σ2P,O-hybrid ligands: Synthesis of the first 4-hydroxy-1,3-benzazaphospholes by ortho-lithiation of M-amidophenyl diethyl phosphates

Article abstract of DOI:10.1002/ejic.201402527

The m-phosphorylanilides 2 are available from anilides 1 by the Atherton-Todd reaction; the selective ortho-lithiation of the o'-methyl-protected phosphorylpivalanilide 2b with tBuLi proceeded in high yield in the presence of ClSiMe₃. The ortho-lithiation is followed by rapid 1,3-migration of the PO₃Et₂ group to yield the phosphonoanilide cis/trans-3b. This compound mainly reacts with excess LiAlH₄ by reductive cyclization to form the 4-hydroxy-1H-1,3-benzazaphosphole 6. The lithiation of the o'-unprotected phosphorylpivalanilide 2a with LDA was unselective and led to 3a and 4a in low yields, whereas additional ortho-lithiation of the benzoyl group occurred for the lithiation of the o'-protected phosphonobenzanilide 2c with tBuLi/LDA to give 7 in rather low yield. The reduction of crude 7 led to (benzylamino)phenol 8 and the 4-hydroxy-1H-1,3-benzazaphosphole 9 as a minor product. The properties, NMR spectroscopy data, and crystal structures of 5b, 6, and 8 are reported.

Full text of DOI:10.1002/ejic.201402527

FULL PAPER
DOI:10.1002/ejic.201402527
σ2P,O-Hybrid Ligands: Synthesis of the First 4-Hydroxy-
1,3-benzazaphospholes by ortho-Lithiation of
m-Amidophenyl Diethyl Phosphates
Bhaskar R. Aluri,[a] Kirti Shah,[b] Neelima Gupta,[b]
Olga S. Fomina,[c] Dmitry G. Yakhvarov,[c,d] Mohammed Ghalib,[a]
Peter G. Jones,[e] Carola Schulzke,[f] and Joachim W. Heinicke*[a]
Dedicated to the memory of Professor Dr. Reinhard Schmutzler
Keywords: Amides / Phosphorylation / Lithiation / Rearrangement / Phosphorus heterocycles
The m-phosphorylanilides 2 are available from anilides 1 by
the Atherton–Todd reaction; the selective ortho-lithiation of
the oЈ-methyl-protected phosphorylpivalanilide 2b with
tBuLi proceeded in high yield in the presence of ClSiMe3.
The ortho-lithiation is followed by rapid 1,3-migration of the
PO3Et2 group to yield the phosphonoanilide cis/trans-3b.
pivalanilide 2a with LDA was unselective and led to 3a and
4a in low yields, whereas additional ortho-lithiation of the
benzoyl group occurred for the lithiation of the oЈ-protected
phosphonobenzanilide 2c with tBuLi/LDA to give 7 in rather
low yield. The reduction of crude 7 led to (benzylamino)-
phenol 8 and the 4-hydroxy-1H-1,3-benzazaphosphole 9 as a
This compound mainly reacts with excess LiAlH4 by re- minor product. The properties, NMR spectroscopy data, and
ductive cyclization to form the 4-hydroxy-1H-1,3-benzaza- crystal structures of 5b, 6, and 8 are reported.
phosphole 6. The lithiation of the oЈ-unprotected phosphoryl-
phorus atoms (σ2P) with OH groups in the ortho position
and complexes thereof have been little investigated; most
Introduction
o-Phosphanylphenols[1,2] are well-known precursors for
of them are o-hydroxy-,[9] o-hydroxymethyl-, or various
transition metal complexes with κP-phosphanylphenol or
hydroxyphenylphosphinine moieties.[10] Five-membered σ2P
κ2P,O-phosphanylphenolate chelate coordination[1,3,4] for a
rings with OH groups are very rare,[11] and OH-substituted
variety of catalysts, for example, for ethylene polymerization
1H-1,3-benzazaphospholes have not yet been reported. The
and copolymerization with α-olefins,[4,5] ring-opening poly-
ring system of the latter is aromatically strongly stabilized
merization (propylene oxide, caprolactam),[6] and hydrogen-
like that of phosphinines[12] but has higher π-density at the
ation (styrene, cyclohexenes),[7] and for assemblies of di-
phosphorus atom because of the conjugation of the σ3N-
topic ligands for rhodium-catalyzed hydroformylation.[8]
donor and the dicoordinated P atom. This changes the co-
However, aromatic compounds of dicoordinated phos-
ordination properties in the direction of π-donor bond con-
tributions.[13] Conjugation with a hydroxy group with a +M
[a] Institut für Biochemie (Anorganische Chemie), Ernst-Moritz-
Arndt-Universität Greifswald,
effect should further increase the π density at the phos-
phorus atom and in addition offer a second OH or O– coor-
dination site. Thus, the synthetic access to 4-hydroxy-1,3-
benzazaphospholes would provide the basis for the study of
the coordination chemistry of new σ2P,O hybrid ligands
with high π density at the phosphorus atom. This prompted
us to extend our longstanding investigations on a variety
of 1,3-azaphospholes[14,15] to 4-hydroxy-1,3-benzazaphos-
Felix-Hausdorff-Str. 4, 17487 Greifswald, Germany
E-mail: heinicke@uni-greifswald.de
http://www.mnf.uni-greifswald.de/institute/institut-fuer-
biochemie/anorganische-chemie.html
[b] Department of Chemistry, University of Rajasthan,
302004 Jaipur, India
[c] A. E. Arbuzov Institute of Organic and Physical Chemistry,
420088 Kazan, Russian Federation
[d] Kazan Federal University,
420008 Kazan, Russian Federation
[e] Institut für Anorganische und Analytische Chemie, Technische pholes with an OH group at the ortho position to the σ2P
Universität Braunschweig,
atom.
Postfach 3329, 38023 Braunschweig, Germany
The routes to 1,3-benzazaphospholes generally start
from 2-diethylphosphonoanilines or -anilides. To access 4-
hydroxybenzazaphospholes, an additional OH group at the
3-position is required. This suggests that the well-known
[f] Institut für Biochemie (Bioanorganische Chemie),
Ernst-Moritz-Arndt-Universität Greifswald
17487 Greifswald, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402527.
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metalation–rearrangement strategy for the synthesis of (o- silica gel furnished only small amounts of the desired phos-
hydroxyaryl)phosphonates could be applied. In the absence phonoanilide 3a (4%) and the regioisomer 4 (13%,
of interfering functional groups, phosphonylation at the or- Scheme 2). The formation of the latter contrasts with the
tho position to an aryl OH group can be achieved in high selective lithiation/substitution pattern of the closely related
yield by the metalation of o-bromoaryl diethyl phosphates N-(3-methoxyphenyl)pivalamide at the 2-position, that is,
with magnesium, sodium,[2b] nBuLi[16] or, more easily, by the common ortho position between the O and N substitu-
the directed ortho-lithiation of aryl diethyl phosphates with ents.[18]
lithium diisopropylamide (LDA) in tetrahydrofuran
(THF).[17] The resulting o-metalated aryl diethyl phosphates
undergo rapid intramolecular 1,3-carbanionic C(Li)
OPO3Et2 to C(PO3Et2)OLi rearrangements to form the cor-
responding o-(diethylphosphono)phenoxides. As pivalanil-
ides similarly undergo rapid directed ortho-lithiation with
nBuLi in THF at –30 to 0 °C,[18] lithiation at the common
ortho position of the O and N substituents was expected to
Scheme 2. Lithiation of 2a by LDA, followed by trapping with
ClSiMe3 and final O-desilylation.
be preferred. A competing carbanionic 1,3-migration of the
N-acyl group was observed only for N-tertiary anilides[19]
and should not interfere. Therefore, we studied the ap-
plicability of the above strategy to access (2-amido-6-
hydroxyphenyl)phosphonates and their conversion to 4-
hydroxy-1H-1,3-benzazaphospholes and report here on the
results and limitations.
