Bis(NHC)-palladium(II) complex-catalyzed dioxygenation of alkenes

Article abstract of DOI:10.1021/om900975a

Bis(NHC)-Pd(II) complexes derived from l,l'-binaphthyl-2,2'-diamine (BINAM) were successfully first used to catalyze the dioxygenation of alkenes under mild conditions tolerant of air and moisture. Cationic NHC-Pd²⁺ diaquo complex 1e showed the highest catalytic activity to give 1,2dioxygenation products with good syn-diastereoselectivity for 1,2-disubstituted alkenes.

Full text of DOI:10.1021/om900975a

928 Organometallics 2010, 29, 928–933
DOI: 10.1021/om900975a
Bis(NHC)-Palladium(II) Complex-Catalyzed Dioxygenation of Alkenes
Wenfeng Wang,† Feijun Wang,† and Min Shi*,†,‡
†Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular
Engineering, East China University of Science and Technology, 130 Mei-Long Road, Shanghai 200237,
People’s Republic of China, and ‡State Key Laboratory of Organometallic Chemistry,
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road,
Shanghai 200032, People’s Republic of China.
Received November 8, 2009
Bis(NHC)-Pd(II) complexes derived from 1,10-binaphthyl-2,20-diamine (BINAM) were success-
fully first used to catalyze the dioxygenation of alkenes under mild conditions tolerant of air and
moisture. Cationic NHC-Pd2þ diaquo complex 1e showed the highest catalytic activity to give 1,2-
dioxygenation products with good syn-diastereoselectivity for 1,2-disubstituted alkenes.
Introduction
the additive and 100 °C harsh conditions were indispensa-
ble.3i Further studies are obviously required to help under-
stand details of the PdII/PdIV catalytic cycle and design
proper achiral and chiral ligands to improve the diastereos-
electivity and achieve enantiocontrol in the above dioxy-
genation of alkenes. Because of several typical features such
as stability to air/moisture, being less toxic, and strong σ-
electron-donating properties,4 N-heterocyclic carbenes
(NHCs) have been prevalent ligands in transition-metal
catalysis.5 Previously, we have synthesized a series of axially
chiral bis(NHC)-Pd(II) complexes from 1,10-binaphthyl-
2,20-diamine (BINAM) and successfully applied them in
several asymmetric catalytic reactions.6 Herein we report
the first example of bis(NHC)-Pd(II) complex-catalyzed
dioxygenation of alkenes under mild conditions.
The osmium-catalyzed cis-dihydroxylation of alkenes and
its asymmetric version developed by Sharpless and co-work-
ers (known as Sharpless AD reaction) are the quintessential
vicinal difunctionalization methods of alkenes.1 Recently,
palladium-catalyzed alkene difunctionalization based on the
Pd(II)/Pd(IV) catalytic cycle2 has also attracted considerable
attention.3 This new approach represented a promising
complement to the Sharpless method without use of toxic
and expensive osmium complexes. For example, Song et al.
reported an olefin hydroxyacetoxylation catalyzed by catio-
nic palladium diphosphine complexes in the presence of
PhI(OAc)2 as an oxidant to oxidize PdII to PdIV.3g Later
on, Jiang’s group developed a process for the Pd-catalyzed
diacetoxylation of alkenes directly using Pd(OAc)2 as palla-
dium source and oxygen as the sole oxidant, wherein KI as
Results and Discussion
*Corresponding author. E-mail: Mshi@mail.sioc.ac.cn. Fax: 86-21-
64166128.
Our initial experiments were carried out to investigate the
catalytic abilities of bis(NHC)-Pd(II) complexes in the dioxy-
genation of trans-stilbene in acetic acid, which is commercially
(1) (a) Jacobsen, E. N.; Marko, I.; Mungall, W. S.; Schroder, G.;
Sharpless, K. B. J. Am. Chem. Soc. 1988, 110, 1968–1970. (b) Kolb, H. C.;
VanNieuwenhze, M. S.; Sharpless, K. B. Chem. Rev. 1994, 94, 2483–2547.
