Two reversible σ-bond metathesis pathways for boron-palladium bond formation: Selective synthesis of isomeric five-coordinate borylpalladium complexes

Article abstract of DOI:10.1021/ja400294m

Two reversible σ-bond metathesis pathways for B-B bond activation to give borylpalladium complexes are demonstrated in the reaction of η²-(Si-H)Pd(0) complexes with B₂pin₂. These two pathways are connected by fluxional beh

Full text of DOI:10.1021/ja400294m

Communication
pubs.acs.org/JACS
Two Reversible σ‑Bond Metathesis Pathways for Boron−Palladium
Bond Formation: Selective Synthesis of Isomeric Five-Coordinate
Borylpalladium Complexes
Naohiro Kirai, Jun Takaya, and Nobuharu Iwasawa*
Department of Chemistry, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan
S
* Supporting Information
Scheme 1. Selective Synthesis of trans-2 and cis-3
ABSTRACT: Two reversible σ-bond metathesis pathways
for B−B bond activation to give borylpalladium complexes
are demonstrated in the reaction of η2-(Si−H)Pd(0)
complexes with B2pin2. These two pathways are connected
by fluxional behavior of the Si−H bond and can be
efficiently controlled by the appropriate choice of
phosphine ligand, enabling the selective synthesis of two
types of five-coordinate borylpalladium complexes.
ate transition metal-promoted activation of the B−B bond
L
of diboron to give borylmetal complexes is an important
Generation of HBpin was also observed by 1H NMR
spectroscopy in both reactions. The structures of these
complexes were characterized by X-ray analyses (Figure 1).
The geometry around the metal center seems to be trigonal-
bipyramidal in both complexes. This is the first example of the
synthesis and isolation of five-coordinate borylpalladium
complexes.9 Facile formation of the five-coordinate structure
would be attributed to the pyramidalized central Si(sp3) atom of
the pincer ligand, which made the four-coordinate square-planar
structure more strained. The stereochemistry of these complexes
was confirmed to be retained in solution, as determined by NMR
analyses [see the Supporting Information (SI)]. The configura-
elementary step in numerous catalytic borylation reactions.1,2
The reaction generally proceeds via (1) oxidative addition of the
B−B bond to the electron-rich metal center or (2) transfer of a
boryl group to M−X (X = O, N, halogen), and a few examples of
σ-bond metathesis with M−R bonds (R = C, B, etc.) have been
reported.3 Especially in the Pd-catalyzed reactions, the formation
of borylpalladiums is mostly limited to the former two pathways,
despite the wide use of Pd in a variety of borylation reactions
using diboron.4,5 The discovery of a new type of B−B bond
activation by Pd complexes would be highly useful for the
development of new catalytic borylation reactions. Herein we
report two reversible σ-bond metathesis pathways for B−Pd
bond formation in the reaction of η2-(Si−H)Pd(0) complexes
with bis(pinacolato)diboron (B2pin2) that are connected by
fluxional behavior of the Si−H bond. The two pathways can be
efficiently controlled by the appropriate choice of phosphine
ligand, enabling for the first time the selective synthesis of two
types of five-coordinate borylpalladium complexes.
We previously reported a new PSiP-pincer−Pd-catalyzed
dehydrogenative borylation of alkenes in which the key
intermediate, a PSiP−borylpalladium complex, is proposed to
be generated by the reaction of B2pin2 with the palladium hydride
generated from the corresponding palladium triflate complex and
AlEt3.6 This finding prompted us to investigate the reaction of
B2pin2 with η2-(Si−H)Pd(0) complexes, which could act as
palladium hydrides via oxidative addition of the Si−H bond.7
Treatment of PMe3-coordinated η2-(Si−H)Pd(0) complex 1a
with 2 equiv of B2pin2 in benzene-d6 at 303 K for 24 h afforded
the five-coordinate borylpalladium complex trans-(B,Si)-2a, in
which the B and Si atoms are in apical positions (Scheme 1).8
Surprisingly, the same reaction of 1b bearing PPh3 instead of
PMe3 selectively gave in good yield another type of five-
coordinate borylpalladium complex, cis-(B,Si)-3b, having the B
and Si atoms at equatorial and apical positions (Scheme 1).
Figure 1. ORTEP diagrams of the five-coordinate borylpalladium
complexes (a) trans-(B,Si)-2a and (b) cis-(B,Si)-3b. Thermal ellipsoids
are shown at the 50% probability level; H atoms and solvent molecules
have been omitted for clarity. Selected bond lengths (Å) and angles
(deg) for trans-(B,Si)-2a: Pd−B, 2.120(8); Pd−Si, 2.396(2); Pd−P1,
2.350(2); Pd−P2, 2.312(2); Pd−P3, 2.304(2); P1−Pd−P2, 113.81(6);
P2−Pd−P3, 131.67(6); P3−Pd−P1, 113.66(6). For cis-(B,Si)-3b: Pd−
B, 2.095(9); Pd−Si, 2.338(2); Pd−P1, 2.371(2); Pd−P2, 2.448(2); Pd−
P3, 2.394(2); P1−Pd−P2, 102.87(6); P2−Pd−P3, 113.11(7); P3−Pd−
P1, 101.93(7).
