Efficient Isomerization of Allyl Ethers and Related Compounds Using Pentacarbonyliron

Full text of DOI:10.1021/jo980581z

J . Org. Chem. 1998, 63, 6745-6748
6745
Efficien t Isom er iza tion of Allyl Eth er s a n d
Rela ted Com p ou n d s Usin g
that they are not catalytic. The catalytic isomerization
of allyl phenyl ether using PdCl
2 2
(PhCN) has been
reported to be quantitative.7
P en ta ca r bon ylir on
Due to the fact that aryl enol ethers undergo facile
hydrolysis and/or polymerization under acidic conditions,
the scope of potential catalysts is limited to those which
are either neutral or basic. Recent studies carried out
J ames V. Crivello* and Shengqian Kong
Center for Polymer Synthesis, Department of Chemistry,
Rensselaer Polytechnic Institute, Troy, New York 12180
in this laboratory have revealed that the tetracarbonyl-
-
hydroferrate(1-) anion [HFe(CO)
4
] , generated by the
Received March 30, 1998
reaction of pentacarbonyliron with sodium hydroxide,
catalyzes efficient carbon-carbon double bond isomer-
ization under mild conditions. We now wish to report a
novel isomerization method based on this inexpensive
and readily available catalyst.
Recent investigations in this laboratory have shown
that alkyl 1-propenyl ethers and related compounds
having the enol ether structure can be rapidly and
efficiently polymerized by onium salt cationic photoini-
tiators.1 These compounds are highly attractive for use
as monomers in a number of photocurable applications
including coatings, inks, and adhesives and in photo-
lithography. Alkyl 1-propenyl ethers can be readily
prepared in high yields by first the condensation of
Pentacarbonyliron has been used extensively for the
8
isomerization of unsaturated organic substrates. How-
9
ever, as pointed out earlier, these reactions suffer from
disadvantages such as the need for high temperatures
or UV irradiation, slow reaction rates, low yields of some
products, and, in many cases, the use of large, noncata-
lytic amounts of pentacarbonyliron. Although nonacar-
bonyldiiron and dodecacarbonyltriiron can effect double
bond isomerizations under milder conditions than those
employed for pentacarbonyliron, they are considerably
more expensive and large mole percentages based on the
substrates are also required.9
2
alkanols with allyl halides followed by a base or ruthe-
nium complex-catalyzed3 isomerization reaction of the
-5
corresponding allyl ethers (eqs 1-4).
,10
It is well-known that pentacarbonyliron can react with
sodium hydroxide to form the hydrido species, Na[HFe-
(CO)
] (eq 5).11 This hydride has many known applica-
4
Fe(CO) + 3NaOH f
5
NaHFe(CO) + Na CO + H O (5)
4
2
3
2
tions12 in organic chemistry such as effecting the deha-
logenation of organic halides, reductive alkylations,
aminations, hydroacylations, and carbon-carbon double
bond hydrogenations of R,â-unsaturated carbonyl com-
pounds. It was also observed that when 1-hexene was
shaken with an ether solution containing preformed
-
[
HFe(CO)
4
] at room temperature for 24 h, 90% isomer-
ization to the corresponding 2- and 3-hexene isomers was
We wished to extend our polymerization studies to
include aryl 1-propenyl ethers. However, the isomeriza-
tion methods described above for alkyl 1-propenyl ethers
were not successful due to the competing Claisen rear-
rangement. In addition, the high cost of the ruthenium
catalyst is also a concern for the large scale and eventual
industrial application of these monomers. Examination
of the literature revealed that aryl 1-propenyl ethers have
been prepared by the base-mediated isomerization of the
13
14
obtained. Interestingly, an early patent showed that
safrole can be isomerized to isosafrole in very good yield
using pentacarbonyliron in the presence of sodium hy-
droxide at temperatures above 110 °C for several hours.
To the best of our knowledge, this is the only case we
found which involves the use of pentacarbonyliron in the
presence of a base as an isomerization catalyst. On the
(
7) Golborn, P.; Scheinmann, F. J . Chem Soc., Perkin Trans. 1 1973,
2870.
(8) (a) Salomon, R. G. Tetrahedron 1983, 39, 485. (b) Pearson, A. J .
corresponding allyl aryl ethers with varying degrees of
success.2,6 These methods suffer from the disadvantage
In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F.
G. A., Abel, E. W., Eds.; Pergamon Press: New York, 1982; Vol. 8, pp
940-942. (c) Hubert, A. J .; Reimlinger, H. Synthesis 1970, 405.
(9) Imanieh, H.; Iranpoor, N.; Forbes, E. J . Synth. Commun. 1989,
19, 2955.
