Chemoselective C-C bond cleavage of epoxide motifs: Gold(I)-catalyzed diastereoselective [4+3] cycloadditions of 1-(1-Alkynyl)oxiranyl ketones and nitrones

Article abstract of DOI:10.1002/chem.201002395

Cutting carbon! A novel facile strategy for the C-C bond cleavage of oxiranyl ketones has been developed. Carbophilic gold(I) activation of the alkyne side chain mediates a heterocyclization and subsequent C-C bond cleavage (see scheme).

Full text of DOI:10.1002/chem.201002395

DOI: 10.1002/chem.201002395
À
Chemoselective C C Bond Cleavage of Epoxide Motifs: Gold(I)-Catalyzed
Diastereoselective [4+3] Cycloadditions of 1-(1-Alkynyl)oxiranyl Ketones
and Nitrones
Tao Wang[a] and Junliang Zhang*[a, b]
In memory of Professor Xian Huang
Epoxides are highly versatile intermediates in organic syn-
thesis due to their easy access and their susceptibility to ring
[1]
À
opening by facile C O bond cleavage. Despite the fact
À
that the C O bond cleavage has been extensively studied in
organic synthesis, C C bond cleavage is rarely reported and
À
has limited applications in synthesis due to the harsh condi-
tions required. a,b-Epoxy ketones are an important class of
compounds with high synthetic utility and potential for fur-
ther functionalization. Several ring-opening pathways for
a,b-epoxy ketones under different conditions have been
demonstrated (Scheme 1A): 1) photoinduced homocleavage
of the epoxide ring to generate a biradical, which can under-
À
go further transformations;[2] 2) Lewis acid mediated Cb
O
bond cleavage to produce a zwitterionic intermediate,[3]
which can react further with nucleophiles to afford various
ketones, or (when R is hydrogen) formation of a-diketones
through hydrogen migration; 3) transition-metal-mediated
À
oxidative addition to the Ca O bond to give a metallaoxe-
tane[4] and subsequent rearrangement to afford 1,3-dike-
À
tones; 4) Ca Cb bond cleavage at high temperatures to
afford carbonyl ylide intermediates, which can be further ap-
plied in 1,3-dipolar cycloadditions.[5–7] Huisgen and March
demonstrated that the [3+2] cycloaddition of aldehydes to
Scheme 1. Previously reported ring-opening models of epoxy ketones (A)
and our working hypothesis (B).
3-phenyloxirane-2,2-dicarboxylate (with two activating
groups) still required 10 days at 1258C to achieve a satisfac-
tory conversion.[6] The development of new strategies to ac-
[a] T. Wang, Prof. Dr. J. Zhang
Shanghai Key Laboratory of Green Chemistry
and Chemical Processes, Department of Chemistry
East China Normal University, 3663 N. Zhongshan Road
Shanghai 200062 (P.R. China)
Fax : (+86)21-6223-5039
E-mail: jlzhang@chem.ecnu.edu.cn
À
complish the C C bond cleavage of epoxides under mild
conditions are clearly highly desirable. Herein, we report a
À
novel strategy to achieve the C C bond cleavage of the ep-
oxide ring by introducing an alkyne that can be activated by
[b] Prof. Dr. J. Zhang
a carbophilic gold(I) complex. To the best of our knowledge,
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
À
this is the first example of a metal-catalyzed, selective C C
bond cleavage of epoxy ketones.
À
It is believed that a,b-epoxy ketones readily undergo C
354 Fenglin Road, Shanghai 200032, (P.R. China)
À
O bond cleavage rather than C C bond cleavage due to the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002395.
preference of coordination of the oxygen atom by the oxo-
86
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Chem. Eur. J. 2011, 17, 86 – 90

