Oxidative addition of a strained C-C bond onto electron-rich rhodium(I) at room temperature

Article abstract of DOI:10.1021/ja403461f

The C-C bond of cyclobutanones undergoes oxidative addition to a T-shape rhodium(I) complex possessing a PBP pincer ligand at room temperature. The remarkable propensity of the rhodium complex for oxidative addition is attributed to the highly electron-donating nature of the boron ligand as well as the unsaturation on the rhodium center.

Full text of DOI:10.1021/ja403461f

Communication
pubs.acs.org/JACS
Oxidative Addition of a Strained C−C Bond onto Electron-Rich
Rhodium(I) at Room Temperature
†
‡
,§
,‡
†
Yusuke Masuda, Maki Hasegawa, Makoto Yamashita,* Kyoko Nozaki,* Naoki Ishida,
,
†
and Masahiro Murakami*
†
Department of Synthetic Chemistry and Biological Chemistry, Kyoto University, Katsura, Kyoto 615-8510, Japan
‡
Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan
§
Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo
12-8551, Japan
1
*
S Supporting Information
Scheme 1. Reaction of Cyclobutanone 2 with 1
ABSTRACT: The C−C bond of cyclobutanones under-
goes oxidative addition to a T-shape rhodium(I) complex
possessing a PBP pincer ligand at room temperature. The
remarkable propensity of the rhodium complex for
oxidative addition is attributed to the highly electron-
donating nature of the boron ligand as well as the
unsaturation on the rhodium center.
temperature for 72 h (Scheme 1). Rhodium carbonyl complex
3
(79% NMR yield) was generated along with cyclopropane 4
81% NMR yield). No intermediary complexesexcept for the
starting rhodium complex 1 and the resulting carbonyl complex
(
elective activation of nonpolar σ-bonds between two
1
S
elements of similar electronegativities, like C−H and C−
31
3
were observed by P NMR analysis during the course of
2
C bonds, has gained increasing attention in organic chemistry.
Whereas a wide variety of transition metal-mediated or
the decarbonylation reaction.
The formation of 3 and 4 from 1 and 2 is explained by
assuming a stepwise mechanism depicted in Scheme 2. The
-
catalyzed reactions cleaving C−H bonds have been developed
in the past decade, examples of C−C bond cleavage with
transition metal complexes are much fewer, probably due to the
kinetic inertness of C−C bonds. Considerably harsh reaction
conditions are generally required for their cleavage. Herein, we
report a striking example of oxidative addition of a C−C bond
to rhodium(I) that occurs even at room temperature.
Scheme 2. Plausible Mechanism for the Formation of 3 and
4
from 1 and 2
We recently reported the synthesis of an infinitely networked
T-shape 14-electron rhodium(I) complex 1 possessing a PBP
3
pincer ligand (Figure 1). It showed a remarkable reactivity at
Figure 1. Structure of rhodium complex 1.
carbonyl group of 2 initially coordinates to the electron-rich
and coordinatively unsaturated rhodium center of 1. The σ-
bond between the carbonyl carbon and its α-carbon undergoes
oxidative addition onto the rhodium(I) center to generate the
five-membered ring acylrhodium(III) species A. The CH2
group of A migrates onto rhodium with a carbonyl ligand left
out to form four-membered ring rhodacyclobutane B.
Reductive elimination gives rise to rhodium carbonyl complex
the rhodium center to insert into O−H bonds of phenols and
primary alcohols through a rapid dissociation of its polymeric
form into the corresponding monomeric form of T-shape
complex. This reactivity suggested that the strongly electron-
4
donating boryl ligand enhanced the electron density on the
rhodium center to facilitate its insertion into the polar O−H
bonds. We next examined the reactivity toward nonpolar
strained C−C bonds. Thus, [PBP]Rh(H)(OTf) complex was
initially treated with Me SiCH Li in C D under an argon
3
and cyclopropane 4. The oxidative addition is suggested to be
3
2
6
6
atmosphere. The resulting [PBP]Rh(I) complex 1 was treated
with cyclobutanone 2, and the mixture was stirred at room
Received: April 13, 2013
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ja403461f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
the rate-determining step, and the ensuing steps would follow
spontaneously. The hexa-coordinated complex B would be
thermodynamically unstable due to the repulsive force which
the four t-Bu groups extend to the other ligands. In contrast,
the tetra-coordinated complex 3 is less congested and gain
stabilization due to π-back-donation from Rh to CO ligand, and
therefore, would be thermodynamically far more stable. This
would provide a major driving force for reductive elimination of
cyclopropane.
reductive elimination in the reaction of 2, the complex 6 failed
to go through those steps. We presume that this difference
arose because the possible reductive elimination product
resulting from the complex 6 is energetically too high.