To prevent lithiation at the competing orthoЈ position of
the ortho-directing OPO3Et2 group, further experiments
were performed with the methyl-protected anilidophosph-
ate 2b. However, all attempts at lithiation with 2–3 equiv. of
LDA, nBuLi, or LDA/BuLi in THF (–80 to 20 °C), fol-
lowed by reaction with ClSiMe3 and MeOH, failed to give
3b or gave only very small amounts. Lithiation with tBuLi
in THF, 1,4-dioxane, or toluene likewise led to mixtures of
varying compositions, whereas the use of diethyl ether as
solvent gave trans-3b (δ31P = 23.5 ppm) and 3bOSi (δ31P =
21.3 ppm) after workup by complete or partial desilylation
(for 3bOSi) by methanol at room temperature (Scheme 3).
Results and Discussion
Synthesis
The starting materials, 3-hydroxy-substituted N-phenyl-
pivalamide (1a) or N-tolylpivalamide (1b) and the analo-
gous benzanilide 1c, were easily accessible from the corre-
sponding hydroxyanilines and acid chlorides in the presence
of a base. The conversion into the corresponding diethyl
(m-acylamido)phenyl phosphates 2a–2c succeeded in high
yields by the Atherton–Todd reaction,[20] that is, the reac-
tion with diethyl phosphite, CCl4, and triethylamine in
THF (Scheme 1), or by direct esterification with ClPO3Et2
in the presence of triethylamine.
Scheme 1. Synthesis of m-amidoaryl diethyl phosphates 2a–2c.
Scheme 3. Lithiation of 2b by tBuLi, followed by trapping with
However, the ortho-lithiation/rearrangement step proved ClSiMe3 and methanolysis.
to be critical. Whereas, as noted above, LDA in THF con-
verts common functionally unsubstituted phenyl diethyl
The yields of the CDCl3-soluble crude products were cal-
phosphates to the respective lithium o-(diethylphosphono)- culated from the relative peak areas of the 31P NMR signals
phenolates in high yields,[17] the reaction with 2a was unse- of trans-3b and 3bOSi (Table 1). The conversion of 2b (δ31P
lective and provided a mixture. 31P NMR spectroscopy = –5.9 ppm) and the relative content of 3b/3bOSi were in-
analysis showed sharp resonances at δ = 21.0, 18.5, and creased noticeably by a small excess of tBuLi (2.1–
–6.5 ppm (unconsumed 2a), as well as a broad signal in the 2.8 equiv.); however, with three or more equivalents or at
range δ = 21–25 ppm. Separation by preparative TLC on larger scales (Ͼ5 g of 2b), the formation of 3b was disfa-
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Table 1. Conversion of 2b to 3b under various conditions, calculated from the relative peak areas of the 31P NMR signals of 2b, 3b, and
3bOSi in the CDCl3-soluble crude products.
Amount of tBuLi,[a] amount of donor
Amount of 2b, amount of
ClSiMe3 [mmol]; solvent
Conversion to 3b and
3bOSi [%][c]
Unconverted 2b [%];[c] major
impurity (%)
[mmol]; reaction mode[b]
4.1 (2.0 equiv.), –; A
11.4 (2.1 equiv.), –; A
30.7 (2.5 equiv.), –; A
10.4 (2.5 equiv.), 12.5 DME; B
52.5 (2.5 equiv.), 63 DME, B
8.15 (2.5 equiv.), 9.8 DME; C
25.5 (2.5 equiv.), 30.6 DME; C
2.05, 4.1; Et2O
5.4, 10.8; Et2O
12.3, 30.7; Et2O
4.15, 10.4; Et2O
21.0, 52.5; Et2O
3.26, 8.15; Et2O
10.2, 25.5; Et2O
2, 22
25, 17
39, 30
26, –
0, 4
35, 9
32, –
47; δ = –5.3 (16)
34; δ = 61.7 (9)
3; δ = 63.3 (15)
32; δ = 19.4 (21)
0; δ –1.0 (60)
9; δ = 15.9 (20)
40; δ = 16.8 (10)
[a] Solution in pentane (1.6 m), equivalents in parentheses are relative to 2b. [b] Reaction mode A: (normal) addition of LiR to 2b at
–80 °C, after 12–15 h at room temp. reaction with ClSiMe3 (–30 to 20 °C, 4 h) and filtration; mode B: (inverse) addition of 2b to LiR at
–78 °C, after 12–15 h at room temp. reaction with ClSiMe3 as in mode A; mode C: inverse lithiation as before, but trapped by MeOH/
ClSiMe3 solution. [c] Percentage of peak areas of 31P NMR signals for the crude filtrate.
vored by increasing Oaryl–P bond cleavage and OEt substi- which could be partly separated and were suitable for struc-
tution at the phosphorus atom to form 1b and tBuP(O) ture determination, whereas good solubility in pentane pre-
species, respectively. Trials with some additives to promote vented their quantitative separation.
the CH-lithiation gave rise to mixtures with a low content
The improved ortho-lithiation/P–C coupling in the pres-
of 3b for tBuLi/KOtBu or tBuLi/tetramethylethylenedi- ence of ClSiMe3 is attributed to the very slow reaction of
amine (tBuLi/TMEDA). Only tBuLi/1,2-dimethoxyethane tBuLi with ClSiMe3 at low temperature; whereas the sil-
(tBuLi/DME), generated in hexane before the lithiation, ylation is sufficiently fast for the reactive lithium intermedi-
showed similar or slightly improved results compared to ates that interfere with the ortho-CH-lithiation. Thus, the
those with tBuLi/Et2O in an inverse lithiation mode (the lithiation of 2b with 1 equiv. of tBuLi in the presence of
addition of 2b in diethyl ether to the tBuLi/DME complex). ClSiMe3 at –60 °C led to 2bOSi as the main product (88%)
The viscous oily trans-3b was purified by silica gel column and proves that primary lithiation occurs at the NH func-
chromatography, but losses on the column diminished the tion. A lack of color change to orange-red, as reported for
isolated yield of trans-3b to a maximum of 26%.
lithium N-arylimidates,[19] hints at rapid trapping by
Almost complete ortho-lithiation, followed by instan- ClSiMe3. Therefore, it can be assumed that the formation
taneous rearrangement with P–C bond formation, was of 2bOSi during the lithiation of 2b with excess tBuLi in the
achieved by the addition of excess tBuLi (2.8 equiv.) at presence of excess ClSiMe3 in combination with the phos-
–80 °C to an ethereal solution of 2b and an excess of phono group allows efficient ortho-lithiation, followed by
ClSiMe3 (2.8 equiv.), followed by slow warming. The reac- immediate intramolecular migration of the diethylphos-
tion with a smaller excess of tBuLi (2.5 equiv.) at –60 °C phono group from the oxygen atom to the carbon atom.
was less suitable and led to an increased formation of aryl– An excess of tBuLi is required for complete conversion of
tBuP(O)(OEt) side products at the expense of the conver- 2b, but it also causes side reactions such as CH-lithiation at
sion of 2b. The primary silyl compounds, that is, the highly the 2-methyl group and substitution of an OEt group at the
viscous crude product 3bSi2 and others, were somewhat sen- phosphorus atom, which lead to small amounts of bis(tri-
sitive to moisture, alcohols, or active OH groups on silica methylsilyl)methyl and aryl–tBuP(O)(OEt)2 species, respec-
gel and were, thus, unsuitable for separation by chromatog- tively (Scheme 4).
raphy. Therefore, the crude mixture was heated at reflux
The conversion of the phosphonoanilides 3b to 4-hy-
with EtOH to yield mainly cis- and trans-3b (56 and 28% droxy-1,3-benzazaphosphole (6) was accomplished by re-
by 13C NMR integration) along with small amounts of the ductive cyclization with excess lithium aluminum hydride
partly desilylated 3bOSi, 5b, and aryl–tBuP(O)(OEt) species. (3.0 equiv.) and hydrolytic workup. The use of pure trans-
After evaporation of the solvent and volatile products un- 3b and final purification by crystallization from THF/hex-
der vacuum, the semisolid residue was extracted with pent- ane gave 6 in 60% yield (Scheme 5). The reduction of cis-
ane/diethyl ether (20:1) to leave the sparingly soluble cis-3b 3b with excess LiAlH4 (3.5 equiv.) under similar conditions
in 49% yield as a high-melting powder. It is readily soluble proved less suitable. After hydrolysis, a mixture of trans-3b,
in CDCl3 and did not isomerize in dry CDCl3 at room tem- 6, and a primary phosphanylaniline was detected (31P
perature over 3 d because of its sufficiently high barrier for NMR peak area ratio 41:32:27). Even more side products
cis/trans isomerization.[21] However, in the presence of were observed for the direct reduction of the crude mixture
moisture, slow conversion to trans-3b and small amounts of formed by ortho-lithiation in the presence of excess
other species with 31P NMR signals at δ = 16.0 and ClSiMe3. In this case, pure 6 was isolated in low yield (19%
17.2 ppm was observed. In the pentane extract, trans-3b, relative to 2b) by column chromatography and extraction
3bOSi, 5b, and two aryl–tBuP(O)(OEt) compounds were en- of N-basic impurities with cold 10% aqueous sulfuric
riched. Concentration under vacuum led to crystals of 5b, acid.
