An experimental and theoretical study into the facile, homogenous (N-heterocyclic carbene)2-Pd(0) catalyzed diboration of internal and terminal alkynes

Article abstract of DOI:10.1039/c6cy01266c

Pd(ITMe)₂(PhCCPh) acts as a highly reactive pre-catalyst in the unprecedented homogenous catalyzed diboration of terminal and internal alkynes, yielding a number of novel and known syn-1,2-diborylalkenes in a 100% stereoselective manner. DFT calculations suggest that a similar reaction pathway to that proposed for platinum phosphine analogues is followed, and that destabilization of key intermediates by the NHCs is vital to the overall success for the palladium-catalyzed B-B addition to alkynes.

Full text of DOI:10.1039/c6cy01266c

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DOI: 10.1039/C6CY01266C
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An Experimental and Theoretical Study into the Facile,
Homogenous (N-Heterocyclic Carbene)2-Pd(0) Catalyzed
Diboration of Internal and Terminal Alkynes†
Received 00th January 20xx,
Accepted 00th January 20xx
Melvyn B. Ansell,a Vitor H. Menezes da Silva,b Gabriel Heerdt,b,c Ataualpa A. C. Braga,*b John
Spencer,*a and Oscar Navarro*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Pd(ITMe)2(PhC≡CPh) acts as a highly reacꢀve pre-catalyst in the unprecedented homogenous catalyzed diboration of
terminal and internal alkynes, yielding a number of novel and known syn-1,2-diborylalkenes in a 100% stereoselective
manner. DFT calculations suggest that a similar reaction pathway to that proposed for platinum phosphine analogues is
followed, and that destabilization of key intermediates by the NHCs is vital to the overall success for the palladium-
catalyzed B-B addition to alkynes.
extensive studies, a number of general limitations remain: the
use of elevated temperatures, high catalyst loadings and long
reaction times. Only two examples of palladium catalysed
alkyne diborations have been described in the literature, both
Introduction
The transition metal catalysed π-insertion of homo and hetero
element-element (E-E’) bonds into alkynes provides the most
atom economical route for the stereoselective synthesis of tri-
and tetrasubstituted alkenes.1 Among the various E-E’
reagents used in such transformations, B-B bonds (diborons) in
particular have attracted substantial interest.2 The resulting
1,2-diboryl alkenes, due to their participation in Suzuki-
Miyaura cross-coupling,3 are recognized as important building
blocks in, for example, the synthesis of pharmaceuticals,4
chirotopical devices5 and optically/electronically active
polymeric materials.6 A number of transition metals have been
utilized in both homogenous and heterogenous catalytic
addition of B-B bonds (diboration) to alkynes including cobalt,7
copper,8 iridium,9 rhodium,9 iron,10 platinum,11 and
palladium.12 To date, platinum is by far the most effective and
widely studied;13 this is attributed to the facile cleavage of the
B-B bond and the lability of the corresponding
by Braunschweig and co-workers and involving the
heterogenous catalysed diboration of alkynes using
[2]borametalloarenophanes.12 The reactions required 6 mol%
of Pd/C and proceeded over a period of 5-16 days at
temperatures of 95-100 ˚C (Scheme 1). The dearth of reported
bis(boryl)platinum complexes.14 As
a result, even easily
handled and often air stable tetraalkoxy- and
tetraaryloxydiboron reagents can be utilized, regardless of
their relatively high B-B bond strength.15 However, despite the
Scheme 1 Palladium Catalyzed Diboration of alkynes
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palladium examples is attributed to the energetics of the B-B using lower catalyst loadings, milder temperatures and in
oxidative addition. The process is endothermic with a very low higher or comparable yields to the highest yielding protocol in
reverse activation barrier16 and therefore kinetically and the literature (2 mol% nanoporous gold, 100 ˚C),22 and
5
and
higher
We recently reported the synthesis of the N-heterocyclic stereoselectivities. Low reaction temperatures have been
carbene bearing17 complex Pd(ITMe)2(PhC≡CPh) (ITMe
reported for the synthesis of these compounds using both
1,3,4,5-tetramethylimidazol-2-ylidene) ( ) and its high catalytic homo- and heterogenous platinum complexes, although at the
reactivity in bis-silylation18 and silaboration of internal and expense of lower yields and in many cases higher catalyst
terminal alkynes.19 This prompted us to investigate its loadings.13,23,24 Compound
was synthesized in a higher yield
potential in the diboration of alkynes. Herein, we report the than the highest yielding protocol in the literature (0.2 mol%
use of
in the unprecedented palladium catalysed diboration Pt/TiO2, 70 ˚C, 16 h).13 The highest yielding synthesis for
of sterically demanding internal and terminal alkynes, compound
6
thermodynamically unfavourable.
