Cationic gold(I)-catalyzed intermolecular [4+2] cycloaddition between dienes and allenyl ethers

Article abstract of DOI:10.1002/adsc.201000597

Simple allenyl ethers have been shown to be efficiently activated by cationic gold(I) catalysts to form reactive dienophiles in intermolecular Diels-Alder reactions. A range of different dienes give the cycloaddition products in moderate to excellent yields and good selectivity.

Full text of DOI:10.1002/adsc.201000597

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DOI: 10.1002/adsc.201000597
Cationic Gold(I)-Catalyzed Intermolecular [4+2] Cycloaddition
between Dienes and Allenyl Ethers
Guan Wang,a Yue Zou,a,b Zhiming Li,a Quanrui Wang,a,* and Andreas Goekeb,*
a
Department of Chemistry, Fudan University, Shanghai 200433, Peopleꢀs Republic of China
Fax : (+86)-216-564-1740; e-mail: qrwang@fudan.edu.cn
Givaudan Fragrances (Shanghai) Ltd, 298 Li Shi Zhen Road, Shanghai 200433, Peopleꢀs Republic of China
b
Fax : (+86) 215-080-0269; e-mail: andreas.goeke@givaudan.com
Received: November 3, 2010; Revised: December 24, 2010; Published online: March 2, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000597.
Abstract: Simple allenyl ethers have been shown to
be efficiently activated by cationic gold(I) catalysts
to form reactive dienophiles in intermolecular
Diels–Alder reactions. A range of different dienes
give the cycloaddition products in moderate to ex-
cellent yields and good selectivity.
pressive methods for the rapid assembly of novel
cyclic and polycyclic structures. However, relatively
few gold-catalyzed intermolecular cycloaddition reac-
tions have been reported.[2] In connection with our
ongoing program of investigating novel syntheses of
functionalized odorants, we became interested in de-
veloping efficient and selective Au-catalyzed transfor-
mations to provide cyclohexene derivatives which
constitute an important class of substructures in fra-
grance chemistry.[3] In this light we considered the
possibility of adopting allenes as dienophiles in Diels–
Alder reactions: they are known to be excellent sub-
strates for gold-promoted transformations[1] and de-
Keywords: allenyl ethers; cycloaddition; homogene-
ous gold catalysis
Over the past decade, homogeneous gold catalysis has rivatives are easily accessible. Although [4+2]cyclo-
become a powerful tool to access unprecedented addition reactions of allenes substituted with elec-
structural and mechanistic diversity.[1] In particular, tron-withdrawing group (EWG) have been extensive-
remarkable advances have been achieved in the de- ly studied,[4] the poor reactivity of unactivated allenyl
velopment of Au-catalyzed intramolecular cyclizations ethers as dienophiles has for a long time limited their
or cycloisomerizations that, in turn, have provided im- use in Diels–Alder reactions.[5] However, recent re-
Scheme 1. Ligand-controlled Au(I)-catalyzed [2+2], [4+2] and [4+3]cycloaddtion reactions of allene-dienes.
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Cationic Gold(I)-Catalyzed Intermolecular [4+2] Cycloaddition between Dienes and Allenyl Ethers
ports show that Pt and Au complexes induce simple tions with diene 3 in a standard [p4s +p2s] mode. The
allene-dienes I to undergo intramolecular [2+2], [4+ resulting Au-substituted oxonium intermediate
4
2] and [4+3]cycloaddition reactions (Scheme 1).[6]
A
could then undergo an elimination of a cationic Au(I)
subsequently performed DFT calculation suggested species to provide the desired six-membered carbocy-
that both the [4+2] and [4+3]cycloadditions proceed cles 5.
through an initial gold-catalyzed concerted [4+3]cy-
Herein, we report a novel intermolecular [4+2]cy-
cloaddition of a gold-activated allene with a diene to cloaddition of allenyl ethers with dienes. This Au(I)-
a carbene intermediate like III.[7] The selectivity for catalyzed ionic Diels–Alder strategy shows different
either pathway depends on a preference for either selectivities compared to thermal reactions and pro-
1,2-H or 1,2-alkyl shifts that the gold-stabilized inter- vides a convenient way to synthesize 4-(alkoxy-
mediate III experiences. Such a mechanistic specula- methylidene)cyclohex-1-enes.