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Scheme 4. Products formed by the lithiation of 2b in the presence of ClSiMe3 and subsequent alcoholysis (intermediates supposed).
the P=C bond. Crystalline 6 is not markedly attacked by
air on contact for a few minutes, whereas in solution the
compound is rapidly oxidized.
Attempts to ortho-lithiate the phosphonobenzanilide 2c
with LDA or nBuLi failed similarly to the attempt reported
above for 2b. A well-defined product was obtained by reac-
tion with excess tBuLi/LDA, generated from tBuLi and
iPr2NH in cold diethyl ether before addition to the ethereal
solution of 2c (–78 °C). The resulting lithium species was
quenched with ClSiMe3 to provide the o-trimethylsilylated
phosphonobenzanilide 7, which indicates that a second
competing ortho-lithiation occurs at the benzimidate struc-
tural unit (Scheme 6). Like that of trans-3b generated in the
absence of ClSiMe3, the yield of 7 was low (28% after puri-
fication by column chromatography), and substantial
amounts of the P–O bond-cleavage product 1c formed. The
direct reduction of the crude product led to a mixture; after
separation of 5-benzyl-2-methylphenol (8, the reduction
product of 1c), a minor amount of 4-hydroxy-2-[(o-trimeth-
ylsilyl)phenyl]benzazaphosphole (9) was detected by its
characteristic NMR spectroscopic data. Attempts to purify
9 by column chromatography were unsuccessful.
Scheme 5. Reductive cyclization of 3b to 6.
The reaction of 3b to 6 proceeds by primary reduction
of the phosphono group and intramolecular nucleophilic
addition of the resulting PH2 or PHLi group at the lithiated
amide (imidate) group, as discussed earlier.[22] N-Basic
phosphane side products were separated by extraction with
dilute aqueous sulfuric acid. This is possible, because 6 is
not basic and is stable to the addition of water at the P=C
bond, even in cold diluted aqueous acids. The N lone pair
is involved in the aromatic π system of 6[12] and stabilizes
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7.48–7.86 ppm) or at ortho position to the amido group of
3a, trans-3b, 5b, 7, and particularly 3bOSi (δ5-H = 7.59–7.74,
8.25 ppm) appear at low field because of the anisotropic
influence of the C=O group,[21] which is turned towards
these protons by an efficient intramolecular N–H···O=P
hydrogen bond. In cis-3b, the opposite orientation of the
C=O group is probable, as indicated by the rather low-fre-
quency chemical shift of 5-H (δ = 7.10 ppm). Much
stronger low-frequency shifts are observed for the protons
at the ortho positions of the silylimidate 2bOSi (δ = 6.52,
6.77 ppm). The silylimidate nature is further indicated by
the C=N 13C NMR resonance[25] at δ = 163.97 ppm, which
is markedly different from the C=O signal of 2b at δ =
176.69 ppm. The acidic NH···O=P proton of 3bOSi is ob-
served at δNH = 11.48 ppm (cf. δ = 9.63–11.93 ppm for re-
lated compounds),[15c] whereas the NH proton signals of
the o-hydroxyphosphonoanilides 3a, 3b, 4a, and 7 appear
at δ = 8.15–8.45 ppm. In these compounds, the phosphono
group favors an intramolecular hydrogen bond with the OH
proton (δ = 10.58–11.67 ppm). For 6 in solution, an intra-
molecular O–H···P hydrogen bond was not detectable, in
Scheme 6. Formation of 7 by double ortho-lithiation of 2c and of
8 and 9 by reduction of the crude product mixture.
Structural Aspects
The structure elucidation of the new compounds is based accordance with the low basicity of the dicoordinated neu-
on conclusive multinuclear solution NMR spectroscopic tral phosphorus atom. The broad signals for the OH group
data, supplemented by satisfactory elemental analyses or at δ = 4.96 ppm and for the NH group at δ = 8.40 ppm
HRMS data (for purity, see NMR spectra, Figures S1–S34 (cf. ref.[14,15]) suggest rather weak intermolecular hydrogen
in the Supporting Information) and by crystal structure bonds.
1
analyses for 5b, 6 and 8. The H and 13C nuclei of the aro-
Information on the solid-state structures of 5b, 6, and
matic rings of the new compounds display the expected 8 was provided by single-crystal X-ray diffraction. For 5b
NMR signal splittings with 3JH,H and typical 4JP,H (5–6 Hz) (Figure 1), the conformation of the phosphono group is
1
and JP,C (170–180 Hz) coupling constants and chemical controlled by an intramolecular O–H···O=P hydrogen
shifts in accordance with values estimated from those of bond. The π planes of the amide group and the benzene
related compounds[14,15] and substituent effects. Compared ring are twisted along the N1–C6 bond by 98°. This is pre-
to those of phosphinines,[9,10,23] the 31P NMR resonances dominantly caused by N1–H···O2 intermolecular hydrogen
of benzazaphospholes are dramatically shifted to low fre- bonding of the amide moiety and further stabilized by
quency by the π donation of the σ3N atom in conjugation intermolecular hydrogen bonding between C-9 of the tBu
to σ2P; the resonance is further shifted in 6 by Δδ = group and O-2; that is, O-2 of one molecule is hydrogen-
–12 ppm in comparison with that of 2-tert-butyl-5-methyl- bonded both to C-9–H and to N-1–H of a second molecule.
1H-1,3-benzazaphosphole.[22] This is attributed to the +M The bond lengths and angles within the molecule are in the
effect of the OH group rather than to steric shielding, as usual ranges.
the steric effects of Me, tBu, or Ph groups versus an H atom
at the 2-position to the phosphorus atom are much smaller
than the electronic effects of –M or +M substituents
(COOH or NHPh) at the same position.[14,15] Thus, 6 is an
example of a functionally substituted, π-electron-rich σ2P
heterocycle.
The 31P NMR chemical shifts of the phosphorylanilides
2a–2c and the phosphonylanilides 3a, 3b, 4a, and 7 display
characteristic values in the range δ = –5.9 to –6.3 ppm and
δ = 22.7 to 24.0 ppm, respectively. In proton-acceptor sol-
vents such as [D6]dimethyl sulfoxide ([D6]DMSO) or [D7]-
N,N-dimethylformamide ([D7]DMF), the 31P NMR reso-
nance of cis-3b is shifted to higher frequency (δ = 28,
29 ppm). The aryl–tBuP(O)(OEt) side products are indi-
cated by 31P NMR resonances in the range δ = 50–63 ppm
Figure 1. Molecular crystal structure of 5b. Ellipsoids represent
and by tBu(P) and i-C(P) doublets in the 13C NMR spectra
50% probability levels; protons, except those bound to N and O,
have been omitted for clarity. Selected bond lengths [Å] and angles
[°]: P1–C1 1.775(6), P1–O3 1.480(5), C6–N1 1.428(8), N1–C7
1.342(8), C7–O2 1.233(7), C2–O1 1.362(7); C6–N1–C7 122.5(5),
with 1JP,C = 99–104 Hz, both characteristic of aryl–tBuP(O)
X compounds.[24] The proton signals of the aryl CH groups
between the phosphoryl and amido groups in 2a–2c (δ = O3–P1–O5 116.0(3), O4–P1–O5 102.6(2), O3–P1–C1 110.7(3).
