Low-valent titanium-mediated stereoselective alkylation of allylic alcohols

Article abstract of DOI:10.1021/ja806440m

We have developed low-valent titanium-mediated 1,3-transpositive cross-coupling reactions of acyclic and cyclic allylic alcohols for the stereoselective introduction of ethyl, 2-silylethyl, 2-phenethyl, and alkenyl groups. Cross-coupling of an allylic alcohol with a vinylsilane or styrene derivative is particularly noteworthy, as an efficient cross-selective coupling of two alkenes has been elusive. The stereochemistry of the cross-coupling alkylation is consistent with syn addition/β-elimination.

Full text of DOI:10.1021/ja806440m

Published on Web 11/06/2008
Low-Valent Titanium-Mediated Stereoselective Alkylation of
Allylic Alcohols
Ivan L. Lysenko, Keunho Kim, Hyung Goo Lee, and Jin Kun Cha*
Department of Chemistry, Wayne State UniVersity, 5101 Cass AVenue, Detroit, Michigan 48202
Received July 8, 2008; E-mail: jcha@chem.wayne.edu
Abstract: We have developed low-valent titanium-mediated 1,3-transpositive cross-coupling reactions of
acyclic and cyclic allylic alcohols for the stereoselective introduction of ethyl, 2-silylethyl, 2-phenethyl, and
alkenyl groups. Cross-coupling of an allylic alcohol with a vinylsilane or styrene derivative is particularly
noteworthy, as an efficient cross-selective coupling of two alkenes has been elusive. The stereochemistry
of the cross-coupling alkylation is consistent with syn addition/ꢀ-elimination.
Scheme 1
Introduction
Direct carbometalation of unactivated alkenes enables ver-
satile C-C bond forming reactions and has been typically
catalyzed by late-transition-metal complexes.1 Group IVA
metals have received less attention, but the Negishi reagent is
particularly effective for cyclic carbozirconation of dienes,
enynes, and diynes.2 The use of a titanacyclopropane intermedi-
ate (Kulinkovich reagent) has been directed primarily at
alkynes.3 Alkenes containing heteroatoms at allylic positions
undergo synthetically useful transformations by the action of
the Negishi or Kulinkovich reagent.2-4 Despite many elegant
applications in transition-metal-catalyzed transformations; how-
ever, an intermolecular cross-selective coupling of two alkenes
stands as a challenging goal.5 As part of research programs on
synthetic applications of the Kulinkovich reagent, we have
developed cross-coupling of an allylic alcohol with a vinylsilane
or styrene derivative (eq 1). Also included are stereoselec-
tive ethyl- and alkenylation reactions of allylic alcohols with
the ethyl Grignard reagent and alkynes, respectively, along with
stereochemical studies.
case of di- or trisubstituted olefins (i.e., R1 * H), formation of
the corresponding titanacyclopropane was believed to be
unfavorable due to steric effects. This structural hypothesis on
ligand exchange led to the development of an olefin exchange-
mediated variant of the Kulinkovich cyclopropanation of
carboxylic acid derivatives.7,8 When R1 * H, it was further
hypothesized that a different reaction pathway of converting 3
(and 3′) to 6 by ꢀ-elimination could become dominant. Ad-
ditionally, a judicious choice of R2 was deemed to be a critical
element of successful implementation and regiocontrol.
The Kulinkovich group first reported a formal SN2′ ethylation
reaction by the ethyl Grignard reagent (R2 ) H),9 but no
stereochemical studies have been reported to date. In situ
formation of a Ti-O tether from an allylic alcohol, which had
been exploited in the directed Kulinkovich cyclopropanation
of homoallylic alcohol,8c,10 could be presumed to direct syn
addition/ꢀ-elimination. The stereochemical outcome was un-
certain for allylic ethers, however, as the ꢀ-elimination step
might require an ate complex by addition of a Grignard reagent.
