Palladium-catalyzed selective acyloxylation using sodium perborate as oxidant

Article abstract of DOI:10.1021/jo1024199

Sodium perborate (SPB), a principal component of washing powders, was employed as an inexpensive and eco-friendly oxidant in the palladium-catalyzed C-H acyloxylation of alkenes in excellent regio-and stereochemistry. The reactions used anhydrides as acyloxy sources. The method applies to both terminal and internal alkenes, and even benzylic C-H oxidation.(Figure Presented)

Full text of DOI:10.1021/jo1024199

pubs.acs.org/joc
interest by White,11,12 Stahl,3,13 Muzart,14 Kaneda,15
Palladium-Catalyzed Selective Acyloxylation Using
Sodium Perborate as Oxidant
Stambuli,16 Bercaw,17 and ourselves.18 Early studies focused
on finding efficient catalyst systems and reaction conditions.
The prevailing system in these studies involved simple palla-
dium salts (e.g., Pd(OAc)2) and benzoquinone (BQ) as essen-
tial components and the reactions were performed in AcOH as
solvent.6,10 BQ played a crucial role as an oxidant for Pd(0) and
an activator ligand to promote nucleophilic attack.3,8,9 AcOH
was also indispensible for the reduction of BQ to hydroqui-
none and to provide acetate nucleophiles. This combination
was successful in many reactions, but the acidic conditions
were problematic for some substrates, which limited synthetic
scope. The selectivity of the nucleophilic attack was sometimes
unsatisfactory, leading to mixtures of isomers. More recent
studies have addressed these problems, varying the solvents
and/or increasing the selectivity of the C-H functionalization
process.11-13,16,18 Many of these studies led to the use of new
ligand systems and/or alternative oxidants in place of BQ.
Recently we published a new solution for palladium-catalyzed
C-H acetoxylation and benzoyloxylation of alkenes with
PhI(OAc)2 as oxidant, enabling us to replace AcOH with
MeCN.18 We envisioned that the cost and environmental
impact would be reduced if PhI(OAc)2 were generated in situ
in substoichiometric amounts.19-21
€
Lukasz T. Pilarski, Par G. Janson, and Kalman J. Szabo*
ꢀ
ꢀ
ꢀ
Department of Organic Chemistry, Stockholm University,
Arrhenius Laboratory, SE-10691 Stockholm, Sweden
kalman@organ.su.se
Received December 8, 2010
Sodium perborate (SPB), a principal component of wash-
ing powders, was employed as an inexpensive and eco-
friendly oxidant in the palladium-catalyzed C-H acylox-
ylation of alkenes in excellent regio- and stereochemistry.
The reactions used anhydrides as acyloxy sources. The
method applies to both terminal and internal alkenes, and
even benzylic C-H oxidation.
The synthesis of PhI(OAc)2 using sodium perborate
tetrahydrate (SPB, NaBO3 4H2O),22 in AcOH reported
3
by Kitamura and co-workers23 inspired us to use SPB
with PhI in our initial attempts to this end. SPB is a cheap
(about $2.9/kg,24 bulk price) and safe oxidant, produced
in bulk amounts in the chemical industry (∼0.5 million
ton/year in EU25) and is an important component of
washing powder. On its own a weak oxidant, SPB may
be activated by AcOH to become a synthetically useful
oxidant.22,26
Allylic C-H acetoxylation is a synthetically and mecha-
nistically interesting field in palladium catalysis.1-4 Excellent
5
6,7
8,9
work by Tsuji, Akermark, Backvall, McMurry,10 and
˚
€
Kocovsky10 established this field in the 90s, with more recent
Although the palladium-catalyzed C-H acetoxylation
of 1a and 1i worked well using a SPB/PhI mixture, we
quickly found that PhI was superfluous. To avoid AcOH,
we used an excess of acetic anhydride (Ac2O)26,27 in MeCN
solvent to boost the oxidation power of SPB. We found
that various alkenes (1a-j) could be C-H acetoxylated
using SPB (2) and Ac2O (3a) in the presence of cata-
lytic amounts of Pd(OAc)2 (Scheme 1). Best results were
obtained using four equivalents of SPB and 16 equivalents
of 3 with respect to the alkene. To the best of our knowl-
edge, this is the first use of 2 as oxidant in palladium
catalysis.
