Synthesis of bis(σ-aryl)dirhodium(III) caprolactamates by oxidative arylation with arylboronic acids

Article abstract of DOI:10.1039/b806283h

Recently discovered stable bis(σ-phenyl)dirhodium(III) caprolactamate and its substituted derivatives are conveniently prepared in high yields from dirhodium(II) caprolactamate and commercially available arylboronic acids in a copper-catalyzed process. The Royal Society of Chemistry.

Full text of DOI:10.1039/b806283h

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COMMUNICATION
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Synthesis of bis(r-aryl)dirhodium(III) caprolactamates by oxidative
arylation with arylboronic acidswz
Jian-Hua Xie, Jason M. Nichols, Conrad Lubek and Michael P. Doyle*
Received (in Cambridge, UK) 14th April 2008, Accepted 25th April 2008
First published as an Advance Article on the web 14th May 2008
DOI: 10.1039/b806283h
Recently discovered stable bis(r-phenyl)dirhodium(III) caprolac-
tamate and its substituted derivatives are conveniently prepared
in high yields from dirhodium(II) caprolactamate and commer-
cially available arylboronic acids in a copper-catalyzed process.
the synthesis of bis(s-aryl)dirhodium(III) caprolactamates
(Scheme 1). When phenylboronic acid replaced sodium tetra-
phenylborate in the previously reported procedure [10 equiv.
PhB(OH)2, 15 mol% (CuOTf)2ꢀC6H6, reaction in 4 : 1
CH2Cl2–MeOH], 1 was formed in modest yield (66%). Mod-
erately strong bases aided complete product formation, and
sodium methoxide was selected. Optimization of the reaction
conditions showed that no reaction occurred in the absence of
copper catalyst. The use of at least 5 equiv. of phenylboronic
acid was required. When this reaction was performed at room
temperature with the bis-acetonitrile complex of dirhodium(II)
caprolactamate and 5.0 equiv. of phenylboronic acid in the
presence of 15 mol% of [(CuOTf)2ꢀC6H6] with 10 equiv. of
sodium methoxide in 4 : 1 CH2Cl2–MeOH, 1 was isolated in
98% yield following a 12 h reaction time. A low yield of 1
(18%) was formed when phenylboronic acid 1,3-propylene
glycol ester was used in place of phenylboronic acid, and there
was no conversion to 1 with sodium phenyltrifluoroborate.
Furthermore, only methanol promoted high product yields for
1; a reaction performed with sodium methoxide in ethanol
gave 1 in only 29% yield, and none of 1 was obtained from a
reaction performed in 2-propanol.
We recently reported the discovery of bis(phenyl)dirhodiu-
m(III) caprolactamate (1),1 and characterized this novel di-
nuclear paddlewheel complex as the first unequivocal example
of a dirhodium(III) compound. Extensive analyses established
that 1 does not have a metal–metal bond. Although dimeta-
l(III) compounds exist,2 few have metal–carbon bonds with the
most extensive listings being those of ruthenium3 and os-
mium.4 Diruthenium(III) organometallic structures have been
constructed from 1-alkynes and stable diruthenium(III) com-
pounds, and they have shown potential as molecular wires.5
Arylrhodium(III) compounds are well known, but they are
generally formed by transmetallation;6 however, these same
methods are not applicable to the formation of their paddle-
wheel dirhodium counterparts that include 1.
Bis(phenyl)dirhodium(III) caprolactamate (1) was prepared
from either dirhodium(II) caprolactamate [Rh2(cap)4] or its
oxidized dirhodium(II,III) analog with sodium tetraphenyl-
borate using copper catalysis. Oxidative arylation does not
occur in the absence of copper.1 However, although sodium
tetraphenylborate is commercially available, this compound
and substituted tetraarylborates are difficult or impossible to
prepare by known methods.7 Consequently, we sought an
alternative process for the synthesis of these compounds, and
now report a convenient general methodology for the synthesis
of bis(s-aryl)dirhodium(III) caprolactamates in high yields.
With the wide availability and synthetic versatility of
arylboronic acids,8 we explored their possible utilization for
Scheme 1 Preparation of bis(s-aryl)dirhodium(III) caprolactamates.
Representative arylboronic acids were surveyed to deter-
mine the generality of this procedure, and the results of this
study are presented in Table 1. With the exception of the
4-dimethylaminophenyl-substituted product (2g), this metho-
dology is tolerant of both electron-donating and electron-
withdrawing substituents on the aromatic ring. Reaction times
longer than 12 h showed no obvious advantage in product
yield, but doubling the amount of arylboronic acid did provide
modest increases in product yield in certain cases. However,
arylboronic acids with ortho substituents (Me or MeO) did not
undergo this reaction, nor did ferrocenylboronic acid and
2-furanylboronic acid.
Department of Chemistry and Biochemistry, University of Maryland,
College Park, MD 20742, USA. E-mail: mdoyle3@umd.edu; Fax:
+1 301-314-2779; Tel: +1 301-405-8388
w Electronic supplementary information (ESI) available: Experimental
details and data for new compounds. See DOI: 10.1039/b806283h
z Dedicated to Professor Andrew B. Holmes on the occasion of his
65th birthday.
Examination of the suitability of alternatives to the rela-
tively expensive and air-sensitive (CuOTf)2ꢀC6H6 was
ꢁc
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Chem. Commun., 2008, 2671–2673 | 2671

