PPh3O as an Activating Reagent for One-Pot Stereoselective Syntheses of Di- and Polybrominated Esters from Simple Aldehydes

Article abstract of DOI:10.1021/acs.joc.5b02618

(Chemical Equation Presented) An efficient one-pot method for the syntheses of di- and polybrominated esters from readily available aldehydes is reported. The direct use of the in situ generated byproduct PPh₃O in the following reactions greatly improves the efficiency of the cascade. Also, the substrate scope of the reaction is proved to be broad.

Full text of DOI:10.1021/acs.joc.5b02618

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Article
PPh3O as an Activating Reagent for One-pot Stereoselective
Syntheses of Di- and Poly- brominated Esters from Simple Aldehydes
Tian-Yang Yu, Hao Wei, Yong-Chun Luo, Yao Wang, Zhu-Yin Wang, and Peng-Fei Xu
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b02618 • Publication Date (Web): 15 Mar 2016
Downloaded from http://pubs.acs.org on March 16, 2016
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PPh3O as an Activating Reagent for Oneꢀpot Stereoselective
Syntheses of Diꢀ and Polyꢀ brominated Esters from Simple Aldehydes
TianꢀYang Yu,a Hao Wei,a YongꢀChun Luo, b Yao Wang, b ZhuꢀYin Wang c and
PengꢀFei Xu*b
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a Key Laboratory of Synthetic and Natural Functional Molecule Chemistry,
College of Chemistry and Materials Science, Northwest University
Xi’an 710127, P. R. China.
b State Key Laboratory of Applied Organic Chemistry, College of Chemistry and
Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China.
Fax : (+86) 931ꢀ8915557; Eꢀmail: xupf@lzu.edu.cn
c College of Mechanics and Materials, Hohai University, Nanjing 210098, P. R.
China.
ABSTRACT: An efficient oneꢀpot method for the syntheses of diꢀ and polyꢀ
brominated esters from readily available aldehydes is reported. The direct use of the
in situ generated byproduct PPh3O in the following reactions greatly improves the
efficiency of the cascade. Also, the substrate scope of the reaction is proved to be
broad.
INTRODUCTION
During many commonly used reactions, a large amount of waste is
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simultaneously produced along with the formation of the desired products.1 As a result,
the waste reduces the synthetic efficiency significantly and then restricts the large
scale applications of these reactions. Therefore, one of the fundamental goals for
synthetic chemists is to minimize the reaction waste.2 So far, many elegant
methods/strategies have been developed to achieve this goal.3−4 Among those novel
synthetic methods, one of the most efficient ways is to use the in situ generated waste
as a catalyst or reactant for the following reaction steps.4 Therefore, this strategy can
enable the construction of complex structures from simple starting materials in a
oneꢀpot multiꢀstep synthesis which effectively improves the overall efficiency of the
cascade reaction.5
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PPh3O is often unavoidably generated as a stoichiometric waste in some most
commonly used reactions such as Wittig,6 Mitsunobu,7 Staudinger8 and also some
PPh3ꢀtriggered reactions.9 However, the atom efficiency of these reactions is not
satisfactory due to the relative high molecular weight of PPh3O. Meanwhile, in many
cases, the separation of desired product will be complicated, which requires a tedious
isolation, extra inputs of manpower and materials. To find the solutions to these
problems, much effort has been devoted to finding practical usage of the waste
products.10 To improve potꢀ and stepꢀeconomy, we envisioned that it would be
possible to develop an effective oneꢀpot reaction wherein simple and commercially
available starting materials are transformed to the desired substrates, and the waste
product, PPh3O, then acts as an activating reagent in the next step to afford
structurally complex products.
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Organobromine compounds can often be found in nature products,
pharmaceuticals and agrochemicals (Figure 1).11 Meanwhile, they are also useful
building blocks in some fundamental chemical transformations. Therefore, the
development of novel bromination methods has attracted considerable attention from
synthetic chemists. The employment of molecular bromine, however, has a limited
application due to its potentially hazardous risk. During the past years, many
innovative bromination methods have been developed, which mainly involved the use
of specially tailored bromine carrying reagents and the in situ generation of bromine
with oxidizing reagents.12 Despite these significant advances, several problems still
remain, which are often associated with the generation of stoichiometric amount of
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wastes and the employment of toxic metal reagents.
Br
O
Br
Br
Br
Br
O
Br
Br
Antibacterial activity
OH Br
O
O
Antimicrobial activity
Br OH
HO OH
O
O
Br
Br
Antimicrobial activity
Figure 1. Some bromineꢀsubstituted compounds
Recently, our group developed PPh3Oꢀcatalyzed stereoselective halogenation
methods.13 These works have complemented the research in the fields of halogenation
and PPh3O catalysis, however, the potꢀ and stepꢀeconomy is still not satisfactory since
the preparation of a pure and suitable compound for halogenation is required. In this
context, it is highly desirable to develop a more ‘ideal’ synthesis method, which only
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employs simple commercially available reagents to generate reactive intermediates
and the byproduct which will then act as an effective activating reagent in the next
reaction to afford the desired products in a single operation. Although the same final
products will be obtained, the strategies and starting points differ significantly.
Herein, we report the stereoselective syntheses of Diꢀ and Polyꢀbrominated esters
by a tandem Wittig/PPh3Oꢀ activated bromination reaction sequence using
commercially available aldehydes as the starting materials.
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RESULTS AND DISCUSSION
Initially, the most commonly used benzaldehyde (1a) and phosphorus ylide 2a
were employed as the model substrates to test our design. The reaction was carried
out in DCE at room temperature. Oxalyl bromide was added upon the complete
consumption of the aldehyde. After 24h, dibrominated ester 3a was obtained in 73%
yield and >19:1 dr. Encouraged by this positive result, we began to optimize this
cascade reaction. First, after careful screening of temperature, amount of 2a and the
solvent (Table 1), the optimal reaction conditions for the synthesis of unsaturated ester
were found to be the use of aldehyde 1a and 1.2 equivalent of phosphorus ylide 2a at
room temperature in DCE.