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The crystal structure analysis of the deuteriobenzene of the decomposition product 1c, which was detected for
solvate of 6 (Figure 2) reveals a planar ring system and the reaction of 2c with tBuLi/LDA. The formation during
bond lengths and angles similar to those of 2-tert-butyl- the reduction of the phosphonoanilide 7 by cleavage of the
1H-1,3-benzazaphosphole,[22] and no noticeable changes are P–C bond would require additional cleavage of the Si–C
induced by the 4-OH group. The C-4–O bond length is in bond. The structure data are typical for a benzylaniline and
the typical range for the C–O bonds of phenols. The OH are consistent with sp2 hybridization of the nitrogen atom;
group forms intermolecular N–H···O hydrogen bonds and which, in combination with the approximately coplanar ar-
weak O–H···P contacts, as illustrated in the packing (Fig- rangement of the N–C17 bond and the plane of the phenol
ure 3).
ring [dihedral angles C6–C1–N–C17 169.36(9), C2–C1–N–
C17 13.00(14)°], allows conjugation with the phenol π sys-
tem. In contrast, the phenyl group is nearly perpendicular
to the C17–N bond [C12–C11–C17–N 81.80(13)°, C16–
C11–C17–N 100.81(12)°]. The packing of the molecules
(see Supporting Information) is controlled by intermo-
lecular N–H···O and O–H···N hydrogen bonds.
Figure 2. Molecular crystal structure of 6. Ellipsoids represent 50%
probability levels. Selected bond lengths [Å] and angles [°]: P3–
C2 1.7397(11), P3–C3A 1.7878(11), C2–N1 1.3629(14), N1–C7A
1.3780(14), C3A–C7A 1.4071(15), C4–O 1.3859(13); N1–C2–P3
112.86(8), C2–P3–C(3A) 89.02(5), P3–C2–C9 128.10(8), N1–C2–
C9 118.86(9).
Figure 4. Molecular crystal structure of 8. Ellipsoids represent 50%
probability levels. Selected bond lengths [Å] and angles [°]: C(1)–
C(2) 1.3937(14), C(1)–N 1.4258(13), C(17)–N 1.4780(14); C(2)–
The crystal structure analysis of 8 (Figure 4) confirmed
the absence of a trimethylsilyl group at the phenyl ring and
suggests that the compound originates from the reduction C(1)–N 121.54(9), C(1)–N–C(17) 116.52(8).
Figure 3. Crystal packing of 6. Dashed lines indicate hydrogen bonds.
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standard procedures and freshly distilled before use. Other chemi-
cals were used as purchased. NMR spectra were recorded with an
Avance II 300 (Bruker) multinuclear FT-NMR spectrometer at
300.1 (1H), 75.5 (13C), and 121.5 (31P) MHz. Chemical shifts are
given in ppm and are referenced to SiMe4 or solvent signals and
Conclusions
The lithiation of N-secondary m-phosphorylpivalanilides
at the common ortho position (between the O and N sub-
stituents), followed by 1,3-O,C migration of the PO3Et2
group as is typical for o-metalated aryl phosphates,[17] is calibrated with this standard for 1H and 13C NMR spectra and
with H3PO4 (85%) for 31P NMR spectra. Coupling constants refer
strongly disfavored compared to the well-established di-
rected ortho-lithiations of aryl diethyl phosphates or N-sec-
ondary pivalanilides without interfering functional groups.
This is shown by the very low yields of 3a and 3b and the
low yield of 7 for the classic lithiation strategy with LDA;
the latter case is accompanied by additional ortho-lithiation
of the benzoic acid residue. The ortho-lithiation is slightly
improved with tBuLi in diethyl ether and is much more ef-
ficient with tBuLi in the presence of ClSiMe3. With excess
1
to JH,H in H NMR spectra and JP,C in 13C NMR spectra unless
indicated otherwise. The assignment numbers follow the nomencla-
ture numbering scheme. For phenyl groups, indication of position
by i/o/m/p is used. The relative peak areas of 31P NMR signals
(measured with AQ = 0.65 s, D1 = 2.0 s) are given in %Σintegral
and may deviate from the quantitative ratios. Low-resolution mass
spectra were recorded with an AMD40 (Maurer) instrument, and
HR mass (ESI) spectra were recorded in Göttingen with an APEX
IV (Bruker Daltonics) 7 T Fourier transform ion cyclotron reso-
tBuLi (2.8 equiv. relative to 2b), the degree of phosphonyl- nance mass spectrometer or with a micrOTOF instrument. Melting
points were determined with a Sanyo Gallenkamp melting point
apparatus, and elemental analysis was performed with a CHNS-
932 analyzer from LECO under standard conditions.
ation at the common ortho position reached 84%. Owing to
the competing partial lithiation/silylation of the oЈ-methyl
group (a new type of aryl phosphate directed lithiation) and
tert-butylation of the diethylphosphono group by the excess
tBuLi that is necessary to achieve complete conversion of
2b, some side products formed. Concerning the strong effect
of ClSiMe3, we suppose that the polar Li species that hinder
the ortho-lithiation, for example, by interactions with the
O=P donor group, are trapped by silylation, and this then
allows the phosphate-directed ortho-lithiation, followed by
the well-known rapid 1,3-intramolecular shift of the PO3Et2
group. It still has to be investigated whether a primarily
formed silylimidate group additionally supports the ortho-
lithiation at the common ortho position of m-phosphorylar-
ylsilylimidates and allows the selective lithiation even of 2a,
2c, and related compounds in the presence of ClSiMe3.
The reductive cyclization of the o-hydroxyphosphonoani-
lide 3b to form the first 4-hydroxy-1H-1,3-benzazaphos-
phole, 6, suggests that this synthetic route is also suitable
for related 2-tert-butyl-4-hydroxy-substituted benzazaphos-
pholes. The limitations for the synthesis of 2-aryl-substi-
tuted 4-hydroxybenzazaphospholes by this route are the ad-
ditional ortho-lithiation of the benzoic acid residue, which
was observed for lithiation with tBuLi/LDA, and the low
yields of 9 after reduction. The application of the tBuLi/
ClSiMe3 lithiation strategy, which was developed in the fi-
nal stage of these studies, might also improve the synthesis
of 2-aryl derivatives. The knowledge of a synthetic access
to 4-hydroxy-1,3-benzazaphospholes, represented by 6,
paves the way for future studies of the coordination behav-
ior of these new π-electron-rich σ2P,O hybrid ligands. Al-
though solutions prepared from 6 and Ni(COD)2 (COD =
cyclooctadiene) were unstable and induced only low cata-
lytic conversion of ethylene to low-molecular-weight oligo-
mers in batch-screening tests,[2d] there is a good chance that
benzazaphosphole hybrid complexes will be obtained with
d10 coinage metal compounds, which form isolable com-
plexes with monodentate benzazaphosphole ligands.[13]
N-(3-Hydroxyphenyl)-2,2-dimethylpropionamide (1a) and N-(3-Hy-
droxy-4-methylphenyl)-2,2-dimethylpropionamide (1b): Compound
1a was prepared according to a literature procedure.[26] The synthe-
sis of compound 1b was performed analogously.[27] The 13C NMR
spectra of these starting materials are depicted in the Supporting
Information.
N-(3-Hydroxy-4-methylphenyl)benzamide (1c): Benzoyl chloride
(4.98 mL, 43 mmol) was added dropwise at 0 °C to a mixture of 5-
amino-2-methylphenol (5.05 g, 41 mmol), NaHCO3 (84.0 g,
123 mmol), ethyl acetate (160 mL), and water (180 mL). The mix-
ture was stirred for 2 h, and the organic phase was separated and
washed with 1 n HCl, followed by water and brine. The resulting
solution was dried with Na2SO4 and concentrated at reduced pres-
sure to give 7.94 g (91%) of an off-white solid, m.p. 206–207 °C.
The NMR spectra in [D6]acetone display cis/trans isomers; the less
abundant isomer does not have a hydrogen bond, and the other
isomer does have a hydrogen bond. 1H NMR ([D6]acetone): δ =
2.17 (s, 3 H, 2-CH3), 2.88 (s, OH, H2O of solvent), 2.91 (s, OH),
3
3
4
7.02 (d, J = 8.1 Hz, 1 H, 5-H), 7.10 (dd, J = 8.1 Hz, J = 2.1 Hz,
1 H, 6-H), 7.42–7.58 (m, 3 H, 2 m-H, p-H), 7.56 (superimposed d,
4J = 2.0 Hz, 1 H, 2-H), 7.95 (m, J ≈ 6.8 Hz, J = 1.5 Hz, 2 H, o-
H), 8.29 (s, 0.25 H, NH), 9.36 (br. s, half-width = 15 Hz, 0.75 H,
NH···O) ppm. 13C{1H} NMR ([D6]acetone): δ = 16.4 (2-CH3),
108.42, 108.51, 112.67, 112.76 (C-2, C-4), 121.11 (br., cis- and
trans-Cq-2 superimposed), 128.90, 129.86, 131.9, 132.8 (C-o,m,p, C-
3), 137.1 (br., cis- and trans-Cq-5), 139.57, 139.66 (Cq-i), 156.73,
156.75 (C-1), 166.79, 166.86 (CO) ppm. MS (EI, 70 eV, 340 °C):
m/z (%) = 227 (36) [M]+, 105 (100), 77 (49), 51 (12). C14H13NO2
(227.26): calcd. C 73.99, H 5.77, N 6.16; found C 73.96, H 5.79, N
6.07.
