An unusually unstable ortho-phosphinophenol and its use to prepare benzoxaphospholes having enhanced air-stability

Article abstract of DOI:10.1039/c3dt51919h

The primary phosphine 3,5-di-tert-butyl-2-phosphinophenol has been prepared and characterized. Oddly, the presence of a sterically demanding tert-butyl group adjacent to the PH₂ centre renders the molecule very sensitive to loss of PH₃ and formation of 3,5-di-tert-butyl-phenol in chloroform solutions in the presence of air. The process was catalyzed by HCl and dependent on the purity of CDCl₃. Despite the instability of 3,5-di-tert-butyl-2-phosphinophenol, this material could be employed to produce a series of luminescent 2-R-4,6-di-tert-butyl-1,3-benzoxaphospholes having greater air stability than corresponding less bulky 2-R-1,3-benzoxaphospholes.

Full text of DOI:10.1039/c3dt51919h

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PAPER
An unusually unstable ortho-phosphinophenol and its
use to prepare benzoxaphospholes having enhanced
air-stability†
Cite this: Dalton Trans., 2013, 42, 14866
Shanshan Wu, Nihal Deligonal and John D. Protasiewicz*
The primary phosphine 3,5-di-tert-butyl-2-phosphinophenol has been prepared and characterized.
Oddly, the presence of a sterically demanding tert-butyl group adjacent to the PH2 centre renders the
molecule very sensitive to loss of PH3 and formation of 3,5-di-tert-butyl-phenol in chloroform solutions in
the presence of air. The process was catalyzed by HCl and dependent on the purity of CDCl3. Despite the
instability of 3,5-di-tert-butyl-2-phosphinophenol, this material could be employed to produce a series of
luminescent 2-R-4,6-di-tert-butyl-1,3-benzoxaphospholes having greater air stability than corresponding
less bulky 2-R-1,3-benzoxaphospholes.
Received 15th July 2013,
Accepted 16th August 2013
DOI: 10.1039/c3dt51919h
www.rsc.org/dalton
Introduction
Within the realm of multiply bonded main group compounds,
a key paradigm has been the understanding that compounds
bearing sterically demanding ligands shall possess enhanced
kinetic stability. Thus over the years much effort has been
given to the design and synthesis of bulky aryl and alkyl
ligands that have been employed in the successful isolation of
new types of element–element bonds.1–5 In particular, com-
pounds having PvC (phosphaalkene) and PvP (diphosphene)
functionalities have become readily characterized when suit-
ably decorated with sterically demanding groups. It was this
approach that led to the first successful isolation of the dipho-
sphene Mes*PvPMes*.6
Fig. 1 Structural distortions induced by steric interactions of bulky groups.
One strategy to enforce coplanarity of PvC units with attached
aryl frameworks involves incorporation of the phosphaalkene
residue into a fused ring. Several examples of such materials
are shown in Chart 1.21–39 The chemistry of the 1,3-benzaza-
phospholes (BAP-R) in particular is quite extensive.25,33 Our
group has become interested in the 1,3-benzoxaphospholes
(BOP-R) due to their highly fluorescent nature when R =
aryl.9,10
At the same time that sterically demanding ligands can
provide kinetic stabilization of inter-element multiple bonds,
such groups can also inhibit the ability of PvE (E = C or P)
bonds to participate in π-conjugation with CvC bonds.
Specifically, large ortho-substituents (R′) will exert steric press-
ures to move attached PvC units out of alignment for
maximal π-conjugation (Fig. 1).
We have also created new benzoxaphosphole analogues
with expanded conjugation, as shown in Chart 2.8,10 All of
Our group7–16 and the Gates group17–20 have been very
interested in conjugated materials featuring PvC bonds.
Many of these materials, however, display molecular distor-
tions that diminish pπ overlaps necessary for π-conjugation.
Chart 1
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106,
USA. E-mail: protasiewicz@case.edu; Tel: +1 216-368-5060
†Electronic supplementary information (ESI) available: Details of synthesis,
characterization, crystallographic analyses, and computations. CCDC 913439
and 913440. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3dt51919h
Chart 2
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these cyclic phosphaalkenes offer in comparison to acyclic
phosphaalkenes (a) enforced coplanarity of PvC bonds with
attached aromatic residues (b) enhanced air and water stability
compared to many other phosphaalkenes and (c) unique
fluorescent properties. We have found that while many of
these benzoxaphospholes are air-stable as solids, they will
slowly degrade in solution (days).
In order to facilitate employment of these types of materials
into devices, we sought to increase the stability and solubility
of benzoxaphospholes by introduction of a sterically demand-
ing tert-butyl group ortho to the low coordinate phosphorus
center. This article communicates the results of this strategy.