COMMUNICATION
À
philic Lewis acid to make the C O bond more labile
(Scheme 1A, route 2). Thus, if there were a way to circum-
vent this binding and activate the epoxide ring by binding
À
the side chain, C C bond cleavage might be realized.
Encouraged by previous reports,[8] we envisaged that this
issue could be addressed by introducing an alkyne to the a
position of a,b-epoxy ketones, that is, 1-(1-alkynyl)oxiranyl
ketones 1 (Scheme 1B). There are two different reaction
pathways in the presence of a carbophilic transition metal or
a Lewis acid. Path I shows the carbonyl oxygen attacking
the metal-activated alkyne to mediate a heterocyclization to
produce intermediate IA, which upon further aromatization
À
through C C bond cleavage affords the furanyl intermediate
IB. Alternatively, the oxygen atom of the epoxy can attack
the metal-activated alkyne (path II) to mediate a heterocyc-
À
lization and subsequent Ca O cleavage of intermediate IIA
Scheme 3.
will generate intermediate IIB.[9]
With this hypothesis in mind, we first developed a general
and efficient route to b-alkyne-a,b-epoxy ketones 1. Gratify-
ingly, these substrates can be easily prepared from a,b-unsa-
turated ketones. The successive bromination, elimination,
and Sonagashira cross-coupling gives 2-(1-alkynyl)-2-alken-
identified product (Scheme 3C). Inspired by this result, we
further optimized the reaction conditions by using ketone
1b and nitrone 4a as model substrates (Table 1). The effect
1-ones in high yields according to a known procedure.[10]
A
Table 1. Optimization of the reaction conditions.[a]
subsequent epoxidation reaction produces the desired prod-
ucts in moderate to high yields (Scheme 2). The structure of
1b was established by spectroscopic analysis and further
confirmed by single-crystal X-ray analysis.[11]
Entry
Catalyst
[AuPPh3]
[AuPPh3]
[AuPPh3]
[AuIPr][OTf]
AuCl3
[AuPPh3]
[AuPPh3]
[AuPPh3]
[AuPPh3]
[AuPPh3]
[AuPPh3]
[AuL1]
[SbF6]
[AuL2]
[SbF6]
Solvent
Time
Yield[e] [%]
1
2
3
4
5
6
7
8
G
N
DCE
DCE
DCE
DCE
DCE
CH2Cl2
toluene
CH3CN
THF
DCE
DCE
DCE
DCE
10 min
10 min
10 min
4 h
80 (75)
84 (79)
83 (76)
38
R
E
N
ACHTUNGTRENNUNG
11 h
51
N
10 min
10 min
10 min
10 min
10 min
10 min
1 h
79 (74)
87 (75)
42
R
N
9
G
33
10[b]
11[c]
12[d]
13[d]
R
81 (79)
83 (77)
98 (90)
100 (93)
E
Scheme 2. The general route to b-alkyne-a,b-epoxy ketones 1.
T
ACHTUNGTRENNUNG
A
A
1h
[a] Unless otherwise noted, reactions were performed with 1b
(0.3 mmol), 4a (0.33 mmol), and catalyst (5.0 mol%) in DCE at RT.
[b] 1.5 equiv of 4a. [c] 2.0 equiv of 4a. [d] Generated in situ from
Me2SAuCl, ligand, and AgSbF6. [e] Yield obtained by NMR spectrosco-
py; the values given in parentheses indicate isolated yields.
We initiated our investigation with the cycloisomerization
of 1a. When 1a was subjected to AgBF4 in a solvent mix-
ture (CH2Cl2/MeOH=10:1), cycloisomerization occurred
and gave the trisubstituted furan 2 in 84% yield through
À
Ca O bond cleavage (Scheme 3A). Interestingly, upon reac-
tion
with
5 mol%
[AuPPh3]
G
(OTf=triflate;
Scheme 3B), compound 1a afforded a highly substituted
furan-3(2H)-one 3 in 59% yield and 3:2 diastereoisomeric
of different counteranions were examined by the combina-
tion of [AuPPh3]Cl with different silver salts, such as
AgSbF6 (Table 1, entry 2) and AgBF4 (Table 1, entry 3). Of
ACHTUNGTRENNUNG
À
À
ratio (d.r.). Both C C and Cb O bond cleavage are involved
in this process.
these two catalysts, [AuPPh3]
yield (79%, Table 1, entry 2). [AuIPr]ACTHUNGTRENNNUG
ACHTUNGTRENNUNG
À
We then studied the other more difficult C C bond cleav-
age pathway. To our delight, this pathway was achieved
when a mixture of 1a and nitrone 4a were subjected to
[AuPPh3]Cl and AgOTf in 1,2-dichloroethane (DCE) at RT
to give the desired bicyclic compound 5aa along with an un-
diisopropylphenyl)imidazol-2-yliden) gave only 38% yield
(Table 1, entry 4). The reaction took 11 h to go to comple-
tion when AuCl3 was used, with only a 51% yield observed
by NMR spectroscopy (Table 1, entry 5). Various solvents
Chem. Eur. J. 2011, 17, 86 – 90
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87

