Ruthenium catalyzed hydroboration of terminal alkynes to Z-vinylboronates

Article abstract of DOI:10.1021/ja307233p

The nonclassical ruthenium hydride pincer complex [Ru(PNP)(H) ₂(H₂)] 1 (PNP = 1,3-bis(di-tert-butyl-phosphinomethyl) pyridine) catalyzes the anti-Markovnikov addition of pinacolborane to terminal alkynes yielding Z-vinylboronates at mild conditions. The complex [Ru(PNP)(H)₂(HBpin)] 2 (HBpin = pinacolborane), which was identified at the end of the reaction and prepared independently, is proposed as the direct precursor to the catalytic cycle involving rearrangement of coordinated alkyne to Z-vinylidene as a key step for the apparent trans-hydroboration.

Full text of DOI:10.1021/ja307233p

Communication
pubs.acs.org/JACS
Ruthenium Catalyzed Hydroboration of Terminal Alkynes to
Z‑Vinylboronates
Chidambaram Gunanathan,†,§ Markus Holscher,† Fangfang Pan,‡ and Walter Leitner*,†
̈
‡
†Institut fur Technische Chemie und Makromolekulare Chemie and Institut fur Anorganische Chemie, RWTH Aachen University,
Aachen 52074, Germany
̈
̈
S
* Supporting Information
under mild conditions. Z-Vinylboronate products are obtained
in high yields and with high turnover numbers up to 970. X-ray
diffraction data and NMR spectroscopy together with
deuterium labeling studies suggest initial formation of a
ruthenium borane complex and rearrangement of coordinated
alkyne to vinylidene as key steps in the catalytic cycle.
Complex 1 confines the structural motifs of a tridentate
pincer ligand14 at ruthenium with two classical hydrides and a
nonclassical hydrogen ligand.15 It is readily synthesized by
hydrogenation of commercially available [Ru(cod)(metallyl)2]
in the presence of the pincer ligand.13 It has been shown to
catalyze the H/D exchange of aromatic compounds16 and the
hydrogenation of nitriles to primary amines.17
ABSTRACT: The nonclassical ruthenium hydride pincer
complex [Ru(PNP)(H)2(H2)] 1 (PNP = 1,3-bis(di-tert-
butyl-phosphinomethyl)pyridine) catalyzes the anti-Mar-
kovnikov addition of pinacolborane to terminal alkynes
yielding Z-vinylboronates at mild conditions. The complex
[Ru(PNP)(H)2(HBpin)] 2 (HBpin = pinacolborane),
which was identified at the end of the reaction and
prepared independently, is proposed as the direct
precursor to the catalytic cycle involving rearrangement
of coordinated alkyne to Z-vinylidene as a key step for the
apparent trans-hydroboration.
When complex 1 (0.1 mol %) was dissolved in cold
pinacolborane (3 mmol, −15 °C), the colorless solution turned
yellow with concomitant gas evolution. Upon dropwise
addition of phenylacetylene (2.5 mmol) to this solution at rt,
the color turned reddish-brown immediately and an exothermic
reaction was observed. After the reaction mixture was stirred for
24 h, GC analysis showed 99% conversion of alkyne and 96%
selectivity to the Z-vinylboronate, which subsequently was
isolated by column chromatography in 92% yield (Table 1,
entry 1). Similar results were obtained when the reaction was
performed in 3 mL of benzene or toluene for better
temperature control. Reactions between phenylacetylene (1
mmol) and pinacolborane (1.5 mmol) without complex 1
under similar reaction conditions provided only 5% conversion
after 24 h and exclusive formation of E-vinylboronate,
confirming the efficient formation of the Z-vinylboronate to
be the result of a metal complex catalyzed pathway.
rganoboron compounds are versatile building blocks in
1
Oorganic synthesis. Among them, vinylboron reagents are
finding wide application as stable vinyl anionic or cationic
synthons,2 as Michael donors,3 in aldol reactions,4 and in
various coupling reactions. Several methods for the synthesis of
vinylboron compounds have been developed.5−8 Hydrobora-
tion of terminal alkynes is a straightforward method for the
synthesis of vinylboranes, resulting in E-vinylboronates as the
main product via anti-Markovnikov and syn-addition of the
boron reagents.5,6,9 Dehydrogenative borylation of alkenes also
provides E-vinylboranes as the major products.10 The synthesis
of Z-vinylboron compounds is currently not possible by direct
borylation but requires an elaborate two-step method.11 Thus,
while direct hydroboration of alkynes provides efficient access
to vinylboron compounds, regio- and stereoselective control for
Z-vinylboronates remains a challenge.12
Various terminal alkynes were subjected to the hydro-
boration reactions to explore the scope of the nonclassical
ruthenium hydride complex 1 for the selective synthesis of Z-
vinylboronates, and the results are summarized in Table 1.
