Oxypalladation Initiating the Oxid-ative Heck Reaction with Alkenyl -Alcohols: Synthesis of Isocoumarin–Alkanones

Article abstract of DOI:10.1002/ejoc.201501451

Highly regioselective nucleopalladation for the oxidative coupling of internal alkynes with alkenyl alcohols by using green and low-costing oxygen as the sole oxidant was investigated. This one-pot cascade cyclization proceeds through Pd-catalyzed intramolecular C–O bond cyclization, insertion of nonbiased alkenyl alcohols, β-H elimination, and reinsertion of a HPdX species, which is finally transferred to the target ketones. This method has the advantages of mild conditions, good functional group tolerance, and can be performed with unactivated alkenes to afford isocoumarin derivatives.

Full text of DOI:10.1002/ejoc.201501451

DOI: 10.1002/ejoc.201501451
Communication
Oxidative Heck Coupling
Oxypalladation Initiating the Oxidative Heck Reaction with
Alkenyl Alcohols: Synthesis of Isocoumarin–Alkanones
Meifang Zheng,[a] Liangbin Huang,[a] Qizhen Tong,[a] Wanqing Wu,[a] and
Huanfeng Jiang*[a]
Abstract: Highly regioselective nucleopalladation for the oxid- nonbiased alkenyl alcohols, ꢀ-H elimination, and reinsertion of
ative coupling of internal alkynes with alkenyl alcohols by using a HPdX species, which is finally transferred to the target ket-
green and low-costing oxygen as the sole oxidant was investi- ones. This method has the advantages of mild conditions, good
gated. This one-pot cascade cyclization proceeds through Pd- functional group tolerance, and can be performed with unacti-
catalyzed intramolecular C–O bond cyclization, insertion of vated alkenes to afford isocoumarin derivatives.
Introduction
Palladium-catalyzed cascade reactions have emerged as valua-
ble tools in modern organic synthesis, as they help to construct
well-defined complex synthetic targets with simple starting ma-
terials in a one-pot operation with high efficiency.[1] The elec-
trophilic activation of alkynes by coordination with a palladium
catalyst continues to be a fascinating arena because of the high
efficiency with which multiple new chemical bonds can be con-
structed.[2] These transformations are generally initiated by
electrophiles or nucleophiles such as amines,[3] carbo nucleo-
philes,[4] halides,[5] and carbonates;[6] this leads to nucleopalla-
dation to afford the key vinyl–Pd species, which undergo the
Heck reaction. For example, Larock et al. made a great contribu-
tion to C≡ C bond directed oxypalladation transformations.[7]
However, challenges still remain in the control of chemoselect-
ivity (5-, 6-, exo, endo, etc. cyclization), limitation of alkenes
bearing electron-withdrawing groups to capture the vinylpalla-
dium intermediates generated from the nucleopalladation reac-
tion of alkynes (owing to the inert reactivity of unactivated ole-
fins and the selectivity of ꢀ-H elimination)[8] (Scheme 1, a), and
Scheme 1. Nucleopalladation triggering oxidative Heck reactions.
the use of an equivalent amount of extra oxidants.[9] Thus, the
development of oxidative cross-coupling reactions between in-
we wanted to broaden the application of allylic alcohols, which
ternal alkynes and nonbiased alkenes initiated by oxypallada-
are abundant in nature and have important synthetic applica-
tion to construct oxygen-containing motifs by using green and
tions. Up to now, synthetic applications of allylic alcohols have
cheap oxidant (e.g., oxygen, air, etc.) as the sole oxidant is al-
been classified into three typical synthetic methods: decoration
ways in high demand.
of C=C bonds,[12] nucleophilic substitution of –OH groups,[13]
Since the initial reports of the Mizoroki–Heck reaction over
and use as ꢀ-carbonyl alkylating agents.[14] Our continued inter-
est in allylic alcohols[15] prompted us to develop a convenient
and novel protocol for Heck-type C–C bond formation with
these unactivated alkenes (Scheme 1, b). On the basis of the
precedent of palladium-catalyzed bromoalkylation of alkyno-
ates via a vinylpalladium complex with allylic alcohols,[15a] we
reasoned that using an intramolecular ester moiety as a nucleo-
phile might provide an avenue for a palladium-catalyzed tan-
dem reaction with alkenyl alcohols to access a range of iso-
coumarin carbonyls (Scheme 1, c).
40 years ago,[10] the palladium-catalyzed coupling of olefins and
organohalides has become one of the most effective strategies
for the generation of new carbon–carbon bonds.[11] Therefore,
[a] School of Chemistry and Chemical Engineering, South China University of
Technology,
Guangzhou, P. R. China
E-mail: jianghf@scut.edu.cn
http://www2.scut.edu.cn/cee/lshxyjzx/index.html
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201501451.
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Oxygen-containing heterocyclic compounds are significant aryl-substituted benzoate was also transformed into the ex-
subunits in numerous natural products.[16] Among them, iso- pected ketone product in good yield (see product 3ua).
coumarin skeletons represent one of the most significant
Table 1. Scope of the internal alkynes.[a]
heterocycles, as they are widespread in natural products, phar-
maceuticals, and agrochemicals.[17] For example, cytogenin
(Scheme 2, c) is used as an oral antitumor drug, and its mecha-
nism of antiangiogenic action contributes to its suppressive ef-
fects on both tumor growth and rheumatoid arthritis.[18c] Con-
sidering the large spectrum of fascinating applications, the de-
velopment of methods for direct and efficient access to highly
substituted and specifically functionalized frameworks is always
of great interest in modern synthetic organic chemistry.[19]
Scheme 2. Examples for widely prevailing ꢀ-aryl carbonyl and isocoumarin
compounds.
Results and Discussion
Next, we investigated the scope of this transformation under