3
4
3-(2,2-Dimethylpropanamido)phenyl Diethyl Phosphate (2a): Diethyl
phosphite (0.36 mL, 2.85 mmol) was added to a mixture of 1a
(0.5 g, 2.59 mmol) and Et3N (1.08 mL, 7.76 mmol) in CCl4 (5 mL)
and THF (5 mL). The mixture was heated to 50 °C for 24 h and
cooled to room temperature, and the precipitated Et3NH+Cl– was
removed by filtration and washed thoroughly with ethyl acetate.
The combined filtrates were concentrated and purified by flash col-
umn chromatography (30% ethyl acetate/hexane) to give a pale yel-
low oil (780 mg, 91%). 1H NMR (CDCl3): δ = 1.29 (s, 9 H, CMe3),
Experimental Section
General: All operations were performed under nitrogen by Schlenk
3
4
3
1.35 (td, J = 7.1 Hz, J = 1.0 Hz, 6 H, CH3), 4.21 (quintd, J =
3
techniques. The solvents CCl4 and triethylamine were dried by
3JPH = 7.1 Hz, J = 1.0 Hz, 4 H, OCH2), 6.93 (d unresolved dt, J
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4
= 8.2 Hz, JP,H = 2.3, 4J + 4ЈJ ≈ 2.3 Hz, 1 H, 6-H), 7.20 (t, 3J = the mixture warmed to room temperature overnight. Then, excess
3
4
8.2 Hz, 1 H, 5-H), 7.35 (d unresolved dt, J = 8.2 Hz, J = 1.8 Hz,
Me3SiCl (0.39 mL, 3.06 mmol) was added at –30 °C. After 8 h at
4J, JP,H ≈ 1.5 Hz, 1 H, 4-H), 7.58 (br., unresolved t with small J room temperature, the precipitate was removed by filtration and
6
4
≈
4JP,H, 1 H, 2-H), 7.90 (br. s, half-width = 5 Hz, NH) ppm. washed with diethyl ether. The pale yellow filtrate was concentrated
13C{1H} NMR (CDCl3): δ = 15.87 (d, 3J = 6.7 Hz, CH3), 27.29
under reduced pressure and separated by silica gel column
chromatography. Elution with 15% ethyl acetate/hexane gave trans-
3b as a colorless oil (115 mg, 26%). 1H NMR (CDCl3): δ = 1.31
(s, 9 H, CMe3), 1.33 (t, 3J = 7.1 Hz, 6 H, CH3), 2.20 (s, 3 H, CH3),
2
(CMe3), 39.45 (CqMe3), 62.48 (d, J = 6.2 Hz, OCH2), 111.88 (d,
3J = 5.5 Hz, C-2), 114.93 (d, 3J = 4.4 Hz, C-6), 116.46 (s, C-4),
129.42 (s, C-5), 139.59 (Cq-3), 150.63 (d, 2J = 6.9 Hz, Cq-1), 176.84
(CO) ppm. 31P{1H} NMR (CDCl3): δ = –6.3 ppm. C15H24NO5P 4.00–4.26 (m, 4 H, OCH2), 7.29 (d, 3J = 8.3 Hz, 1 H, 4-H), 7.59
3
4
(329.33). MS (EI, 70 eV, 150 °C): m/z (%) = 330 (12), 329 (62) (dd, J = 8.2 Hz, JP,H = 5.6 Hz, 1 H, 5-H), 8.12 (br. s, half-width
[M]+, 246 (12), 245 (99.6), 189 (34), 119 (40), 57 (100).
= 11 Hz, NH), 10.80 (s, OH) ppm. 13C{1H} NMR (CDCl3): δ =
4
3
15.72 (d, J = 1.7 Hz, CH3), 15.98 (d, J = 6.7 Hz, CH3), 27.26 (s,
2
5-(2,2-Dimethylpropanamido)-2-methylphenyl Diethyl Phosphate
(2b): Diethyl phosphite (13.6 mL, 106 mmol) was added to a mix-
ture of 1b (20.0 g, 96.5 mmol) and Et3N (40.3 mL, 289 mmol) in
CCl4 (200 mL). This reaction mixture was heated to reflux for 4 h
and then warmed to room temperature. The solids were removed
by filtration and washed thoroughly with ethyl acetate. The filtrate
was concentrated and purified by column chromatography with
30% ethyl acetate/hexane to elute impurities and unconverted 1b
and with 35% ethyl acetate/hexane to elute the product. The re-
moval of the solvent under vacuum gave a highly viscous colorless
substance (22.9 g, 69%). 1H NMR (CDCl3): δ = 1.28 (s, 9 H,
CMe3), 39.61 (s, CqMe3), 63.00 (d, J = 4.4 Hz, OCH2), 98.08 (d,
1J = 171.3 Hz, Cq-1), 113.60 (d, J = 10.3 Hz, C-5), 122.25 (d, J
= 11.5 Hz, Cq-3), 136.07 (d, 4J = 1.7 Hz, C-4), 137.60 (d, 2J =
1.6 Hz, Cq-6), 160.58 (d, 2J = 5.4 Hz, Cq-2), 176.27 (CO) ppm.
31P{1H} NMR (CDCl3): δ = 23.9 ppm. MS (EI, 70 eV, 80 °C): m/z
(%) = 343 (14) [M]+, 286 (15), 259 (11), 185 (13), 148 (17), 147
(100), 73 (24), 57 (70). HRMS (ESI, MeOH/H2O, HCOOH): calcd.
for C16H27NO5P [M + H]+ 344.16214; found 344.16220; calcd. for
[M + Na]+ 366.14408; found 366.14397.
3
3
Diethyl cis/trans-[6-(2,2-Dimethylpropanamido)-2-hydroxy-3-meth-
ylphenyl]phosphonate (cis-3b, trans-3b) and the Detection of 5b and
tBuP(O)OEt-Substituted Side Products: tBuLi in pentane (9.6 mL,
1.9 m, 18.2 mmol) was added dropwise to a solution of 2b (2.24 g,
6.52 mmol) and ClSiMe3 (2.3 mL, 18.2 mmol) in diethyl ether
(20 mL) at –85 °C. The resulting orange-red solution was warmed
slowly to room temperature (still immersed in the cooling medium).