Scheme 2
Compound 4 is readily characterized by its signature
31P NMR signal at −169.7 ppm (t, JHP = 198 Hz). Unfortunately,
the isolation of 4 is also accompanied by small amounts of
3,5-di-tert-butyl-phenol (ca. 2–5%). Oddly, solutions of 4 in
CDCl3 open to air quickly begin to decompose, and is comple-
tely consumed in 6 days. By contrast, the less hindered ortho-
phosphinophenol PP1 (Chart 3) shows no evidence of
decomposition under the same conditions and time frame.
Another surprising fact is that the major product (>90%
1H NMR) of aerobic oxidation in CDCl3 is 3,5-di-tert-butyl-
phenol (i). The final 31P NMR spectra are uninformative due to
the presence of numerous signals of low intensity.
Similar observations have been made by Heinicke during
isolation of certain substituted ortho-phosphinophenols, such
as PP245 and PP346 (Chart 3).47 The presence of traces HX
(from the acid hydrolysis workup after the LiAlH4 reduction of
corresponding hydroxyphenylphosphonates) was suspected as
catalysts for the decomposition process. This hypothesis was
corroborated by the finding that replacing HCl with acetic acid
during the workup produced less phenol. Attempts to further
purify PP3 and remove contaminating 2,4-di-tert-butylphenol
by distillation led to further formation of 2,4-di-tert-butyl-
phenol as well as formation of dihydro-1,2,3-benzoxadiphosp-
hole DBDP (Chart 3). The fate of the phosphorus atoms in
these reactions was presumed to be PH3. The presence of steri-
cally demanding groups, especially ortho to the OH group, was
Results and discussion
Synthesis & stability of ortho-phosphinophenol 4
The synthesis of 1,3-benzoxaphospholes begins the appropri-
ate ortho-phosphinophenol. We chose 3,5-di-tert-butyl-phenol
(i) as
a convenient starting point towards this target
(Scheme 1). Phosphorylation of i by diethyl chlorophosphate
readily affords 3,5-di-tert-butylphenylphosphate (ii). We were
not able, however, to find conditions to effect the selective
ortho-deprotonation of ii to generate iii. Intermediate iii is
expected to undergo the anionic phospha-Fries rearrangement
to yield iv. Attempts to access iv from ii using BuLi, Pr2NLi,
or LiTMP (TMP = 2,2,5,5-tetramethylpiperidinyl) as bases led
instead to mixtures of products (as ascertained by 31P NMR
spectroscopy). Some of the signals observed in the 31P NMR
spectra were consistent suggested that the bases were effecting
dephosphorylation instead of deprotonation.
Access to key intermediate iii was achieved by an alternative
strategy. Bromination of 3,5-di-tert-butyl-phenol (i) using
Br2 affords 3,5-di-tert-butyl-2-bromophenol (1).40–42 Reaction
with diethyl chlorophosphate and NaH leads to diethyl
(2-bromo-3,5-di-tert-butylphenyl)phosphate (2) in excellent
yield. Addition of n-butyllithium initiates lithium–bromine
exchange (generating intermediate iii), and rapid migration of
the phosphorus to the phenyl ring via the anionic phospha-
Fries rearrangement43,44 to produce 3. Reduction of 3 using
LiAlH4 affords the ortho-phosphinophenol 4 in good yield
(Scheme 2).
n
i
Scheme 1
Chart 3
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suggested to induce destabilizing OH⋯P interactions and
makes these systems susceptible to carbon–phosphorus bond
cleavage. Carbon–phosphorus bond cleavage is well estab-
lished for organophosphorus compounds having the bulky
Mes* (Mes* = 2,4,6-tBu3C6H2) group.48 The primary phosphine
Mes*PH2, however, is a rare example of an air-stable primary
phosphine.
We were thus interested in determining why compound 4 is
unstable. Early investigations of the air stability of 4 showed
that solutions of 4 in C6D6 and DMSO-d6 were more stable
than solutions in CDCl3 (compound 4 was found to persist up
to 120 days in DMSO-d6 solvent). Upon further investigations,
however, the air stability of 4 varied somewhat depending on
the specific bottle of CDCl3 and/or the means used to purify
the CDCl3 used to prepare solutions.
Fig. 2 31P{1H} NMR spectrum of 4 in CH3COOD showing 4-d1 (minor) and
4-d2 (major).
Among the suspected impurities that can arise from the
decomposition of CDCl3 are HCl and phosgene.49,50 Phosgene
would be expected to hydrolyse in moist air to produce HCl.
HCl could also be present as a trace contaminant from the syn-
thesis of 4 (aqueous NH4Cl was used to quench reaction
mixture from the reduction of 3 by LiAlH4).
successive deuterium added to compound 4, a shift of
0.86 ppm in the 31P NMR spectrum is observed.
Reaction of 4 with 5% HOTs in unpurified CDCl3 was also
slow and produced i and PH3 (5 days). Unlike the case with acetic
acid, the reaction of 4 with 1 equivalent of HOTs occurs over
5 hours and shows the formation of 3,5-di-tert-butylphenol, and
PH3 as the main phosphorus containing species (Fig. S6, ESI†).
For comparison, we find that PP1 is stable to acid in all
We thus tested the impact of HCl on solutions of 4.
