using Ni(0)[P(OEt)3]n and Pd(0)[P(OEt)3]n
(n = 3,4) catalysts (6 mol%), formed by heating
NiBr2 or PdCl2 with excess triethyl phosphite be-
fore the addition of N-formyl-2-bromoanilides 1a,b,
and under somewhat milder coupling conditions
(10 min, 180◦C), allowed access to the N-formyl-2-
phosphonoanilides 2a,b in low (∼30% with NiBr2)
or good yields (72–75% with PdCl2). The reduction
of 2a,b with excess LiAlH4 at –10 to 20◦C (3d), how-
ever, took another course than the reduction of N-
acyl-o-phosphonoanilides derived from aliphatic or
benzoic carboxylic acids. For the latter, N-secondary
o-phosphinoanilines were minor side products and
benzazaphospholes were the main products. Here,
the opposite was observed; 3a,b formed the main
product and 4a,b formed the side product (molar
ratio ∼75%:25%). This may be explained by the eas-
ier reduction of formyl- than aliphatic acylanilides
or benzanilides. Formation of benzazaphospholes
requires primary reduction of the phosphono to a
phosphino or phosphido group, which then under-
goes cyclocondensation with the imidate group [8],
here formed by lithiation of NH CHO. To convert
the highly air-sensitive primary phosphines 3a,b to
benzazaphospholes 5a,b, the mixtures were heated
for 24 h at 50◦C with excess dimethylformamide
dimethylacetal (DMFA) in the presence of Me3SiCl
as the catalyst (Scheme 1). The mixtures of 4a/5a and
4b/5b, respectively, were then separated by column
chromatography on silica gel under inert conditions.
After it became clear that the reduction of 2a,b is
unsuitable for the preparation of 1,2-unsubstituted
1H-1,3-benzazaphospholes, we studied the use of
N-formanilides for the synthesis of 1-methyl-1,3-
benzazaphospholes. Reduction of 1a,b with LiAlH4
in diethyl ether under mild conditions (–10 to
SCHEME
1 Formation and LiAlH4 reduction of
o-phosphono-formanilides 2a,b (R H, Me).
20◦C, 1 d) provided good yields of N-methyl-2-
bromotoluidines 6a,b, which were phosphonylated
to 7a [6b] and 7b by heating with triethyl phosphite
in the presence of 5–6 mol% Pd(OAc)2 or PdCl2 at
180–200◦C for 0.5–3 h. The coupling yield was rather
moderate (50% and 40%, respectively). The selec-
tive direct reduction of o-phosphono formanilides
with milder reducing agents rather than LiAlH4,
studied for 2a with excess diisobutylaluminium hy-
dride, provided N-methyl o-phosphonoaniline 7a in
a higher yield (∼70%), but was contaminated by
small amounts of o-phosphonoaniline, formed in a
side reaction. Use of this mixture in the synthesis of
5a requires time-consuming purification by column
chromatography for separation from 4a as described
above. Therefore, the preparation of benzazaphosp-
holes via the reduction of 2-bromoformanilides 1 to
N-methyl-2-bromanilines 6, Pd-catalyzed phospho-
nylation to N-methyl-o-phosphonoanilines 7, reduc-
tion with LiAlH4 to N-methyl-o-phosphinoanilines 3
and final cyclization with DMFA is the more con-
venient route to N-methyl-1,3-benzazaphospholes 5
(Scheme 2). The last three steps have already been
reported by us for the synthesis of 5a [6b] and
are likewise suitable for the access to 5b. Using
this route, purification requires usually only the
SCHEME 2 Syntheses of N-methyl-o-phosphonoanilines
and conversion to benzazaphospholes.
Heteroatom Chemistry DOI 10.1002/hc

454 Ghalib et al.
ical shift), they still possess considerably higher
net donicity than CO ligands. In comparison to 2-
hydroxy-, 2-bromo-, or 2-chlorophosphinine ligands,
however, they are weaker coordinated as indicated
by slightly longer P—W bonds of 8b compared with
those in the respective phosphinine W(CO)5 com-
plexes (P—W 2.4586(17), 2.4477(13) [13b], 2.457(2)
CHART 1 Compound 8b.
˚
A [13a]). Only in a (2-o-diphenylphosphinophenyl-
benzazaphosphole)W(CO)4 PˆP´-chelate complex
[5], due to the push-pull-effect of the σ3P donor
and the CO- and σ2P-acceptor sites, respectively, the
P—W bond of the azaphosphole P atom (2.4219(5)
removal of minor N-basic impurities, which can be
accomplished by the extraction of ethereal prod-
uct solutions with dilute aqueous sulphuric acid.
This is possible because 2-unsubstituted 1H-1,3-
benzazaphospholes are stable to hydrolysis and do
not exhibit basic properties by the involvement of
the nitrogen lone pair in the aromatic system.
˚
A) is shorter than in the phosphinine W(CO)5 com-
plexes and, corresponding to the smaller radius of
sp2- compared with sp3-hybridized phosphorus, is
shorter than the W—P bond to the Ph2P group
˚
The dicoordinated phosphorus atom in the ben-
zazaphospholes receives π density from nitrogen, as
mentioned above, but is a weak σ donor. Attempts
to bind BH3 (from Me2S–BH3) at room tempera-
ture, which is typically for phosphine donors [9],
failed with benzazaphospholes. Only electron-rich
transition metals, which allow efficient back bond-
ing, form η1P-benzazaphosphole complexes, for ex-
ample, M(CO)5 complexes (M=Cr, Mo, W) [1a,1b,
7,10]. Complex 8b (Chart 1), recently obtained from
W(CO)5(THF) (THF–tetrahydrofuran) and alterna-
tively synthesized 5b [7], could now be crystallized.
