Synthesis of an Azaphosphatriptycene and Its Rhodium Carbonyl Complex

Article abstract of DOI:10.1021/acs.organomet.9b00081

A 10-aza-9-phosphatriptycene is accessible on a gram scale, in three laboratory steps from commercially available precursors. Infrared spectroscopy of a rhodium(I) carbonyl complex bearing this ligand reflects the weak σ-donor/strong π-acceptor character of the phosphine; the solid-state structure reveals moderate steric demand. This ligand supports highly active catalysts for the hydroformylation of cyclic alkenes.

Full text of DOI:10.1021/acs.organomet.9b00081

Communication
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Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis of an Azaphosphatriptycene and Its Rhodium Carbonyl
Complex
Yu Cao,† Jonathan W. Napoline,†,§ John Bacsa,‡ Pamela Pollet,† Jake D. Soper,†
and Joseph P. Sadighi*,†
†School of Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
‡X-ray Crystallography Center, Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta Georgia 30322, United
States
S
* Supporting Information
ABSTRACT: A 10-aza-9-phosphatriptycene is accessible on a gram
scale, in three laboratory steps from commercially available
precursors. Infrared spectroscopy of a rhodium(I) carbonyl complex
bearing this ligand reflects the weak σ-donor/strong π-acceptor
character of the phosphine; the solid-state structure reveals
moderate steric demand. This ligand supports highly active catalysts
for the hydroformylation of cyclic alkenes.
Herein we report the preparation of an azaphosphatripty-
olycyclic phosphines have been the subject of considerable
Pinterest for their combination of structural rigidity and cene ligand via a straightforward sequence. This sequence
affords the ligand reproducibly, on a gram scale. We have
examined this phosphine as a supporting ligand for rhodium-
catalyzed hydroformylation, in which weak σ-donor and strong
π-acceptor ligands have enabled remarkable advances in
activity.9−21 Although the rigid, polycyclic phosphabarrelene
framework has been found to be highly effective,22,23
phosphatriptycenes had not been explored in this context. A
combination of this ligand with a suitable rhodium precatalyst
gives rise to an isolable complex for spectroscopic and
structural study or to a highly active catalyst system for
hydroformylation of terminal and cyclic olefins.
unusual electronic properties. Phosphatriptycenes, for example,
possess a high fraction of s orbital character in the phosphorus
lone pair as a result of their constrained C−P−C angles.1,2 The
initial report on the first ligand in this class, 10-aza-9-
phosphatriptycene, noted its extremely high field 31P chemical
shift of −80 ppm.1 A 9-phosphatriptycene variant displayed
electronic character more similar to that of triphenylstibine
than that of triphenylphosphine.3 Tsuji, Tamao, et al.
synthesized a series of 9-phospha-10-silatriptycenes, studying
a phosphine selenide and a platinum(II) complex to gain
insight into their electronic properties.4
Scheme 1a shows the synthesis of 2,7,14-tri-tert-butyl-10-
aza-9-phosphatriptycene (1) from the corresponding tris(2-
bromoaryl)amine, prepared in two facile and high-yield
steps24,25 from commercial precursors. Triple lithium−
bromine exchange, as used to prepare tris(2-lithiophenyl)-
phosphine,4,5 proved effective in generating the tris(2-
lithiophenyl)amine intermediate. Crystalline, air-stable tris-
(2,4-di-tert-butylphenyl) phosphite proved more convenient
than triphenyl phosphite as the electrophile and gave higher
yields; we believe that increased steric demand favors closing
of the bicyclic framework over intermolecular side reactions.