Addition of 5% HCl (in Et2O) to a solution of 4 in CDCl3
exposed to air does promote the decomposition of 4. Within
30 minutes 4 is completely consumed and 3,5-di-tert-butyl-
phenol is formed in >90% yield (1H NMR, Fig. S1, ESI†). Analy-
sis of the 31P NMR spectra reveals PH3 as the major product,
as identified by a characteristic quartet ( JHP = 189 Hz) at
δ −237.3. Repeating this experiment after treating the CDCl3
with molecular sieves (Sigma-Aldrich 3 Å, 4–8 mesh, dried at
110 °C) slowed the decomposition of 4 (and production of
PH3) to 7 hours. Under the same conditions but under either
nitrogen or dried air instead of air showed little or no
decomposition of 4. A solution of 4 in dry CDCl3 under nitro-
gen with 5% HCl, in the presence of 1 equiv. of degassed
water, slowly produces PH3 (days–weeks). In addition, the pres-
ence of a new species identified by a doublet in the 31P NMR
spectra at δ 34.4 ppm with a JPH coupling constant of 490 Hz is
noted. This signal is consistent with the presence of 2-hydroxy-
phenylphosphinic acid.51
1
cases. The H NMR spectra of mixtures of PP1 and HCl show
broadening of both PH and OH resonances suggesting rapid
proton transfer.
These collective experimental data present a challenge for a
single clear mechanistic interpretation. The role of HX is
further complicated by the uncertainty in the aggregation and
dissociation of ionic species in CDCl3. One simple starting
point for discussion is depicted in Scheme 3. If the added HX
protonates the phosphorus atom, the resulting phosphonium
salt might be considered “protected” towards oxidation.52 This
hypothesis would be consistent with the fact that compound 4
is relatively air-stable in neat acetic acid. We have not been
able to observe such phosphonium salts by 31P NMR spectro-
scopy, however. The role of the anion might be critical in initi-
ating carbon–phosphorus bond cleavage from [4]+ (Scheme 3,
right). Alternatively, the role of the acid might lie with steps
occurring after oxidation by O2/H2O of 4 (Scheme 3, left).
These observations might suggest the presence of an
unknown contaminant in unpurified CDCl3 is responsible for
catalyzing the decomposition of 4. This hypothesis can only be
partly true, for addition of 1 equiv. of HCl (in Et2O) to 4 in dry
CDCl3 induces the immediate decomposition of 4 to i and
PH3, in either air or under nitrogen.
In order to ascertain the impact of choice of acid, the
impact of acetic acid and p-toluenesulfonic acid on 4 was
examined. Solutions of 4 and 5% acetic acid-d1 in unpurified
CDCl3 produce PH3 over the period of a couple of days. Oddly,
a solution of 4 and 1 equiv. of CH3COOD in unpurified CDCl3
does not produce any PH3 in over 20 days open to air (some
slight oxidation is noted). In fact, a solution of compound 4 in
neat CH3COOD (Sigma-Aldrich, 99 atom% D) open to the air is
remarkably stable. The 31P{1H} NMR analysis of such solutions
reveals a facile PH/PD exchange process (Fig. 2). For each
Scheme 3
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While the specific details of the carbon–phosphorus bond
cleavage in 4 remain unknown, it is interesting to note that the
very closely related ortho-phosphinoanisole MP1 (Chart 3)
undergoes aerobic oxidation retaining its phosphorus center
to produce MP2.40
Another means to assess the air stability of primary phos-
phines has been reported by Higham and coworkers.53,54 In
this model two electronic factors were identified as critical to
predict the air stability of a primary phosphine. Primary phos-
phines that have HOMOs that contained little phosphorus
lone pair character displayed enhanced air-stability. In
addition, if calculations on [RPH2]˙+ revealed SOMO energies
above −10.0 eV, then enhanced air-stability was predicted.
We thus undertook similar calculations on phosphino-
phenols PP1 and 4. Minimization of the structure of PP1
revealed the presence of two low energy conformations (Fig. S1,
ESI†) that differ by only 1.0 kcal mol−1. The HOMO for lowest
energy isomer has little contribution from the phosphorus lone
pair (Fig. 3, left). The OH group projects a hydrogen atom
towards the phosphorus lone pair, suggesting OH⋯P hydrogen
bonding. Previous studies, however, suggest limited stability is
gained for EH⋯P hydrogen bonding interactions.55–57
Fig. 4 Calculations results for the radical cations of ortho-phosphinophenol
(left) and 3,5-di-tert-butyl-2-phosphinophenol
4 (right, DFT B3LYP/6-31G*).