Most crystals were twins but a suitable single crys-
tal allowed a crystal structure analysis of 8b, the
first of a benzazaphosphole M(CO)5 complex. The
compound crystallized in the triclinic space group
(2.5442(5) A). The P—C2 and P—C3 bond lengths of
8b are slightly shortened and the C3a—P—C2 angles
are slightly increased by the coordination, in com-
parison to usual values in free benzazaphospholes,
whereas the other bond lengths and angles are not
significantly changed [1a,5,6].
CONCLUSIONS
It has been shown that the higher reactivity of
the N-formyl substituent compared with aliphatic
N-acyl or benzoyl groups suppresses the synthe-
sis of 1,2-unsubstituted 1H-1,3-benzazaphospholes
by the reductive cyclization of o-phosphono-
formanilides with excess LiAlH4, and instead fur-
nishes mainly N-methyl-2-phosphinoanilines. The
reason is the easier reduction of the formyl com-
pared with the diethylphosphono group, as demon-
strated by the selective reduction of the formyl
group with milder reducing agents such as iBu2AlH.
A minor side reaction, cleavage of the formyl
group instead of reduction to N-methyl, dimin-
ishes the value of this reduction for the prepa-
ration of o-methylamino benzenephosphonic acid
diethylesters, for example, as precursors for the
synthesis of N-methylbenzazaphospholes. The re-
duction of o-bromoformanilides with LiAlH4 and
subsequent Pd-catalyzed phosphonylation with tri-
ethyl phosphite was found to be more suitable for the
further use in the synthesis of benzazaphospholes by
purer products. Apart from the synthetic aspects, the
first crystal structure investigation of a benzazaphos-
phole M(CO)5 complex displayed slightly tilted and
somewhat weaker coordination (as indicated by the
slightly longer P—W bond lengths) of the “π-excess”
σ2P-benzazaphosphole ligands compared with the
better known phosphinine ligands.
¯
P1 with two independent molecules in the asym-
metric unit. The benzazaphosphole ligands are pla-
nar (deviation of the nine ring atoms from planarity
˚
0.005 and 0.006 A), but the P—W bonds are slightly
tilted out of the plane. The tungsten atoms devi-
˚
ate from the ring planes by 0.33 A, correspond-
ing to 7.5◦ and 7.8◦ angles of the P—W vectors
to the ring planes. In structurally related Cr(CO)5
complexes with 1,5-dimethyl-1,2,3-diazaphosphole
[11] and pivaloyl-2-phosphaindolizine [12] ligands,
where phosphorus profits less from the π-donor ef-
fect of the σ3N atom, the deviation of the P–Cr bonds
from the ring planes are small and not more sig-
◦
◦
˚
nificant, 2 [11] and 1.5 /0.12 A [12], respectively,
as they are in the known o-substituted phosphi-
◦
˚
nine W(CO)5 complexes (0.7–3.3 /0.02–0.12 A) [13].
The W—C bonds trans to the benzazaphospholes
are shortened and the trans-C—O bonds elongated
as compared with the corresponding bonds of cis-
CO ligands. This indicates that although the σ2P-
benzazaphosphole ligands display rather π-acidic
properties relative to usual phosphine donors (up-
field instead of downfield 31P coordination chem-
Heteroatom Chemistry DOI 10.1002/hc

Syntheses of 2-Unsubstituted 1H-1,3-Benzazaphospholes from N-Formyl-2-bromoanilides 455
11.3 Hz, CHO). 13C{1H}NMR (CDCl3)): δ 20.42 (s,
4-CH3), 114.63 (s, Cq-2), 119.37 (s, C-6), 129.22 (s,
C-5), 132.35 (s, Cq-4), 133.67 (s, C-3), 136.71 (s,
EXPERIMENTAL
General
The syntheses of air-sensitive P(III)-compounds
were carried out under dry nitrogen using Schlenk
techniques and freshly distilled dry solvents.
2-Bromoformanilide (1a) was prepared by CuO-
catalyzed conversion of formic acid and 2-
bromoaniline [14]. Commercially available starting
materials (2-bromoaniline and LiAlH4 from Al-
pha Aesar, p-toluidine, Me2NCH(OMe)2(DMFA) and
ClSiMe3 from Acros) were used as received unless
indicated otherwise. Nuclear magnetic resonance
(NMR) spectra were recorded on a multinuclear
FT-NMR spectrometer ARX300 (Bruker, Karlsruhe,
Germany) at 300.1 (1H), 75.5 (13C), and 121.5 (31P)
MHz. Chemical shifts (δ) are given in ppm rela-
tive to Me4Si and H3PO4 (85%), respectively. Cou-
pling constants refer to JHH in 1H and JPC in 13C
NMR data unless stated otherwise. High resolution
mass spectrometric (HRMS) measurements were
performed in the Institut fu¨r Organische Chemie,
Universita¨t Go¨ttingen, Greifswald, Germany, using
a double focusing sector-field instrument MAT 95
(Finnigan, Fisher Scientific) (EI 70 eV) or a 7T
Fourier transform ion cyclotron resonance mass
spectrometer APEX IV (Bruker Daltonics, Bremen,
Germany) (ESI). Elemental analyses were carried
out with a CHN analyser Vario Micro Cube (Elemen-
tar, Hanau, Germany) under standard conditions.