(1) Tsuji, J. Palladium Reagents and Catalysts. New Perspectives for the
21st Century; Wiley: Chichester, U.K., 2004.
(2) Hartwig, J. Organotransition Metal Chemistry; University Science
Books: Sausalito, CA, 2010.
(3) Popp, B. V.; Stahl, S. S. Top. Organomet. Chem. 2007, 22, 149.
(4) Liu, G.; Wu, Y. Top. Curr. Chem. 2010, 292, 195.
(5) Tsuji, J.; Sakai, K.; Nagashima, H.; Shimuzu, I. Tetrahedron Lett.
1981, 22, 131.
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(6) Hansson, S.; Heumann, A.; Rein, T.; Akermark, B. J. Org. Chem.
1990, 55, 975.
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(7) Akermark, B.; Larsson, E. M.; Oslob, J. D. J. Org. Chem. 1994, 59,
5729.
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(8) Grennberg, H.; Backvall, J.-E. Chem.;Eur. J. 1998, 4, 1083.
(9) Grennberg, H.; Simon, V.; Backvall, J.-E. J. Chem. Soc. Chem.
€
Commun. 1994, 265.
(10) McMurry, J. E.; Kocovsky, P. Tetrahedron Lett. 1984, 25, 4187.
(11) Chen, M. S.; White, M. C. J. Am. Chem. Soc. 2004, 126, 1346.
(12) Delcamp, J. H.; White, M. C. J. Am. Chem. Soc. 2006, 128, 15076.
(13) Cambell, A. N.; White, P. B.; Guzei, I. A.; Stahl, S. S. J. Am. Chem.
Soc. 2010, 132, 15116.
(14) Thiery, E.; Aouf, C.; Belloy, J.; Harakat, D.; Le Bras, J.; Muzart, J.
J. Org. Chem. 2010, 75, 1771.
(15) Mitsudome, T.; Umetani, T.; Nosaka, N.; Mori, K.; Mizugaki, T.;
Ebitani, K.; Kaneda, K. Angew. Chem., Int. Ed. 2006, 45, 481.
(16) Henderson, W. H.; Check, C. T.; Proust, N.; Stambuli, J. P. Org.
Lett. 2010, 12, 824.
(19) Ochiai, M.; Takeuchi, Y.; Katayama, T.; Sueda, T.; Miyamoto, K.
J. Am. Chem. Soc. 2005, 127, 12244.
(20) Dohi, T.; Maruyama, A.; Yoshimura, M.; Morimoto, K.; Tohma,
H.; Kita, Y. Angew. Chem., Int. Ed. 2005, 44, 6193.
(21) Dohi, T.; Kita, Y. Chem. Commun. 2009, 2073.
(22) McKillop, A.; Sanderson, W. R. Tetrahedron 1995, 51, 6145.
(23) Hossain, M. D.; Kitamura, T. J. Org. Chem. 2005, 70, 6984.
(24) Sodium perborate, SPB, Price. http://www.chemistrystore.com/So-
dium_Perborate-Sodium_Perborate_55lbs.html.
(25) http://ecb.jrc.ec.europa.eu/documents/Existing-Chemicals/RIS-
K_ASSESSMENT/SUMMARY/perboricacidsodiumsaltsum301.pdf.
(26) Muzart, J. Synthesis 1995, 1325.
(17) Lin, B. L.; Labinger, J. A.; Bercaw, J. E. Can. J. Chem. 2009, 87, 264.
€
(18) Pilarski, L. T.; Selander, N.; Bose, D.; Szabo, K. J. Org. Lett. 2009,
11, 5518.
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DOI: 10.1021/jo1024199
r
Published on Web 01/20/2011
J. Org. Chem. 2011, 76, 1503–1506 1503
2011 American Chemical Society

Pilarski et al.
JOCNote
SCHEME 1. Palladium-Catalyzed Acyloxylation of Alkenes
Using Sodium Perborate (SPB)
TABLE 1. Palladium-Catalyzed C-H Acyloxylation using SPB as
Oxidanta
A particular advantage of using SPB (Figure 1) is that it
generates nontoxic boric acid and water as byproducts from
the oxidation reaction, which also simplifies purification.