Upper structure in each panel displays lowest energy conformation and lower
structure shows SOMO orbital.
of [4]˙+ shows a SOMO energy level at −10.5 eV. Both SOMOs
are lower in energy than the −10.0 eV threshold that predicts
enhanced air-stability. These criteria alone thus do not illumi-
nate any special electronic reasons for the differences in the
air-stability between PP1 and 4, and argue that the importance
lies more in their special reactivity with acid. The underlying
factors dictating stability of primary phosphines can be very
subtle. For example, we recently reported on 3-phosphino-2-
naphthol which shows no signs of decomposition in CDCl3
open to air for over a month.8 On the other hand, attempts to
produce isomeric 1-phosphino-2-naphthol by analogous
reduction of 1-(diethylphosphonate)-2-naphthol led only to
2-naphthol and loss of phosphorus.47
The lowest energy isomer of 4 is shown in Fig. 3 (right).
Like PP1, little phosphorus lone pair contribution is found for
the HOMO. The ortho-tert-butyl group directs the hydrogen
atoms on the phosphorus atom away from the bulky group.
The hydroxyl hydrogen atom, in response, now directed away
from the phosphorus atom and not available for any possible
OH⋯P hydrogen bonding interaction. These results thus meet
the first of the two criteria for enhanced air stability for a
primary phosphine.
Calculations on [PP1]˙+ and [4]˙+ produced the results
shown in Fig. 4. The lowest energy isomer of [PP1]˙+ shows a
SOMO energy level at −11.3 eV, while the lowest energy isomer
Synthesis and characterization of new 1,3-benzoxaphospholes
A series of bulky tert-butyl substituted 1,3-benzoxaphospholes
(5a–e) were synthesized by refluxing solutions of ortho-phos-
phinophenol 4 with imidoyl chlorides (Scheme 4) in THF.10
Compounds 5a–e were isolated as colorless or yellow crystal-
line solids. Each compound displays a 31P{1H} diagnostic reso-
nance between δ 77.2–92.2 ppm that is consistent with the
reported values for other benzoxaphospholes. The isolated
yields are modest, ranging from 20.9% to 46.4%. Under the
reaction conditions that generate HX, some additional 3,5-di-
Fig. 3 Calculations results for ortho-phosphinophenol (left) and 3,5-di-tert-
butyl-2-phosphinophenol 4 (right DFT B3LYP/6-31G*). Upper structure in each
panel displays lowest energy conformation and lower structure shows HOMO
orbital.
Scheme 4
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Table 1 Absorption and fluorescence data for 5a–e in CH2Cl2
R
λmax (nm)
ε (M−1 cm−1
)
λF,max (nm)
ΦF
5a
5b
5c
5d
5e
6a
4-MeC6H4
4-MeOC6H4
C6H5
4-BrC6H4
Ad
348
354
345
353
289
343
23 650
12 190
12 110
21 680
14 440
20 000
430
438
425
427
424
426
0.13
0.12
0.12
0.07
0.04
0.69
Fig. 5 ORTEP representations of the molecular structure of 5a (left) and 5c
(right). Selected bond lengths (Å) and angles (°) for 5a: P(1)–C(1), 1.708(1);
P(1)–C(3) 1.795(1); O(1)–C(1), 1.368(1); O(1)–C(2), 1.376(1); C(1)–P(1)–C(3),
88.64(5); C(1)–O(1)–C(2), 110.40(9). Selected bond lengths (Å) and angles (°) for
5c: P(1)–C(3), 1.703(2); P(1)–C(2) 1.799(2); O(1)–C(1), 1.377(2); O(1)–C(3),
1.374(2); C(2)–P(1)–C(3), 88.68(8); C(1)–O(1)–C(3), 110.4(1).
4-MeC6H4
a Data from ref. 10.
the optical properties depend on the nature of the R group.
The data reveal π–π* transitions for the compounds 5a–d
varied from 348 to 354 nm, and at 289 nm for compound 5e.
Aryl substituted compounds 5a–d displayed λem from 425 to
438 nm, while alkyl substituted compound 5e showed λem at
424 nm. While compounds 5 display comparable absorption
and fluorescence properties to previously reported BOP-Rs, the
quantum yields (ΦF) were noticeably lower, possibly due to
extra degrees of vibrational energy relaxation provided by
additional tert-butyl groups. Unlike the compounds such as 6,
compounds 5 show appreciable fluorescence in the solid state.
This fact is attributable to the presence of the tert-butyl groups
which prevent π-stacking and fluorescence quenching in the
solid state.
tert-butylphenol is formed, which not only decreases yields but
added challenges in purification of the benzoxaphospholes.
The single-crystal structures of 5a and 5c are very similar
and are shown in Fig. 5. The PvC bond lengths in 5a and 5c
are 1.708(1) and 1.703(2) Å, respectively. These distances are
slightly shorter than the PvC bond distance of 1.712(1) Å
determined for 2-(p-chlorophenyl)-1,3-benzoxaphosphole.58
Neither of these two structures shows evidence for π-stacking
in the crystal lattice. Within each structure the 2-aryl group is
rotated from the plane of the benzoxaphosphole by 24.3° and
17.6° for 5a and 5c, respectively. By contrast, the structures
of 2-(p-chlorophenyl)-1,3-benzoxaphosphole is completely
planar.58 The tert-butyl-groups in 5a and 5c are rather remote
to the 2-aryl substituents, thus subtle packing forces in the
crystals are likely responsible for deviations from planarity in
the conjugated backbones of these molecules (Fig. 4).
