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Room-Temperature Selective Aliphatic Carbon Carbon 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
bS Supporting Information ABSTRACT: Selective aliphatic carbon(R) 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(R) C(β) bond. The reaction is general for both straight- and branch-chain ethers.
S
elective aliphatic carbon carbon bond activation (ACCA) by transition metals under mild conditions has long been a desirable, but very challenging goal.1 Transition-metal-mediated carbon carbon bond activation of various substrates has been extensively studied.2 6 A classical example of the cleavage of a C(sp3) C(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) 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 C C 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,5,9 11 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 characterized. 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). Initially, Rh(tmp) 1 was found to cleave the weakest C(R) C(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 efficiently at 25 °C within 10 min to give 2a in up to 83% yield (eq , Table 1, entry 5). r 2011 American Chemical Society
Scheme 1. ACCA of Ether with Rh(tmp)
Table 1. Base and Water Loading Effect
entry PPh3 equiv KOH equiv H2O equiv Ph4PBr equiv 2a yield/%a 1 2
0 1
0 0
0 0
0 0
15 35
3
1
10
0
0
40
4
1
10
50
0
54
5b
1
10
50
0.1
83
Average of at least duplicate runs. b Reaction time ∼1 10 min, 10 min to ensure complete reaction. a
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) C(β) bond at least 4 kcal mol 1 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 C CH3 bond cleaved rather than the weaker but hindered internal C C 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 3691
dx.doi.org/10.1021/om200280w | Organometallics 2011, 30, 3691–3693
Organometallics
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Table 2. Scope of Ethers
entry
ethers
Scheme 2. Proposed Mechanism
Rh(tmp)R
time
yield (%)a
1
(MeCH2)2O
Rh(tmp)Me (2b)
1d
10
2
(EtCH2)2O
Rh(tmp)Et (2c)
∼1 10 minb
62
3
(PrCH2)2O
Rh(tmp)Pr (2a)
∼1 10 minb
83
4
(BuCH2)2O
Rh(tmp)Bu (2d)
∼1 10 minb
79
5
(AmCH2)2O
Rh(tmp)Am (2e)
2h
54
6
(Me2CH)2O
Rh(tmp)Me (2b)
∼1 10 minb
85
7
(MeCH2OCH2)2
Rh(tmp)Me (2b)
∼1 10 minb
74
2h
39
8
i
( BuCH2)2O
i
Rh(tmp) Bu (2f)
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.
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 entry
a
ether
Rh(tmp)Rb
esterc
1
(PrCH2)2O
Rh(tmp)Pr/83%
HCO2Bu/48%
2
(BuCH2)2O
Rh(tmp)Bu/79%
HCO2Am/36%
3
(Me2CH2)2O
Rh(tmp)Me/85% b
CH3CO2iPr/72% c
Average of at least duplicate runs. Isolated yield. GC yield. d 10 min to ensure complete reaction.
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 coproducts 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 disproportionation 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)OH and RhIII(tmp)H. The highly reactive Rh(tmp)OH once formed in nonpolar
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 C C bond cleavage of ethers by Rh(tmp) can be ruled out due to the high energy barrier in cleaving the strong C C bond (see SI). The observation of organic co-product firmly supports Rh(tmp)OH as the intermediate in cleaving the carbon carbon bond of ethers (Scheme 2). 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 disproportionation 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) C(β) bond of 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 basepromoted dehydrogenation (Scheme 2, eq v).22,23 In summary, we have discovered the mild, selective, and facile C(R) C(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
bS
Supporting Information. Detailed experimental section, characterization data for compounds prepared in this paper, calculation of thermodynamics of the reaction, and physical constants
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dx.doi.org/10.1021/om200280w |Organometallics 2011, 30, 3691–3693
Organometallics of ethers. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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(22) For equilibrium between Rh(por)OH, Rh(por)H, and Rh2(por)2, see: Fu, X.; Wayland, B. B. J. Am. Chem. Soc. 2004, 126, 2623–2631. (23) The analogous hydrogen evolution from electrochemical reduction of rhodium(II) porphyrin hydrides has been suggested by Saveant 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.
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