FIGURE 1. Structure of SPB (twice the empirical formula,
NaBO3 4H2O).22,26
3
The catalytic reaction proceeds under mild conditions
(typically at 40 °C) with high functional group tolerance
(Table 1); the ester (entries 1 and 7-9), lactone (entry 10),
amide (entry 4) ketone (entry 3), and aromatics (entries 5-6)
survived the allylic C-H functionalization unchanged.
A particularly important feature of this procedure is that it
is suitable for the C-H functionalization of both terminal
(1a-1f) and internal alkenes (1g-1j). Without using AcOH
as solvent and BQ as oxidant very few efficient C-H
acetoxylation methods for oxidation of internal alkenes have
been reported in the literature.3,4 Furthermore, not only
allylic but also benzylic C-H bonds28,29 can be functiona-
lized by the above methodology (entry 11). The benzylic
C-H functionalization of 1k requires harsher conditions
than do the allylic C-H bonds; the reaction temperature had
to be increased to 100 °C and AcOH used as solvent.
Previous studies used PhI(OAc)2 as oxidant under similar
reaction conditions for this transformation.30 Using Piv2O
(3b) both terminal and internal alkenes could be functiona-
lized (entries 12-14), and by application of benzoic anhy-
dride (3c) benzoyloxylation could be performed (entries
15-16). In these reactions we employed only four equiva-
lents of the corresponding anhydrides.
The reactions were highly selective, giving only one regio-
and stereoisomer, with substitution always at the allylic γ-
position (relative to the EWG or aryl group) and trans
double bond geometry (determined from the 1H NMR
spectrum of the crude reaction mixture) for all acyclic alkene
substrates (1a-i and 1l). The reactions also proved to be
easily scalable. Tenfold scale-ups gave only a small decrease
in yields (entries 1 and 13).
Replacing SPB with H2O2-urea led to a sharp decrease of
the catalytic efficiency. For example, acetoxylation of 1a
with H2O2-urea under our standard conditions (entry 1) gave
4a in only 20% yield. Na2S2O8 and Oxone proved to be
totally inefficient for the conversion of alkenes (1) to allylic
acetates under our conditions in absence of SPB. Conducting
aBn = benzyl; Bz = benzoyl, Piv = pivaloyl. Unless otherwise stated,
a mixture of 1 (0.3 mmol), SPB (1.2 mmol), Ac2O (4.8 mmol) and
Pd(OAc)2 (5 mol %) in MeCN (0.5 mL) was stirred for 18 h at the
indicated temperature. bThe reaction is scaled up to 3 mmol of 1. cAcOH
used as solvent, reaction conducted for 22 h. dPiv2O (1.2 mmol) was used
instead of Ac2O. eBz2O (1.2 mmol) was used instead of Ac2O.
the reactions using 1, SPB and Ac2O in the absence of
Pd(OAc)2, we could not detect even traces of allylic acetate
products. Under these conditions using internal alkenes
(e.g., 1g) we could detect epoxidation of the double bond,26,31
in accordance with previous reports using SPB-OAc mixtures
(28) Jiang, H.; Chen, H.; Wang, A.; Liu, X. Chem. Commun. 2010, 46,
7259.
(29) Zhang, J.; Khaskin, E.; Anderson, N. P.; Zavalij, P. Y.; Vedernikov,
A. N. Chem. Commun. 2008, 3625.
(30) Dick, A. R.; Hull, K. L.; Sanford, M. S. J. Am. Chem. Soc. 2004, 126,
2300.
1504 J. Org. Chem. Vol. 76, No. 5, 2011

Pilarski et al.
JOCNote
similar oxidative functionalization procedure was reported
by Ritter and co-workers32,33 for pyridine derivatives analo-
gous to 1k.
The alternative Pd(0)/Pd(II) cycle may begin with the
formation of an allyl-Pd(II) complex, subsequently acti-
vated by SPB or 5 to give the product and Pd(0), which could
be reoxidized by 7. However, it should be noted that addition
of PhI to the reaction mixture does not influence the yield
(64%) of the reaction. Our initial studies for in situ genera-
tion of PhI(OAc)2 involved addition of PhI in large excess. It
is well-known that PhI rapidly undergoes oxidative addition
with Pd(0) species but much more reluctantly with Pd(II)
species.34,35 That the active catalyst was not trapped by PhI
suggests Pd(0) species might not be catalytically relevant.
