(N-Heterocyclic Carbene)2-Pd(0)-Catalyzed Silaboration of Internal and Terminal Alkynes: Scope and Mechanistic Studies

Article abstract of DOI:10.1021/acscatal.6b00127

Pd(ITMe)₂(PhC≡CPh) acts as a highly reactive precatalyst in the silaboration of terminal and internal alkynes to yield a number of known and novel 1-silyl-2-boryl alkenes. Unprecedented mild reaction temperatures for terminal alkynes, short reaction times, and low catalytic loadings are reported. During mechanistic studies, cis-Pd(ITMe)₂(SiMe₂Ph)(Bpin) was directly synthesized by oxidative addition of PhMe₂SiBpin to Pd(ITMe)₂(PhC≡CPh). This represents a very rare example of a (silyl)(boryl)palladium complex. A plausible catalyst decomposition route was also examined.

Full text of DOI:10.1021/acscatal.6b00127

Letter
pubs.acs.org/acscatalysis
(N-Heterocyclic Carbene)2‑Pd(0)-Catalyzed Silaboration of Internal
and Terminal Alkynes: Scope and Mechanistic Studies
Melvyn B. Ansell, John Spencer,* and Oscar Navarro*
Department of Chemistry, University of Sussex, Brighton BN1 9QJ, United Kingdom
S
* Supporting Information
ABSTRACT: Pd(ITMe)2(PhCCPh) acts as a highly reactive
precatalyst in the silaboration of terminal and internal alkynes to
yield a number of known and novel 1-silyl-2-boryl alkenes.
Unprecedented mild reaction temperatures for terminal alkynes,
short reaction times, and low catalytic loadings are reported. During
mechanistic studies, cis-Pd(ITMe)2(SiMe2Ph)(Bpin) was directly
synthesized by oxidative addition of PhMe2SiBpin to Pd-
(ITMe)2(PhCCPh). This represents a very rare example of a
(silyl)(boryl)palladium complex. A plausible catalyst decomposition
route was also examined.
KEYWORDS: N-heterocyclic carbene, silaboration, homogeneous catalysis, alkyne, palladium, synthetic methods
he regio- and stereoselective synthesis of multisubstituted
alkenes is a challenging reaction, recurrent in the
Scheme 1. Silaboration of Terminal Alkynes
T
formation of complex organic structures. In particular, tri-
and tetra-substituted alkenes are present in many pharmaceut-
icals,1 dipeptide mimetics,2 polymers,3 and columnar liquid
crystals.4 There are now many reported methods for the
synthesis of such alkenes including olefin metathesis5 and
carbonyl olefination,6 among others.7 Notably, the transition-
metal catalyzed π-insertion of a bond between two elements of
the p-block (e.g., Si−Si, Si−Sn, Sn−Sn, B−B, and Si−B) into
an alkyne has received a significant amount of attention.8 One
of the most interesting examples is arguably the 1,2-addition of
a silicon−boron bond (silaboration).9 The resulting 1-silyl-2-
boryl alkenes have the potential to independently undergo, for
example, a cross-coupling reaction at the boryl (Suzuki−
Miyaura) fragment10 and a Fleming−Tamao oxidative addition
or cross coupling (Hiyama) at the silyl fragment.11,12 Arguably,
the most effective alkyne silaboration protocol is the palladium
diacetate/isocyanide combination reported by Ito and co-
workers (Scheme 1).13,14 The reactions proceed with high
stereoselectivity toward the syn-1,2-addition products and in the
case of terminal alkynes high regioselectivity, with the boryl
fragment attached to the terminal position. Recently, Suginome
and co-workers reported that the reverse regioselectivity was
possible by changing the palladium source and using a sterically
encumbered phosphine ligand, albeit using the more reactive
(chlorodimethylsilyl)boronic acid pinacol ester.15 “Abnormal”
regioselectivity was also reported by Stratakis and co-workers
using a supported gold nanoparticle catalyst.16 However, alkyne
silaboration protocols have been largely limited to high reaction
temperatures, long reaction times, and moderately high catalyst
loadings. The most challenging aspect to silaboration chemistry
remains the silaboration of unsymmetrical internal alkynes and
the resulting formation of regioisomeric mixtures; there are
limited examples that remedy this.13b,17,18
N-Heterocyclic carbenes (NHCs) have replaced phosphines
in many catalytic reactions. They are known to exhibit
equivalent or better σ-donor character than the most common
phosphines, and the resulting (NHC)-M complexes are also
often more robust toward decomposition.19 We recently
reported the use of NHCs as a set of ligands in the first
isolation of a bis(trimethylsilyl)palladium complex, cis-Pd-
(ITMe)2(SiMe3)2 (ITMe = 1,3,4,5-tetramethylimidazol-2-yli-
dene) and the first example of a bis(NHC)-palladium alkyne
complex, Pd(ITMe)2(PhCCPh) (1).20 Both complexes
acted as highly active precatalysts for the cis-bis-silylation of
Received: January 14, 2016
Revised: February 18, 2016
© XXXX American Chemical Society
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Letter
sterically and electronically demanding internal and terminal
alkynes. The high reactivity exhibited by 1 prompted us to
investigate its effectiveness at catalyzing the silaboration of
alkynes. Herein, we report the use of Pd(ITMe)2(PhCCPh)
in the silaboration of sterically and electronically demanding
terminal and symmetrical internal alkynes. Unprecedented low
catalytic loadings, short reactions times, and mild reaction
temperatures for terminal alkynes are presented. Initial
experimental investigations into the mechanism of the reaction
and the isolation of important intermediates are also described.