The cognate zirconocyclopentane intermediates were reported
to prefer anti elimination, but geometrical constraints appeared
Previous Work and Mechanistic Hypothesis. Treatment of
(monosubstituted) allylic alcohol derivatives 1 (R1 ) H) with
the Kulinkovich reagent 2 was previously shown to provide a
convenient method for generating allyltitanium reagents 5 in
situ via ꢀ-elimination of the presumed intermediate 4 (Scheme
1).3 The initial titanacyclopentane intermediates 3 and 3′ were
believed to be in equilibrium with 4, and steric effects (e.g.,
the degree of substitution in alkenes) appeared to be the
dominant factor in controlling the position of equilibria.6 In the
9
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2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 15997–16002 15997

A R T I C L E S
Lysenko et al.
Table 1. Ethylation of Allylic Alcohols and Ethersa,b
ethers. Also included were related alkylation and alkenylation
reactions of allylic alcohols.
Results and Discussion
Stereochemistry of Ethylation. In view of the different
conformational preferences of E- and Z-olefins, both cyclic and
acyclic substrates 7 and 9 were selected for allylic ethylation
by the action of the Kulinkovich reagent (Table 1). At the oustet
significant solvent effects were noted that THF afforded poor
yields. Thus, diethyl ether was used as the solvent both for the
reaction solvent and the Grignard reagent in subsequent studies.
Several trends were apparent from Table 1. The protecting
groups R of the allylic alcohol exerted little influence on yields.
Ethylation of acyclic Z-allylic alcohol derivatives proceeded with
good selectivity for the E-double bond geometry, but that of
E-substrates was nonselective (entries 4-6 vs 7-9). Interest-
ingly, E-selectivity was increased for the BOM ethers (entries
6 and 9), and this observation was in accord with Kulinkovich’s
earlier report on the THP protecting group.9a
The stereochemical course of the ethylation reaction could
be probed by employing nonracemic allylic alcohol derivatives
and measuring the degree of chirality transfer. One drawback
of this customary method stems from the fact that E- and
Z-olefin products (e.g., 10 and 11) are typically inseparable.
Additionally, accurate measurements of ee’s of both major and
minor products were deemed to be nontrivial according to our
mechanistic conjecture that the configuration of the major
ethylation product 10 from an E-olefin substrate would be
enantiomeric to that of the minor isomer 11. An alternate method
was thus chosen to exploit the diastereomeric nature of the two
products (e.g., 10 and 11). Directed (syn) addition/syn elimina-
tion at the double bond was first established by 2-cyclohexen-
1-ol derivatives (entries 1-4 in Table 2). Thus, the ethylation
reaction of allylic ethers (entries 2 and 4) was unequivocally
ascertained to proceed with the identical stereochemistry as that
of the respective allylic alcohols (entries 1 and 3). Comparative
evaluation of diastereomeric substrates 17-20 was next under-
taken (entries 5-12). Diastereomers 21 and 22 were obtained
selectively from Z-anti- and Z-syn-isomers 17 and 19, respec-
tively, and their stereochemical assignment was made initially
on the basis of syn addition/syn elimination. The corresponding
E-anti substrates 18 afforded a mixture of two diastereomers
22 and 23 (entries 7 and 8), which were different from a mixture
of 21 and 24 obtained from E-syn-compounds 20 (entries 11
and 12). Hydrogenation of 21 and 22 was carried out individu-
ally in addition to that of separate mixtures of 22/23 and 21/24
to reveal their stereochemical relationship (eq 2). The 1H NMR
spectra of the resulting two alkane products were unmistakably
distinguishable so as to firmly establish the configurations of
21-24.
a Stereochemistry assigned by analogy to Table
2 (see text).
b Reaction conditions: ClTi(OiPr)3 (1 equiv), EtMgBr (3 equiv), ether,
rt. cReaction temperature: -78 °C to rt.
to be the overriding stereocontrol element.11 As no unequivocal
stereochemical information was available on these reactions,
we decided to determine the stereochemistry of the 1,3-
transpositive ethylation reaction of both allylic alcohols and
(1) For reviews, see: Transition Metals for Organic Synthesis; Beller, M.,
Bolm, C., Eds.; Wiley-VCH: Weinheim, 1998; Vols. 1 and 2.
(2) (a) For reviews, see: Negishi, E. In Catalytic Asymmetric Synthesis;
Ojima, I., Ed.; Wiley-VCH: New York, 2000; pp 165-189. (b)
Negishi, E. In Titanium and Zirconium in Organic Synthesis; Marek,
I., Ed.; Wiley-VCH: New York, 2002; pp 1-49. (c) Hoveyda, A. H.;
Heron, N. In ComprehensiVe Asymmetric Catalysis; Jacobsen, E. N.,
Pfaltz, A., Yamamoto, H., Eds.; Springer: Berlin, 1999; pp 431-454.