Table 1. Optimization of Wittig reactiona
Entry
Solvent
DCE
DCE
DCE
DCE
Temp.
rt
Time (h) Yield (%)b
E/Zc
1
2d
3
3
3
3
2
86
86
86
80
>19 : 1
>19 : 1
>19 : 1
>19 : 1
rt
40 oC
rt
4
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DCE
rt
rt
rt
rt
4
3
3
3
86
83
77
74
>19 : 1
>19 : 1
>19 : 1
>19 : 1
CH3CN
CHCl3
toluene
7
8
9
a Unless otherwise noted, the reactions were carried out with 1a (0.2 mmol), 2a (0.24 mmol) and
4Å MS (40.0 mg) in the indicated solvent (0.5 mL). b Isolated yield after flash chromatography. c
Determined by 1H NMR spectroscopic analysis of the crude reaction mixture. d 0.3 mmol 2a was
used.
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Then, we optimized this oneꢀpot tandem reaction which produced dibrominated
ester 3a from simple aldehyde 1a and ylide 2a (Table 2). Different reaction
temperatures were tested to determine the effect on the reaction (Table 2, entries 1−3).
As shown in Table 2, the data revealed that 40oC was the best temperature for the
second step. Then the amount of oxalyl bromide was investigated and 3.0 equiv. was
found to be optimal. Finally, several solvents (CHCl3, CH3CN, toluene and THF)
were evaluated (Table 2, entries 6−9) and DCE was proved to be the most promising
one.
Table 2. Optimization of oneꢀpot dibromination reactiona
Br
O
O
O
4Å MS
(COBr)2
Ph3P
+
O
solvent, rt
3h
O
temp., 4Å MS
24h
Br
3a
1a
2a
entry
solvent
DCE
Temp.
Yield (%)b
dr (anti/syn)c
>19 : 1
>19: 1
1
2
rt
73
85
59
30
85
53
51
DCE
40 oC
60 oC
40 oC
40 oC
40 oC
40 oC
3
DCE
>19 : 1
>19 : 1
>19 : 1
>19 : 1
>19 : 1
4d
5e
6
DCE
DCE
CHCl3
CH3CN
7
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toluene
THF
40 oC
40 oC
47
>19 : 1
n.d.
trace
a Unless otherwise noted, the reactions were carried out with 1a (0.2 mmol), 2a (0.24 mmol) and
8
9
4Å MS (40.0 mg) in the indicated solvent (0.5 mL) at rt for 3h, followed by the addition of oxalyl
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b
bromide (0.6 mmol) and stirring at indicated temperature for 24h. Isolated yield after flash
chromatography. c Determined by 1H NMR spectroscopic analysis of the crude reaction mixture. d
e
0.4 mmol oxalyl bromide was used. 0.8 mmol oxalyl bromide was used. n.d. = not determined,
MS = molecular sieve.
With optimal conditions in hand, the substrate scope of the reaction was then
investigated (Table 3). A variety of functionalized aromatic and aliphatic aldehydes
were employed to evaluate the generality of this oneꢀpot reaction. As shown in Table
3, in all cases, the reaction proceeded smoothly to produce the antiꢀdibrominated13b
ester 3 with moderate to good yields (41%−88%) and excellent diastereoselectivities
(d.r.>19:1). The results demonstrated that no alphaꢀepimerization took place under the
reaction conditions. The substitution position (orthoꢀ, metaꢀ, and paraꢀ) of the
substituents on aromatic ring had little influence on the results (Table 3, entries 2−4,
10−12). Both of the electronꢀwithdrawing and electron–donating functional groups in
aromatic ring were tolerated in this reaction, although the substrates containing
electronꢀneutral and electronꢀdonating substituents proceeded faster than those
containing electronꢀwithdrawing substituents. The aliphatic substrate also worked
well and the product was obtained with excellent dr and moderate yield (Table 3,
entry 15−16). Another phosphorus ylide was found to work well with 1a, which
afforded 3q in good yield with excellent diastereoselectivity (Table 3, entry 17).
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Table 3. Substrate scope of oneꢀpot dibromination reaction.a
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Entry
1
2
3
4
5
6
7
8
R1
R2
Product
3a
Time (h) Yield (%)b dr (anti/syn)c
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
24
36
36
36
36
36
36
36
36
24
24
24
24
24
24
24
24
85
80
77
80
88
85
75
69
66
70
74
67
56
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66
42
83
>19 : 1
>19: 1
>19 : 1
>19 : 1
>19 : 1
>19 : 1
>19 : 1
>19 : 1
>19 : 1
>19 : 1
>19 : 1
>19 : 1
>19 : 1
>19 : 1
>19 : 1
>19 : 1
>19 : 1
2ꢀClC6H4
3ꢀClC6H4
4ꢀClC6H4
3,4ꢀCl2C6H3
4ꢀBrC6H4
4ꢀFC6H4
4ꢀNO2C6H4
4ꢀCNC6H4
2ꢀMeC6H4
3ꢀMeC6H4
4ꢀMeC6H4
3,4ꢀMe2C6H3
4ꢀtBuC6H4
nꢀhexyl
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
9
10
11
12
13
14
15d
16d
17
iꢀpropyl
3p
3q
Ph
a
Unless otherwise noted, the reactions were carried out with 1 (0.2 mmol), 2 (0.24 mmol) and
4Å MS (40.0 mg) in dry DCE (0.5 mL) at rt for 3h, followed by the addition of oxalyl bromide
(0.6 mmol) and stirring at 40 C. Isolated yield after flash chromatography. Determined by 1H
o
b
c
d
NMR spectroscopic analysis of the crude reaction mixture. The reaction temperature of second
step is 60 oC. MS= molecular sieve.
To further demonstrate the synthetic value of this oneꢀpot bromination method,
we decided to explore the utilization of this bromination reaction in the syntheses of
polyꢀbrominated molecules. Therefore, we applied this method to the reaction of
α,βꢀunsaturated aldehyde 4 and ylide 5, which proceeded smoothly to produce the
desired polyꢀbrominated product. After reaction condition optimization (see the
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Supporting Information), the substrate scope of this oneꢀpot polyꢀbromination
reaction was established. As shown in Table 4, this method was applicable to a wide
range of α,βꢀunsaturated aldehydes with different aryl substituents. In general,
moderate to good yields and good diastereoselectivities were attainable for the
examples examined. As a consequence of different reactivities in Wittig step,
substrates bearing electronꢀwithdrawing groups had higher yields than those bearing
electronꢀneutral and electronꢀdonating groups. Then, we investigated the effect of
substituent position on the reaction yield and diastereoselectivity (Table 4, entries
2−4). As the data revealed, metaꢀsubstituted substrate had best yield and
diastereoselectivity due to a synergistic effect of inductive effect and steric effect
(Table 4, entry 3). We also observed an increase in yield but a decrease in
diastereoselectivity as a result of substituent’s inductive effect (−F > −Cl > −Br)
(Table 4, entries 4, 6−7). Meanwhile, disubstituted aldehyde (disubstitution is on the
aromatic portion) also worked well under the optimized reaction conditions, both of
the yield and diastereoselectivity were satisfactory (Table 4, entry 5). Finally, we
tested another phosphorus ylide and the reaction proceeded smoothly to provide the
desired product 6i (Table 4, entry 9). The Xꢀray crystallographic structure of 6a was
shown in Figure 2.14 The relative configuration of this product (6a) described in our
previous paper13b is misassigned and should be corrected accordingly and the relative
stereochemistry in the minor tetrabromide diastereomer is unknown yet.
