Preparation of 2-Trialkylsiloxy-substituted 1,3-dienes and their Diels - Alder/cross-coupling reactions

Article abstract of DOI:10.1021/ol070089e

2-Triethylsiloxy-substituted 1,3-butadiene has been prepared in gram quantities from chloroprene via a simple synthetic procedure. Silatrane and catechol-substituted analogues of this main group element substituted diene were prepared by ligand exchange a

Full text of DOI:10.1021/ol070089e

ORGANIC
LETTERS
2007
Vol. 9, No. 9
1623-1626
Preparation of 2-Trialkylsiloxy-
Substituted 1,3-Dienes and Their
Diels−Alder/Cross-Coupling Reactions
Ramakrishna R. Pidaparthi, Mark E. Welker,* Cynthia S. Day, and
Marcus W. Wright
Department of Chemistry, Wake Forest UniVersity, P.O. Box 7486,
Winston-Salem, North Carolina 27109
welker@wfu.edu
Received January 11, 2007
ABSTRACT
2-Triethylsiloxy-substituted 1,3-butadiene has been prepared in gram quantities from chloroprene via a simple synthetic procedure. Silatrane
and catechol-substituted analogues of this main group element substituted diene were prepared by ligand exchange and characterized by
X-ray crystallography in addition to standard spectroscopic techniques. Diels−Alder reactions of these dienes are reported as well as subsequent
TBAF assisted/Pd-catalyzed Hiyama cross-coupling reactions of those Diels
−Alder adducts.
Reports of main group element substituted 1,3-dienes and
their reaction chemistry are not widespread in organic
chemistry. With respect to silyl-substituted 1,3-dienes, 1-tri-
methylsilyl-1,3-butadiene was originally reported and trapped
with maleic anhydride in 1957.1 Fleming and co-workers then
reported an alternate preparation of this compound and a
number of its Diels-Alder reactions over the 1970s and early
1980s.2,3 Subsequent to these initial reports a number of
1-silyl-1,3-diene preparations that rely on olefination of R,â-
unsaturated aldehydes, nickel-catalyzed coupling reactions,
etc. have also been reported and reviewed.4,5 Reports of
2-silyl-substituted 1,3-dienes are 3- to 4-fold less frequent
than their 1-substituted counterparts. 2-Triethylsilyl-1,3-
butadiene and a few of its Diels-Alder reactions were
reported by Ganem and Batt in 1978.6 Paquette and Daniels
reported some 2-silyl-substituted-1,3-cyclohexadienes in
1982 but none of their Diels-Alder chemistry.7 Trost and
Mignani reported the Pd-catalyzed elimination and cycliza-
tion reactions of 3-acetoxy-2-trimethylsilyl-1-butene in 1986.8
Two other reports of the preparation of 2-trialkylsilyl-1,3-
dienes have appeared since a 1995 review but neither of these
studies report Diels-Alder reactions.9,10 Reports of the
preparation and use of 2-trialkoxysilyl-1,3-dienes are ex-
tremely rare. We find a report of the use of the Ganem and
Batt protocol to make 2-trimethoxysilyl- and 2-triethoxysilyl-
1,3-butadiene in 198411 and then a report of the polymeri-
zation of these materials in 1989.12 Given the known
propensity of trialkoxysilyl aryls and alkenyls to participate
(6) Batt, D. G.; Ganem, B. Tetrahedron Lett. 1978, 3323-3324.
(7) Daniels, R. G.; Paquette, L. A. Organometallics 1982, 1, 1449-
1453.
(1) Sadykh-Zade, S. I.; Petrov, A. D. Dokl. Akad. Nauk SSSR 1957, 112,
(8) Trost, B. M.; Mignani, S. J. Org. Chem. 1986, 51, 3435-3439.
(9) Kahle, K.; Murphy, P. J.; Scott, J.; Tamagni, R. J. Chem. Soc., Perkin
Trans. 1 1997, 7, 997-999.
662.
(2) Fleming, I.; Percival, A. J. Chem. Soc., Chem. Commun. 1976, 681-
681.
(10) Ikeda, Z.; Oshima, K.; Matsubara, S. Org. Lett. 2005, 7, 4859-
(3) Carter, M. J.; Fleming, I.; Percival, A. J. Chem. Soc., Perkin Trans.
1 1981, 2415-2434.
(4) Luh, T. Y.; Wong, K. T. Synthesis 1993, 349-370.
(5) Stadnichuk, M. D.; Voropaeva, T. I. Russ. Chem. ReV. 1995, 64, 25-
46.
4861.
(11) Sato, F.; Uchiyama, H.; Samaddar, A. K. Chem. Ind. (London) 1984,
743-744.
(12) Takenaka, K.; Hattori, T.; Hirao, A.; Nakahama, S. Macromolecules
1989, 22, 1563-1567.
10.1021/ol070089e CCC: $37.00
© 2007 American Chemical Society
Published on Web 04/05/2007

