Palladium-catalyzed oxidative carbocyclization-carbonylation of allenynes and enallenes

Article abstract of DOI:10.1002/chem.201402688

A highly efficient oxidative carbocyclization-carbonylation reaction cascade of allenynes and enallenes has been developed using a Pd^II salt in low catalytic amounts under ambient temperature and pressure (1 atm of carbon monoxide). The use of DMSO as an additive was found to be important for an efficient reaction. A wide range of alcohols as trapping reagents were used to give the corresponding esters in good yields. A highly efficient oxidative carbocyclization-carbonylation reaction cascade of allenynes and enallenes has been developed using a low catalyst loading of Pd(OAc)₂ at room temperature and atmospheric pressure (1 atm of carbon monoxide). The use of DMSO as an additive was found to be important for an efficient reaction. A wide range of alcohols as trapping reagents were used to give the corresponding esters in good yields.

Full text of DOI:10.1002/chem.201402688

DOI: 10.1002/chem.201402688
Communication
&
Homogeneous Catalysis
Palladium-Catalyzed Oxidative Carbocyclization–Carbonylation of
Allenynes and Enallenes
Chandra M. R. Volla, Javier Mazuela, and Jan-E. Bꢀckvall*[a]
Dedicated to the centennial of the MPI fꢀr Kohlenforschung
Abstract: A highly efficient oxidative carbocyclization–car-
bonylation reaction cascade of allenynes and enallenes
has been developed using a PdII salt in low catalytic
amounts under ambient temperature and pressure (1 atm
of carbon monoxide). The use of DMSO as an additive was
found to be important for an efficient reaction. A wide
range of alcohols as trapping reagents were used to give
the corresponding esters in good yields.
The development of methodologies for the synthesis of poly-
unsaturated carbo- and heterocycles is an important challenge
in modern organic chemistry.[1] In this context, palladium(II)-
catalyzed oxidative carbocyclizations have proven to be effi-
cient and useful for the preparation of these compounds.[2,3]
Palladium(II)-catalyzed oxidative carbocyclizations have also
been used successfully as key-steps in the total synthesis of
several natural products.[4] Our research group has been previ-
Scheme 1. E=CO2Me a) Palladium-catalyzed oxidative carbocyclizations of
enallenes or allenynes. b) Palladium-catalyzed carbocyclization–arylation,
ously involved in the development of various palladium-cata-
lyzed oxidative carbocyclization reactions.[5–7] We have extend-
ed the synthetic potential of these carbocyclization reactions
by quenching the intermediates with appropriate coupling
partners. Reaction of enallenes[8] or allenynes[9] 1 with a PdII
catalyst leads to allylic CÀH activation to give intermediate 2,
which can be trapped by either an arylboronic acid or bis(pina-
colato)diboron (B2pin2) to afford 3 (Scheme 1a). More recently,
intermediate 2 resulting from allenynes 1 was also quenched
by terminal alkynes to realize a domino carbocyclization–alky-
nylation reaction.[10] In contrast, alkyl-substituted allenynes 4
follow a different activation pathway, in which the propargylic
CÀH bond was cleaved to give intermediate 5. By careful
choice of reaction conditions, this intermediate was selectively
formed, and subsequent reaction with either an arylboronic
acid, B2pin2,[11] or acetic acid[12] provided the corresponding vi-
nylallene products 6 (Scheme 1b).
–borylation and –acyloxylation of allenynes. c) Oxidative palladium-catalyzed
carbocyclization–carbonylation of allenynes and enallenes.
alcohol nucleophiles to provide cyclic a,b-unsaturated esters 7
or vinylallene esters 8 (Scheme 1c). To the best of our knowl-
edge, there is no report in the literature combining Pd-cata-
lyzed oxidative carbocyclization and carbonylation reactions of
unsaturated substrates.
Transition-metal-catalyzed carbonylation has emerged as
a powerful tool for the preparation of carbonyl compounds.[13]
In this respect, palladium-catalyzed carbonylation with carbon
monoxide has been extensively studied.[14] In particular, palladi-
um-catalyzed oxidative carbonylations have been widely em-
ployed for the introduction of carbonyl functionality in unsatu-
rated substrates.[15]
For the initial studies, we chose allenyne 9a with a terminal
alkyne as a model substrate in the presence of 1 atm of CO
(Table 1). Treatment of 9a with Pd(OAc)2 (5 mol%) and 1,4-ben-
zoquinone (BQ; 2 equiv) in CD3OD at room temperature with
a CO balloon afforded product [D4]-10aa in 48% NMR yield.
An unexpected incorporation of deuterium was observed in
the product. The explanation of this observation is that the
