Catalytic activity of a large Rhodium metallaborane towards the [2+2+2] cycloaddition of alkynes

Article abstract of DOI:10.1016/j.ica.2018.05.041

Rhodadecaborane [6-(η⁵-C₅Me₅)-nido-6-RhB₉H₁₃] (1) was found to be able to catalyze the [2+2+2] cycloaddition of a series of terminal and internal alkynes to yield mixtures of 1,2,4- and 1,3,5-substituted benzene. The reactivity of compound 1 with alkynes demonstrates that decaborane based metallaborane can be used as the catalyst for [2+2+2] cycloaddition of alkynes. All compounds are characterized by NMR spectroscopy and MS spectrometry methods.

Full text of DOI:10.1016/j.ica.2018.05.041

Accepted Manuscript
Catalytic activity of a large Rhodium metallaborane towards the [2+2+2] cy-
cloaddition of alkynes
Pei Ma, James T. Spencer
PII:
S0020-1693(18)30564-4
https://doi.org/10.1016/j.ica.2018.05.041
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Inorganica Chimica Acta
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Catalytic activity of a large Rhodium metallaborane towards the
[2+2+2] cycloaddition of alkynes
Pei Ma1, James T. Spencer∗
Department of Chemistry and the Forensic and National Security Sciences Institute (FNSSI), 1-014 Centre
for Science and Technology, Syracuse University, Syracuse, NY 13244, USA
Abstract
Rhodadecaborane [6-(η5-C5Me5)-nido-6-RhB9H13] (1) was found to catalyze the [2+2+2]
cycloaddition of a series of terminal and internal alkynes to yield mixtures of 1,2,4- and
1,3,5-substituted benzene. The reactivity of 1 with alkynes demonstrates that decaborane
based metallaborane can be used as the catalyst for [2+2+2] cycloaddition of alkynes. All
compounds are characterized by NMR spectroscopy and MS spectrometry methods.
Keywords:
metallaborane, catalyst, cyclotrimerization, cycloaddition, Rhodium
∗Corresponding author
Email address: jtspence@syr.edu (James T. Spencer)
1Current address: Foundation Medicine Inc, 150 Second St, Cambridge, MA 02141, USA
Preprint submitted to Inorganica Chimica Acta
May 29, 2018

Introduction
Metallaborane and metallacarborane cluster chemistry has been a fascinating field of
study which finds applications in medicine, materials science, catalyst, nano chemistry,
and beyond.[1–13] The synthesis, reactivities and applications of them continue to attract
attention.[14–23]
5
Owing to uncommon structures as well as their unique bonding, catalytic activities
of metallaborane have attracted significant attentions.[24] Yan and coworkers previously
reported that dirhodaboranes such as 2,3-(C5Me5Rh)2B3H7 are able to catalyze the cy-
clotrimerization of both terminal and internal alkynes.[24–29] Recently, it was reported
10 that the catalytic activity of compound nido-[(Cp∗Rh)2B6H10] towards the cycloaddition
of alkynes while no activity for its isoelectronic counterpart [(Cp∗Ru)2B6H12].[30] Ghosh
and coworkers reported that arachno-[(CO)8Re2B2H6]) acts as an active catalyst for the
cyclotrimerization of various terminal and internal alkynes under mild conditions.[31]
During our investigation of the interaction between metallaborane and alkynes, we found
15 that under elevated temperature, the 10-vertex rhodadecaborane [6-(η5-C5Me5)-nido-6-RhB9H13]
(Compound 1, Figure 1) is able to catalyze the [2+2+2] cycloaddition of alkynes. This
discovery of the catalytic reactivity of large rhodadecaborane 1 for [2+2+2] cycloaddition
demostrates the rich chemistry of metallaborane and suggest that large borane cluster should
be further investigated in catalytic activities.
2