At –30 to –20 °C, a color change to yellow and the formation of a
white precipitate was observed. After the solution had been stirred
overnight, the fine precipitate (LiCl) was separated by centrifuga-
tion, and the solvent was removed under vacuum to give a pale
yellow, highly viscous crude product mixture (3.32 g). This was de-
silylated by heating to reflux in dry EtOH (10 mL) for 15 h. The
solvent was removed under vacuum, and 31P NMR monitoring of
the highly viscous substance in CDCl3 showed an intense signal at
δ = 23.8–24.2 ppm (relative peak area 84%, cis/trans-3b) and small
signals at δ = 20.1 (6%, 3bOSi), 59.7 (4%, A), and 63.1 ppm (7%,
B). The ratio of cis-3b/trans-3b was roughly 2:1 on the basis of the
ratio of 13C NMR signals and corresponded to crude yields of 56
and 28%. The mixture was treated with pentane/Et2O (20:1 mL),
filtered, washed with pentane (20 mL), and dried to furnish cis-3b
as a beige powder (1.22 g, m.p. 220–222 °C), which still contained
ca. 10% of trans-3b (corrected yield of cis-3b 49%). cis-3b: 1H
NMR (CDCl3): δ = 0.91 (br. t, 3J = 7 Hz, 3 H, CH3), 1.22–1.34
(superimposed m, 3 H, Me), 1.23 (s, 9 H, CMe3), 2.22 (s, 3 H,
3
4
CMe3), 1.35 (td, J = 7.1 Hz, J = 1.0 Hz, 6 H, CH3), 2.25 (s, 3 H,
3
CH3), 4.17–4.27 (m, 4 H, OCH2), 7.09 (d, J = 8.3 Hz, 1 H, 3-H),
7.38 (dd, 3J = 8.3 Hz, J = 1.7 Hz, 1 H, 4-H), 7.48 (br., unresolved
4
t, 1 H, 6-H), 7.62 (br. s, half-width = 5 Hz, NH) ppm. 13C{1H}
3
NMR (DEPT, CDCl3): δ = 15.84 (s, CH3), 16.10 (d, J = 6.7 Hz,
CH3), 27.55 (CMe3), 39.55 (Cq–CMe3), 64.67 (d, 2J = 6.2 Hz,
3
4
OCH2), 111.94 (d, J = 2.3 Hz, C-6), 116.89 (d, J = 0.9 Hz, C-4),
3
4
124.62 (d, J = 6.4 Hz, Cq-2), 131.17 (C-3), 137.16 (d, J = 1.5 Hz,
Cq-5), 148.92 (d, 2J = 7.2 Hz, Cq-1), 176.69 (CO) ppm. 31P{1H}
NMR (CDCl3): δ = –5.9 ppm. MS (EI, 70 eV, 100 °C): m/z (%) =
344 (8) [M]+, 343 (46), 259 (78), 231 (24), 230 (11), 122 (18), 105
(75), 57 (100). HRMS (ESI, MeOH/H2O/HCOOH): calcd. for
C16H26NO5P [M + H]+ 344.16214; found 344.16220; calcd. for [M
+ Na]+ 366.14408; found 366.14409.
5-Benzamido-2-methylphenyl Diethyl Phosphate (2c): Diethyl phos-
phite (4.22 mL, 33 mmol) was added to a mixture of 1c (5.0 g,
22 mmol) and Et3N (9.19 mL, 66 mmol) in CCl4 (150 mL) and
THF (30 mL). The mixture was heated to reflux for 4 h, and
workup as described for 2a gave 2c as a white solid (5.5 g, 70%);
m.p. 124–125 °C. 1H NMR (CDCl3): δ = 1.35 (td, 3J = 7.1 Hz,
4JP,H = 1.1 Hz, 6 H, CH3), 2.20 (s, 3 H, 2-CH3), 4.19 (dquint, 3JP,H
3
3
≈ J = 7.1 Hz, 4 H, OCH2), 7.11 (d, J = 8.2 Hz, 1 H, 3-H), 7.38–
7.53 (m, 4 H, 2 m-H, p-H, 4-H), 7.63 (br., unresolved t, 1 H, 6-H),
7.86 (m, 3J ≈ 8.0 Hz, J = 1.3 Hz, 2 H, o-H), 8.43 (br. s, half-width
4
3
CH3), 3.10–3.45 (m, 2 H, OCHAHB), 4.03 (br. t, J = 6.4 Hz, 2 H,
= 4.7 Hz, 1 H, NH) ppm. 13C{1H} NMR (CDCl3): δ = 15.82 (s, 2-
CH3), 16.08 (d, 3J = 6.7 Hz, OCH2CH3), 64.69 (d, 2J = 6.5 Hz,
OCH2), 4.0–4.3 (superimposed br. s, 1 H, OH), 7.08–7.17 (m, 2 H,
4-H, 5-H), 8.11 (br. s, half-width 4 Hz, 1 H, NH) ppm. 13C{1H}
NMR (CDCl3): δ = 15.90, 16.13 (2 superimposed d, 3J ≈ 8 Hz,
CH3), 18.26 (s, CH3), 27.42 (CMe3), 39.55 (CMe3), 61.94, 62.01 (2
3
3
OCH2), 111.94 (d, J = 2.3 Hz, C-6), 116.9 (s, C-4), 124.8 (d, J =
6.6 Hz, Cq-2), 127.28, 128.55 (2s, C-o,m), 131.28 (s, C-3), 131.6 (s,
C-p), 134.97 (s, Cq-i), 137.23 (s, Cq-5), 148.9 (d, J = 6.8 Hz, Cq-
2
2
1
superimposed d, J ≈ 6 Hz, OCH2), 101.63 (d, J = 177.8 Hz, Cq-
1), 110.51 (d, 3J = 10.6 Hz, C-5), 126.44 (d, 3J = 10.6 Hz, Cq-3),
134.58 (br. s, C-4), 138.57 (br. d, 2J = 5.3 Hz, Cq-6), 170.73 (br.,
Cq-2), 175.97 (CO) ppm. 31P{1H} NMR: δ = 24.2 (CDCl3), 28.0
([D6]DMSO), 29.0 (moist [D7]DMF) ppm. HRMS (ESI): calcd. for
C16H25NO5P [M – H]– 342.1476; found 342.1480; calcd. for [M –
H – C2H4]– 314.1152; found 342.1161. C16H26NO5P (343.36): calcd.
C 55.97, H 7.63, N 4.08; found C 55.43, H 7.21, N 4.24.
1), 165.9 (CO) ppm. 31P{1H} NMR (CDCl3): δ = –6.3 ppm. MS
(EI, 70 eV, r.t.): m/z (%) = 364 (7) [M]+, 149 (7), 105 (21), 58 (100).
HRMS (ESI, MeOH/NaOAc): calcd. for [M + Na]+ 386.11278;
found 386.11287. C18H22NO5P (363.34): calcd. C 59.50, H 6.10;
found C 59.12, H 6.06.
Diethyl
trans-[6-(2,2-Dimethylpropanamido)-2-hydroxy-3-methyl-
phenyl]phosphonate (trans-3b): A solution of tBuLi in pentane
(1.80 mL, 1.7 m, 3.06 mmol) was added dropwise to a solution of
2b (0.50 g, 1.46 mmol) in diethyl ether (5 mL) at –80 °C. A yellow
precipitate formed after a few minutes. Stirring was continued as
Removal of the solvent from the filtrate left a pale yellow, viscous
oil (1.09 g) containing various components: 31P NMR (CDCl3): δ
= 23.5 (27%, trans-3b), 24.3 (24%, 5b), 59.9 (10%, A), 63.2 (15%,
Eur. J. Inorg. Chem. 2014, 5958–5968
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Table 2. Crystal data and structure refinement for 5b, 6, and 8.
Compound
5b
6·C6D6
8
Empirical formula
Formula weight
Temperature [K]
Wavelength [Å]
Crystal system
Space group
a [Å]
C22H42NO5PSi2
487.72
170(2)
0.71073
monoclinic
P21/n
C18H16D6NOP
305.78
133(2)
0.71073
monoclinic
C2/c
C14H15NO
213.27
103(2)
1.54184
monoclinic
P21/c
13.802(3)
90
26.048(2)
90
11.1345(3)
90
α [°]
b [Å]
β [°]
c [Å]
10.097(2)
94.74(3)
20.215(4)
90
2807.4(10)
4
1.154
10.9824(11)
98.471(3)
11.8799(11)
90
3361.4(5)
8
7.7752(2)
109.543(4)
13.8287(4)
90
1128.22(5)
4
γ [°]
Volume [Å3]
Z
ρ (calcd.) [Mg/m3]
1.208
0.163
1.256
0.619
µ [mm–1]
0.212
F(000)
1056
1280
456
Crystal size [mm]
θ range for data collection [°]
Index ranges
0.5ϫ0.09ϫ0.08
1.86 to 23.67
–15 Յ h Յ 14
–11 Յ k Յ 11
–21 Յ l Յ 22
16899
0.5ϫ0.2ϫ0.2
1.58 to 30.50
–37 Յ h Յ 37
–15 Յ k Յ 15
–16 Յ l Յ 16
27747
0.24ϫ0.16ϫ0.03
4.21 to 75.53
–13 Յ h Յ 13
–5 Յ k Յ 9
–17 Յ l Յ 16
10717
2239 [R(int) = 0.022]
98.9% to θ = 67.5°
Reflections collected
Independent reflections
Completeness
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [IϾ2σ(I)]
4218 [R(int) = 0.112]
99.3% to θ = 23.67°
numerical
0.9802 and 0.7756
5117 [R(int) = 0.033]
99.7% to θ = 30.00°
semi-empirical from equivalents semi-empirical from equivalents
0.9682 and 0.9088
1.00000 and 0.77727
full-matrix least squares on F2
2239/0/154
1.08
R1 = 0.0386
wR2 = 0.108
R1 = 0.0416
wR2 = 0.111
0.21 and –0.20
full-matrix least squares on F2 full-matrix least squares on F2
4218/40/316
1.11
R1 = 0.0849
wR2 = 0.240
R1 = 0.105,
wR2 = 0.250
5117/0/175
1.13
R1 = 0.0405
wR2 = 0.109
R1 = 0.0558
wR2 = 0.116
0.48 and –0.19
R indices (all data)
Largest diff. peak and hole [eÅ–3] 0.59 and –0.49
B), 20.2 (6%), 23.9 (7%), 24.1 (7%), 37.8 (2%), 51.8 (2%). Within
a few days a small part of the substance crystallized and was sepa-
rated from the solution and identified as 5b by crystal structure
analysis. The crystal data are compiled in Table 2, and selected
bond lengths and angles are given in Figure 1. The NMR spectro-
scopic data of 5b were obtained after TLC separation on silica gel
stirred at room temperature for 3 d. Then, degassed water was
added dropwise until the evolution of H2 gas had ceased. The solids
were removed by filtration and washed thoroughly with diethyl
ether. The organic layer was dried with Na2SO4, filtered, and con-
centrated to give a colorless solid (654 mg), which was crystallized
from THF/hexane to give colorless 6 (500 mg, 60%); mp. 134–
135 °C. The data of single crystals grown by slow concentration of
a C6D6 solution in a plastic-capped NMR tube are compiled in
Table 2, and selected bond lengths and angles are given in Figure 2.