(d) Hoveyda, A. H. In Titanium and Zirconium in Organic Synthesis;
Marek, I., Ed.; Wiley-VCH: New York, 2002; pp 180-229.
(3) For reviews, see: (a) Sato, F.; Urabe, H.; Okamoto, S. Chem. ReV.
2000, 100, 2835. (b) Sato, F.; Urabe, H. In Titanium and Zirconium
in Organic Synthesis; Marek, I., Ed.; Wiley-VCH: New York, 2002;
pp 319-354.
(4) (a) Ito, H.; Motoki, Y.; Taguchi, T.; Hanzawa, Y. J. Am. Chem. Soc.
1993, 115, 8835. (b) Hanzawa, Y.; Kiyono, H.; Taguchi, T. Hetero-
cycles 2004, 62, 297.
(5) For recent advances in transition-metal-catalyzed couplings of 2π
components, see, inter alia: (a) RajanBabu, T. V. Chem. ReV. 2003,
103, 2845. (b) Montgomery, J. Angew. Chem., Int. Ed. 2004, 43, 3890.
(c) Iida, H.; Krische, M. J. Top. Curr. Chem. 2007, 279, 77. (d) Ng,
S.-S.; Ho, C.-Y.; Schleicher, K. D.; Jamison, T. F. Pure Appl. Chem.
2008, 80, 929.
(6) Lee, J.; Cha, J. K. Tetrahedron Lett. 1996, 37, 3663.
(7) For reviews, see: (a) Kulinkovich, O. G.; de Meijere, A. Chem. ReV.
2000, 100, 2789. (b) Kulinkovich, O. G. Russ. Chem. Bull., Int. Ed.
2004, 53, 1065.
(8) (a) Lee, J.; Kang, C. H.; Kim, H.; Cha, J. K. J. Am. Chem. Soc. 1996,
118, 291. (b) Lee, J.; Kim, H.; Cha, J. K. J. Am. Chem. Soc. 1996,
118, 4198. (c) Quan, L. G.; Kim, S.-H.; Lee, J. C.; Cha, J. K. Angew.
Chem., Int. Ed. 2002, 41, 2160.
(9) (a) Kulinkovich, O. G.; Epstein, O. L.; Isakov, V. E.; Khmel’nitskaya,
E. A. Synlett 2001, 49. (b) Matyushenkov, E. A.; Churikov, D. G.;
Sokolov, N. A.; Kulinkovich, O. G. Russ. J. Org. Chem. 2003, 39,
478. (c) Kulinkovich, O. G.; Shevchuk, T. A.; Isakov, V. E.;
Prokhorevich, K. N. Russ. J. Org. Chem. 2006, 42, 659. (d)
Kulinkovich, O. G. Pure Appl. Chem. 2000, 72, 1715.
As indicated, the stereochemistry of the acyclic products in
Table 1 was initially assigned by analogy and subsequently
(10) (a) Savchenko, A. I.; Kulinkovich, O. G. Russ. J. Org. Chem. 1997,
33, 846. (b) Isakov, V. E.; Kulinkovich, O. G. Synlett 2003, 967.
(11) (a) See, inter alia: Owen, D. R.; Whitby, R. J. Synthesis 2005, 2061.
(12) (a) Marshall, J. A.; Yu, R. H.; Perkins, J. F. J. Org. Chem. 1995, 60,
5550. (b) Enders, D.; Kipphardt, H.; Fey, P. Org. Synth. 1987, 65,
183. (c) Enders, D.; Eichenauer, H. Chem. Ber. 1979, 112, 2933.
´
(b) Barluenga, J.; Alvarez-Rodrigo, L.; Rodr´ıguez, F.; Fan˜ana´s, F. J.
Angew. Chem., Int. Ed. 2004, 43, 3932.