Conclusion
The bulky ortho-phosphinophenol 3,5-di-tert-butyl-2-phosphino-
phenol 4 has been prepared and characterized. Compound 4 is
unusually susceptible to an acid catalysed carbon–phosphorus
bond cleavage reaction that produces 3,5-di-tert-butyl-phenol
and PH3. Despite the sensitive nature of 4, it has been success-
fully utilized to produce a series of highly luminescent 2-R-4,6-
di-tert-butyl-1,3-benzoxaphospholes. Two of these new benzox-
aphospholes have been structurally characterized. Examination
of the air stability of the benzoxaphospholes shows that they
are more stable than derivatives not possessing tert-butyl
groups adjacent to the phosphorus atoms.
The tert-butyl groups, however, were predicted to increase the
air stability of the benzoxaphospholes in solution. Solutions of
5a and compound 6 (Chart 4), the direct analogue of 5a
bearing no tert-butyl groups in CDCl3 (0.71 g mL−1) were left
exposed to the atmosphere at room temperature and the
1
decomposition processes were monitored by 31P and H NMR
spectroscopy (as CDCl3 evaporated more CDCl3 was added). It
was found that 6 started to decompose after only 1 hour. After
3 days little 6 remains. Sterically encumbered 5a, however,
exhibits much greater air-stability. The solution of 5a required
33 days to decompose to the same extent that 6 does in 3 days.
1,3-Benzoxaphospholes are one of a small class of com-
pounds that having phosphaalkene units that display signifi-
cant fluorescence.10 The presence of the added tert-butyl
groups was not expected to greatly influence this key property.
Indeed, the new benzoxaphospholes 5, like previous examples,
are blue when irradiated with UV light. The specific absorption
and fluorescence spectral data for 5a–e in CH2Cl2 are pre-
sented in Table 1. As found for non-hindered BOP-R (Chart 1),
Experimental
General information
All air-sensitive operations were performed under nitrogen
atmosphere, using Schlenk line techniques or within a Vacuum
Atmospheres MBraun glove box unless otherwise stated. Tetra-
hydrofuran, hexanes and toluene were dried by distillation
from sodium benzophenone ketyl. Dichloromethane was dried
by distillation from CaH2 prior to use for UV-vis and fluore-
scence measurements. Benzimidoyl chlorides were prepared
according to literature procedures.59 2-Bromo-3,5-di-tert-butyl-
phenol was prepared by the reaction of 3,5-di-tert-butylphenol
with Br2.41 1H, 13C{1H} and 31P NMR spectra were recorded on
a Varian INOVA AS-400 or 600 spectrometers, and all chemical
shifts are referred to residual CHCl3 signals (for 1H NMR
Chart 4
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spectra) or to an external sample of 85% H3PO4 (for 31P NMR
3,5-Di-tert-butyl-2-phosphinophenol (4). To a solution of
spectra). Melting points were measured on a Mel-temp melting LiAlH4 (1.03 g, 27.0 mmol) in 150 mL THF was slowly added a
point apparatus. Elemental analyses were performed by solution of diethyl (2-hydroxy-4,6-di-tert-butylphenyl)phospho-
Robertson Microlit Laboratories (Madison, NJ). High resolu- nate (3.70 g, 10.8 mmol) in 50 mL THF at 0 °C. The mixture
tion mass spectrometry was performed by the University of was stirred at room temperature overnight. To the reaction
Michigan Mass Spectrometry facility using a VG (Micromass) mixture was slowly added a degassed saturated NH4Cl solution
70-250-S magnetic sector spectrometer. UV-vis and fluore- (∼5 mL). The mixture was then extracted with degassed
scence data were recorded using a Cary 50 Bio UV-Visible diethylether (∼100 mL) and the organic layer was separated
spectrophotometer and a Cary Eclipse spectrometer, respect- and dried over MgSO4. The solvent was removed in vacuo to
1
ively. Anthracene in ethanol was used as the quantum yield yield a colorless oil. Yield: 1.42 g (55.3%). H NMR (399.7 Hz):
standard.60
δ 7.11 (m, 1H), 6.86 (d, 1H, 4JHH = 1.6 Hz), 5.53 (br s, 1H), 3.68
(d, 2H, 1JPH = 198.4 Hz), 1.55 (s, 9H), 1.30 (s, 9H). 31P{1H} NMR
(161.8 Hz): δ −169.7 (s). 13C{1H} NMR (100.5 Hz): δ 158.2,
Experimental procedures
Diethyl (2-bromo-3,5-di-tert-butylphenyl)phosphate (2). To a 155.6 (d, Jpc = 16.7 Hz), 153.1, 116.0 (d, Jpc = 6.3 Hz), 109.8,
suspension of NaH (60% in mineral oil, 1.47 g, 36.8 mmol) in 107.4 (d, Jpc = 22.2 Hz), 37.3, 34.8, 31.7 (d, Jpc = 11.9 Hz), 31.1.
