Gold(I)-Catalyzed Intramolecular Carbon-Oxygen Bond Cleavage Reaction via Gold Carbenes Derived from Vinylidenecyclopropanes

Article abstract of DOI:10.1002/adsc.201600483

Under gold(I) catalysis, the in situ generated gold carbene from vinylidenecyclopropanes bearing a methoxymethyl group can undergo an intramolecular nucleophilic attack to give the carbon-oxygen bond cleavage product in good yield within 15 min. and different phosphine ligands could control both Z- and E-configurations, respectively. The reaction mechanism has been clarified on the basis of deuterium- and¹⁸O-labeling experiments. This method provides a new and efficient approach to stereoselectively synthesize alkylidenecyclobutanones under mild conditions. (Figure presented.).

Full text of DOI:10.1002/adsc.201600483

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DOI: 10.1002/adsc.201600483
Gold(I)-Catalyzed Intramolecular Carbon-Oxygen Bond Cleavage
Reaction via Gold Carbenes Derived from Vinylidenecyclopro-
panes
De-Yao Li,a Yin Wei,a and Min Shia,*
a
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 345 Lingling Lu, Shanghai 200032, Peopleꢀs Republic of China
Fax : (+86)-21–6416-6128; e-mail: mshi@mail.sioc.ac.cn
Received: May 6, 2016; Revised: June 27, 2016; Published online: && &&, 0000
Supporting information for this article can be found under: http://dx.doi.org/10.1002/adsc.201600483.
Abstract: Under gold(I) catalysis, the in situ gener-
ated gold carbene from vinylidenecyclopropanes
bearing a methoxymethyl group can undergo an in-
tramolecular nucleophilic attack to give the carbon-
oxygen bond cleavage product in good yield within
15 min. and different phosphine ligands could con-
trol both Z- and E-configurations, respectively. The
reaction mechanism has been clarified on the basis
of deuterium- and 18O-labeling experiments. This
method provides a new and efficient approach to
stereoselectively synthesize alkylidenecyclobutan-
ACHTUNGTRENNUNGones under mild conditions.
Keywords: carbon-oxygen bond cleavage; gold car-
benes; nucleophilic attack; vinylidenecyclopropanes
Gold carbene chemistry is an important research
topic in the field of homogeneous gold catalysis.[1] As
a kind of Fisher-type carbene, accepting nucleophilic
attack is the very nature of the gold carbene.[2]
Among gold carbenes generated from different pro-
cesses,[3] their electrophilic properties have been stud-
ied extensively in comparison to the other reactivities
such as insertion and addition reactions.[4] Echavarren
et al. reported that the gold carbene derived from
a 1,6-enyne can react with 3rd position of indole to de-
liver the corresponding nucleophilic addition pro-
ducts;[4c] Moreover, in 2014, his group also discovered
that the gold carbene generated from a retro-Buchner
reaction can accept an intramolecular attack of
a phenyl group to give the 9H-fluorene[3k] (Scheme 1).
In 2009, Tosteꢀs group disclosed that the gold carbene
center generated from pivalate migration can undergo
intramolecular allyloxy attack and further allyl-trans-
fer to produce benzopyran derivatives[3i] (Scheme 1).
Scheme 1. Selected nucleophilic reaction types of in situ gen-
Coming to the gold carbenes generated from an N- erated gold carbenes.
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oxide with an alkyne, similar reaction processes can age takes place at a different site as compared with
take place, and the in situ generated gold carbenes Yeꢀs work.
can be trapped by intramolecular hydroxy groups or
Next, we moved to a screening of the reaction con-
intermolecular carboxylic acids, or even allylsulfur ditions and found that when 10 mol% JohnPhosAuCl/
species can react with the gold carbene to deliver a- AgNTf2 was used as the catalyst, product 3a,[8] the Z
thiocarbonyl compounds[4e,f,m] (Scheme 1).
isomer of 2a, can be obtained in 52% yield. The Z/E
Compared to the above results, our initial studies configurations have been confirmed by NOESY cor-
have established a brand new pathway for generating relations (Scheme 3, also see the Supporting Informa-
the gold carbene from vinylidenecyclopropanes tion for the details).
(VDCPs),[5] however, their electrophilic properties
are still vague. Therefore, we have attempted to inves-
tigate the electrophilic properties of this new gold car-
bene species and aimed at expanding their reaction
mode. In this paper, we wish to report our new find-
ings.