in fluoride-assisted, metal-catalyzed cross-coupling reac-
tions,13 we felt that an easily accessible preparation of
2-trialkoxysilyl-1,3-dienes would be subsequently useful for
tandem Diels-Alder/cross-coupling chemistry. We report our
initial studies in this area in the work that follows.
and C(27)-C(28)-C(29)-C(30) was 170.3(3)°) whereas the
third molecule had an s-cis like diene torsion angle (C(37)-
C(38)-C(39)-C(40)) of 36.3(4)° (Figure 1). Diene 4
2-Trialkoxysilyl-Substituted 1,3-Diene Preparation and
Characterization. 2-Triethoxysilyl-1,3-butadiene (2) was
prepared in high yield by the nucleophilic addition of 1,3-
butadienyl-2-magnesium chloride (generated in situ from
chloroprene (1) and Mg) to triethoxysilyl chloride.12,14 The
title compound (2) was isolated as a colorless liquid after
distillation under reduced pressure. This compound slowly
polymerized on standing at room temperature over a period
of 10-15 days but is quite stable at low temperature, -20 °C.
2-Triethoxysilyl-1,3-butadiene (2) can be used in ligand
exchange reactions to make other siloxy dienes (3, 4) as air
stable crystalline solids. Alcoholysis of compound 2 with
triethanolamine in the presence of a catalytic amount of KOH
resulted in the formation of (buta-1,3-dien-2-yl)silatrane (3)
as a light yellow solid. Treatment of 2 with catechol in the
presence of KOH yielded potassium [bis(1,2-benzenediolato)-
1,3-butadien-2-yl]silicate (4) as a white amorphous powder.
At room temperature, compounds 3 and 4 show no signs of
decomposition over a period of a few weeks.
Figure 1. View of a molecule of the s-cis conformer of diene 3.
crystallizes with potassium coordinated THF molecules
(Figure 2). The Si-C13 bond length is 1.889(5) Å and the
All the silyl butadienes (2, 3, and 4) were structurally
characterized by 1D and 2D NMR techniques. On the basis
of the absence of NOESY cross-peaks between H1 T H4
and H3 T H1 and no observable peak broadening at
temperatures down to -60 °C, we concluded that the s-cis
to s-trans interconversion is too fast to observe in solution.
Dienes 3 and 4 were both also characterized by single-crystal
X-ray diffraction. Diene 3 proved to be unusual in that there
were three independent molecules per asymmetric unit. Two
of the three molecules in this unit had s-trans like diene
torsion angles (C(17)-C(18)-C(19)-C(20) was 178.5(3)°
Figure 2. View of a molecule of diene 4 showing the coordinating
THF solvent molecules.
diene torsion angle is very close to the s-trans 176.7(8)°
(Figure 2).
Diels-Alder Reactions. We initially compared dienes 2,
3, and 4 in reactions with N-phenylmaleimide. We found
that whereas the triethoxysilyl diene 2 showed only about a
2% conversion to cycloadduct 5 by NMR after 30 min at
25 °C in THF, the silatrane 3 and catechol containing dienes
4 showed complete conversion to cycloadducts 6 and 7 by
1H NMR and the cycloadducts were subsequently isolated
in almost quantitative yield.
(13) Hiyama, T.; Shirakawa, E. Top. Curr. Chem. 2002, 219, 61-85.
(14) Nunomoto, S.; Yamashita, Y. J. Org. Chem. 1979, 44, 4788-
4791.
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Org. Lett., Vol. 9, No. 9, 2007

