Synthesis of N,P-Disecondary o-Arylphosphanylanilines via o-R1NHC6H4P(R)O2Et Precursors and Preliminary Study of Cyclocondensations with (EtO)3CH/NH4PF6

Article abstract of DOI:10.1002/ejic.201901069

N-Secondary o-aminophenylarylphosphinates were prepared by Pd-catalyzed cross coupling of aryl-P(OEt)₂ with the corresponding o-bromoanilines and reduced with LiAlH₄ to o-phosphanylanilines. For the N-mesityl compounds a two-step reduction was necessary because of competing P–N condensation reactions. Preliminary studies of cyclocondensations of o-(RPH)C₆H₄NHR¹ with (EtO)₃CH/NH₄PF₆ showed that, in contrast to benzimidazolium salts, accessible in this way from disecondary o-phenylenediamines, the homologous 1,3-benzazaphospholium salts are highly reactive and undergo instantaneous oxidative addition of EtOH, liberated in the condensation. The resulting 3-ethoxy-benzazaphospholinium salts, which for R = Ph, R¹ = Np are relatively stable and were detected by NMR and HRMS, decompose to yield P-oxo-1,3-benzazaphospholine·xHPF₆ salts, and for R¹ = Me additionally the rearranged N-ethyl-P-oxo-benzazaphospholinium salt. Detection of triethylphosphate indicates partial acid-catalyzed conversion of PF₆^– with EtOH. Alternative cyclocondensation of o-(MesPH)C₆H₄NHNp with excess Me₂NCH(OMe)₂ in the presence of HCl/Et₂O led to the 2-methoxy-, or after removal of MeOH under vacuum to the corresponding 2-dimethylamino-1,3-benzazaphospholine. The structure elucidation of the new compounds is based on multinuclear NMR data and confirmed for an o-anilinophenylphosphinate by crystal structure analysis.

Full text of DOI:10.1002/ejic.201901069

DOI: 10.1002/ejic.201901069
Full Paper
Benzazaphospholinium Species
Synthesis of N,P-Disecondary o-Arylphosphanylanilines via
o-R1NHC6H4P(R)O2Et Precursors and Preliminary Study of
Cyclocondensations with (EtO)3CH/NH4PF6
Bhaskar R. Aluri,[a][‡] Mohammed Ghalib,[a][‡‡] Peter G. Jones,[b] Holm Frauendorf,[c] and
Joachim W. Heinicke*[a]
Abstract: N-Secondary o-aminophenylarylphosphinates were R = Ph, R1 = Np are relatively stable and were detected by NMR
prepared by Pd-catalyzed cross coupling of aryl-P(OEt)2 with and HRMS, decompose to yield P-oxo-1,3-benzazaphosphol-
the corresponding o-bromoanilines and reduced with LiAlH4 to ine·xHPF6 salts, and for R1 = Me additionally the rearranged N-
o-phosphanylanilines. For the N-mesityl compounds a two-step ethyl-P-oxo-benzazaphospholinium salt. Detection of triethyl-
reduction was necessary because of competing P–N condensa- phosphate indicates partial acid-catalyzed conversion of
–
tion reactions. Preliminary studies of cyclocondensations of PF6 with EtOH. Alternative cyclocondensation of o-(MesPH)-
o-(RPH)C6H4NHR1 with (EtO)3CH/NH4PF6 showed that, in con- C6H4NHNp with excess Me2NCH(OMe)2 in the presence of
trast to benzimidazolium salts, accessible in this way from disec- HCl/Et2O led to the 2-methoxy-, or after removal of MeOH un-
ondary o-phenylenediamines, the homologous 1,3-benzaza- der vacuum to the corresponding 2-dimethylamino-1,3-benz-
phospholium salts are highly reactive and undergo instantane- azaphospholine. The structure elucidation of the new com-
ous oxidative addition of EtOH, liberated in the condensation. pounds is based on multinuclear NMR data and confirmed for
The resulting 3-ethoxy-benzazaphospholinium salts, which for an o-anilinophenylphosphinate by crystal structure analysis.
–
Introduction
salt (Et3O+BF4 ). Alkyl halides do not attack these σ2P-heterocy-
cles, even on prolonged heating, since their nucleophilicity is
low, because of the energetically low lone pair at phosphorus.[5]
Catalysis by suitable transition metal compounds, however,
studied mainly for N-neopentyl-1,3-benzazaphosphole, enabled
P-alkylation with neopentyl iodide or P-arylation with various
aryl iodides or bromides, but led in all known examples to 1,3-
benzazaphospholine-P-oxides IV (R2 = Me), which is obviously
attributable to traces of moisture in the reaction mixture.
Whether the extreme sensitivity to addition of water was also
catalyzed by the transition metal, or induced by addition of the
polarizable halide anion at the benzazaphospholium cation and
subsequent reaction with moisture, is not clear. To learn more
about this aspect, we have extended our studies on syntheses
and cyclocondensations of N-secondary o-aminoarylphos-
phanes[6,7] to the preparation of N,P-disecondary o-phosphanyl-
anilines and their cyclization with orthoformates. A first com-
munication in this direction was published recently by Light-
burn, Dombrowski and Tan,[8] who synthesized 2-alkoxy-1-
N,N′-Disubstituted benzimidazolium salts I are well known com-
pounds and highly useful precursors for benzanellated N-het-
erocyclic carbenes (bNHC) and transition metal complexes or
catalysts with bNHC ligands.[1,2] The homologous 1,3-benzaza-
phospholium salts II (R = Et, R1 = Me, neopentyl),[3] however,
have scarcely been investigated, and benzodiphospholium salts
are known only as semi-ylide salts III (R = Me, Ph, X = Br, PF6)
with pentavalent phosphorus[4] (Scheme 1). Both II and III react
readily with water to form phosphine oxides IV (R2 = H) [3] and
ring-opened products, respectively, with cleavage at the ylidic
PCP bridge. The two examples of II were obtained by reaction
of N-substituted 1H-1,3-benzazaphospholes with Meerwein′s
[a] Institut für Biochemie, Universität Greifswald,
Felix-Hausdorff-Str. 4, 17487 Greifswald, Germany
E-mail: heinicke@uni-greifswald.de
[b] Institut für Anorganische und Analytische Chemie,
Technische Universität Braunschweig,
38023 Braunschweig, Germany
[c] Institut für Organische und Biomolekulare Chemie, Zentrale Analytik/
Massenspektrometrie, Georg-August-Universität Göttingen,
37077 Göttingen, Germany
[‡] Current address: SABIC Technology Centre,
31961 Al-Jubail, Saudi Arabia
[‡‡] Current address: University of Hodeidah, Department of Chemistry,
4513 Hodeidah, Yemen
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. ·
This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
Scheme 1. Related benzimidazolium (I), benzazaphospholium (II) and benzo-
diphospholium salts (III) and benzazaphospholine-P-oxides (IV, R2 = H, Me).
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methyl-3-phenyl-1,3-benzazaphospholines, further studied as li-
gands for Rh-catalyzed branch-selective hydroformylation of
various 3-mono- and 1,3-disubstituted 4-hydroxyalkenes.
Results and Discussion
Synthesis and Reduction of N-Secondary o-
Aminophenylphosphinates
Syntheses of N-methyl-o-phenylphosphanylaniline[8,9] and two
N-aryl o-phenylphosphanylanilines[10] are known but are me-
thodically limited by the final cleavage of a P-phenyl group
from the corresponding o-diphenylphosphanylaniline precur-
sors by Na or Li. This prompted us to re-examine our earlier
reported synthetic route to N-secondary alkyl- and aryl-substi-
tuted o-phosphanylanilines[7] for the preparation of N,P-disec-
ondary representatives. The synthesis involved the palladium-
catalyzed P–C cross-coupling of N-secondary o-bromoanilines
with excess triethylphosphite or aryl-diethylphosphonite and
subsequent reduction of the resulting o-anilinophosphinates
1a–e with excess LiAlH4; P-alkyl derivatives such as 2a[11] addi-
tionally required the final P-alkylation of the P-primary precur-
sor[7a] (Scheme 2). The coupling of neopentyl-o-bromoaniline
with P(OEt)3 to 1a in the presence of Pd(OAc)2 (4 mol-%) was
performed under quite harsh conditions, namely heating at
200 °C for 1 h[7a]. The catalyst formation from the Pd salt and
the phosphite was observed by color change and NMR moni-
toring. Heating of the primarily formed yellow [Pd{P(OEt)3}2X2]
complex[12] in the absence of the aryl halide led to colorless
[Pd0{P(OEt)3}3-4].[13] The 31P signal of this Pd0 complex disap-
peared instantaneously upon addition of aryl halide, indicating
rapid oxidative addition, which opens the catalytic cycle.[7c] The
reactions with aryldiethylphosphonites proceed analogously
but require somewhat lower temperature to avoid thermal de-
compositions, which are indicated by a color change to dark
brown and by low yields of coupling products. Thus, the con-
version of mesityl-o-bromoaniline with MesP(OEt)2 at 200 °C/
30 min. gave only 24 % 1e after workup by column chromatog-
raphy. Coupling temperatures at 160–165 °C avoided rapid de-
composition but required prolonged reaction times. NMR-moni-
toring of the formation of 1b from neopentyl-bromoaniline and
PhP(OEt)2 in the presence of PdCl2 (5 mol-%) showed a 28 %
yield after 1 h and 58 % after 2 h (after workup 55 % in two
Scheme 2. Synthesis of N-secondary o-anilinoarylphosphinates 1a–e and N,P-
disecondary o-arylphosphanylanilines 2a–d (1a,[7a] 2a[11]).