(c) Johnson, R. A.; Sharpless, K. B. In Catalytic Asymmetric Synthesis, 2nd
ed.; Ojima, I., Ed.; VCH: New York, 2000. (d) Li, G.; Wei, H.-X.; Kim, S. H.
Org. Lett. 2000, 2, 2249–2252. (e) Li, G.; Wei, H.-X.; Kim, S.-H.; Carducci,
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C.; Wei, H.-X.; Li, G. J. Org. Chem. 2003, 68, 5742–5745.
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Yang, C.; Nolan, S. P. J. Organomet. Chem. 2002, 653, 69–82. (e) Diez-
Gonzalez, S.; Nolan, S. P. Coord. Chem. Rev. 2007, 251, 874–883.
(5) For selected reviews on NHC ligands, see: (a) Nolan, S. P., Ed.
N-Heterocyclic Carbenes in Synthesis; Wiley-VCH: Weinheim, Germany,
2006. (b) Glorius, F. N-Heterocyclic Carbenes in Transition Metal Cata-
lysis; Springer: Berlin, 2007. (c) Herrmann, W. A. Angew. Chem., Int. Ed.
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1767–1769. (f) Cesar, V.; Bellemin-Laponnaz, S.; Gade, L. H. Chem. Soc.
Rev. 2004, 33, 619–636. (g) K€uhl, O. Chem. Soc. Rev. 2007, 36, 592–607.
(h) Kantchev, E. A. B.; O'Brien, C. J.; Organ, M. G. Angew. Chem., Int. Ed.
2007, 46, 2768–2813. (i) Diez-Gonzalez, S.; Marion, N.; Nolan, S. P. Chem.
Rev. 2009, 109, 3612–3676.
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Tetrahedron 2008, 64, 2412–2418. (d) Zhang, T.; Shi, M. Chem.;Eur. J.
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(2) For more recent selected reports about the mechanism of the PdII/
PdIV catalytic cycle, see: (a) Dick, A. R.; Hull, K. L.; Sanford, M. S.
J. Am. Chem. Soc. 2004, 126, 2300–2301. (b) Desai, L. V.; Hull, K. L.;
Sanford, M. S. J. Am. Chem. Soc. 2004, 126, 9542–9543. (c) Dick, A. R.;
Kampf, J. W.; Sanford, M. S. J. Am. Chem. Soc. 2005, 127, 12790–12791.
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2009, 131, 10974–10983. (g) Arnold, P. L.; Sanford, M. S.; Pearson, S. M.
J. Am. Chem. Soc. 2009, 131, 13912–13913.
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K. J. Am. Chem. Soc. 2005, 127, 14586–14587. (c) Liu, G. S.; Stahl, S. S.
J. Am. Chem. Soc. 2006, 128, 7179–7181. (d) Desai, L. V.; Sanford, M. S.
Angew. Chem., Int. Ed. 2007, 46, 5737–5740. (e) Muniz, K. J. Am. Chem.
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J. Am. Chem. Soc. 2008, 130, 763–773. (g) Li, Y.; Song, D. T.; Dong, V. M.
J. Am. Chem. Soc. 2008, 130, 2962–2964. (h) Jensen, K. H.; Sigman, M. S.
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r
pubs.acs.org/Organometallics
Published on Web 01/25/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 4, 2010 929
Figure 1. Bis(NHC)-Pd(II) complexes 1a-e.
Table 1. Bis(NHC)-Pd(II) Complex-Catalyzed Dioxygenation of
trans-Stibenea
available and inevitably contains minor water, using PhI-
(OAc)2 as the terminal oxidant. Bis(NHC)-Pd(II) complexes
1a-e,6f bearing different counterions on the palladium center,
were examined under the above conditions to seek out the most
effective catalyst (Figure 1). The results of these experiments are
summarized in Table 1. In the absence of palladium(II) cata-
lyst, no background reaction was observed even with prolong-
ing the time to 84 h (Table 1, entry 1). Using 4 mol % NHC-
Pd(II) 1a as catalyst after 2.5 days at 50 °C afforded a mixture
of products 4 and 5a in 82% yield (syn:anti = 95:5) and 7%
yield (syn:anti = 40:60) in wet acetic acid, respectively (Table 1,
entry 2). Other NHC-Pd(II) complexes such as 1b, 1c, and 1d
were not very effective in this reaction (Table 1, entries 3-5),
although they were better catalysts than 1a in umpolung
allylation of aldehydes with cyclohexenyl acetate6f or conjugate
addition of arylboronic acids to cyclic enones.6d Cationic
NHC-Pd2þ diaquo complex 1e6e was found to be more reactive
than 1a to consume trans-stilbene completely within 12 h,
giving 4 in 63% yield with good syn-diastereoselectivity (syn:
anti = 90:10), along with 5a in 8% yield (Table 1, entry 6).