Two independent molecules of complex 6 with almost
identical structure were contained in the asymmetric unit. The
acyl ligand is located on the apical position of a distorted square
pyramidal structure. A vacant coordination site located trans to
the acyl ligand is effectively blocked by the two bulky t-Bu
groups in the pseudoaxial positions. The Rh−B [av. 2.044 Å]
and Rh−P [av. 2.3588 Å] bonds of 6 are slightly longer than
An analogous decarbonylation reaction of cyclobutanones is
mediated by various rhodium(I) complexes bearing PPh or
3
5
,6
3a
NHC ligands. It generally requires refluxing in toluene or in
xylene, even when a stoichiometric amount of a rhodium(I)
complex is used. The [PBP]Rh complex 1 is apparently more
active. We assume that the strongly σ-donating nature of the
those of 1 [Rh−B, 1.948(3) Å; Rh−P, av. 2.2958 Å], probably
due to the steric repulsion between the rhodabenzocyclopente-
none moiety and PBP ligand. Whereas the CO (av. 1.212 Å)
and Rh−C(benzoyl) (av. 1.995 Å) lengths are within their
7
8
9,10
1
boryl ligand as well as the unsaturation on the metal center
typical values,
the Rh−(η -benzyl) bond (av. 2.213 Å)
facilitates the intrinsically sluggish oxidative addition of the C−
C σ-bond.
located trans to boron is considerably longer than typical Rh−
(η -benzyl) bonds, clearly indicating a strong trans influence
1
9,11
We next examined the reaction of 1 with benzocyclobute-
none 5. Again, oxidative addition occurred at room temperature
and single crystals of the resulting complex were successfully
obtained using a benzene/hexane solution for recrystallization.
An X-ray crystallographic analysis revealed that the bond
between the carbonyl carbon and the sp α-carbon was site-
selectively cleaved to afford five-membered acylrhodium(III)
complex 6 (Scheme 3, Figure 2). Although an analogous
of the boryl ligand.
The H NMR spectrum in C D showed two signals of the t-
Bu groups. This magnetic inequivalency is consistent with its
Cs-symmetrical structure in the solid state. The C NMR
1
6
6
1
3
spectrum exhibits a resonance assigned to the carbonyl carbon
3
1
at δ 211.6 with two distinct couplings of J (d, 33 Hz) and
C
RhC
2
JPC (t, 5 Hz). The signal of the methylene carbon derived from
benzocyclobutenone was observed at δ 23.5 with couplings of
C
1
2
31
JRhC (d, 26 Hz) and J (t, 6 Hz). In the P NMR spectrum, a
PC
1
Scheme 3. Stoichiometric Reaction of Rhodium Complex 1
with Benzocyclobutenone 5
doublet signal (δ 93.7) with J of 139 Hz was observed. The
P RhP
1
coupling constant is smaller than that of 1 ( J = 192 Hz),
RhP
which reflects weaker π-back-donation from the Rh(III) center
than that from Rh(I). The 11B nucleus resonated at around 53
ppm. This value is similar to those of [PBP]Rh(CO) and
[
PBP]Rh(CH CH ) rather than those of [PBP]Rh(H)(Cl)
2
2
and [PBP]Rh(H)(OTf), which possess interaction between
boron and hydride atoms. The strong signal observed at 1639
cm in the IR spectrum was similar to those of the related
acylrhodium complexes, in accordance with ν values
predicted by DFT calculation.