tion was partially validated by the generation of cy-
We initially chose the cationic complex
cloheptenone VI in the presence of an excess amount [(Ph3P)Au]SbF6 in CH2Cl2 as solvent for preliminary
of diphenyl sulfoxide which can efficiently oxidize studies of the above proposal (Table 1). Treatment of
gold-carbene intermediates.[6a,8]
a mixture of the allenyl ether 1a and 1,3-cyclopenta-
Furthermore, it has been known for a long time diene (3a) with 1 mol% [(Ph3P)Au]SbF6 (A) afforded
that acetals of acrolein are convenient precursors of the desired [4+2]cycloaddition adduct 5a in 76%
allylidene oxonium cations, which have been success- yield after 3 min at room temperature, with the (Z)-
fully employed as dienophiles in ionic Diels–Alder re- isomer highly predominating (Table 1, entry 1). Low-
actions.[9] In 2007, Zhang established an intramolecu- ering the reaction temperature to 08C neither im-
lar [3+2]cycloaddition of allenyl MOM ethers and proved the yield nor the Z/E selectivity. In fact, inferi-
enones/enals towards a cyclopentanone enol ether
containing an all-carbon quaternary center. In the ra-
tionale, a selective AuACTHNUTRGNEU(GN III) activation of the allenyl
Table 1. Optimization of reaction conditions.[a]
ether moiety to form an oxocarbenium species was
proposed.[10] A recent work by Iwasawa and co-work-
ers has also described a novel Pt(II)-catalyzed [3+
2]cycloaddition reaction of silyl propadienyl ethers
and alkenyl ethers. Again, a p-allyl cationic inter-
mediate was postulated to account for the reactivity
of the allenes as a three-carbon unit.[11] These results
encouraged us to study the possibility of using allenyl
ethers as precursors of ionic dienophiles in Au-pro-
moted intermolecular [4+2]cycloaddition reac-
tions.[12] Our mechanistic working hypothesis
(Scheme 2) consisted of an initial Au(I)-activation of
the allenyl ether 1 to form a cationic gold-stabilized
allylidene oxonium dienophile 2, which is in reso-
nance with allyl cation 2’. We speculated that species
2 would participate in Diels–Alder cycloaddition reac-
Entry Catalyst
(mol%)
Solvent Time Yield [%][b]
(Z:E)[c]
1
A (1)
A (1)
A (1)
A (1)
A (1)
A (1)
B (1)
C (1)
–
CH2Cl2 3 min 76 (90:10)
CH2Cl2 30 min 60 (85:15)
Toluene 30 min 45 (80:20)
CH3NO2 3 min 60 (91:9)
CH3CN 3 min 48 (88:12)
2[d]
3
4
5
6
7
8
9
THF
5 min 30 (89:11)
CH2Cl2 3 d
CH2Cl2 2 d
CH2Cl2 2 d
CH2Cl2 2 d
CH2Cl2 3 d
CH2Cl2 3 d
CH2Cl2 3 d
33 (52:48)[e]
0[f]
0[e]
10
11
12
13
14
–
65 (22:78)[g]
<5[e]
D (5)
E (5)
F (5)
G (5)
<5[e]
<5[e]
CH2Cl2 30 min 11 (20:80)
[a]
Experimental conditions: a mixture of 1a (1.0 mmol), 3a
(3.0 mmol) and a catalyst (0.01 mmol) in 2 mL CH2Cl2 in
a sealed tube was stirred for the time stated.
Isolated yield of 5a.
[b]
[c]
[d]
[e]
[f]
Z:E ratios were determined by GC and 1H NMR.
Performed at 08C.
Starting material was recovered.
Decomposition.
Performed at 858C in an autoclave.
Scheme 2. Postulated mechanaism of an Au(I)-catalyzed
ionic Diels–Alder reaction of dienes with allenyl ethers.
[g]
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Table 2. Reactions of ethers 1b–g with cyclopentadiene.[a]
or results were obtained (Table 1, entry 2). In the sol-
vent screen, we found that dichloromethane per-
formed better in terms of the product yield than any
of the other solvents tested (Table 1, entries 3–6).
Other control experiments showed that the counter
anions of the gold complexes play a vital role regard-
ing the speed of the transformation. For example, em-
ploying 1 mol% [(Ph3P)Au]Cl resulted in a yield of
only 33% after 3 days (Table 1, entry 7).
Entry Allene R
Product Yield [%][b] (Z:E)[c]
Since the cationic complex [(Ph3P)Au]SbF6 was
generated from [(Ph3P)Au]Cl and AgSbF6 in CH2Cl2,
a control experiment using 1 mol% AgSbF6 as the
sole catalyst was performed to exclude the possibility
of silver catalysis. In this case, only decomposition of
the substrate allene was observed (Table 1, entry 8). It
is notable that, without catalyst, no conversion oc-
curred even after 2 days at room temperature
(Table 1, entry 9). However, in the absence of any cat-
alyst, the desired adduct 5a was also formed in a ther-
mal reaction in 65% yield after 2 days at 858C. Inter-
estingly, the (E)-isomer was formed as the major
isomer under these conditions (Table 1, entry 10).