Similar results were observed at room temperature, albeit after
a longer time (Table 1, entry 7).
4
5a
entry catalyst time (h) yield (%)b syn:antic yield (%)d syn:antic
1
2
3
4
5
6
none
1a
1b
1c
1d
1e
1e
1e
1e
84
61
72
68
49
12
35
70
22
20
0
82
35
35
11
63
61
0c
0
7
2
2
1
8
10
0
12
73b
95:5
92:8
89:11
82:18
90:10
92:8
40:60
53:47
62:38
47:53
69:31
73:27
7e
8f
9g
59
0c
90:10
69:31
85:15
10g,h 1e
a Reaction conditions: All reactions were performed with 0.3 mmol of
2, 4 mol % of NHC-Pd(II) catalyst, 1.1 equiv of PhI(OAc)2, and 3.0
equiv of H2O in 3 mL of acetic acid (available commercially) at 50 °C.
b Isolated yield. c Determined by 1H NMR integration of the crude
product. d Determined by 1H NMR integration of crude product 5a
compared with 4. e At room temperature. f No oxidant PhI(OAc)2.
g Without the addition of 3.0 equiv of H2O. h Anhydrous acetic acid
was used.
High catalytic ability of NHC-Pd(II) catalyst 1e is possibly
attributed to its cationic character, facilitating activation of
the double bond of alkenes and enhancing the electrophili-
city of the subsequent palladated alkyl group.3f,g Surpris-
ingly, catalyst 1a, bearing an iodide counterion, was also
effective despite a longer reaction time required to complete
the transformation. Titration experiments suggested that 1a
in CDCl3 solution was converted to a new complex by
also ineffective in this reaction.7 On the basis of these results,
a process based on a Pd(II)/Pd(0) catalysis appeared im-
probable here.
1
To evaluate the influence of water for the distribution of
products 4 and 5a, the control experiments were carried out.
It was found that the yield of hydroxyacetate 4 (59% yield)
slightly decreased along with the increase of 5a (12% yield)
without the additional 3 equiv of H2O (Table 1, entry 9), and
moreover, using anhydrous AcOH as solvent (without H2O)
the dioxygenation of trans-stilbene exclusively afforded dia-
cetate 5a in 73% yield (syn:anti = 85:15) (Table 1, entry 10).
These results suggested that water was required for the
formation of hydroxyacetate 4.
Possible Mechanism for the Dioxygenation of trans-Stil-
bene. By an isotopic labeling study using 97% 18O-enriched
water, Song et al. have proposed a Pd(II)/Pd(IV) catalytic
mechanism involving hydrolysis of an acetoxonium inter-
mediate to lead to hydroxyacetate 4.3g To investigate the
origin of hydroxyacetate 4 herein, the 18O-labeling experi-
ment was repeated by treatment of trans-stilbene with 3.0
equiv of H218O (97% 18O-enriched) in anhydrous acetic acid
(see the Supporting Information). Dioxygenation products 4
and 5a were isolated as isotopic mixtures in 67% and 10%
yield, respectively. As shown in Table 2, the formation of
addition of stoichiometric PhI(OAc)2, with the H NMR
spectroscopic signal at 3.82 ppm for the N-CH3 of NHC-
Pd(II) complex 1a slightly downfield shifted to 3.88 ppm
after 5 min (see the Supporting Information). Formation of
reddish I2 was observed simultaneously, which possibly
originated from oxidation of the coordinating iodide coun-
terion on the palladium center by PhI(OAc)2. Prompted by
these results, we assumed that the new complex generated
from 1a with PhI(OAc)2 featured a cationic palladium center
bearing noncoordinating counterions. Although details re-
main to be elucidated further, this assumption is consistent
with our experimental observations.3i
Without the addition of PhI(OAc)2 (Table 1, entry 8), no
detectable product was formed, even with 20 mol % of NHC-
Pd(II) catalyst 1e. Using benzoquinone as the oxidant was
(7) Herein, H2O2 (30 wt%, aqueous) tested to be an effective oxidant.