We had previously synthesized Pd(ITMe)2(PhCCPh) (1)
in what was effectively a three-step process.20 We have since
devised an improved synthesis of 1: Pd(ITMe)(methallyl)Cl
was reacted with one equivalent of each of potassium tert-
butoxide, isopropanol, and ITMe at room temperature forming
Pd(ITMe)2, which was then exposed in situ to a slight excess of
diphenylacetylene at room temperature for 18 h in toluene.
After workup, 1 was isolated in an 85% yield (Scheme 2).
Scheme 3. Silaboration of Terminal and Internal Alkynes
Scheme 2. Improved Synthesis of 1
With large quantities of 1 in hand, we proceeded to
investigate its capacity to catalyze the silaboration of alkynes.
Diphenylacetylene and (dimethylphenyl)silyl boronic acid
pinacol ester (PhMe2SiBpin) were chosen as model substrates
for the optimization of the initial reaction parameters. The
reaction was carried out in C6D6 in order to monitor its
progression. To our delight, (E)-(1,2-diphenyl-2-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2yl)vinyl)dimethyl(phenyl)-
silane (2) was obtained in 96% yield (100% stereoselectivity)
using 0.5 mol % of 1 at room temperature and in less than 30
min (Scheme 3). A comparable yield was obtained in benzene.
The only report for a catalytic synthesis of this compound
required 2 mol % of Pd(OAc)2/30 mol % tert-octyl isocyanide
at 110 °C over 2 h.13b To scope the versatility of this protocol,
a series of sterically and electronically challenging alkynes were
reacted with PhMe2SiBpin. The silaboration of terminal aryl,
alkyl, silyl, and even diterminal alkynes proceeded at room
temperature using 0.5 mol % of 1 in less than 30 min with
100% regio- and stereoselectivity. As for compound 2, the only
previous synthesis of compounds 3−6 required 2 mol % of
Pd(OAc)2/30 mol % tert-octyl isocyanide at 110 °C, in reaction
times varying from 1 to 4 h,13b whereas compound 7 has not
been previously reported, to the best of our knowledge.
a
b
Yield in C6H6 under same reaction conditions; Major isomer
isolated from a mixture that included 20% of the other regiosomer;
c
Major isomer isolated from a mixture that included 7% of the other
regioisomer.
the silaboration of symmetrical alkynes that are electronically
challenging has not been reported. Albeit requiring temper-
atures of 100 °C, the novel compounds 8−11 were all
synthesized with 100% cis-stereoselectivity as established by
NOESY NMR. Both alkyl−alkyl and aryl−aryl internal alkynes
bearing functionalities such as carboxylic ester, boronate ester,
pyrrole, and ether reacted well under these conditions. Two
unsymmetrical alkynes were also subjected to these reaction
conditions. The silaboration of 1-phenyl-1-propyne afforded
compound 13, isolated as the major regioisomer of a mixture
containing 7% of the other regioisomer. This is a similar result
There are only a few examples of catalytic silaborations of
symmetrical and unsymmetrical internal alkynes in the
literature: namely, Ito and co-workers’ silaboration of
diphenylacetylene, 1-phenyl-1-propyne and dec-5-yne,13b Sa-
wamura and co-workers’ organocatalytic silaboration of polar-
coordinating internal alkynes,17 and Sato and co-workers’
ynamide silaboration.18 Mixtures of regioisomers are usually
observed in the silaboration of unsymmetrical internal alkynes.
However, to our knowledge, a more thorough investigation into
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to that obtained by Ito,13b albeit in a shorter reaction time and
using a lower catalyst loading. On the other hand, the
silaboration of unsymmetrical alkyne 1-phenyl-2-trimethylsily-
lacetylene resulted in the isolation of the novel compound 12 as
a major product from an 80:20 mixture of regioisomers.