Other common Lewis acids showed insufficient cata-
lytic activity (Table 1, entries 11–13). When the same
reaction was conducted in the presence of 5 mol% of
platinum(II) chloride, only an 11% yield of the prod-
uct was achieved after 30 min, leading to the predomi-
nant formation of the (E)-isomer 5a (Table 1,
entry 14).
1
2
3
1b
1c
1d
allyl
5b
5c
5d
75 (88:12)
77 (84:16)
73 (77:23)
Me
4
1e
5e
72 (99:1)[d]
5
6
1f
1g
t-Bu
adamantyl
5f
5g
68 (86:14)
62 (82:18)
[a]
Experimental conditions: a mixture of 1b–g (1.0 mmol),
3a (3.0 mmol) and catalyst A (0.01 mmol) in 2 mL
CH2Cl2 in a sealed tube was stirred for 30 min.
Isolated yield of 5b–g.
[b]
[c]
[d]
Z:E ratios were determined by GC and 1H NMR.
dr=1:1.
Table 3. Reaction of allenyl ethers 1 with substituted cyclo-
pentadienes.[a]
The scope and limitations of this cycloaddition
were then investigated using cyclopentadiene 3a and
different allenyl ethers 1b–g in the presence of
1 mol% of the cationic gold(I) complex A in CH2Cl2.
The results are shown in Table 2.
Interestingly, the electronic and steric properties of
the substituents R of the allenyl ether 1 have little in-
fluence on the selectivities and yields of bicyclic prod-
ucts 5. In general, the reactions proceeded smoothly
with good yields to give exclusively products 5b–g
without detection of any [2+2] and [4+3]products.
Notably, (ꢀ)-menthyl-substituted allene 1e resulted in
product 5e with an outstanding Z-selectivity of the
exo-double bond. The diastereoselectivity with regard
to the configuration of the bicyclic moiety was poor
(Table 2, entry 4, dr=1:1).
To further explore the reactivities of allenyl ethers
with cyclopentadienes, several mono-substituted cy-
clopentadienes and allenyl ethers were treated with
Au(I) catalyst A under standard reaction conditions.
Excellent yields were achieved with such reactive
dienes and conversions were complete within 3 min
Entry R1
R2
Product (ortho:para)[b] Yield [%][c]
1
2
3
4
5
6
Bn
Me
Me 5ho:5hp (35:65)
Me 5io:5ip (42:58)
96
95
95
90
88
89
3-MMB Me 5jo:5jp (40:60)
Bn
Me
i-Pr 5ko:5kp (44:56)
i-Pr 5lo:5lp (47:53)
3-MMB i-Pr 5mo:5mp (39:61)
[a]
Experimental conditions: a mixture of 1 (1.0 mmol), 3
(3.0 mmol) and catalyst A (0.01 mmol) in 2 mL CH2Cl2
in a sealed tube was stirred for 3 min.
[b]
[c]
Z:E ratios were determined by GC and 1H NMR.
Isolated yield of 5o and 5p.
(Table 3, entries 1–6). It is noteworthy that for mono- substitution usually comprises the bridgehead posi-
substituted cyclopentadienes only ortho- and para-re- tion.[13]
gioisomers were formed with complete Z-selectivity
We also extended this gold-catalyzed [4+2]process
at the exo-cyclic double bond. These ortho:para ratios to acyclic dienes (Table 4). With 2,3-dimethlybuta-
were much better than those obtained in classical diene as substrate, the reaction resulted in only a
Lewis acid-catalyzed Diels–Alder reactions in which 13% yield of 5n after 20 min (Table 4, entry 1). A pro-
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Cationic Gold(I)-Catalyzed Intermolecular [4+2] Cycloaddition between Dienes and Allenyl Ethers
Table 4. Reaction of allenyl ethers 1 with acyclic dienes.[a]
Entry
Product
Time (min)
Yield [%][b] (Z:E)[c]
1
2
3
4
5
5n: R1 =Bn, R2 =R3 =Me, R4 =H
20
12
12
3
13 (99:1)
56 (99:1)
41 (99:1)
45 (99:1)
36 (99:1)
5o: R1 =Bn, R3 =H, R4 =Me, R2 =4-methylpent-3-enyl
5p: R1 =allyl, R3 =H, R4 =Me, R2 =4-methylpent-3-enyl
5q: R1 =3-MMB, R3 =H, R4 =Me, R2 =4-methylpent-3-enyl
5r: R1 =Me, R3 =H, R4 =Me, R2 =4-methylpent-3-enyl
3
[a]
Experimental conditions: a mixture of 1 (1.0 mmol), 3 (3.0 mmol) and catalyst A (0.01 mmol) in 2 mL CH2Cl2 in a sealed
tube was stirred for the time stated.