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Table 4. Substrate scope of oneꢀpot tetraꢀbromination reaction.a
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Br
Br
DCE, 50 o
C
(COBr)2
CO2R4
O
R4
R3
R3
Ph3P
O +
3h, N2
reflux, 36h
4Å M.S.
O
5
Br
Br
4Å M.S.
4
6
Entry
R3
Ph
R4
Me
Me
Me
Me
Me
Me
Me
Me
Et
Product
6a
Yield (%)b
dr c
13 : 1
7 : 1
>19 : 1
15 : 1
11 : 1
13 : 1
>19 : 1
4 : 1
1
2
3
4
5
6
7
8
9
58
70
76
67
74
70
58
20
58
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2ꢀClC6H4
3ꢀClC6H4
4ꢀClC6H4
3,5ꢀCl2C6H3
4ꢀBrC6H4
4ꢀFC6H4
4ꢀMeC6H4
Ph
6b
6c
6d
6e
6f
6g
6h
6i
11 : 1
a Unless otherwise noted, the reactions were carried out with 4 (0.2 mmol), 5 (0.24 mmol) and 4Å
o
MS (40.0 mg) in dry DCE (0.5 mL) at 50 C for 3h, followed by the addition of oxalyl bromide
(1.0 mmol) and stirring under reflux condition for 36h. b Isolated yield after flash chromatography.
c
1
Determined by H NMR spectroscopic analysis of the crude reaction mixture. MS= molecular
sieve.
Figure 2. Xꢀray crystallographic structure of 6a
To extend the substrate scope of the reaction, we also used this bromination
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method to produce dibrominated ketone and cyclic compound. As shown in Figure 3,
2,3ꢀdibromoꢀ1,3ꢀdiphenylpropanꢀ1ꢀone (8) was produced in 22% yield and 8:1 d,r,
and the yield of dibrominated product 10 is 9%.
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O
Br
O
reflux
(COBr)2
+
Ph
O
PPh3
O
22% yield
d.r. = 8:1
Ph
Ph
Ph
DCE
4Å M.S.
Br
8
7
H
Br
PPh3
O
O
NaHCO3
H2O, rt
(COBr)2
DCE, 40 oC
4Å M.S.
O
9% yield
Br
10
O
Br
9
Figure 3 Two applications of the bromination method.
To further investigate the reaction mechanism, methyl (
was employed in this dibromination reaction, compared with methyl
)ꢀ3ꢀphenylacrylate13b, it was found that both of the yield and dr decreased (Figure 4),
Z
)ꢀ3ꢀphenylacrylate17
(E
which was believed to be caused by steric hinderance effect. Two substituents which
are originally at the same side of the double bond will be an obstacle to the bromide
ion attacking on the threeꢀmembered cyclic bromonium ion intermediate20. On the
basis of our previous related study13 and the data observed, a possible reaction
mechanism is proposed. As shown in Figure 5, initially, aldehyde reacts with
phosphorus ylide to afford unsaturated ester and triphenylphosphine oxide (Wittig
reaction). Next, oxalyl bromide is added to the mixture and reacts with
triphenylphosphine oxide to produce M1. Bromotriphenylphosphonium bromide (M1)
reacts with the unsaturated ester, which is produced in former Wittig reaction, to
generate a threeꢀmembered cyclic bromonium ion intermediate M2. Finally, M2 is
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attacked by bromide ion via SN2 path way to form dibrominated product.
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Figure 4. Two dibromination reactions.
Figure 5. Proposed mechanism of the oneꢀpot reaction.
CONCLUSIONS
In summary, by using easily available reagents as starting materials, we have
successfully developed a practical tandem oneꢀpot Wittig/bromination reaction
procedure which efficiently produces diꢀ and polyꢀbrominated esters with satisfactory
yields, diastereoselectivties and broad substrate scope. Ph3PO, which is the
byꢀproduct in Wittig reaction, directly serves as the activating reagent for the next
bromination reaction, which results in a great improvement in the efficiency of this
reaction. Considering that PPh3O is usually generated as the waste product in many
reactions, it should not be surprising that this general strategy can be applied to a
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variety of such reactions thus to improve the efficiencies of these reactions.
EXPERIMENTAL SECTION
7
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General Information. Chemicals and solvents were either purchased from
commercial suppliers or purified by standard procedures as specified in Purification
of Laboratory Chemicals, 4th Ed (Armarego, W. L. F.; Perrin, D. D. Butterworth
Heinemann: 1997). Analytical thinꢀlayer chromatography (TLC) was performed on
silica gel plates with Fꢀ254 indicator and compounds were visualized by irradiation
with UV light. Flash column chromatography was carried out using silica gel (200–
300 mesh) at increased pressure. The NMR spectra were recorded in CDCl3 as the
solvent at room temperature (400 MHz 1H, 100 MHz 13C). 1H and 13C chemical shifts
are reported in ppm relative to either the residual solvent peak (13C) or TMS (1H) as
an internal standard. IR spectra were recorded using FTꢀIR instrument. HRMS were
performed on ORBITRAP ELITE instrument (ESI). The unsaturated aldehyde
substrates were prepared according to the literature procedures.15
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General Procedure for the Preparation of Compounds 3a−3q. Aldehyde 1
(0.20 mmol), Ylide 2 (0.24 mmol) and 4Å molecular sieve (40.0 mg) were added to a
flameꢀdried Schlenk tube. The vessel was placed under vacuum and the atmosphere
was exchanged with N2 three times before dry DCE (0.5 ml) was added. The mixture
was stirred at room temperature for 3h. Then, oxalyl bromide (0.6 mmol) was added
to the stirred reaction mixture. The final reaction mixture was stirred at 40 oC for 24h
or 36h. After the reaction was complete, the reaction mixture was purified by flash
column chromatography using petroleum ether/EtOAc (80:1) to obtain the desired
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product 3.