Reduction of 1a–e with excess LiAlH4 (3–5 equivalents) and
hydrolytic workup furnished the N,P-disecondary o-arylphos-
phanylanilines 2a–e. The N-mesityl compounds 2d and 2e were
accompanied by substantial amounts (25 and 33 mol-%) of the
corresponding P-oxides 2d(O) and 2e(O). Separation by vac-
uum distillation, attempted for 2e, failed to give a pure product,
whereas a second reduction of the crude mixture with LiAlH4,
as shown for 2d, gave a good overall yield. The formation of
such P-oxides had not yet been observed in the reduction of
N-primary or N-secondary o-anilinophosphonates with excess
LiAH4. It was presumably caused by slower reduction of the o-
mesitylamino-diarylphosphinates and a competing condensa-
tion step of NLi- and PIII-OEt functional intermediates to eight-
membered cyclic dimers or acyclic P–N species, which are more
stable to attack by LiAlH4 than P–O compounds but are cleaved
by hydrolytic workup to the NH-functional phosphine oxides
2d(O) and 2e(O). A closer examination of this side reaction was
not performed within this study.
Reactions with Triethylorthoformate in the Presence of
NH4PF6
runs). Increasing yields with growing steric demand of the A standard preparation of N,N′-disubstituted benzimidazolium
N-substituents, observed in the earlier P–C couplings with salts I is to heat a mixture of the corresponding disecondary
P(OEt)3,[7a] suggest that the lower yield of 1c from the reaction o-phenylendiamine, a suitable ammonium salt and excess tri-
of methyl-o-bromoaniline with PhP(OEt)2 may be attributed to ethylorthoformate while removing the ethanol that is liberated
a stronger susceptibility to side reactions in the case of small in the cyclocondensation.[1,2] Analogous treatment of 2a with
N-substituents. To examine whether addition of auxiliary phos- NH4PF6 and CH(OEt)3, heating for 6 h at 130 °C in a slow stream
phane ligands improves the coupling yields, the synthesis of 1b of N2, furnished not the homologous benzazaphospholium
was also performed in the presence of tris-o-tolylphosphane hexafluorophosphate but instead the P-oxo-benzazaphos-
under otherwise identical conditions. The yield was nearly the pholine 6a·xHPF6. This displayed, even in the crude product
same. Test of BINAP/PdCl2 (7.5/5 mol-%) in the synthesis of 1d spectra, the strongest phosphorus resonance at δ = 41.3, ac-
led to a similar result (58 %) but a shorter reaction time (1 h); companied by small signals at δ = –0.1 (TEP: triethylphosphate),
this suggests that additional phosphane ligands may improve 86 and various trace signals, 31P integral ratio (without PF6 ) 65,
–
the coupling yield whereas purification of the product is then 4, 13 and <3 %. The molar ratio of 6a and TEP was ca. 9:1,
more complicated.
based on integration of PCH and OCH2 proton signals. Column
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chromatography on silica gel furnished an impure and an al- uct of 5c, along with a small amount of TEP (δ31P = 36.9/37.4,
most pure fraction, the latter still with a trace of TEP (2–3 mol- 29.6/29.8, –0.7; peak heights 20:7:33:19:21; PF6– δ = –143.2). The
1
% by H integration). The varying 6a/PF6 31P integral ratio indi- formation of the N-ethylation product 7c from 5c reflects the
cates that no stable 6a(H+)PF6 salt was formed but instead a high electrophilicity of alkoxyphosphonium salts, known mainly
–
labile association of 6a·xHPF6 (x < 1). NMR monitoring of the from the alkylation of halide ions by intermediate RP+(OAlk)3
reaction of 2b with NH4PF6 and CH(OEt)3 was even more in- species in the Michaelis–Arbuzov reaction.[16] The occurrence
formative, allowing the identification of a reasonably stable in- of stereoisomers is attributed to lower stereoselectivity of the
termediate ethoxyphosphonium salt 5b. This, together with reaction by the small steric demand of the NMe group, allowing
6b·xHPF6, formed the main components of the crude product syn- and anti-position towards the P-substituents Ph and O.
mixture after 3–5 h heating at 80–90 °C, with either a slight or Crystallization from refluxing ethanol provided solid 7c, isomer
a larger excess of CH(OEt)3 (1.1 or 4.8 equivalents). In addition, ratio 2:1. Reaction of 2d with NH4PF6 and excess CH(OEt)3 under
a substantial amount of (EtO)3PO and various minor or trace the same conditions furnished a similar mixture with phos-
products were detected. 5b exhibits a downfield 31P resonance phorus resonances at δ = 36.6, 28.3, –1.1(TEP) and –144 (PF6
–
at δ = 73, lying close to that of the zwitterionic P-F3B–O-substi- anion) but was not further investigated. The origin of TEP was
tuted P-ethyl-benzazaphospholium compound (δ = 71.5),[3] not studied in detail, but formation by H+-catalyzed reaction of
–
formed from moisture and II (R = Et, R1 = CH2CMe3), close to PF6 with EtOH is probable since solutions of 5b/6b·xHPF6 in
the 86 ppm signal observed in the reaction of 2a, and between MeOH display in the ESI-HRMS spectra peaks for
the P resonances of Ph3POEt+BF4 (δ = 62) and nBu3POEt+BF4
[(MeO)3PO + H/Na+] beside the somewhat stronger peaks for
–
–
(δ = 98),[14] a range that is often observed for C3PX+ species [(EtO)3PO + H/Na+]. This reaction, together with PF6-salt forma-
(X = OR etc.).[15] Storage for a week at r.t. led to a slow decrease tion of side products, caused a decreasing content x of HPF6 in
of the content of 5b compared to 6b, and column chromatog- 6a,b·xHPF6 during workup and furnished in late fractions of
raphy caused complete decomposition of 5b during elution column chromatographic separations small amounts of HPF6-
and poor separation of 6b from impurities including (EtO)3PO. free 6a and 6b, already obtained by us using catalytic P-alkyl-
This suggests that compounds of type 5 are more or less stable ation or arylation of 1H-1,3-benzazaphospholes.[3]
intermediates in the formation of the P-oxides of type 6 and
are formed via cyclocondensation of the starting materials to
2-alkoxybenzazaphospholinium and benzazaphospholium in-
termediates 3 and 4 as proposed in Scheme 3.
As an alternative to triethyl orthoformate, excess dimethyl-
formamide dimethyl acetal (DMFA) was used in the cyclocon-
densation with 2d, with an ethereal solution of HCl as catalyst
(cf. lit.[7a]). 31P and 1H NMR reaction monitoring after heating
for 2 d at 60 °C indicated the almost quantitative conversion of
2d to 8d, MeOH and Me2NH·HCl. Removal of volatiles under
vacuum led to strongly diminished signals of excess DMFA and
replacement of the 2-methoxy-doublet of 8d by a partly super-
imposed NMe2 singlet, without significant changes of other
benzazaphospholine proton signals. This indicates rapid H+-cat-
alyzed reactions at CH-2 and conversion with the less volatile
Me2NH·HCl or excess DMFA to 9d during removal of MeOH
(Scheme 4). In addition, a trace of moisture in the NMR solvent
led to minor replacement of NMe2 by OH, indicated by a slightly
diminished integral of the superimposed p-Me and NMe2
Scheme 3. Cyclocondensations of 2a–c with (EtO)3CH/NH4PF6 and proposed
intermediates.
The analogous treatment of 2c with NH4PF6 and excess
CH(OEt)3 led to the discovery of a further feature of this reac-
tion. The crude product, obtained after removal of excess tri-
ethyl formate under vacuum, proved to be a mixture of two
stereoisomers each of 6c·xHPF6 and 7c, a rearrangement prod-
Scheme 4. Reaction of 2d with dimethylformamide dimethyl acetal to 8d and
9d (x <1).
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singlets and an OH/H2O trace signal,[17] followed by slow con- 145.5(16)°], typical for o-phosphonoanilines and related o-
version to 6d. These observations are in good accordance with aminoarylphosphonates,[7,23] causes an unsymmetrical inter-
the ready replacement of the methoxy group of 8c, prepared secting arrangement of the O2–P–C21 fragment towards the
from 2c and trimethyl orthoformate, by alkoxy groups of less central aromatic ring, which is associated with the larger spatial
volatile alcohols.[8] The reversible substitution reactions at C-2 demand of the phenyl compared to the ethoxy group [O2–P–
might also be the basis for the usefulness of the 2-alkoxy-1,3- C2–C3 39.41(12)°, C21–P–C2–C3 66.60(12)°]. The two o-methyl
benzazaphospholines as ligands for Rh-catalyzed branch-selec- groups of the N-mesityl substituent force a nearly perpendicular
tive hydroformylations of 4-hydroxyalkenes, reported in the orientation towards the aniline ring plane [C1–N–C11–C12
same paper.