Received: January 10, 2013
Published: February 5, 2013
© 2013 American Chemical Society
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Communication
tional stability of 2a and 3b is noteworthy since such five-
coordinate Pd(II) complexes are often proposed as transient
intermediates in cis−trans isomerization reactions of 16-electron
Pd(II) complexes via an associative mechanism.10 Thus, this is a
quite rare example of the selective formation and isolation of
either a trans or a cis complex simply by changing one of the
ligands in a five-coordinate Pd complex.11,12
Several experimental results shed light on the mechanism and
the origin of the stereoselectivity as follows. Monitoring of the
time dependence of the reaction of PPh3-coordinated complex
1b with B2pin2 at 303 K revealed that trans-(B,Si)-2b was actually
generated in the beginning along with HBpin, and 2b gradually
disappeared as cis-(B,Si)- was formed with time (Table 1). At
found that PMe3-coordinated trans-(B,Si)-2a scarcely reacted
with 1 equiv of HBpin, giving 1a in only 3% yield after 1 h
(Scheme 2b). This result indicates that in the case of PMe3-
coordinated complexes, the reverse reaction of the kinetic
product, trans-(B,Si)-2a, is very slow at room temperature
because of the stronger coordination of PMe3 than PPh3.13,14
Thus, the two reaction pathways to give the trans- or cis-(B,Si)
product are controllable by the appropriate choice of the
monophosphine ligand (Figure 2). The relative energy differ-
Table 1. Time Course Analysis of the Reaction of 1b with
B2pin2
Figure 2. Two reversible B−Pd bond formation reactions.
ences of 1−3 were estimated from the equilibrium constants for
1b and 3b and for 1a and 2a (HBpin/B2pin2-mediated reaction)
and for the phosphine-exchange reactions of 1a−3a with PPh3 at
298 K (Table 2; see the SI for details). These studies clarify that
(1) cis-(B,Si)-3 is more stable than trans-(B,Si)-2 by 2−3 kcal/
mol for both phosphine ligands; (2) the PMe3-coordinated
borylpalladium complex is more stable than PPh3-coordinated
one by ca. 6 kcal/mol for each isomer; and (3) PPh3-coordinated
trans-(B,Si)-2b is less stable than the starting complex 1b,
whereas PMe3-coordinated trans-(B,Si)-2a is more stable than
1a.
entry
time
1b
trans-(B,Si)-2b
cis-(B,Si)-3b
1
2
3
4
30 min
1 h
77%
71%
59%
28%
18%
15%
9%
5%
14%
32%
70%
2 h
5 h
3%
first, we considered the possibility that the initially formed trans-
(B,Si)-2b underwent thermal isomerization to cis-(B,Si)-3b.
However, isolated trans-(B,Si)-2b itself was confirmed to be
mostly stable in benzene-d6 at 303 K even after 24 h (14% of 2b
was converted to 3b), indicating that such thermal isomerization
was negligible under the reaction conditions.
In view of the slow isomerization of trans-(B,Si)-2b under
thermal conditions, the possibility that B2pin2 or HBpin might be
involved in this isomerization process was investigated. Although
treatment of 2b with B2pin2 caused no reaction, it was a surprise
to find that 2b actually reacted with 1 equiv of HBpin within 1 h at
303 K to give 1b in 65% yield along with the formation of B2pin2
(65% as determined by GC) (Scheme 2a). Thus, the B−Pd bond
Table 2. Energies of 1−3 (in kcal/mol) Relative to 1a at 298 K
2a
2b
−1.9
+4.4
1a
1b
0.0
3a
3b
−4.1
+1.5
+2.5
Density functional theory (DFT) calculations indicated that
there exist two distinct reaction pathways from 1 and B2pin2 to
give trans-(B,Si)-2 and cis-(B,Si)-3 through reversible oxidative
addition/reductive elimination of the Si−H bond with Pd (i.e.,
fluxional behavior of Si−H bond) (Figure 3).15−17 The five-
coordinate trans-(B,Si) borylpalladium complex E is formed
through the reaction of η2-(Si−H)Pd(0) complex A and B2 [a
model of B2pin2 in which B = B(ethylene glycolato)] via
transition state TSBC, leading to square-planar borylpalladium
complex D. Coordination of PMe3 to D affords E.18 On the other
hand, A can function as a palladium hydride, F, through
reversible oxidative addition/reductive elimination of the Si−H
bond with Pd via TSAF, as previously proposed.7 The pathway to
Scheme 2. Reversibility of B−Pd bond formation
cis-(B,Si) complex I involves the reaction of F and B2 via TSGH
.
