cis-selective single-cleavage skeletal rearrangement of 1,6-enynes reveals the multifaceted character of the intermediates in metal-catalyzed cycloisomerizations

Article abstract of DOI:10.1002/anie.200803269

A skeleton in the closet: The unprecedented title rearrangement of 1,6-enynes has been observed with gold and platinum catalysts (see scheme, [M] = metal catalyst, Z = C(CO₂Me)₂, R = electron-donating group). This reaction is propose

Full text of DOI:10.1002/anie.200803269

Communications
DOI: 10.1002/anie.200803269
Cycloisomerization Reactions
cis-Selective Single-Cleavage Skeletal Rearrangement of 1,6-Enynes
Reveals the Multifaceted Character of the Intermediates in Metal-
Catalyzed Cycloisomerizations**
Eloísa JimØnez-Nfflæez, Christelle K. Claverie, Christophe Bour, Diego J. Cµrdenas, and
Antonio M. Echavarren*
Skeletal rearrangements which are catalyzed by electrophilic
metals are the most emblematic transformations of enynes.[1,2]
For 1,6-enynes 1 two main types of products, 2 (single exo-
cleavage) and 3 (double exo-cleavage), were initially identi-
fied (Scheme 1).[3–10] A third type of product 4 (single endo-
cleavage) was found when using AuI,[11] InCl3,[4e,f] FeIII,[11b] or
RuII [12] as the catalysts. The factors that control the selectivity
in this rearrangement manifold are not yet clearly under-
stood.
The single exo-cleavage rearrangement is superficially
similar to the metathesis of enynes,[13] although these reac-
tions are very different.[14] For AuI, the rearrangement was
proposed to proceed via intermediates 5 (Scheme 1)[15] by a
mechanism that is consistent with previous work.[3–6] This
mechanism also explains the stereospecificity of this reaction,
as observed with AuI and other metal catalysts.[2,11] On the
other hand, the double exo-cleavage skeletal rearrangement
usually leads to diene 3 with a predominant[2–4,11] or exclusive
Z configuration.[16] For AuI, formation of 3 was proposed to
proceed by evolution of 5 to form a new rearranged carbene 6,
which undergoes proton loss and protodemetalation.[15]
Trapping of intermediate 6 has been carried out with
olefins,[17] indole,[18] and carbonyl compounds.[19] Proton loss
from 6 can form a 1,4-diene in InCl3-catalyzed reactions of
substrate in which R’ is an alkyl group.[4f]
The Janus-like character of intermediates 5 has been
recently discussed, stressing their carbocationic nature.[1c,20]
Conventionally, these intermediates are often depicted as
cyclopropyl gold carbenes, although DFT calculations show
that these species have highly distorted structures that are
inbetween cyclopropyl gold carbenes and gold-stabilized
homoallylic carbocations.[11c,15,21] Intermediates of type 5 are
involved in other processes such as nucleophilic additions of
heteronucleophiles[1,2b,11,20,22,23] inter- and intramolecular
cyclopropanations,[24] and intramolecular [4+2] cycloaddi-
tions of arylalkynes with alkenes.[25] All of these processes
are stereospecific.[26]
If open cations such as 5’ are involved in the above-
mentioned reactions, then the question of stereospecificity
arises as bond rotation could occur prior to rearrangement.
Herein, we show that this is indeed the case for 1,6-enynes
(E)-1 bearing R groups at the alkene moiety that are electron-
donating, and which react non-stereospecifically with metal
catalysts. Interestingly, the single-cleavage skeletal rearrange-
ment of (E)- or (Z)-1 give dienes (Z)-2 in an unexpected cis-
selective process (Scheme 2).
Scheme 1. The three types of skeletal rearrangement of 1,6-enynes and
the key reaction intermediates.