Melting points were determined in sealed capillaries
using a Sanyo Gallenkamp melting point apparatus
(Gallenkamp, Loughborough, England).
Cq-1), 161.78 (s, CO). Elemental analysis: calcd. for
C8H8BrNO (214.06), C 44.89, H 3.77; found: 45.12,
H 3.81.
(2-Formylamido)benzenephosphonic Acid Di-
ethylester (2a). A mixture of triethyl phosphite (7.71
mL, 45.0 mmol) and palladium(II) chloride (160 mg,
0.90 mmol, 6 mol% relative to 1a) was heated un-
der nitrogen atmosphere for 10 min at 180◦C. The
color turned to pale yellow, indicating the Pd(0) cat-
alyst complex. After cooling, 1a (3.0 g, 15.0 mmol)
was added and the mixture heated at 180–185◦C for
20–25 min. NMR monitoring indicated the conver-
sion of all triethyl phosphite to the product, some
diethyl phosphite, diethyl ethylphosphonate (minor
by-product by Michaelis–Arbuzov reaction) and mi-
nor amounts of triethyl phosphate (from catalyst
formation). The crude product was prepurified by
flash column chromatography on silica using n-
hexane/diethyl ether (18%) for elution and subse-
quently distilled at 10–2 mbar/100◦C bath tempera-
ture to give 2.9 g (75%) pale yellow oil. When NiBr2
was used instead of PdCl2 in the same procedure,
the yield was 1.20 g (31%). 1H NMR (CDCl3): δ 1.30
3
(t, J = 7.0 Hz, 6 H, CH3), 4.07 (m, 4 H, POCH2),
3
4
4
7.13 (tdd, J = 7.6, JPH = 3.0, J = 1 Hz, 1 H, H-5),
3
7.51 (br t, J = 7.9 Hz, 1 H, H-4), 7.55 (ddd, super-
3
3
4
imposed by H 4, JPH ≈ 14–16, J = 7.5, J = 1.5
Hz, 1 H, H-6), 8.44 (d, J = 1.5 Hz, CHO), 8.60 (t,
3J ≈ JPH = 8.1, 6.9 Hz, 1 H, H-3), 10.56 (br s, 1
4
H, NH). 13C{1H}NMR (CDCl3): δ 16.05 (d, J = 6.6
3
2-Bromo-4-methyl-formanilide
(1b). Com-
2
Hz, CH3), 62.60 (d, J = 5.3 Hz, POCH2), 113.75 (d,
pound 1b was prepared in analogy to 1a [14]
by heating 2-bromo-4-methylaniline (2.05 g, 11.0
mmol) with formic acid (1.25 mL, 33.1 mmol) and
CuO for 90 min at 70◦C. Extraction of the product
with dichloromethane (DCM), separation of excess
formic acid by treatment with aqueous KHCO3,
and removal of the solvent from the Na2SO4-dried
DCM solution furnished 2.25 g (95%) solid anti/syn-
isomers (by 1H integration ∼70:30 mol%) of 1b.
Major isomer – 1H NMR (CDCl3): δ 2.30 (s, 3 H,
1J = 179.1 Hz, Cq-1), 121.34 (d, J = 10.6 Hz, C-3),
3
123.44 (d, 3J = 14.6 Hz, C-5), 132.39 (d, 2J = 6.6 Hz,
C-6), 133.96 (d, 4J = 2.6 Hz, C-4), 141.60 (d, 2J = 8.0
Hz, Cq-2), 159.30 (s, CO). 31P{1H}NMR (CDCl3): δ
19.0, 18.7, (anti-syn isomer ratio ∼ 90:10%). HRMS
(ESI in MeOH+FA): C11H16NO4P, calcd. for [M+H]+
258.0890; found: 258.0891.
2-Formylamido-5-methyl-benzenephosphonic
3
4-CH3), 7.11 (dd, superimposed, J ≈ 8.3,4J ≈ 1 Hz,
Acid
Diethylester
(2b). Triethyl
phosphite
1 H, H-5), 7.36 (d, 4J = 1.1 Hz, 1 H, H-3), 7.70
(4.8 mL, 28 mmol) and palladium(II) chloride
(99 mg, 0.56 mmol, 6 mol% rel. to 2b) were heated
under nitrogen atmosphere for 10 min at 180◦C.
After cooling, 2b (2.0 g, 9.3 mmol) was added and
the mixture heated at 185◦C. The color gradually
turned orange from pale yellow after some minutes.
After 25 min, NMR monitoring indicated the con-
version of all triethyl phosphite. The crude product
was prepurified by flash column chromatography
(br s,
1
H
NH), 8.21 (d, 3J
=
8.3 Hz,
1 H, H-6), 8.46 (d, 3Janti = 1.5 Hz, CHO). 13C{1H}NMR
(CDCl3): δ 20.48 (s, 4-CH3), 112.92 (s, Cq-2), 122.08
(s, C-6), 128.94 (s, C-5), 132.17 (s, Cq-4), 132.52 (s,
C-3), 135.72 (s, Cq-1), 158.81 (s, CO). Minor isomer
– 1H NMR (CDCl3): δ 2.32 (s, 3 H, 4-CH3), 7.11, 7.13
3
(2 d, superimposed, J ≈ 8 Hz, 2 H, H-5, H-6), 7.42
(br s, 1 H, H-3), 7.70 (br s, 1 H, NH), 8.62 (d, 3Jsyn
=
Heteroatom Chemistry DOI 10.1002/hc