trans)ꢀmethyl 2,3ꢀdibromoꢀ3ꢀphenylpropanoate (3a). A white solid (55 mg, 85%,
(
7
8
9
d.r. > 19:1), m.p. 109−110 oC; 1H NMR (400 MHz, CDCl3): δ 3.90 (s, 3H), 4.85 (d,
J
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= 11.6 Hz, 1H), 5.34 (d,
J
= 12.0 Hz, 1H), 7.36−7.42 (m, 5H); C NMR (100 MHz,
CDCl3): δ 168.3, 137.5, 129.4, 128.9, 128.0, 53.4, 50.6, 46.7; IR (KBr): 3460, 3347,
3010, 1738, 1434, 1380, 1274, 1220, 1152, 700, 587cmꢀ1; HRMS (ESI+) exact mass
calculated for [M+Na]+ (C10H10Br2NaO2) requires m/z 342.8940
trans)ꢀmethyl 2,3 ꢀdibromoꢀ3ꢀ(2ꢀchlorophenyl) propanoate (3b). A white solid (57
, found m/z 342.8949.
(
mg, 80%, d.r. > 19:1), m.p. 80−81 C; 1H NMR (400 MHz, CDCl3): δ 3.91 (s, 3H),
4.94 (s, 1H), 5.91 (s, 1H), 7.27−7.36 (m, 2H), 7.40−7.42 (m, 1H), 7.46−7.48 (m, 1H);
13C NMR (100 MHz, CDCl3): δ 168.0, 135.1, 134.0, 130.3, 130.2, 128.9, 127.6, 53.5,
45.4; IR (KBr): 3351, 3016, 2954, 2921, 2377, 1745, 1593, 1435, 1377, 1273, 1151,
1038, 763, 736, 605, 575 cmꢀ1; HRMS (ESI+) exact mass calculated for [M+Na]+
o
(C10H9Br2ClNaO2) requires m/z 376.8550
trans)ꢀmethyl 2,3ꢀdibromoꢀ3ꢀ(3ꢀchlorophenyl)propanoate (3c). A white solid
(55mg, 77%, d.r. > 19:1), m.p. 63−64 oC ; 1H NMR (400 MHz, CDCl3): δ 3.90 (s, 3H),
4.78 (d, = 12.0 Hz, 1H), 5.28 (d, = 11.6 Hz, 1H), 7.27−7.35 (m, 3H), 7.40 (s, 1H);
, found m/z 376.8560.
(
J
J
13C NMR (100 MHz, CDCl3): δ 168.0, 139.5, 134.7, 130.1, 129.6, 128.2, 126.3, 53.5,
49.2, 46.3; IR (KBr): 3473, 3007, 2953, 1748, 1596, 1576, 1478, 1435, 1377, 1302,
1268, 1149, 790, 700, 610, 573 cmꢀ1; HRMS (ESI+) exact mass calculated for
[M+Na]+ (C10H9Br2ClNaO2) requires m/z 376.8550
, found m/z 376.8556.
(
trans)ꢀmethyl2,3ꢀdibromoꢀ3ꢀ(4ꢀchlorophenyl)propanoate (3d). A white solid (57
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mg, 80%, d.r. > 19:1), m.p. 96−97 C; 1H NMR (400 MHz, CDCl3): δ 3.89 (s, 3H),
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4.79 (d,
J
= 11.6 Hz, 1H), 5.31 (d, J = 12.0 Hz, 1H), 7.33 (d, J = 8.8 Hz, 2H), 7.37 (d,
8
9
13
J
= 9.2 Hz, 2H); C NMR (100 MHz, CDCl3): δ 168.1, 136.1, 135.2, 129.4, 129.2,
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53.5, 49.5, 46.5; IR (KBr): 3460, 3012, 2957, 1742, 1594, 1742, 1594, 1493, 1435,
1377, 1277, 1231, 1146, 1088, 982, 835, 735, 587 cmꢀ1; HRMS (ESI+) exact mass
calculated for [M+Na]+ (C10H9Br2ClNaO2) requires m/z 376.8550
, found m/z
376.8557.
(trans)ꢀmethyl 2,3ꢀdibromoꢀ3ꢀ(3,4ꢀdichlorophenyl) propanoate (3e). A white solid
(69 mg, 88%, d.r. > 19:1), m.p. 83−84 oC; 1H NMR (400 MHz, CDCl3): δ 3.90 (s, 3H),
4.75 (d,
7.47 (d,
J
= 11.6 Hz, 1H), 5.26 (d,
= 8.4 Hz, 1H), 7.50 (d,
J
= 11.6 Hz, 1H), 7.24 (dd, J = 8.4 Hz, 2.0 Hz, 1H),
13
J
J
= 2.0 Hz, 1H); C NMR (100 MHz, CDCl3): δ
167.9, 137.7, 133.6, 133.1, 130.9, 130.1, 127.3, 53.6, 48.4, 46.1; IR (KBr): 3008,
2954, 1748, 1563, 1473, 1405, 1364, 1298, 1267, 1216, 1144, 1032, 883, 823, 739,
600, 475 cmꢀ1; HRMS (ESI+) exact mass calculated for [M+Na]+ (C10H8Br2Cl2NaO2)
requires m/z 410.8160
trans)ꢀmethyl 2,3ꢀdibromoꢀ3ꢀ(4ꢀbromophenyl) propanoate (3f). A white solid (68
mg, 85%, d.r. > 19:1), , m.p. 106−107 oC; 1H NMR (400 MHz, CDCl3): δ 3.89 (s, 3H),
, found m/z 410.8171.
(
4.78 (d,
J
= 12.0 Hz, 1H), 5.30 (d, J = 12.0 Hz, 1H), 7.27 (d, J = 8.4 Hz, 2H), 7.52 (d,
13
J
= 8.4 Hz, 2H); C NMR (100 MHz, CDCl3): δ 168.1, 136.6, 132.1, 129.6, 123.4,
53.5, 49.5, 46.4; IR (KBr): 3454, 3009, 2376, 1747, 1590, 1378, 1279, 1148, 1073,
1011, 827, 583 cmꢀ1; HRMS (ESI+) exact mass calculated for [M+Na]+
(C10H9Br3NaO2) requires m/z 420.8045, found m/z 420.8051.