Crystallization of 1 from methanol effectively removes the
byproduct 2,4-di-tert-butylphenol and allows isolation of the
air-stable product in 71% yield based on tris(2-bromoaryl)-
amine. The 31P NMR resonance for 1 appears at very high
field, δ −77.0 ppm (CDCl3), similar to that of 10-aza-9-
phosphatriptycene.1 This value falls well upfield from those of
The difficulty of early phosphatriptycene syntheses may have
limited their exploration in catalysis, but more recent
approaches afford easier access. Efficient generation of tris(2-
lithiophenyl)phosphine enabled the synthesis of 10-sila-9-
phosphatriptycenes4,5 and, more recently, salts of the anionic
9-phosphatriptycene-10-phenylborate.6,7 Phosphatriptycenes
have been examined as ligands in catalytic reactions such as
the Stille coupling.3 More recently, a silica-tethered 10-sila-9-
phosphatriptycene supported the effective Suzuki−Miyaura
coupling of chloroarenes.5
The parent 10-aza-9-phosphatriptycene was synthesized
from tris(2-bromophenyl)amine,1 but more substituted tris-
(2-bromoaryl)amines are easier to prepare. Hellwinkel et al.
synthesized the trimethyl-substituted 10-aza-9-phosphatripty-
cene from tri-p-tolylamine by a sequence of bromination,
halogen−lithium exchange, and quenching with triphenyl
phosphite.8 They reported difficulties in reproducing this
route, however, and subsequently adapted Bickelhaupt’s
elegant but synthetically demanding route for 9-phospha-
triptycene.
Received: February 7, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00081
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
95.00(13) to 95.14(13)°; those in [(Ph3P)Rh(acac)(CO)]
range from 103.37(9) to 104.24(9)°. The effective cone angle
for 1 in complex 2 is about 151°, somewhat larger than that of
145° for triphenylphosphine.29 The C−N−C angles range
from 107.5(2) to 108.9(2)°.
Scheme 1. Synthesis of a 10-Aza-9-phosphatriptycene and
Its Rh(I) Carbonyl Complex
Combinations of 1 with the precatalyst [Rh(acac)(CO)2]
were examined in the hydroformylation of 1-octene, for
comparison with the well-studied triphenylphosphine.30 The
results are summarized in Table S1. Ligand 1 proved
comparable to triphenylphosphine in several respects. The
key advantage of 1 was the high activity of the catalyst system
formed in situ, at ligand:metal ratios lower than those preferred
for triphenylphosphine.31
In light of the modest selectivity but high activity of the 1:
[Rh] system, we examined the hydroformylation of less
reactive substrates. The Ph3P/[Rh] catalyst system exhibits
low activity in the hydroformylation of cyclic alkenes under
mild conditions.31 In contrast, bulky π-acidic phosphite ligands
enable high catalyst activity toward these substrates.12
Cyclohexene was examined first (Table 1). Because
a
b
Conditions: THF/n-C5H12, −78 °C, 2 h. Conditions: Ar = 2,4-di-
tert-butylphenyl; THF/n-C5H12, −78 °C to room temperature, then
reflux, 96 h. Yield for reactions 1 and 2: 71%. Conditions: PhCH3,
room temperature, 20 min. Yield: 58%.
c
9-phosphatriptycene (δ −64.8 ppm),2 9-phospha-10-silatripty-
cenes (δ ca. −44 ppm),4,5 and the 9-phosphatriptycene-10-
phenylborate anion (also δ ca. −44 ppm).6,7
The rthodium(I) complex [(1)Rh(acac)(CO)] (2) was
prepared by reaction of 1 with [Rh(acac)(CO)2] (Scheme 1b).
The 31P NMR spectrum of 2 displays a doublet resonance at δ
a
Table 1. Hydroformylation of Cyclohexene
1
−1.3 ppm, with JP−Rh = 189 Hz. For comparison, the
reaction
time (h)
reaction
pressure
(bar)
aldehyde
yield (%)
aldehyde
corresponding triphenylphosphine complex gives δ +48.6 ppm
TOF (h−1
)
and 1JP−Rh = 177 Hz.26 The infrared stretching frequency νCO
,
entry
1
2
3
4
5
temp (°C)
2
2
2
2
4
100
100
80
100
100
20
20
20
15
20
85
73
49
64
100
2130
1830
1230
1610
1250
reflecting the σ-donor and π-acceptor character of supporting
ligands,27 is 1985 cm−1 for complex 2. This value falls between
those of 1975 cm−1 for [(Ph3P)Rh(acac)(CO)] and 2006
cm−1 for {[(PhO)3P]Rh(acac)(CO)}.28
The solid-state structure of 2 was determined by single-
crystal X-ray diffraction (Figure 1). Complex 2 is planar with a
Rh−P bond distance of 2.2080(8) Å, slightly shorter than that
of 2.2418(9) Å in [(Ph3P)Rh(acac)(CO)].26 Within the
bound phosphatriptycene the C−P−C angles range from
b
a
Conditions unless specified otherwise: [Rh(acac)(CO)2] 0.020 mol
b
%; ligand 1 0.040 mol %; [cyclohexene]initial = 0.86 M. Conditions:
[cyclohexene]initial = 0.56 M.