The reaction of cyclopropylenyne (E)-7a proceeded non-
stereospecifically to give 8a as a mixture of E/Z isomers,
along with the product of endo-skeletal rearrangement 9a
(Table 1). Although (Z)-8a was formed as a minor product
with moderately active AuCl (Table 1, entry 1), counterintui-
tively, the use of the more electrophilic cationic AuI catalysts
provided (Z)-8a as the major product (Table 1, entries 2 and
[*] E. JimØnez-Nfflæez, Dr. C. K. Claverie, Dr. C. Bour,
Prof. A. M. Echavarren
Institute of Chemical Research of Catalonia (ICIQ)
Av. Països Catalans 16, 43007 Tarragona (Spain)
Fax: (+34)97-792-0225
E-mail: aechavarren@iciq.es
E. JimØnez-Nfflæez, Dr. D. J. Cµrdenas, Prof. A. M. Echavarren
Departamento de Química Orgµnica, Universidad Autónoma de
Madrid (UAM), Cantoblanco, 28049 Madrid (Spain)
[**] We thank the MEC (projects CTQ2007-60745/BQU; Consolider
Ingenio 2010 (grant no. CSD2006-0003); predoctoral fellowship to
E.J.-N.), the AGAUR (2005 SGR 00993), the ICIQ Foundation, and
the Centro de Computación Científica (UAM) for computation time.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803269.
Scheme 2. The cis-selective single-cleavage rearrangement of 1,6-
enynes (E)-and ( Z)-1.
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Angewandte
Chemie
Table 1: Metal-catalyzed skeletal rearrangement of cyclopropylenynes
(E)-7a,b.[a]
Table 2: Metal-catalyzed skeletal rearrangement of enynes (E)-14a–f.[a]
Boc=tert-butoxycarbonyl.
Entry
7
[M]
t
Yield (Z)-8/(E)-8/9
[min] [%]
1
2
3
4
(E)-7a AuCl
5
5
5
5
5
95
76
98
94
93
100
40
n.d.
n.d.
n.d.
0
11:83:6
66:7:27
81:9:10
34:26:40
88:6:6
ꢀ99:ꢁ1:ꢁ1
65:6:29
22:76:2
43:41:16
60:36:4
–
Entry
14
[M]
t [min]
Yield [%]
(Z)-15/(E)-15/16
(E)-7a [AuCl(PPh3)]/AgSbF6
(E)-7a [AuCl(oTol3P)]/AgSbF6
(E)-7a 10
(E)-7a 11a/AgSbF6
(E)-7a 11a/AgSbF6
(E)-7a 12/AgSbF6
(E)-7a PtCl4
(E)-7a GaCl3
(E)-7a InCl3
(E)-7a AgSbF6
(E)-7b 10
1
2
3
4
(E)-14a
(E)-14a
(E)-14a
(E)-14a
(E)-14b
(E)-14b
(E)-14b
(E)-14b
(E)-14b
(E)-14c
(E)-14c
(E)-14c
(E)-14c
(E)-14c
(E)-14d
(E)-14d
(E)-14d
(E)-14d
(E)-14e
(E)-14e
(E)-14 f
AuCl
10
11b
PtCl4
AuCl
10
11b
PtCl4
13
AuCl
10
11b
13
PtCl4
AuCl
10
11b
PtCl4
10
210
40
20
50
n.d.
100
85
29
70
98
80
100
88
88
94
100
97
100
83
86
100
n.d.
96
2:72:26
4:17:79
4:33:63
11:79:10
90:10:0
99:0:1
5
6[b]
7
5
5
210
240
120
20
180
180
360
10
960
270
180
180
10
5
6[b]
7
8
9
10
11
12
240
180
960
240
5
90:5:5
8
9
70:30:0
88:10:2
92:5:3
94:0:6
85:3:12
93:7:0
60:40:0
93:7:0
86:5:9
74:9:17
53:44:17
75:7:16
96:4:0
96
ꢀ99:ꢁ1:ꢁ1
10
11
12
13
14
15
16
17
18
19
20
21
[a] Reaction conditions: catalyst (2 mol%) or AuCl (7 mol%), and PtCl4
(5 mol%) in CH2Cl2, at room temperature. [b] Reaction carried out at
À208C. Mes=mesityl, n.d.=not determined, Tol=tolyl.
15
180
20
90
90
13
10
89
82:18:0
[a] Reaction conditions: catalyst (2 mol%) or AuCl (7 mol%), and PtCl4
(5 mol%) in CH2Cl2, at room temperature. [b] Reaction carried out at
À208C. n.d.=not determined.