Scheme 1. Reactions of 2-Trialkoxysilyl-1,3-dienes
Figure 3. View of a molecule of major cycloadduct isomer 9.
We next wanted to get an idea of relative reactivity of
these most reactive silicon-substituted dienes 3 or 4 in
comparison to known, reactive dienes such as Danishefsky’s
diene (1-methoxy-3-trimethylsiloxy-1,3-butadiene).15 We
found that silatrane diene 3 reacted with N-phenylmaleimide
at 0 °C with a kobs of 3.7 × 10-2 min-1 and t1/2 of 18.8 min
whereas Danishefsky’s diene reacted under identical condi-
tions with a kobs of 2.1 × 10-2 min-1 and a t1/2 of 33 min.
These data place the silatrane diene 3 at almost twice as
reactive as Danishefsky’s diene.
Diels-Alder/Cross Coupling. We have demonstrated that
it is possible to effect Hiyama cross-coupling reactions of
these silicon-substituted Diels-Alder cycloadducts.16 Sila-
trane-substituted cycloadduct 6 was treated with iodobenzene
or p-iodoanisole in the presence of Pd(II), PPh3, and TBAF
to produce the cross-coupled cycloadducts 13 and 14 in 83%
and 68% isolated yield.
Single-point-energy, semiempirical (AM1) calculations of
HOMO energies for a number of dienes, while constraining the
1,3-diene dihedral angles to 0°, were also performed with
SPARTAN2.0.1,3-Butadiene,2-methoxybutadiene,andDani-
shefsky’s diene have HOMO energies of -9.35, -9.09, and
-8.82 eV, respectively. Dienes 2, 3, and 4 have HOMO ener-
gies of -9.21, -7.87, and -5.04 eV, respectively. These ener-
gies are consistent with our observations that 2 is less reactive
than Danishefsky’s diene whereas 3 is more reactive than
Danishefsky’s diene and diene 3 is less reactive than diene 4.
Additional Diels-Alder reactions with the unsymmetrical
dienophile, citraconic anhydride 8, were then performed to
assess any differences in rate or regioselectivity of cycload-
dition reactions. The silatrane-substituted diene 3 produced
a 2.0:1 mixture of para:meta regioisomers (9:10) in 78%
isolated yield after heating to 120 °C in THF for 48 h. The
catechol silane-substituted diene (4) reacted under slightly
milder conditions (90 °C for 36 h) to produce a 4.8:1 mixture
of 11:12 in 78% isolated yield.
We have also looked very briefly at the possibility of using
these dienes in one-pot sequential reaction sequences rather
than the two-pot Diels-Alder cross-coupling sequences
described above. The first attempt to do transmetallation/
Diels-Alder/cross coupling by treating 2 with N-phenyl-
maleimide, iodobenzene, and TBAF in the presence of Pd(II)
and CuI just yielded the cross-coupling product of the silyl
diene, 2-phenyl-1,3-butadiene (15).17 The implication of this
experiment is that transmetallation/oxidative addition/reduc-
tive elimination could not be intercepted by the Diels-Alder
reaction under these conditions. A less ambitious transmet-
allation/Diels-Alder/protonolysis scheme did yield some
Diels-Alder product 1618,19 from a one-pot reaction.
Major and minor isomer regiochemistry and stereochem-
istry assignments were originally performed with NOESY
data which showed strong NOEs between the CH3 and both
the ring junction H and one of the two diastereotopic H’s
on the CH2 R to the alkene C-H for both 9 and 11. This
assignment was subsequently confirmed for the major isomer
(9) by X-ray crystallography (Figure 3).
In summary, we have prepared new, stable, crystalline
silyl-substituted dienes in high yield and find that they readily
Org. Lett., Vol. 9, No. 9, 2007
1625

participate in Diels-Alder/cross-coupling tandem reactions.
We will report the transition-metal-catalyzed reaction chem-
istry of these main group element substituted dienes in much
more detail in due course.
trometry performed high-resolution mass spectral analyses.
We thank Professor Al Rives for assistance with the HOMO
energy calculations for the dienes.
Supporting Information Available: Experimental pro-
cedures and characterization data for all new compounds and
CIF files for compounds 3, 4, and 9. This material is available
free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We thank the National Science Foun-
dation for their support of this work (CHE-0450722) and
the NMR instrumentation used to characterize the compounds
reported here. The Duke University Center for Mass Spec-
OL070089E
(15) Danishefsky, S.; Kitahara, T.; Yan, C. F.; Morris, J. J. Am. Chem.
Soc. 1979, 101, 6996-7000.
(16) Denmark, S. E.; Butler, C. R. Org. Lett. 2006, 8, 63-66.
(17) Lee, P. H.; Lee, S. W.; Seomoon, D. Org. Lett. 2003, 5, 4963-
4966.
(18) Fahmy, A. F.; Aly, N. F.; Abd El-Aleem, A. H. Indian J. Chem.
1978, 16B, 992.
(19) Fahmy, A. F.; Aly, N. F.; Orabi, M. O. Indian J. Chem. 1983, 22B,
912.
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