[b]
[c]
Isolated yield of 5n–r.
Z:E ratios were determined by GC and 1H NMR.
longed reaction time led to decomposition of the
Although our results can be rationalized by the
allene. However, the more reactive homo-myrcene [4+2]mechanism depicted in Scheme 2, alternative
participated well in the reaction to provide the cyclic pathways through [4+3]cycloadditions of species 2’
adducts with nearly complete Z-selectivity, although with dienes may also be suggested. In one aspect the
the yields were generally low (Table 4, entries 2–5).
initially formed Au-carbenoid intermediates III’ could
Recent reports have demonstrated that, under cat- be formed by a concerted [4+3]cycloaddition mode
alysis of gold or platinum complexes, furans carrying followed by 1,2-alkyl shift and deauration leading to
an allene group can undergo intramolecular cycload- the observed [4+2]adducts (Scheme 4, route a). A
dition reactions to give a mixture of [4+3] and [4+ different possibility could be a stepwise reaction
2]products.[6,7] Therefore, we became interested in the (route b) via allylic cation VII. However, this is re-
behaviour of furan derivatives in the presence of al- garded to be the less likely option since vinylcyclobu-
lenyl ethers and cationic gold(I) catalyst A. Allenyl tanes 8, formed via 7 by the attack of the organome-
ether 1b was allowed to react with furan under our tallic anion at the allylic position in VII, have never
standard reaction conditions which furnished a Mu- been identified in our chemistry.
kaiyama–Micheal type 1,4-addition product 6a in
To differentiate the concerted [4+3] versus [4+
38% yield. The treatment of 2-methylfuran did not 2]pathways, a trapping experiment of the hypothetical
alter the reaction pattern, and 1,4-addition product 6b Au-carbenoid intermediates III’ was attempted
was obtained in a better yield (59%). These results (Scheme 5). Thus, when the reaction of substrates 1a
were comparable to those of AuCl3-catalyzed 1,4-ad- and 3a was performed in the presence of an excess
dition reactions between acroleins and furans amount of diphenyl sulfoxide, which has proved to be
(Scheme 3).[14] These results are in accord with pro- an efficient oxidant to intercept gold-carbene inter-
posed intermediate 2 (Scheme 2) as an equivalent of mediates in many gold-catalyzed transformations,[6a,8]
an activated acrolein derivative.
the formation of ketone 9 could not be detected
(Scheme 5). This suggests that the formation of the
six-membered ring compounds occurs via a favoured
[4+2]pathway. However, it could be still argued that
the [4+3]cycloaddition/1,2-rearrangement sequence
proceeds much faster than the oxidation of intermedi-
ate III’.
In order to shed light on this mechanistic conun-
drum, the Au(I)-catalyzed intermolecular [4+2]cyclo-
addition reaction between 1d and 1,3-butadiene with
PMe3 as the ancillary ligand was theoretically investi-
gated by means of DFT calculations (Scheme 6). Our
results indicate that [(Me3P)Au]+ firstly coordinates
to the terminal double bond of allene 1d to give
1dAu1, from which the Au-stabilized allylic cation
Scheme 3. Au(I)-catalyzed reaction of furan and allenyl
ether.
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Guan Wang et al.
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Scheme 4. The alternative ways leading to [4+2]products.
cation. These are evidences for the conjugation inter-
action of MeO group with the allyl cation system, in-
dicating that the structure 1dAu3 is a significant reso-
nance contributor.
1,3-Butadiene may react with 1dAu3 to deliver the
Au-coordinated Z- and E-products through two con-
certed transition state structures: 1dTSconcZ and
1dTSconcE, respectively (Figure 1). The barrier
height for the former one is 2.2 kcalmolꢀ1 which is
smaller than that for the latter (4.2 kcalmolꢀ1). These
data suggest that the reaction occurs resulting in the
preferential formation of the Z-product. This is just
what we observed experimentally.
Scheme 5. Trials to trap [4+3]Au-carbenoid intermediates.