81.49(19)°, C1–N–C11–C16 97.71(17)°]. Twists around the N–C11
axis will lead to interactions of the methyl protons with the
protons at C6 and C5 and be responsible for the broad singlets
of the o-methyl groups.
Structure Elucidation
The identification and structural characterization of the new
compounds is based on conclusive NMR data and for 1d addi-
tionally on crystal structure analysis. The benzo-proton and 13C
signals, strongly influenced by the amino- (+M/–I) and the PHR-,
o-phosphonium- or P=O-group, appear in the region typical for
o-anilinophosphono, -phosphanyl and -phosphonium com-
pounds or 2,3-dihydrobenzazaphospholes[6d,11] and with the
expected coupling pattern. Characteristic data are presented in
1
Table S1. The JPC coupling constants of 1b–e (128–140 Hz)
correspond to typical values for diarylphosphinates and those
of 6a,b·xHPF6 and 7c·xHPF6 to values characteristic of tertiary
2
phosphine oxides.[18] The different JPH coupling constants of
the PCHaHb proton signals of the latter reflect the cis- (larger)
and trans-orientation towards the O-atom of the P=O group.[19]
The 13C and 31P data of 6a,b·xHPF6 are similar to those of neu-
Figure 1. Molecular structure of 1d in the crystal. Ellipsoids represent 50 %
probability levels. Selected bond lengths [Å] and angles (°): P–C2 1.7862(13),
N–C1 1.3725(17), C11–N 1.4293(17), C21–P 1.7910(13); P–C(2)–C(1) 123.35(10),
C(2)–C(1)–N 121.79(12), C(2)–P–C(21) 107.74(6), C(1)–N–C(11) 123.03(12).
tral P-oxo-1,3-benzazaphospholines 6a,b (Δδ
≤
2)[3] and
[20]
6Bn,Ph
.
The three P-Cq doublets of the intermediate 5b ap-
pear significantly upfield from those of 6b·xHPF6, as is typical
for phosphonium compounds compared to related phosphine
oxides (cf. Table S1), and confirm the 3-ethoxy-1,3-benzaza-
phospholinium structure of 5b, suggested by the aforemen-
tioned similarity of the phosphorus resonance δ = 73.4 to the
chemical shift values of R3P+OR species.[3,14,15] The N-ethylation
to the syn- and anti-stereoisomers of 7c was concluded from
Conclusions and Outlook
A synthetic route to N-secondary o-aminophenylarylphosphinic
acid esters by Pd-catalyzed P–C cross coupling of N-secondary
o-bromoanilines with Ph(OEt)2 and MesP(OEt)2 was demon-
strated. Though not yet optimized, yields near 60 % (monitored
by 31P NMR) were achieved for N-neopentyl compounds. Reduc-
tion with LiAlH4 and aqueous workup provided N,P-disecondary
o-phosphanylanilines. In the case of the N-mesityl compounds,
this reaction was accompanied by formation of the respective
P-secondary phosphine oxide. This can be attributed to steric
hindrance of the reduction and competing formation of P–N
bonds by reaction of N–Li with an intermediate EtO-PIII species.
The PIII–N bonds are rather stable to LiAlH4 but are hydrolyzed
by the aqueous workup to P(O)H compounds and furnished
pure o-phosphanylanilines after a second reduction step. Pre-
liminary studies of the cyclocondensation of the disecondary
o-phosphanylanilines with triethylorthoformate/NH4PF6 show a
major difference in the reactivity compared to analogous N,N′-
disecondary o-phenylene diamines. Whereas the latter provide
high yields of stable benzimidazolium salts, ideal precursors for
the downfield shifted N+Me, N+CH2 and N+CH2P proton and 13
C
resonances, typical for quaternary alkylammonium salts, and
the upfield CH3(Et) 13C signals at δ = 9–9.5 (cf. Et4N+ I–[21]).
The structure of 9d was indicated by the P–C–N 13C doublet
at δ = 95.4, close to the acetalic carbon of the related 2-meth-
oxy-1,3-benzazaphospholine 8c, synthesized by Lightburn et
1
al.[8] The larger JPC coupling in 9d (21.9 Hz vs. 4.7 Hz in 8c)
may be attributed to the angle-dependence of this coupling
and larger spatial demand of the 2-NMe2 and N-mesityl groups
2
in 9d compared to 2-OMe and N-methyl in 8c. The lack of JPH
coupling of the PCHN proton signal of 8d and 9d indicates
trans-orientation towards the P electron lone-pair, i.e. trans-con-
figuration. The other 13C and proton NMR data of the ring
framework are in accordance with those of other 1,3-benzaza-
phospholines, studied earlier by us[6d,11,22] (Table S1, see there
also for hints at earlier erroneous assignments of CH5/6).
More detailed information on the steric situation is provided benzanellated N-heterocyclic carbenes,[2] the benzazaphos-
for the single crystalline 1d by the XRD-data (Figure 1). The pholium salts II are highly sensitive to moisture[3] and obviously
compound crystallizes in P21/c with Z = 4 and displays normal also towards EtOH, which is liberated in the reaction with
values of bond lengths and angles. The intramolecular P=O···H– CH(OEt)3. Thus, they could not be observed directly. The forma-
N hydrogen bond [N–H 0.838(19) Å, H···O 2.079(18) Å, N–H···O tion of the heterocyclic phosphine oxides, as in the hydrolysis
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reactions,[3] suggests their occurrence as intermediates and, in
contrast to the reversible addition of alcohols to “classic”
immonium compounds, instantaneous “inverse” oxidative addi-
tion to form P-ethoxybenzazaphospholinium salts with an
–RP+(OEt)–CH2–NR substructure. These display, similarly to the
RnP+(OR′)3-n intermediates of the Michaelis–Arbuzov-reac-
tion,[16] limited stability because of strong electrophilicity
[–RP(=O+Et)–CH2–NR–] and facile Et+ transfer to sufficiently re-
active nucleophiles, or in the case of 5c partial N-ethylation to
7c. The driving force is the high stability of the P=O bond. The
key to the addition step of EtOH at the –P(R)–CH=N+R– function
is the s-character of the low-lying P lone-pair (HOMO-2[5]). This
disfavors sp2 hybridization and leads to a weak hyperconjuga-
tive (PCP-LUMO-Cp)–π-bond (“soft double bond”[24]) with signifi-
cant positive charge at P compared to C2, which is responsible
for the facile “inverse addition”. Preliminary studies of the alter-
native HCl-catalyzed condensation with excess Me2NCH(OMe)2
(DMFA), for 2d leading to 8d and 9d, demonstrates access to
2-methoxy-1,3-benzazaphospholines or 2-dimethylamino-1,3-
benzazaphospholines. These hydrolyze easily and convert to
P-oxo-1,3-benzazaphospholines, as detected for 6d. However,
as demonstrated by the group of Tan,[8] they can also be used
as ligands in catalytic transformations. To find out if even elimi-
nation of ROH (or Me2NH) is possible, as known for the genera-
tion of triazol-ylidenes from 3-methoxy-1,2,4-triazoles,[25] and if
N,P-heterocyclic carbenes as potential 4e-donor ligands[26] can
be trapped, may be a challenging task for future research. The
knowledge of the reactivity of N,P-disecondary o-phosphanyl-
anilines may pave the way for such studies.
1.04 mmol), PdCl2 (9 mg, 4 mol-%) and PhP(OEt)2 (0.25 mL,
1.31 mmol) was heated at 160 °C for 2 h and thereafter purified
directly at ambient temperature using column chromatography on
silica gel with 25 % ethyl acetate/hexane as eluant to give 200 mg
(55 %) pale yellow oil. The NMR data are in good accordance to
those given below.
b) A dry Schlenk flask was charged with N-neopentyl-2-bromo-
aniline (4.30 g, 17.8 mmol), PhP(OEt)2 (3.75 mL, 19.5 mmol), tris-o-
tolylphosphane (405 mg, 7.5 mol-%) and PdCl2 (157 mg, 5 mol-%).