Figure 1: Structure of compound 1 (each vertex represents a B-H, Cp⋆ =η5-C5Me5).
20 Results and Discussion
Compound 1 was first reported by Fontaine and coworkers in 1986.[32] It has been shown
that a rich chemistry is related to this nido-6 rhodadecaborane in literature. For example,
the reaction of compound 1 with an excess of PMe2Ph in CH2Cl2 solution at room tempera-
ture yielded the air-stable, red-orange [5-(η5-C5Me5)-nido-5-RhB9H11-7-(PMe2Ph)2] complex
25 with a yield of 35%.[33] Isonitrile compounds such as MeNC or p-MeC6H4NC were also re-
ported to react with compound 1 under mild conditions to give 6-(η5-C5Me5)-6,9-(MeNC)2-
arachno-6-RhB9H11.[34] The compound was reported recently that it’s active under both
thermal and photolytic conditions.[35]
When catalytic amounts (1 mol%) of compound 1 with phenylacetylene was refluxed in
30 xylene (138 ◦C) for 2 hours, cyclotrimerization products of the phenylacetylene were isolated
from the mixture (Figure 2).
We systematically explored the reactivity of rhodadecaborane 1 towards alkynes that
led to the formation of a mixture of 1,2,4- and 1,3,5- substituted arenes. The activity
and selectivity for the cycloaddition of a variety of internal and external alkynes are listed
35 in Table 1. Entry 5 and entry 6 imply the presence of a significant steric factor caused the
decreasing activities of compound 1. Catalytic activity is higher for terminal than for internal
alkynes, and that the presence of electron-withdrawing groups enhances the reactivity of an
3

Ph
Ph
Ph
Ph
Ph
Ph
(1)
reflux, xylene
Ph
1,2,4-cyclotrimer
1,3,5-cyclotrimer
Figure 2: [2+2+2] cycloaddition of phenylacetylene mediated by compound 1.
alkyne. In all entries, the 1,2,4-isomer was found to be the major isomer.
Table 1: 2+2+2 cycloaddition of alkynes catalyzed by 1
entry
alkyne yield(%) ratioa
−
2 3
1
2
3
4
5
6
7
8
CH (CH ) C−CH
60
43
64
55
36
0c
1:3
NAb
1:4
−
3
−
CH CH C−CCH CH
−
3
2
2
3
−
CH (CH ) C−CH
−
3
2 2
−
C H CH CH C−CH
1:3
−
6
5
2
2
−
Si(CH ) C−CH
1:3
−
3 3
Si(CH ) C−CSi(CH )
NAc
1:2
−
−
3 3
3 3
−
C H -C−CH
74
64
−
6
5
C H -C−CC H
NAb
−
−
6
5
6
5
a ratio of 1,3,5- to 1,2,4-trisubstituted benzene.
b single isomer isolated.
b no cyclotrimerization.
We believe that all the cycloaddition reactions reported here involve the initial formation
40 of a metallacycle intermediate and subsequent alkyne insertion.[29] The involvement of the
metal center of metallaborane in initialization of a reaction pathway is not unusually. It
has been reported multiple times that acetylene moieties can react with metal centers of
metallaborane and can be also incorporated into metallaborane cluster frameworks.[36–39]
In fact, Rhodium-based organometallic catalysts have been one of the most actively
45 pursed catalysts.[40–47] Although rhodium metallaborane catalysts were reported previously,
It should be noted that previous reports of metallaborane with similar catalytic reactivity
all have small boron cluster.[25, 26, 29, 30] Our discovery suggest that larger cluster met-
allaborane also have similar catalytic reactivity. Large-vertex metallaborane complexes are
4