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Table 1 Copper(I) triflate catalyzed oxidative arylation of dirhodiu-
m(II) caprolactamate with arylboronic acidsa
Compound
Ar =
equiv. ArB(OH)2
2, yield %b
2a
2b
2b
2d
2c
2e
2f
2g
2h
2h
3-NO2C6H4
3-HCOC6H4
3-HCOC6H4
4-HCOC6H4
4-BrC6H4
4-MeOC6H4
4-(BocNH)C6H4
4-Me2NC6H4
3-Thiophenyl
3-Thiophenyl
10
10
5
10
5
5
10
5
64
54
53
99
91
89c
71
34
72
80d
Scheme 2 Phenyl transfer from boronate ester to rhodium.
greater than those when (CuOTf)2ꢀC6H6 was employed. Ar-
ylboronic acids with both electron-donating and electron-
withdrawing substituents were suitable substrates, but those
with strong electron-withdrawing substituents like nitro in the
para position led to a reduction in product yield. All of these
compounds are air, water, and thermally stable.
5
10
a
Reactions were performed at room temperature in
4 : 1
CH2Cl2–MeOH with 15 mol% (CuOTf)2ꢀC6H6 and 10 equiv. NaOMe
b
with a reaction time of 12 h. Isolated yield after column chromato-
graphy on silica gel or crystallization from the reaction solution by
Although mechanisms for aryl transfer involving p-complex
formation between metals with an open coordination site and
phenylboronates9 or via boron metallocycles10 are well known
for rhodium(I) complexes, the evidence obtained from this
study points to a scheme in which the aryl group is transferred
from boron to rhodium through a boronate complex with the
oxidized dirhodium(II,III) caprolactamate (Scheme 2). Solvo-
lysis of the boronic acid is rapid11 and expected to form 3 in
equilibrium with other boronate species. The requirement of a
species like 3 is consistent with the lack of reactions with
phenylboronic acid ester or with sodium phenyltrifluorobo-
rate. Furthermore, use of the dimethyl phenylboronic acid
ester resulted in a 50% reduction in yield of 1. Coordination of
3 with dirhodium caprolactamate and oxidation forms a
rhodium(III) boronate complex that undergoes phenyl transfer
from boron to give an intermediate monophenyl dirhodium
complex that subsequently undergoes a second oxidative
arylation reaction. The preference for methanol over other
alcohol solvents suggests a significant steric effect for phenyl
transfer.
c
trituration with methanol. In a separate reaction under the same
d
reaction conditions, the yield of 2e was 87% after 24 h. Reaction
time was 24 h.
undertaken. Under the same conditions as reported in Table 1,
but with only 10 mol% copper reagent, Cu(OAc)2ꢀH2O,
CuCl2, CuCl, CuBr and CuSO4ꢀ5H2O all gave the same yield
of 1 (ꢂ2%) from the reaction between PhB(OH)2 and
Rh2(cap)4. Since these oxidations are catalytic only in the
presence of molecular oxygen, the primary role of copper
appears to be that of providing the Cu(II)/Cu(I) redox couple
for the oxidation of Rh(II) to Rh(III).
Based on the survey of copper catalysts suitable for oxida-