THF (50 mL) was slowly added a solution of 2-bromo-3,5-di- HRMS (EI, 70 eV): m/z 238.1487 (calc. 238.1487).
tert-butylphenol (10.0 g, 35.1 mmol) in THF (50 mL) at 0 °C.
The yellow mixture was stirred at 0 °C for 30 min, and then (5a). To
4,6-Di-tert-butyl-2-(4-methylphenyl)-1,3-benzoxaphosphole
solution of 3,5-di-tert-butyl-2-phosphinophenol
a
diethylchlorophosphate (5.40 mL, 36.8 mmol) was added. The (0.562 g, 2.36 mmol) in 10 mL THF was added a solution of
mixture was stirred at room temperature for 1 hour to yield a 4-methyl-N-phenylbenzimidoyl chloride (1.08 g, 4.68 mmol) in
brown solution. To the reaction mixture was slowly added satu- 20 mL THF. The reaction was refluxed for 96 hours to produce
rated NH4Cl solution (∼5 mL), followed by the addition of a yellow cloudy solution. After the reaction was cooled to room
100 mL of diethylether, then by addition of 100 mL of a satu- temperature, the mixture was filtered to remove a white pre-
rated brine solution. The organic layer was separated and then cipitate. The solvent was removed under vacuum from the fil-
dried over anhydrous Na2SO4. The solvent was removed under trate to yield a yellow solid. The solid was extracted with
vacuum to yield a brown oil. Yield: 13.9 g (94.0%). 1H NMR hexanes and filtered through Celite, and then passed through
(399.7 Hz): δ 7.34 (m, 1H), 7.27 (m, 1H), 4.27 (m, 4H), 1.52 (s, basic alumina to yield a colorless solution. The solvent was
4
3
9H), 1.35 (td, 6H, JPH = 1.2 Hz, JHH = 7.2 Hz), 1.29 (s, 9H). removed in vacuo to yield a colorless crystalline 5a. Yield:
1
31P{1H} NMR (161.8 Hz): δ −6.3 (s). 13C{1H} NMR (100.5 Hz): δ 0.167 g (20.9%). Mp: 92–94 °C. H NMR (399.7 Hz): δ 7.92 (m,
3
150.7, 148.8, 148.2 (d, Jpc = 5.6 Hz), 121.1, 115.7 (d, Jpc = 2.1 2H), 7.62 (m, 1H), 7.31 (m, 1H), 7.23 (d, 2H, JHH = 8 Hz), 2.39
Hz), 112.0 (d, Jpc = 7.9 Hz), 64.6 (d, Jpc = 6.3 Hz), 37.3, 34.7, (s, 3H), 1.59 (s, 9H), 1.41 (s, 9H). 31P{1H} NMR (161.8 Hz):
31.0, 29.7, 16.0 (d, Jpc = 6.8 Hz). HRMS ([M + H]+): m/z δ 85.8 (s). 13C{1H} NMR (100.5 Hz): δ 193.9 (d, Jpc = 54.1 Hz),
421.1138 (calc. 421.1138).
161.2 (d, Jpc = 4.0 Hz), 151.1 (d, Jpc = 9.7 Hz), 150.7 (d, Jpc =
Diethyl (2-hydroxy-4,6-di-tert-butylphenyl)phosphonate 3.1 Hz), 139.6 (d, Jpc = 4.7 Hz), 132.0 (d, Jpc = 13.6 Hz), 130.6
(3). To a solution of diethyl (2-bromo-3,5-di-tert-butylphenyl)- (d, Jpc = 44.3 Hz), 129.5, 124.6 (d, Jpc = 14.2 Hz), 117.2 (d, Jpc
phosphate (2.53 g, 6.01 mmol) in THF (75 mL) was added 7.5 Hz), 108.0, 36.9 (d, Jpc = 2.3 Hz), 35.2, 31.5, 31.0 (d, Jpc
=
=
3.40 mL n-BuLi (2.5 M in hexane, 8.41 mmol) at −78 °C. The 8.8 Hz), 21.5. HRMS (EI, 70 eV): m/z 338.1806 (calc. 338.1800).
mixture was stirred at −78 °C for 30 minutes, and then was UV(CH2Cl2): λmax, nm (ε, M−1 cm−1) 348 (23 650). Fluorescence
poured into a room temperature mixture of saturated NH4Cl (CH2Cl2): λex, 430 nm. Quantum yield (CH2Cl2): Φ 0.13.