were
synthesized
with
comparatively
=
1
7
1
8
was reported by Miyaura and co-workers (94%
employing low catalytic loadings and mild reaction yield)13 using 3 mol% Pt(CO)2(PPh3)2 at 80 ˚C in DMF over 24 h.
temperatures. In addition, a thorough density functional
theory (DFT) study was conducted in order to establish a likely
mechanistic pathway explaining this reactivity.
Results and Discussion
The
reaction
parameters
and
were
optimized
using
diphenylacetylene
commercially
available
bis(pinacalato)diboron (B2pin2) as the model substrates, with
C6D6 as solvent in order to monitor the progression by 1H
NMR. To our delight, 100% stereoselective conversion to (Z)-
1,2-diphenyl-1,2-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)ethane (2) was observed using 0.5 mol% of 1 at room
temperature in 21 h. Unfortunately, initial work-up procedures
proved troublesome, with the use of either silica and alumina
columns resulting in very low isolated yields presumably due
to reactivity with, or strong binding to the stationary phase.
Kugelrohr distillation is an alternative methodology reported in
the literature,20 but is generally applicable to small quantities
of material and therefore unviable as a scalable procedure. In
our case, the more noticeable impurity was unreacted B2pin2.
To remove it, the crude dry material was stirred in deionized
H2O at room temperature over 24 h.21 Subsequent filtration
and drying resulted in the clean isolation of
2 in a 99% yield.
While there are several protocols in the literature for the
synthesis of 2, the highest yielding was reported by Jin and co-
workers22 who obtained a comparable yield to ours using 2
mol% of nanoporous gold at 100 ˚C over 12 h. To test the
potential of this protocol for scaling-up, the synthesis of
2 was
also carried out in non-deuterated benzene and toluene on a
larger practical scale, resulting in comparable isolated yields
(Table 1). The potential of
other diboron reagents was also investigated, but
unfortunately neither bis(catecolato)diboron nor
1 to catalyse this reaction using
bis(neopentylglycolato)diboron afforded any of the desired
product.
With this information in hand, a series of sterically and
electronically demanding alkynes were reacted with B2pin2
(Table 1). The diboration of alkyl and aryl terminal alkynes
proceeded using 0.5 mol% of 1 at room temperature over 1-48
B2pin2: 1-1.5 equiv (see Supporting Information for details). aScale-up synthesis
of : 0.8 mmol of substrate, benzene, r. t., 24 h (92%); 0.95 mmol of substrate,
toluene, r. t., 24 h (95%). bConversion of starting alkyne to 15
h with 100% stereoselectivity. A wide range of functionalities
on the aryl moiety was tolerated including fluoro,
trifluoromethyl, methoxy and alkyl groups in the ortho, meta
2
.
