Gold-catalyzed reactions of 1,5- and 1,6-enynes with carbonyl compounds: cycloaddition vs. metathesis

Article abstract of DOI:10.1002/chem.200900668

Gold-catalyzed reactions of 1,5- and 1,6-enynes with carbonyl compounds was demonstrated. It was observed that 1,n-enynes (n = 5-7) achieved skeletal rearrangement in the presence of a variety of electrophilic metals as catalysts, where-as in the presence

Full text of DOI:10.1002/chem.200900668

DOI: 10.1002/chem.200900668
Gold-Catalyzed Reactions of 1,5- and 1,6-Enynes with Carbonyl Compounds:
Cycloaddition vs. Metathesis
Ana Escribano-Cuesta,[a] Verꢀnica Lꢀpez-Carrillo,[a] Dominic Janssen,[a] and
Antonio M. Echavarren*[a, b]
1,n-Enynes (n=5–7) undergo skeletal rearrangement in
the presence of
catalysts,[1] whereas in the presence of hetero- or carbo-
nucleophiles, addition products are formed.[2–7] Carbonyl
compounds also act as nucleophiles in intra-[8] and inter-
molecular reactions of 1,6-enynes.[9] In the latter case, 1,6-
enynes 1, unsubstituted at the terminal alkene (R1 =R2 =H),
a variety of electrophilic metals as
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
reacted with aldehydes to give tricyclic compounds
2
(Scheme 1).[9] Products 3 of formal [2+2+2] cycloaddition,
related to those formed in the intramolecular reaction,[8]
were not obtained in this reaction.
In the gold(I)-catalyzed reaction of 1,6-enynes with alde-
hydes, products 3 of [2+2+2] cycloaddition were indeed
formed. However, unexpectedly, 1,3-dienes 4 were the
major products in a transformation that corresponds to a
formal metathesis between the enyne and the aldehyde. 1,5-
Enynes 5 reacted to give cycloadducts 6 and/or 6’ by an
initial 5-endo-dig cyclization (Scheme 1). For 1,5-enynes
with aryl substituents at the alkyne, an unprecedented frag-
Scheme 1. Different pathways in the gold(I)-catalyzed reactions of 1,5-
and 1,6-enynes with aldehydes. [AuL]+ =gold catalyst.
ACHTUNGTRENNUNG
[2+2+2] cycloaddition were isolated from enyne 1a in 21–
85% yield, along with dienes 4a–h (Table 1, entries 1–18).
The reaction proceeded readily with electron-rich aldehydes,
whereas, in contrast with that shown before for 1,6-enynes 1
without substituents at the alkene,[9] no adduct could be ob-
tained in the reaction of 1a with o-nitrobenzaldehyde
(Table 1, entry 17). Interestingly, enynes 1b and 1c, with an
aryl substituent at the alkene, gave the 1,3-dienes 4 by inter-
molecular metathesis with the aldehydes (Table 1,
entries 19–21). Enynes 1d and 1e, which in the absence of
nucleophiles reacted by a 6-endo-dig pathway,[1a,2] reacted
the presence of AuCl (5 mol%) in CH2Cl2 gave exclusively
the product of skeletal rearrangement,[2a,b] whereas very low
conversions (ꢀ7%) were obtained when using PtCl2, PtCl4,
InCl3, or GaCl3 as catalysts. Better results were achieved
with cationic gold(I) catalysts 8–10 (Table 1). Products 3 of
[a] A. Escribano-Cuesta, V. Lꢀpez-Carrillo, Dr. D. Janssen,
Prof. A. M. Echavarren
Institute of Chemical Research of Catalonia (ICIQ)
Av. Paꢁsos Catalans 16, 43007 Tarragona (Spain)
Fax : (+34)977920225
by
a 5-exo-dig process to give dienes 4i–n (Table 1,
entries 22–27).
E-mail: aechavarren@iciq.es
Reactions were carried out routinely with two equivalents
of aldehyde. In the metathesis-like reactions of substrates
1a, 1d, and 1e, acetone was released, but did not efficiently
compete with the aldehydes R3CHO. Therefore, this reac-
tion is applicable to the ready synthesis of C1-substituted
1,3-dienes that would be difficult to prepare by other
[b] Prof. A. M. Echavarren
Departamento de Quꢂmica Orgꢃnica, Universidad Autꢀnoma de
Madrid (UAM)
Cantoblanco, 28049 Madrid (Spain)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900668.
