Room-temperature selective aliphatic carbon-carbon bond activation and functionalization of ethers by rhodium(II) porphyrin

Article abstract of DOI:10.1021/om200280w

Selective aliphatic carbon(α)-carbon(β) bond activation of ethers by (5,10,15,20-tetramesitylporphyrinato)rhodium(II) (Rh(tmp) (1)) was achieved at room temperature to yield corresponding rhodium porphyrin alkyls and the functionalized esters. Rh(tmp)OH was the proposed intermediate responsible for cleaving the C(α)-C(β) bond. The reaction is general for both straight- and branch-chain ethers.

Full text of DOI:10.1021/om200280w

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Room-Temperature Selective Aliphatic CarbonÀCarbon Bond
Activation and Functionalization of Ethers by Rhodium(II) Porphyrin
Siu Yin Lee, Tsz Ho Lai, Kwong Shing Choi, and Kin Shing Chan*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, People’s Republic of China
S Supporting Information
b
ABSTRACT: Selective aliphatic carbon(R)Àcarbon(β) bond acti-
vation of ethers by (5,10,15,20-tetramesitylporphyrinato)rhodium(II)
(Rh(tmp) (1)) was achieved at room temperature to yield corre-
sponding rhodium porphyrin alkyls and the functionalized esters.
Rh(tmp)OH was the proposed intermediate responsible for cleaving the C(R)ÀC(β) bond. The reaction is general for both
straight- and branch-chain ethers.
Scheme 1. ACCA of Ether with Rh(tmp)
elective aliphatic carbonÀcarbon bond activation (ACCA)
S
by transition metals under mild conditions has long been a
desirable, but very challenging goal.1 Transition-metal-mediated
carbonÀcarbon bond activation of various substrates has been
extensively studied.2À6 A classical example of the cleavage of a
C(sp3)ÀC(sp3) bond is driven by releasing the ring strain in
cubane with [RhI(CO)2Cl]2.7 Other nonaliphatic CCA by
transition-metal complexes include nickel(0)-catalyzed C(sp2)À
C(sp2) bond activation of biphenylene with acetylenes to afford
9,10-disubstituted phenanthrenes developed by Jones et al.8 In
contrast, there is little precedent for the cleavage of aliphatic
CÀC bonds with subsequent functionalization into organics.
We have reported the ACCA chemistry of RhII(tmp) (tmp =
5,10,15,20-tetramesitylporphyrinato dianion) with various organic
substrates such as nitroxides,5,9À11 nitriles,12 and c-octane.13
These reactions occur at temperatures higher than 70 °C, and
the cleaved non-Rh(por)-containing R fragments are not char-
acterized. It is desirable to achieve functionalization of the
cleaved alkyls into organic compounds. We now report the
room-temperature, selective ACCA of ethers into Rh(tmp)alkyls
and esters14,15 (Scheme 1).
Table 1. Base and Water Loading Effect
entry PPh3 equiv KOH equiv H2O equiv Ph4PBr equiv 2a yield/%a
1
0
1
1
1
1
0
0
0
0
0
15
35
40
54
83
2
0
3
10
10
10
0
0
4
5b
50
50
0
0.1
a Average of at least duplicate runs. b Reaction time ∼1À10 min, 10 min
to ensure complete reaction.
Initially, Rh(tmp) 1was found to cleave the weakest C(R)ÀC(β)
bond of n-butyl ether to give Rh(tmp)Pr 2a in only 15% yield in 1
day at 25 °C (eq , Table 1, entry 1). The more electron-rich (PPh3)-
Rh(tmp)16,17 improved the yield to 35% (eq , Table 1, entry 2).
Encouraged by the promoting effect of base in ACCA of ethers
by rhodium(III) porphyrin,15 we thus examined the effect of
KOH on the ACCA. However, the addition of 10 equiv of KOH
was not beneficial, likely due to its poor solubility in n-butyl ether
at room temperature (eq , Table 1, entry 3). However, further
addition of H2O to dissolve KOH enhanced the reaction yield to
54% in 1 day (eq , Table 1, entry 4).
To further improve the homogeneity of the reaction mixture,
Ph4PBr (0.1 equiv) was added as the phase transfer catalyst. To
our delight, both the rate and yield of the reaction were enhanced
significantly. Selective ACCA of n-butyl ethers occurred effi-
ciently at 25 °C within 10 min to give 2a in up to 83% yield
(eq , Table 1, entry 5).
The ACCA is general for a variety of ethers (eq , Table 2). The
ACCA worked well in both straight-chain and branch-chain
ethers. Diethyl ether having a C(R)ÀC(β) bond at least 4 kcal
molÀ1 stronger than other ethers gave poorer yield (Table 2,
entry 1, see SI). Branching on the C(R) atom had little effect on
rate, but branching on the C(β) site lowered the rate significantly
(Table 2, entry 6 vs 8). Selective ACCA can be achieved in
sterically hindered ethers but in a lower yield (Table 2, entry 4
vs 8). In 2-ethoxylethyl ether, the stronger but unhindered
terminal CÀCH3 bond cleaved rather than the weaker but
hindered internal CÀC bond (Table 2, entry 7).
By successfully identifying the cleaved C(β) fragment as
esters, the stoichiometry of the ACCA reaction was established.