(10) Iranpoor, N.; Mottaghinejad, E. J . Organomet. Chem. 1992, 423,
399.
(11) King, R. B. In Organometallic Synthesis; Academic Press: New
York, 1965; Vol. I, p 96.
(12) Alper, H. Tetrahedron Lett. 1975, 27, 2257 and references
therein.
(1) (a) Crivello, J . V.; J o, K. D. J . Polym. Sci. Part A: Polym. Chem.
Ed. 1993, 31, 1473. (b) Crivello, J . V.; J o, K. D. J . Polym. Sci. Part A:
Polym. Chem. Ed. 1993, 31, 1483. (c) Crivello, J . V.; J o, K. D. J . Polym.
Sci. Part A: Polym. Chem. Ed. 1993, 31, 2143. (d) Crivello, J . V.;
L o¨ hden, G. J . Polym. Sci., Polym. Chem. Ed. 1996, 34(6), 1015. (e)
Crivello, J . V.; Yang, B.; Kim, W.-G. J . Macromol. Sci., Pure and Appl.
Chem, 1966, A33(4), 399. (f) Crivello, J . V.; L o¨ hden, G. J . Polym. Sci.,
Polym. Chem. Ed. 1996, 34(10), 2051.
(
(
(
(
(
2) Prosser, T. J . J . Am. Chem. Soc. 1961, 83, 1773.
3) Reuter, J . M.; Salomon, R. G. J . Org. Chem. 1977, 42(21), 3360.
4) Sasson, Y.; Blum, J . J . Org. Chem. 1975, 40(13), 1887.
5) Zoran, A.; Sasson, Y.; Blum, J . J . Org. Chem. 1981, 46(2), 255.
6) Endo, K.; Otsu, T. Polymer 1991, 32(15), 2856.
(13) Sternberg, H. Z.; Markby, R.; Wender, I. J . Am. Chem. Soc.
1956, 78, 5704.
(14) Radlove, S. B. U.S. Patent 2,575,529, 1951, Maytag & Co.;
Chem. Abstr. 1952, 46, 4812.
S0022-3263(98)00581-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/02/1998

6
746 J . Org. Chem., Vol. 63, No. 19, 1998
Notes
Ta ble 1. Isom er iza tion of Allyl Eth er s a n d Rela ted Com p ou n d s w ith 5 m ol % Ir on Hyd r id e Ca ta lyst
basis of this background, we have recently developed a
straightforward, general method for the isomerization of
simple alkyl and aryl allyl ethers and several other types
of related compounds which is described in this Note.
The method consists of simply combining the substrate
with 5 mol % of pentacarbonyliron and 10 mol % of
sodium hydroxide in an ethanol-water solution (15:1 by
volume) and refluxing the mixture under an atmosphere
of nitrogen until the isomerization is complete. The
reaction can be conveniently monitored by gas chroma-
tography or 1H NMR. For the isomerization of simple
alkyl or aryl allyl ethers, the reaction time is typically
anion, which is a strong nucleophile. However, no
byproducts of this reaction were observed and high yields
of the desired isomerized products were obtained. This
method can also be used to carry out “zip” reactions which
involve multiple step double bond isomerizations (7a ) in
which the double bond migrates over two or three carbon
atoms to give the enol ether in good yield. The isomer-
ization reactions are driven toward the formation of the
desired 1-propenyl ether products due to the stabilization
provided by the oxygen and the higher thermodynamic
stability gained by the conversion of a terminal to an
internal double bond. Calculations have given a ∆H
value of -4 kcal/mol for this reaction.15
0
.5-1.5 h.
As demonstrated in Table 1, this new method is very
In each case shown in Table 1, a Z,E isomeric mixture
was obtained with a small excess of the Z isomer except
for the isomerizations of 8a and 9a which gave predomi-
nately the E isomer. In addition, we were delighted to
observe that 2-propenyloxybenzene (10a ) can be smoothly
isomerized to the corresponding 1-propenyl ether in high
efficient not only for the isomerization of mono-, di-, and
trifunctional allyl ethers (1a -3a ) but also for crotyl
ethers (4a ) and ethers bearing an internal allylic carbon-
carbon double bond (5a ). The successful isomerization
of allyl glycidyl ether (2-propenyloxymethyloxirane, 6a )
was surprising to us because we were concerned that the
epoxide ring may open in the presence of the iron hydride
(15) Taskinen, E. Tetrahedron 1993, 48, 11389.

Notes
J . Org. Chem., Vol. 63, No. 19, 1998 6747
yield. This allows the facile synthesis of 1-propenyl aryl
ethers from the corresponding easily prepared allyl ethers
avoiding the usual complication of competing Claisen
rearrangements. Previous attempts in our group to
isomerize this class of compounds using either strong
base or ruthenium catalysis were not successful due to
this side reaction. Investigations of the polymerization
of 1-propenyl aryl ethers are currently in progress in this
laboratory.
Sch em e 1
Our brief investigation of the scope of this new isomer-
ization method showed that the activity of this catalyst
system is highly dependent on the structure of the
substrate. For example, the isomerization of 2-methyl-
4
-oxadodec-1-ene (11a ) requires a longer reaction time
than a simple alkyl allyl ether. In addition, the catalyst
is deactivated over the long reaction time and the
addition of a second portion of catalyst was required to
give satisfactory yields of the corresponding enol ether
-
4
a dimeric ion is formed by condensation of two [HFe(CO) ]
ions, which subsequently decomposes into [Fe
and hydrogen (eq 6).
2 8
(CO)
]2
-
(11b). In our attempts to isomerize but-3-en-2-yl ether
1
2a , an unidentified byproduct was observed according
1
to GC and H NMR analysis which could not be at-
tributed to one of the isomers of the expected products,
i.e., E,Z 3-methyl-4-oxatetradec-2-ene or 2-ethyl-3-oxa-
tridec-1-ene. Separation and isolation of this byproduct
from the Z and E isomers of the product was difficult
because of the similarity of the boiling points. We also
observed that all the isomerized products rapidly decom-
posed by hydrolysis on silica gel or alumina chromatog-
raphy columns. Consequently, chromatographic methods
were not successful means of separation for the reaction
products, although different solvent systems and chro-
matographic supports were tried. In a recent paper,
Boons et al.16 reported that the reaction of the Wilkinson’s
The structure of the divalent [Fe
electronic with that of Co (CO)
2
(CO)
8
]2 ion is iso-
-
catalyst [(PPh
3
)
3
RhCl] with n-butyllithium results in a
rhodium hydride catalyst which can isomerize a wide
range of unsubstituted and substituted allyl ethers. We
conducted the isomerization of 12a with this catalyst
under similar reaction conditions and observed the same
byproduct. It should be noted that isomerization of 12a
2
8
, which also catalyzes
olefin isomerizations.8 By analogy with this latter cobalt
complex, we would like to propose the mechanism in
Scheme 1 for the isomerization of allylic ethers by
pentacarbonyliron in the presence of base.
c
using (PPh
3
)
3
RuCl
2
at 130 °C led to the formation of many
Last, by combining this new isomerization method with
the Williamson ether synthesis, it is now possible to
prepare 1-propenyl ethers using a streamlined, two-step
one-pot reaction. First, the alcohol and allyl bromide are
combined with base in the presence of a phase transfer
catalyst. When the etherification reaction is complete,
pentacarbonyliron is added to isomerize the allyl ether.
Using this method, 1-propenyl ether 1b was conveniently
prepared in 83% yield (eq 7).
byproducts. Our catalyst was also ineffective in convert-
ing propargyl ether 13a into its 1-decyloxyallene isomer.
In this case, only a trace of n-decyl alcohol, which appears
to be derived from the hydrolysis of the starting material,
was observed. Surprisingly, the reaction of 1,4-dibutoxy-
2
-butyne (14a ) in the presence of the iron catalyst was
1
7
very complicated. We have previously reported that the
corresponding 1,4-dibutoxy-1,2-butadiene was not formed
using ruthenium catalysis. Isomerization of 2-phenyl-
4
,7-dihydro-1,3-dioxepin (15a ) was accompanied by de-
composition to benzaldehyde. Other screening experi-
ments also ruled out substrates such as 2-ethenyl-1,3-
dioxolane (16a ) and allyl acetate (17a ) as candidates for
isomerization by this method due to unknown reasons.
To confirm that the iron hydride anion is involved in
the isomerization reaction, tetraethylammonium tetra-
carbonylhydroferrate was prepared by an established
procedure.18 The compound thus obtained was used
successfully to catalytically isomerize alkyl allyl ethers
to 1-propenyl ethers under the same reaction conditions
In conclusion, we have developed a novel method of
carrying out the efficient isomerization of allyl ethers and
several related classes of compounds. This method has
several advantages such as high reaction rate, mild
conditions, and low cost which make it attractive for both
academic and industrial uses. It is also worth noting that
the difference in the reactivity and reaction rates of
different substrates toward isomerization could be used
to carry out selective isomerizations. This new method
should also find use in the deblocking of allyl ethers used
as hydroxyl protecting groups since the 1-propenyl ether
group can be readily removed by acid-catalyzed hydroly-
1
3
used here. In an early paper, Sternberg proposed that
(
16) Boons, G.; Burton, A.; Isles, S. J . Chem. Soc., Chem. Commun.