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Stereoselective Alkylation of Allylic Alcohols
A R T I C L E S
Table 2. Stereochemical Studies on Ethylation of Allylic Alcohols
confirmed by the following chirality transfer experiment em-
ploying enantiomerically enriched (95% ee) E-(+)- and Z-(+)-
9a, which were prepared from the corresponding known
propargyl alcohol12a by LAH reduction and hydrogenation with
the Lindlar catalyst (eq 3). Thus, (S)-12 was obtained from (R)-
(+)-Z-9a as a single isomer and its S configuration was
unequivocally established by straightforward derivatization with
(+)-RAMP12b and comparison with an authentic sample12c
prepared from commercially available (S)-2-methyl-1-butanol.
The cognate experiment with (R)-(+)-E-9a gave a mixture of
(R)-10 and (S)-11, and their configurations were determined by
the formation of two diastereomeric chiral hydrazones. The
minor hydrazone product derived from (S)-11 was identical to
that obtained from (S)-12.
and Ethersa
a Reaction conditions: ClTi(OiPr)3 (1 equiv), EtMgBr (3 equiv),
ether, rt (entries 1, 2, 3, 4, 6, 8, 10, and 12) or -78 °C to rt (entries
3, 4, 5, 7, 9, and 11). b4-Methylcyclohexene was also isolated in 40%
yield.
Additional examples were obtained to show a broad scope
with respect to allylic alcohols (Table 3). The use of a
trisubstituted olefin allows convenient construction of a qua-
ternary center (entry 1). A tertiary alcohol is also a suitable
substrate (entry 2). Selective ethylation of an allylic alcohol is
achieved in competition with an allylic ether (entry 4). As
expected, enantioselective ethylation is readily available by using
nonracemic allylic alcohols (entries 4 and 5).13
Alkylation and Alkenylation. A broad scope for installing
functionalized groups other than the ethyl moiety at an allylic
position is highly desirable and synthetically valuable. When a
homologous Grignard reagent (e.g., n-BuMgBr) was employed
in place of EtMgBr, a ca. 1:1 mixture of regioisomeric allylation
products (including both C1 and C2 positional isomers: R2 )
Et in Scheme 1) was obtained in modest (ca. 40-50%) yields
from allylic alcohols. In sharp contrast to the aforementioned
ethylation, however, allylic ethers failed to undergo alkylation.
The directing effects by a temporary alkoxide-Ti tether were
clearly necessary to override unfavorable steric effects engen-
dered by a homologous Grignard reagent.
attractive conceptually and operationally. Our attention was thus
focused on the development of such cross-coupling reactions
instead of optimizing the alkylation with nBuMgBr. In view of
the well-documented facile preparation (even at low tempera-
ture) and utility of titanium-alkyne complexes,3 coupling of
7a and an alkyne was first examined toward this end. Efficient
alkenylation products were isolated in satisfactory yields from
allylic alcohols (eq 4). As was the case with the aforementioned
ethylation, the alkenylation of Z-9a proceeded with high
diastereoselectivity, but the respective reaction with E-9a was
stereorandom to give a 1.6:1 mixture of Z- and E-double bond
isomers. Allylic ethers gave only trace amounts of the coupling
products. Only internal alkynes were amenable to the coupling
reaction, and regiocontrol was expected to be problematic with
unsymmetrical alkynes. Because similar alkenylation examples
appeared in the literature,14 no additional studies were pur-
sued.
Cross-coupling between allylic alcohols and olefins (or
alkynes) by means of a sacrificial Grignard reagent is more
(14) Recently, Micalizio reported this coupling reaction of alkynes, which
affords enantio- and diastereoselective alkenylation: (a) Kolundzic,
F.; Micalizio, G. C. J. Am. Chem. Soc. 2007, 129, 15112. (b) Shimp,
H. L.; Hare, A.; McLaughlin, M.; Micalizio, G. C. Tetrahedron 2008,
64, 3437.
(13) This allylic ethylation was reminiscent of Dzhemilev’s carbomagne-
sation, which was elegantly developed by Hoveyda as a powerful tool
in asymmetric synthesis.2c,d
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A R T I C L E S
Lysenko et al.