456 Ghalib et al.
on silica gel using n-hexane/diethyl ether (18%) and
then distilled at 10–2 mbar/100◦C bath temperature
to give 1.83 g (72%) pale yellow oil. With NiBr2
instead of PdCl2, the same procedure gave 0.79 g
(30%). 1H NMR (CDCl3): δ 1.31 (t, 3J = 7.0 Hz, 6 H,
CH3), 2.31 (s, 3 H, 5-CH3), 4.08 (m, 2 H, POCH2),
7.32 (br d, 3J = 7–8 Hz, 1 H, H-4), 7.35 (br dd,
was separated by column chromatography on sil-
ica gel using n-hexane/18% CH2Cl2 for elution.
1
3
4a – H NMR (CDCl3): δ 7.25 (tdd, J = 8.1,
4
4
7.2, JPH = 2–3, J = 1.2 Hz, 1 H, H-5), 7.41 (tt,
3J = 7.8, 7.2, J ≈ JPH = 1.1 Hz, 1 H, H-6), 7.59
(dq, 3J = 8.1, 4JPH = 2, 4J = 1 Hz, 1 H, H-7), 8.17
(ddd, 3J = 7.9, 3JPH = 3.2, 4J = 0.9 Hz, 1 H, H-4),
8.57 (dd, 2JPH = 38.1, 3J = 5.1 Hz, 1 H, H-2), 9.21
(br s, 1 H, NH). 31P{1H}NMR (CDCl3): δ 81.25. 5a
4
4
3
4
superimposed with H-4, JPH = 12–14, J = 1–2 Hz,
3
1 H, H-6), 8.41 (d, J = 1.9 Hz, CHO), 8.48 (dd, J
3
1
4
≈ JPH = 8.7, 6.9 Hz, 1 H, H-3), 10.44 (br s, 1 H,
– H NMR (CDCl3): δ = 3.97 (d, JPH = 0.9 Hz,
3
3 4
NH). 13C{1H}NMR (CDCl3): δ 16.07 (d, J = 6.6 Hz,
3 H, NMe), 7.25 (tdd, J = 7.8, 6.9, JPH = 1.8,
4J = 1.2 Hz, 1 H, H-5), 7.45 (tt, 3J = 7.8, 7.2, 4JPH
CH3), 20.60 (s, 5-CH3), 62.51 (d, 2J = 5.3 Hz, OCH2),
1
3
4
3
113.61 (d, J = 177.8 Hz, Cq-1), 121.35 (d, J = 11.9
≈ J = 1.8, 1.2 Hz, 1 H, H-6), 7.58 (dq, J = 8.2,
Hz, C-3), 132.39 (d, 2J = 6.6 Hz, C-6), 133.10 (d, 3J =
4JPH = 2.4, J = 1.2 Hz, 1 H, H-7), 8.17 (dddd,
4
4
3
4
5
13.3 Hz, Cq-5), 134.67 (d, J = 2.7 Hz, C-4), 139.15
3J = 8.1, JPH = 3.9, J = 1.2, J = 0.9 Hz, 1 H,
H-4), 8.46 (d, 2JPH = 37.5 Hz, 1 H, H-2). 31P NMR
(CDCl3): δ = 72.0. The data of 4a and 5a are in
accordance within the measuring accuracy with
values of these compounds, obtained by conden-
sation with formiminomethylester hydrochloride
[6,15].
(d, J = 8.0 Hz, Cq-2), 159.12 (s, CO). 31P{1H}NMR
2
(CDCl3): δ 19.3 (18.7 minor isomer). HRMS (DEI):
C12H18NO4P, calcd. for [M]+ 271.0973; found:
271.0962.
Reduction of 2a to N-methyl-2-phosphinoaniline
(3a) and 1H-1,3-benzazaphosphole (4a) and Conver-
sion of 3a to 1-methyl-1H-1,3-benzazaphosphole (5a).
Reduction of 2b to N-methyl-5-methyl-2-
phosphinoaniline
(3b)
and
5-methyl-1H-1,3-
benzazaphosphole (4b) and Conversion of 3b to
1,5-dimethyl-1H-1,3-benzazaphosphole (5b).
a) A solution of 2a (1.0 g, 3.9 mmol) in diethyl
ether (20 mL) was added dropwise to pellets of
LiAlH4 (0.44 g, 11.6 mmol) in diethyl ether (150
mL) at –10◦C. After stirring for 3 days, the mix-
ture was hydrolyzed by water drop-by-drop until
the evolution of hydrogen (monitored by bubble
counter) ceased. The precipitate was filtered off
and washed thoroughly with diethyl ether. After
drying the ethereal solution with Na2SO4, the sol-
vent was removed to give 0.42 g pale yellow oil,
according to the NMR data consisting of 3a and
4a, molar ratio 75%:25% (by 1H NMR integration
of characteristic aryl proton signals), correspond-
ing to crude yields of 58% and 20%, respectively.
a) A solution of 2b (1.0 g, 3.69 mmol) in diethyl
ether (20 mL) was added dropwise at –10◦C to
LiAlH4 pellets (0.42 g, 11.1 mmol) in diethyl ether
(150 mL). After stirring for 3 days, water was
added drop-by-drop until the evolution of hydro-
gen ceased. The solid precipitate was filtered off
and thoroughly washed with ether. The filtrate
was dried with Na2SO4 and separated by filtra-
tion, and the solvent was removed in vacuum to
give 0.45 g yellow oil. The NMR spectra displayed
signals of 3b (δ31P = –153.0 ppm) and 4b (δ31P =
79.7 ppm), molar ratio 75%:25% (by 1H NMR in-
tegration of 2-CH- and PH-signals, respectively),
corresponding to crude yields of 0.34 g (60%) 3b
and 0.11 g (19%) 4b.