According to our NBO calculation results, both
1dAu2 was formed with an activation free-energy bar- 1dTSconcZ and 1dTSconcE are clearly stabilized by
rier of 3.9 kcalmolꢀ1. Based on the NBO (natural the interaction between Au and C2 (the carbon atom
ꢀ
bond orbital) analysis, the C O bond length is short- connected with Au atom directly). On the other hand,
ened from 1.374 ꢂ in 1d to 1.302 ꢂ in the cation. although the 1dTSconcE is endo and 1dTSconcZ is
Consistently, the charge on the oxygen atom is also exo, the stabilization energy due to the second order
reduced from ꢀ0.52844e in 1d to ꢀ0.43860e in the perturbation (SOP) of LP*C3!BD*C27-C28 and BD
Scheme 6. Model reaction pathway.
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Cationic Gold(I)-Catalyzed Intermolecular [4+2] Cycloaddition between Dienes and Allenyl Ethers
Figure 1. DFT energy profile for [4+2]reaction of 1d/butadiene and Me2C=C=CH2 (1h)/butadiene initiated by Au(I) cata-
lyst. A: 1,3-butadiene.
C27-C28!LP*C3 in 1dTSconcZ is much higher than addition reaction of allenyl ethers and dienes mediat-
those in 1dTSconcE (2.67 kcalmolꢀ1), which stabilizes ed by a cationic Au(I) catalyst. The reactions are cat-
1dTSconcZ to a greater degree than the E counter- ionic in nature and the concept of Au-stabilized allyli-
part. This may be the main reason for the Z-product dene oxonium activation was applied to account for
predominance (for the numbering system and the the observed unusual reactivity of allenyl ethers. This
structures of the transition states, see Supporting In- methodology represents an unprecedented strategy to
formation).
access unique cyclohexene derivatives which have
The similar pathway for the Au(I)-catalyzed inter- great synthetic value.
molecular [4+2]cycloaddition reaction of 1,3-buta-
diene with dimethylallene 1h was also evaluated for
comparison. It has been found that the activation bar-
rier for 1hTSconc is considerable larger than that for
Experimental Section
the reaction of allenyl ether 1d. This difference in ac-
General Procedure for the Gold-Catalyzed Diels–
tivation energy predicts that the same reaction with
1h will occur less feasibly. According to the SOP
result, there is no similarly significant stabilization
effect originated from the interaction between Au
and C2 as in the 1dTSconcZ and 1dTSconcE. This
may be accounted for by the less electron-donating
property of methyl group than the methoxy group in
1d. The intermolecular character of our reactions and
the electron-donating alkoxy group to assist the for-
Alder Reaction of Allenyl Ethers and Dienes
To a solution of allenyl ether 1 (1.0 mmol) and diene 3
(3.0 mmol) in CH2Cl2 (10 mL) under argon atmosphere was
added catalyst [(Ph3P)Au]SbF6 (0.01 mmol, 1.0 mL of 0.01M
solution in CH2Cl2) at room temperature. The reaction mix-
ture was stirred at room temperature for the time displayed
in the tables. Triethylamine was then added to quench the
reaction and the reaction mixture was concentrated under
reduced pressure. The crude product was purified by kugel-
mation of oxonium cation 2 may be key factors dictat- rohr distillation or column chromatography (SiO2, hex-
AHCTUNGTRENNUNG
ing the reaction pathway.
We also located a stepwise pathway for the reaction
with allenyl ether 1d, but the free activation energy in
the rate-limiting step (7.6 and 7.4 kcalmolꢀ1 for Z and
E, respectively) is not favourable in comparison to
the concerted one. The concerted [4+3]process fol-
lowed by a 1,2-alkyl shift pathway is not located in
our case.
functional theory (DFT) level with the hybrid B3LYP func-
tional as implemented within the Gaussian09 package.[15]
For geometry optimizations, the main group elements (C, O,
P, H) were described using the 6–31G(d) basis set. The Au
metal centre was described by the SDD combination of rela-
tivistic effective core potentials and valence functions.[16]
Solvation effects (dichloromethane) were evaluated through
PCM calculations (with UFF atomic radii) on the gas-phase
In conclusion, we have developed a highly regiose-
lective and stereoselective intermolecular [4+2]cyclo- optimized geometries. Quoted energies are Gibbs free ener-
Adv. Synth. Catal. 2011, 353, 550 – 556
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555

Guan Wang et al.
COMMUNICATIONS
gies at 298 K and 1 atm. The energies we discussed are re-
ferred to the CH2Cl2 solution.
wa, K. Aso, M. Shiro, K. Kanematsu, J. Am. Chem.
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Acknowledgements
We are grateful to Dr. Gerhard Brunner for multi-dimension-
al NMR measurements.
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Adv. Synth. Catal. 2011, 353, 550 – 556