Heating this initially colorless mixture for 3 h at 160 °C led to a
color change to yellow and later to brown. After cooling at ambient
temperature the crude product was purified using column chroma-
tography on silica gel with 10 % ethyl acetate/hexane as eluant to
give 3.0 g (51 %) of a pale yellow oil. Repetition in a smaller scale
(27 % of the above quantities) furnished 1b in 52 % yield. IR (KBr):
ν = 3314 (m), 2955 (st), 1599 (st), 1584 (st), 1452 (st), 12094 (st),
˜
1034 cm–1 (vst). 1H NMR (CDCl3): δ = 1.00 (s, 9 H, CMe3), 1.39, (t,
3J = 7.1 Hz, 3 H, CH3), 2.86 (d, 3J = 5.1 Hz, 2 H, NCH2), 4.05–4.26 (m,
3
2 H, OCH2), 6.54–6.64 (m, 2 H, aryl), 7.05 (t br, J ≈ 5 Hz, 1 H, NH),
7.29 (t br, 3J = 8.3, 7.3 Hz, 1 H, aryl), 7.34–7.53 (m, 4 H, aryl), 7.80
3
(m, JPH = 12.4, 3J = 8.3, 4J = 1.5 Hz, 2 H, o-H). 13C{1H} NMR and
DEPT-135 (CDCl3): δ = 16.52 (d, 3J = 6.7 Hz, CH3), 27.71 (s, CMe3),
2
31.68 (s, CMe3), 55.14 (s, NCH2), 61.04 (d, J = 6.1 Hz, OCH2), 109.98
(d, 1J = 134.9 Hz, Cq-1), 110.80 (d, 3J = 10.2 Hz, CH-3), 114.86 (d,
3
2
3J = 12.5 Hz, CH-5), 128.37 (d, J = 13.3 Hz, 2 CH-m), 131.18 (d, J =
10.4 Hz, 2 CH-o), 131.87 (d, 4J = 2.5 Hz, CH-p), 132.57 (d, 1J =
139.3 Hz, Cq-i), 133.25 (d, 2J = 9.0 Hz, CH-6), 133.94 (br, CH-4), 152.93
(d, J = 8.0 Hz, Cq-2). 31P{1H} NMR (CDCl3): δ = 36.2. MS (EI, 70 eV,
2
80 °C): m/z (%) = 332 (2) [M + 1]+, 331 (8) [M]+, 275 (100), 246 (20),
228 (19), 168 (11), 150 (15), 91 (55), 77 (14). HRMS (ESI in MeOH/
H2O, HCOOH): C19H26NO2P (331.39), calcd. for [M + H]+: 332.17739,
found 332.17732.
2-(Methylaminophenyl)phenylphosphinic Acid Ethyl Ester (1c):
A dry Schlenk flask was charged with N-methyl-2-bromoaniline
(200 mg, 1.07 mmol), Pd(OAc)2 (9 mg, 4 mol-%) and PhP(OEt)2
(0.23 mL, 1.20 mmol). This mixture was converted and purified as
Experimental Section
General: All operations except purification of phosphinic acid es-
ters were carried out under nitrogen atmosphere using Schlenk
techniques. Ether and THF were ketyl-dried and freshly distilled be-
fore use, dichloromethane was dried over P4O10 and recondensed
under vacuum. Glassware was warmed before evacuation and filling
with nitrogen, but complete removal of traces of moisture seems
not to be achieved in this way. NMR solvents were stored under
N2 in greased Schlenk bottles. 2-Bromoaniline, PhP(OEt)2, LiAlH4,
HC(OEt)3, Me2NCH(OMe)2, NH4BF4 and HC(Cl)=NMe2+Cl– (Arnold′s
reagent) were purchased and used without further treatment. N-
Methyl-,[27] N-neopentyl- and N-mesityl-2-bromoanilines[7a] were
prepared as reported earlier, likewise 1a[7a] and 2a.[11] NMR spectra
were recorded on a multinuclear FT-NMR spectrometer ARX300
(Bruker) at 300.1 (1H), 75.5 (13C), and 121.5 (31P) MHz. Chemical
shifts (δ) are given in ppm relative to Me4Si and H3PO4 (85 %), re-
spectively, or to solvent signals, calibrated with the aforementioned
standards. Coupling constants refer to JHH in 1H and JPC in 13C NMR
data unless stated otherwise. Assignment numbers follow the no-
menclature of the numbering scheme and are (partly) indicated in
Scheme 1 and Scheme 2. Low-resolution mass spectra were re-
corded with a single focusing sector-field mass spectrometer
AMD40. HRMS were measured in Göttingen by using a double fo-
cusing sector-field instrument MAT 95 (Finnigan) (EI 70 eV) or a 7T
Fourier transform ion cyclotron resonance mass spectrometer APEX
IV (Bruker Daltonics) (ESI in MeOH).
1
described for 1b yielding 117 mg (40 %) 1c as a pale yellow oil. H
3
NMR (CDCl3): δ = 1.39 (t, J = 7.1 Hz, 3 H, CH3), 2.83 (s, 3 H, NCH3),
4.01–4.25 (m, 2 H, OCH2), 6.57–6.64 (m, 2 H, aryl), 7.06 (vbr s, NH),
3
3
7.28–7.53 (m, 5 H, aryl), 7.78 (m, JPH = 12.4, J ≈ 7, 4/5J ≤ 1.6 Hz, 2
H, o-H). 13C{1H} NMR and DEPT-135 (CDCl3): δ = 16.45 (d, 3J = 6.8 Hz,
CH3), 29.95 (s, NCH3), 61.09 (d, 2J = 5.8 Hz, OCH2), 109.69 (partly
superimp. d, 1J = 129 Hz, Cq-1), 110.48 (partly superimp. d, 3J =
3
3
10 Hz, CH-3), 115.07 (d, J = 12.7 Hz, CH-5), 128.37 (d, J = 13.2 Hz,
2 CH-m), 131.07 (d, 2J = 10.5 Hz, 2 CH-o), 131.89 (d, 4J = 2.8 Hz, CH-
p), 132.58 (d, 1J = 140.6 Hz, Cq-i), 133.20 (d, 2J = 9.1 Hz, CH-6), 133.99
(d, 4J = 2.0 Hz, CH-4), 153.60 (d, 2J = 7.9 Hz, Cq-2). 31P{1H} NMR
(CDCl3): δ = 37.3. MS (EI, 70 eV, 210 °C): m/z (%) = 276 (13) [M +
1]+, 275 (100) [M]+, 229 (32), 184 (34), 156 (32), 147 (79), 106 (25),
91 (25), 77 (56). HRMS (EI, 70 eV): calcd. for [C15H18NO2P]+: 275.1070,
found 275.1072.
2-(Mesitylaminophenyl)phenylphosphinic Acid Ethyl Ester (1d):
PhP(OEt)2 (0.13 mL, 0.69 mmol) was added to a mixture of N-
mesityl-2-bromoaniline (200 mg, 0.69 mmol) and catalytic amounts
of PdCl2 (6 mg, 5 mol-%) and BINAP (32 mg, 7.5 mol-%). This mix-
ture was heated at 160 °C for 1 h and purified using column chro-
matography with 13 % ethyl acetate/hexane as eluant to give
150 mg (58 %) of a pale yellow oil. This solidified within few hours
and was crystallized by slow concentration of a saturated solution
3
of 1d in ethyl acetate. 1H NMR (CDCl3): δ = 1.43 (t, J = 7.0 Hz, 3 H,
2-(Neopentylaminophenyl)phenylphosphinic Acid Ethyl Ester CH3), 1.93 (s, 3 H, o′-CH3), 2.17 (s, 3 H, o′-CH3), 2.29 (s, 3 H, p′-CH3),
3
4
(1b): a)
A
mixture of N-neopentyl-2-bromoaniline (253 mg, 4.15–4.30 (m, 2 H, OCH2), 6.15 (br dd, J ≈ 8, JPH ≈ 6 Hz, 1 H, H-3),
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3
4
4
6.66 (tdd, J = 8.3, 7.9, JPH = 2.8, J = 0.9 Hz, 1 H, H-5), 6.88 (s br,
calcd. for [M + H]+: 272.1563, found 272.1562; calcd. for [M + Na]+:
294.1382, found 294.1390 (strong).
1 H, H-m′), 6.92 (s br, 1 H, H-m′), 7.15 (tt, 3J = 8.4, 7.3, 4J + JPH
=
=
5
2.5 Hz, 1 H, H-4), 7.40–7.55 (m, 4 H, H-6, 2 H-m, H-p), 7.85 (m, 3JPH
N-Methyl-2-(phenylphosphanyl)aniline (2c): Reduction of 1c
(0.80 g, 2.91 mmol) in Et2O (5 mL) with LiAlH4 tablets (0.33 g,
8.70 mmol) in Et2O (10 mL) for 2 d at 20–24 °C changed the color
of the mixture to green, followed by yellow and finally pale brown.
Hydrolytic workup as described for 2b furnished 450 mg (72 %)
12.4 Hz, 2 H, H-o), 8.16 (br s, NH). 13C{1H} NMR and DEPT-135
(CDCl3): δ = 16.55 (d, 3J = 6.7 Hz, CH3), 17.98 (s, o′-CH3), 18.16 (s,
o′-CH3), 20.92 (s, p′-CH3), 61.16 (d, 2J = 6.0 Hz, OCH2), 110.89 (d, 1J =
135.1 Hz, Cq-1), 112.39 (d, 3J = 9.6 Hz, CH-3), 116.23 (d, 3J = 12.9 Hz,
3
CH-5), 128.38 (d, J = 13.3 Hz, 2 CH-m), 129.09 (s, 2 CH-m′), 131.33
1
colorless oily 2c. H NMR ([D8]THF): δ = 2.73 (br s, 3 H, NCH3), 4.69
(d, 2J = 10.6 Hz, 2 CH-o), 131.97 (d, JPH = 3 Hz, CH-p), 132.38 (d,
4
1
(br s, NH), 5.09 (d, JPH = 220 Hz, 1 H, PH), 6.54–6.61 (m, 2 H, aryl),
1J = 139.0 Hz, Cq-i), 133.04 (d, 2J = 8.1 Hz, CH-6), 133.73 (d, 4J =
2.1 Hz, CH-4), 134.85, 135.60 (2 s, Cq-p′, Cq-i′), 136.06, 136.15 (2 s, 2
7.19–7.26 (m, 4 H, aryl), 7.28–7.35 (m, 3 H, aryl). 31P{1H} NMR
([D8]THF): δ = –61.8 (br). These data are in accordance with those
of 2c as prepared from 2-methylaminophenyl-diphenylphosphane
by cleavage of a phenyl group with lithium in liquid ammonia.[8,9]
Cq-o′), 150.97 (d, J = 7.6 Hz, Cq-2). 31P{1H} NMR (CDCl3): δ = 35.7.