3). This outcome is in contrast to the results reported for the
reaction of the ethyl ester analogue of (E)-7a with an IrI
catalyst, which gave only the E diene.[4e] The best yields of
(Z)-8a were obtained using [AuCl(oTol3P)] (Table 1, entry 3)
or 11a (Table 1, entries 5 and 6). Reactions of (E)-7a with
PtCl4, GaCl3, or InCl3 also gave substantial amounts of (Z)-
8a, although these transformations were slower (Table 1,
entries 8–10). No reaction was observed with AgSbF6
(Table 1, entry 11). In contrast to results observed in the
reaction of enyne (E)-7a with catalyst 10, enyne (E)-7b
reacted very cleanly to exclusively afford (Z)-8b in excellent
yield (Table 1, compare entries 4 and 12). Deuterium labeling
experiments confirmed that the Z configured products are the
result of a single-cleavage skeletal rearrangement.[27a] In
addition, (E)-8a does not undergo isomerization in the
presence of 10 (2 mol%) in CD2Cl2.
The formation of Z dienes was also observed with
substrates (E)-14a–f bearing electron-rich aryl substituents
at the alkene group (Table 2).[27b] Interestingly, in contrast to
the stereospecific reaction of enyne (E)-14g (Ar= Ph),[4c,11b,28]
the reaction of (E)-14a also gave (Z)-15a, along with the
expected (E)-15a (Table 2, entries 1–4). Substrates 14b–f
selectively gave (Z)-15b–f in good yields with either gold or
platinum catalysts (Table 2, entries 5–21).[29] In general, the
best results were obtained with cationic gold catalysts 10 or
11b, although in the case of enynes (E)-14b–d, the less
electrophilic AuCl also led to (Z)-15b–d as major isomers
(Table 2, entries 5, 10, and 15).
The reaction of (Z)-7a with catalyst 10 gave (Z)-8a in
96% yield (Scheme 3). Only traces of (E)-8a and 9 were
detected in this reaction. Similarly, (Z)-14b led cleanly to (Z)-
15b (84%–89% yield) with catalysts 11b or 13. The reaction
of (Z)-14b with catalyst 13 (5 mol%) also led cleanly to (Z)-
15b (89% yield). In contrast to the cis-selective rearrange-
ment observed for (E)-7a and (E)-7b, cyclopentyl-substituted
enyne (E)-17 was treated with catalyst 10 to exclusively give
(E)-18 (92% yield).
These results for enynes bearing electron-donating groups
at the alkene moiety are consistent with the formation of open
carbocations that undergo facile bond rotation prior to the
rearrangement. According to DFT calculations, for cationic
gold intermediates 5a (R = H) and 5b (R = Me), carbocation
5’’ is the more relevant canonical structure, whereas for 5d
(R = c-C3H5) and 5e (R = p-MeOC6H4) the structure actually
resembles that of 5’ (Table 3). In contrast, neutral intermedi-
ate 5c shows a more regular structure resembling 5 with
elongated b and c bonds.
It is interesting to compare the high barrier of rotation
around bond d of the neutral intermediate 5c (L = ClÀ) with
that of cationic complex 5d (Table 3), which correlates
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Table 4: Gold(I)-catalyzed alkoxycyclization of enynes (E)-7a and
(E)-14b,f,g.
Entry
Enyne
Solvent
t [min]
Product
anti/syn
(yield [%])
1
2
3
4
5
6
(E)-7a
MeOH
MeOH
5
60
60
5
5
60
19a (86)
100:0
100:0
60:40
100:0
52:48
100:0
(E)-14b
(E)-14b
(E)-14 f
(E)-14 f
(E)-14g
19b (100)
19b (95)
Scheme 3. Gold(I)-catalyzed skeletal rearrangement of enynes (Z)-7a,
(Z)-14b, and (E)-17. Z=C(CO2Me)2.
CH2Cl2/MeOH[a]
EtOH
19c (98)[b]
19c (82)[b]
19d (84)[b]
CH2Cl2/EtOH[a]
CH2Cl2/MeOH[a]
qualitatively with the barrier observed in the reaction of (E)-
7a with AuCl (preferential retention of the E configuration)
and cationic AuI complexes (preferential inversion). These
theoretical results support the hypothesis which suggests that
cationic intermediates with strongly electron-donating R
groups, such as c-C3H5 and p-MeOC6H4, are open carbocat-
ions 5’, which can undergo bond rotation.[30] The origin of the
high selectivity observed for the formation of the Z isomers
might result from the higher reactivity of the Z rotamers of
intermediates 5 in the single-cleavage rearrangement. Nota-
bly, in all cases, products of endocyclic rearrangement 9a, 9b,
and 16a–e were obtained as single stereoisomers, which
indicates that these dienes arise from cleavage of bond b in
intermediates 5 prior to bond rotation.
