Asymmetric 1,4-dihydroxylation of 1,3-dienes by catalytic enantioselective diboration

Article abstract of DOI:10.1021/ja809610h

(Chemical Equation Presented) Asymmetric 1,4-dihydroxylations of 1,3-dienes, and other transformations, are initiated by the Pt-catalyzed enantioselective addition of bis(pinacolato)diboron (B₂(pin) ₂) to conjugated dienes. The studies reported in this communication suggest that both cyclic and acyclic substrates will participate in this reaction; however, dienes which are unable to adopt the S-cis conformation are unreactive. For most substrates, 1,4-addition is the predominant pathway. In addition to oxidation to the derived 2-buten-1,4-diol, stereoselective carbonyl allylation with the intermediate bis(boronate) ester is also described.

Full text of DOI:10.1021/ja809610h

Published on Web 06/08/2009
Asymmetric 1,4-Dihydroxylation of 1,3-Dienes by Catalytic Enantioselective
Diboration
Heather E. Burks, Laura T. Kliman, and James P. Morken*
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467
Received December 10, 2008; E-mail: morken@bc.edu
Table 1. Catalytic Enantioselective Diboration/Oxidation of
Catalytic enantioselective oxidation reactions provide an impor-
tant strategy for the conversion of simple unsaturated hydrocarbon
substrates to synthetically useful organic compounds. While highly
selective asymmetric oxidation of hydrocarbons may be accomplished
in many ways, the 1,4-dihydroxylation of 1,3-dienes (Scheme 1),
a synthetically valuable process, remains a largely unsolved
problem. To accomplish this transformation, [4+2] cycloaddition
1,3-Dienesa
1
1
of dienes with O2 is often employed; however, this reaction has
not been accomplished under the influence of a chiral catalyst.2
Likewise, the Pd(II)-catalyzed 1,4-diacetoxylation and related
reactions,3 very promising transformations for asymmetric synthesis,
have not yet been accomplished with useful levels of enantiose-
lectivity.4 Our research program has focused on the enantioselective
diboration of unsaturated hydrocarbons,5 and in this communication
we describe an enantioselective 1,4-diboration of 1,3-dienes,6,7
a
transformation that furnishes the above-described 1,4-dihydroxy-
lation products in an economical fashion, with good yields, and
with high levels of enantiomeric purity.8
Scheme 1
We recently developed a Pd-catalyzed enantioselective diboration
of allenes, a process that benefits from accelerated catalysis when
done in the presence of monodentate phosphine ligands. Detailed
mechanistic studies suggest this reaction proceeds by oxidative
addition of the diboron to palladium, a step which is followed by
migratory insertion of the terminal alkene into a Pd-B bond
(Scheme 2, eq 1).9 DFT studies revealed that the insertion step
proceeds by an unusual elementary reaction that directly provides
a stable, coordinatively saturated η3-allyl palladium complex 2 (see
structure 1; rotation of the terminal alkene occurs concomitantly
with insertion, allowing π-bonding with the adjacent alkene to
develop in the transition state). Considering that olefin insertion
involving a 1,3-diene likely benefits from a similar incipient π bond
(3), it was of interest to determine whether catalysts developed for
the asymmetric allene diboration would be effective in catalytic
diene diboration.
Scheme 2
a Unless otherwise indicated, (R,R)-L1 was employed with reaction at
60 °C for 12 h, followed by oxidation with 30% H2O2 and 3 M NaOH
for 3 h. b Percent yield of purified material. Value is an average of two
experiments. c Determined by GC or SFC analysis employing a chiral
stationary phase. d The product depicted corresponds to that obtained
with (S,S)-L2. e Reaction at room temperature; value in parentheses is
after a single recrystallization. f Reaction at room temperature. g 3 equiv
of B2(pin)2 employed. h (S,S) enantiomer of ligand L1 employed for this
experiment.