We then turned our attention to the mechanism of these
reactions. The proposed catalytic cycle for “normal” silabora-
tion using Pt group catalysts involves an initial oxidative
addition resulting in a cis-(silyl)(boryl)M(II) complex. The
alkyne then undergoes migratory insertion into the M-B bond
to form the corresponding (silyl)-MII-(borylvinyl) species,
followed by a reductive elimination to form the 1-silyl-2-boryl-
alkene.21 The isolation of the oxidative addition products for Pt
group complexes is extremely rare due to their low stability: to
our knowledge, the only reported examples to date are a series
of (phosphine)-Pt complexes reported by Ozawa and co-
workers21a and one Pd complex reported by Onozawa and
Tanaka.22 We decided to investigate the stoichiometric
reactivity of 1 in the hope of isolating this important
intermediate in the catalytic cycle. On reacting two equivalents
of PhMe2SiBpin with 1 in toluene, cis-Pd(ITMe)2(SiMe2Ph)
(Bpin) (14) and 2 formed at room temperature in under 30
min. (Scheme 4). Single crystals of 14 were isolated from a
Figure 1. Molecular structure of 14 with thermal ellipsoids at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths [Å] and angles [°]: Pd1−CA: 2.091(3), Pd1−C9:
2.120(3), Pd1−BT: 2.038(4), Pd1−Si2: 2.3352(9), CA−Pd1−C9:
103.26(12), C9Pd1−Si2: 94.71(9), BT−Pd1−Si2: 81.49(11), CA−
Pd1−BT: 80.89(13).
Scheme 4. Synthesis of Compounds 14 and 2
Scheme 5. Decomposition of 14 in Solution
With all this information in hand, we propose the mechanism
depicted in Scheme 6, starting with the activation of complex 1
leading to the formation of complex 14, as shown in our
mechanistic studies. We then suggest the approach of the
alkyne above the plane of the molecule because, due to their
nature, it is unlikely that neither the NHCs, silyl nor boryl
groups would detach prior the coordination of the alkyne. A
subsequent migratory insertion of the boryl group into the
alkyne would then form a borylvinyl−palladium−silyl inter-
mediate. This preferred boryl migration over silyl migration has
been thoroughly investigated in the literature and is believed to
be both a kinetically and thermodynamically favorable
process.21,24 A weak coordination of the boryl moiety to the
Pd center would stabilize the borylvinyl palladium intermediate
and allow for a stereoselective reductive elimination,25
generating the desired silaborated product and 14 e− complex
Pd(ITMe)2, the catalytically active species in the cycle.
In conclusion, we have shown that complex 1 is a very
reactive precatalyst in the silaboration of sterically and
electronically demanding internal and terminal alkynes,
proceeding at much lower catalyst loadings, milder temper-
atures (in the case of terminal alkynes), and in much faster
reaction times than in previous protocols reported in the
literature. Investigations into the mechanism for this reaction
resulted in synthesis of cis-Pd(ITMe)2(SiMe2Ph)(Bpin). This
represents a very rare example of a (silyl)(boryl)palladium
complex isolated from the oxidative addition of a Si−B reagent
to a Pd(0) center. Studies into the catalytic potential of 1 in
double recrystallization in acetonitrile at −30 °C. X-ray analysis
indicated a distorted square planar geometry with the NHCs
orthogonal to the Si−Pd−B plane (Figure 1). To gain further
insights on the reactivity of 14, we carried out its stoichiometric
reaction with diphenylacetylene, leading to the quantitative
formation of 1 and 2 at room temperature in only 10 min (see
Supporting Information). Unfortunately, attempts of isolating
the borylvinyl-Pd-silyl intermediate generated after the
migratory insertion step were unsuccessful.
Complex 14 seems indefinitely stable to decomposition as a
solid under inert conditions. It however rapidly decomposes in
solution in nonpolar aromatic solvents such as toluene and
benzene and at a slower rate in acetonitrile. By monitoring the
1
mixture in C6D6 by H NMR, we assigned the decomposition
products as Pd(ITMe)2, B2pin2, palladium black and an
unknown (NHC)-Pd complex, which we tentatively identified
as cis-Pd(ITMe)2(SiMe2Ph)2 (15) (Scheme 5). The identity of
15 was confirmed by an independent synthesis, reacting
Pd(ITMe)2 and 1,1,2,2-tetramethyl-1,2-diphenyldisilane
(PhMe2SiSiMe2Ph). Single crystals of 15 were isolated from a
saturated solution of toluene/benzene (10:1) at room temper-
ature (see Supporting Information).20,23 This decomposition of
14 in solution leading to Pd black could very well explain
catalyst death with time as the concentration of the alkyne in
solution decreases.
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Scheme 6. Proposed Catalytic Cycle
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b00127.
X-ray data (CIF)
X-ray data (CIF)
X-ray data (CIF)
Experimental procedures, characterization data for all
compounds, and NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: j.spencer@sussex.ac.uk.
*E-mail: o.navarro@sussex.ac.uk.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We wish to thank Dr. Iain Day, Dr. Alaa Abdul-Sada, and Dr.
George Kostakis (University of Sussex) for helpful discussions.
M.B.A. is funded as an EPSRC Standard Research Student
(DTG) under grant no. EP/L505109/1.
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