generally more stable, thus making more practical.
50 Conclusion
To conclude, we demonstrated the application of rodadecaborane 1 in catalyzing the
[2+2+2] cycloaddition of both internal and external alkynes with wide functional-groups
under thermolytic condition. This discovery shows that large-vertex metallaborane can be
useful in catalytic reactivity for the [2+2+2] cycloaddition of alkynes. Fruitful results can
55 be anticipated given the available large variety of metallaboranes and the huge potential
applications of the [2+2+2] cycloaddition reactions.
Experimental
Physical Measurements
All NMR spectra were recorded on a Bruker Avance 300 MHz NMR equipped with a 5 mm
60 OXI probe. The 11B NMR spectra were recorded at 96.3 MHz and were referenced at 96.3
MHz. Spectra were referenced to BBr3 at +40.0 ppm (relative to BF3Et2O at δ = 0.0 ppm)
with positive chemical shifts indicating downfield resonances. Typical 11B NMR acquisition
parameters employed were a relaxation delay of 0.1 ms and a 90◦ pulse of 10 µs. Proton (1H)
NMR spectra were recorded at 300.15 MHz with chemical shifts referenced to an internal
65 standard of tetramethylsilane at δ=0.0 ppm. Carbon (13C) NMR spectra were obtained at
77.47 MHz. Unit resolution mass spectra were obtained on a Hewlett Packard model 5989B
gas chromatograph/mass spectrometer (GC/MS) using an ionization potential of between
11 and 70 eV. FT-IR spectra in the range of 400 to 4000 cm−1 were measured on a Mattson
Galaxy 2020 spectrometer and were referenced to the 1602.8 cm−1 band of polystyrene. The
70 spectra were observed as KBr mulls and are reported in cm−1. UV-Vis spectral data were
collected on a Cary 1 UV-Vis spectrophotometer in a quartz cell.
Materials
All solvents used were reagent grade or better. Tetrahydrofuran (THF) and hexane
were distilled over potassium metal prior to use. TLC plates were purchased from Fisher
75 Scientific. The nido-decaborane(14) (B10H14), was purchased from the Callery Chemical
5

Company and was purified by vacuum sublimation at 40 ◦C prior to use. Appropriate care
was taken in handling the boron hydrides under inert atmosphere conditions.[48] All reactions
were done in an inert atmosphere unless otherwise noted. Deuterated NMR solvents were
purchased from Cambridge Isotope Laboratories, Inc. and dried over molecular sieves before
80 use, unless otherwise noted. All other commercially available reagents were used as received.
The compound 1 ([6-(η5-C5Me5)-nido-6-RhB9H13]) was synthesized by a modification of a
previously described procedure.[32]
Experimental Procedures
General procedures for the alkynes in the presence of catalytic amount (1
85 mol %) of 1: Complex 1 (3.0 mg, 0.0086 mmol) and alkynes (0.86 mmol) dissolved in 10
mL of dry xylene was refluxed under nitrogen for 2 h. All volatile components were then
evaporated under reduced pressure at room temperature and the residue was analyzed by
GCMS and NMR spectroscopy. The data revealed the conversion of the phenylacetylene
to its cyclotrimerization products 1,2,4-triphenylbenzene and 1,3,5-triphenylbenzene. The
90 solvent was evaporated under vacuum and the residue was extracted into hexane and passed
through Celite. After removal of the hexane, the filtrate was subjected to chromatographic
silica gel TLC plate with hexane:CH2Cl2 (3:1) as eluent to give the substituted benzenes.
1
The characterization of the products was completed by H, 13C NMR spectroscopy, mass
spectrometry and by a comparison with literature data.[49, 50] The yield of substituted
1
95 benzenes was determined by H NMR spectroscopy employing a reference known weight
compound using a similar literature method.[30]
Acknowledgment
We wish to thank the Department of Chemistry and the Forensic and National Security
Sciences Institute (FNSSI) for their generous support.
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URL:
http://pubs.acs.org/doi/abs/10.1021/om00032a012. doi:10.1021/om00032a012.
arXiv:http://pubs.acs.org/doi/pdf/10.1021/om00032a012.
12

Catalytic activity of a large Rhodium metallaborane towards the
[2+2+2] cycloaddition of alkynes

lResearch Highlights:
 Known Rhodadecaborane of formula [6-(η5-C5Me5)-nido-6-RhB9H13] is found to
catalyze cyclotrimerization of alkynes
 The metallaborane is larger in size than previously reported 8-vertex systems and is
more stable and ready for easier handling
 The reaction yields are comparable with other reported catalytic systems.
 It indicates the catalyst application of large vertex metallaborane can be a direction
in research