Organometallics 2009, 28, 6845–6846 6845
DOI: 10.1021/om900270v
Base-Promoted, Selective Aliphatic Carbon-Carbon Bond Cleavage of
Ethers by Rhodium(III) Porphyrin Complexes
Tsz Ho Lai and Kin Shing Chan*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories,
Hong Kong, People’s Republic of China
Received April 9, 2009
Summary: Base-promoted, selective aliphatic carbon(R)-
(C2H4), as the proposed intermediate to afford the rhodium
acyl porphyrin complexes.8a In addition, the base-promoted
benzylic CHA of toluenes8b and CHA of alkane9 by rhodi-
um(III) porphyrins have also been documented. Therefore,
we sought to examine the reactivities of rhodium hydroxo
porphyrin complexes by reacting Rh(tmp)I (1a) with KOH
in ether solvents. To our delight, the unstrained, aliphatic
C(R)-C(β) bonds of ethers were cleaved and the β-alkyl
groups were transferred to rhodium porphyrins to yield
Rh(tmp) alkyls. We now report the discovery of the new
reactivity pattern in organometallic chemistry.
carbon(β) bond activation (CCA) of ethers by (5,10,15,20-
tetramesitylporphyrinato)rhodium(III) iodide was achieved.
Carbon-carbon bond activation (CCA) by late-transi-
tion-metal complexes is important and challenging in the
field of organometallic chemistry.1 Most examples involve
ring-strained cubane,2a cyclopropane,2b cyclobutanone,2c
and biphenylene2d or chelating substrates of pincer type
ligands,2e,f amine,2g and cycloalkanone imine2h with low-
valent group 9 transition-metal complexes.
Examples with high-valent group 9 transition-metal com-
plexes are still rarely documented. Bergman and co-workers
have reported the activations of aliphatic nitrile bonds by
rhodium(III) complexes3a,b and cyclopropane carbon-
carbon bonds by iridium(III) complexes,3c respectively.
Additionally, the intermediate valency of rhodium(II)
mesotetramesitylporphyrin Rh(tmp) is also known to acti-
vate the aliphatic carbon-carbon bonds of unstrained
ketone,4 amide,5 ester,5 nitroxide,6 and nitrile.7
Recently, we have reported mild carbon-hydrogen bond
activation (CHA) of benzaldehyde by using (5,10,15,20-
tetratolylporphyrinato)(β-hydroxyethyl)rhodium(III) with
a rhodium hydroxo porphyrin complex, Rh(ttp)(OH)-
Initially, Rh(tmp)I reacted with n-butyl ether solvent at
40 °C in 1 day to give a low yield of 4% Rh(tmp)Pr as the
selective C(R)-C(β) bond activation product (Table 1, entry 1).
We then examined the promoting effect of bases (Table 1,
eq 1). The weak base Cs2CO3 was not advantageous (Table 1,
entry 2). Stronger bases such as KOtBu, NaOMe, NaOH,
and KOH were much more effective, with KOH producing
the highest product yield (Table 1, entries 3-5 vs 7). A lower
loading of KOH (5 equiv) gave a lower yield, while a higher
loading (20 equiv) resulted in little enhancement over the use
of 10 equiv (Table 1, entries 6 and 8). In benzene solvent,
n-butyl ether (50 equiv) yielded less than 1% of Rh(tmp)Pr at
40 °C in 1 day. Therefore, base and neat ether are necessary.
The counteranion of Rh(tmp)X (X = I, Cl) strongly
affected the yield of CCA (eq 2). In the presence of KOH
(10 equiv), Rh(tmp)I gave Rh(tmp)Pr in 53% yield while
Rh(tmp)Cl produced Rh(tmp)Pr in 23% yield. It is likely
that Rh(tmp)I undergoes much more facile ligand substitu-
tion with KOH for further CCA reaction. Furthermore,
Rh(tmp)Cl, being less soluble than Rh(tmp)I in ether, gave
a lower CCA yield.
*To whom correspondence should be addressed. E-mail: ksc@cuhk.
edu.hk.
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The reaction temperature for CCA of straight-chain ethers
with Rh(tmp)I was varied from 40 to 100 °C (Table 2, eq 3).
The lower yields for n-propyl ether as compared to those for
n-butyl ether (Table 2, entries 1 and 2 vs 3-5) were due to the
poorer solubility of Rh(tmp)I in n-propyl ether. The highest
product yield was observed for the more soluble n-butyl ether
in 86% yield at 100 °C (Table 2, entry 5). Therefore, the
optimal reaction temperature was found to be 100 °C.
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r
2009 American Chemical Society
Published on Web 11/25/2009
pubs.acs.org/Organometallics
Lai, Tsz Ho
