Extremely electron-rich, boron-functionalized, icosahedral carborane-based phosphinoboranes

Article abstract of DOI:10.1021/om301116x

We have prepared the first examples of B9-connected trivalent aryl and alkyl phosphinoborane species via Pd-catalyzed phosphination of 9-iodo-meta-carborane. Our studies highlight the unique electronic features of the B9-connected meta-carboranyl moiety a

Full text of DOI:10.1021/om301116x

Communication
pubs.acs.org/Organometallics
Extremely Electron-Rich, Boron-Functionalized, Icosahedral
Carborane-Based Phosphinoboranes
Alexander M. Spokoyny,* Calvin D. Lewis,† Georgiy Teverovskiy, and Stephen L. Buchwald*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S
* Supporting Information
ABSTRACT: We have prepared the first examples of B9-connected trivalent aryl and
alkyl phosphinoborane species via Pd-catalyzed phosphination of 9-iodo-meta-
carborane. Our studies highlight the unique electronic features of the B9-connected
meta-carboranyl moiety as compared to its C1-based analogue. This work suggests that
the B9-functionalized meta-carboranyl substituent in these ligands exhibits more
electron-releasing character than any other known carbon-based substituent, ultimately
laying the foundation for a new class of phosphine ligands with extremely electron-rich
character.
To evaluate this hypothesis, access to both C−P and B−P
carboranyl-based species is required. While the chemistry of C-
functionalized icosahedral carborane-based phosphines is well
established,9 its B-functionalized counterparts are largely
unknown. The only experimental evidence to support the
existence of such a species was provided by Bregadze and
Kabachnik in 1992,10 who reported the syntheses of B9-
functionalized meta- and ortho-carboranyl species (C2B10H11-
PCl2) in 5% and 10% yields, respectively, upon UV photolysis
of mercury carboranyl complexes and phosphorus trichloride.
We envisioned that transition metal-catalyzed cross-coupling
would provide a suitable route to phosphorus-functionalized B-
based phosphinocarborane derivatives. Recent work by
Hawthorne, for example, utilized large, electron-rich phosphine
ligands capable of supporting active Pd(0)/Pd(II) catalytic
species in a facile catalytic amidation of B-iodocarboranes,11
wherein difficulties associated with the oxidative addition of the
B−I moiety could be circumvented by judicial choice of the
ligand.
risubstituted phosphines (PR3) represent an important
Tclass of ligands in modern chemistry largely due to the
ease with which their steric and electronic properties can be
rationally tuned using various carbon-rich functional groups
(R).1,2 Yet, the current pool of substituents provides only a
finite window for the modulation of the ligand’s electronic
character.3 Thus, chemistry faces a fundamental challenge in
identifying new and robust substituents that would overcome
these limitations.
Three-dimensional aromatic icosahedral dicarba-closo-dodec-
aborane clusters (also referred to as carboranes, general formula
C2B10H12) have been known to exhibit dramatically different
electronic effects on the substituents attached to different
vertices (for cluster numbering scheme, see SI).4 As a
consequence of a nonuniform electron distribution and
electronegativity differences between boron and carbon,
substituents attached at the carbon vertices usually experience
a strong electron-withdrawing effect, whereas the substituents
on boron atoms located furthest from the carbons experience a
strong electron-donating effect. This fundamental property of
carboranes was explored by Hawthorne for tuning the Lewis
acidity of Hg(II) in mercurocarborand species5 and later by
In order to access B-functionalized phosphines, we utilized a
modified Pd-catalyzed protocol used previously for phosphina-
tion of aryl halides.12
Upon heating 9-iodo-meta-carborane (2, Figure 1A, synthe-
sized in a manner analogous to the one reported by the Jones
and Hawthorne groups13) with diphenylphosphine in toluene
at 120 °C in the presence of Pd2dba3/DIPPF (6 mol %/12 mol
% ratio; dba, dibenzylideneacetone; DIPPF, 1,1′-bis(di-
isopropylphosphino)ferrocene) catalyst and Cs2CO3, we
observed the formation of a new species via in situ 31P NMR
spectroscopy (Figure 1).14
A broad signal resembling a quartet (JP−B = 58 Hz) suggested
that a B−P bond was likely formed (see SI). After prolonged
heating of this reaction mixture for 36 h, complete
consumption of the phosphine starting material was observed
Teixidor and Vinas, who observed that a methyl substituent
̃
attached to several boron vertices in these clusters acts as an
electron-withdrawing group.6 This effect has been somewhat
overlooked in the past, resulting in a number of papers
incorrectly generalizing the electronic properties of icosahedral
carboranes as “electron-withdrawing”.7 A recent report
suggested that icosahedral carborane substituents could enable
electronic tunability of chalocogen-based ligands that is greater
in magnitude than with any known carbon-based functional
groups.8 Specifically, by virtue of vertex differentiation one can
access ligands that are sterically invariant, yet either extremely
electron-poor or electron-rich.8 This begs the question whether
such tunability can be translated to other heteroatom-based
ligands such as phosphines.