Table 1 Diboration of Terminal and Internal Alkynes
and para positions. Compounds 3, 4, 5 and 6 were synthesized
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The novel compounds
9,
10 and 11 were synthesized with PMe3 at the same level of theory was also undertaken (Figure
(C9-C7 = 3.11 Å
spectroscopy. In the case of 11 chemoselectivity is achieved and C9-Pd-C7 = 94.9o) are similar 1-PMe3 (P1-P2 = 3.74 Å and
since the olefin remains unreacted. Unsymmetrical internal P1-Pd-P2 more
105.8o). Furthermore, in 1-PMe3
100% syn-stereoselectivity as established by NOESY NMR 1). The optimized bond lengths and angles of
1
=
a
alkynes also reacted well under these conditions, albeit at accentuated out-of-plane distortion of the square planar
higher -but still mild- temperatures (50 ˚C). The novel geometry around the Pd centre, caused by the phosphine
compounds 12 and 13 were synthesized with 100% syn- ligands, was observed. While the dihedral C1-C6-C7-C9 angle in
stereoselectivities.
The
diboration
of
1-phenyl-2-
1
is 1.5o, in 1-PMe3 the dihedral C1-C6-P2-P1 angle is 7.5o.
trimethylsilane, resulting in the formation of 14, required an
increased catalyst loading of 2 mol% and a much higher
temperature (100 ˚C). The best procedure for the synthesis of
14 was detailed by Nishihara, obtaining a comparable yield
using 5 mol% of Pt(PPh3)4 at 80 ˚C.13 Finally, the diboration of
4-octyne resulted in a maximum conversion to 15 of 39%. Even
lower conversions and the formation of palladium black were
observed when we carried out the reaction at higher
temperatures. We presume that the electron-rich nature of
the alkyne results in a low binding affinity to the very electron-
rich, active catalyst and therefore discourages diboration.
We decided then to investigate the reasons behind this
unprecedented activity. The accepted experimental and
theoretical mechanism for platinum group transition metal
catalysed diboration of alkynes involves: (i) oxidative addition
of the B-B bond to a M(0)L2 centre forming L2M(II)(B)2, (ii)
dissociation of an L ligand (a phosphine) and coordination of
the alkyne in its place, (iii) insertion of the alkyne into the M-B
bond, (iv) isomerization of the resulting complex, followed by
re-coordination of the
L ligand, and (v) stereoselective
reductive elimination.25 This mechanism is general and applies
to other E-E’ bond addition to alkynes.26 We recently proposed
that the use of NHCs as a ligand set results in a different
mechanism, in which both NHCs remained coordinated
throughout. This alternative pathway was used as an
explanation for the observed increase in reactivity of
1
compared to their phosphine and isocyanide analogues in
alkyne bis-silylations18 and silaborations.19 To gain further
Figure 1 Optimized structures of
1
and 1-PMe3 at M06-L/BSI
) and angles (o).
level of theory with selected bond lengths (
Å
insight into the mechanism and role of
1 in the diboration of
alkynes, computational studies were carried out on the
optimized model substrates (see Supporting Information).
Additionally, a simultaneous study of Pd(0)(PMe3)2(PhC≡CPh)
The oxidative addition of bis(pinacolato)diboron to
1 begins
with the dissociation of the alkyne from the η2-complex,
resulting in the formation of the 14 electron complex I1 (Figure
2). This dissociation is favourable at 6.4 and 3.5 kcal mol-1 for
the NHC and PMe3 complexes, respectively. The reaction
continues through the incorporation of bis(pinacolato)diboron
(
1-PMe3) was performed to establish a direct comparison with
the NHC ligand set.
Initially, the geometry of 1 was optimized at M06-L/BSI level of
theory and compared to X-ray diffraction data.18 The
optimized Pd-alkyne bond lengths Pd-C1 and Pd-C6 are longer,
around 0.01 Å, than the results obtained by X-ray data. Both
bond angles C6-C1-C5 and C1-C6-C25 are equal to 147.1o, in
excellent agreement with the experimental values of 147.5(3)o
in the coordination sphere of I1, achieving the intermediate I2
.