1
with 10% ethyl acetate/hexane (1:9). H NMR (CDCl3): δ = 0.02
3
(s, 18 H, 2 SiMe3), 1.30 (s, 9 H, CMe3), 1.32 (t, J = 7.1 Hz, 6 H,
3
CH3), 2.26 (s, 1 H, 3-H), 3.95–4.25 (m, 4 H, OCH2), 7.13 (d, J =
4
8.3 Hz, 1 H, 4-H), 7.62 (dd, 3J = 8.3 Hz, JP,H = 6.0 Hz, 1 H, 5-
4
1H NMR (C6D6): δ = 1.20 (d, JP,H = 1.2 Hz, 9 H, CMe3), 2.19 (s,
H), 8.14 (v. br. s, NH), 10.76 (br. s, OH) ppm. 31P{1H} NMR
(CDCl3): δ = 24.3 ppm. HRMS (ESI): calcd. for C22H41NO5PSi2
[M – H]– 486.2266; found 486.2253; calcd. for [M – H – C2H4]–
458.1942; found 458.1948. The side products A and B were iden-
tified as aryl–tBuP(O)(OEt) species.[24] 13C NMR (CDCl3): A: δ
3
3 H, 5-CH3), 4.96 (br. s, half-width = 12 Hz, OH), 6.84 (dd, J =
4
3
8.2 Hz, JP,H = 1.3 Hz, 1 H, 7-H), 7.05 (d, J = 8.2 Hz, 1 H, 6-H),
8.40 (v. br. s, half-width = 18 Hz, NH) ppm. 13C{1H} NMR
(DEPT, C6D6): δ = 15.91 (d, J = 0.6 Hz, 5-CH3), 31.87 (d, 3J =
4
9.2 Hz, CMe3), 36.24 (d, 2J = 14.0 Hz, CqMe3), 106.62 (s, C-7),
1
= 23.23 (s, CMe3), 35.13 (d, JP,C = 98.2 Hz, CMe3) ppm, Cq-1
3
4
113.06 (d, J = 6.5 Hz, Cq-5), 129.14 (d, J = 2.5 Hz, C-6), 130.42
(d, J = 40.9 Hz, Cq-3a), 144.11 (d, J = 4.7 Hz, Cq-7a), 154.24 (d,
2J = 12.6 Hz, Cq-4), 188.94 (d, J = 57.3 Hz, Cq-2) ppm. 31P{1H}
1
superimposed; B: δ = 23.35 (s, CMe3), 35.22 (d, JP,C = 99.5 Hz,
1
2
CMe3), 99.09 (d, 1JPC = 103.5 Hz, Cq-1) ppm. HRMS (ESI): calcd.
for C18H29NO4P [M – H]– 354.1840; found 354.1837. A minor sig-
nal set (Δδ1H ≈ 0.01–0.02 ppm, δ31P = 24.1 ppm) belongs to the 3-
trimethylsilylmethyl derivative. HRMS (ESI): calcd. for
C19H33NO5PSi for [M – H+]– 414.1871; found 414.1838; calcd. for
[M – H – C2H4]– 386.1547; found 386.1543.
1
NMR: δ = 51.5 (C6D6), 48.0 (CDCl3) ppm. MS (EI, 70 eV, 100 °C):
m/z (%) = 222 (13) [M]+, 221 (75) [M]+, 206 (100), 168 (30), 136
(33). HRMS (ESI, MeOH/H2O, HCOOH): calcd. for C12H17NOP
[M + H]+ 222.10423; found 222.10432. (b) A solution of tBuLi in
pentane (3.6 mL, 1.9 m, 6.9 mmol) was added at –80 °C to an ethe-
real solution (25 mL) of 2b (0.79 g, 2.3 mmol) and ClSiMe3
(0.9 mL, 6.9 mmol). The mixture was slowly warmed to room tem-
perature and stirred overnight. The crude product was added slowly
to LiAlH4 (0.26 g, 6.9 mmol) in diethyl ether (150 mL) at 0 °C. The
2-tert-Butyl-4-hydroxy-5-methyl-1H-1,3-benzazaphosphole (6): (a)
A solution of pure trans-3b (1.3 g, 3.74 mmol) in diethyl ether was
added in small portions at 0 °C to LiAlH4 tablets (430 mg,
11.4 mmol) in diethyl ether (20 mL). This reaction mixture was
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mixture was stirred at room temp. for 2 d, air-free water was added
until the hydrogen evolution had ceased, and the mixture was fil-
tered. 31P NMR spectroscopy showed the signal of 6 (28%) along
with the signals of primary (δ = –173.4 ppm, 11%) and secondary
phosphanes (δ = –84.3 ppm, 8%; –67.1, 6%) and further side prod-
ucts. The diethyl ether was removed under vacuum, and the resid-
ual pale yellow oil was separated by silica gel column chromatog-
raphy with 15% ethyl acetate/hexane for elution. Compound 6 was
collected along with 2-methyl-5-(neopentylamino)phenol; the crude
product was treated with 10% aqueous sulfuric acid, washed with
a small amount of water, and dried with Na2SO4 (yield 98 mg, 19%
referred to 2b). The NMR spectroscopic data are in good agree-
ment with those given above.
for 9. The extraction of the crude product with diethyl ether led to
the enrichment of 8, which crystallized from a concentrated toluene
solution; m.p. 138–139 °C (yield not determined). The crystal data
are compiled in Table 2, and selected bond lengths and angles are
given in Figure 4. The removal of the solvent from the yellow
mother liquor led to a sticky residue, which according to the NMR
spectra consisted mainly of 8 with a small amount of 9 (δ31P =
59.1 ppm) and traces of two PVOEt compounds (δ31P = 8.1 and
3.1 ppm). An attempt to separate 9 by silica gel column chromatog-
raphy with up to 30% ethyl acetate/hexane failed. Compound 8:
1H NMR (CDCl3): δ = 2.12 (s, 3 H, 2-CH3), 3.3 (v. br., NH, OH),
4
3
4.28 (s, 2 H, NCH2), 6.14 (d, J = 2.2 Hz, 1 H, 6-H), 6.18 (dd, J
4
3
= 8.1 Hz, J = 2.2 Hz, 1 H, 4-H), 6.89 (d, J = 8.1 Hz, Hz, 1 H, 3-
H), 7.26–7.38 (m, 5 H, Ph) ppm. 13C{1H} NMR (CDCl3): δ = 14.78
(2-CH3), 48.66 (NCH2), 100.15 (C-6), 105.95 (C-4), 112.68 (Cq-2),
127.21 (C-p), 127.51, 128.60 (2 C-o, 2 C-m), 131.36 (C-3), 139.24
(Cq-i), 147.50 (Cq-5), 154.59 (Cq-1) ppm. HRMS (ESI, MeOH,
NaOAc): calcd. for [M + H]+ 214.12264; found 214.12271; calcd.
for [M + Na]+ 236.10459; found 236.10471. C14H15NO (213.28):
calcd. C 78.84, H 7.09, N 6.57; found C 79.10, H 7.43, N 6.33.