Received: November 20, 2012
Published: November 30, 2012
© 2012 American Chemical Society
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Figure 2. (A, B) Comparison of the reactivity of ligands 3a and 3a′
with Pt(COD)Cl2. (C) Crystallographically derived molecular
structure of 4a depicted with 50% thermal ellipsoid probabilities (H
atoms omitted for clarity). (D) 31P{1H} NMR spectrum of 4a.
reactive (see SI, Section 3). In total, these observations are
consistent with recent work on carborane-based ligands
featuring sulfur and selenium atoms on B9 and C1 vertices of
the meta-carboranyl cluster (vide supra).
Figure 1. (A) Synthesis and (B, C) selected spectroscopic data for B9-
functionalized 9-meta-carboranyl phosphine species 3a (R = Ph). (D)
Crystallographically derived molecular structure of 3a depicted with
50% thermal ellipsoid probabilities.
Using the developed synthetic protocol, we were able to
cross-couple 2 with four electron-rich secondary phosphines,
thus obtaining alkyl-based phosphinoboranes 3b−3e (Figure
t
3A and SI). Attempts to cross-couple Bu2PH using these
(31P NMR). Subsequent workup and purification of the
reaction mixture yielded a crystalline substance, which was
subjected to a full NMR spectroscopic analysis. In particular,
the 11B NMR spectrum of 3a exhibits the characteristic pattern
of the B9-substituted meta-carborane cluster (Figure 1B),
where the signal at ca. δ −3 displays no coupling to the proton
and integrates 1:9 to the rest of the resonances in the spectrum.
A definitive structural confirmation for 3a was established using
single-crystal X-ray diffraction. The solid-state structure of 3a
features a trivalent phosphorus center connected to two carbon
atoms on the phenyl rings and a B9 atom of the meta-
carboranyl moiety (Figure 1D). While considering the
previously studied phosphinoboranes by Noth and Power, we
̈
found that the B−P bond length in 3a (1.948 Å) best
corresponds to the presence of a purely single-bond interaction
between the B and P atoms.14 This interaction is also
reminiscent of the diphenylphosphineborabenzene species
described by Fu,14c where delocalization of the LUMO orbital
on the B atom renders π-donation from the P atom-centered
lone electron pair unfavorable. Importantly, 3a was found to be
stable both in solution and as a solid under ambient conditions
for several weeks.
Drastic differences between 3a and 3a′ (C1-based analogue
of 3a;15 see SI and Figure 2) were observed upon reaction with
Pt(COD)Cl2 (COD, 1,4-cyclooctadiene) under similar con-
ditions. Stirring one or two equivalents of 3a and Pt(COD)Cl2
in dichloromethane resulted in a mixture of Pt(II) bis-
phosphine complexes 4a and 4b (Figure 2 and SI).
Figure 3. (A) Synthesized alkyl-based phosphinoboranes (3b−3e).
For all compounds (except for 3e), the yield is calculated based on the
combination of protection/deprotection steps. (B) Syntheses of m-
carborane-based phosphinoborane-BH3 adducts and their subsequent
deprotection. (C, D) Selected NMR spectroscopic data for 5d. (E)
Crystallographically derived molecular structure of 5d depicted with
50% thermal ellipsoid probabilities.
On the other hand, no coordination of 3a′ to the Pt(II)
precursor was observed (Figure 2C). Prolonged reaction times
(3 days) or using an excess of 3a′ (3 equiv) resulted in no
conversion. Given that ligands 3a and 3a′ are isosteric, the
observed differences in chemical reactivity can most reasonably
be attributed to the electronic influences of B9- and C1-
functionalized meta-carboranyl moieties. Indeed, natural bond
orbital analysis of the optimized structures of 3a and 3a′ further
corroborates the drastic differences in partial charges and lone
pair energies located on the phosphorus atoms of these ligands,
showing that 3a is significantly more electron-rich and therefore
conditions did not provide any phosphinoborane product. This
is likely due to the size of Bu2PH, ultimately rendering the
t
transmetalation step ineffecient. Due to difficulties separating
the DIPPF ligand from the target phosphinoboranes on silica
gel, 3b−3d mixtures were converted to their BH3 adducts and
purified by recrystallization, yielding pure 5b−5d. In all cases, a
characteristic shift of approximately 25−37 ppm in the 31P
NMR spectra was observed, consistent with the formation of
borane-based adducts (Figure 3B and SI).