1
Detection of 3a in the crude product – H NMR
1
(CDCl3): δ 2.99 (s, 3 H, NCH3), 3.65 (d, JPH
=
201.4 Hz, 2 H, PH2), 4.28 (br s, 1 H, NH), 6.72 (br
d, 3J = 7.9 Hz, 1 H, H-6), 6.79 (m, 1 H, H-4), 7.43
(m, 1 H, H-5), 7.58 (m, 1 H, H-3); 31P{1H}NMR
(CDCl3): δ –153.7.
b) The mixture of 3b and 4b was heated with ex-
cess Me2NCH(OMe)2 (3.50 mL, 26.1 mmol) and
Me3SiCl (0.39 mL, 3.1 mmol) for 24 h at 50◦C
and worked up as described above for 3a/4a. Fi-
nal separation of 4b and 5b was performed by
column chromatography on silica gel using n-
hexane/40% CH2Cl2. The yield of 5b was 0.25 g
(42%).
b) Excess Me2NCH(OMe)2 (3.50 mL, 26.1 mmol)
and Me3SiCl (0.39 mL, 3.1 mmol) were added
to the above mixture of 3a and 4a and heated
for 24 h at 50◦C. Then volatiles were removed
in vacuum, the residue was dissolved in Et2O,
the ethereal solution was washed with degassed
10% aqueous H2SO4 to remove basic impuri-
ties, and was dried with Na2SO4. After filtration
and washing with ether, the solution was con-
centrated, and the product mixture of 4a and 5a
4b – 1H NMR (CDCl3): δ 2.50 (s, 3 H, 5-CH3), 7.23
3
3
(br d, J = 8.7 Hz, 1 H, H-6), 7.50 (br d, J = 8.7
Hz, 1 H, H-7), 7.93 (m, 1 H, H-4), 8.56 (dd, 2JPH
=
38.0, 3J = 4.9 Hz, 1 H, H-2), 9.12 (br s, 1 H, NH).
13C{1H}NMR (CDCl3): d 21.16 (s, 5-CH3), 113.39
Heteroatom Chemistry DOI 10.1002/hc

Syntheses of 2-Unsubstituted 1H-1,3-Benzazaphospholes from N-Formyl-2-bromoanilides 457
4
(2-Methylamino)benzenephosphonicAcidDiethyl-
ester (7a).
(s, C-7), 126.86 (d, J = 2.7 Hz, C-6), 128.32 (d,
2J = 21.3 Hz, C-4), 129.72 (s, J = 11.9 Hz, Cq-
3
2
1
5), 140.51 (d, J = 6.6 Hz, Cq-7a), 140.51 (d, J =
a) The phosphonylation of 6a with triethyl phos-
phite by heating (0.5 h, 200◦C) in the presence
of Pd(OAc)2 yielding 7a (50% yield) was reported
earlier by us [5b].
1
42.5 Hz, Cq-3a), 157.54 (d, J = 53.1 Hz, Cq-2).
31P{1H}NMR (CDCl3): δ 79.7. Calcd. For C8H8NP
(149.13): C 64.43, H 5.41, N 9.39; found: C 64.62,
H 5.50, N 9.24.
b) A solution of 2a (4.8 g, 18.7 mmol) in di-
ethyl ether (20 mL) was added dropwise at –
10◦C to a solution of diisobutylaluminum hy-
dride (25w% in hexane, 43.0 mL, 64.2 mmol,
d = 0.848 g/mL) in diethyl ether (100 mL). After
stirring for 3 days, the mixture was hydrolyzed
until the evolution of hydrogen ceased. Na2SO4
was added for drying, the solids were filtered off
and thoroughly washed with ether. The solvent
was removed in a vacuum to give 3.36 g (74%)
pale yellow oily 7a, contaminated by ∼10 mol%
of 2-aminobenzenephosphonic acid diethyl ester
(δ31P = 21.2). 1H NMR (CDCl3): δ1.31 (t, 3J = 7.2
The NMR data of 3b and 5b were in good agree-
ment with those given below.
2-Bromo-N-methyl-aniline (6a). A solution of
1a [14] (16.0 g, 81 mmol) in THF was added drop-
wise at 0◦C into a stirred solution of LiAlH4 (3.7 g,
97.4 mmol) in diethyl ether. The progress of the re-
duction was monitored by thin layer chromatogra-
phy. After stirring overnight at room temperature,
distilled water was added drop-by-drop until the evo-
lution of H2 gas ceased. The solids were filtered off
and washed with ether, the filtrate was dried over
Na2SO4, and concentrated in vacuum. The com-
pound was then purified by column chromatogra-
phy on silica gel using hexane/2% Et2O to give 13.5
4
Hz, 6 H, CH3), 2.83 (d, J = 4.9 Hz, 3 H, NCH3),
4.06 (m, 4 H, OCH2), 5.30 (br s, NH), 6.61 (dd br,
4
3J ≈ JPH = 8.7, 7.5 Hz, 1 H, H-3), 6.65 (tdd, 3J =
4
4
7.2, JPH = 3.3, J = 1 Hz, 1 H, H-5), 7.36 (tdd,
1
g (91%) colorless oil. H NMR (CDCl3): δ = 2.91 (s,
3J = 7.2, J = 1.5, JPH = 0.9 Hz, 1 H, H-4), 7.43
4
5
3 H, CH3), 4.36 (br, 1 H, NH), 6.59 (td, 3J = 7.6, 4J =
3
3
4
(ddd, JPH = 15, J = 7.5, J = 1.5 Hz, 1 H, H-6).