Received: March 29, 2011
Published: June 27, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/om200280w Organometallics 2011, 30, 3691–3693
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Table 2. Scope of Ethers
Scheme 2. Proposed Mechanism
entry
ethers
Rh(tmp)R
time
yield (%)a
1
2
3
4
5
6
7
8
(MeCH2)2O
(EtCH2)2O
(PrCH2)2O
(BuCH2)2O
(AmCH2)2O
(Me2CH)2O
Rh(tmp)Me (2b) 1 d
10
62
83
79
54
85
74
39
Rh(tmp)Et (2c)
∼1À10 minb
Rh(tmp)Pr (2a)
∼1À10 minb
benzene solvent decomposes to regenerate Rh(tmp), H2O,
and O2.16,17 Rh(tmp) likely further reacts with H2O to give
Rh(tmp)H finally in high yield.
Rh(tmp)Bu (2d) ∼1À10 minb
Rh(tmp)Am (2e) 2 h
Rh(tmp)Me (2b) ∼1À10 minb
(MeCH2OCH2)2 Rh(tmp)Me (2b) ∼1À10 minb
(iBuCH2)2O
Rh(tmp)iBu (2f) 2 h
a Isolated yield of average of at least duplicate runs. b 10 min to ensure
complete reaction.
Table 3. Organic Co-productd
\eqno (3
yielda
Rh(tmp) and Rh(tmp)H are unlikely intermediates in ACCA.
Rh(tmp)H alone has been shown to be ineffective in ACCA
(eq ). It is not an intermediate since the expected cleaved C(β)
fragment BuOCH3 is inconsistent with the ester observed. Direct
CÀC bond cleavage of ethers by Rh(tmp) can be ruled out due to
the high energy barrier in cleaving the strong CÀC bond (see SI).
The observation of organic co-product firmly supports Rh(tmp)-
OH as the intermediate in cleaving the carbonÀcarbon bond of
ethers (Scheme 2).
entry
ether
Rh(tmp)Rb
esterc
1
2
3
(PrCH2)2O
(BuCH2)2O
(Me2CH2)2O
Rh(tmp)Pr/83%
Rh(tmp)Bu/79%
Rh(tmp)Me/85%
HCO2Bu/48%
HCO2Am/36%
i
CH3CO2 Pr/72%
a Average of at least duplicate runs. b Isolated yield. c GC yield. d 10 min
to ensure complete reaction.
Based on the above findings, Scheme 2 illustrates a proposed
mechanism for the ACCA of n-butyl ether by Rh(tmp). Facile
precoordination of PPh3 with Rh(tmp) generates (Ph3P)Rh-
(tmp)18,19 (Scheme 2, eq i), which then undergoes dispropor-
tionation with H2O to afford Rh(tmp)H and the proposed key
intermediate Rh(tmp)OH16 (Scheme 2, eq ii). Rh(tmp)OH
immediately activates the C(R)ÀC(β) bondof Bu2O, presumably
by sigma-bond metathesis, to selectively yield Rh(tmp)Pr and
BuOCH2OH (Scheme 2, eq iii). Condensation of BuOCH2OH
with Rh(tmp)OH yields the metalloether, Rh(tmp)OCH2OBu.20
Subsequent β-H or β-proton elimination generates Rh(tmp)H
and butyl formate (Scheme 2, eq iv).21 The Rh(tmp)H generated
is then recycled back to Rh(tmp) through thermally and base-
promoted dehydrogenation (Scheme 2, eq v).22,23
Alkyl formate or alkyl acetate were observed respectively by
GC/MS analysis of the reaction mixture of Rh(tmp) and ethers
(Table 3). Some extent of alkaline hydrolysis of the less stable
alkyl formate (eq 4) accounts for the lower yields of organic co-
products for straight-chain ethers (Table 3, entries 1 and 2), but
isopropyl acetate was obtained in high yield (Table 3, entry 3).
To gain more mechanistic understandings, the role of water in
yield enhancement was investigated. The reaction of Rh(tmp)
and H2O in the inert and less polar solvent benzene-d6 required
more harsh reaction conditions to give Rh(tmp)H (eq ). In the
presence of PPh3, Rh(tmp) in the form of more electron-rich
Rh(tmp)(PPh3) reacted with KOH and H2O at 120 °C to give
Rh(tmp)H 3 in 93% yield (eq ). On the basis of the dispropor-
tionation mechanism of Rh(II) porphyrin complexes with D2O
reported by Wayland et al.,16 it suggests that water induces
RhII(tmp) disproportionation into RhIII(tmp)OHandRhIII(tmp)H.
The highly reactive Rh(tmp)OH once formed in nonpolar
In summary, we have discovered the mild, selective, and facile
C(R)ÀC(β) bond activation and functionalization of ethers by
Rh(tmp). Studies on the detailed origin of the regioselectivity
and the scopes of ACCA are ongoing.
’ ASSOCIATED CONTENT
S
Supporting Information. Detailed experimental section,
b
characterization data for compounds prepared in this paper, cal-
culation of thermodynamics of the reaction, and physical constants
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of ethers. This material is available free of charge via the Internet
at http://pubs.acs.org.
(22) For equilibrium between Rh(por)OH, Rh(por)H, and
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(23) The analogous hydrogen evolution from electrochemical
reduction of rhodium(II) porphyrin hydrides has been suggested by
Savꢀeant et al.; see: (a) Grass, V.; Lexa, D.; Saveant, J.-M. J. Am. Chem. Soc.
1997, 119, 7526–7532. (b) Grass, V.; Lexa, D.; Momenteau, M.;
Saveant, J.-M. J. Am. Soc. 1997, 119, 3536–3542.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: ksc@cuhk.edu.hk.
’ ACKNOWLEDGMENT
We thank the Research Grants Council of Hong Kong
(CUHK 400309) for financial support.
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