For example, by using H2O2 (6 equiv) instead of PhI(OAc)2, catalyst 1a
exclusively afforded hydroxyacetate 4 in 70% yield (syn:anti = 90:10) at
ambient temperature for 72 h. A PdII/IV-catalyzed oxidative cyclization
of enyes with H2O2 as the oxidant: Yin, G. Y.; Liu, G. S. Angew. Chem.,
Int. Ed. 2008, 47, 5442–5445.

930 Organometallics, Vol. 29, No. 4, 2010
Wang et al.
Table 2. Results of Isotopic Labeling Experiment with
18O-Enriched Water
Scheme 1. Hydrolysis of Isotopic Mix-4
a Integration of peak area. b ESI ([M þ Na]þ).
isotopic mix-4
m/za
abundance (%)b
derived from PdIV species anti-B was most likely operating
in this system,2f,3c,g and this process was similar to mechan-
ism a, involving an intramolecular cyclization to form acet-
oxonium D. Moreover, an external nucleophilic attack
pathway was also possible with acetate derived from the
dissociated anion in a solvent of acetic acid.2c,f,3f
unlabeled
18O-labeled
279.1
281.1
283.1
73.1
26.9
0
doubly 18O-labeled
a ESI/MS, [M þ Na]þ. b Integration of peak area.
doubly 18O-labeled hydroxyacetate 4 was not observed. The
relative amounts of unlabeled 4 (73.1%) and 18O-labeled 4
(26.9%) were determined using mass spectrometry (ESI,
[M þ Na]þ). As shown in Scheme 1, hydrolysis of the iso-
topic mixture of hydroxyacetate 4 resulted in significant
removal of the 18O-labeling. This result demonstrated that
the 18O-labeling was selectively incorporated into the carbo-
nyl group of the acetate rather than the hydroxyl group
of 4. On the basis of this 18O-labeling experiment and the
syn-diastereoselectivity observed here, NHC-Pd(II) com-
plex-catalyzed dioxygenation of alkenes to give hydroxya-
cetate 4 showed the same mechanism as that published by
Song and co-workers (Figure 2, a).3g
However, this mechanism (Figure 2, path a) cannot clearly
illustrate the formation of diacetate 5a, which was always
observed as a concomitant product in wet AcOH and,
moreover, as the exclusive product in anhydrous AcOH.
Several possible scenarios were discussed to explain the
observed diacetoxylation product 5a, as below.
Alternatively, a mechanism involving cis-acetoxypallada-
tion8 of alkenes to generate the alkyl-PdII intermediate syn-A
cannot be precluded herein, and therefore it leads to dimin-
ishment of the diastereoselectivity of diacetate 5a. As shown
in Figure 2, since hydroxyacetate 4 and diacetate 5a both
originated from the identical assumed alkyl-PdIV species B
and alkyl-PdII species A, the good syn-diastereoselectivity of
hydroxyacetate 4 suggested that trans-stilbene dominantly
underwent trans-acetoxypalladation to form anti-A followed
by oxidation to give anti-B.3g Thus, cis-acetoxypalladation
of alkenes was less likely to occur here than trans-acetox-
ypalladation.
Additionally, 5a could be formed via the same mechanism
as the hydroxyacetoxylation product 4, with acetate attack-
ing at the benzylic position of intermediate D rather than
water attacking at the carbonyl group of acetate (Figure 2,
path a). But this possibility can be excluded on the basis of
the following: Water is a stronger nucleophile than acetate;
the 18O-labeling was selectively incorporated into the carbo-
nyl group of the acetate of 4, which demonstrated water
attacking at the carbonyl group of acetate rather than the
benzylic position of intermediate D. Thus, it was also im-
probable for acetate to attack at the benzylic position of
intermediate D to form diacetate 5a.