Quantitative conversion and high selectivities were observed
consistently, providing excellent isolated yields above 80% for a
wide variety of electronically and sterically different substituents
at the triple bond (Table 1, entries 1−6, 9−11). Oxygen and
nitrogen functionalities adjacent to the reactive sites were also
tolerated (Table 1, entries 7−8).
In the present paper we describe the synthesis of Z-
vinylboronates via a chemo-, regio-, and stereoselective
borylation of terminal alkynes with pinacolborane catalyzed
by the nonclassical ruthenium hydride pincer complex
[RuH2(H2)(PNP)] 1 (Scheme 1).13 This selective hydro-
boration reaction proceeds for a broad scope of substrates
Scheme 1. Z-Selective Borylation of Terminal Alkynes with
Pinacolborane (HBpin) Using Catalyst 1
The hydroboration reactions catalyzed by complex 1 are
chemoselective for terminal alkynes. Terminal alkenes and
internal18 alkynes did not react. In an equimolar mixture of
phenylacetylene and styrene, only the alkyne was converted to
Received: July 27, 2012
Published: August 22, 2012
© 2012 American Chemical Society
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Communication
a
Table 1. Hydroboration of Terminal Alkynes Catalyzed by 1
a
Conditions: To a cold solution (−15 °C) of complex 1 and pinacolborane (3 mmol), in 3 mL of toluene, 2.5 mmol of precooled alkyne (1.25 mmol
b
in cases of dialkyne) was added dropwise and the reaction mixture was stirred at rt for 24 h. Selectivity for the Z-isomer based on GC analysis of
crude reaction mixture; regioisomers are the main side products. Isolated yields after column chromatography, based on alkynes. Reactions carried
out under neat conditions. Conversion of alkyne is only 72%. Reaction completed in 12 h. E-Vinylboronates formed in 32%.
c
d
e
f
g
Z-vinylboronates according to GC and NMR analysis of the
crude reaction mixture (85% isolated yields).19 When 2-allyl-2-
propargyl diethylmalonate was subjected to the hydroboration
reaction, the reaction took place at the terminal alkyne
functionality exclusively (Table 1, entry 12).20 However, the
Z/E-ratio of the reaction was significantly lower (Z/E, 66:32)
than that for other substrates.
Scheme 2. Synthesis and Single Crystal X-ray Structure of
[Ru(PNP)(H){(μ-H)2Bpin}] 2
a
The hydroboration could also be carried out successfully on
terminal dialkynes. Complete conversion of the C−C triple
bonds was observed (GC), and very high Z-selectivities for the
bis-vinylboronates were obtained in all reactions (Table 1,
entries 13−15). When 1,4-diethynylbenzene was reacted with
pinacolborane in the presence of 1, the known styryl-bis-
boronate, which was prepared earlier in 22% yield in two
steps,11a was obtained directly in 85% isolated yields (Table 1,
entry 13) with excellent stereocontrol (89% Z-selectivity;
GC).21 Similarly, 1,6-heptadiyne and 1,9-decadiyne gave the
bis-boronate derivatives in very good isolated yields (Table 1,
entries 14, 15).
a
Selected bond lengths (Å) and angles (deg); values from DFT
Complex 1 reacts under pinacolborane with concomitant
evolution of gas to complex 2 that was obtained in 96% yield in
pentane (Scheme 2). Crystals of 2 suitable for single crystal X-
ray diffraction studies could be obtained from toluene. The unit
cell contains two independent molecules, which could be fully
refined and show only minor structural differences.22 The
ruthenium atom occupies the center of a distorted octahedron.
calculations in parentheses: Ru−H1 1.620 (1.619), Ru−H2 1.521
(1.623), Ru−H3 1.625 (1.781), Ru−B 2.125 (2.128), B−H2 1.458
(1.508), B−H3 1.403 (1.415); H1−Ru−H2 90.2 (82.3), H2−Ru−B
43.3 (44.9), H3−Ru−B 41.3 (41.3).