In summary, we have described an inexpensive, highly
selective catalytic method for the C-H acyloxylation of
alkenes using SPB and carboxylic anhydrides. As far as we
know, this is the first application of SPB in palladium
catalysis. The reaction has a broad synthetic scope and it
can be used for both external and internal alkenes and
benzylic acetoxylation. The acyloxy groups can be varied
by selecting different anhydrides. The products of the cata-
lytic C-H functionalization reactions can be employed as
useful substrates in palladium-catalyzed allylic substitution
reactions.1,2 The C-H functionalization of internal alkenes
suggests a large potential for asymmetric induction, for
which few efficient protocols currently exist.
FIGURE 2. Proposed catalytic cycle.
in dichloromethane.31 However, using Pd(OAc)2 and MeCN as
solvent strongly suppresses substrate epoxidation. Analysis of
the crude products indicated that for internal alkenes 1g-i
traces of epoxy-products were formed, but these could be
removed by chromatography. The structure of the ester func-
tionality in 1a, 1g-i, and 1l had little effect on the acyloxylation
reactions. It affected only the purification of the products. The
benzyl esters (4a, 4g, 4h) are less volatile than the methyl esters
reducing the purification losses. When the boiling point of the
product (4i) is increased by a substituent, the methyl ester was
employed. Compared to other products 4k showed a relatively
high tendency for hydrolysis of the acetate, which led to a
decrease in yield.
The regio- and stereoselectivity of the reactions are con-
sistent with an allyl-Pd mechanism.1,2 However, we were
unable unambiguously to determine the mechanistic path-
way through monitoring the stoichiometric reactivity of
model allyl-Pd(II) complexes (see Supporting Information).
Accordingly, we recognize that the C-H activation may be
occurring through either a Pd(II/IV) catalytic cycle,30 as
postulated for the acetoxylation with PhI(OAc)2,18 or via
the classical Pd(0/II) pathway.
Our proposed mechanism (Figure 2) assumes the first step
to be oxidative addition of 5, formed from the reaction
between SPB (2) and Ac2O (3a),31 to the precatalyst (most
probably X = OAc, L = MeCN), affording complex 6. The
electron-withdrawing acetate groups in 5 likely encourage
conjugation between the peroxide oxygen lone pairs (nπ) and
the empty pπ orbital of boron, increasing the oxidation
potential. The next steps are boronate-acetate exchange
and alkene coordination to give complex 8, followed by
internal deprotonation. The Pd(IV) center in 8 is highly
electrophilic, which favors generation of the strongly π-
donating η3-allyl moiety in 10. Reductive elimination from
10 gives product 4a with a high regio- and stereoselectivity. A
closely related mechanistic alternative could be oxidation of
the Pd(II) species by 5 to bimetallic Pd(III) intermediates. A
Experimental Section
General Method A. Acetoxylation of Alkenes. A mixture
of Pd(OAc)2 (3.4 mg, 0.015 mmol, 5 mol%), SPB, 2 (185 mg,
1.2 mmol), MeCN (0.5 mL), anhydride 3a (489 mg, 4.8 mmol),
and 1 (0.3 mmol) in a screwtop vial was stirred for 18 h at the
indicated temperature. The crude was partitioned between
water (10 mL) and Et2O (10 mL), washed with water (3 ꢀ
10 mL), dried (MgSO4), filtered, concentrated, and purified by
column chromatography.
General Method B. Pivaloylation and Benzoylation of Alkenes.
The reactions were performed as in general method A except
pivalic acid anhydride (1.2 mmol) or benzoic acid anhydride
(1.2 mmol) were used. One ml of MeCN was used instead of
0.5 mL and saturated aqueous NaHCO3 was used instead of
water in the work up.
(E)-3-(Phenylsulfonyl)Allyl Acetate (4b): Synthesized accord-
ing to general method A. Isolated by column chromatography
(Et2O/pentane 1:3) as a colorless oil (75%). 1H NMR (400 MHz,
CDCl3): δ 2.10 (3H, s), 4.77 (2H, dd, 3JHH = 4.0 Hz, 4JHH = 2.0
0
Hz), 6.56 (1H, dt, 3JHH = 15.1 Hz, 2.0 Hz), 6.98 (1H, dt, 3JHH
=
15.1, 4.0 Hz), 7.52-7.59 (2H, m), 7.61-7.67 (1H, m), 7.87-7.92
(2H, m). 13C NMR (100 MHz, CDCl3): δ 20.7, 61. 6, 127.9,
129.5, 131.4, 133. 8, 139.6, 140.0, 170.0. HRMS (ESI): m/z calcd.
for [C11H12O4S þ Na]þ 263.0349, found 263.0355.