J. Zhang and T. Wang
Table 3. Reactions of 1b with various nitrones.[a]
and different amounts of nitrone were then investigated, but
such changes did not improve the process (Table 1, en-
tries 6–11). Considering ligand effects in gold-catalyzed reac-
tions,[12,13] the 2-dicyclohexylphosphino-2’,6’-dimethoxy-1,1’-
biphenyl (L1, S-Phos) and 2-dicyclohexylphosphino-2’,6’-dii-
sopropoxy-1,1’-biphenyl (L2, Ruphos)-derived gold com-
plexes were then examined (Table 1, entries 12 and 13); the
latter gave the best result (93% yield).
Entry
R4/R5
Time [h]
Product (Yield[b] [%])
1
2
3
4
5
6
7
8
9
4-MeOC6H4/Ph (4b)
4-NO2C6H4/Ph (4c)
furyl/Ph (4d)
5
10
10
10
0.5
0.5
0.5
1
5bb (89)
5bc (82)
5bd (79)
5be (80)
5bf (97)
5bg (89)
5bh (90)
5bi (86)
5bj (89)
A wide variety of 1-(1-alkynyl)oxiranyl ketones 1 were ex-
amined under the optimal conditions. The results are sum-
marized in Table 2. The ketone substituent R1 was either an
styryl/Ph (4e)
4-BrC6H4/Ph (4 f)
Ph/4-MeC6H4 (4g)
Ph/4-BrC6H4 (4h)
Ph/3-ClC6H4 (4i)
4-OMeC6H4/4-BrC6H4 (4j)
Table 2. Variation of 1-(1-alkynyl)oxiranyl ketones 1 component.[a]
0.5
[a] Unless otherwise noted, reactions were performed with 1b
(0.3 mmol), 4 (0.33 mmol), [AuL2]Cl (5 mol%), and AgSbF6 (5 mol%) in
DCE (3.0 mL) at RT. [b] Isolated yield.
Entry
R1/R2/R3
Time [h]
Product (Yield[b] [%])
1
2
3
4
5
6
7
8
9
Ph/4-MeOC6H4/Ph (1c)
Ph/4-NO2C6H4/Ph (1d)
4-MeOC6H4/Ph/Ph (1e)
4-ClC6H4/Ph/Ph (1 f)
Ph/Ph/nBu (1g)
Ph/Ph/cyclohexenyl (1h)
Ph/Ph/4-MeOC6H4 (1i)
Me/Ph/4-MeOC6H4 (1j)
Me/4-ClC6H4/Ph (1k)
1.5
8
3
1
2
1
1
3
10.5
5ca (71)
5da (81)
5ea (88)
5 fa (98)
5ga (84)
5ha (81)
5ia (76)
5ja (69)
5ka (64)
[a] Unless otherwise noted, reactions were performed with 0.3 mmol of 1,
0.33 mmol of 4a, 5.0 mol% of [AuL2]Cl, and AgSbF6 (1:1) in 3.0 mL of
DCE at RT. [b] Isolated yield.
aromatic ring or an alkyl group. For those substrates with an
aromatic R1, the reactions normally gave the corresponding
cycloadducts in good to excellent yields (Table 2, entries 1–
7); for alkyl-substituted substrates, the reactions proceeded
smoothly to afford the desired products in good yields
(Table 2, entries 8 and 9). Both electron-withdrawing and
-donating groups could be introduced to the phenyl ring
(R1–3). Note that cis adducts were only detected for 5, indi-
cating that this reaction is highly chemo-, regio-, and stereo-
selective. The structure of 5ja was established by spectro-
scopic analysis and further confirmed by single-crystal X-ray
analysis.[11]
The scope of this reaction was next examined by varying
nitrone 4 (Table 3). para-Methoxy- or para-nitrobenzalde-
hyde-derived nitrones gave the corresponding products in
high yields with excellent diastereoselectivity (>20:1)
(Table 3, entries 1 and 2). Furyl and stryryl could also be in-
corporated as the R4 group into the products (Table 3, en-
tries 3 and 4). Halogens at the R4 and R5 positions makes
further functionalizations possible (Table 3, entries 5, and 7–
9).
Scheme 4. Plausible mechanism.
1 enhances the electrophilicity of the alkyne. Subsequent nu-
cleophilic attack of the carbonyl oxygen on the gold(I)-acti-
vated alkyne forms the oxonium-containing vinyl–gold inter-
mediate IA.[8f,14] Intermediate IA may undergo two different
reaction pathways (cycle I and II). In cycle I, the regioselec-
tive homo-Michael addition of nitrone 4 at the Cb position
of the epoxy motif would generate furanyl–gold intermedi-
ate IC, which in turn, upon ring closure through the favored
chairlike conformation, would give the formal [4+3] cyclo-
adduct in a highly diastereoselective fashion and regenerate
the gold catalyst. Alternatively, intermediate IA could un-
À
dergo aromatization through C C bond cleavage of the
epoxy motif to produce furanyl–gold intermediate IB with
an oxygen-stabilized carbocation (cycle II), which would
react with nitrone 4 to give the same intermediate IC as in
cycle I.
A plausible mechanism that accounts for this gold(I)-cata-
lyzed, highly diastereoselective [4+3] cyclization is depicted
in Scheme 4. The gold(I) coordination of the triple bond of
88
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Chem. Eur. J. 2011, 17, 86 – 90

