wide range of organic transformations.9 Only substoichio-
metric amounts of catalyst are sufficient for complete
conversions, and catalysts can be readily recycled.9 Never-
theless, Ln(OTf)3-catalyzed processes typically require toxic,
highly polar, moderately coordinating solvents, which incur
significant safety and cost-associated disadvantages.9
The use of room temperature ionic liquids (RTILs),
thermally stable fluids comprised solely of ions, as chemical
reaction media in place of conventional volatile organic
solvents, has grown dramatically in recent years.10 In addition
to low volatility, a potential environmental benefit, ionic
liquids are extremely polar, usually aprotic, and often afford
unique reaction efficiencies and selectivities.11 For Ln(OTf)3-
mediated intramolecular HO/cyclization of alkenols, RTILs
based on noncoordinating anions might be expected to
provide increased Ln3+ Lewis acidity, due to minimal solvent
coordination to the Ln3+ center, and to offer catalyst and
ionic liquid recyclability, as well as ease of product separa-
tion. In this contribution we report that lanthanide triflates
in RTIL media are very efficient and selective catalysts for
the HO/cyclization of unactivated alkenols, and present a
preliminary discussion of reaction scope, selectivity, and
kinetics for this new catalytic process.
The efficiency of the Ln(OTf)3-catalyzed 1 f 2 conversion
was first optimized as a function of solvent (Table 1), and
an optimum protocol was identified. Initial screening of
Ln(OTf)3-mediated intramolecular HO/cyclization of 1 in
nitromethane revealed exclusive formation of 2 (eq 1, Table
1); the formation of ꢀ-hydride elimination /isomerization
products is not observed. Ln(OTf)3 complexes exhibit modest
catalytic activity for conversion 1 f 2 in nitromethane (Table
1, entries 1-3), with relative ordering of catalytic activity,
Yb3+ > Sm3+ > La3+.
Table 1. Screening of Ln(OTf)3 Complexes and Reaction Media
for Intramolecular Hydroalkoxylation/Cyclization of 1
Ln3+
time
(h) (°C, yield %)a,b
Nt h-1
entry (mol %) M3+ (Å)
solvent
1.
2.
3.
4.
5.
6.
7.
8.
La(5)
Sm(5)
Yb(5)
-
1.172 CD3NO2
18
18
18
18
18
3
3
3
3
3
0.01(100,1)c
0.04(100,2)c
0.10(100,9)c
–(120,-)
1.098 CD3NO2
1.008 CD3NO2
-
-
[C2mim][OTf]d
-
[C1dbu][OTf]e
–(120,-)
La(1)
Yb(1)
Yb(1)
Yb(1)
Yb(1)
Yb(1)
1.172 [C1dbu][OTf]e
1.008 [C1dbu][OTf]e
1.008 [C2mim][OTf]d
1.008 [C4mim][OTf]f
1.008 [C2mim][NTf2]g
1.008 [C4mim][NTf2]h
0.67(120,48)
5.37(120,88)
6.37(120,93)
6.01(120,86)
1.18(120,59)
1.07(120,52)
9.
10.
11.
3
a Turnover frequencies determined by 1H NMR spectroscopy using
1,1,2,2-tetrachloroethane as internal standard. b Isolated yield of the purified
product. c Percent formation of the final product was determined by 1H NMR
integration vs internal standard due to low observed product formation.
d 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate. e 1-Methyl-1,3-
diazabicyclo[5.4.0]undec-7-enium trifluoromethanesulfonate. f 1-Butyl-3-
methyl imidazolium trifluoromethanesulfonate. g 1-Ethyl-3-methylimidazolium
trifluoromethanesulfonyl amide. h 1-Ethyl-3-methylimidazolium trifluo-
romethanesulfonyl amide.
highly polar yet relatively noncoordinating,10 thus likely
enhancing Ln3+ Lewis acidity/unsaturation as suggested by
the increased turnover frequencies (Table 1, entries 6-9).
The present large rate enhancements in the hydrophilic10
ionic liquid, [Cnmim][OTf], vs the hydrophobic10 [Cnmim]-
[NTf2] analogues (Table 1, entries 9,10) are reasonably
attributable to differences in solvation.11b
Investigating the scope of Ln(OTf)3-catalyzed HO/cycliza-
tion of primary/secondary aliphatic/aromatic alkenols (Table
2) reveals efficacy for the formation of five- and six-
membered oxygen heterocycles with the Markovnikov-type
selectivity also observed in Ru-,4c Pt-,4d Au-,4b and Brønsted
acid5c-based catalytic systems. Cyclizations of diverse alk-
enyl alcohols proceed with near-quantitative conversions and
reasonably large turnover frequencies at 1 mol % Ln(OTf)3
loading in 1-24 h at 60-120 °C. The final product can be
isolated either by simple diethyl ether extraction or by
vacuum transfer, thus allowing efficient catalyst and ionic
liquid recycle.
In marked contrast to the catalytic results in nitromethane,
the 1 f 2 catalytic conversion in RTILs based on weakly
-
coordinating OTf- and NTf2 anions proceeds with large
rate enhancements, as reflected by ∼70-fold increases in
turnover frequencies (Table 1, entries 6-11). The marked
rate sensitivities to the nature of the solvent can be attributed
to differences in the Ln3+ Lewis acidity in solvents that differ
in cation-coordinating strength.12 Nitromethane is a polar,
aprotic, moderately cation-coordinating solvent,12 which
through coordination to Ln3+, is expected to decrease the
-
Lewis acidity. RTILs based on OTf- and NTf2 anions are
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of structural effects on this catalytic transformation. The
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