9
9134 J. AM. CHEM. SOC. 2009, 131, 9134–9135
10.1021/ja809610h CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Scheme 4
Initial experiments probed the reaction between bis(pinacolato)-
diboron (B2(pin)2) and 1,3-dienes. While no reaction was observed
with a catalyst composed of Pd2(dba)3 and a chiral TADDOL-derived
phosphoramidite, the most effective catalyst for enantioselective allene
diboration, when the palladium complex was replaced with Pt2(dba)3,
a complex that generally exhibits higher reactivity in diboration
reactions,10 efficient catalysis and modest levels of enantioselectivity
were observed. Several permutations of reaction conditions and ligand
structure revealed that a catalyst composed of Pt2(dba)3 and chiral
TADDOL-derived phosphonite L111 provides good reactivity for a
range of substrates and modest to high levels of enantioselection for
many (Table 1). As can be observed in Table 1, acyclic dienes bearing
aryl or alkyl substitution at the terminus react efficiently and with high
selectivity. With ligand L1, butadienes bearing substitution at both
C1 and C3 react with lower stereocontrol, however, with L2 high
selectivity is obtained (entry 5). Notably, cyclic dienes can be suitable
substrates for the asymmetric diboration/oxidation sequence and
provide otherwise difficult-to-access products with high levels of
asymmetric induction. As noted in entry 10, the 1,2-diol is the
predominant product when one terminus of the substrate is disubsti-
tuted, an outcome which likely results from enhanced steric congestion.
Lastly, the lack of reaction with cis-piperylene (entry 11) suggests that
only dienes able to adopt an S-cis conformation will participate in the
Pt-catalyzed diene diboration. This observation may reflect the
importance of structure 3 (Scheme 2) in the reaction mechanism.
In addition to 2-buten-1,4-diols, other important scaffolds can be
easily prepared from simple dienes through the asymmetric diboration
reaction. Enantiomerically enriched butenolides and derived butyro-
lactones are prominent structural elements in natural products, and
straightforward methods for their preparation are scarce.12 Diene
diboration provides a new approach: subsequent to diboration and
oxidation, the unpurified material was subjected to perruthenate-
catalyzed oxidation13 and furnished the derived butenolide in good
yield and without compromising the integrity of the carbinol stereo-
center.
for support of the BC Mass Spectrometry Center. We also thank
AllyChem, Co., Ltd. for a generous donation of B2(pin)2.
Note Added after ASAP Publication. The uncorrected proof
version was published June 8, 2009. Minor text corrections have been
made in the version published on June 12, 2009.
Supporting Information Available: Characterization and proce-
dures. This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) For recent applications of the singlet oxygen [4+2] cycloaddition in
synthesis, see: (a) Lee, J. S.; Fuchs, P. L. J. Am. Chem. Soc. 2005, 127,
13122. (b) Barbarow, J. E.; Miller, A. K.; Trauner, D. Org. Lett. 2005, 7,
2901. (c) Ono, K.; Nakagawa, M.; Nishida, A. Angew. Chem., Int. Ed.
2004, 43, 2020. (d) Kusama, H.; Hara, R.; Kawahara, S.; Nishimori, T.;
Kashima, H.; Nakamura, N.; Morihira, K.; Kuwajima, I. J. Am. Chem. Soc.
2000, 122, 3811. (e) Zhou, G.; Gao, X.; Li, W. Z.; Li, Y. Tetrahedron
Lett. 2001, 42, 3101. (f) Izzo, I.; Meduri, G.; Avallone, E.; De Riccardis,
F.; Sodano, G. Eur. J. Org. Chem. 2000, 439. (g) For recent reviews, see:
(h) Clennan, E. L.; Pace, A. Tetrahedron 2005, 61, 6665.
(2) For the enantioselective desymmetrization of 1O2 cycloadducts, see: Staben,
S. T.; Xin, L.; Toste, F. D. J. Am. Chem. Soc. 2006, 128, 12658.
(3) For a recent review of this process: Ba¨ckvall, J.-E. Palladium-Catalyzed
1,4-Additions to Conjugated Dienes. In Metal Catalyzed Cross-Coupling
Reactions, 2nd ed.; De Meijere, A., Diederich, F., Eds.; Wiley-VCH:
Weinheim, 2004; pp 479-529.
(4) (a) Thorarensen, A.; Palmgren, A.; Itami, K.; Ba¨ckvall, J.-E. Tetrahedron
Lett. 1997, 38, 8541. (b) Itami, K.; Palmgren, A.; Thorarensen, A.; Ba¨ckvall,
J.-E. J. Org. Chem. 1998, 63, 6466. (c) Verboom, R. C.; Plietker, B. J.;
Ba¨ckvall, J.-E. J. Organomet. Chem. 2003, 687, 508. (d) El-Qisairi, A. K.;
Qaseer, H. A.; Fennelly, J. P.; Chiarelli, M. P.; Henry, P. M. Catal.
Commun. 2008, 9, 1661.
Scheme 3
(5) Recent reviews: (a) Burks, H. E.; Morken, J. P. Chem. Commun. 2007,
4717. (b) Ramirez, J.; Lillo, V.; Segarra, A. M.; Fernandez, E. Comp. Rend.
