Facile Rh-C Bond Cleavage of Rhodium(III) Benzyl Porphyrin Complex in DMSO with Strong Bases

Article abstract of DOI:10.1002/ijch.201500039

Rhodium-carbon bond cleavage represents an essential step for small molecule activation and transformation by rhodium porphyrin complexes. This article reports on a new method for the cleavage of porphyrin rhodium(III)-carbon bonds in DMSO solvent with st

Full text of DOI:10.1002/ijch.201500039

Communication
DOI: 10.1002/ijch.201500039
Facile Rh-C Bond Cleavage of Rhodium(III) Benzyl
Porphyrin Complex in DMSO with Strong Bases
Zikuan Wang, Haolin Yin, Lin Yun, Xu Liu, and Xuefeng Fu*[a]
1
Abstract: Rhodium-carbon bond cleavage represents an es-
sential step for small molecule activation and transforma-
tion by rhodium porphyrin complexes. This article reports
on a new method for the cleavage of porphyrin rhodi-
um(III)-carbon bonds in DMSO solvent with strong bases.
UV-Vis, in situ H NMR spectrum and GC-MS analysis con-
firmed generation of rhodium(I) species and monodeuterat-
ed toluene. Mechanistic studies revealed that strongly re-
ducing CD3SOCD2À, formed in situ from DMSO-d6 and
strong bases, promoted Rh-C bond cleavage.
Keywords: porphyrin · reduction · Rh-C bond cleavage · rhodium
Rhodium(III) and related group nine Co(III) and Ir(III)
porphyrin complexes are drawing increasing interest in
view of their rich reactivities toward various substrates.[1,2]
Our research interest lies in exploring the scope and
mechanism of small molecule activation and catalytic
transformations,[2h,l,3] of which the cleavage of M-X (X=C,
O, N, etc.) bonds stands as an essential step for efficient
and selective conversion. A remarkable scope of C-H[4]
and C-C[1d,j,5] bond activation of organic molecules to
form organo-rhodium porphyrin complexes were report-
ed. However, the high stability of resulting Rh-C bonds
inhibits further applications in catalytic transformations.
Current state-of-the-art methods for cleaving Rh-C
bonds can be classified into homolytic and heterolytic
pathways. Photolysis of rhodium alkyl porphyrin com-
plexes is a well-established method for homolytic cleav-
age of Rh-C bonds to generate porphyrin rhodium(II)
metallo-radicals.[6] We have recently reported visible light
induced Rh-C bond cleavage at ambient temperature in
a catalytic Si-C bond hydroxylation process.[3b] Study of
heterolytic Rh-C bond cleavage is relatively rare.[7] Gro-
ves[1h,1i] and our group[3b]achieved Rh-C bond cleavage via
an intramolecular SN2 pathway in basic benzene and
DMSO solutions. In this communication, we report
a unique Rh-C bond cleavage reactivity of a rhodium
benzyl porphyrin complex in DMSO with strong bases.
The complex, benzylrhodium(III) tetra (p-sulfonato-
phenyl) porphyrin ((TSPP)RhIII-Bn), was synthesized
from (TSPP)RhI and PhCH2Br.[3b] Heating a DMSO-d6
solution containing (TSPP)Rh-Bn and excess NaOMe at
808C under argon resulted in a color change from orange
to brown over a period of 12 h. The intensity of UV-Vis
Figure 1. UV-Vis spectral changes of a DMSO-d6 solution contain-
ing 3”10À5 molLÀ1 of (TSPP)Rh-Bn and 6”10À3 molLÀ1 of NaOMe
under room temperature.[8]
(lmax =468, 624 nm) assigned to a new species arose with
isosbestic points at 407, 449, 513, 565 and 580 nm. In situ
1H NMR monitoring revealed the gradual consumption
of (TSPP)Rh-Bn, together with the appearance of a sin-
glet at 7.92 ppm and a 1:1 :1 triplet at 2.27 ppm (J=
2 Hz), with integral ratio 4 :1, assigned to pyrrole CH of
[a] Z. Wang, H. Yin, L. Yun, X. Liu, X. Fu
College of Chemistry and Molecular Engineering
Peking University
Beijing 100871 (P. R. China)
absorption peaks corresponding to (TSPP)Rh-Bn (lmax
=
e-mail: fuxf@pku.edu.cn
424, 536 nm) decreased as the reaction proceeded
(Figure 1). Concomitantly, red-shifted Soret and Q bands
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/ijch.201500039.
Isr. J. Chem. 2016, 56, 188 – 191
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Communication
Scheme 1. NaOMe-promoted Rh-C bond cleavage of (TSPP)Rh-Bn in DMSO-d6.
(TSPP)RhI and benzyl CH2 of PhCH2D, respectively
(Scheme 1). GC-MS analysis confirmed the mono-deuter-
ated nature of toluene product. Performing the reaction
in normal DMSO, on the other hand, led to exclusive for-
mation of PhCH3 at a much faster rate than that in
DMSO-d6 (complete conversion was achieved within
5 min in DMSO-h6 at room temperature; see Supporting
Information), confirming the origin of the deuterium in
PhCH2D to be DMSO solvent.
To establish the mechanism, the role of NaOMe was
probed by comparison studies. Treatment of a DMSO-d6
solution of (TSPP)Rh-Bn with NaOMe at room tempera-
ture caused an upfield shift of porphyrin and benzyl reso-
nances; for example, resonances of pyrrole CH and
benzyl CH2 shifted from 8.60 and À4.59 ppm to 8.33 and
À5.26 ppm, respectively (see Supporting Information for
detailed NMR characterization data). Similar upfield
shifts were observed upon addition of pyridine (py) or