The transition state TSA01 represents the step where the B-B
bond is cleaved with concomitant formation of two σ Pd-B
bonds. This process has a free energy activation barrier
relative to the separated reactants at ΔG‡ = 9.7 kcal mol-1 for
and 146.03(3)o, respectively. A comparison between
1 and 1-
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Figure 2 Free energy profile (in kcal mol-1) for the oxidative addition pathways with bis-ligand complexes.
the NHC and 11.1 kcal mol-1 for the PMe3 bis-ligand complexes. Information for more details on the free energy profile of
These reaction barrier heights suggest that the oxidative reaction pathways).
addition for the phosphines is kinetically less favoured than
the NHC ligands. Bis(boryl)palladium(II) complex (I3) is the
product of the oxidative reaction step for both systems, and,
energetically, is 8.7 kcal mol-1 and 6.5 kcal mol-1 above the
reactants with NHC and PMe3 ligands, respectively.
Alternatively, the oxidative addition step could proceed
through the mono-ligand complexes. Scheme 2 depicts two
possibilities (Pathway A and Pathway B) related to dissociative
reaction routes. Dissociation of alkynes from
electron complex I1 is an exergonic process for the NHC (
kcal mol-1) and the PMe3 ( 3.5 kcal mol-1) ligands (Pathway A).
However, in Pathway B, the dissociation of ligand (L) from to
1
to form the 14-
−
6.4
−
1
form the mono-ligand alkyne palladium(0) complex 1_alkyne is
an endergonic process at 5.9 kcal mol-1 for L = NHC and 8.9 kcal
mol-1 for L = PMe3. The thermodynamic driving force for the
dissociation of L is greater going through Pathway A over
Pathway B. Furthermore, the breaking of the second Pd-L bond
to form the reactive mono-ligand Pd(0)-L (I4) is easier for L =
PMe3, than for L = NHC systems. Indeed, the Pd-NHC bond
typically is stronger than the Pd-phosphine bond. The bis-
ligand complex is energetically preferred for both NHC and
phosphine systems, in the oxidative addition of
bis(pinacolato)diboron to the Pd centre (see Supporting
Scheme 2 Dissociation pathways for the activation of catalyst
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Journal Name
N
Ph
Ph
N
N
N
O
N
N
N
Pd
Ph
Ph
N
N
O
O
Pd
B
B
N
NHC dissociation
activation
N
N
N
N
1
oxidative
addition
O
O
I3
B
N
N
O
O
Pd
Pd
B
O
reductive
elimination
I6
N
O
N
I1
O
B
alkyne coordination
N
O
N
N
Ph
Ph
Pd
B
Ph
Ph
N
O
O
I13
Ph
Ph
O
B
O
alkyne insertion
Pd
B
N
N
O
NHC re-coordination
cis/trans
isomerization
N
N
Ph
O
O
Pd B
O
I9
O
O
Ph
O
Pd B
Ph
B
B
O
O
I11
Ph
I12
N
N
Scheme 3 Proposed mechanism for the (NHC)2Pd(0) catalysed diboration of alkynes.
The next step is the alkyne insertion on the metal centre. The
first proposal is that the insertion of alkyne occurs directly to
the bis(boryl)palladium(II) complex I3. The product of the
alkyne insertion can be obtained through the penta-coordinate
transition state TSIA1 (Figure 3 – see supporting information
for further details and Scheme 3 for the proposed mechanism).
Another possibility for the insertion of the alkyne proceeds via
a dissociate pathway (TSIA2). Kinetic results on Pt(0)-catalyzed
diboration reactions with phosphine ligands suggested that the
alkyne insertion occurs from the three-coordinate species in
the oxidative addition.27 Following this, it is a reasonable
assumption that the dissociation of one L takes place from
complex I3 to generate the mono(boryl)palladium(II) complex
I6 (Scheme 3).The transition state TSIA2 is associated to the
migratory insertion of the alkyne with one ligand attached on
the metal centre. The alkyne triple bond and Pd-B bonds are
broken forming a new C-B.
The dissociative pathway (TSIA2) has a lower relative reaction
free energy barrier than on the associative pathway (TSIA1) at
ΔΔG‡ = 4.0 kcal mol-1 for L = PMe3 and at ΔΔG‡ = 15.4 kcal mol-1
for L = NHC system. Based on these results, the reductive
elimination should occur by the cis-complex I12 from the
Figure 3 Associative (TSIA1) and dissociative (TSIA2) transition
states for the alkyne insertion, respectively. Distances for
selected bonds are given in Angstrom units (
energies are given at 298.15 K.
