A Sequential Route to Cyclopentenes from 1,6-Enynes and Diazo Ketones through Gold and Rhodium Catalysis

Article abstract of DOI:10.1002/adsc.201600980

This work reports the construction of cyclopentene cores from 1,6-enynes and aryl diazo ketones through two new reaction sequences involving initial gold-catalyzed cyclization of 1,6-enynes with diazo species, followed by rhodium-catalyzed skeletal rearrangement of the resulting 3-cyclopropyl-2-en-1-ones. In most instances the rhodium-catalyzed reactions afforded cyclopentene derivatives whereas several n-alkyl- or ortho-substituted phenyl ketones delivered seven-membered oxacycles. A plausible mechanism provides rationales for these two distinct products. (Figure presented.).

Full text of DOI:10.1002/adsc.201600980

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DOI: 10.1002/adsc.201600980
A Sequential Route to Cyclopentenes from 1,6-Enynes and Diazo
Ketones through Gold and Rhodium Catalysis
Balaji S. Kale,a Hsin-Fu Lee,a and Rai-Shung Liua,*
a
Department of Chemistry, National Tsing-Hua University, Hsinchu, Taiwan
E-mail: rsliu@mx.nthu.edu.tw
Received: September 5, 2016; Revised: November 8, 2016; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600980.
Abstract: This work reports the construction of cy-
clopentene cores from 1,6-enynes and aryl diazo ke-
tones through two new reaction sequences involving
initial gold-catalyzed cyclization of 1,6-enynes with
diazo species, followed by rhodium-catalyzed skele-
tal rearrangement of the resulting 3-cyclopropyl-2-
en-1-ones. In most instances the rhodium-catalyzed
reactions afforded cyclopentene derivatives whereas
several n-alkyl- or ortho-substituted phenyl ketones
delivered seven-membered oxacycles. A plausible
mechanism provides rationales for these two dis-
tinct products.
their Rh(I)-catalyzed skeletal rearrangement. In our
findings, most diazo ketones delivered [2+2+1]-cyclo-
adducts, but alkyl- or ortho-aryl-substituted diazo ke-
tones surprisingly produced seven-membered oxacy-
cles 5 from unexpected [2+2+3]-modes [Eq. (3)].
Keywords: cyclopropylgold carbenes; diazo ke-
tones; 1,6-enynes; formal [2+2+1]-cycloaddition;
gold; rhodium-catalyzed process
Metal-catalyzed [2+2+n]-cycloadditions of readily
available 1,n-enynes (n=6 or 7) with p-bond motifs
are powerful tools to access medium-sized carbo- and
heterocyclic frameworks.[1] Considerable success has
been achieved for catalytic [2+2+2]-cycloadditions of
1,n-enynes (n=6, 7) with p-bond species, but their
[2+2+1]-reactions remain largely unexplored.[2,3] Be-
sides the well-known Pauson–Khand reactions [Eq.
(1)],[3] isocyanides are the only applicable substrates
for these [2+2+1]-reaction modes.[4] These two cyclo-
additions were implemented commonly with low-
valent metal complexes including Ru(0), Co(I), Rh(I),
Notably, this cascade sequence comprises reactions
Ir(I), Ti(II) and Ni(0), as depicted in Eqs. (1) and of two new types. In the presence of gold catalysts,
(2).[3,4] Diazo species have been used as one-carbon 1,6-enynes are known to form cyclopropylgold car-
building units for several formal [n+1]-cycloadditions benes A; the trapping of these cations with common
(n=2–4),[5] but their formal [2+2+1]-reactions with C, N or O-based nucleophiles normally occurs at the
1,n-enynes remain unknown. Here, we report the de- cyclopropyl carbons,[6,7] with few organic oxides at-
velopment of formal [2+2+1]-cycloadditions of 1,6- tacking at gold carbenes [Eq. (4)].[8] We aimed to re-
enynes with diazo ketones involving two sequential alize the attack of diazo species at gold carbenes.[9,10]
steps; initial Au-catalyzed transformations of 1,6- Alkyl- or aryl-substituted cyclopropylalkenes can be
enynes into 3-cyclopropyl-2-en-1-ones 3, followed by induced by various Rh(I), Ni(0) or Pd(0) catalysts to
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form cyclopentene derivatives,[11] but we are aware of For IPrAuCl/AgNTf2, toluene and THF were inappli-
no example for cyclopropylvinyl esters or ketones cable solvents whereas dichloroethane (DCE) provid-
[Eq. (5)]. We envisaged that the strain of a bicyclic ed a satisfactory yield (85%) of species 3a (entries 6–
framework as in 3 [Eq. (3)] would promote the cleav- 8). In the absence of diazo species 2a, treatment of
age of its cyclopropane ring.