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(trans)ꢀmethyl 2,3ꢀdibromoꢀ3ꢀ(4ꢀfluorophenyl)propanoate (3g). A white solid (51
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o
mg, 75%, d.r. > 19:1), m.p. 63−64 C; 1H NMR (400 MHz, CDCl3): δ 3.90 (s, 3H),
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4.79 (d,
(m, 2H); 13C NMR (100 MHz, CDCl3): δ 168.2, 163.0 (d,
10 Hz), 130.0 (d, = 10 Hz), 116.0 (d, = 20 Hz), 53.5, 49.7, 46.8; IR (KBr): 3461,
J
= 12.0 Hz, 1H), 5.33 (d,
J
= 12.0 Hz, 1H),7.08 (t,
J
= 8.8 Hz, 2H), 7.37ꢀ7.41
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J
= 248 Hz), 133.6 (d, J =
J
J
3011, 2953, 1893, 1751, 1604, 1512, 1437, 1378, 1225, 985, 857, 592, 512 cmꢀ1;
HRMS (ESI+) exact mass calculated for [M+Na]+ (C10H9Br2FNaO2) requires m/z
360.8846
, found m/z 360.8851.
(trans)ꢀmethyl 2,3ꢀdibromoꢀ3ꢀ(4ꢀnitrophenyl)propanoate (3h): A white solid (51
1
mg, 69%, d.r. > 19:1); H NMR (400 MHz, CDCl3): δ 3.92 (s, 3H), 4.81 (d,
J = 12.0
Hz, 1H), 5.40 (d, = 11.6 Hz, 1H), 7.60 (d, = 8.8 Hz, 2H), 8.27 (d, = 8.8 Hz, 2H);
J
J
J
13C NMR (100 MHz, CDCl3): δ 167.7, 148.1, 144.4, 129.2, 124.1, 53.7, 48.0, 45.7
HRMS (ESI+) exact mass calculated for [M+H]+ (C10H10Br2NO4) requires m/z
365.8971
, found m/z 365.8977.
(trans)ꢀmethyl 2,3ꢀdibromoꢀ3ꢀ(4ꢀcyanophenyl)propanoate (3i): A white solid (46
1
mg, 66% yield, d.r. > 19:1); H NMR (400 MHz, CDCl3): δ 3.91 (s, 3H), 4.79 (d,
J =
11.6 Hz, 1H), 5.34 (d,
J = 12.0 Hz, 1H), 7.53 (d, J = 8.0 Hz, 2H), 7.70 (d, J = 8.0 Hz,
13
2H); C NMR (100 MHz, CDCl3): δ 167.8, 142.5, 132.7, 128.9, 118.0, 113.2, 53.6,
48.5, 45.7; HRMS (ESI+) exact mass calculated for [M+H]+ (C11H10Br2NO2) requires
m/z 345.9073
, found m/z 345.9076.
(trans)ꢀmethyl 2,3ꢀdibromoꢀ3ꢀ(oꢀtolyl)propanoate (3j). A white solid (47mg, 70%,
d.r. > 19:1), m.p. 83−84 oC; 1H NMR (400 MHz, CDCl3): δ 2.45 (s, 3H), 3.90 (s, 3H),
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4.93 (d,
(100 MHz, CDCl3): δ 168.4, 136.5, 135.7, 130.8, 129.1, 126.9, 126.8, 53.4, 46.2, 19.4;
IR (KBr): 3456, 3023 2952 1745 1436 1377 1271 1150 983 769
726 604
497 cmꢀ1; HRMS (ESI+) exact mass calculated for [M+Na]+
(C11H12Br2NaO2) requires m/z 356.9096 found m/z 356.9101.
J = 11.6 Hz, 1H), 5.64 (d, J = 12.0 Hz, 1H), 7.17−7.41 (m, 4H); C NMR
，
，
，
，
，
，
，
，
，
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，
，
,
(trans)ꢀmethyl 2,3ꢀdibromoꢀ3ꢀ(mꢀtolyl)propanoate (3k). A white solid (50 mg, 74%,
d.r. >19:1), m.p. 69−70 C; 1H NMR (400 MHz, CDCl3): δ 2.37 (s, 3H), 3.89 (s, 3H),
o
4.84 (d, J = 11.6 Hz, 1H), 5.31 (d, J = 11.6 Hz, 1H), 7.15−7.20 (m, 3H), 7.25−7.29 (m,
1H); 13C NMR (100 MHz, CDCl3): δ 168.4, 138.7, 137.4, 130.2, 128.7, 128.6, 125.1,
53.4, 50.8, 46.7, 21.4; IR (KBr): 3089, 2921, 2392, 1747, 1436, 1377, 1273, 1145,
1019, 983, 700, 611 cmꢀ1; HRMS (ESI+) exact mass calculated for [M+Na]+
(C11H12Br2NaO2) requires m/z 356.9096
trans)ꢀmethyl 2,3ꢀdibromoꢀ3ꢀ(pꢀtolyl)propanoate (3l). A white solid (45 mg, 67%,
d.r. > 19:1), m.p. 98−99 oC; 1H NMR (400 MHz, CDCl3): δ 2.36 (s, 3H), 3.89 (s, 3H),
, found m/z 356.9102.
(
4.84 (d,
J
= 11.6 Hz, 1H), 5.33 (d, J = 12.0 Hz, 1H), 7.19 (d, J = 8.0 Hz, 2H), 7.29 (d,
13
J
= 8.0 Hz, 2H); C NMR (100 MHz, CDCl3): δ 168.4, 139.5, 134.6, 129.6, 127.9,
53.4, 50.8, 46.8, 21.3; IR (KBr): 3472, 3008, 1911, 1746, 1437, 1268, 1146, 1012,
740, 590, 516 cmꢀ1; HRMS (ESI+) exact mass calculated for [M+Na]+
(C11H12Br2NaO2) requires m/z 356.9096
trans)ꢀmethyl 2,3ꢀdibromoꢀ3ꢀ(3,4ꢀdimethylphenyl) propanoate (3m). A white
solid (39 mg, 56%, d.r. > 19:1), m.p. 121−122 oC; 1H NMR(400 MHz, CDCl3): δ 2.26
(s, 3H), 2.28 (s, 3H), 3.89 (s, 3H), 4.85 (d, = 12.0 Hz, 1H), 5.31 (d, = 12.0 Hz, 1H),
, found m/z 356.9101.