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Chem. Eur. J. 2009, 15, 5646 – 5650

COMMUNICATION
Table 1. Gold-catalyzed reaction of 1,6-enynes with aldehydes.[a]
A proposed mechanism for the formation of 2 involved
the rearrangement of the cyclopropyl gold carbene 11 to
form 12,[10] which is then trapped by the carbonyl compound
to give 13 (Scheme 3).[9] A Prins cyclization would then
Entry
1
R3
ACHTUNGTNER[NUNG AuL]SbF6 3 (Yield [%]) 4 (Yield [%])
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
b
c
Ph
Ph
Ph
4-MeC6H4
8
9
10
8
9
8
9
8
9
8
9
10
8
9
8
9
8
8
8
8
8
8
8
10
10
10
10
3aa (29)[b]
3aa (21)[b]
3aa (35)[b]
3ab (22)
3ab (41)
3ac (41)
3ac (77)
3ad (53)
3ad (85)
3ae (30)
3ae (58)
3ae (25)
3af (50)
3af (63)
3ag (22)
3ag (39)
–
3ah (24)
–
–
–
–
–
–
–
–
4a (61)
4a (11)[c,d]
4a (25)[c,d]
4b (71)
4b (21)[c,d]
4c (59)[c]
4c (19)
4d (39)
4d (9)
4e (23)
4e (12)
4e (27)
4 f (34)[c]
4 f (29)
4g (76)
4g (57)
–
4h (70)
4d (70)
4b (41)
4d (81)
4i (65)
4j (63)
4k (91)
4 L (67)
4m (34)
4n (60)
4-MeC6H4
2,4-Me2C6H3
2,4-Me2C6H3
2,4,6-Me3C6H2
2,4,6-Me3C6H2
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
3,4-(MeO)2C6H3
3,4-(MeO)2C6H3
3,4,5-(MeO)3C6H2
3,4,5-(MeO)3C6H2
2-O2NC6H4
c-C3H5
2,4,6-Me3C6H2
4-MeC6H4
2,4,6-Me3C6H2
Ph
4-MeC6H4
2,4,6-Me3C6H2
4-BrC6H4
Scheme 3. Mechanisms for the intermolecular gold(I)-catalyzed reactions
of 1,6-enynes with aldehydes.
form cation 14, which could evolve by cyclopropane forma-
tion to give 2.[9] In the case of 1,6-enynes substituted at the
alkene, direct reaction of the aldehyde with 11 would lead
to oxonium cation 15, which could undergo a Prins cycliza-
tion to form tetrahydropyranyl cation 16.[8,11] Elimination of
the metal fragment would then give 3, whereas a fragmenta-
tion reaction would provide 4. Products derived from inter-
mediate 17, which would be formed in the intramolecular
Prins reaction occurring with the opposite regiochemistry,
were not detected.
A similar reaction of 1,5-enynes via gold carbenes 18
would give intermediates 19 (Scheme 4), which undergo a
Prins reaction to form 20. Intermediates 20 would then un-
dergo proton elimination and protodemetalation to give 6
or a 1,2-shift of R1 to yield 6’ via carbocations 21. Formation
of tetrahydrofurans by the Prins reaction occurs relatively
c
d
d
d
d
e
e
4-MeC6H4
2,4,6-Me3C6H2
–
[a] Reactions conditions: Aldehyde (2 equivalents) and [AuL]SbF6 (2
mol%) in CH2Cl2 at À408C, 12 h. [b] 9:1–1:1 mixture of 3aa and its
1
D[4a,5] isomer. [c] Yield determined by H NMR spectroscopy (1,3,5-trime-
thoxybenzene as standard). [d] Skeletal rearrangement product was also
formed (10–50% yield).
methods. Thus, for example, the reaction of 1,6-enyne 1d
with 1-pyrenecarboxaldehyde in the presence of catalyst 10
(2 mol%) at À408C gave pyrenyl diene 4o in 76% yield
(Scheme 2).
Scheme 2. Gold-catalyzed reaction of 1,6-enyne 1d with 1-pyrenecarbox-
aldehyde.
Scheme 4. Mechanism for the intermolecular gold(I)-catalyzed reactions
of 1,5-enynes with aldehydes.