optimized conditions. First, we tested different alkynyl termini,
and the results are listed in Table 1. As shown, the optimal
conditions were applied successfully to various 2-alkynyl-
benzoates, and the corresponding products were formed in
good to excellent yields. The data illustrates the use of both
alkyl-substituted alkynes (see products 3aa–ha and 3va–xa)
and aryl-substituted alkynes (see products 3ia–ua) as efficient
precursors and good tolerance to electron-withdrawing and
electron-donating groups on the aryl groups. The tested carb-
oxylic acid, ethyl acrylate, and tert-butyl acrylate underwent the
nucleopalladation smoothly to afford 3aa in good to excellent
yields. Furthermore, a chloro-functionalized alkyne substrate
was also examined, and corresponding product 3fa was iso-
lated in 84 % yield. Moreover, the substrate containing a cyano
group was successfully transformed into 3ga in 50 % yield,
which greatly expanded the scope of the products. Various 2-
alkynylbenzoates with electron-withdrawing and electron-do-
nating groups on the aromatic ring performed well under the
standard conditions. Additionally, substituents in the ortho posi-
tion of 2-arylylbenzoates were tolerated (see products 3la and
3qa). Different functional groups including NO2, Ac, Cl, Me, Et,
CF3, NMe2, and tBu were all compatible with the reaction sys-
[a] Reaction conditions: Reactions were performed with 1 (0.5 mmol),
Pd(OAc)2 (5 mol-%), KI (0.5 equiv.), NaHCO3 (1.5 equiv.), and 2a (4 equiv.) in
MeCN (1 mL) at 80 °C under an atmosphere of O2 (balloon) for 16–24 h.
tem (see products 3ia–ua). Particularly, compound 3ka with
an ortho-fluoro group was obtained in 82 % yield. Moreover,
benzoates bearing various substituents in different positions of
the benzoates were utilized in this reaction (see products 3ia–
Then, we tried to further expand the scope of alkenyl alco-
ua). For instance, halo-substituted benzoates underwent the hols for isocoumarin synthesis (Table 2). Alkyl-substituted allyl
process well and were converted into desired products 3va and alcohols were found to be suitable substrates for this transfor-
3wa in yields of 74 and 73 %, respectively. Interestingly, another mation, and they were transformed into the desired products
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in good yields (see products 3aa to 3ad). 2-Methylprop-2-en-1- tions of homoallylic alcohols proceeded smoothly to give the
ol was tested, and 3-(2-methyl-2-formylethyl)isocoumarin (3ab) corresponding alkylated isocoumarins in good yields (see pro-
was obtained in 56 % yield. Furthermore, even bulky aryl-substi- ducts 3am and 3an). Furthermore, an alkenyl alcohol with an
tuted allylic alcohols were employed to couple with ortho-alk- even longer chain, such as oct-7-en-4-ol, was transformed into
ynylbenzoates (see products 3ae and 3ah). For example, the product 3ao in 48 % yield. However, the oxidative Heck reaction
reaction of 1-phenylprop-2-en-1-ol proceeded well and was of an internal olefin was difficult and only a trace amount of
converted into isocoumarin 3ae in 77 % yield. Besides, the reac- 3al was detected by GC–MS.
On the basis of our previous reports on reactions performed
with the use of allylic alcohol species as alkyl resources,[15]
a
Table 2. Scope of the alkenyl alcohols.[a]
possible catalytic cycle is proposed in Scheme 3. Initial coordi-
nation of the internal alkyne to PdII affords intermediate A,
which subsequently undergoes intramolecular 6-endo-dig cycli-
zation by nucleophilic attack of the carbonyl oxygen atom onto
the distal carbon atom of the triple bond to generate key oxy-
palladation intermediate B. With the loss of the alkyl group of
the ester in the presence of base and subsequent Mizoroki–
Heck reaction of the alkenyl alcohol inserting into the carbon–
palladium bond of vinylpalladium species B, intermediate C is
formed. Then, it undergoes ꢀ-H elimination to form intermedi-
ate D, and this is followed by reinsertion of the hydrated palla-
dium species into the enol to generate intermediate E.[14,15] Fur-
ther ꢀ-H elimination of intermediate E produces final product
3 through a Wacker-like process, and the PdII species is regener-
ated in the presence of the oxidizing reagent, O2, to complete
the catalytic cycle.
Scheme 3. Proposed mechanism.
Conclusions
In conclusion, we developed a novel oxyalkylation reaction of
alkynes by using alkenyl alcohols as alkylating agents that al-
lows the facile synthesis of ꢀ-, γ-, and δ-isocoumarin ketones.
This reaction proceeds by oxypalladation and sequential Heck-
type insertions of alkenyl alcohols. The use of oxygen as the
sole oxidant makes the reaction practical and environmentally
friendly. Notable features of this transformation include the use
[a] Reaction conditions: Unless otherwise noted, the reaction was performed
with 1 (0.5 mmol), Pd(OAc)2 (5 mol-%), KI (0.5 equiv.), NaHCO3 (1.5 equiv.),
and 2 (4 equiv.) in MeCN (1 mL) at 80 °C under an atmosphere of O2 (balloon)
for 16–24 h.
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Supporting Information (see footnote on the first page of this
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Acknowledgments
The authors thank the National Nature Science Foundation of
China (NSFC) (grant numbers 21172076, 21202046, and
21420102003), the Fundamental Research Funds for the Central
Universities (grant number 2015Y001), and the Doctoral Fund
of the Department of Education of Guangdong (grant number
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Keywords: Synthetic methods · Palladium · Oxygen
heterocycles · Heck reaction · Elimination · Natural products
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Received: November 18, 2015
Published Online: January 5, 2016
Eur. J. Org. Chem. 2016, 663–667
www.eurjoc.org
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim