Unconventional isomerization of allylic alcohols to allylcarbinols mediated by lanthanide catalysts

Article abstract of DOI:10.1016/j.tetlet.2013.01.111

We report here the catalytic isomerization of aryl-substituted allylic alcohols mediated by lanthanide alkoxide complexes. The conversion yields allylcarbinols as the products of the olefin isomerization process, but with different olefinic positions than those afforded by typical transition metal catalysts. The average isolated product yields in these reactions are 65-81%. The catalytic cycle and the source of the unusual olefinic positioning are proposed to involve a strong interaction between the Lewis acidic La ³⁺ ion and the substrate hydroxyl group.

Full text of DOI:10.1016/j.tetlet.2013.01.111

Tetrahedron Letters 54 (2013) 1828–1831
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Tetrahedron Letters
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Unconventional isomerization of allylic alcohols to allylcarbinols mediated by
lanthanide catalysts
SungYong Seo a,b, Xianghua Yu b, Tobin J. Marks b,
⇑
a Department of Chemistry, Pukyong National University, Busan 608-737, Republic of Korea
b Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
We report here the catalytic isomerization of aryl-substituted allylic alcohols mediated by lanthanide
alkoxide complexes. The conversion yields allylcarbinols as the products of the olefin isomerization pro-
cess, but with different olefinic positions than those afforded by typical transition metal catalysts. The
average isolated product yields in these reactions are 65–81%. The catalytic cycle and the source of the
unusual olefinic positioning are proposed to involve a strong interaction between the Lewis acidic La3+
ion and the substrate hydroxyl group.
Received 2 January 2013
Revised 17 January 2013
Accepted 21 January 2013
Available online 31 January 2013
Keywords:
Isomerization
Ó 2013 Elsevier Ltd. All rights reserved.
Allylic alcohol
Allylcarbinol
Organolanthanide catalyst
Olefin isomerization is a fundamental organic transformation of
significant value and can be catalyzed by a variety of acids, bases,
and organometallic compounds.1 However, due to the complexity
and difficulty in controlling such processes, attempted isomeriza-
tions frequently afford unwanted side-reactions.2 For the same
reason, there are only few industrial olefin isomerization processes
currently in commercial practice.3 Nevertheless, the conversion of
allylic alcohols into carbonyl compounds via enols is a useful syn-
thetic process, and such conversions can be catalyzed by a number
of late transition metal catalysts.4 The driving force for these olefin
isomerization processes is the formation of more thermodynami-
cally stable carbonyl compounds.
Lanthanide compounds have attracted increasing attention due
to the unique chemistry of f-block elements, such as stable oxida-
tion states, high kinetic lability, tunable ionic radii, strongly polar
metal–ligand bonding, and strong electro/oxophilicity.5 In regard
to catalysis, organolanthanide complexes have shown remarkable
success in catalyzing hydrogenation, polymerization, and hydro-
amination reactions.6 Furthermore, homoleptic La[N(SiMe3)2]3 lan-
thanide amido complexes7 are versatile catalysts for Tischenko
reactions,8 amidations,9 as well as diverse hydroelementation pro-
cesses, such as hydroamination,6,8a,10 hydrosilylation,11 hydrobora-
tion,12 hydrophosphination,13 and hydroalkoxylation.14 Here we
report the isomerization of allylic alcohols to allylcarbinols medi-
ated by lanthanide catalysts. Importantly, different product regio-
chemistries are produced than with conventional transition
metal catalysts, and the new isomerization orientations suggest
that the isomerization direction can be controlled by selection of
the catalyst. To the best of our knowledge, such controlled posi-
tioning of olefin isomerizations has not been reported before for
this type of substrate.
As judged by in situ 1H NMR spectroscopy, the homoleptic lan-
thanide amide complex La[N(SiMe3)2]3 is found to undergo instan-
taneous protonolysis by allylic alcohols (substrates 1–10, entries
1–10 in Table 1) at room temperature to form the corresponding
alkoxides and free HN(SiMe3)2. At 90 °C, in situ NMR spectroscopic
analysis indicates that these substrates reach an equilibrium distri-
bution of starting E and Z allylic alcohols, no matter which config-
uration is used initially. However, on a slower timescale, more
extensively isomerized allylcarbinol products appear and eventu-
ally grow in as the major products, with conversion halting at
the point where thermodynamic equilibrium is reached (Eq. 1,
direction b). Isolated yields vary from 65% to 81% as shown in
Table 1. In marked contrast, it has been reported that the same
substrates 1, 2, 9, and 10, undergo isomerization in direction a
(Eq. 1), with the formation of aldehydes in 81–96% yields, when
rhodium catalysts are used:15a,b
OH
OH
OH
O
a
b
ð1Þ
this work
Note that a variety of allylic alcohols also undergo conversion
via pathway a to afford aldehydes in catalytic reactions meditated
by other transition metal catalysts.4,15 In the present study where
⇑
Corresponding author. Tel.: +1 847 491 5658; fax: +1 847 491 2990.
E-mail address: t-marks@northwestern.edu (T.J. Marks).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.01.111