À
C C Bond Cleavage of Epoxide Motifs
COMMUNICATION
To gain further insight into the mechanism, enantioen-
riched substrate 1b’ (with 73% enantiomeric excess (ee))
and 4a were subjected to the standard reaction conditions.
After 1 h, compound 5ba’ was obtained in 95% yield, with
only 18% ee. This indicates that both pathways (cycles I and
II) are involved in the process (Scheme 5).
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In conclusion, we have successfully developed a novel
À
facile strategy towards chemoselective C C bond cleav-
age[15] of epoxides by introducing a side chain (alkyne) that
can be selectively activated by a catalyst. Furthermore,
novel hetero-bicyclics 5 can be efficiently prepared in a
highly diastereoselective fashion from readily available 1-(1-
alkynyl)oxiranyl ketones and nitrones. The application of
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À
this novel activation strategy towards chemoselective C C
activation of epoxides to other oxiranyl derivatives is being
carried out in our laboratory and will be reported in due
course.
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Experimental Section
General procedure for the synthesis of 5ba: [AuL2]Cl (10.5 mg,
0.015 mmol) and AgSbF6 (5.2 mg, 0.015 mmol) were mixed and stirred
for 10 min in anhydrous DCE (1 mL) under N2. The mixture was trans-
ferred to 1b (97.3 mg, 0.3 mmol) and 4a (65.1 mg, 0.33 mmol) in anhy-
drous DCE (2 mL), the resulting reaction mixture was stirred at RT for
1 h under N2. After evaporation under reduced pressure, the residue was
purified by column chromatography on silica gel (hexane/EtOAc=20:1)
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to afford 5ba as
a white solid (145.6 mg, 93%). M.p. 171–1738C.
1H NMR (CDCl3, 300 MHz): d=7.97 (d, J=7.8 Hz, 2H), 7.68–7.65 (m,
2H), 7.50–7.29 (m, 11H), 7.24–7.18 (m, 7H), 6.99–6.92 (m, 3H), 6.47 (s,
1H), 5.99 ppm (s, 1H); 13C NMR (CDCl3, 75 Hz): d=149.0, 145.9, 143.2,
138.7, 136.5, 135.5, 130.7, 130.2, 130.0, 129.2, 128.73, 128.68, 128.58, 128.4,
127.9, 127.9, 127.8, 126.7, 126.6, 126.2, 124.0, 123.1, 118.5, 118.1, 108.8,
70.4 ppm; IR (neat): n˜ =3061, 3030, 1597, 1490, 1345, 1190, 989, 851, 762,
691 cmÀ1; MS (EI, 70 ev): m/z (%): 521 (0.17) [M]+, 77 (100); HRMS: m/
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Acknowledgements
We are grateful to NSFC (20972054), Ministry of Education of China
(20090076110007, NCET), STCSM (08dj1400100) and 973 Program
(2009CB825300) for financial support.
À
Keywords: C C bond cleavage · cycloaddition · epoxides ·
gold · nitrones
Chem. Eur. J. 2011, 17, 86 – 90
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89

J. Zhang and T. Wang
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[16] The enantioenriched substrate 1b’ was prepared from the corre-
sponding 2-(1-alkynyl)-2-alken-1-one according to the procedure
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À
[15] For reviews of C C bond activations, see: a) W. D. Jones, Nature
Received: August 19, 2010
Published online: December 1, 2010
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90
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