Inspired by Yeꢀs work,[6] we designed and synthe-
sized a new VDCP, which is substituted by a methoxy-
methyl group at the ortho position of the benzene
Scheme 3. Different NOESY correlations between CH3 and
group, compound 1a. At beginning, we proposed that CH2 in compounds 2a and 3a.
the oxygen atom can act as the nucleophile to attack
the generated gold carbene species during the reac-
tion, giving the cyclized product. However, as shown
Upon optimizing the reaction conditions, we first
in Scheme 2, a-methylenecyclobutanone 2a is fixed the silver salt as AgNTf2 and examined gold cat-
formed[7] via cleavage of the methoxymethyl group in alysts with different ligands in the presence of
the presence of 10 mol% IPrAuCl/AgNTf2 in 1,2- 1.0 equiv. of water as additive. IPrAuCl was excellent
À
DCE for 15 minutes. Obviously, the C O bond cleav- for generating product 2a without formation of 3a.
JohnPhosAuCl, JakiephosAuCl and t-BuXphosAuCl
gave 3a in moderate yields and could not suppress the
formation of 2a (Table 1, entries 2, 3 and 5). The use
of PPh3AuCl afforded 3a in 25% yield without forma-
tion of 2a (Table 1, entry 6). A favorable turn was ob-
served when BrettPhosAuCl was utilized as the cata-
lyst, giving 3a in 79% yield along with 2a in 6% yield
(Table 1, entry 4). After screening the gold catalysts,
Ag salts and NaBArF were also examined in this re-
action. The results indicated that AgNTf2 was the best
one for the formation of both 2a and 3a (Table 1, en-
tries 7–14). Thus, we identified that IPrAuCl/AgNTf2
was the best catalyst system for the production of 2a
and BrettPhosAuCl/AgNTf2 for 3a.
With the best catalyst systems for both products in
hand, we next examined the influence of additives
and solvents in this reaction. Since water is required
in this transformation, different kinds of alcohols
were added to promote the possible hydrolysis. As for
the formation of 2a, adding 1.0 equiv. of MeOH in-
creased the yield of 2a to 80% (Table 2, entry 1). For
3a, the addition of 1.0 equiv. of t-BuOH improved the
yield of 3a to 82% (Table 2, entry 10). Solvent screen-
ing demonstrated that DCM is the best solvent for
obtaining 2a, and 1,2-DCE is best suited for the for-
mation of 3a (Table 2, entries 1–14). Thus, we identi-
fied that with conditions A 2a could be obtained in
highest yield in the presence of 10 mol% IPrAuCl/
AgNTf2, 1.0 equiv. of H2O and 1.0 equiv. of MeOH as
additives in DCM at room temperature for 15
Scheme 2. Different reaction sites between Yeꢀs work and
ours.
minutes. With conditions B 3a could be obtained
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Table 1. Au catalyst and Ag salt screening for the reaction of 1a afford 2a and/or 3a.
[a]
Reaction conditions unless specified otherwise: 1a (0.10 mmol, 1.0 equiv), Au cat.
(0.01 mmol, 0.10 equiv.), Ag salt (0.01 mmol, 0.10 equiv.), H2O (0.10 mmol,
1.0 equiv), 0.1M in 1,2-DCE, room temperature for 15 min.
Isolated yield.
[b]
in highest yield in the presence of 10 mol% comes. As for the 6-methyl substituted substrate 1l,
BrettPhosAuCl/AgNTf2, 1.0 equiv. of H2O and the reaction failed to give the desired product proba-
1.0 equiv. of t-BuOH as additives in DCE at room bly due to the steric hindrance.
temperature for 15 minutes.
Similar reactions were conducted using conditions
Under the optimized conditions, we surveyed the B and the corresponding products 3 were obtained in
substrate scope of this reaction and the results are good to excellent yields (Table 4). Firstly, substrates
shown in Table 3. When R2 is H, we first synthesized bearing Me, Et, Pr, Bu, Bn, 4-FBn at R1 were subject-
the alkyl-substituted VDCPs. As for substrates 1a–1d, ed to the optimized reaction conditions. The substitut-
1f, 1g and 1i, the reactions all proceeded smoothly to ed a-methylenecyclobutanones 3a–3d and 3f, 3g could
furnish the corresponding products 2 in the yields be obtained in 64–88% yields. In the case of 1e, the
ranging from 51% to 88%. When fluorine was intro- intramolecular Friedel–Crafts reaction took place to
duced at the para position of the benzyl group, the give 3e similarly. Changing the R3 group to Et also af-
yield decreased probably due to the electronic proper- forded 3h in 81% yield. Different linear alkyl groups
ties of the substituent. In the case of 1e in which R1 = such as 3-methylbutanyl and homoallylic group substi-
Ph, the reaction underwent another different pathway tuted VDCPs underwent the reaction smoothly,
to give 2e in 70% yield through an intramolecular giving the desired products in good yields. Electron-
Friedel–Crafts reaction without gold carbene genera- donating or electron-withdrawing groups on the ben-
tion.[9] This result also occurred under conditions B. zene ring of substrates 1 did not have any effects on
On changing the R3 group to Et, the reaction also the reaction outcome. Only the substrate bearing
proceeded efficiently to give 2h in 80% yield (the a methyl group at the 6-position of the benzene ring
product is the same as 2a). Next, we surveyed the sub- did not give the desired product due to the steric
stituents on the benzene ring: regardless of whether effect.