solution (50 mL) and diethylether (50 mL). The organic layer
was separated and then dried over anhydrous Na2SO4. The (5b). To
4,6-Di-tert-butyl-2-(4-methoxyphenyl)-1,3-benzoxaphosphole
solution of 3,5-di-tert-butyl-2-phosphinophenol
a
solvent was removed under vacuum to yield a yellow oil. The (0.458 g, 1.92 mmol) in 10 mL THF was added a solution of
material was further purified by distillation using Buchi glass 4-methoxy-N-phenylbenzimidoyl chloride (0.519 g, 2.11 mmol)
oven (B-585 kugelrohr, 190 °C, 0.05 mm Hg) to initially afford in 20 mL THF. The reaction was refluxed for 38 hours to
a colorless oil, which after cooling to room temperature, produce a yellow cloudy solution. After the reaction was cooled
yielded a colorless crystalline solid. Yield: 1.55 g (75.2%). Mp: to room temperature, the mixture was filtered to remove a
39–40 °C. 1H NMR (399.7 Hz): δ 11.9 (br s, 1H, 4JPH = 0.8), 7.14 white precipitate. The solvent was removed by vacuum to yield
4
4
4
(dd, 1H, JHH = 2.0 Hz, JPH = 6.0 Hz), 6.85 (dd, 1H, JHH
=
a yellow solid. The solid was extracted with hexanes and fil-
4
2.0 Hz, JPH = 4.8 Hz), 4.19 (m, 4H), 1.46 (s, 9H), 1.35 (td, 6H, tered through Celite and then passed through basic alumina
4JPH = 0.4 Hz, JHH = 6.8 Hz), 1.28 (s, 9H). 31P{1H} NMR (161.8 to give a yellow solution. The solvent was removed in vacuo to
4
Hz): δ 27.6 (s). 13C{1H} NMR (100.5 Hz): δ 164.6 (d, Jpc
=
yield a yellow solid. Recrystallization from toluene afforded a
7.9 Hz), 157.1 (d, Jpc = 2.8 Hz), 154.3 (d, Jpc = 8.2 Hz), 117.1 (d, yellow crystalline 5b. Yield: 0.250 g (36.8%). Mp: 99–100 °C. 1H
Jpc = 14.2 Hz), 114.0 (d, Jpc = 12.1 Hz), 103.6 (d, Jpc = 177.1 Hz), NMR (399.7 Hz): δ 7.96 (m, 2H), 7.59 (m, 1H), 7.29 (m, 1H),
62.4 (d, Jpc = 5.8 Hz), 38.2 (d, Jpc = 3.1 Hz), 35.0, 32.6, 30.7, 16.1 6.94 (m, 2H), 3.87 (s, 3H), 1.59 (s, 9H), 1.40 (s, 9H). 31P{1H}
(d, Jpc = 6.9 Hz). Elemental Analyses Calc. for C18H31O4P, NMR (161.8 Hz): δ 80.9 (s). 13C{1H} NMR (150.0 Hz): δ 193.8 (d,
C 63.14%, H 9.13%; found: C 63.02%, H 8.88%.
Jpc = 54 Hz), 161.1 (d, Jpc = 3.8 Hz), 160.7 (d, Jpc = 4.5 Hz), 151.0
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(d, Jpc = 9.8 Hz), 150.5 (d, Jpc = 3.0 Hz), 130.6 (d, Jpc = 44.1 Hz), 1-adamantane-benzimidoyl chloride (1.32 g, 4.82 mmol) in
127.8 (d, Jpc = 13.4 Hz), 126.2 (d, Jpc = 14.0 Hz), 117.1 (d, Jpc 20 mL THF. The reaction was refluxed for 17 hours to produce
=
7.4 Hz), 114.1, 107.9, 55.4, 36.9 (d, JPC = 2.1 Hz), 35.2, 31.5, a yellow cloudy solution. After the reaction was cooled down to
31.0 (d, JPC = 8.9 Hz). HRMS (EI, 70 eV): m/z 354.1735 (calc. room temperature a white precipitate was removed by fil-
354.1749). UV(CH2Cl2): λmax, nm (ε, M−1 cm−1) 354 (12 190). tration, and the solvent was removed under vacuum to yield a
Fluorescence (CH2Cl2): λex, 438 nm. Quantum yield (CH2Cl2): yellow solid. The solid was extracted with hexanes and the
Φ 0.12.