(
J
J
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7.11−7.16 (m, 3H); 13C NMR (100 MHz, CDCl3): δ 168.4, 138.2, 137.3, 134.9, 130.1,
129.1, 125.4, 53.4, 51.0, 46.8, 19.8, 19.6; IR (KBr): 3477, 3010, 2972, 2920, 1748,
1505, 1432, 1377, 1267, 1143, 740, 701, 663, 608, 541 cmꢀ1; HRMS (ESI+) exact
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mass calculated for [M+Na]+ (C12H14Br2NaO2) requires m/z 370.9253
, found m/z
370.9259.
(trans)ꢀmethyl 2,3ꢀdibromoꢀ3ꢀ(4ꢀ(tertꢀbutyl)phenyl) propanoate (3n). A white
solid (56 mg, 74%, d.r. > 19:1), m.p. 104−105 oC; 1H NMR (400 MHz, CDCl3): δ 1.32
(s, 9H), 3.89 (s, 3H), 4.86 (d,
8.4 Hz, 2H), 7.39 (d,
J
= 12.0 Hz, 1H), 5.35 (d, J = 12.0 Hz, 1H), 7.32 (d, J =
13
J
= 8.4 Hz, 2H); C NMR (100 MHz, CDCl3): δ 168.4, 152.6,
134.4, 127.7, 125.8, 53.4, 50.8, 46.8, 34.7, 31.2; IR (KBr): 3476, 3002, 2964, 1916,
1748, 1609, 1436, 1269, 1145, 1020, 835, 739, 590, 499 cmꢀ1; HRMS (ESI+) exact
mass calculated for [M+Na]+ (C14H18Br2NaO2) requires m/z 398.9566
, found m/z
398.9570.
(trans)ꢀmethyl 2,3ꢀdibromononanoate (3o). A colorless oil (30 mg, 45%, d.r. > 19:1);
1H NMR (400 MHz, CDCl3): δ 0.90 (t,
J = 6.8 Hz, 3H), 1.32−1.39 (m, 6H), 1.46−1.49
(m, 1H), 1.56−1.59 (m, 1H), 1.77−1.86 (m, 1H), 2.20−2.28 (m, 1H), 3.83 (s, 3H),
4.34−4.43 (m, 2H); 13C NMR (100 MHz, CDCl3): δ 168.4, 53.2, 52.8, 47.7, 35.1, 31.5,
28.4, 26.2, 22.5, 14.0; IR (KBr): 3001, 2955, 2929, 2858, 1753, 1437, 1378, 1268,
1150, 1023, 720, 565 cmꢀ1; HRMS (ESI+) exact mass calculated for [M+Na]+
(C10H18Br2NaO2) requires m/z 350.9566
, found m/z 350.9572.
(
trans)ꢀmethyl 2,3ꢀdibromoꢀ4ꢀmethylpentanoate (3p)16 : A colorless oil (24mg,
42%, d.r. > 19:1); 1H NMR (400 MHz, CDCl3): δ 0.90 (d, J = 6.8 Hz, 3H), 1.09 (d, J
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= 6.8 Hz, 3H), 2.35 (m, 1H), 3.84 (s, 3H), 4.44 (s, 2H); 13C NMR (100 MHz, CDCl3):
δ 168.6, 61.3, 53.2, 46.1, 29.4, 22.0, 15.1;
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(trans)ꢀethyl 2,3ꢀdibromoꢀ3ꢀphenylpropanoate (3q). A colorless solid (56 mg, 83%,
10
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o
d.r. > 19:1), m.p.73−74 C; 1H NMR (400 MHz, CDCl3): δ 1.37 (t,
J
= 7.2 Hz, 3H),
4.35 (q,
J
= 6.8 Hz, 2H), 4.83 (d,
J
= 11.6 Hz, 1H), 5.34 (d,
J
= 12.0 Hz, 1H),
7.35−7.41 (m, 5H); 13C NMR (100 MHz, CDCl3): δ 167.8, 137.6, 129.3, 128.9, 128.0,
62.6, 50.7, 47.0, 13.9; IR (KBr): 3453, 3011, 2986, 1741, 1456, 1379, 1272, 1148,
1026, 697, 602, 562 cmꢀ1; HRMS (ESI+) exact mass calculated for [M+Na]+
(C11H12Br2NaO2) requires m/z 356.9096, found m/z 356.9100.
General Procedure for the Preparation of Compounds 6a−6i. Unsaturated
aldehyde 4 (0.20 mmol), Ylide 5 (0.24 mmol) and 4Å molecular sieve (40.0 mg) were
added to a flameꢀdried Schlenk tube. The vessel was placed under vacuum and the
atmosphere was exchanged with N2 three times before dry DCE (0.5 ml) was added.
o
The mixture was stirred at 50 C for 3h. Then, oxalyl bromide (1.0 mmol) was added
to the stirred reaction mixture. The final reaction mixture was stirred under reflux
condition for 36h. After the reaction was complete, the reaction mixture was purified
by flash column chromatography using petroleum ether/EtOAc (100:1) to obtain the
desired product 6.
Compound 6a. A white solid (59 mg, 58%, d.r. = 13:1), m.p. 148−150 C; 1H NMR
o
(400 MHz, CDCl3): δ 3.88 (s, 3H), 4.72 (d,
J
= 11.2 Hz, 1H), 5.06 (dd,
J = 11.2Hz,
2.0 Hz, 1H), 5.23 (d, = 10.8 Hz, 1H), 5.31 (dd,
J
J
= 11.0 Hz, 1.6 Hz, 1H), 7.34−7.42
(m, 5H); 13C NMR (100 MHz, CDCl3): δ 167.6, 139.0, 129.2, 128.9, 128.1, 57.0, 55.0,
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54.7, 53.6, 47.2; IR (KBr): 3338, 3001, 2952, 2925, 2372, 1748, 1592, 1433, 1382,
1265, 1142, 1021, 690, 601, 576 cmꢀ1; HRMS (ESI+) exact mass calculated for
[M+Na]+ (C12H12Br4NaO2) requires m/z 526.7463, found m/z 526.7471.