electron-donating or electron-withdrawing groups
The following control experiments were performed
were introduced, the reactions went through smoothly to gain more insights into the reaction mechanism.
to afford the desired products 2j, 2k, 2m and 2n in First, as judged by the reaction outcomes, ambient
yields ranging from 72–91%, indicating that electronic water was involved in this reaction. A control experi-
effects had no obvious impact on the reaction out- ment with participation of D2O was conducted.
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Table 2. Solvent and additive screening for the reaction of 1a to afford 2a and/or 3a.
[a]
Reaction conditions: 1a (0.10 mmol, 1.0 equiv), Au cat. (0.01 mmol, 0.10 equiv.),
AgNTf2 (0.01 mmol, 0.10 equiv.), H2O (0.10 mmol, 1.0 equiv.), alcohol (0.10 mmol,
1.0 equiv.), 0.1M in 1,2-DCE, room temperature for 15 min.
Isolated yield.
[b]
Table 3. Substrate scope for the reaction of 1 to 2.[a,b]
[a]
[b]
Reaction conditions: 1 (0.30 mmol, 1.0 equiv.), IPrAuCl (0.03 mmol, 0.1 equiv.), AgNTf2 (0.03 mmol, 0.1 equiv.), H2O
(0.30 mmol, 1.0 equiv.), MeOH (0.30 mmol, 1.0 equiv.), 0.1M in DCM, room temperature, 15 min.
Isolated yield.
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Table 4. Substrate scope for the reaction of 1 to 3.[a,b]
[a]
Reaction conditions: 1 (0.30 mmol, 1.0 equiv.), BrettPhosAuCl (0.03 mmol, 0.1 equiv.), AgNTf2 (0.03 mmol, 0.1 equiv.),
H2O (0.30 mmol, 1 equiv.), t-BuOH (0.30 mmol, 1 equiv.), 0.1M in 1,2-DCE, room temperature, 15 min.
Isolated yield.
[b]
Under the optimized conditions A, adding 1.0 equiv. 1.0 equiv. of H218O was added into the reaction
of D2O gave D-2a in 66% yield along with deuterium system still under conditions A. We found that the re-
incorporation of >90% [Scheme 4, Eq. (1)]. This action proceeded smoothly to give the 18O labeled
result illustrated that ambient water indeed takes part product 18O-2a in 71% yield along with 89% 18O in-
into this reaction through a proton shift. The oxygen corporation on the basis of LR-MS analysis
source in the carbonyl group has been also examined. [Scheme 4, Eq. (2)]. To make the conclusion more
Considering that water participated in this reaction, convincing, the 18O labeled starting material 18O-1a
with 53% 18O content was synthesized through eight
steps (see the Supporting Information for the details).
The reactions using 18O-1a as substrate were per-
formed both under conditions A and B. The results
indicated that the corresponding products 2a and 3a
were formed in 80% and 77% yields, respectively
without 18O incorporation [Scheme 4, Eq. (3)]. These
Scheme 4. Isotope labeling experiments.
Scheme 5. Control experiments.
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experimental results unambiguously proved that 18O gold catalysis has been proposed in Scheme 6. Taking
comes from the water phase rather than the intramo- 3a as an example, the allenic motif in VDCP 1a
lecular methoxymethyl group.
is activated by an Au+ species generated from
Next, we conducted several control experiments to BrettPhosAuCl and AgNTf2 to produce gold carbene
explain the Z/E selectivity of the reaction. As shown species C through intermediates A and B.[5] Then the
1
in Scheme 5, on the basis of H NMR spectroscopic intramolecular nucleophilic attack of the methoxy-
analysis, we found that both 3d (Z-configuration) and methyl group to the carbene center produces the
À
2d (E-configuration) are stable upon heating at 608C seven-membered oxonium intermediate D. The C O
within 15 minutes. Furthermore, under conditions A bond is then cleaved along with dearomatization to
or conditions B, 3d and 2d could not be transformed give intermediate E (path A). On the other hand, an
each other, suggesting that the thermodynamic or ki- alternative reaction mechanism is also reasonable to
netic factors did not account for the Z/E selectivity.