2
MS (EI, 70 eV, 20 °C): m/z (%) = 380 (20) [M + 1]+, 379 (100) [M]+,
210 (48), 209 (65), 208 (82), 124 (33), 120 (48), 77 (29). HRMS (EI,
70 eV): calcd. for [C23H26NO2P]+: 379.1696, found 379.1697.
N-Mesityl-2-(phenylphosphanyl)aniline (2d) and 2d(O): Reduc-
tion of 1d (100 mg, 0.264 mmol) in Et2O (2 mL) with LiAlH4 tablets
(50 mg, 1.32 mmol) in Et2O (2 mL) and hydrolytic workup as de-
scribed for 2b furnished 81 mg of a grayish oil, containing a mixture
of 2d and the P-oxide 2d(O) (molar ratio 3:1). Treatment of this
mixture with a further portion of LiAlH4 (50 mg) and workup in the
same way as before gave 60 mg (71 %) pure 2d as a colorless vis-
2-(Mesitylaminophenyl)mesitylphosphinic Acid Ethyl Ester (1e):
Mesityl-P(OEt)2 (δ31P = 176.4), prepared according to ref.,[28] was
contaminated by ca. 15 mol-% Mes2POEt (δ31P = 116.7) and distilled
at 50 °C/10–4 mbar, bath temperature 80 °C. The distillate (719 mg,
2.99 mmol, 1.5 equiv.) was added to N-mesityl-2-bromoaniline
(575 mg, 1.98 mmol) and Pd(OAc)2 (22 mg, 5 mol-%), the mixture
was heated for 0.5 h at 200 °C and the product purified using col-
umn chromatography on silica gel with 8 % ethyl acetate/hexane
as eluant; yield 200 mg (24 %) 1e of a pale yellow oil. 1H NMR
(CDCl3): δ = 1.38 (t, 3J = 7.1 Hz, CH3), 2.12, 2.17 (2 s, each 3 H, p-
and p′-CH3), 2.28, 2.30, 2.569, 2.571 (4 s, each 3 H, o- and o′-CH3),
1
cous oil. H NMR (C6D6): δ = 1.72 (br s, 3 H, o′-CH3), 1.98 (br s, 3 H,
1
o′-CH3), 2.14 (s, 3 H, p′-CH3), 5.27 (d, JPH = 218 Hz, 1 H, PH), 5.50
3
4
4
(br s, NH), 6.29 (ddd, J = 8.2, JPH = 2.2, J = 1.0 Hz, 1 H, H-6), 6.69
3
4
4
3
(tt, J = 7.3, 7.5, JPH ≈ J ≈ 2 Hz, 1 H, H-4), 6.75 (d br, J = 8.3 Hz,
2 H, H-o), 6.93–6.99 (m, 3 H, H-p, 2 H-m′), 7.02 (td, 3J = 8.2, 7.3, J =
4
3
3
1.6 Hz, 1 H, H-5), 7.31 (m, 2 H, 2 H-m), 7.64 (ddd, JPH = 11.2, J =
7.4, 4J = 1.6 Hz, 1 H, H-3). 13C{1H} and DEPT-135 NMR (C6D6): δ =
18.38, 18.85 (2 s, o′-CH3), 21.65 (s, p′-CH3), 112.96 (s, CH-6), 117.48
4.09 (m, 2J = 10.0, 3JPH ≈ 3J = 7.2 Hz, 1 H, OCHA), 4.25 (m, J = 10.1,
2
3JPH ≈ J = 7.1 Hz, 1 H, OCHB), 6.14 (dd br, J = 8, JPH = 6 Hz, 1 H,
3
3
4
3
4
4
H-3), 6.54 (tdd, J = 7.5, JPH = 2.6, J = 0.9 Hz, 1 H, H-5), 6.89 (d br,
1
3
(d, J = 11.9 Hz, Cq-2), 119.06 (d, J = 10.1 Hz, CH-4), 128.95 (s, CH-
4JPH = 4.1 Hz, 2 H, H-m), 6.94 (br s, 2 H, H-m′), 7.09 (superimposed
3
p), 129.73 (d, J = 5.3 Hz, 2 CH-m), 130.26 (s, 2 CH-m′), 132.68 (br s,
3
3
4
ddd JPH = 15, J = 7.6, J = 1.4 Hz, 1 H, H-6), 7.11 (superimposed t
br, 3J = 7.5 Hz, 1 H, H-4). 13C{1H} NMR and DEPT-135 (CDCl3):
CH-5), 133.39 (d, 2J = 14.9 Hz, 2 CH-o), 135.39 (d, 1J = 11.7 Hz,
Cq-i), 136.12 (s, Cq-p′), 136.24, 136.64 (2 br s, 2 Cq-o′), 136.51 (s,
3
δ = 16.47 (d, J = 7.0 Hz, CH3), 18.12, 18.19 (2 s, 2 o′-CH3), 20.94 (s,
2
2
NCq-i′), 139.39 (d, J = 28.7 Hz, CH-3), 150.23 (d, J = 4.1 Hz, Cq-1).
31P{1H} NMR (C6D6): δ = –60.1. MS (EI, 70eV, 20 °C): m/z (%) = 320
(14) [M + 1]+, 319 (6.5) [M]+, 304 (15), 191 (19), 177 (19), 158 (100),
142 (51), 141 (85), 105 (40), 77 (40). HRMS (ESI, 70 eV): calcd. for
[C21H22NP + H]+: 320.1563, found 320.1566.
p′-CH3), 21.09 (d, 5J = 1.1 Hz, p-CH3), 23.56 (d, 3J = 3.9 Hz, 2 o-CH3),
60.40 (d, 2J = 6.0 Hz, OCH2), 111.83 (d, 3J = 9.1 Hz, CH-3), 112.71 (d,
1J = 128.4 Hz, Cq-1), 115.93 (d, J = 13.7 Hz, CH-5), 125.95 (d, J =
3
1
139.2 Hz, Cq-i), 129.08, 129.13 (2 s, 2 CH-m′), 130.73 (d, 3J = 13.3 Hz,
2
4
2 CH-m), 132.44 (d, J = 11.6 Hz, CH-6), 133.09 (d, J = 2.1 Hz, CH-
4), 135.04, 135.57 (2 s, Cq-p′, Cq-i′), 136.21 (s, 2 Cq-o′), 141.63 (d, 4J =
2.9 Hz, Cq-p), 142.97 (d, J = 11.1 Hz, 2 Cq-o), 151.14 (d, J = 6.2 Hz,
Cq-2). 31P{1H} NMR (CDCl3): δ = 42.7. MS (EI, 70 eV, 340 °C): m/z
(%) = 422 (22) [M + 1]+, 421 (78) [M]+, 208 (62), 184 (81), 43 (98),
41 (100). HRMS (ESI in MeOH/H2O, HCOOH): Calcd. for [C26H32NO2P
+ H]+: 422.22434, found 422.22428.
Characteristic NMR data of 2d(O) in the crude product (assignment
numbering as in 2d): 1H NMR (C6D6): δ = 1.91, 2.14, 2.24 (3 s,
2
2
3
4
4
o′-CH3, p′-CH3), 6.48 (ddd, J = 7.5, JPH = 2.6, J = 1.0 Hz, 1 H, H-4),
7.62 (ddd, 3JPH = 13.6, J = 7.8, J = 1.6 Hz, 1 H, H-3), 7.90 (d, JPH
=
3
4
1
480 Hz, PH), 8.46 (NH); other aryl proton signals superimposed by
signals of 2d. 31P{1H} NMR (C6D6): δ = 26.3. HRMS (ESI, 70 eV):
C21H22NOP (335.38), calcd. for [M + H]+: 336.1512, found 336.1515.