1,6-enyne 1a with IPrAuCl/AgNTf2 (5 mol%) in
DCM (208C) produced 3-styryl-2,5-dihydrofuran 1a’
in 81% yield very rapidly; the conversion was com-
plete in 5 min. This cycloisomerization was complete-
ly suppressed by diazo ketone 2a that seems to inter-
cept cyclopropylgold carbene A efficiently to produce
ther desired product 3a. This new process represents
a coupling of two carbene precursors.[12]
We assessed the scope of such gold-catalyzed reac-
tions using various 1,6-enynes and diazo ketones; the
results are summarized in Table 2. Most resulting al-
kenylcyclopropane derivatives 3 were present as Z/E
isomers. For various trans-styryl-derived 1,6-enynes
Table 1 shows our initial tests on the cyclizations of 1b–1d, their corresponding products 3b–3d were ob-
1,6-enyne 1a with phenyl diazomethyl ketone 2a to tained in 70–73% yields (entries 1–3). We tested also
yield the desired 3-cyclopropyl-2-enone 3a. With less the reactions on additional 1,6-enynes 1e–1g bearing
acidic IPrAuCl/AgNTf2, the reaction of the two reac- alterable cycloalkyl substituents [R, R=–(CH2)n–; n=
tants proceeded smoothly in dichloromethane (DCM, 3–5], affording expected products 3e–3g in good
208C) to give species 3a as a mixture of Z/E isomers yields (86–88%, entries 4–6). For additional phenyl-
(Z/E=1:2, entry 1); the yield was up to 92%. With substituted diazo ketones 2b–2c (R=4-XC6H4, X=
Ph3PAuCl/AgNTf2 and P(t-Bu)2(o-biphenyl)AuCl/ OMe, Ph), their corresponding products 3h and 3i
AgNTf2, we observed a rapid decomposition of diazo were produced smoothly (78–84%, entries 7 and 8).
species 2a to produce but-2-ene-1,4-dione. In these Such gold-catalyzed reactions were operable not only
two cases, the initial 1,6-enyne was recovered in 76– with alkyl ketone-substituted diazo species 2d–2f, but
81% because the liberated 1,4-diphenylbut-2-ene-1,4- also with heteroaryl ketone analogues 2g–2i, yielding
dione probably reduced the acidity of the gold cata- the desired products 3j–3o in good yields (84–94%)
lysts (entries 2 and 3). IPrAuCl/AgBF4 and IPrAuCl/ except for 2-furyl derivative 3n that was obtained in
AgSbF6 were also catalytically active to yield target 56% yield (entries 9–14). For ortho-substituted phenyl
3a in satisfactory yields (81 and 82%, entries 4 and 5). diazo ketones 2j–2l (X=Me, Cl, Br), their resulting
Table 1. Cyclizations of 1,6-enyne 1a with diazo ketone under Au catalysts.
Entry
[Au][a] (5 mol%)
Solvent
Time [h]
Compounds[b] (yield [%])
1
2
3
4
5
6
7
8
IPrAuCl/AgNTf2
Ph3PAuCl/AgNTf2
LAuCl/AgNTf2
IPrAuCl/AgBF4
IPrAuCl/AgSbF6
IPrAuCl/AgBF4
IPrAuCl/AgBF4
IPrAuCl/AgBF4
DCM
DCM
DCM
DCM
DCM
toluene
DCE
3
3a (92%, Z/E=1:2)
1a (76%)
1a (81%)
3a (82%, Z/E=1:2.8)
3a (81%, Z/E=1:2.2)
1a (76%)
24
24
1
2
10
1
3a (85%, Z/E=1:1.8)
complicated mixture
THF
16
[a]
[1a]=0.17M.