tive arylation, copper(II) sulfate was selected as the most
suitable. Results from the use of 10 mol% CuSO4ꢀ5H2O with
Rh2(cap)4 and an array of representative boronic acids are
described in Table 2. Even with the use of less copper (mol%)
and reaction times of 12 h, product yields were consistently
The UV–visible spectra of these compounds exhibit a
substituent-dependent absorption that is at or greater than
450 nm with electron-donating substituents and at or below
425 nm with electron-withdrawing substituents (Table 3). This
spectral dependence suggests a distinct p-interaction through
Table 2 Copper(II) sulfate hydrate catalyzed oxidative arylation of
dirhodium(II) caprolactamate with arylboronic acidsa
Table 3 Spectral changes as a function of aryl substituent for
representative bis(s-aryl)dirhodium(III) caprolactamatesa
lmax (e ꢃ 10ꢄ3 b
)
13C, d (ppm)c
Compound
Ar =
equiv. ArB(OH)2
2, yield %b
Compound
Substituent
2c
2i
2j
2k
2l
4-HCOC6H4
4-NO2C6H4
4-MeOOCC6H4
4-F3CC6H4
4-MeC6H4
10
10
10
10
5
95
51
97
96
92
93
97
1
H
4-Me
4-Ph
4-HCO
4-MeOOC
4-CF3
4-MeO
4-Me2N
430 (4.5)
440 (8.1)
440 (1.3)
422 (7.9)
425 (8.2)
420 (4.2)
452 (6.2)
485 (3.0)
148.0 (d, 37.0 Hz)
142.8 (d, 37.0 Hz)
146.6 (d, 37.0 Hz)
161.5 (d, 37.0 Hz)
157.6 (d, 37.0 Hz)
167.9 (d, 23.3 Hz)
134.2 (d, 38.7 Hz)
131.4 (d, 37.6 Hz)
2a
2m
2c
2j
2k
2e
2g
a
2m
2n
a
4-Ph-C6H4
6-MeO-2-naphthyl
5
5
Reactions were performed at room temperature in
4 : 1
CH2Cl2–MeOH with 10 mol% CuSO4ꢀ5H2O and 10 equiv. NaOMe
Spectral data for additional compounds can be found in Supple-
b
with a reaction time of 12 h. Isolated yield after column chromato-
b
c
mentary Information.w
to rhodium.
l . Carbon bound
max in nm; e in Mꢄ1 cmꢄ1
graphy on silica gel.
ꢁc
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the phenyl ring onto the HOMO–LUMO of the bis(s-aryl)-
dirhodium(III) caprolactamates. Similarly, the 13C chemical
shift of the carbon directly attached to rhodium is also
dependent on para substituents. With electron-withdrawing
substitutions higher chemical shifts are observed; lower che-
mical shifts are found when para substituents are electron-
donating. Except for that of 2k, the JRh–C coupling constants
from mononuclear phenylrhodium(III) are much lower than
those reported here for bis(s-aryl)dirhodium(III).12
In summary, a broad selection of bis(s-aryl)dirhodium(III)
caprolactamates is formed in high yield by copper(II) catalyzed
reactions of arylboronic acids with dirhodium(II) caprolacta-
mate, and their spectral properties suggest delocalization of
aryl substituents to rhodium. The overall process is a net two
one-electron transfers coupled with aryl transfer from
arylboronic acid.
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1 J. M. Nichols, J. Wolf, P. Zavalij, B. Varughese and M. P. Doyle,
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ꢁc
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