31P{1H}NMR (CDCl3): δ22.0 ppm.
3
4
1.5 Hz, 1 H, H-4), 6.64 (dd, J = 8.1, J = 1.4 Hz, 1
3
4
H, H-6), 7.22 (td, J = 8.5, J = 1.5 Hz, 1 H, H-5),
3
4
7.44 (dd, J = 7.9, J = 1.5 Hz, 1 H, H-3). Calc. for
C7H8BrN: 184.98 (100%), 186.98 (97%); found: MS
(EI, 70 eV, 30◦C): m/z (%) = 187 (53) [M]+, 185 (58)
[M]+, 106 (12), 104 (12), 77 (100), 52 (32).
5-Methyl-2-methylamino-benzenephosphonic
Acid Diethylester (7b). Compound 6b (1.5 g, 7.5
mmol), triethyl phosphite (3.86 mL, 22.5 mmol),
and PdCl2 (80 mg, 0.45 mmol, 6 mol% rel. to 6b)
were added to a device for fractional distillation
and heated for 3h at 180–200◦C so that the main
part of P(OEt)3 was refluxing. Purification of the
crude product on silica gel using hexane/ethyl
acetate (10%) for elution furnished 0.78 g (40%)
pale yellow oil. 1H NMR (CDCl3): δ 1.29 (t, 3J =
7.0 Hz, 6 H, CH3), 2.20 (s, 3 H, 5-CH3), 2.79 (d,
3J = 5.0 Hz, 3 H, NCH3), 4.04 (m, 4 H, OCH2),
6.35 (br q, 3J = 4.5 Hz, 1 H, NH), 6.52 (br t, 3J
2-Bromo-N-methyl-4-methylaniline (6b). Com-
pound 1b (1.99 g, 9.3 mmol), dissolved in Et2O/THF
(20 mL), was added at –10◦C to LiAlH4 pellets (1.06 g,
27.9 mmol) in diethyl ether (150 mL). After stirring
for 1 day, water was added drop-by-drop until the
evolution of hydrogen ceased. The mixture was dried
with Na2SO4, was filtered, and the solid residue was
washed with diethyl ether (three times). The solvent
was removed in vacuum and the crude product was
purified by column chromatography on silica using
n-hexane/ethyl acetate (10%) for elution to give 1.38
≈
4JPH = 8.1, 7.8 Hz, 1 H, H-3), 7.17 (br d, 3J =
3
4
8.1 Hz, 1 H, H-4), 7.23 (dd, JPH = 15.0, J = 2.1
Hz, 1 H, H-6). 13C{1H}NMR (CDCl3): δ 16.12 (d,
3J = 6.6 Hz, 3 H, CH3), 19.97 (s, 5-CH3), 29.99
(s, NCH3), 61.74 (d, 2J = 5.3 Hz, OCH2), 107.02
(d, 1J = 181.8 Hz, Cq-1), 110.34 (d, 3J = 11.9 Hz,
C-3), 123.93 (d, 3J = 13.3 Hz, Cq-5), 133.19 (d,
2J = 8.0 Hz, C-6), 134.98 (d, 4J = 2.7 Hz, C-4),
150.76 (d, 2J = 8.0, Cq-2). 31P{1H}NMR (CDCl3):
δ 22.3. HRMS (ESI in MeOH+FA): C12H21NO3P
(257.27) calcd. for [M+H]+ 258.1254; found:
258.1254.
1
g (74%). H NMR (CDCl3): δ 2.27 (s, 3 H, 4-CH3),
2.90 (s, 3 H, NCH3), 4.21 (br s, 1 H, NH), 6.58 (br d,
3
4
3J = 8.1 Hz, 1 H, H-6), 7.05 (ddd, J = 8.1, J = 1.8,
4
J = 0.6 Hz, 1 H, H-5), 7.33 (br d, J = 1.5 Hz, 1 H,
H-3). 13C{1H}NMR (CDCl3): δ 19.95 (s, 4-CH3), 30.76
(s, NCH3), 109.44 (s, Cq-2), 110.69 (s, C-6), 127.04 (s,
Cq-4), 128.97 (s, C-5), 132.56 (s, C-3), 143.71 (s, Cq-
1). HRMS (ESI in MeOH+FA): C8H10BrN (200.08)
calcd. for [M+H]+ 200.0069; found: 200.0068 (for
79Br).
Heteroatom Chemistry DOI 10.1002/hc

458 Ghalib et al.
Synthesis of N-methyl-5-methyl-2-phosphinoani-
line (3b) from 7b. A solution of 7b (1.0 g, 3.89
mmol) in diethyl ether (20 mL) was added at –10◦C
to LiAlH4 pellets (0.44 g, 11.6 mmol) in diethyl ether
(150 mL). After stirring for 3 days, water was added
drop-by-drop until the evolution of hydrogen ceased.