1
2-Methyl-5-(neopentylamino)phenol: H NMR (C6D6): δ = 0.81 (s,
4
9 H, CMe3), 2.09 (s, 3 H, Me), 2.67 (s, 2 H, NCH2), 5.89 (d, J =
2.3 Hz, 1 H, 2-H), 6.12 (dd, 3J = 8 Hz, 4J = 2.3 Hz, 1 H, 6-H), 7.06
3
(d, J = 8.3 Hz, 1 H, 5-H) ppm. The formation of this compound
is attributed to the reduction of unconverted 2b.
Diethyl
{2-Hydroxy-3-methyl-6-[2Ј-(trimethylsilyl)benzamido]-
1
Compound 9: H NMR (CDCl3): δ = –0.07 (s, SiMe3), 2.27 (s, 2-
phenyl}phosphonate (7): A solution of tBuLi in pentane (1.6 m,
2.85 mL, 4.56 mmol) was added dropwise to iPr2NH (0.39 mL,
2.77 mmol) in diethyl ether (5 mL) at –30 °C. After 30 min, this
mixture was added slowly to a solution of 2c (500 mg, 1.38 mmol)
in Et2O (10 mL) at –78 °C. After a few minutes, a yellow precipitate
formed. Stirring was continued as the mixture warmed to room
temperature (overnight). Then, after the mixture was cooled to
–30 °C, excess Me3SiCl (0.58 mL, 4.57 mmol) was added, and the
mixture was warmed to room temperature and stirred for 4 h. The
precipitate was removed by filtration and washed with diethyl ether;
the pale yellow filtrate was concentrated under vacuum and puri-
fied by column chromatography. Elution with 2% ethyl acetate/
hexane gave 7 (170 mg, 28%) as a pale yellow oil. 1H NMR
3
3
Me), 6.69 (d, J ≈ 8 Hz, 7-H), 6.95 (d, J ≈ 8 Hz, 6-H), 7.70–7.73
(2 d, 3Ј-H, 6Ј-H), 9.55 (br. s, OH) ppm; the other signals are super-
imposed. 13C{1H} NMR (CDCl3): δ = –1.56 (d, 5J = 2.6 Hz,
3
SiMe3), 15.11 (2-Me), 106.18 (C-7), 112.43 (d, J = 7.7 Hz, Cq-5),
129.11 (C-6), 130.59 (d, 1J = 40.0 Hz, Cq-3a), 143.40 (d, 2J =
5.5 Hz, Cq-7a), 152.73 (d, 2J = 12.0 Hz, Cq-4), 173.36 (d, 1J =
50.2 Hz, Cq-2); 2-aryl signals uncertain in the presence of aryl sig-
nals of impurity 7: 125.37, 128.83 (d, J = 2.6 Hz), 129.2 (superim-
posed), 131.80 (br.), 134.88 (d, J = 16.4 Hz, Cq-1Ј), 136.32 ppm.
31P{1H} NMR (CDCl3): δ = 59.1 ppm.
Crystal Structure Analysis of 5b, 6, and 8: The crystal data are sum-
marized in Table 2. The diffraction data for 5b were recorded at
–103 °C with a STOE-IPDS 2T diffractometer with graphite-mo-
nochromated Mo-Kα radiation. The structure was solved by direct
methods (SHELXS-97) and refined by full-matrix least-squares
techniques (SHELXL-97).[28] All non-hydrogen atoms were refined
with anisotropic displacement parameters. The hydrogen atoms at
N1 and O1 were found and refined isotropically without any re-
straints. Methyl groups were refined as idealized rigid groups al-
lowed to rotate but not tip. Other hydrogen atoms were placed at
calculated positions and refined by using a riding model. One ethyl
substituent (C14, C15) is disordered over two positions with 60
and 40% occupancy, respectively. The diffraction data for 8 were
recorded at –173 °C with an Oxford Diffraction Nova A dif-
fractometer with mirror-focused Cu-Kα radiation, and those for 6
were recorded at –130 °C with a Bruker SMART 1000 CCD dif-
fractometer with monochromated Mo-Kα radiation. The structure
solutions and refinements proceeded as above, except that the sol-
vent of 6 required special attention; one of the two half-molecules
of deuteriobenzene had to be removed with the SQUEEZE[29] rou-
tine because of severe disorder.
3
(CDCl3): δ = 0.37 (s, 9 H, SiMe3), 1.27 (t, J = 7.1 Hz, 6 H, CH3),
2
3
3
2.23 (s, 3 H, 2-CH3), 4.09 (ddq, J = 10.2 Hz, JP,H = 8.5 Hz, J =
7.1 Hz, 2 H, Ha of OCH2), 4.20 (ddq, 2J = 10.2 Hz, 3JP,H = 7.9 Hz,
3J = 7.1 Hz, 2 H, Hb of OCH2), 7.35 (d, J = 8.2 Hz, 1 H, 4-H),
3
3
3
4
7.44, 7.48 (2 superimposed td, J = 7.5 Hz, J ≈ 7 Hz, J = 1.6 Hz,
1 H, 4Ј-H, 5Ј-H), 7.62–7.74 (m, 3 H, 5-H, 3Ј-H, 6Ј-H), 8.45 (s, 1
H, NH), 10.78 (s, 1 H, OH) ppm. 13C{1H} and DEPT-135 NMR
(CDCl3): δ = –0.33 (d, J = 5 Hz, SiMe3), 15.72 (d, 3J = 5.2 Hz,
2
1
CH3), 15.84 (s, CH3), 63.05 (d, J = 4.3 Hz, OCH2), 98.51 (d, J =
173.5 Hz, Cq-1), 113.43 (d, 3J = 10.4 Hz, C-5), 122.74 (d, 3J =
11.8 Hz, Cq-3), 125.19, 128.53, 129.76, 135.54 (C-6Ј, C-5Ј, C-4Ј, C-
3Ј), 136.12 (C-4), 137.23 (s, Cq-6), 140.80, 141.71 (Cq-1Ј, Cq-2Ј),
160.50 (d, 2J = 5.3 Hz, Cq-2), 168.09 (CO) ppm. 31P{1H} NMR
(CDCl3): δ = 23.2 ppm. HRMS (ESI, MeOH, NaOAc): calcd. for
C21H30NNaO5PSi [M + Na]+ 458.1523; found 458.1526.
5-(Benzylamino)-2-methylphenol (8) and 5-Methyl-4-hydroxy-2-[2-
(trimethylsilyl)phenyl]-1H-1,3-benzazaphosphole (9): A solution of
crude 7, containing 1c, was prepared as described above by lithi-
ation of 2c (4.81 g, 13 mmol) in Et2O (30 mL) at –78 °C with a
solution formed from tBuLi (1.6 m in pentane, 27.3 mL, 43 mmol)
and iPr2NH (3.90 mL, 27 mmol) in diethyl ether and subsequent
reaction with ClSiMe3 (4.9 mL, 44 mmol). To avoid the loss of 7
during workup, the crude product was directly subjected to re-
duction. The ethereal solution was added in small portions to Li-
AlH4 (1.5 g, 39 mmol) in THF (20 mL) at 0 °C. This reaction mix-
ture was stirred at room temperature for 3 d. Then, the mixture
was cooled to 0 °C, and degassed water was added dropwise until
the evolution of H2 gas had ceased. The solid was removed by
filtration, and the filtrate was concentrated to give a yellow oil. 1H
NMR spectroscopy revealed strong signals for 8 and weak signals
CCDC-1007401 (for 5b), -1000343 (for 6), and -1000342 (for 8)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_requ-
est/cif.
Supporting Information (see footnote on the first page of this arti-
cle): Supplementary experimental data for 3a, 4a, 2bOSi, and 3bOSi
;
1
13C NMR spectra of 1a and 1b; H and 13C NMR spectra of 1c,
2a–2c, 3a, 3b, 4a, 6, 7, and 8; 31P NMR spectra of P compounds;
crystal packing of 8.
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Received: June 11, 2014
Published Online: October 28, 2014
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