Å). Relative free
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insertion of the alkyne via the dissociative mechanism for both
ligands (further details in Supporting Information). The re-
coordination of other ligand is reasonable, since the 16-
electron configuration is achieved forming the palladium(II)
complex I13 with a square planar geometry (Scheme 3).
In order to obtain the anti-adduct is necessary to proceed with
a consecutive isomerization processes involving the C=C bond
and the C-Bpin moiety in the allyl ligand after the alkyne
insertion is accomplished. Cui and co-workers suggested these
isomerization pathways are energetically prohibitive because
of the rigidity of the C=C bond.25 Therefore, the substrate
controls the stereoselectivity in the Pt(0)-catalyzed diboration
reaction towards the syn-1,2-diborated product. Analogously,
this mechanism for selectively could be expanded for the
present reaction, since the same substrate was used (alkyne).
In this case, if these isomerization processes take place very
quickly, the selectivity would be defined solely by the relative
energies of the transition states associated with the reductive
elimination steps. Figure 4 shows the optimized geometry of
the transition states TSRE1 and TSRE2 reacted with the syn
and anti-adducts, respectively. For the NHC complex, the
relative free energy activation of ΔΔG‡ = 7.4 kcal mol-1,
favouring the transitions state TSRE1, is in perfect agreement
with the product detected experimentally.
Figure
4
Calculated transition states for the reductive
elimination step associated with syn- and anti-1,2-diborated
adduct, respectively. Distances for selected bonds are given in
Angstrom units (
K.
Å). Relative free energies are given at 298.15
DFT calculation suggest the Pd(0)-catalysed alkyne diboration
supported by NHC ligands proceeds through the same
mechanism as the phosphine ligands (the free energy profile of
the overall catalytic cycle is presented in the Supporting
Information). This mechanism (Scheme 3) can therefore be
summarized as: (i) the activation of the catalyst by alkyne
catalysts than the analogous phosphine-based Pd(0)
complexes. Based on these results, we propose that the
considerably increased reactivity of NHC-bearing complex 1 in
the alkyne diboration is a consequence of the oxidative
addition step; more specifically, on the destabilization of the
(diboron)Pd(0)L2 complex I2 by the NHC ligands resulting in a
lower activation free energy for the oxidative addition (3.9 kcal
mol-1) compared to PMe3 (13.3 kcal mol-1).
dissociation from 1, (ii) oxidative addition of the B-B to Pd(0),
(iii) ligand dissociation from bis(boryl)palladium(II) complex
I3, (iv) insertion of the alkyne into a Pd-B bond via migratory
insertion, (V) cis-trans isomerization involving the C-Bpin and
the allyl ligands, and (vi) reduction of Pd(II) to Pd(0) with the
elimination of the syn-1,2-diborylated product.
Cui and co-workers proposed that a reversible oxidative
addition step is the reason of the null reactivity of a Pd(0)L2
catalyst (L = phosphine) in alkyne diboration reactions.16 The
oxidative additions step was predicted to have an activation
barrier of 8.6 kcal mol-1. However, the B-B oxidative addition
to Pd(0) was characterized as an endothermic process with a
reverse barrier of only 0.1 kcal mol-1. The cause of this low
reverse barrier is attributed to the promotion energy from
d10Pd(0)L2 with linear geometry (singlet – ground state) to d9s1
Pd(0)L2 with bent geometry (triplet – excited state). The
energy between these two electronic configurations is larger
for Pd(0)L2 than for Pt(0)L2 with phosphines. Sasaki and co-
workers,27 studying the activity of Pd(0)L2 and Pt(0)L2 catalyst
(L = phosphine) in the C-H activation of methane by oxidative
addition, reported the destabilization of the M(0)L2 complexes
as an important factor in smoother oxidative additions.