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Compound 6b. A white solid (76 mg, 70%, d.r. = 7:1), m.p. 100−102 C; 1H NMR
(400 MHz, CDCl3): δ 3.89 (s, 3H), 4.72 (d,
J
= 11.2 Hz, 1H), 5.06 (dd,
J = 11.2 Hz,
2.0 Hz, 1H), 5.29 (dd, = 11.2 Hz, 1.6 Hz, 1H), 5.94 (d,
J
J
= 11.2 Hz, 1H), 7.26−7.31
(m, 1H), 7.34−7.41 (m, 2H), 7.56−7.58 (m, 1H); 13C NMR (100 MHz, CDCl3): δ
167.6, 136.8, 133.6, 130.1, 129.8, 129.1, 127.7, 56.3, 54.2, 53.6, 49.3, 47.2; IR (KBr):
3478, 3007, 2954, 1748, 1478, 1437, 1276, 1147, 1039, 983, 734, 611, 581, 457 cmꢀ1;
HRMS (ESI+) exact mass calculated for [M+Na]+( C12H11Br4ClNaO2) reqiures m/z
560.7073, found m/z 560.7086.
o
Compound 6c. A white solid (82 mg, 76%, d.r. > 19:1), m.p. 96−97 C; 1H NMR
(400 MHz, CDCl3): δ 3.88 (s, 3H), 4.70 (d,
J
= 11.2 Hz, 1H), 4.98 (dd,
J = 11.2 Hz,
2.0 Hz, 1H), 5.17 (d, = 11.2 Hz, 1H), 5.27 (dd,
J
J
= 11.2 Hz, 2.0 Hz, 1H), 7.28−7.34
(m, 3H), 7.40 (s, 1H); 13C NMR (100 MHz, CDCl3): δ 167.5, 140.9, 134.6, 130.1,
129.4, 128.3, 126.4, 56.6, 54.4, 53.7, 53.6, 47.1; IR (KBr): 3479, 3006, 2954, 1749,
1595, 1576, 1476, 1435, 1273, 1146, 1022, 895, 739, 693, 582 cmꢀ1; HRMS (ESI+)
exact mass calculated for [M+Na]+ ( C12H11Br4ClNaO2) reqiures m/z 560.7073, found
m/z 560.7084.
o
Compound 6d. A white solid (73 mg, 67%, d.r. = 15:1), m.p.141−142 C; 1H NMR
(400 MHz, CDCl3): δ 3.88 (s, 3H), 4.70 (d,
J
= 11.2 Hz, 1H), 4.99 (dd,
J = 10.8 Hz,
1.6 Hz, 1H), 5.20 (d, = 11.2 Hz, 1H), 5.28 (dd,
J
J
= 10.8 Hz, 1.6 Hz, 1H), 7.35 (d, J =
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9.6 Hz, 2H), 7.37 (d,
J
= 9.2 Hz, 2H); C NMR (100 MHz, CDCl3): δ 167.6, 137.6,
5
6
7
8
9
135.0, 129.5, 129.1, 56.8, 54.5, 53.9, 53.6, 47.1; IR (KBr): 3473, 2953, 1746, 1595,
1491, 1437, 1413, 1267, 1084, 1022, 831, 741, 602, 576, 533, 410 cm−1; HRMS
(ESI+) exact mass calculated for [M+Na]+ ( C12H11Br4ClNaO2) reqiures m/z 560.7073,
found m/z 560.7087.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
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41
42
43
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45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
o
Compound 6e. A white solid (85 mg, 74%, d.r. = 11:1), m.p. 150−151 C; 1H NMR
(400 MHz, CDCl3): δ 3.89 (s, 3H), 4.69 (d,
J
= 11.2 Hz, 1H), 4.92 (dd,
J = 11.2 Hz,
1.6 Hz, 1H), 5.11 (d, = 11.2 Hz, 1H), 5.23 (dd,
J
J
= 11.2 Hz, 1.6 Hz, 1H), 7.29−7.30
(m, 2H), 7.34−7.36 (m, 1H); 13C NMR (100 MHz, CDCl3): δ 167.5, 142.1, 135.3,
129.4, 126.8, 56.3, 54.1, 53.6, 52.7, 46.9; IR (KBr): 3481, 3080, 2953, 1750, 1572,
1435, 1278, 1204, 1146, 1023, 860, 804, 741, 691, 585, 542 cmꢀ1; HRMS (ESI+)
exact mass calculated for [M+Na]+( C12H10Br4Cl2NaO2) reqiures m/z 594.6684, found
m/z 594.6694.
o
Compound 6f. A white solid (82 mg, 70%, d. r. = 13:1), m.p.153−154 C; 1H NMR
(400 MHz, CDCl3): δ 3.88 (s, 3H), 4.70 (d,
J
= 11.2 Hz, 1H), 4.99 (dd,
= 11.2 Hz, 1H), 5.27 (dd, = 10.8 Hz, 1.6 Hz, 1H), 7.29 (d,
= 8.4 Hz, 2H); C NMR (100 MHz, CDCl3): δ 167.6, 138.1,
J = 11.0 Hz,
2.0 Hz, 1H), 5.18 (d,
8.4 Hz, 2H), 7.52 (d,
J
J
J =
13
J
132.1, 129.8, 123.2, 56.7, 54.4, 54.0, 53.6, 47.1; IR (KBr): 3348, 2997, 2951, 2371,
1747, 1589, 1486, 1436, 1379, 1264, 1146, 1070, 1022, 827, 741, 603, 524 cmꢀ1;
HRMS (ESI+) exact mass calculated for [M+Na]+( C12H11Br5NaO2) requires m/z
604.6568, found m/z 604.6578.
o
Compound 6g. A white solid (61 mg, 58%, d.r. > 19:1), m.p.112−113 C; 1H NMR
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(400 MHz, CDCl3): δ 3.88 (s, 3H), 4.71 (d,
J
= 11.2 Hz, 1H), 5.00 (dd, J = 11.0 Hz,
5
6
7
8
9
1.8 Hz, 1H), 5.22 (d,
J
= 11.2 Hz, 1H), 5.29 (dd,
J
= 10.8 Hz, 2.0 Hz, 1H), 7.08 (t,
J
J
=
=
8.4 Hz, 2H), 7.38−7.42 (m, 2H); 13C NMR (100 MHz, CDCl3): δ 167.6, 162.8 (d,
250 Hz), 135.1 (d, = 3 Hz), 130.0 (d, = 8 Hz), 115.9 (d,
10
11
12
13
14
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17
18
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48
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52
53
54
55
56
57
58
59
60
J
J
J
= 22 Hz), 57.2, 54.6,
54.1, 53.6, 47.2; IR (KBr): 3480, 3005, 2955, 2848, 1749, 1604, 1511, 1437, 1275,
1232, 1146, 1018, 838, 740, 607, 630, 468 cmꢀ1; HRMS (ESI+) exact mass calculated
for [M+Na]+( C12H11Br4FNaO2) requires m/z 544.7369, found m/z 544.7375.
o
Compound 6h. A white solid (21 mg, 20%, d.r.=4:1), m.p. 89−90 C; 1H NMR (400
MHz, CDCl3): δ 2.37 (s, 3H), 3.88 (s, 3H), 4.72 (d,
10.8 Hz, 1.6 Hz, 1H), 5.22 (d, = 11.2 Hz, 1H), 5.31 (dd,
7.20 (d, = 8.0 Hz, 2H), 7.30 (d,
J
= 11.2 Hz, 1H), 5.05 (dd,
= 11.0 Hz, 1.6 Hz, 1H),
J = 8.0 Hz, 2H); C NMR (100 MHz, CDCl3): δ
J =
J
J
13
J
167.7, 139.4, 136.2, 129.6, 128.0, 57.2, 55.2, 54.8, 53.6, 47.3, 21.3; IR (KBr): 3005,
2953, 1750, 1593, 1436, 1380, 1273, 1146, 1022, 819, 768, 723, 606, 578 cmꢀ1;
HRMS (ESI+) exact mass calculated for [M+Na]+( C13H14Br4NaO2) requires m/z
540.7620, found m/z 540.7626.
o
Compound 6i. A white solid (61 mg, 58%, d.r. = 11:1), m.p. 100−101 C; 1H NMR
(400 MHz, CDCl3): δ 1.36 (t,
(d, = 11.2 Hz, 1H), 5.06 (dd,
5.32 (dd,
= 10.8 Hz, 1.6 Hz, 1H), 7.35−7.42 (m, 5H); 13C NMR (100 MHz, CDCl3):
J
= 7.2 Hz, 3H), 4.34 (qd,
J
= 7.0 Hz, 1.6 Hz, 2H), 4.69
J
J
= 10.8 Hz, 1.6 Hz, 1H), 5.23 (d,
J
= 10.8 Hz, 1H),
J
δ 167.1, 139.1, 129.2, 128.9, 128.1, 62.8, 57.1, 55.0, 54.8, 47.6, 13.8; IR (KBr): 3469,
2984, 1745, 1455, 1372, 1267, 1144, 1026, 858, 767, 738, 695, 603, 578, 507 cmꢀ1;
HRMS (ESI+) exact mass calculated for [M+Na]+( C13H14Br4NaO2) reqiures m/z
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540.7620 , found m/z 540.7626.
5
6
7
8
9
Procedure for the Preparation of Compound 8. Benzaldehyde (0.20 mmol),
Ylide 7 (0.24 mmol) and 4Å molecular sieve (40.0 mg) were added to a flameꢀdried
Schlenk tube. The vessel was placed under vacuum and the atmosphere was
exchanged with N2 three times before dry DCE (0.5 ml) was added. The mixture was
stirred under reflux for 24h. Then, oxalyl bromide (0.6 mmol) was added to the stirred
reaction mixture. The final reaction mixture was stirred at room temperature for 5h.
After the reaction was complete, the reaction mixture was purified by flash column
chromatography using petroleum ether/EtOAc to obtain the desired product 8.
Compound 8.18 A white solid (16 mg, 22%, d.r. = 8:1); H NMR (400 MHz, CDCl3): δ
10
11
12
13
14
15
16
17
18
19
20
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5.65 (d,
J
= 11.2 Hz, 1H), 5.84 (d,
J
J
= 11.2 Hz, 1H), 7.36−7.45 (m, 3H), 7.52−7.57 (m,
13
4H), 7.65−7.68 (m, 1H), 8.11 (d,
= 7.4 Hz, 2H); C NMR (100 MHz, CDCl3): δ
191.2, 138.2, 134.5, 134.2, 129.3, 129.0, 128.9, 128.9, 128.4, 49.8, 46..8.
Procedure for the Preparation of Compound 10. Wittig salt 919 (0.5 mmol)
was added to a Schlenk tube, and then 1.25 mL saturated aqueous NaHCO3 was
o
poured into the tube. The mixture was vigorously stirred for 1h at 30 C. Then the
crude mixture was extracted with DCM, washed with water and brine, and dried over
Na2SO4. The solvent was evaporated under reduced pressure and the residue was
transferred to a flameꢀdried Schlenk tube. Then 4Å molecular sieve (40.0 mg), DCE
(1.0 mL) and oxalyl bromide (1.5 mmol) were added. The reaction mixture was
o
stirred at 40 C for 24h. After the reaction was complete, the reaction mixture was
purified by flash column chromatography using petroleum ether/EtOAc (11:1) to
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obtain the desired product 10.
Compound 10.18 A white solid (14 mg, 9%); 1H NMR (400 MHz, CDCl3): δ 4.97 (d,
7
8
9
J
= 2.4 Hz, 1H), 5.35 (d,
J
= 2.4 Hz, 1H), 7.19 (d,
J
= 8.0 Hz, 1H), 7.23−7.26 (m, 1H),
10
11
12
13
14
15
16
17
18
19
20
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49
50
51
52
53
54
55
56
57
58
59
60
13
7.39 (d, = 7.6 Hz, 1H), 7.46 (t,
J
J
= 8.0 Hz, 1H); C NMR (100 MHz, CDCl3): δ
160.7, 150.3, 131.6, 128.6, 125.6, 119.9, 117.7, 43.6, 39.2.
ACKNOWLEDGEMENTS
We are grateful to the NSFC (21172097, 21202070, 21302075 and 21372105), the
International S&T Cooperation Program of China (2013DFR70580), the National
Natural Science Foundation from Gansu Province of China (no. 1204WCGA015), and
the “111” program from MOE of P. R. China.
ASSOCIATED CONTENT
Supporting Information
Optimization, copies of spectra and Xꢀray crystallographic data for 6a. This material
is available free of charge via the Internet at http://pubs.acs.org.
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