996, 141.
17) Crivello, J . V.; Yoo, T. J . Polym. Sci., Polym. Chem. Ed. 1995,
3, 2493.
18) Cole, T. E.; Pettit, R. Tetrahedron Lett. 1977, 9, 781.
1
(
3
(

6
748 J . Org. Chem., Vol. 63, No. 19, 1998
Notes
1H NMR (200 MHz, CDCl
): δ 6.21 (dd, E isomer), 5.93 (dd,
sis under mild conditions. We briefly examined the scope
of this method and confirmed the role of the iron hydride
as the catalytically active species in this reaction. This
catalyst system, which has high activity even in the
presence of water and other impurities, allows us to
prepare 1-propenyl ethers simply and directly by a one-
pot reaction.
3
Z isomer), 4.84-4.65 (m, E isomer), 4.44-4.28 (m, Z isomer),
3
1
.70 (t, Z isomer), 3.60 (t, E isomer), 1.65-1.48 (m, 5H), 1.45-
.08 (m, 14H), 0.87 (t, 3H).
On e-P ot Syn th esis of 1-P r op en yl Eth er 1b. Into a 500
mL round-bottom three-necked flask equipped with an overhead
stirrer, thermometer, condenser, and nitrogen inlet were placed
1.6 g (0.2 mol) of 1-decanol, 24.2 g (0.2 mol) of allyl bromide,
5 mL of toluene, and 8.8 g (0.22 mol) of sodium hydroxide. The
3
7
Exp er im en ta l Section
reaction mixture was stirred at room temperature for 15 min.
Then, 0.9 g (3.0 mmol) of tetra-n-butylammonium bromide was
added and the reaction mixture slowly heated to 50 °C and
maintained at that temperature overnight. The reaction mix-
ture was sparged with a flow of nitrogen for 10 min and 100
mL of an ethanol-water solution (15:1 v/v) of 1.96 g (0.01 mol)
of pentacarbonyliron was added. The reaction mixture was
brought to reflux temperature and maintained at that temper-
ature for 1 h. The solvent was evaporated under reduced
pressure. The same workup as described above gave 32.9 g
Gen er a l. All the organic reagents employed in this investi-
gation as well as substrates 6a , 8a , 9a , 10a , 16a , and 17a were
used as purchased from the Aldrich Chemical Co. without
additional purification. General procedures for the preparation
of allyl ethers were described in previous publications.1
Gas chromatographic analyses were performed using a high-
performance capillary HP-1 (cross-linked silicone gum) column
a-c
1
and a flame ionization detector. H NMR spectra were obtained
3
at 200 MHz at room temperature in CDCl with tetramethyl-
(83%) 1-propenyl ether 1b as a clear, colorless liquid.
silane (TMS) as an internal standard. Elemental analyses were
performed by Atlantic Microlabs, Inc., Norcross, GA. The enol
ethers prepared during this work were fully characterized by
Ack n ow led gm en t. Support of this work by the
1
H NMR spectroscopic and elemental analysis and compared
National Science Foundation and the Petroleum Re-
search Foundation administered by the American Chemi-
cal Society is gratefully acknowledged.
with authentic compounds prepared as described previously.1a-f
Isom er iza tion of Allyl Eth er s a n d Rela ted Com p ou n d s.
A typical experimental procedure for the isomerization of the
compounds shown in Table 1 is given below.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra of
all compounds listed in Table 1 with the exception of purchased
substrates 16a and 17a and elemental analyses of new
compounds 10b a n d 11b (4 pages). This material is contained
in libraries on microfiche, immediately follows this article in
the microfilm version of the journal, and can be ordered from
the ACS; see any current masthead page for ordering
information.
Isom er iza tion of Allyl Eth er 1a . An ethanol-water (15:1
v/v) solution (15 mL) of pentacarbonyliron (0.495 g, 2.53 mmol)
and allyl ether 1a (10.0 g, 50.5 mmol) was mixed with solid
sodium hydroxide (0.2020 g, 5.05 mmol) under a nitrogen
atmosphere. The solution was refluxed for 0.5 h, and the solvent
was evaporated under reduced pressure. The remaining solution
was mixed with 100 mL of hexane and filtered, dried over
anhydrous sodium sulfate, concentrated under reduced pressure,
and distilled (bp 55 °C/0.05 mmHg; lit. bp) to give 9.6 g (96%)
product, 1-propenyl ether 1b as clear, colorless liquid.
J O980581Z