N-Neopentyl-2-(phenylphosphanyl)aniline (2b): A solution of 1b
(2.70 g, 8.15 mmol) in Et2O (10 mL) was added dropwise at 0–5 °C
N-Mesityl-2-(mesitylphosphanyl)aniline (2e) and 2e(O): Reaction
to LiAlH4 tablets (0.97 mg, 25.6 mmol) in Et2O (20 mL) and stirred of 1e (200 mg, 0.475 mmol) in Et2O (2 mL) with LiAlH4 tablets
for 4 d at r.t. At ca. 10 °C degassed H2O was added dropwise until
the evolution of H2 gas ceased. Solids were filtered off and washed
thoroughly with ether. The organic layer was dried with Na2SO4,
the solution separated and the solvent removed under vacuum to
give 1.61 g (73 %) off-white viscous oily 2b (purity by 31P integra-
(54 mg, 1.42 mmol) in Et2O (2 mL) as described for 2b furnished
144 mg of a colorless viscous oil, containing a mixture of 2e and
the P-oxide 2e(O) (molar ratio ca. 2:1). Extraction with hexanes gave
a small amount of a solid residue, according to the NMR data 2e(O)
with residual 2e (molar ratio ca. 8:2). In the extract the 2e/2e(O)
ratio was marginally changed. Removal of solvent and volatile side
products (with only one mesityl group) at 10–5 mbar/75 °C bath
temperature furnished 144 mg of a colorless viscous oil, containing
2e/2e(O) in a 70:30 % molar ratio (based on integration of separate
proton signals of the components). 2e: 1H NMR (CDCl3): δ = 1.92
(vbr, 6 H, 2 o-CH3 of PMes), 2.27, 2.28 (2 s, 6 H, 2 o-CH3 of NMes),
1
tion 98 %). H NMR (C6D6): δ = 0.88 (br s, 9 H, CH3), 2.79 and 2.83
(br dd, 2J = 11.4, 3J = 4.5 Hz, 2 H, NCH2), 4.40 (br s, NH), 5.33 (d,
3
1JPH = 216.4 Hz, 1 H, PH), 6.72 (br d, J = 8.1 Hz, 1 H, H-6), 6.69 (br
t, 3J = 7.2 Hz, 1 H, H-4), 7.16 (br s, 3 H, aryl-H), 7.35–7.55 (m, 3 H,
3
3
aryl-H), 7.79 (br dd, JPH = 11.2, J = 7.9 Hz, 1 H, H-3). 13C{1H} NMR
(C6D6): δ = 28.18 (s, CMe3), 32.17 (s, CMe3), 56.25 (s, NCH2), 111.19
1
3
1
(s, CH-6), 116.07 (d, J = 11.0 Hz, Cq-2), 117.57 (d, J = 11.6 Hz, CH-
2.38 (br s, 6 H, 2 p-CH3), ca. 5.40 (br d, JPH = 226 Hz, PH), 5.41 (br
3
2
3
4
4
4), 128.76 (s, CH-p), 129.61 (d, J = 5.0 Hz, 2 CH-m), 132.75 (d, J = s, NH), 6.07 (ddd, J = 8.2, JPH = 2.8, J = 1 Hz, 1 H, H-6), 6.65 (tt,
14.9 Hz, 2 CH-o), 132.89 (s, CH-5), 135.27 (d, 1J = 11.0 Hz, Cq-i), 3J = 7.4, JPH ≈ J ≈ 2 Hz, 1 H, H-4), 6.86–6.96 (superimposed, H-m,
4
4
2
2
3
4
3
139.67 (d, J = 30.8 Hz, CH-3), 152.09 (d, J = 3.3 Hz, Cq-1). 31P{1H} H-m′), 7.03 (td, J = 8.3, 7, J = 1.6 Hz, 1 H, H-5), 7.28 (ddd, JPH
=
3
4
NMR (C6D6): δ = –61.5. HRMS (ESI in MeOH): C17H22NP (271.34)
9.4, J = 7.4, J = 1.6 Hz, 1 H, H-3). 31P{1H} NMR (CDCl3): δ = –90.3.
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NMR data of 2e(O) in the solid residue (assignment numbering as
in 2e): 1H NMR (CDCl3): δ = 2.17, 2.21, 2.31, 2.33 (4 s, each 3 H, 4
strong 31P NMR signals at δ = 40.0 (6b), –1.4 (TEP) and +0.2 (not
identified), integral ratio 42:35:23, and some weaker signals, but not
3
4
–
CH3), 2.57 (s, 6 H, 2 Me), 6.21 (dd, J = 8.3, JPH = 5.2 Hz, 1 H, H-6), those of PF6
.
3
4
4
6.57 (tdd, J = 7.8, 7.0, JPH = 2.5, J = 1 Hz, 1 H, H-4), 6.83 (partly
superimposed ddd, JPH ≈ 17, 3J = 7.5, 4J = 1.5 Hz, H-3), 6.9–7.0
3
b) Reaction with larger excess of CH(OEt)3 (ca. 1:1:5). NH4PF6 (400 mg,
2.45 mmol) and CH(OEt)3 (2.0 mL, 12.0 mmol) were added to 2b
(680 mg, 2.51 mmol) at r.t. After stirring overnight the mixture was
heated for 5 h at 80–90 °C while passing a slow stream of N2
through the mixture. The progress of the reaction was monitored
(superimposed, 4 H, H-m, H-m′), 7.15 (tt, J = 8.4, 7.0, J ≈ 5JPH = 1–
3
4
2 Hz, 1 H, H-5), 8.18 (br s, NH), 8.73 (d, JPH = 483 Hz, PH). 31P{1H}
1
NMR(CDCl3): δ = 15.6. In a C6D6 solution of the crude product mix-
ture the NH signal of 2e(O) appears at δ = 9.28 (br s), other signals
are less strongly shifted, δ(31P) = 14.2.
1
by H and 31P NMR (see discussion). After 5 d at r.t. no significant
change was observed, whereas a sample, measured after 1 month
at r.t. in [D6]acetone, displayed a decrease of the amount of 5b in
favor of 6b (Pint 31, 41 %). The mixture was then subjected to col-
umn chromatography on silica gel with gradient elution by hexane/
ethyl acetate (85:15 to 0:100). Decomposition of 5b in the column
prevented a clean separation of the components. The largest frac-
tion (0.43 g of a pale yellow oil) consisted mainly of 6b·xHPF6, a
minor amount of (EtO)3PO and traces of 5b and unidentified spe-
cies with δ31P = 26.6 and 88.8. The next fraction (0.12 g) was 6b[3]
with a trace content of HPF6.
Detection of 1,3-Dineopentyl-3-oxo-1,3-benzazaphospholine
Hexafluorophosphate Salt (6a·xHPF6): A mixture of 2a (85 mg,
0.32 mmol) with CH(OEt)3 (60 μL, 0.36 mmol) and NH4PF6 (52 mg,
0.32 mmol) was heated for 6 h at 130 °C (bath temp.) using a distil-
lation apparatus and slow stream of N2 to remove EtOH. NMR moni-
toring of the yellow crude product (123 mg) in C6D6 displayed com-
plete conversion of 2a. The crude product was transferred to a silica
gel column and fractioned on SiO2 by elution with ethyl acetate
(10 vol.-%)/hexane. The second fraction furnished 60 mg of a still
contaminated product, the third fraction (43 mg) was spectroscopi-
cally almost pure 6a·xHPF6. Attempts to crystallize the viscous oil
failed. In (d8)THF solution a solvate with 0.5 THF was formed after
evaporation of volatiles under vacuum at room temperature
1
5b (data from crude product mixture) – H NMR (CD2Cl2/CH(OEt)3):
δ = 1.06 (s, 9 H, CH3), 1.44 (td, 3J = 7.0, 4JPH = 0.6 Hz, 3 H, CH3(POEt)),
3.24 (br d, 2J = 15 Hz, 1 H, NCHa), 3.43 (br d, 2J = 15.0, 4JPH = 2.9 Hz,
1 H, NCHb), 4.09 (superimp. br d, 2J ≈ 18 Hz, 1 H, PCHaN, trans to
1
(30 min, 1 Torr). H NMR (C6D6): δ = 0.82 (s, 9 H, CH3), 1.09 (s, 9 H,
2
CH3), 1.68 (dd, 2J = 15.1, JPH = 10.6 Hz, 1 H, PCHa), 2.02 (t, 2J =
2
2
O), 4.59 (dd, J = 18.2, JPH = 15.4 Hz, 1 H, PCHbN, cis to O), 4.05–
15.1, 2JPH = 14.0 Hz, 1 H, PCHb), 2.54 (br d, 2J = 14.7 Hz, 1 H, NCHa),
4.38 (superimposed m, P+OCH2), 7.02 (tdd, J = 7.8, 7.2, JPH = 3.4,
3
4
4
2.84 (dd, 2J = 14.7, JPH = 1.5 Hz, 2 H, NCHb), 3.50 (superimposed
4J = 0.6 Hz, 1 H, H-5), 7.07 (dd br, J = 8.5, JPH = 4.4 Hz, 1 H, H-7),
3
4
2
2
7.45–7.95 (superimposed m, H-4, H-6, Ph-H). 13C{1H} NMR (CD2Cl2/
dd, J = 14.4, JPH = 5.7 Hz, 1 H, PCHtrans to lp), 3.53 (superimposed
t, 2J = 14.1, JPH = 12.5 Hz, 1 H, PCHcis to lp), 6.45 (dd br, 3J = 8.4,
2
3
CH(OEt)3): δ = 16.28 (d, J = 6.6 Hz, CH3), 28.34 (s, CMe3), 34.63 (s,
4JPH = 3.8 Hz, 1 H, H-7), 6.63 (br td, 3J = 7.4, 7.2, JPH = 3.0, 4J =
4
1
3
CMe3), 51.37 (d, J = 70.4 Hz, PCH2N), 62.60 (d, J = 7.2 Hz, NCH2),
0.8 Hz, 1 H, H-5), 7.10 (ddt, J = 8.4, 7.2, |4J + JPH| = 2.4 Hz, 1 H, H-
3
5
68.50 (d, 2J = 7.2 Hz, P+OCH2), 99.24 (d, 1J = 110.1 Hz, Cq-3a), 112.59
3
3
4
3
3
1
6), 7.64 (ddd, JPH = 10.2, J = 7.4, J = 1.1 Hz, 1 H, H-4); 1.42 (m, 2
(d, J = 9.9 Hz, CH-7), 120.34 (d, J = 12.1 Hz, CH-5), 118.22 (d, J =
124.9 Hz, Cq-i), 130.54 (d, 3J = 15.4 Hz, 2 CH-m), 130.74 (d, 2J =
7.2 Hz, CH-4), 133.30 (d, 2J = 12.7 Hz, 2 CH-o), 136.65 (d, 4J = 3.3 Hz,
H, CH2, THF), 3.57 (partial superimposed m, 2 H, OCH2, THF). 13C
3
NMR (C6D6): δ = 28.89 (s, NCCMe3), 32.09 (d, J = 8.0 Hz, PCCMe3),
32.31 (d, 2J = 4.0 Hz, PCCMe3), 34.90 (s, NCCMe3), 45.25 (d, 1J =
4
2
CH-p), 140.04 (d, J = 1.7 Hz, CH-6), 161.55 (d, J = 19.8 Hz, CH-7a);
1
4
CH(OEt)3: 15.18, 59.86, 113.03. 31P NMR (CD2Cl2/CH(OEt)3): δ = 73.4,
70.3 Hz, PCH2), 57.29 (d, J = 67.7 Hz, PCH2N), 64.01 (d, J = 6.6 Hz,
NCH2), 110.85 (d, J = 9.3 Hz, CH-7), 118.21 (d, J = 96.9 Hz, Cq-3a),
118.34 (d, J = 10.4 Hz, CH-5), 130.84 (d, J = 6.6 Hz, CH-4), 134.52
3
1
1
–144.4 (sept, JPF
=
711 Hz, PF6–). HRMS (ESI in MeOH):
3
2
C20H27F6NOP2 (473.37), calcd. for [C20H27NOP + H+] 328.1825, found
2
–
(s, CH-6), 157.04 (d, J = 18.6 Hz, CH-7a); 26.44 (m, THF), 68.47 (m,
328.1809 (m); detection of alcoholysis of PF6 by signals for
THF). 31P NMR (C6D6): δ = 46.3, –142.8 (sept, 1JPF = Hz, PF6–); integral
ratio 3.8:1. HRMS (ESI): C17H28NOP·xHPF6, calcd. for [C17H29NOP]+
294.1981, found 294.1983.