Dissociation of HB (a model of HBpin) from complex H
followed by coordination of PMe3 results in the formation of cis
complex I as a more stable product than the trans complex E by
4.8 kcal/mol. Generation of the coordinatively unsaturated η2-
(Si−H)Pd(0) complex A in the initial step is strongly supported
by the experimental result that addition of 1 equiv of phosphine
significantly retarded the reaction of 1 with B2pin2 (see the SI).19
Both Pd−B bond-forming steps (TSBC and TSGH) are easily
feasible energetically, while the conversion of A to F (TSAF)
requires the highest activation energy through the reaction
(TSBC = +16.5, TSGH = +15.8, TSAF = +24.1 kcal/mol).
Therefore, the formation of E via TSBC is the kinetically favored
formation is reversible. The same reaction of cis-(B,Si)-3b with
HBpin also afforded 1b and B2pin2, but in lower yield (ca. 20%).
Both reactions finally came to equilibrium after 24 h to give a ca.
70:30 1b:3b mixture. These results indicate that the formation of
trans-(B,Si)-2b is kinetically favored but also reversible and that
the reaction affords the thermodynamically more stable product,
cis-(B,Si)-3b, as the major product in 24 h. Furthermore, it was
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Figure 3. Free energy profiles calculated using DFT at the PW91PW91/[6-31G(d,p)/LANL2DZ] level in THF(PCM). B2 is a model of B2pin2 in which
the pinacolato moiety is replaced by ethylene glycolato.
pathway, and the formation of I via TSAF followed by TSGH is the
thermodynamically favorable process.20 It is clear that the kinetic
pathway becomes easily reversible in the case of the PPh3 system
according to the energy relationships clarified in Table 2.
The core structures of the transition states involved in B−Pd
bond formation are depicted in Figure 4. TSBC is best described
Pd−P2 angles largely bent away from 180° (115.5° in TSBC;
124.2° in TSGH).22 This reaction is quite unique in that (1) two
reversible σ-bond metathesis pathways for B−Pd bond formation
via B−B bond activation are demonstrated; (2) the two pathways
are connected via fluxional behavior of the Si−H bond; and (3)
the appropriate choice of monophosphine ligand enables
efficient control of the two pathways, leading to the stereo-
selective synthesis of two types of borylpalladium complexes.
In conclusion, we have demonstrated two reversible σ-bond
metathesis pathways for B−Pd bond formation in the reaction of
η2-(Si−H)Pd(0) complexes with B2pin2 that are connected by
fluxional behavior of Si−H bond. The two pathways can be
efficiently controlled by the appropriate choice of the phosphine
ligand, enabling the selective synthesis of two types of five-
coordinate borylpalladium complexes. We envision that such σ-
bond metathesis reactivity of PSiP−Pd(II) complexes in B−B
bond activation will open up new possibilities for the
development of new borylation reactions. Further mechanistic
studies and the development of new synthetic reactions utilizing
these phenomena are ongoing in our group.
Figure 4. Core structures of (a, b) TSBC and (c, d) TSGH: (a, c) side
views; (b, d) front views.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental computational details; spectral and analytical data
for 1−3; and crystallographic data for 1a, 2a, 2b, 3a, and 3b
(CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
as a structure in which the Si−H bond is mostly cleaved and the
B1−H bond of the borane is forming on Pd (Figure 4a,b). This
structure can be regarded as the TS for a σ-bond metathesis
reaction between the B1−B2 bond and the H−Pd bond of the
transient trigonal-pyramidal palladium hydride generated by
oxidative addition of the Si−H bond to Pd(0) (similar to TSAF).
In TSGH, the B2−H, H−Pd, and B1−Pd bond lengths are 1.40,
1.74, and 2.26 Å, respectively, indicating that formation of the
B2−H and B1−Pd bonds and cleavage of the B1−B2 and H−Pd
bonds proceed simultaneously in a σ-bond metathesis reaction
(Figure 4c,d). No Pd(IV) intermediates were found before and
after TSBC and TSGH by intrinsic reaction coordinate
calculations. This is the first demonstration of a σ-bond
metathesis mechanism involvinig a B−B bond and Pd−H
bond,21 and it should provide new mechanistic aspects of the
activation of B−B bonds. It should be noted that the PSi(sp3)P
pincer ligand would facilitate the formation of a six-coordinate-
like structure around Pd in these transition states, with the P1−
AUTHOR INFORMATION
Corresponding Author
niwasawa@chem.titech.ac.jp
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “Molecular Activation Directed
toward Straightforward Synthesis” (22105006), a Grant-in-Aid
for Scientific Research (A) (24245019), and a Grant-in-Aid for
Young Scientists (A) (24685006) from the Ministry of
Education, Culture, Sports, Science, and Technology of Japan.
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