[b]
Product yields are obtained after purification from a silica column. IPr=1,3-bis(diisopropylphenyl)imidazol-2-ylidene,
L=P(t-Bu)2(o-biphenyl).
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Table 2. Scope of the gold-catalyzed reactions.[a,b]
[a]
[1]=0.17M.
[b]
Product yields are reported after separation from a silica column.
products 3p–3r were produced in 84–91% yields bicyclic framework to increase the cyclopropane
(entry 15–17).
strain, thus likely enabling the ring cleavage. Our ini-
Our next task was to achieve a skeletal rearrange- tial test with species 3a and Pd(PPh3)4 (10%) in hot o-
ment of vinylcyclopropanes to cyclopentenes[11] for xylene (1508C, 30 h) led to a complete decomposition
the preceding cyclopropylvinyl ketones 3. Although of starting 3a (Table 3, entry 1). The use of
cyclopropylvinyl esters or ketones are known to be in- [Rh(COD)Cl]2 (5 mol%) and [Rh(COD)Cl]2/dppe (5/
active for this arrangement, compound 3a has a small 10 mol%) only implemented the Z!E isomerization
Table 3. Screening of various catalysts.
Entries
Catalysts (x mol%)
Temp./time [oC]/[h]
Product yields [%][b]
3a
4a
E-3a
1
2
3
4
5
6
7
8
Pd(PPh3)4 (10)
[Rh(COD)Cl]2 (5)
[Rh(COD)Cl]2/dppe (5/10)
ClRh(PPh3)3 (10)
[Rh(CO)2Cl]2 (5)
[Rh(CO)2Cl]2 (5)
[Rh(COD)Cl]2/AgNTf2 (5/10)
ClRh(PPh3)3/AgNTf2 (5/10)
150/30
150/24
150/30
150/30
150/22
120/24
150/24
150/24
–
–
–
60
–
–
–
–
–
–
–
–
82
–
–
–
86
85
–
–
84
83
72
–
[a]
[3a]=0.10M.
[b]
Product yields are reported after separation from a silica column. COD=cyclooctadiene, dppe=1,2-bis(diphenylphosphi-
no)ethane.
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of initial 3a whereas RhCl(PPh3)3 showed no chemical
reactivity at all (entries 2–4). To our pleasure,
[RhCl(CO)2]2 enabled an efficient transformation of
species 3a into the desired bicyclopentene species 4a
in hot toluene (1508C) as a single diastereomer (dr>
20:1); the corresponding yield was up to 82%
(entry 5). At 1208C, [RhCl(CO)2]2 was active only in
the Z!E isomerization of initial 3a (entry 6). Ac-
cordingly, this Rh(I)-catalyzed ring expansion in-
volves a prior Z!E isomerization of species 3a to these substrates provided bicyclic pentenes 4 efficient-
attain a high stereoselectivity. We tested other cation- ly whereas a few instances afforded seven-membered
ic Rh(I) catalysts including [RhCl(COD)]2/AgNTf2 oxacyclic products 5 instead. We first tested these
and ClRh(PPh3)3/AgNTf2, but they led only to the Rh(I)-catalyzed reactions with various phenyl-substi-
Z!E isomerization (entries 7 and 8). Weakly acidic tuted cyclopropyl species 3b–3d (Ar=4-XC6H4, X=F,
“ClRh(CO)2” seems to be appropriate for this ring Cl and Me), yielding the desired 4b–4d in 60–62%
expansion. The molecular structure of compound 4a yields (entries 1–3). For cycloalkyl-substituted sub-
was inferred from that of its 4-chlorophenyl analogue strates 3e–3g [R, R=–(CH2)n–, n=3–5], their result-
4c (see Table 4, entry 2).[13]
ing products 4e–4g were produced with 78–82%
We tested a one-pot reaction using 1,6-enyne 1a yields (entries 4–6). For varied phenyl ketone sub-
and diazo ester 2a through initial treatment with strates 3h and 3i (R1 =4-XC6H4, X=OMe and Ph),
IPrAuCl/AgNTf2 in DCE for 3 h, followed by treat- their resulting products 4h and 4i were obtained in
ment with a two-fold excess of an o-xylene solution 72–74% yields (entries 7 and 8). Interestingly, a dis-
containing [Rh(CO)2Cl]2 (5 mol%); this mixture was tinct chemoselectivity was observed for n-alkyl
heated in a sealed tube at 1508C for 30 h to afford ketone analogues 3j–3l, which afforded seven-mem-
the desired 4a in 51% yield which is less than the bered oxacyclic species 5j, 5k and 5l in 65–85% yields,
combined yield (75%) of the two-step reaction [Eq. respectively (entries 9–11). We tested the reactions on
(6)].
heteroaryl ketone analogues 3m–3o (R1 =2- or 3-
Inspired by the success of this ring expansion, we thienyl and 2-furyl), yielding the desired bicyclopen-
tested the applicability of cyclopropylvinyl ketones tenes 4m–4o in 62–84% yields, respectively (en-
3b–3p with [Rh(CO)2Cl]2 (5 mol%) in hot o-xylene; tries 12–14). For ortho-substituted phenyl ketone de-
the results are depicted in Table 4. In most instances, rivatives 3p–3r, their Rh(I)-catalyzed reactions afford-
Table 4. Rh-catalyzed ring expansions.[a,b]
[a]
[1]=0.10M.
[b]
Product yields are reported after separation from a silica column.
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Scheme 1. Reactions of diazo esters and other 1,6-enynes
ed seven-membered oxacyclic products 5p–5r in 58– cessful examples highlight the difficulty of the ring-ex-
84% yields, regardless of the types of X substituents pansion of cyclopropylvinyl esters or ketones. This re-
(entries 15–17).
action sequence failed to work with all-carbon 1,6-
Various types of 1,6-enynes and diazocarbonyl spe- enynes bearing C(CO2Et)2 as a bridging unit because
cies greatly affected the efficiency of these Rh-cata- their first reaction with diazo ketone was unsuccess-
lyzed reactions (Scheme 1). Gold-catalyzed reactions ful.
of 1,6-enyne 1a with diazo ester 2m yielded trans-cy-
Scheme 2 rationalizes the formation of two distinct
clopropylvinyl ester 3s in 87% yield, but its subse- products, including cyclopentene derivatives 4 and
quent Rh(I)-catalyzed reaction was unsuccessful in seven-membered oxacycles 5, depending on the type
hot toluene over a long period (1508C, 30 h). Similar- of ketone. Before a ring expansion, ClRh(CO)2 en-
ly, cyclopropylvinyl ketones 3t and 3u generated from
1,6-enynes 1h and 1i failed to undergo Rh(I)-cata- envisage that acidic RhCl(CO)2 can coordinate with
lyzed rearrangement in hot o-xylene. These unsuc- the E-configured enones at the ketone or alkene
G
Scheme 2. Rationales for two distinct products.
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2H), 6.74 (d, J=12.0 Hz, 1H), 5.91 (d, J=12.0 Hz, 1H),
4.11 (dd, J=8.1, 2.7 Hz, 1H), 3.87 (d, J=8.4 Hz, 1H), 2.39
(d, J=4.2 Hz, 1H), 2.25 ~2.24 (m, 1H), 1.45 (s, 3H), 1.31 (s,
3H); 13C NMR (150 MHz, CDCl3): d=191.6, 137.8, 137.6,
137.2, 132.9, 131.4, 128.6, 128.4, 127.8, 125.7, 82.9, 67.0, 44.6,
31.3, 30.4, 24.4, 23.5; ESI-MS: m/z=319.1695, calcd. for
C22H23O2 [M+H]: 319.1698.
moiety, as in species A and A’. This process facilitates
a cyclopropane cleavage to form key intermediate B.