solution was filtered through Celite and then passed through
4,6-Di-tert-butyl-2-phenyl-1,3-benzoxaphosphole (5c). To a basic alumina to give a colorless solution. The solvent was
solution of 3,5-di-tert-butyl-2-phosphinophenol (1.00 g, removed in vacuo to yield white solid 5e. Yield: 0.776 g
1
4.21 mmol) in 20 mL THF was added a solution of N-phenyl- (46.4%). Mp: 103–104 °C. H NMR (399.7 Hz): δ 7.54 (m, 1H),
benzimidoyl chloride (0.998 g, 4.63 mmol) in 20 mL THF. The 7.27 (m, 1H), 2.11 (m, 3H), 2.09 (m, 6H), 1.80 (m, 6H), 1.56 (s,
reaction was refluxed for 13 hours to produce a yellow cloudy 9H), 1.38 (s, 9H). 31P{1H} NMR (161.8 Hz): δ 77.2 (s). 13C{1H}
solution. After the reaction was cooled to room temperature, a NMR (100.5 Hz): δ 210.8 (d, Jpc = 61.3 Hz), 161.4 (d, Jpc
white precipitate was removed by filtration, and the solvent 3.1 Hz), 151.2 (d, Jpc = 9.7 Hz), 149.9 (d, Jpc = 2.5 Hz), 129.5 (d,
was removed by vacuum to yield a yellow solid. The solid was Jpc = 45.1 Hz), 116.7 (d, Jpc = 6.9 Hz), 107.8, 42.0 (d, Jpc
=
=
extracted with hexanes and filtered through Celite and then 9.3 Hz), 39.6 (d, Jpc = 10.8 Hz), 36.9 (d, Jpc = 2.4 Hz), 36.7, 35.1,
passed through basic alumina to yield a colorless solution. 31.5, 31.0 (d, Jpc = 9.0 Hz), 28.4. HRMS (EI, 70 eV): m/z
The solvent was removed in vacuo to yield a colorless crystal- 382.2422 (calc. 383.2455). UV(CH2Cl2): λmax, nm (ε, M−1 cm−1
)
1
line solid 5c. Yield: 0.630 g (46.2%). Mp: 98–100 °C. H NMR 289 (14 440). Fluorescence (CH2Cl2): λex, 424 nm. Quantum
(399.7 Hz): δ 8.03 (m, 2H), 7.63 (m, 1H), 7.43 (m, 2H), 7.38 (m, yield (CH2Cl2): Φ 0.04.
1H), 7.32 (m, 1H), 1.60 (s, 9H), 1.41 (s, 9H). 31P{1H} NMR
Stability studies. Air-stability tests were performed in NMR
tubes with deuterated solvent and opened to air at room temp-
(161.8 Hz): δ 89.4 (s). 13C{1H} NMR (100.5 Hz): δ 193.5 (d, Jpc
=
54.0 Hz), 161.4 (d, Jpc = 4.0 Hz), 151.3 (d, Jpc = 9.8 Hz), 150.9 erature. As CDCl3 evaporated, more CDCl3 was added to
(d, Jpc = 3.0 Hz), 134.7 (d, Jpc = 13.2 Hz), 130.6 (d, Jpc = 44.5 replenish the solution. For solutions studied under nitrogen,
Hz), 129.3 (d, Jpc = 4.6 Hz), 128.7, 124.6 (d, Jpc = 14.4 Hz), 117.2 the samples were prepared in glove bags and transferred to
1
(d, Jpc = 7.6 Hz), 108.1, 36.9 (d, Jpc = 2.1 Hz), 35.2, 31.5, 31.0 (d, J-Young NMR tubes. Samples were monitored by H NMR and
Jpc = 8.8 Hz). HRMS (EI, 70 eV): m/z 324.1644 (calc. 324.1643). 31P NMR spectroscopy.
UV(CH2Cl2): λmax, nm (ε, M−1 cm−1) 345 (12 110). Fluorescence
(CH2Cl2): λex, 425 nm. Quantum yield (CH2Cl2): Φ 0.12.
Computational studies. Calculations were undertaken using
DFT (6-31G*) level of theory as implemented in the program
4,6-Di-tert-butyl-2-(4-bromophenyl)-1,3-benzoxaphosphole SPARTAN’10 (Wavefunction, Inc., Irvine, CA) and as previously
(5d). To
solution of 3,5-di-tert-butyl-2-phosphinophenol described.53
a
(1.00 g, 4.20 mmol) in 20 mL THF was added a solution of
4-bromo-N-phenyl-benzimidoyl chloride (1.36 g, 4.62 mmol) in
20 mL THF. The reaction was refluxed for 13 hours to produce
a yellow cloudy solution. After the reaction was cooled down to
Acknowledgements
room temperature a white precipitate was removed by fil- We are grateful to the National Science Foundation for support
tration, and the solvent was removed under vacuum to yield a of this work (NSF-CHE 1150721).
yellow solid. The solid was extracted with hexanes and the
solution was filtered through Celite and then passed through
basic alumina to yield a yellow solution. The solvent was
removed in vacuo to yield a yellow solid. Recrystallization from
Notes and references
hexanes provided a yellow crystalline 5d. Yield: 0.730 g
(43.1%). Mp: 102–103 °C. H NMR (399.7 Hz): δ 7.88 (m, 2H),
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7.61 (m, 1H), 7.54 (m, 2H), 7.32 (m, 1H), 1.59 (s, 9H), 1.40 (s,
9H). 31P{1H} NMR (161.8 Hz): δ 92.2 (s). 13C{1H} NMR (150.0
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151.4 (d, Jpc = 2.3 Hz), 133.6 (d, Jpc = 13.4 Hz), 131.9 (d, Jpc
1.1 Hz), 130.4 (d, JPC = 44.3 Hz), 126.0 (d, Jpc = 14.4 Hz), 123.2
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=
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14872 | Dalton Trans., 2013, 42, 14866–14874
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