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A. M. Echavarren et al.
rarely.[12] A Prins reaction of 19 with the alternative regio-
chemistry would afford 22,[8,11] which would fragment, re-
leasing acetone, to give rise to dienes 7.
Enyne 5a reacted with electron-rich 3,4,5-trimethoxyben-
zaldehyde in the presence of catalyst 10 to give an unexpect-
ed tricyclic product 23 in 47% yield, along with 6ae (13%
yield; Scheme 5). The structure of 23 was confirmed by X-
ray diffraction. A plausible mechanism for the formation of
23 involves trapping of the gold carbene in intermediate 24
to give 25 in a Friedel–Crafts process, followed by a proton-
promoted cleavage (Scheme 5).
Different types of intramolecular trapping of a gold car-
bene were found in the reaction of enyne 5h with aldehydes
in the presence of catalyst 10 (Scheme 6). Thus, in the reac-
tion of 5h with cyclopropanecarbaldehyde, in addition to
adduct 6ha (1.5:1 mixture of epimers, 41% yield), tricyclic
compound 26 was isolated in 30% yield. However, reaction
of 5h with mesitaldehyde gave 27 as a single stereoisomer in
43% yield. Tricyclic compounds 26 and 27 were formed by
1,5-Enyne 5a reacted with aldehydes to give adducts 6aa–
ai as the major products (Table 2, entries 1-13). In some
cases, the isomerization products 6’ were obtained in low
yield (Table 2, entries 1, 8, and 10). Interestingly, 1,5-enyne
5b afforded the products of 1,2-alkyl shift, 6’ba, in 75%
yield, along with the expected adduct 6ba as the minor
product (Table 2, entry 15). Reaction of 5a with ketones
also afforded mixtures of the corresponding adducts, 6aj/
6’aj and 6ak/6’ak (Table 2, entries 14 and 15).
Gold-catalyzed reactions of aryl-substituted 1,5-enynes
5c–g with carbonyl compounds afforded trienes 7 by a frag-
mentation process, via intermediate 22 (Scheme 4). The
most efficient transformations occurred when trans-cinna-
maldehyde was used as the carbonyl compound (Table 2,
entries 17-23). Compounds 7 were obtained as single E ster-
eoisomers, as established by NOESY experiments, which is
in accord with a fragmentation occurring via 22 with an
equatorial R3 substituent (Scheme 3).
À
formal insertions of gold carbenes 28a and b into C H
bonds of neighboring groups.[13]
In summary, a clearer picture of the reaction pathways
occurring in the gold(I)-catalyzed additions of carbonyl
compounds to enynes has emerged from this study, which
complements that leading to tricyclic compounds 2.[9] The
reaction of 1,6-enynes with al-
dehydes gave the expected
products of [2+2+2] cycloaddi-
Table 2. Gold-catalyzed reaction of 1,5-enynes with carbonyl compounds.[a]
tion. However, in most cases,
1,3-dienes 4 were also formed
by a metathesis-type reaction of
the enyne with the aldehyde.