Table 3. Additional Examples of Ethylation of Allylic Alcoholsa
Cross-Coupling of Two Alkenes. The respective coupling
reaction of allylic alcohols with alkenes, which was one of
our two main objectives, proved to be challenging. Reductive
dimerization of homoallylic alcohols was achieved by the
Kulinkovich group by harnessing preassociation of a ho-
moallylic alkoxide to a titanacyclopropane intermediate.9
However, a cross-coupling reaction between two alkenes has
been elusive and undoubtedly hinges on a judicious choice
of two reactants. As a key prerequisite, alkene substrates
should display a strong affinity for the Kulinkovich inter-
mediate 2 to readily form the requisite titanacyclopentane 3.
The positional preference of R2 in 3 (Scheme 1) would also
be crucial to regioselective cross-coupling in conjunction with
the formation of a temporary alkoxide linker to the titana-
cyclopropane intermediate and the thermodynamically favor-
able ꢀ-elimination step. The aforementioned alkylation
reaction with nBuMgBr clearly indicated that an alkyl group
would exert little positional preference. After considerable
experimentation, vinylsilane and styrene were selected for
allylic alkylation. Our choice was guided by notable observa-
tions made in the olefin exchange-mediated cyclopropanation
of esters: treatment with a mixture of methyl 6-heptenoate
and trimethyl(vinyl)silane with the Kulinkovich reagent
(derived from iPrMgCl) gave the intermolecular cyclopro-
panation product at the expense of the intramolecular
counterpart.15 The olefin exchange-mediated cyclopropanation
of an ester with styrene was achieved by the ethyl Grignard
reagent,16 whereas other terminal ω-alkenes require cyclo-
hexyl or cyclopentyl Grignard reagents for successful Kulink-
ovich cyclopropanation reactions.7,8 These previous obser-
vations hinted at the distinctive yet unexplored utility of
vinylsilane (or vinylsiloxane) and styrene in forming the
requisite titanacyclopropane and titanacyclopentane inter-
mediates (i.e., 2 and 3 in Scheme 1). Moreover, high
positional selectivity was noted for silyl and phenyl substit-
uents in the low-valent titanium-mediated transformations of
the respective alkyne derivatives, as illustrated by eq 4.3
a Reaction conditions: ClTi(OiPr)3 (1 equiv), EtMgBr (3 equiv),
ether, rt. bReaction conditions: ClTi(OiPr)3 (1 equiv), EtMgBr (4 equiv),
ether, rt.
Ultimately, trimethyl(vinyl)silane (39a), vinylsiloxanes 39b,
and styrene (40) were found to be well suited for reductive cross-
coupling reactions with allylic alcohols (Table 4). Under typical
reaction conditions, a 1:1 mixture of an allylic alcohol and 39a
or 39b in ether was treated sequentially with commercially
available ClTi(OiPr)3 and an ether solution of cyclopentylmag-
nesium chloride at -78 °C, followed by allowing the resulting
mixture to slowly warm to 0 °C or rt. To ensure complete
conversion of the allylic alcohol substrate, 2 equiv of ClTi(O-
iPr)3 and 5 equiv of the cyclopentyl Grignard reagent were
satisfactory. Alternatively, MeTi(OiPr)3 (1 equiv) and the
cyclopentyl Grignard reagent (2 equiv) were also used.17
Coupling between 7a and styrene (40) was subsequently studied
by employing MeTi(OiPr)3 to give a 9:1 mixture of the coupling
product 43 and its dehydrostyrene derivative 44 in 79% yield
(eq 5). The minor byproducts are not shown in Table 4 for
convenience. As was the case with the ethylation reaction, the
scope was broad regarding allylic alcohols to allow a convenient,
stereoselective preparation of quaternary centers (entries 11 and
12) and an enantioselective synthesis of coupling products
(entries 15 and 16). Comparable reactivity was noted for 39a
and 40 on the basis of their competition experiment. A similar
competition experiment between the two styrenes 40 and 41
indicated the considerably lower reactivity of the latter compared
to the former (cf. entries 3 and 4).18
(17) Subsequent to the publication of the trans-dialkyl selective cyclopro-
panation of esters with homoallylic alcohols,8c the use of MeTi(OiPr)3
was also found to be convenient and effective.
(18) (a) Electronic effects of other substituents on the benzene ring will
be examined in due course. (b) Interestingly, Z-disubstituted alkenyl-
silanes also undergo stereoselective coupling, albeit in moderate (40-
53%) yields, and optimization is currently in progress.