To support the hypothesis that bond rotation of carbocat-
ions 5’ causes the lack of stereospecificity in these reactions,
we carried out the reaction of (E)-7a, (E)-14b, and (E)-14 f
with catalyst 10 (Table 4). The alkoxycyclizations proceeded
stereospecifically to give adducts 19a–c in good yields when
the reactions were carried out in pure MeOH or EtOH
(Table 4, entries 1, 2, and 4),[31] which is in keeping with the
[a] Reaction conditions: CH2Cl2 with MeOH (5 equiv) or EtOH (5 equiv).
[b] Traces of skeletal rearrangement products were also observed.
entries 3 and 5). Interestingly, under these reaction condi-
tions, enyne (E)-14g reacted stereospecifically to provide
anti-19d exclusively[11b] (Table 4, entry 5). On the other hand,
when enynes (E)-20a,b were treated with cationic gold(I)
catalysts 10 or 11b they gave 21a,b as trans/cis mixtures of
stereoisomers (Table 5), thus indicating that bond rotation of
the carbocationic intermediate is faster than attack by the
phenyl or p-nitrophenyl groups. This result is in contrast with
all other examples of [4+2] cycloadditions that are catalyzed
by gold for substrates bearing other substituents at the alkene
group.[25]
In summary, the cis-selective single-cleavage rearrange-
ment of enynes has revealed an unrecognized aspect of gold
intermediates in cycloisomerization and related reactions of
enynes. In general, reactions of 1,6-enynes with electrophilic
metal catalysts can be interpreted as stereospecific additions
of electrophiles (the h2-alkyne-metal complex) to alkenes. For
enynes containing alkenes that bear strongly electron-donat-
ing substituents these reactions are non-stereospecific, and
proceed through open carbocations of the type 5’. Remark-
general behavior observed by other 1,6-enynes in similar
[11a,b]
reactions catalyzed by gold
or platinum;[9] however,
when the concentration of the nucleophile was decreased,
anti/syn mixtures of stereoisomers were obtained (Table 4,
Table 3: Calculated bond distances and barriers of rotation for inter-
mediates 5.[a] L=ligand.
Table 5: Intramolecular [4+2] cycloaddition of enynes (E)-20a,b.
R
5
L
a [Š]
b [Š]
c [Š]
DE°
[kcalmolÀ1 [b]
]
H[c]
5a
5b
5c
5d
5e
PH3
PH3
ClÀ
PH3
PH3
1.378
1.372
1.401
1.356
1.344
1.742
1.720
1.621
1.586
1.578
1.569
1.622
1.606
1.987
2.328
–
–
Me[d]
Entry
20
[Au]
t
Yield [%]
trans-21/cis-21
c-C3H5
c-C3H5
p-MeOC6H4
28.1 (14.7)
11.3 (8.8)
8.1 (7.1)
1[a]
2[b]
3[a]
4[b]
(E)-20a
(E)-20b
(E)-20b
(E)-20b
10
10
10
11b
6 h
8 min
1 h
87
85
60
82:12
67:33
72:28
46:54
[a] DFT calculations at the B3LYP/6-31G(d) (C,H,P), LANL2DZ (Au)
level. Electronic energies corrected with zero point energy (ZPE).
[b] Barriers of rotation around the d bond. Values in parentheses include
the effect of solvent (CH2Cl2, PCM). PCM=polarizable continuum
model. [c] Reference [11c]. [d] Reference [15].
8 min
100
[a] Reaction conditions: catalyst (2 mol%) in CHCl3, at room temper-
ature. [b] Reaction conditions: catalyst (3 mol%) in CH2Cl2, microwave
irradiation, 808C.
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Chemie
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Keywords: carbocations · enynes · gold · metal carbenes ·
rearrangement
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Angew. Chem. Int. Ed. 2008, 47, 7892 –7895
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7895