account for the formation of intermediate E. The
Based on the above control experiments and previ- allene moiety is activated by gold(I) complex (inter-
ously reported literature results,[6] a plausible mecha- mediate A’), and then the nucleophilic attack of the
nism for the formation of 2 and 3 from VDCP under oxygen atom onto the terminal position gives inter-
Scheme 6. A plausible reaction mechanism.
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mediate B’. The resulting 7-membered ring then pro- Acknowledgements
duces the corresponding a-aurated species C’ through
This work was supported by the Joint NSFC-ISF Research
ring expansion of the cyclopropane along with cleav-
age of the benzylic C O bond. Elimination of Au
+
Program, jointly funded by the National Natural Science
Foundation of China and the Israel Science Foundation. We
are also grateful for the financial support from the National
Basic Research Program of China [(973)-2015CB856603],
and the National Natural Science Foundation of China
(20472096, 21372241, 21361140350, 20672127, 21421091,
21372250, 21121062, 21302203, 20732008, 21572052).
À
can also give intermediate E (path B). Upon hydroly-
sis and keto-enol tautomerization, cyclobutanone in-
termediate G is formed. Au+L acts as a Lewis acid as
well to promote the aromatization along with 1,5-H
transfer to deliver the product 3a. We speculated that
steric bulkiness of ligands in the gold(I) catalyst could
control the Z/E configuration to give product 2 or 3.
However, at present stage, we could not gather any
experimental evidence to exclude path A or path B.
Further DFT calculations on the reaction mechanism
may be required to identify the exact reaction path-
way.
In summary, we have developed a novel carbon-
oxygen bond cleavage process based on a new gold
carbene generation process using VDCP as a staring
material. This work not only disclosed the new reac-
tivity of gold carbene species but also extended the
reaction mode of the new gold carbene generated
from VDCP. Moreover, different reaction sites as
compared to Yeꢀs work have been observed and
a new method for the synthesis of alkylidenecyclobu-
tanones has been also established, subsequently en-
riching the gold carbene chemistry.
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Experimental Section
General Procedure for Synthesis of 2 and 3
2a for example: To a flame-dried, argon-purged Schlenk
tube were added IPrAuCl (23.1 mg, 0.03 mmol) and Ag salt
(11.4 mg, 0.03 mmol). Then DCM (2 mL) was added to gen-
erate a white suspension in which AgCl was generated.
After 5 min, MeOH (12 mL, 0.03 mmol) and H2O (5.4 mL,
0.03 mmol) were added and compound 1a (0.3 mmol) dis-
solved in DCM (1 mL) was added into the reaction tube.
The reaction was carried out at room temperature for 15 mi-
nutes. After that, the reaction mixture was filtered through
a thin layer of celite. The organic phase was concentrated
under reduced pressure and the residue was purified by
a silica gel flash column chromatography with petroleum
ether as an eluent to afford compound 2a as a light yellow
oil; yield: 15.9 mg (84%). 1H NMR (CDCl3, 400 MHz,
TMS): d=2.27 (s, 3H, CH3), 2.31–2.35 (m, 5H, CH3, CH2),
2.87–2.90 (m, 2H, CH2), 7.05–7.07 (m, 1H, Ar), 7.18–7.23
(m, 3H, Ar); 13C NMR (CDCl3, 100 MHz, TMS): d=19.3,
21.1, 21.2, 43.3, 125.9, 126.8, 127.7, 130.4, 134.1, 139.9, 143.8,
143.9, 200.3; IR (neat): n=3015, 2931, 2853, 1734, 1655,
1600, 1487, 1433, 1393, 1295, 1184, 1107, 757 cmÀ1; HR-MS
(EI): m/z=186.1047, calcd. for (C13H14O)+: 186.1045.
Supporting Information
Detailed descriptions of experimental procedures and the
spectroscopic data of the products as well as their crystal
structures are presented in the Supporting Information.
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Gold(I)-Catalyzed Intramolecular Carbon-Oxygen Bond
Cleavage Reaction via Gold Carbenes Derived from
Vinylidenecyclopropanes
9
Adv. Synth. Catal. 2016, 358, 1 – 9
De-Yao Li, Yin Wei, Min Shi*
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