The PNP pincer ligand adopts a regular mer-coordination with
two phosphines in axial positions and the pyridyl N at one of
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Communication
the four coordination sites in the equatorial plane. Electron
densities consistent with three hydride ligands were located at
the remaining sites. One is situated in a terminal position,
whereas the other two are bridging to the boron center,
resulting in a Ru−B distance of 2.125 Å.15c,23
Scheme 4. Proposed Mechanism for Z-Selective trans-
Hydroboration of Terminal Alkynes
DFT calculations on the molecular structure of 2 (B97-D/
def2-TZVP(ECP); Scheme 2) are fully in line with the
experimentally derived structure, supporting that the free
refinement of the hydrogen centers reflects correct positions.22
The calculated structure implies that the formulation as a
ruthenium dihydride complex with a σ-bonded B−H group
may also contribute to the overall bonding pattern. The
structure of 2 as shown in Scheme 2 is also corroborated
spectroscopically in solution. The 31P {1H} NMR spectrum
exhibits a singlet at 95.7 ppm, which is shifted downfield by 14
ppm relative to 1.13 In the H NMR of 2 two broad singlets
1
appeared at −11.72 and −5.02 ppm, attributed to the two
bridging hydrides and one terminal hydride, respectively.24
NMR spectroscopic investigation of the reaction mixture
revealed the presence of 2 as the only P-containing species after
catalysis. Furthermore, when 2 (0.1 mol %) was used as the
catalyst for the reaction of pinacolborane with phenylacetylene
under standard conditions, the corresponding Z-vinylboronate
was obtained in 93% (87% isolated) yield. These results
indicate that 2 acts as the actual entry point into the catalytic
cycle of the Z-selective hydroboration of terminal alkynes.22
Subjecting 1-deuterio-2-phenylacetylene and pinacolborane
to the catalytic reaction lead to exclusive formation of the Z-
vinylboronate with deuterium at the internal carbon (Scheme
3). The proton signal at a chemical shift of 7.2 ppm was absent
chemoselectivity for the hydroboration of terminal alkynes with
this system. The Z-stereochemistry in the product is
determined in the reaction sequence from I to III, presumably
reflecting steric interactions in the formation of complex II.
In conclusion, the ruthenium pincer complex 1 bearing a
nonclassical hydride and its borane analog 2 catalyze the
hydroboration of terminal alkynes to give selectively Z-
vinylboronates in high yields under mild conditions. Mecha-
nistic studies suggest a 1,2-hydrogen shift from an η2-alkyne to
a vinylidene complex as a key step prior to the C−B bond
formation. Further work to elucidate the scope of this principle
and the details of the stereochemical discrimination is currently
underway.
Scheme 3. Reaction with Deuterium Labeled Terminal
Alkyne
in the 1H NMR spectrum while the 31C NMR spectrum
displayed a 1:1:1 triplet at 147.9 ppm (JDC = 23.4 Hz)
confirming the position of deuterium at the phenyl-substituted
vinylic carbon (PhCD).
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, spectral and X-ray data for
intermediate complex 2, and NMR data of Z-vinylboronates.
This material is available free of charge via the Internet at
http://pubs.acs.org.
On the basis of these data a catalytic cycle for the Z-selective
hydroboration of terminal alkynes with pinacolborane is
postulated (Scheme 4). The reaction of 1 with pinacolborane
leads to the immediate formation of the ruthenium−borane
complex 2. This can undergo a σ-bond metathesis-type
rearrangement to a ruthenium hydride with a covalent Ru−B
bond and a nonclassically bonded dihydrogen molecule. The
H2 ligand is replaced with the alkyne to generate complex I.
The η2-coordinated terminal alkyne in I reacts under 1,2-
hydrogen migration25 to the η1-vinylidene intermediate II.
Coupling of the vinylidene and pinacolborate ligands generates
the C−B bond in complex III. Coordination of pinacolborane
in IV followed by σ-bond metathesis liberates the vinylboronate
product and regenerates V to close the catalytic cycle.
AUTHOR INFORMATION
Corresponding Author
leitner@itmc.rwth-aachen.de
■
Present Address
§School of Chemical Sciences, National Institute of Science
Education and Research, Bhubaneswar 751005, India.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The mechanism shown in Scheme 4 provides a rationale for
the experimental observation that the apparent trans-addition of
the borane results in fact from a 1,2-hydrogen shift at the alkyne
and a geminal addition of the boron and hydrogen centers of
the pinacol borane reagent. It also explains the very high
■
C.G. is grateful to the AvH Foundation for the award of a
Humboldt Research Fellowship. We thank J. Wurlitzer and H.
Eschmann for the GC measurements and Prof. U. Englert for
X-ray diffraction studies.
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(17) Gunanathan, C.; Holscher, M.; Leitner, W. Eur. J. Inorg. Chem.
̈
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