addition/elimination gives no change, and no alcohol or
alkane byproducts were detected using this system, alkene
conversion and the yield of aldehyde are identical. Quantitative
conversion was observed using 0.020 mol % [Rh] precatalyst
and 0.040 mol % 1 (entry 5).
Next we examined the reactivity of this catalyst system
toward electron-rich heteroatom-substituted alkenes. Formyl
furan and formyl pyran derivatives, for example, are relevant to
natural product synthesis.13 Results for the hydroformylation
of 2,3-dihydrofuran and 3,4-dihydro-2H-pyran are shown in
Scheme 2.
The conversion of 2,3-dihydrofuran to a mixture of the 2-
and 3-formyl derivatives proceeded under mild conditions. The
selectivity for 2-formyltetrahydrofuran, while moderate, was
higher than that of the Ph3P/[Rh] system under comparable
conditions.13 The hydroformylation of 3,4-dihydro-2H-pyran
proceeded to lower conversion under similar conditions,
perhaps due to greater hindrance about the CC bond and
the absence of ring strain,33 and displayed no significant
preference for functionalization at the 2- or 3-positions.
In conclusion, a functionalized azaphosphatriptycene has
been synthesized by a concise route from inexpensive
precursors. The broad synthetic accessibility of triaryl-
amines34−36 should permit considerable variation on this
framework. Consistent with its expected weak σ basicity and
strong π acidity, this ligand combines with rhodium to give
Figure 1. Solid-state structure of [(1)Rh(acac)(CO)], with ellipsoids
at 50% probability. Calculated hydrogen atoms are omitted for clarity.
Selected interatomic distances (Å) and angles (deg): Rh1−P1
2.2080(8), Rh1−C1 1.821(3), Rh1−O2 2.067(2), Rh1−O3
2.044(2), C1−O1 1.142(4); P1−Rh1−CO 86.43(10), C2−P1−C13
95.00(13), C2−P1−C22 95.14(13), C13−P1−C22 95.13(13).32
B
DOI: 10.1021/acs.organomet.9b00081
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
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* Supporting Information
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Accession Codes
CCDC 1857239 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for J.P.S.: joseph.sadighi@chemistry.gatech.edu.
ORCID
Jake D. Soper: 0000-0002-1961-8076
Joseph P. Sadighi: 0000-0003-1304-1170
■
Present Address
§Department of Chemistry, Saint Anselm College, Manchester,
New Hampshire 03102, United States.
Notes
́
́
(17) Fernandez, E.; Ruiz, A.; Claver, C.; Castillon, S.; Polo, A.;
Piniella, J. F.; Alvarez-Larena, A. Regio- and Stereoselective
Hydroformylation of Glucal Derivatives with Rhodium Catalysts.
Organometallics 1998, 17, 2857−2864.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
̈
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Oxyfunctionalized Phosphonite Ligands for the Hydroformylation of
Isomeric n-Olefins. Angew. Chem., Int. Ed. 2000, 39, 1639−1641.
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conditions with new classes of π-acceptor ligands. Chem. Commun.
1996, 2071−2072.
We thank the American Chemical Society Petroleum Research
Fund (Award Number 54739-ND3) for generous support of
this project and Peach State Laboratories, Inc. for support to
J.W.N. Profs. Charles L. Liotta and Christopher W. Jones
kindly allowed us the use of their groups’ Parr reactors and gas
chromatographs, and Dr. Robert A. Braga allowed the use of an
FT-IR spectrometer. We thank them and Prof. E. Kent
Barefield for helpful discussions.
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catalysts for the efficient hydroformylation of terminal and internal
olefins. J. Chem. Soc., Perkin Trans. 1 1997, 1, 2681−2682.
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