Chelating phosphines were used to destabilize the M(0)L2
complexes by bringing the reactants closer in order to
promote the oxidative addition transition state. (NHC)-Pd(0)
catalysts were also investigated in the activation of methane
by oxidative addition,28 and considered better candidates as
Conclusions
We have shown that complex
1 acts as highly active catalyst in
the diboration of sterically and electronically demanding
alkynes. For terminal alkynes, low catalyst loadings and
temperatures were used for the 100% stereoselective
synthesis of syn-1,2-diborylalkenes. Internal alkynes can react
using this protocol, albeit requiring elevated temperatures.
This represents the first example of homogenous palladium
catalysed diboration of alkynes. DFT calculations, based on
M06 suite density functionals, were performed to understand
the activity of the NHC-bearing catalyst
1. The results suggest
that 1 proceeds through the same mechanistic pathway as the
corresponding phosphine analogues. The dissociation of one
NHC is crucial part of the mechanism, unaccounted for in our
proposed pathways for the other E-E’ bond additions to
alkynes.18,19 Despite their strong coordination to metal
centres, it has been previously shown that the reversible
dissociation of an NHC from an oxidative addition products is a
mechanistic possibility.29 The DFT study also showed that the
destabilization of the (diboron)Pd(0)L2 adduct by the NHCs was
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Braunschweig, M. Kaupp, C. J. Adams, T. Kupfer, K. Radacki
and S. Schinzel, J. Am. Chem. Soc. 2008, 130, 11367.
key to the successful oxidative addition of the B-B bond.
Further investigations into the scope and limitation of
1 in the
13 K. M. Anderson, M. J. G. Lesley, N. C. Norman, A. G. Orpen
and J. Starbuck, New J. Chem. 1999, 23, 1053; T. Ishiyama, N.
Matsuda, M. Murata, F. Ozawa, A. Suzuki and N. Miyaura,
Organometallics 1996, 15, 713; R. L. Thomas, F. E. S. Souza
and T. B. Marder, J. Chem. Soc. Dalton Trans. 2001, 1650; A.
B-B bond additions to other unsaturated organic substrates
are currently ongoing in our laboratories.
Grirrane, A. Corma and H. Garcia, Chem. Eur. J. 2011, 17
2467; F. Alonso, Y. Moglie, L. Pastor-Pérez and A. Sepúlveda-
Escribano, ChemCatChem. 2014, , 857; J. Jiao, K. Hyodo, H.
,
Acknowledgements
The authors wish to thank Dr. Iain Day and Dr. Alaa Abdul-Sada
(University of Sussex) for helpful discussions. M.B.A. is funded
as an EPSRC Standard Research Student (DTG) under grant no.
EP/L505109/1. V.H.M.S (grants #2013/04813-6 and
#2015/11840-5) and A.A.C.B (grant #2015/01491-3) are
thankful to Fundação de Amparo à Pesquisa do Estado de São
6
Hu, K. Nakajima, and Y. Nishihara, J. Org. Chem. 2014, 79
285.
,
14 C. N. Iverson and M. R. Smith III, J. Am. Chem. Soc. 1995, 117,
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Scott, W. Clegg and N. C. Norman, Organometallics 1996, 14
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,
Paulo for financial support, and to the LCCA-USP for the 15 A. Finch, P. J. Gardner and A. F. Webb, J. Chem.
Thermodynamics 1972, 4, 497.
computational support.
16 Q. Cui, D. G. Musaev and K. Morokuma, Organometallics
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17 N-heterocyclic Carbenes: Effective Tools for Organometallic
Synthesis, Ed. S. P. Nolan, Wiley-VCH, Weinheim, 2014; N-
Heterocyclic Carbenes in Transition Metal Catalysis and
Organocatalysis, Ed. C. S. J. Cazin, Springer, Heidelberg, 1st
edn, 2011; N-Heterocyclic Carbenes in Transition Metal
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18 M. B. Ansell, D. E. Roberts, F. G. N. Cloke, O. Navarro and J.
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