(MeO)3PO·Na+, (MeO)2(EtO)PO·Na+, (MeO)(EtO)2PO·Na+ and
(EtO)3PO·Na+, relative intensities ca. 3.7 × 105, 0.5 × 105, 0.25 × 105
and 2.1 × 105; high resolution for strong peaks: (MeO)3PO·Na+ calcd.
163.0131, found 163.0130; (EtO)3PO·Na+ calcd. 205.0600, found
205.0602.
Detection of 3-Ethoxy-1-neopentyl-3-phenyl-1,3-benzazaphospho-
linium Hexafluorophosphate (5b), 1-Neopentyl-3-oxo-3-phenyl-1,3-
benzazaphospholine Hexafluorophosphate Salt (6b·xHPF6) and
(EtO)3PO
6b·xHPF6 (data after enrichment to ca. 70 mol-% by column chro-
matography) – 1H NMR (CD2Cl2): δ = 0.98 (s, 9 H, CH3), 2.77 (br d,
2
4
a) Reaction with 10 % excess of CH(OEt)3. NH4PF6 (488 mg,
2.99 mmol) and CH(OEt)3 (0.55 mL, 3.3 mmol) were added to 2b
2J = 15 Hz, 1 H, NCHa), 3.22 (br d, J = 15, JPH ≈ 1 Hz, 1 H, NCHb),
3.66–3.67 (unresolved superimp. br dd, 2 H, PCH2N), 6.68 (br td, 3J =
4
3
4
(812 mg, 2.99 mmol) at 22 °C. After 4 d at r.t. (only trace conversion 7.3, JPH = 3 Hz, 1 H, H-5), 6.86 (dd br, J = 8.5, JPH = 4 Hz, 1 H, H-
3
3
of 2b) the mixture was heated for 3.5 h at 70–80 °C (complete
dissolution of NH4PF6 within 1 h). An unidentified white precipitate
7), 7.24 (br dd, JPH = 10.4, 3J = 7.8 Hz, 1 H, H-4), 7.39 (br td, J =
4
3
7–8, JPH = 3 Hz, 2 H, H-m), 7.47 (br dd, J = 8.3, 7.3 Hz, 1 H, H-6),
3
3
3
(0.45 g, CHN ratio ca. 13:7:2), formed during storage for 2 weeks, 7.57 (br td, J = 7.5, J = 1 Hz, 1 H, H-p), 7.67 (br dd, JPH = 13, J =
was filtered off and washed with DCM. Removal of volatiles (at
7.3 Hz, 2 H, H-o). 13C{1H} NMR (CD2Cl2): δ = 28.35 (s, CMe3), 34.67
1 Torr) from the filtrate gave a pale yellow viscous oil (1.3 g). A (s, CMe3), 56.28 (d, 1J = 73.7 Hz, PCH2N), 63.21 (d, 3J = 7.2 Hz, NCH2),
small amount of crystals, formed in this oil, dissolved on washing
with DCM. Overlayering of the solution of the oil in a small amount
of Et2O with hexane provided 0.2 g of white crystals with CHN
values corresponding to 6b·0.44HPF6 (calcd. C 61.82, H 6.44, N 4.01;
found C 61.70, H 6.20, N 4.00 %), which is close to a 2:1 ratio with
H+-bridged 6b. It is sparingly soluble in [D6]acetone or [D6]DMSO,
displaying NMR signals of 6b. The pale yellow oil, left after removal
of the solvents under vacuum (not further worked up), displayed
110.89 (d, 3J = 9.4 Hz, CH-7), 113.78 (d, 1J = 104.5 Hz, Cq-3a), 118.27
3
3
(d, J = 11.6 Hz, CH-5), 129.24 (d, J = 12.7 Hz, 2 CH-m), 129.61 (d,
2J = 6.6 Hz, CH-4), 131.11 (d, J = 11.0 Hz, 2 CH-o), 131.54 (d, J =
2
1
108.5 Hz, Cq-i), 133.30 (d, 4J = 2.2 Hz, CH-p), 135.52 (br s, CH-6),
157.74 (d, J = 19.8 Hz, CH-7a). 31P NMR (CD2Cl2): δ = 42.0, –144.2
2
(sept, 1JPF = 711 Hz, PF6–). HRMS (ESI): C18H22F6NOP2 (444.31), calcd.
for [C18H22NOP + H+] 300.1512, found 300.1516 (m); calcd. for
[C18H22NOP + Na+] 322.1331, found 322.1334.
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Full Paper
NMR Detection of (EtO)3PO in the Main Fraction of 6b·x HPF6:
1.2–1.5 Hz, 1 H, H-4); signals of excess DMFA: 2.29 (s, NMe2), 3.33 (s,
4
1H NMR (CD2Cl2): δ = 1.29 (td, 3J = 7.1, JPH = 0.9 Hz, CH3), 4.00–
2 OMe), 4.36 (s, CH). 31P{1H} NMR (CDCl3): δ = –38.1.
4.15 (superimp. m, OCHaHb). 13C{1H} NMR (CD2Cl2): δ = 16.15 (d,
3J = 6.6 Hz, CH3), 64.36 (d, 2J = 6.1 Hz, OCH2). 31P (CD2Cl2): δ = –1.4.