(E)-4-Morpholino-4-oxobut-2-en-1-yl Acetate (4d): Synthe-
sized according to general method A without aqueous workup.
Isolated by column chromatography (EtOAc) as a colorless oil
(52%). 1H NMR (CDCl3, 400 MHz): δ 2.11 (3H, s), 3.50-3.76
(8H, m), 4.74 (2H, dd, 3JHH = 5.1 Hz, 4JHH = 1.8 Hz), 6.43 (1H,
dt, 3JHH = 15.2 Hz, 4JHH = 1.8 Hz), 6.86 (1H, dt, 3JHH = 15.2,
(32) Powers, D. C.; Geibel, M. A. L.; Klein, J. E. M. N.; Ritter, T. J. Am.
Chem. Soc. 2009, 131, 17050.
(33) Powers, D. C.; Ritter, T. Nat. Chem. 2009, 1, 302.
(34) Phan, N. T. S.; Van Der Sluys, M.; Jones, C. W. Adv. Synth. Catal.
2006, 348, 609.
(31) Xie, G.; Xu, L.; Hu, J.; Ma, S.; Hou, W.; Tao, F. Tetrahedron Lett.
1988, 29, 2967.
ꢀ
(35) Szabo, K. J. J. Mol. Cat. A 2010, 324, 56.
J. Org. Chem. Vol. 76, No. 5, 2011 1505

Pilarski et al.
JOCNote
5.1 Hz).13C NMR (100 MHz, CDCl3): δ 21.0, 42.5, 46.4, 63.3,
66.9 (two overlapping signals, see attached HSQC spectrum in
Supporting Information), 121.3, 139.0, 164.9, 170.5. HRMS
(ESI): m/z calcd. for [C10H15NO4 þ Na]þ 236.0893, found
236.0896.
tm, JHH = 7.4 Hz), 7.89 (2H, dm, JHH = 7.3, 1.8 Hz). 13C
NMR (100 MHz, CDCl3): δ 27.3, 39.0, 61.6, 127.9, 129.5, 131.2,
133.8, 140.0, 140.2, 177.6. HRMS (ESI): m/z calcd. for [C14-
H18O4S þ Na] þ 305.0818, found 305.0806.
3
3
(E)-Benzyl 4-pivaloxypent-2-enoate (4n): Synthesized accord-
ing to general method B. Isolated by column chromatography
(Et2O/pentane 1:9) as a pale yellow oil (58%). 1H NMR (CDCl3,
400 MHz): δ 1.22 (9H, s), 1.34 (3H, d, 3JHH = 6.7 Hz), 5.19 (2H,
d, 3J HH= 2.5 Hz), 5.47 (1H, m), 5.99 (1H, dd, 3JHH = 15.8 Hz,
4JHH = 1.8 Hz), 6.93 (1H, dd, 3JHH = 15.8, 4.6 Hz), 7.30 - 7.41
(5H, m). 13C NMR (100 MHz, CDCl3): δ 19.7, 27.3, 39.0, 66.6,
68.6, 120.4, 128.5, 128.5, 128.7, 136.0, 147.5, 166.1, 177.6.
HRMS (ESI): m/z calcd. for [C17H22O4 þ Na]þ 313.1410,
found 313.1396.
(E)-Benzyl 4-acetoxyhex-2-enoate (4h): Synthesized accord-
ing to general method A. Isolated by column chromatography
(Et2O/pentane 1:5) as a colorless oil (70%). 1H NMR (400 MHz,
CDCl3): δ 0.93 (3H, t, 3JHH = 7.4 Hz), 1.64-1.75 (2H, m), 2.09
(3H, s), 5.18 (2H, s), 5.48 (1H, m), 5.99 (1H, dd, 3JHH = 15.8 Hz,
4JHH =1.6 Hz), 6.88 (1H, dd, 3JHH = 15.8, 5.29 Hz), 7.30-7.40
(5H, m). 13C NMR (100 MHz, CDCl3): δ 9.4, 21.1, 27.0, 66.6,
121.4, 128.5, 128.7, 135.9, 146.0, 166.0, 170.2. HRMS (ESI): m/z
calcd. for [C15H18O4 þ Na]þ 285.1097, found: 285.1101.