First, the control experiment showed that hydroxyacetate
4 was converted into 5a in only very low yield (<5%) under
the standard conditions of Table 1, indicating that 5a does
not fully originate from hydroxyacetate 4.
Second, on the basis of above experimental results and
previous mechanistic proposals for the Pd(II)/Pd(IV) cata-
lytic cycle,2 the pathway involving C-OAc bond-forming
reductive elimination was assumed to deliver diacetate 5a
from the PdIV species anti-B (Figure 2, path b). Two possible
mechanisms were considered for C-OAc coupling reductive
elimination from PdIV species anti-B to give diacetate 5a
(Figure 2, path b). Path I is a concerted three-centered
mechanism, which would involve direct C-OAc reductive
elimination from the octahedral species anti-B to give anti-5a
with retention of the stereochemistry (a rare process in both
PtIV and PdIV chemistry).3a,c,d,9 Recently, a computational
study by Liu and co-workers suggested that path I was
possible in this C-OAc bond-forming reaction.2e Path II
would proceed by an SN2-type mechanism with reversion of
configuration at the palladated alkyl carbon center to give
syn-5a. An intramolecular nucleophilic attack of acetate
Scope of the Dioxygenation Method. We next examined the
generality of this dioxygenation method with other alkenes.
As shown in Table 3, treatment of the resulting reaction
mixture of trans-stilbene with Ac2O afforded 5a in 79% yield
(syn:anti = 8:1) (Table 3, entry 1). Other 1,2-disubstituted
alkenes such as cinnamyl ether and indene were highly
reactive to provide products bearing vicinal stereogenic
centers with good diastereoselectivity under the standard
conditions, affording 5b and 5c both in 99% yield with 10:1
dr and 4:1 dr, respectively (Table 3, entries 2 and 3). How-
ever, R-methylstyrene as a representative 1,1-disubstituted
olefin was less reactive to give diacetate 5d in 64% yield. In
addition, tertiary alcohol 5e was obtained exclusively with-
out treatment of Ac2O (Table 3, entry 4). Moreover, an
intramolecular dioxygenation of 1-phenylbut-3-en-1-ol pro-
ceeded smoothly to give tetrahydrofuran product 5f in 60%
yield, albeit in a longer time and with lower diastereoselec-
tivity (1.5:1 dr) (Table 3, entry 5).
As shown in Table 4, styrene and its derivatives were also
highly reactive under the same conditions as mentioned
above, but their dioxygenation resulted in both 1,2- and
1,1-diacetoxylation products, 7 and 8, respectively (Table 4,
entries 1-6). Notably, a clear relationship was indicated
between the electronic nature of the styrene substrates and
the resulting proportion of 1,2- and 1,1-diacetoxylation
products. Electron-rich styrene derivatives were more liable
(8) For examples for cis-oxypalladation of alkenes, see ref 3i and: (a)
Hamed, O.; Henry, P. M.; Thompson, C. J. Org. Chem. 1999, 64, 7745–
7750. (b) Hayashi, T.; Yamasaki, K.; Mimura, M.; Uozumi, Y. J. Am. Chem.
Soc. 2004, 126, 3036–3037. (c) Ney, J. E.; Wolfe, J. P. J. Am. Chem. Soc.
2005, 127, 8644–8651. (d) Hay, M. B.; Wolfe, J. P. J. Am. Chem. Soc. 2005,
127, 16468–16476.
(9) For rare direct reductive elimination from PtIV, see: (a) Edelbach,
B. L.; Lachicotte, R. J.; Jones, W. D. J. Am. Chem. Soc. 1998, 120, 2843–
2853. (b) Crumpton-Bregel, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2003,
125, 9442–9456. (c) Luedtke, A. T.; Goldberg, K. I. Inorg. Chem. 2007, 46,
8496–8498.

Article
Organometallics, Vol. 29, No. 4, 2010 931
Figure 2. Possible mechanism for the dioxygenation of alkenes: X = OAc, I, -OTf (H2O), or solvent.