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The 11B NMR spectra of 5b−5d revealed characteristic
11BH3 resonances at ca. δ 38−41 (Figure 3C and SI), which
integrate to a 1:10 ratio to the corresponding borane regions of
the meta-carboranyl moieties. Relatively small values for
coordination chemical shifts (ccs) observed from 31P NMR
spectra of these species compared to their free phosphine
analogues (+25−37 ppm) is consistent with the ccs values
reported for other bulky phosphine-BH3 adducts.16 Borane
adducts 5b−5d were found to be bench stable both in solution
and in the solid state. This observed stability is in contrast to
several previously reported phosphineborane-BH3 species,
wherein scrambling of a BH3 and a boryl moiety via hydride
transfer occurred, ultimately resulting in the decomposition of
the parent species.17 This further reinforces the notion that the
meta-carboranyl boryl moiety serves as an innocent substituent
and is chemically more stable than many previously studied
boryl systems. Upon reacting 5b−5d with an excess of a weakly
nucleophilic base (1,4-diazabicyclo[2.2.2]octane, DABCO) for
18 h in THF at 70 °C, we observed nearly quantitative
conversion of the parent borane species to the free phosphine
species 3b−3d via in situ 31P NMR spectroscopy. These
substances were then purified via flash column chromatography
on silica gel, yielding the pure phosphines. Overall,
phosphinoborane species 3b−3e were found to be stable
enough to be manipulated at ambient conditions without
noticeable oxidation. All other characterization data for 3b−3e
are consistent with the proposed structural formulations (see
SI).
Cl (2.456 Å) interaction (for comparison, in trans-Rh-
(PCy3)2(CO)Cl, the Rh−C bond is 1.748 Å and Rh−Cl is
2.388 Å),18 suggesting that phosphine 3b is more electron-rich
than PCy3. Finally, the IR spectroscopic data for ν[CO] in
complexes 6a−6e provide strong evidence supporting our
computational analyses (Figure 4B). We found that ν[CO] in
species 6b−6e are consistently lower than in any other known
trans-Rh(PR3)2(CO)Cl complexes (the lowest value reported
to date is at 1943 cm−1 for PCy3).18 Significantly, ν[CO] of
1981 cm−1 in 6a′ indicates that ligand 3a′ is drastically less
electron-rich than its B9-connected analogue, 3a. Thus
crystallographic and IR spectroscopy data obtained for Rh(I)
complexes bearing ligands 3b−3e suggest that these
phosphinoborane species are more electron-rich than any
known trivalent phosphine species containing only carbon-rich
substituents.
In conclusion, we have discovered a new class of stable and
extremely electron-rich phosphinoboranes featuring the B9-
functionalized meta-carboranyl substituent. Consistent with the
recent study on carboranyl thioether and selenol ligands,8 we
observe that the B9-based meta-carboranyl moiety exhibits
more electron-releasing character than any carbon-based
substituent and thus can potentially provide chemists with a
new approach to overcome the fundamental electronic property
limitations associated with carbon-based substituents pertaining
to ligand design. The phosphinoboranes reported herein can
also be potentially interesting molecules to explore in the
context of transition metal-based1 and -free19 catalysis.
Density functional theory calculations on the geometry-
optimized structures of 3b−3e suggest that these species are
more electron-rich than any other known alkyl-based
phosphine ligands (vide supra and SI). To experimentally
determine the electronic properties of 3a−3e, we synthesized
trans-Rh(PR3)2(CO)Cl complexes, bearing the carbonyl (CO)
ligand as a spectroscopic handle. The infrared (IR) carbonyl
stretching frequency was previously shown to correlate
extremely well with the electronic properties of the phosphine
ligands in this type of Rh(I) complexes.18 Reacting ligands 3a−
3e with the Rh(I) precursor in CH2Cl2 resulted in a
quantitative formation of 6a−6e as observed by in situ 31P
NMR spectroscopy (Figure 4).
ASSOCIATED CONTENT
* Supporting Information
Experimental details, crystallographic data (CIF), NMR and IR
spectra, and details for computational experiments. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: spokoyny@mit.edu; sbuchwal@mit.edu.
■
Present Address
†School of Medicine, University of Missouri, Columbia,
Missouri 65211, United States
Single crystals grown from solutions of 6a and 6b further
confirmed our structural assignment for these species through
X-ray diffraction studies. Notably, for 6b we observed an
extremely short Rh−C bond (1.717 Å) and an elongated Rh−
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Institutes of Health for financial support
of this project (GM46059) and a fellowship to A.M.S.
(1F32GM101762). A.M.S. is grateful to Prof. Vincent Lavallo
(UCR) for helpful discussions and to Dr. Tina Li (Dow
Chemicals) and Mr. James Colombe (MIT) for commenting
on the paper. We thank Dr. Peter Muller (MIT) for assistance
̈
with X-ray crystallography. The departmental X-ray diffraction
instrumentation was purchased with the help of funding from
the National Science Foundation (CHE-0946721).
REFERENCES
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Figure 4. (A) Syntheses of Rh(I) complexes bearing m-carborane-
based phosphinoborane ligands. (B) IR spectroscopic data for
carbonyl stretches ν[CO] in 6a−6e. (C, D) Crystallographically
derived molecular structures of 6b and 6a depicted with 50% thermal
ellipsoid probabilities (H atoms are omitted for clarity).
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