PPh3 to (TSPP)Rh-Bn (albeit less pronounced), suggest-
ing that the upfield shift resulted from MeOÀ coordina-
tion to RhIII. Ligand-induced homolysis of analogous por-
phyrin cobalt(III)-carbon bonds are well documented.[9]
However, donor ligands other than NaOMe (py, NEt3,
PhCH2NH2, and PPh3) were ineffective in promoting this
transformation, despite that coordination was observed in
all four cases (Table 1, entries 2, 4–6). Moreover, blocking
the vacant coordination site of (TSPP)Rh-Bn with py
prior to addition of NaOMe had no apparent inhibitory
effect on the formation of (TSPP)RhI and PhCH2D
(Table 1, entry 3). These evidences collectively argue
against a coordination-induced Rh-C bond scission mech-
anism.
Table 1. Reaction of various ligands or bases with (TSPP)Rh-Bn in
DMSO-d6.[a]
Entry
Ligand or base
Products[b]
1
2
3
4
5
6
7
8
9
NaOMe
py
py+NaOMe
NEt3
PhCH2NH2
PPh3
LDA
(TSPP)RhI, PhCH2D
(TSPP)Rh(py)-Bn
(TSPP)RhI, PhCH2D
(TSPP)Rh(NEt3)-Bn
(TSPP)Rh(NH2CH2Ph)-Bn
(TSPP)Rh(PPh3)-Bn
(TSPP)RhI, PhCH2D
LiHMDS
KOH(saturated)
(TSPP)RhI, PhCH2D
27% (40%, 3d), (TSPP)RhI, PhCH2D
[a] Reaction conditions: 10 eq. ligand or base, Ar atmosphere,
1
808C, 12 h. [b] Determined by H NMR and GC-MS; yield >95%
unless specified.
and organoradicals. Hence, various strong bases of com-
parable or superior basicity than NaOMe were tested
(Table 1, entries 7–9). It should be noted that the sterical-
ly bulky amides, including lithium diisopropylamide
(LDA) and lithium bis(trimethylsilyl)amide(LiHMDS),
were also found to be highly effective, affording the prod-
ucts (TSPP)RhI and PhCH2D quantitatively (Table 1, en-
tries 7–8). This result also rules out a coordination-in-
duced mechanism. Retarded conversion was observed for
KOH (Table 1, entry 9), which is presumably due to its
low solubility in DMSO. In parallel to the applicability of
various strong bases, the sulfoxide moiety in DMSO was
found to be crucial for the reduction. Solvents without
sulfoxide functional groups, including protic (CD3OD,
D2O) and polar aprotic solvents (DMF) led to no conver-
sion (Table 2, entries 2–4); however, tetrahydrothio-
phene-1-oxide was found to be as effective as DMSO-d6
for this reaction (Table 2, entry 5).
Subsequently, we investigated the role of NaOMe as
a base to deprotonate DMSO. It is well known that
DMSO can react wiÀth strong bases to give the strongly
reducing CH3SOCH2 anion,[10] which is capable of reduc-
ing O2 and organic substrates via single electron trans-
fer.[11] Meanwhile, alkylcobalamins[12] and organocobalt
phthalocyanines,[13] analogous to organorhodium porphyr-
ins,[14] have been shown to undergo CoÀC bond cleavage
upon single electron reduction, affording Co(I) complexes
Additionally, the reaction was found to be considerably
slower in the presence of air (reaching only 70% conver-
sion after 24 h) than under argon atmosphere and com-
pletely inhibited by 1 atm of O2. This result indicates the
possible radical nature of the reaction. Indeed, perform-
ing the reaction in the presence of 2 eq. of 2,2,6,6-tetra-
methylpiperidineoxy (TEMPO) resulted in the quantita-
Isr. J. Chem. 2016, 56, 188 – 191
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www.ijc.wiley-vch.de
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Communication
Scheme 2. Proposed mechanism for Rh-C bond cleavage of (TSPP)Rh-Bn in DMSO-d6.
Table 2. Reaction of NaOMe with (TSPP)Rh-Bn in various solvent-
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Entry
Solvent
Product[b]
(TSPP)RhI, PhCH2D
No reaction
No reaction
No reaction
1
2
3
4
DMSO-d6
CD3OD
D2O
DMF
5
(TSPP)RhI, PhCH3
[a] Reaction conditions: 10 eq. NaOMe, Ar atmosphere, 808C, 12 h.
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(TSPP)Rh-Bn in moderately polar and non-polar solvents. [b] Deter-
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dine (evidenced by HRMS, see Supporting Information),
supporting the formation of a benzyl radical intermediate
after RhII-Bn bond cleavage.
Based on the above results, we propose the following
mechanism, as depicted in Scheme 2. Deprotonation of
DMSO-d6 by NaOMe led to CD3SOCD2À, which further
underwent single electron transfer to (TSPP)RhIII-Bn.
Homolysis of resulting RhII-Bn intermediate gave
(TSPP)RhI and a benzyl radical, which subsequently ab-
stracted a deuterium atom from DMSO-d6 to afford
PhCH2D.
In conclusion, we developed a new method for the
cleavage of porphyrin rhodium(III)-carbon bond using
a strong reductant, CD3SOCD2À, generated in situ from
the solvent DMSO-d6 and strong bases. Further efforts to
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Acknowledgement
This work was supported by NSFC 21171012 and
21322108.
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www.ijc.wiley-vch.de
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Received: May 10, 2015
Accepted: June 12, 2015
Published online: August 3, 2015
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191