Table 3. Bis(NHC)-Pd(II) Complex-Catalyzed Dioxygenation of Alkenesa
a Reaction conditions: 0.3 mmol of substrate, 4 mol % of catalyst 1e, 1.1 equiv of PhI(OAc)2, and 3.0 equiv of H2O in 3 mL of wet HOAc at proper
temperature; then Ac2O, rt. b Isolated yield. c Catalyst 1a was used. d No Ac2O was used. e No H2O was added.
to form 1,1-diacetate 8; particularly, the most electron-rich
substrate 6e led to the highest ratio of 8 to 7 (Table 4, entries
1-5). Catalyst 1a, bearing iodide counterions on the palla-
dium center, afforded similar results with 8e in 70% yield and
7e in only 4% yield (Table 4, entry 6).
To reveal the origin of diacetates 7 and 8, we next investi-
gated the reaction mixtures without treatment with Ac2O
upon completion. In the presence of H2O in wet acetic acid,
both styrene and 4-methylstyrene were mainly transformed
into hydroxyacetates 9 and 10 (Table 4, entries 7 and 8).
Particularly, no 1,2-diacetate 7a and minor 1,1-diacetate 8a
(4% yield) were detected for styrene (Table 4, entry 7), and
products 7d and 8d were isolated in 9% and 24% yield,
respectively, for 4-methylstyrene (Table 4, entry 8). To our
surprise, electron-rich 4-methoxystyrene exclusively gave
diacetate products 7e (6% yield) and 8e (65% yield) even in
the presence of H2O (Table 4, entry 9). Furthermore, if the
dioxygenationreactions were performed inanhydrousAcOH,

932 Organometallics, Vol. 29, No. 4, 2010
Table 4. Bis(NHC)-Pd(II) Complex-Catalyzed Dioxygenation of Styrene Derivativesa
Wang et al.
entry
X
temperature
time (h)
yield of 7 (8) (%)b
yield of 9 (10) (%)b
1
2
3
4
H (6a)
Cl (6b)
F (6c)
Me (6d)
MeO (6e)
MeO (6e)
H (6a)
Me (6d)
MeO (6e)
H (6a)
rt
rt
rt
rt
rt
rt
rt
50 °C
rt
6
6
6
83 (12)
85 (12)
83 (11)
66 (33)
7 (61)
4 (70)
0 (4)e
9 (24)
6 (65)
95 (4)
77 (23)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
50 (46)
38 (17)
0 (0)e
0 (0)e
0 (0)e
27
24
37
9
22
24
15
40
5
6c
7d
8c,d
9d
10d,f
11d,f
rt
rt
Me (6d)
a Reaction conditions: 0.3 mmol of 6, 4 mol % of catalyst 1e, 1.1 equiv of PhI(OAc)2, and 3.0 equiv of H2O in 3 mL of wet AcOH at proper
temperature; then Ac2O, rt. b Isolated yield. c Catalyst 1a was used. d No Ac2O was used. e Determined by 1H NMR integration of the crude product.
f Anhydrous acetic acid was used and no H2O was added.
Scheme 2. Dioxygenation of Styrene-β,β-d2 (84% deuterated at Cβ)
1,1-diacetoxylationproducts8a and 8d wereisolated in almost
the same yields as those in wet AcOH, but 1,2-diacetoxylation
products 7a and 7d were given in 95% and 77% yields,
respectively, without formation of any hydroxyacetates 9
and 10 (Table 4, entries 10 and 11). The presence of H2O
was required for the formation of hydroxyacetates 9 and 10,
but it was detrimental to 1,2-diacetate 7.