This reaction proceeded by a
fragmentation of the tetrahy-
dropyranyl cations formed by
an intramolecular Prins reac-
tion. In contrast with the 5-exo-
dig cyclization of 1,6-enynes,
1,5-enynes reacted with carbon-
yl compounds to give adducts
by an initial 5-endo-dig cycliza-
tion. Fragmentation of the in-
termediates also occurred with
1,5-enynes in a few cases. Inter-
estingly, the intermediate cat-
ionic gold carbenes can also un-
Entry
5
R2, R3
[AuL]SbF6
t [h]
Product(s) (ratio,[b] yield [%])
1
2
3
4
5
6
7
8
9
10
11
12
13
14[c]
15[d]
16
17
18
19
20
21
22
23
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
b
c
c
c
d
e
f
2,4-Me2C6H3, H
2,4-Me2C6H3, H
2,4,6-Me3C6H2, H
2,4,6-Me3C6H2, H
2,4,6-Me3C6H2, H
4-BrC6H4, H
9
10
8
9
10
9
9
9
10
9
9
10
9
9
9
9
8
9
10
9
8
2.5
30
4
4
4
2.5
1
1
6aa (7.5:1; 79)+6’aa (15)
6aa (7:1; 81)
6ab (7:1; 74)
6ab (95)
6ab (85)
6ac (1:1, 78)
6ad (1:1, 62)
6ae (1:1; 59)+6’ae (9)
6af (2.7:1, 66)
6ag (2.5:1; 51)+6’ag (7)
6ah (1.5:1, 72)
6ah (2:1, 59)
6ai (3:1, 58)
6aj+6’aj (1:1, 55)
6ak+6’ak (1:2, 67)[e]
6ba (2:1)+6’ba (1:4.7; 75)
7c (75)
7c (50)
7c (75)
7d (80)
7e (79)
7 f (67)
7g (78)
3-BrC6H4, H
2-BrC6H4, H
3,4,5-(MeO)3C6H2, H
2-Naph, H
(E)-CH=CHPh, H
(E)-CH=CHPh, H
CH=CMe2, H
7
2.5
0.5
7
0.5
32
20
2.5
1
1
2
1
1
À
dergo formal C H insertion or
electrophilic aromatic substitu-
tion to form complex carbo-
CD3, CD3
(CH2)5
A
a stereo-
(E)-CH=CHPh, H
(E)-CH=CHPh, H
(E)-CH=CHPh, H
(E)-CH=CHPh, H
(E)-CH=CHPh, H
(E)-CH=CHPh, H
(E)-CH=CHPh, H
(E)-CH=CHPh, H
AHCTUNGTRENNUNG
Experimental Section
9
9
2
1
General procedure for the reaction of
1,6-enynes with aldehydes (Table 1): A
solution of 1,6-enyne (80 mg) and the
corresponding aldehyde (2 equiva-
lents) in CH2Cl2 (ca. 0.1m) was cooled
to À408C and after 15 min the gold
g
[a] Reaction conditions, unless otherwise stated: Catalysts (5 mol%) with aldehyde (2 equivalents) in CH2Cl2
at 238C. [c] Ratios for 6aa–ba refer to epimers at C1. [d] Reaction with [D6]acetone (4 equivalents). [e] Re-
action with cyclohexanone (4 equivalents) and catalyst 9 (3 mol%). [f] Cycloisomerization products were also
obtained (27%). Naph=naphthyl.
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Gold-Catalyzed Reactions of 1,5- and 1,6-Enynes with Carbonyl Compounds
COMMUNICATION
complex 8, 9, or 10 (2 mol%) was added The solution was kept at À408C
for 12 h. A 0.1m solution of Et3N in hexane (1 mL) was added to deacti-
vate the gold catalyst and the solution was filtered through Celite. After
evaporation of the solvent, the residue was purified by flash chromatog-
raphy on silica gel (hexane/EtOAc eluent).
General procedure for the reaction of 1,5-enynes with carbonyl com-
pounds (Table 2): The starting 1,5-enyne was dried before the reaction by
repeated evaporation of a solution of the compound in toluene (1 mL/
10 mg of 1,5-enyne) under vacuum. A solution of the 1,5-enyne (30 mg)
and the corresponding aldehyde (1.5–2 equivalents) or ketone (4 equiva-
lents) in CH2Cl2 (1 mL) was slowly added at room temperature to a solu-
tion of the gold complex 9 or 10 in CH2Cl2 (0.5 mL). The reaction mix-
ture was stirred for the time stated in Table 2 (monitored by thin-layer
chromatography). Work-up was carried out as described above.
See the Supporting Information for details and spectroscopic data on the
compounds.
Acknowledgements
This work was supported by the MEC (projects CTQ2004-02869, Consol-
ider Ingenio 2010 grant CSD2006-0003, predoctoral fellowships to A. E.-
C and V. L.-C., and Torres Quevedo contract to D. J.), the AGAUR
(2005 SGR 00993), and the ICIQ Foundation. We also thank Eloꢂsa Jimꢅ-
nez-NfflÇez for helpful insights and Dr. J. Benet-Buchholz and E. Escu-
dero-Adꢃn (X-ray diffraction unit, ICIQ) for the structure of 23.
Scheme 5. Gold-catalyzed reaction of 1,5-enyne 5a with 3,4,5-trimethoxy-
benzaldehyde to give tricyclic compound 23.
Keywords: aldehydes · cycloaddition · enynes · gold ·
homogeneous catalysis
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Received: March 13, 2009
Published online: April 22, 2009
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