Data of 9d: 1H NMR (CD2Cl2) δ = 1.52 (OH/H2O trace);[17] 2.24, (partly
superimposed s, p′-Me), 2.26 (partly superimposed s, NMe2), 2.39 (s,
3
6 H, 2 o′-Me), 4.80 (br s, 1 H, PCHcisN), 6.00 (br d, J = 8.5 Hz, 1 H,
1-Ethyl-1-methyl-3-oxy-3-phenyl-1,3-benzazaphospholinium
Hexafluorophosphate (7c): Compound 2c (430 mg, 1.99 mmol),
CH(OEt)3 (10 mL) and NH4PF6 (325 mg, 1.99 mmol) were placed into
a dry Schlenk flask and heated for 6 h at 130 °C as described for
6a·xHPF6. After cooling to room temperature the volatiles were re-
moved under vacuum to give a pale brown powder. NMR monitor-
ing in CD2Cl2 solution showed 31P NMR peaks at δ = 29.6, 29.8,
3
4
4
H-7), 6.76 (br s, 1 H, m′-CH), 6.80 (tdd, J = 7.5, 7.2, JPH = 2.6, J =
1.3 Hz, 1 H, H-5), 6.92 (br s, 1 H, m′-CH), 7.15–7.29 (m, 6 H, aryl-H),
7.57 (ddd br, 3J = 7.2, 3JPH = 5.3, 4J = 1.5, 5J <1 Hz, 1 H, H-4). 13C{1H}
and DEPT-135 NMR (CD2Cl2): δ = 17.49, 19.58 (2 s, o′-CH3), 20.91 (s,
p′-CH3), 41.25 (d, 3J = 15.1 Hz, 2-NMe2), 95.44 (d, 1J = 21.9 Hz, PCHN),
108.02 (s, CH-7), 117.49 (d, 3J = 6.8 Hz, CH-5), 118.54 (d, 1J = 6.8 Hz,
Cq-3a), 128.79 (d, 3J = 5.7 Hz, 2 CH-m), 128.08, 129.63, 130.46
–
36.9, 37.4 and –0.7(TEP) (peak heights 33:19:20:7:21) and the PF6
2
(CH-p and 2 CH-m′), 131.11 (CH-6), 131.05 (d, J = 24.7 Hz, 2 CH-o),
multiplet. Purification attempts by crystallisation from THF/hexane
and diethyl ether/hexane mixtures failed, but crystallisation from
refluxing ethanol furnished 200 mg (24 %) pale brown crystals of
syn/anti-isomers of 7c, molar ratio ca. 2:1 (by integration of 31P as
well as of corresponding proton and 13C signals). C16H19F6NOP2
(417.27); MS (EI 70 eV, 160 °C): m/z (%) = 257 (3) [Mcation – Me+],
243 (29) [Mcation – Et+]+, 242 (17), 228 (13), 98 (26), 85 (100), 77 (18),
73 (37), 72 (30). Major isomer 7cI. 1H NMR (moist [D6]DMSO): δ =
2
1
133.28 (d, J = 20.4 Hz, CH-4), 137.28 (partly superimposed d, J =
29.7 Hz, Cq-i), 137.48, 138.90, 140.29 (3 s, Cq-i′, Cq-p′, Cq-o′), 141.46
(d, 4J = 23.0 Hz, tentatively assigned to Cq-o′ with through-space
proximity to P lone pair), 154.19 (Cq-7a). 31P{1H} NMR (CD2Cl2): δ =
–38.7. MS (EI, 70 eV, 350 °C): m/z (%) = 331 (28), 330 (78) [M –
NMe2]+, 314 (21), 158 (83), 78 (56), 77 (100), 57 (67). HRMS (EI,
70 eV): C24H27N2P (374.46), calcd. for [M – NMe2]+ 330.14061, found
330.14071. 6d - 31P{1H} NMR (CD2Cl2): δ = +37.4. HRMS (EI, 70 eV):
3
1.23 (br t, J = 7.1 Hz, 3 H, CH3), 3.85 (s, 3 H, N+CH3), 4.02–4.18 (m,
C
22H23NOP (347.39), calcd. for [M + H+] 348.1511, found 348.1512.
2
2
2
2 H, N+CH2), 4.45 (dd, J = 15.7, JPH = 3.8 Hz, PCHa), 4.84 (dd, J =
2
15.7, JPH = 10.1 Hz, PCHb), 7.59–7.69 (m, 2 H, aryl), 7.72–7.92 (m, 5
Crystal Structure Analysis
H, aryl), 8.09 (superimposed dd, J = 8.5, J = 5.7 Hz, 1 H, aryl), 8.27
A crystal of 1d was mounted on a glass fiber in inert oil. Data were
recorded at 100(2) K on a Bruker APEX-2 diffractometer using
Mo-Kα-radiation (0.71073 Å). Crystal data: Monoclinic, space group
P21/c, a = 15.7092(8), b = 8.7962(4), c = 16.2105(11) Å, ꢀ =
115.444(3)°, V = 2022.72(19) Å3, Z = 4, Dx = 1.246 Mg/m3, μ =
0.15 mm–1, F(000) = 808, 100(2). A total of 65604 data (5518 unique,
Rint 0.0543) were recorded in the range θ = 2.52 to 29.30° (100 %
completeness to θ = 28.50°). Absorption corrections were based on
multi-scans. The structure was refined by full-matrix least-squares
on F2.[29] The NH hydrogen was refined freely; other hydrogen at-
oms were included using a riding model or rigid methyl groups.
The final wR2 was 0.0970 for 252 parameters and all reflections,
with R1 [I > 2σ(I)] 0.0398; S 1.05; for all data wR2 was 0.1051, with
(br dd, J = 8.5, JPH = 2.6 Hz, 1 H, H-7). 13C{1H} NMR and DEPT-135
(moist [D6]DMSO): δ = 9.51 (s, CH3), 54.83 (d, 4J = 4.4 Hz, N+CH3),
62.84 (d, 1J = 68.8 Hz, PCH2N+), 67.66 (s, N+CH2), 120.59 (d, 3J =
3
4
1
1
8.3 Hz, CH-7), 126.50 (d, J = 94.6 Hz, Cq-3a or Cq-i), 127.29 (d, J =
3
113.9 Hz, Cq-i or Cq-3a), 129.12 (d, J = 13.2 Hz, 2 CH-m), 130.01 (d,
3J = 6.6 Hz, CH-5), 132.2 (superimposed d, J ≈ 10 Hz, 3 C, 2 CH-o,
2
CH-4), 133.70 (d, 4J = 2.8 Hz, CH-p), 136.23 (d, 4J = 2.2 Hz, CH-6),
149.83 (d, 2J = 13.2 Hz, Cq-7a). 31P{1H} NMR: δ = 31.2 (s), –143.2
(sept, 1J = 711 Hz, PF6–) in moist [D6]DMSO, integral ratio 3:2 by
1
–
different relaxation time; δ = 34.6, –143.8 (sept, J = 708 Hz, PF6
)
in CD3OD. Minor isomer 7cII. 1H NMR (moist [D6]DMSO): δ = 1.35
(br t, 3J = 7.0 Hz, 3 H, CH3), 3.72 (s, N+CH3), 4.27–4.40 (m, N+CH2),
4.48 (dd, 2J = 15.2, JPH = 3.6 Hz, Ha), 4.68 (dd, 2J = 15.2, JPH
=
2
2
R1 0.0551, max. Δρ 0.43 and –0.40 e Å–3
.
10.2 Hz, Hb), aryl range superimposed by signals of 7cI. 13C{1H} NMR
and DEPT-135 (moist [D6]DMSO ): δ = 8.89 (s, CH3), 58.56 (s, N+CH3),
61.28 (d, 1J = 68.8 Hz, PCH2N+), 62.76 (d, 3J = 4.4 Hz, N+CH2), 120.35
CCDC 1812307 (for 1d) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre.
3
1
(d, J = 8.3 Hz, CH-7), 125.59 (d, J = 94.6 Hz, Cq-3a or Cq-i), 126.78
(d, 1J = 112.8 Hz, Cq-i or Cq-3a), 129.23 (d, 3J = 13.2 Hz, 2 CH-m),
Supporting Information (see footnote on the first page of this
article): Comparison of characteristic NMR data of 5b, 6b·xHPF6, 7c
and 9d with those of the benzazaphospholium salt II (R1 = Et, R =
Np), NMR spectra of 1b–e, 2b–d, 6a·xHPF6, 5b/6b·xHPF6, 7c, 8d
and 9d and procedure and NMR data of N,P-dineopentyl-2-phos-
phanylaniline-P-borane.
3
2
130.27 (d, J = 7.2 Hz, CH-5), 132.33 (superimposed d, J ≈ 10 Hz, 2
CH-o), 132.46 (superimposed d, 2J ≈ 10 Hz, CH-4), 133.87 (d, 4J =
4
2
3.0 Hz, CH-p), 136.38 (d, J = 1.7 Hz, CH-6), 151.65 (d, J = 13.2 Hz,
Cq-2). 31P{1H} NMR: δ = 31.4 (s) in moist [D6]DMSO; δ = 34.8 in
CD3OD; anion as for 7cI.
Detection of 8d and 9d: Excess dimethylformamide dimethyl
acetal (DMFA, 0.12 mL, 0.93 mmol) was added to 2d (90 mg,
Acknowledgments
0.28 mmol), followed by HCl solution in Et2O (0.1 mL, 2
M,
Financial support by the Deutsche Forschungsgemeinschaft
(DFG) is gratefully acknowledged (HE1997/14-1). We thank
B. Witt, G. Thede, and M. Redies for NMR spectra, W. Heiden
for LR mass spectra, and G. Sommer-Udvarnoki (Georg-August-
Universität Göttingen, Institut für Organische und Biomoleku-
lare Chemie) for HRMS measurements.
0.2 mmol). This mixture was heated at 60 °C for 2 d. 1H and 31P
NMR spectra displayed almost quantitative conversion to 8d, with
further conversion to 9d after removing volatile products and resid-
ual DMFA under vacuum. Treatment with a small amount of diethyl
ether and hexane and drying under vacuum gave 93 mg (89 %) of
a white solid. After storage for 6 weeks small impurities by
Me2NH·HCl (δ13C = 35.09) and by P-oxide 6d were detected.
Keywords: Phosphinates · Phosphanes · P,N heterocycles ·
Benzazaphospholinium salts · Synthesis design
1
Data of crude product 8d: H NMR (CDCl3) δ = 2.24, (s, 3 H, p′-Me),
4
2.40 (s, 6 H, 2 o-Me), 3.47 (d, JPH = 4.5 Hz, 3 H, OMetrans), 4.81 (s, 1
3
H, PCHcisN), 6.03 (br d, J = 8.5 Hz, 1 H, H-7), 6.73 (br s, 1 H, m-CH),
6.80 (tdd, 3J = 7.2, JPH = 2.6, 4J = 1 Hz, 1 H, H-5), 6.92 (br s, 1 H,
4
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Full Paper
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Received: October 14, 2019
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