5-(Acetyloxy)-5-methyl-2(5H)-furanone (4j): Synthesized ac-
cording to general method A. Isolated by column chromatography
(E)-3-(phenylsulfonyl)allyl Benzoate (4p): Synthesized accord-
ing to general method B. Isolated by column chromatography
(Et2O/pentane 1:1) as a colorless oil (89%). 1H NMR (CDCl3,
400 MHz): δ 5.05 (2H, dd, 3JHH = 4.0 Hz, 4JHH = 2.1 Hz), 6.65
(EtOAc/pentane 1:5) as a colorless oil (62%). 1H NMR (CDCl3,
3
400 MHz): δ 1.82 (3H, s), 2.08 (3H, s), 6.17 (1H, d, JHH
=
5.7 Hz), 7.62 (1H, d, JHH = 5.7 Hz). 13C NMR (100 MHz,
CDCl3): δ 21.5, 23.0, 107.1, 122.9, 153.3, 168.9, 169.3. HRMS
(ESI): m/z calcd. for [C7H8O4 þ Na]þ 179.0315, found 179.0320.
(E)-Benzyl 4-pivaloxybut-2-enoate (4l): Synthesized according
to general method B. Isolated by column chromatography
(Et2O/pentane 1:4) as a colorless oil (72%). 1H NMR (CDCl3,
(1H, dt, 3JHH = 15.2 Hz, 4JHH = 2.1 Hz), 7.12 (1H, dt, 3JHH
=
3
15.2, 4.0 Hz), 7.46 (2H, t, 3JHH = 7.7 Hz), 7.52-7.65 (4H, m),
7.91 (2H, dm, 3JHH = 7.1 Hz), 8.03 (2H, dm, 3JHH = 7.1 Hz).
13C NMR (100 MHz, CDCl3): δ 62.0, 128.0, 128.7, 129.2, 129.5,
129.9, 130.3, 131.5, 133.7, 133.8, 139.9, 140.0, 135.6. HRMS
(ESI): m/z calcd. for [C16H14O4S þ Na]þ 325.0510, found
325.0507.
400 MHz): δ 1.24 (9H, s), 4.73 (2H, dd, 3JHH = 4.4 Hz, 4JHH
=
2.1 Hz), 5.20 (2H, s), 6.07 (1H, dt, 3JHH = 15.8 Hz, 4JHH = 2.1
3
Hz), δ 7.00 (1H, dt, JHH = 15.8 Hz, 4.4 Hz), δ 7.30-7.41
Acknowledgment. A Carl Tryggers Stiftelse stipend for L.
T.P. and the support of the Swedish Research Council is
greatly acknowledged. Financial support from the Founda-
tion Lars Hiertas Minne is also acknowledged.
(5H, m). 13C NMR (100 MHz, CDCl3): δ 27.3, 39.0, 62.6, 66.6,
121.5, 128.5, 128.7, 135.9, 142.4, 165.8, 177.9. HRMS (ESI): m/z
calcd. for [C16H20O4 þ Na]þ 299.1254, found 299.1268.
(E)-3-(phenylsulfonyl)allyl Pivalate (4m): Synthesized accord-
ing to general method B. Isolated by column chromatography
(Et2O/pentane 2:3) as a colorless oil (88%). 1H NMR (CDCl3,
Supporting Information Available: General experimental
details and spectroscopic data for the acyloxylation products.
This material is available free of charge via the Internet at http://
pubs.acs.org.
400 MHz): δ 1.20 (9H, s), 4.76 (2H, dd, 3JHH = 4.0 Hz, 4JHH
=
2.1 Hz), 6.52 (1H, dt, 3JHH = 15.1, 4JHH = 2.1 Hz), 7.00 (1H, dt,
3JHH = 15.1, 4.0 Hz), 7.55 (2H, tm, 3JHH = 7.8 Hz), 7.64 (1H,
1506 J. Org. Chem. Vol. 76, No. 5, 2011