Na2SO4 was added for drying, the mixture was fil-
tered, and the solid residue was thoroughly washed
with ether. Removal of the solvent in vacuum fur-
1
nished 0.44 g (74%) yellow oil. H NMR (CDCl3): δ
2.31 (s, 3 H, 4-CH3), 2.95 (s, 3 H, NCH3), 3.62 (d,
1JPH = 199.9 Hz, 2 H, PH2), 4.04 (br s, 1 H, NH),
6.60 (br dd, 3J = 8.1, JPH = 1.7 Hz, 1 H, H-6),
4
5
7.19 (br ddd, 3J = 8.4, 4J = 1.5, JPH = 0.6 Hz, 1
FIGURE 1 Structure of 8b in the crystal (ellipsoids with 50%
◦
˚
3
probability). Selected bond lengths (A) and angles ( ): W(1)-
P(3) 2.4825(6), W(2)-P(3ꢁ) 2.4783(6), W(1)-C(11) 2.005(3),
W(2)-C(11ꢁ) 2.000(3), cis-W C bonds 2.041(3)-2.058(3);
C(11)-O(1) 1.146(3), C(11ꢁ)-O(1ꢁ) 1.150(3), cis-C O bonds
H, H-5), 7.38 (br dd, JPH = 12.3, 4J = 1.5 Hz, 1
H, H-3). 13C{1H}NMR (CDCl3): δ 20.00 (s, 4-CH3),
30.88 (s, N-CH3), 109.21 (s, C-6), 110.39 (d, 1J = 10.6
3
Hz, Cq-2), 125.55 (d, J = 11.9 Hz, Cq-4), 131.73 (s,
1.133(3)-1.143(3); cis-P
W
C angles 86.49(7)-91.86(7),
C angles 177.83(8), 177.92(8), cis-C
87.64(11)-92.67(11), trans-C
C-5), 138.36 (d, 2J = 31.8 Hz, C-3), 149.16 (s, Cq-
1). 31P{1H}NMR (CDCl3): δ –153.0. HRMS (ESI in
MeOH+FA): C8H12NP (153.16), calcd. for [M+H]+
154.0780; found: 154.0780.
trans-P
angles
W
W
angles
C
W
C
174.78(10)-177.67(11); C(2) P(3) 1.707(3), P(3) C(3A)
1.766(3), N(1) C(2) 1.356(3), N(1) C(7A) 1.384(3),
C(2) P(3) C(3A) 91.33(12), N(1) C(2) P(3) 112.6(2).
light (water cooled 250 W Hg medium pressure
immersion lamp) until one equivalent of CO was
evolved (by gas burette, 1–1.5 h). N2 was bubbled
through the solution to blow off CO before 5b
(50 mg, 0.31 mmol) was added to this solution. After
stirring for 1 day, the volatiles were removed in
vacuum. The product was extracted with n-hexane,
separated from insoluble particles and the solvent
evaporated in a vacuum to give 97 mg (65%) of a
yellow-brown solid. Crystals were grown by slow
partial evaporation of a concentrated solution in
CDCl3. Crystal data are given below, selected bond
lengths and angles are given in Fig. 1. 1H NMR
Synthesis of 1,5-dimethyl-1H-1,3-benzazaphos-
phole (5b) from 3b. Me2NCH(OMe)2 (4.37 mL,
32.6 mmol) and Me3SiCl (0.5 mL, 3.94 mmol)
were added to 3b (0.5 g, 3.26 mmol). After heat-
ing for 24 h at 50◦C, volatiles were removed un-
der vacuum at room temperature. The residue was
dissolved in Et2O, and basic impurities were ex-
tracted with degassed 10% aqueous H2SO4. After
drying with sodium sulfate, the solvent was re-
moved in vacuum yielding 0.38 g (71%) viscous oil.
The NMR data are identical to those of 5b, ob-
tained by acid-catalyzed disproportionative conden-
sation of 4-methyl-2-phosphinoaniline and excess
3
1
(C6D6): δ 2.18 (s, 5-CH3), 2.73 (d, J = 2.6 Hz, 3 H,
paraformaldehyde (ESI [6]). H NMR (CDCl3): δ =
3
4
NCH3), 6.85 (br d, J = 8.7 Hz, 1 H, H-7), 6.99 (dt,
2.39 (s, 3 H, 5-CH3), 3.87 (d, JPH = 0.8 Hz, 3 H,
4
5
3J = 8.7, J ≈ JPH ≈ 2 Hz, 1 H, H-6), 7.25 (br d,
3
NCH3), 7.16 (br d, J = 8.4 Hz, 1 H, H-6), 7.37 (br
3
2J = 33.3 Hz, 1 H, H-2), 7.84 (dm, JPH = 8.4 Hz,
3
d, J = 8.4 Hz, 1 H, H-7), 7.80 (m, 1 H, H-4), 8.32
1 H, H-4). 13C{1H}and 135-DEPT NMR (C6D6): δ
(d, 2JPH = 37.8 Hz, 1 H, H-2). 13C{1H}NMR (CDCl3):
3
3
21.77 (s, 5-CH3), 37.29 (s, N-CH3), 113.97 (d, J =
δ = 21.10 (s, 5-CH3), 37.35 (d, J = 2.7 Hz, NCH3),
2
112.02 (s, CH-7), 126.49 (d, 4J = 2.6 Hz, CH-6),
5.3 Hz, C-7), 127.43 (d, J = 13.3 Hz, C-4), 128.69
4
3
2
3
(d, J = 4.5 Hz, C-6), 131.73 (d, J = 14.6 Hz, Cq-5),
138.82 (d, 1J = 22.6 Hz, Cq-3a), 142.17 (s, Cq-7a),
128.89 (d, J = 21.3 Hz, CH-4), 129.38 (d, J = 11.9
2
Hz, Cq-5), 141.29 (d, J = 5.3 Hz, Cq-7a), 142.55 (d,
1
2
1J = 39.8 Hz, Cq-3a), 161.94 (d, J = 54.4 Hz, CH-
1
156.64 (d, J = 29.2 Hz, CH-2), 195.92 (d, J = 9.3
2
2). 31P{1H}NMR (CDCl3): δ = 69.8. HRMS (ESI in
MeOH+FA): C9H10NP, calcd. for [M+H]+ 164.0624;
found: 164.0624.