Chim. 2007, 10, 138. (c) Beletskaya, I.; Moberg, C. Chem. ReV. 2006, 106,
2320. (d) Ishiyama, T.; Miyaura, N. Chem. Record 2004, 3, 271.
(6) Non-enantioselective diene diboration: (a) Ishiyama, T.; Yamamoto, M.;
Miyaura, N. Chem. Commun. 1996, 2073. (b) Ishiyama, T.; Yamamoto,
M.; Miyaura, N. Chem. Commun. 1997, 689. (c) Clegg, W.; Thorsten, J.;
Marder, T. B.; Norman, N. C.; Orpen, A. G.; Peakman, T. M.; Quayle,
M. J.; Rice, C. R.; Scott, A. J. J. Chem. Soc., Dalton Trans. 1998, 1431.
(d) Morgan, J. B.; Morken, J. P. Org. Lett. 2003, 5, 2573.
(7) For asymmetric silylboration of dienes: (a) Gerdin, M.; Moberg, C. AdV.
Synth. Catal. 2005, 347, 749. For diastereoselective diene diboration using
chiral diboron reagents, see: ref 6c.
(8) B2(pin)2 is a readily available inexpensive reagent ($800/kg, AllyChem,
Co., Ltd. (www.allychem.com)).
(9) (a) Burks, H. E.; Liu, S.; Morken, J. P. J. Am. Chem. Soc. 2007, 129, 8766.
(b) Pelz, N. F.; Woodward, A. R.; Burks, H. E.; Sieber, J. D.; Morken,
J. P. J. Am. Chem. Soc. 2004, 126, 16328.
(10) (a) Ishiyama, T.; Matusuda, N.; Miyaura, N.; Suzuki, A. J. Am. Chem.
Soc. 1993, 115, 11018. (b) Ishiyama, T.; Matsuda, N.; Murata, M.; Ozawa,
F.; Suzuki, A.; Miyaura, N. Organometallics 1996, 15, 713. For compu-
tational studies comparing Pd and Pt in diboration, see: (c) Cui, Q.; Musaev,
D. G.; Morokuma, K. Organometallics 1998, 17, 742. (d) Sakaki, S.;
Kikuno, T. Inorg. Chem. 1997, 36, 226.
The R-chiral allylboronate functionality embedded in diene dibo-
ration products also finds use in stereoselective carbonyl allylation.14
Critical concerns are whether this transformation proceeds with high
chirality transfer and whether it is selective for one constitutional
isomer. To address this, benzaldehyde was added to an unquenched
diboration reaction. After 12 h of reaction and oxidative workup, this
sequence provided a single diastereomer of a single constitutional
isomer, with near-perfect chirality transfer. The product structure
suggests that this transformation proceeds through chair-like transition
state A that minimizes A(1,3) interactions and with C-C bond
formation occurring at the least hindered carbon of the intermediate
diboron.
In conclusion, we have described a catalytic enantioselective
diboration of 1,3-dienes, a process which generally provides
synthetically useful chiral 2-buten-1,4-diols as the reaction product.
Further studies of the substrate scope and reaction utility are in
progress and will be reported in the near future.
(11) (a) Sakaki, J.; Schweizer, W. B.; Seebach, D. HelV. Chem. Acta 1993, 76,
2654. (b) Seebach, D.; Hayakawa, M.; Sakaki, J.; Schweizer, W. B.
Tetrahedron 1993, 49, 1711. (c) Alexakis, A.; Burton, J.; Vastra, J.;
Benhaim, C.; Fournioux, X.; van den Heuvel, A.; Leveque, J.-M.; Maze,
F.; Rosset, S. Eur. J. Org. Chem. 2000, 4011. (d) Bee, C.; Han, S. B.;
Hassan, A.; Iida, H.; Krische, M. J. J. Am. Chem. Soc. 2008, 130, 2746.
(12) Seitz, M.; Reiser, O. Curr. Opin. Chem. Biol. 2005, 9, 285.
(13) Ley, S. V.; Norman, J.; Griffith, W. P.; Marsden, S. P. Synthesis 1994,
639.
(14) (a) Hoffmann, R. W. Pure Appl. Chem. 1988, 60, 123. (b) Hoffmann, R. W.;
Neil, G.; Schlapbach, A. Pure. Appl. Chem. 1990, 62, 1993.
Acknowledgment. Support by the NIGMS (GM-59417) and
Merck is gratefully acknowledged, as is the NSF (DBI-0619576)
JA809610H
9
J. AM. CHEM. SOC. VOL. 131, NO. 26, 2009 9135