The reactivity of various alcohols toward the rhodium
3a
−1
CO
12
3a
complex 1 was examined in our previous study. Whereas
primary alcohols underwent oxidative addition, secondary
alcohols such as cyclohexanol and 2-propanol failed to react
with 1. Much to our surprise, 3,3-diphenylcyclobutanol 7,
slightly smaller than cyclohexanol because of the strained four-
membered ring, facilely reacted with 1 in C D at room
6
6
temperature to generate the new rhodium hydride complex 9
together with unidentified byproducts (Scheme 4). This is the
first example of oxidative addition of the O−H linkage of
secondary alcohols to 1. Furthermore, the ring-opened alkane
11 (10%) was generated together with rhodium carbonyl
Figure 2. ORTEP drawing of 6 (50% thermal ellipsoid, hydrogen
atoms and one of two independent molecules are omitted for clarity)
Selected bond lengths (Å) and angles (°) as averages: B−Rh 2.044,
Rh−C(acyl) 1.955, C(acyl)O 1.212, Rh−C(benzyl) 2.213, Rh−P
13
complex 3 (16%) upon heating at 80 °C. It is assumed that
the rhodium(III) hydride complex 9 undergoes β-hydride
14
2
.3588, B−N 1.430, B−Rh−C(benzyl) 174.89, P−Rh−P 151.10, N−
elimination to form rhodium(I) dihydrogen complex C.
B−N 105.4.
Subsequent oxidative addition affords (dihydrogen)-
rhodacyclopentanone intermediate D. The carbon−rhodium
bond of D is hydrogenolyzed to produce E. Migratory
elimination of a carbonyl group forms F and subsequent
reductive elimination gives 3 and 11. The reaction with N-
toluenesulfonyl azetidinol proceeded more cleanly to afford
carbonyl complex 3 (75%) and dimethylamide 12 (65%).
In summary, we disclose that a thermally stable C−C bond of
cyclobutanones was cleaved, even at room temperature, by
oxidative addition onto T-shape [PBP]Rh complex 1. Crystallo-
reaction of benzocyclobutenone with ClRh(PPh ) has been
3
3
9
reported by Liebeskind et al., the reaction requires heating at
1
30 °C, and oxidative addition occurs with both the C(CO)−
3
2
C(sp ) and C(CO)−C(sp ) bonds to afford a mixture of
isomeric products. Thus, the PBP pincer ligand not only
accelerates the C−C oxidative addition but also confers the site-
selectivity probably due to the bulky P(t-Bu) groups. Whereas
2
the assumed intermediate A underwent elimination of CO and
B
dx.doi.org/10.1021/ja403461f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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ASSOCIATED CONTENT
Supporting Information
■
land, K. Eur. J. Org. Chem. 2012, 2683−2706.
*
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Detailed experimental procedures, physical and spectroscopic
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Internet at http://pubs.acs.org.
(
4) Ref 3b showed that the boryl ligand in PBP pincer could be
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AUTHOR INFORMATION
Corresponding Author
makoto@oec.chem.chuo-u.ac.jp; nozaki@chembio.t.u-tokyo.ac.
jp; murakami@sbchem.kyoto-u.ac.jp
■
[
−1
CO stretch of [PBP]Rh-CO (3) in ref 3a was 1933 cm , similar to
−1
that of benzene-based PCP system (1925 cm , see: Moulton, C. J.;
Shaw, B. L. J. Chem. Soc., Dalton 1976, 1020−1024. ). However, it was
difficult to estimate the difference in donor ability of X ligand in PXP
system (X = B, C) from CO stretch, because the Rh−C bond lengths
in these system were different [3, 1.908(3) Å; PCP, 1.848(16) Å]. It
may lead to difference in orbital overlapping between d-orbital of Rh
and π*-orbital of CO ligand. That is, it may cancel the lower
wavenumber shift come from electron richness at Rh center by strong
σ-donor ability of boryl ligand.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Funding Program for Next
Generation World-Leading Researchers, Green Innovation,
from JSPS (K.N.), Grants-in-Aid for Scientific Research on
Innovative Areas “Molecular Activation Directed toward
Straightforward Synthesis” [22105005 (M.M.); 23105510
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996, 118, 8285−8290. (c) Matsuda, T.; Shigeno, M.; Murakami, M.
Chem. Lett. 2006, 35, 288−289.
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Nature 1965, 206, 295−296. (b) Muller, E.; Segnitz, A.; Langer, E.
(
M.Y.)] from MEXT, Advanced Catalytic Transformation
program for Carbon utilization (ACT-C) from JST (M.M.),
and grants from Natural Sciences from the Mitsubishi
Foundation (M.Y.), Toray Science Foundation (M.Y.), and
Asahi Glass Foundation (M.Y. and M.M.). The computations
were performed at the Research Center for Computational
Science, Okazaki, Japan.
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(
̧
̧
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(
dimethylbut-1-ene (2 equiv), cyclopropane 4 (46%) was obtained
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takes place to generate the dihydrogen complex C, which partially
reacts with the alkene.
D
dx.doi.org/10.1021/ja403461f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX