Silver acetate/TF-biphamphos-catalyzed endo-selective enantioselective 1,3-dipolar cycloaddition of azomethine ylides with vinyl phenyl sulfone

Article abstract of DOI:10.1002/adsc.200900552

The first catalytic endo-selective 1,3-dipolar cycloaddition of azomethine ylides and vinyl phenyl sulfone has been developed successfully. The highly efficient silver acetate (AgOAc)/TF-BiphamPhos catalytic system exhibited high reactivity, excellent diastereoselectivity (>98:2), good enantioselectivity (67-92% ee) and broad substrate scope under mild conditions.

Full text of DOI:10.1002/adsc.200900552

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DOI: 10.1002/adsc.200900552
Silver Acetate/TF-BiphamPhos-Catalyzed endo-Selective
Enantioselective 1,3-Dipolar Cycloaddition of Azomethine
Ylides with Vinyl Phenyl Sulfone
Gang Liang,a Min-Chao Tong,a and Chun-Jiang Wanga,*
a
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Peopleꢀs Republic of China
Fax : (+86)-27-6875-4067; e-mail: cjwang@whu.edu.cn
Received: August 5, 2009; Revised: October 25, 2009; Published online: November 24, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900552.
Abstract: The first catalytic endo-selective 1,3-dipo-
lar cycloaddition of azomethine ylides and vinyl
phenyl sulfone has been developed successfully.
The highly efficient silver acetate (AgOAc)/TF-Bi-
phamPhos catalytic system exhibited high reactivity,
excellent diastereoselectivity (>98:2), good enan-
tioselectivity (67–92% ee) and broad substrate
scope under mild conditions.
fones have been seldom studied in the catalytic asym-
metric 1,3-dipolar cycloaddition[11] although the sulfo-
nyl group possesses a high electron-withdrawing prop-
erty and functional transformation versatility.[12] Two
transition metal-catalyzed asymmetric 1,3-dipolar cy-
cloadditions of azomethine ylides with vinyl phenyl
sulfone have been reported by Carretero using the
Cu(I)/Taniaphos complex[7a] and Fukuzawa using the
Cu(I)/ClickFerrophos complex,[7c] both of which pro-
duced the exo-adducts with high diastereoselectivity
and enantioselectivity (Figure 1). To the best of our
knowledge, there are no reported methods which pro-
vide access to the endo-adducts with both high enan-
tioselectivity and chemical efficiency for the catalytic
asymmetric 1,3-dipolar cycloaddition of azomethine
ylides with vinyl phenyl sulfone. Only one example
Keywords: asymmetric catalysis; azomethine ylides;
cycloaddition; diastereoselectivity; enantioselectivi-
ty; vinyl phenyl sulfone
The asymmetric 1,3-dipolar cycloaddition of azome- reported by Grigg addressed that the endo-adduct
thine ylides to electron-deficient alkenes is a powerful and 70% ee were achieved for the reaction of vinyl
and atom-economical carbon-carbon bond-forming phenyl sulfone with an azomethine ylide in the pres-
reaction that can be used to create structurally and ence of a stoichiometric amount of a diphosphine-
stereochemically rich pyrrolidines.[1] Because the re- Ag(I) complex,[3a] however, no detailed data were
sulting substituted pyrrolidines are prevalent in many given to support the relative and absolute configura-
natural alkaloids, compounds of pharmaceutical sig- tions of the resulted adduct. Recently, we reported a
nificance, organocatalysts, and biologically important novel family of readily available chiral TF-Bipham-
building blocks in organic synthesis,[2] recent research Phos ligands (1) (Figure 1), whose Cu(I) and Ag(I)
has focused on the catalytic asymmetric version of the complexes exhibited highly diastereoselectivity and
1,3-dipolar cycloaddition of azomethine ylides. Since excellent enantioselectivity in the asymmetric 1,3-di-
the original pioneering work of Grigg[3] employing polar cycloaddition of various azomethine ylides with
stoichiometric amounts of a chiral metal complex and conjugated carbonyl dipolarophiles.[13] Extending the
the first catalytic asymmetric version reported by interest in these ligands for asymmetric catalysis and
Zhang[4] using the Ag(I)/xylyl-FAP system, various filling up the literature gap, herein, we described that
types of chiral metal catalysts [containing metals such Ag(I)/TF-BiphamPhos served as an efficient catalyst
ACTHNUTRGNEUNG
as Ag(I),[4,5] Zn(II),[6] Cu(I/II),[7] Ni(II),[8] and for the highly endo-selective 1,3-dipolar cycloaddition
Ca(II),[9]] and organocatalysts[10] have been developed of azomethine ylides and vinyl phenyl sulfone with
to afford moderate to high enantio-/diastereoslectivi- high enantioselectivities of up to 92% ee.
ties. Despite excellent results achieved for this trans-
Initially, the asymmetric 1,3-dipolar cycloaddition
formation, most of the dipolarophiles applied in these of N-benzylidene glycine methyl ester 3a with vinyl
reactions are the derivatives of conjugated unsaturat- phenyl sulfone 2 was examined using different metal
ed esters and maleimides.[3–10] In contrast, vinyl sul- salts as Lewis acids and TF-BiphamPhos as chiral
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Figure 1. Catalytic asymmetric 1,3-dipolar cycloaddition of vinyl phenyl sulfone with azomethine ylides.
ligand in the presence of triethylamine at room tem- AgSbF6, AgOTf and AgClO4, combined with TF-Bi-
perature, and the representative results are summar- phamPhos 1a showed high reactivities, and the reac-
ized in Table 1. Except for the inactivity of Cu
monovalent copper or silver metal precursors, such as perature using PhMe as the solvent (Table 1, en-
Cu(CH3CN)4ClO4, Cu(CH3CN)4BF4, AgOAc, tries 1–8). To our delight, endo-4a was obtained as the
ACHTUNGRTEN(NGNU OTf)2, tions were finished in less than 30 min at room tem-
A
ACHTUNGTRENNUNG
Table 1. Screening studies of the asymmetric 1,3-dipolar cycloaddition of
vinyl phenyl sulfone 2 with azomethine ylide 3a.[a]
[a]
The reactions were carried out with 0.33 mmol of 2 and 0.40 mmol of 3a
in 2 mL of solvent.
CuClO4 =CuACTHNURTGENNUG(CH3CN)4ClO4, CuBF4 =CuAHCUTNGTRNE(NUGN CH3CN)4BF4.
Isolated yield.
Determined by HPLC analysis.
[b]
[c]
[d]
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Silver Acetate/TF-BiphamPhos-Catalyzed endo-Selective 1,3-Dipolar Cycloaddition
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Table 2. Asymmetric 1,3-dipolar cycloaddition of vinyl phenyl sulfone 2
with various azomethine ylides 3 catalyzed by AgOAc/1d.[a]
[a]
The reactions were carried out with 0.33 mmol of 2 and 0.40 mmol of 3
in 2 mL PhMe.
Isolated yield.
Determined by chiral HPLC analysis.
Data in parentheses were achieved after recrystallization.
[b]
[c]
[d]
[e]
In 6 h.
sole product, which was totally different from the exo lar cycloaddition of azomethine ylides with various
preference achieved by Cu(I)/Taniaphos[7a] and Cu(I)/ conjugated carbonyl dipolarophiles,[13] unexpectedly
ClickFerrophos[7c] complexes. The apparent differen- showed the lowest asymmetric induction and catalytic
ces of the proton chemical shifts were observed ability in this transformation (Table 1, entry 16). Strik-
through comparing the 1H NMR spectra of exo- ingly, ligand 1d containing the bulky cyclohexyl group
adduct 4a and endo-adduct 4a: H-2 and H-4 of the on the phosphorus atom, which provided unsatisfacto-
pyrrolidine ring in exo-4a appear around 4.18 ppm ry result in former studies, emerged as the most effec-
and 3.66 ppm, respectively;[7a] however, the signal tive chiral ligand in this reaction and provided endo-
peaks of the corresponding protons in endo-4a merge 4a as the sole product in high yield and excellent
together and appear around 4.00 ppm (see Supporting enantioselectivity of 92% within 20 min (Table 1,
Information for more information). The catalytic abil- entry 15). A preliminary screening of solvent effects
ity and asymmetric induction of the Cu(I)/1a complex showed that toluene and xylene were the best sol-
was generally inferior to that of the Ag(I)/1a com- vents, other solvents such as dichloromethane, diethyl
plex, which was opposite to the trend exhibited in the ether, tetrahydrofuran gave lower enantioselectivities
cases reported by Carretero[7a] and Fukuzawa.[7c] Al- (Table 1, entries 4, 9–12). Reducing the temperature
though the examined four silver salts gave the similar from room temperature to 08C or À208C did not im-
results in terms of yields and enantio-/diastereoselec- prove the enantioselectivity (Table 1, entry 15 versus
tivity, AgOAc was chosen as the best metal precursor entries 17 and 18). Thus, the optimized reaction con-
due to the economy and easy handling. The reaction ditions were established as 3 mol% of AgOAc/1d and
rate was reduced remarkably without Et3N as the 15 mol% Et3N in PhMe or xylene at room tempera-
extra base although the enantioselecivity remained at ture.
the same level (Table 1, entries 4 and 5). We then con-
Having established the optimal reaction condition,
ducted the reaction using other TF-BiphamPhos li- we then investigated a series of representative imino
gands. When the phenyl group on the phosphorus esters 3 derived from glycinate to test the substrate
atom of ligand 1a was replaced by a xylyl group (1b) scope. As shown in Table 2, a wide array of imino
or a 3,5-bis(trifluoromethyl)phenyl group (1c), the esters derived from aromatic aldehyde reacted
enantioselectivity decreased from 80% to 74% and smoothly with vinyl phenyl sulfone 2 to afford the
53%, respectively (Table 1, entries 5, 13 and 14). corresponding endo-adducts exclusively in high yields
Ligand 1e bearing two bromines at the 3,3’-positions and good enantioselectivities (Table 2, entries 1–6). It
of the TF-Bipham backbone, which exhibited the ex- appears that the position and the electronic properties
cellent reactivity and substrate scope for the 1,3-dipo- of the substituents on the aromatic rings have very
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Figure 2. X-ray crystal structure of endo-4f (relative configuration).
limited effects on the enantioselectivities. Azomethine configuration of 4a was assigned as (2R,4R,5R)
ylides from a- or b-naphthylaldehyde also worked through the highly efficient desulfonylation of the N-
well in this transformation producing the endo-4g and methyl derivative 5 and then optical rotation compari-
4h with 85% and 86% ee, respectively (Table 2, en- son of the obtained 6 with the data in a literature re-
tries 7 and 8). An azomethine ylide from an aliphatic port[7a] (Scheme 1). Those of other adducts were ten-
aldehyde has been seldom studied in the asymmetric tatively proposed on the basis of these results.
1,3-dipolar cycloaddition reaction probably due to its
Based on the relative and absolute configurations
lower reactivity. Remarkably, azomethine ylide 3i of 4a, 4f and 4g, the high endo-selectivity observed in
from the aliphatic cyclohexanecarbaldehyde was tol- the AgOAc/(S)-TF-BiphamPhos-catalyzed asymmet-
erated in this reaction, and the endo-adduct 4i could ric 1,3-dipolar cycloaddition reaction of azomethine
be obtained in 77% yield and 67% ee (Table 2, ylides with vinyl phenyl sulfone can be rationalized
entry 9). Notably, azomethine ylide 3j derived from by the proposed tetracoordinated complex[5c,j] shown
the a-substituted amino acid alanine also worked well in Figure 3. The in situ-formed azomethine ylide is co-
in this transformation producing the endo-4j bearing ordinated to the metallic center and oriented in such
a
nitrogen-substituted
quaternary
stereogenic a transition state because of the steric repulsion be-
center[14] with 82% ee in 90% yield (Table 2, tween the phenyl group in the ylide and the cyclohex-
entry 10). Fortunately, all the products are solid, and yl rings on the phosphorus atom of the chiral ligand,
enantioenriched compounds can be easily obtained by and the highly steric congestion imposed by the latter
simple recrystallization of the crude products effectively blocks the dipolarophile approach from
(Table 2, entries 1 and 6).
the Re (C=N) face of the azomethine ylide and forms
The relative configuration of the adducts 4f and 4g the endo-(2R,4R,5R) product through Si face attack
was determined to be endo by X-ray diffraction analy- (See Supporting Information for more information).
sis of the corresponding racemic adducts,[15,16] which
In conclusion, we have reported the first catalytic
were achieved by using racemic TF-BiphamPhos 1a endo-selective 1,3-dipolar cycloaddition of azome-
as the ligand (Figure 2 and Supporting Information). thine ylides and vinyl phenyl sulfone catalyzed by the
However, high quality crystals of enantiomerically en- efficient AgOAc/TF-BiphamPhos. This catalytic
riched endo-adducts for determining the absolute con- system exhibited high reactivity, excellent diastereose-
figuration have not been achieved yet. The absolute lectivity, good enantioselectivity (67–92% ee) and
Scheme 1. Determination of the absolute configuration of 4a through desulfonylation of methylated compound 5.
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Silver Acetate/TF-BiphamPhos-Catalyzed endo-Selective 1,3-Dipolar Cycloaddition
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Figure 3. Proposed transition state leading to the endo-adduct when using vinyl phenyl sulfone as the dipolarophile.
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broad substrate scope under mild conditions. Further
investigations on the mechanism and applications of
TF-BiphamPhos in asymmetric catalysis are ongoing
in our laboratory and will be reported in due course.
Experimental Section
General Procedure
Under an argon atmosphere a solution of ligand (S)-TF-Bi-
phamPhos 1d (7.7 mg, 0.01 mmol) and AgOAc (1.67 mg,
0.01 mmol) in 1 mL toluene was stirred at room temperature
for about 1 h. Then, imine substrate (0.40 mmol) was added
as a solution in 1 mL toluene followed by Et3N (0.05 mmol)
and phenyl vinyl sulfone (37.0 mg, 0.33 mmol). Once the
starting material had been consumed (monitored by TLC),
the mixture was filtered through celite and the filtrate was
concentrated to dryness. The crude product was analyzed by
1H NMR to determine the endo/exo ratio, and then the resi-
due was purified by column chromatography to give the cor-
responding cycloaddition product as a white solid, which
was then directly analyzed by chiral HPLC to determine the
enantiomeric excess.
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Acknowledgements
This work was supported by National Natural Science Foun-
dation of China and the startup fund from Wuhan University.
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