(15) For the Kulinkovich cyclopropanation of vinylsilanes, see: (a) Lee,
K.; Kim, S.-I.; Cha, J. K. J. Org. Chem. 1998, 63, 9135. (b) Mizojiri,
R.; Urabe, H.; Sato, F. Tetrahedron Lett. 1999, 40, 2557. (c) Mizojiri,
R.; Urabe, H.; Sato, F. J. Org. Chem. 2000, 65, 6217.
(16) Kulinkovich, O. G.; Savchenko, A. I.; Sviridov, S. V.; Vasilevskii,
D. A. J. Chem. Soc., MendeleeV Commun. 1993, 230.
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Stereoselective Alkylation of Allylic Alcohols
A R T I C L E S
Scheme 2
The siloxane functionality of the coupling products was
readily converted by Woerpel’s protocol to the hydroxyl group
(eq 6),19 which should be useful for subsequent elaboration.
Table 4. Cross-Coupling between Allylic Alcohols and
Vinylsilanesa or Styrenesb
Thus,this1,3-transpositivecross-couplingandtheTamao-Fleming
oxidation complement the conventional Claisen rearrangement
approach.
Stereochemical Model. The high diastereoselectivity displayed
by Z-allylic alcohols can be rationalized by the involvement of
conformer A, where the allylic strain is minimized (Scheme
2).20 The lack of selectivity for E-allylic alcohols and the
attendant formation of both E- and Z-olefins suggest the
coinvolvement of both conformers B and C. The identical
stereochemical models also account for the stereochemistry of
cross-coupling reactions of allylic alcohols with vinylsilanes,
styrenes, and alkynes.
A priori, an alternate stereochemical pathway of anti addition/
antiperiplanar elimination could be considered for the ethylation
reaction of allylic ethers. However, formation of the identical
a Reaction conditions: allylic alcohol (1 equiv), 39a or 39b (1 equiv),
ClTi(OiPr)3 (2 equiv), c-C5H9MgCl (5 equiv), ether, -78 to 0 °C or rt.
b Reaction conditions: allylic alcohol (1 equiv), 40 or 41 (1 equiv),
(19) Smitrovich, J. H.; Woerpel, K. A. J. Org. Chem. 1996, 61, 6044.
(20) Note added in proof: The stereochemical study of the ethylation just
appeared. Isakov, V. E.; Kulinkovich, O. G. Tetrahedron Lett. 2008,
49, 6959.
MeTi(OiPr)3 (1 equiv), c-C5H9MgCl (2.2 equiv), ether, -78 to
0
°C. cMinor products are not shown; see eq 5.
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A R T I C L E S
Lysenko et al.
product 15 from alcohol 13a and methyl ether 13b (and 16 from
14a and 14b) in a cyclic system (entries 1-4 in Table 2), as
well as a noticeable difference in alkylation yields between cis
and trans isomers (entries 2 vs 4 in Table 2), precludes this
scenario. In the case of the cross-coupling between allylic
alcohols 13a/14a and vinylsilane 39b, a similar divergence in
yields was also noted (entries 7 vs 9 in Table 4). Interestingly,
the corresponding coupling reactions of styrene (40) showed
little variation between cis and trans isomers (entries 8 vs 10 in
Table 4). At present, the origin for high E-selectivity exhibited
by BOM allylic ethers is not clear.
2-phenethyl, and alkenyl groups. The stereochemistry of the
cross-coupling alkylation is consistent with syn addition/ꢀ-
elimination. New cross-coupling reactions between an allylic
alcohol and a vinylsilane or styrene derivative are particularly
noteworthy and demonstrate the unexplored utility of vinylsi-
lanesandstyrenesinlow-valenttitanium-mediatedtransformations.
Acknowledgment. Dedicated to Professor Sir Jack E. Baldwin,
FRS, on the occasion of his 70th birthday. We thank the NIH
(GM35956) and NSF (CHE-0615604) for generous financial
support.
Supporting Information Available: Experimental details and
spectroscopic data for key intermediates. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
Conclusion
In conclusion, we have developed convenient 1,3-transpositive
cross-coupling reactions of acyclic and cyclic allylic alcohols
to allow the stereoselective introduction of ethyl, 2-silylethyl,
JA806440M
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