Hz, 4 cis-CO), 200.66 (d, J = 29.2 Hz, trans-CO).
31P{1H}NMR (C6D6): δ 31.5 (satellites d, JPW
=
1
241.0 Hz). HRMS (DEI+): C14H10NO5PW (487.05),
calcd. for [M(184W)]+ 486.9808; found: 486.9808
(and an isotopic peak pattern with correct relative
abundancies).
η1-P-(1,5-Dimethyl-1H-1,3-benzazaphosphole)
pentacarbonyltungsten(0) (8b). (The synthesis was
communicated in [7]). A solution of W(CO)5(THF)
◦
was prepared at ∼20 C by irradiation of W(CO)6
Crystal Structure Analysis of 8b. Crystal
¯
(119.7 mg, 0.34 mmol) in THF (85 mL) with UV
data: Triclinic, space group P1, a = 7.1378(3),
Heteroatom Chemistry DOI 10.1002/hc

Syntheses of 2-Unsubstituted 1H-1,3-Benzazaphospholes from N-Formyl-2-bromoanilides 459
◦
3
˚
[2] (a) Le Floch, P. Coord Chem Rev 2006, 250, 627–
681; (b) Mu¨ller, C.; Vogt, D. Dalton Trans 2007,
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[4] Ghalib, M. PhD thesis, University of Greifswald,
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G. Tetrahedron 2001, 57, 9963–9972.
b = 13.4542(6), c = 16.1594(7) A, α = 95.895(3) ,
◦
◦
˚
β = 90.501(3) , γ = 90.901(3) , V = 1543.36(12) A ,
Z = 4, Dx = 2.096 Mg/m3, m = 7.609 mm–1, F(000) =
920. Most of the crystals were twins, but a suitable
single crystal 0.20 × 0.15 × 0.03 mm was eventually
found and mounted on a glass fibre in inert oil.
A total of 71178 data (8878 unique, Rint 0.034)
were recorded at low temperature on an Oxford
Diffraction Xcalibur diffractometer using Mo-Kα
radiation (λ = 0.71073 A) to 2θmax 60◦. Absorption
˚
corrections were applied using multiscans. The
structure was solved by the heavy-atom method
and refined by full-matrix least-squares on F2 [16].
Hydrogen atoms were included using a riding model
or rigid methyl groups. The final wR2 was 0.0414
for 401 parameters and all reflections, with R1 (I >
–3
˚
2σ(I)) 0.0194; S 1.12, max. ꢀρ 1.4 e·A .
Complete crystallographic data for 8b have been
deposited with the Cambridge Crystallographic Data
Centre, CCDC 926717. Copies of the data can be ob-
tained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK, (Fax: +44-
(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.Uk).
[9] (a) Andrushko, N.; Bo¨rner, A. In Phosphorus Ligands
in Asymmetric Catalysis; Bo¨rner, A. (Ed.): Wiley–
VCH: Weinheim, Germany, 2008; Vol. 3, pp. 1275–
1347; (b) Gaumont, A. C.; Carboni, B. Science of
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B.; Maffei, M. Coord Chem Rev 1998, 178–180,
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Anorg Allg Chem 2002, 628, 2869–2876; (b) Heinicke,
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[11] Weinmaier, J. H.; Tautz, H.; Schmidpeter, A. J
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ACKNOWLEDGMENTS
We thank Dr. M. K. Kindermann, G. Thede, and M.
Steinich for numerous NMR spectra, Dr. H. Frauen-
dorf and G. Sommer–Udvarnoki (Georg-August-
Universita¨t Go¨ttingen, Institut fu¨r Organische und
Biomolekulare Chemie) and Prof. Dr. H.-C. Bo¨ttcher
and B. Breitenstein (Department Chemie, Ludwig-
Maximilians-Universita¨t Mu¨nchen) for HRMS mea-
surements.
[12] Gupta, N.; Jain, C. B.; Heinicke, J.; Bansal, R. K.;
Jones, P. G. Eur J Inorg Chem 1998, 1079–1086.
[13] (a) Le Floch, P.; Ricard, L.; Mathey, F. Polyhedron
1990, 9, 991–997; (b) Mao, Y.; Mathey, F. Chem Eur
J 2011, 17, 10745–10751.
[14] Thakuria, H.; Borah, B. M.; Das, G. J Mol Catal A:
Chem 2007, 274, 1–10.
[15] Issleib, K.; Vollmer, R.; Oehme, H.; Meyer, H. Tetra-
hedron Lett 1978, 441–444.
[16] Sheldrick, G. M. Acta Crystallogr 2008, A64, 112–122.
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Heteroatom Chemistry DOI 10.1002/hc