S. Seo et al. / Tetrahedron Letters 54 (2013) 1828–1831
1829
Table 1
Lanthanide-catalyzed isomerization of allylic alcohols
Entrya
Allylic alcohol
OH
Product
Reaction time (d)
Yieldb (%)
OH
1
6
68
1
2
11
2
3
4
5
6
7
8
3
8
71
70
74
69
72
65
73
OH
OH
12
OH
3
4
7
OH
OH
OH
10
8
O
O
O
5
13
OH
OH
6
7
OH
12
4
Cl
Cl
14
OH
Cl
8
OH
OH
9
12
4
70 (E:Z = 51:49)
81 (E:Z = 63:37)
9
15
OH
OH
10
10
16
a
General reaction conditions: Allylic alcohol (0.5 mmol), La[N(SiMe3)2]3 (7.5
Isolated yield.
lmol) in C6D6, 90 °C.
b
pathway b is followed, substrates with Z configurations typically
afford higher allylcarbinol yields and proceed more rapidly than
the corresponding E substrates, probably reflecting their greater
thermodynamic energies. In studies of substituent effects, includ-
ing those with electron-withdrawing and -donating groups as in
the phenyl rings of substrates 3–8 (Table 1), it is found that the
reaction rates and the product yields vary to only small extent.
The lowest yield and reaction rate are observed for chlorophenyl
substrate 7, where the slightly basic chloride functionality may
coordinate with/deactivate the highly Lewis acidic La3+ complex.
From the very different catalytic activity and selectivity pat-
terns exhibited by the lanthanide and transition metal catalysts,
it is reasonable that the new isomerization pathway reported here
reflects a strong interaction between the La3+ electrophile and the
substrate hydroxyl oxygen atoms during the catalytic cycle. A plau-
sible pathway is shown in Scheme 1. As the proposed pathway sug-
gests, the alcoholic substrates first protonolytically replace the
amido ligands (such processes are known to be very rapid) to af-
ford homoleptic alkoxide complexes with both the hydroxyl group
and
philic lanthanide center. Deprotonation by a neighboring alkoxide
ligand then yields a
3-allyl intermediate, followed by reprotona-
tion at an internal carbon atom (b to the oxygen) of the
3-allyl li-
p-system coordinated to the strongly oxophilic and electro-
g
g
gand to form a coordinated allylcarbinol. Subsequent ligand
dissociation/trans-alcoholysis then releases the allylcarbinol prod-
uct to complete the catalytic cycle. In contrast to this scenario, for

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S. Seo et al. / Tetrahedron Letters 54 (2013) 1828–1831
typical ‘soft’ late transition metal catalysts, traditional
g
3-allyl
H
R
complexes are more likely to form due to their low oxophilicity
and accessible oxidative addition/reductive elimination pathways1
(Scheme 2). The subsequently formed enol will then quickly con-
vert into an aldehyde, which drives the reaction. However for lan-
thanide complexes, oxidative addition/reductive elimination
pathways are not typically accessible and a strong La3+-hydroxyl
interaction together with strongly basic alkoxide ancillary ligands,
O
M(OR)2
O
M
O
M
R
O
O
H
R
(RO)2
H
(RO)2
OH
O
H
plausibly disfavors the same
g
3-allyl structure formation.
Scheme 2. Proposed reason why allylic alcohols do not form aldehydes in the
All of the present isomerization conversions do not proceed
100% to allylcarbinols, but isolated yields range from 65% to 81%.
From the NMR and GC–MS results, the byproducts are unreacted
E or Z starting materials due to the equilibrium nature of this
catalytic process (Scheme 1). There are also trace amounts of alde-
hydes as indicated by an 1H NMR signal near d 9. Trace ethers
resulting from condensation of two alcohol molecules are also
suggested from the GC–MS ions with molecular weights of
m/z = (2 Mꢀ18)+. Additional evidence for such condensation pro-
cesses is the small quantities of precipitate observed at the bottom
of the NMR tubes which are likely be oxo species derived from
La[N(SiMe3)2]3 hydrolysis by H2O from alcohol condensation
reactions.
The majority of substrates explored in this study (entries 1–8)
are tri-substituted alkenes, and the products are tetra-substituted
alkenes. Note that for simple olefin isomerizations without func-
tional groups, the C@C double bond typically moves into more
substituted positions.4 Keeping this information in mind, we fur-
ther investigated the driving force underlying this new isomeriza-
tion process with substrates 9 and 10 where the product olefins
have the same number of substituents as the starting materials
(entries 9 and 10), for comparison with substrates 1 and 2 (entries
1 and 2). Interestingly, substrates 9 and 10 both afford high yields
of the expected isomerization products but with roughly compara-
ble isolated E:Z ratios for 9 and excess E for 10. It is not clear why
the conversion rate for substrate 10 is somewhat greater.
presence of lanthanide (Ln) catalysts.
products currently under investigation. Aryl groups are well
known to activate proximate double bonds and to interact with
electrophilic lanthanide catalytic centers:16
La[N(SiMe3)2]3
5 mol %
120 oC, C6D6
+
byproducts
OH
OH
17
30%
ð2Þ
In summary, a new allylic alcohol olefin isomerization pathway
is found to be meditated by lanthanide catalysts, and a catalytic cy-
cle is proposed. Thermodynamic effects with less substituted sub-
strates were also studied, and aryl substituents are found to
significantly accelerate the catalytic conversion. Further studies
of mechanism and synthetic applications are underway.
Acknowledgment
This work was supported by the Pukyong National University
Research Fund in 2011(PKS-2011-07).
Supplementary data
A substrate without aryl substituents on the olefinic chain was
also investigated in the isomerization process. No obvious reaction
is observed at 90 °C for 4-methyl-pent-2-en-1-ol (17) with La[N
(SiMe3)2]3 in benzene-d6. On heating to 120 °C, a slow isomerization
is observed and, concurrently, additional amounts of byproducts
are formed (Eq. 2). Only 30% of the desired product is observed
by integration of the 1H NMR spectrum, with the mixture of other
Supplementary data (substrate synthesis and characterization
data for all new compounds) associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2013.01.111.
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Scheme 1. Proposed catalytic cycle for the lanthanide-catalyzed allylic alcohol
isomerization. Alcohol substituents R,R0 are omitted for clearance.

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