A 1,3-Rh shift of allyl species B generates six-mem-
bered carbocycles C containing C-bound Rh-enolates;
such enolates have 18-e configurations and their re-
ductive eliminations are expected to be slow. In con-
trast, their O-bound enolates D have 16-electron con-
figurations[14] that are favorable for a reductive elimi-
nation. Most substrates in Table 4 yield cyclopentene
derivatives 4 because the substrates bearing aryl or
heteroaryl ketones prefer a resonance structures C’;
this feature impedes the tautomerization between C-
and O-bound enolates.
This hypothesis is supported by ortho-phenyl ke-
tones 3p–3r (R=o-XC6H4, X=Me, Cl and Br, en-
tries 15–17, Table 4) that afford seven-membered oxa-
cyclic compounds 5p–5r; their phenyl rings and C=O
are not co-planar with the resonance form C being
the preferable form. For alkyl-substituted substrates
3j–3l, a tautomerization of their C!O enolates is
Standard Catalytic Procedure for Synthesis of
[(3aR,4R,5R)-1,1-Dimethyl-4-phenyl-3,3a,4,5-
tetrahydro-1H-cyclopenta[c]furan-5-yl](phenyl)-
methanone (4a)
A 20-mL flask was charged with rhodium(I) carbonyl chlo-
ride dimer (6.1 mg, 0.015 mmol), and to this was added a so-
lution of 3a (100 mg, 0.314 mmol) in o-xylene (3.0 mL). The
reaction mixture was stirred at 1508C for 22 h before it was
filtered through a silica bed and dried over MgSO4. The
crude product was purified by a silica column (10% EA/
hexane, Rf =0.5) to afford compound 4a as pale yellow oil;
yield: 82 mg (0.257 mmol, 82%). 1H NMR (600 MHz,
CDCl3): d=7.90–7.88 (m, 2H), 7.54 ~7.51 (m, 1H), 7.42–
very facile, rendering their seven-membered oxacycles 7.39 (m, 2H), 7.27–7.23 (m, 4H), 7.17–7.15 (m, 1H), 5.35
(dd, J=2.7, 1.7 Hz, 1H), 5.08 ~5.06 (m, 1H), 4.13 (t, J=
7.9 Hz, 1H), 3.98 (t, J=9.0 Hz, 1H), 3.83 ~3.81 (m, 1H),
3.54 (dd, J=9.7, 8.2 Hz, 1H), 1.36 (s, 3H), 1.32 (s, 3H);
13C NMR (150 MHz, CDCl3): d=198.3, 159.1, 141.7, 136.1,
133.2, 128.6, 128.5, 127.6, 126.6, 115.7, 76.4, 70.3, 67.1, 57.8,
52.1, 27.9, 26.7; ESI-MS: m/z=319.1696 calcd. For C22H23O2
[M+H]: 319.1698.
5j–5l highly productive.
In summary, we report the development of formal
[2+2+1]-cycloadditions of 1,6-enynes with aryl diazo-
methyl ketones[15,16] through two new consecutive re-
actions. Our strategy employs a gold catalyst to imple-
ment initial cyclizations of 1,6-enynes with aryl diazo-
methyl ketones to yield cyclopropylvinyl ketones.
Subsequent Rh-catalyzed ring expansions of these cy-
clopropylvinyl ketones produce cyclopentene deriva-
tives in most instances whereas several n-alkyl- or
ortho-substituted phenyl ketones afforded seven-
membered oxacycles. Rationales for these two distinct
products are provided in a postulated mechanism.
Acknowledgements
The Ministry of Science and Technology supported this work.
References
Experimental Section
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yield: 146.2 mg (0.459 mmol, 92%). 1H NMR (600 MHz,
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A Sequential Route to Cyclopentenes from 1,6-Enynes and
Diazo Ketones through Gold and Rhodium Catalysis
9
Adv. Synth. Catal. 2017, 359, 1 – 9
Balaji S. Kale, Hsin-Fu Lee, Rai-Shung Liu*
Adv. Synth. Catal. 0000, 000, 0 – 0
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