To explain the observed 1,1-diacetoxylation product 8a,
we next carried out the dioxygenation reaction using β,β-
dideuterated styrene [D2]6a as a substrate instead of 6a in
entry 1 of Table 4 (Scheme 2). While both deuterium atoms
were conserved at Cβ of the 1,2-diacetoxylation product 7a,
they were fully shifted from Cβ to CR, and the hydrogen atom
at CR shifted to Cβ for 1,1-diacetate 8a. This result suggested
that a mechanism involving a sequence of β-H elimination
and then reinsertion of the coordinated alkene was incom-
prehensible,10 because the product 8a with only one deuter-
ium atom shifted from Cβ to CR was not observed (see the
Supporting Information). The mechanistic detail for the
formation of 1,1-diacetoxylation product 8a was still indefi-
nite, and further investigation was required herein, but a
tentatively proposed mechanism is presented in the Support-
ing Information (Figure S5).
strong σ-electron-donating properties of NHCs allow for
the formation of strong NHC-metal bonds11 and prevent
decomposition of these catalysts during the reaction (usually
upon heating). Consequently, the robust NHCs ligands are
attractive candidates for investigating the details of the PdII/
PdIV catalytic cycle or facilitating the design of new catalysts
for regio- and stereoselective PdII/PdIV-catalyzed reac-
tions.2g Second, the proposed mechanism involving C-O
coupling reductive elimination from PdIV species anti-B
(Figure 2, b) provided the foundation for the future devel-
opment of enantioselective vicinal dioxygenation of alkenes
by modulating the chiral ligand structures and choosing
proper oxidants or reaction substrates.3,12
In summary, we have successfully developed the first exam-
ple of bis(NHC)-Pd(II) complex-catalyzed dioxygenation of
alkenes. The conveniently operated transformation proceeded
readily under mild conditions tolerant of air and moisture.
Cationic NHC-Pd2þ diaquo complex 1e was found to be an
effective catalyst to give 1,2-dioxygenation products with good
syn-diastereoselectivity for 1,2-disubstituted alkenes. Current
work is underway to elucidate the mechanistic details and
explore the potential for asymmetric induction in the dioxy-
genation process of alkenes.
The work described herein provided several important
features of PdII/PdIV-catalyzed transformations. First,
Experimental Section
General Procedure for Bis(NHC)-Palladium(II) Complex-
Catalyzed Dioxygenation of Alkenes. To the solution of a bis-
(NHC)-Pd(II) catalyst (0.012 mmol, 4 mol %) and an alkene
(10) (a) Kalyani, D.; Sanford, M. S. J. Am. Chem. Soc. 2008, 130,
2150–2151. (b) Urkalan, K. B.; Sigman, M. S. Angew. Chem., Int. Ed. 2009,
48, 3146–3149.
(11) (a) Frenking, G.; Sola, M.; Vyboishchikov, S. F. J. Organomet.
Chem. 2005, 690, 6178–6204. (b) Radius, U.; Bickelhaupt, F. M. Coord.
Chem. Rev. 2009, 253, 678–686. (c) Jacobsen, H.; Correa, A.; Poater, A.;
Costabile, C.; Cavallo, L. Coord. Chem. Rev. 2009, 253, 687–703.
(12) For an asymmetric PdII/PdIV-catalyzed process, see: Tsujihara,
T.; Takenaka, K.; Onitsuka, K.; Hatanaka, M.; Sasai, H. J. Am. Chem.
Soc. 2009, 131, 3452–3453.

Article
Organometallics, Vol. 29, No. 4, 2010 933
(0.3 mmol) in acetic acid (3.0 mL) and H2O (16 μL, 0.9 mmol)
was added PhI(OAc)2 (106 mg, 0.33 mmol, 1.1 equiv) at room
temperature without inert protecting gases. The resulting mix-
ture was stirred at suitable temperature. After the complete
consumption of the alkene, 0.3 mL of acetic anhydride was
added and the reaction mixture was stirred at room temperature
overnight or for 36 h. After the reaction finished, the solvents
were removed under reduced pressure. The product distribution
and the diastereoselectivity were directly determined by 1H
NMR integration of the resulting crude product. The crude
products were purified by flash chromatography on a silica gel
column (petroleum ether/EtOAc, 20:1).
Acknowledgment. We thank the Shanghai Municipal
Committee of Science and Technology (06XD14005,
08dj1400100-2), National Basic Research Program of China
(973-2009CB825300), and the National Natural Science
Foundation of China (20472096, 20872162, 20672127,
20821002, and 20732008) for financial support.
Supporting Information Available: Detailed description of
experimental procedures and full characterization of new com-
pounds shown in tables and figures. This material is available free of
charge via the Internet at http://pubs.acs.org or from the authors.