acidic alkyne CÀH of 9a is exchanged with the deuterium of
CD3OD in the presence of the PdII catalyst as shown by control
experiments (see Supporting Information). Among many PdII
In the present study, we envisioned that intermediates 2 or
5 can be carbonylated by carbon monoxide insertion to form
reactive acylpalladium(II) species, which can further react with
[a] Dr. C. M. R. Volla, Dr. J. Mazuela, Prof. Dr. J.-E. Bꢁckvall
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University, 106 91 Stockholm (Sweden)
E-mail: jeb@organ.su.se
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402688.
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Table 1. Pd-catalyzed carbocyclization–carbonylation.
Entry
Pd catalyst
Pd(OAc)2
mol%
Yield
[%][a]
1
2
5
5
48
59
3
4
[Pd(TFA)2], [Pd(acac)2], PdCl2, [Pd(DMSO)2Cl2]
Pd(OAc)2
5
1
<10
63
[a] The yield was measured by 1H NMR using mesitylene as an internal
standard.
catalysts screened, only Pd(OAc)2 and 1,2-bis(phenylsulfinyl)-
ethanepalladium(II) diacetate (11) gave reasonable yields of
the ester [D4]-10aa. When [Pd(TFA)2] (TFA=trifluoroacetate)
was used in place of Pd(OAc)2, lower yields of the product
were observed. Other palladium salts such as PdCl2, [Pd(acac)2]
or [Pd(DMSO)2Cl2] gave no reaction (Table 1 entry 3). Interest-
ingly, lowering the catalyst loading to 1 mol% increased the
yield of [D4]-10aa to 63% (Table 1 entry 4).
Scheme 2. Effect of additives in the Pd-catalyzed carbocyclization–carbonyla-
tion.
After several unsuccessful attempts to increase the yield by
screening different parameters, such as temperature, catalyst
loading, oxidant, and concentration, we assumed that the de-
composition of PdII to Pd0 under the reducing CO atmosphere
might be the main reason for the low yields (see Supporting
Information). To circumvent this problem, we studied the
effect of various stabilizing ligands in the carbonylation reac-
tion. As can be seen from Scheme 2, addition of 10 mol% of
DMSO (L1) has a beneficial effect on the reaction and under
these conditions the yield increased to 78% (74% isolated
yield). Other sulfoxide (L2–L4) and sulfone ligands (L5–L7)
were also screened, but in all these cases, lower yields were
obtained. We investigated the effect of the amount of DMSO
on the reaction and it was found that the DMSO concentration
plays a crucial role for the carbonylation reaction. While cata-
lytic amounts of DMSO (10–20 mol%) were found to be profit-
able, increasing the amount of DMSO further led to a lower
yield. For example, a 1:1 mixture of CD3OD and DMSO
(50 equiv) gave only 32% of [D4]-10aa along with 60% of un-
reacted 9a after 12 h of reaction (see Supporting Information).
The optimized conditions established for the formation of
a,b-unsaturated ester 10aa were applied to differently substi-
tuted allenynes (Table 2). When both methyl groups of the
allene unit were replaced by a pentamethylene group (9b),
the reaction with CO in MeOH gave the a,b-unsaturated ester
10ba in a 56% yield, but the catalyst loading had to be in-
creased to 5 mol% (Table 2, entry 2). Unsymmetrical allene 9c
also showed a similar reactivity and gave a mixture of isomers
10ca and 10ca’ in a 3:1 ratio (Table 2, entry 3).
matic substituted allenynes 9d–9 f required a slightly elevated
temperature (558C) for the carbonylation reaction to occur
and afforded the corresponding products 10da–10 fa in good
yields (Table 2, entries 4–6). We then turned our attention to
the use of enallenes 12a–12c in the domino carbocyclization–
carbonylation reaction. In contrast to the allenyne substrates,
enallenes have a propensity for b-elimination after the carbo-
cyclization because of the presence of b-hydrogen atom(s).
However, we were pleased to find that under the optimized re-
action conditions enallenes, 12a and 12b selectively gave the
corresponding esters 13aa and 13ba in 85 and 63% yields, re-
spectively, without giving the b-elimination product (Table 2,
entries 7 and 8). Enallene 12c gave the ester 13ca in 71%
yield (Table 2, entry 9).
The scope of the alcohol partners in the carbocyclization–
carbonylation reaction was then explored using allenyne 9a or
enallene 12a (Table 3). In addition to MeOH, other aliphatic al-
cohols reacted smoothly to provide the desired a,b-unsaturat-
ed esters in good yields. It should be noted that the reaction
could be carried out in dichloroethane as solvent using five
equivalents of the alcohol to obtain comparable results (see
Supporting Information). Similar yields were obtained when in-
creasing the chain length of the alcohol partner (products
10aa vs. 10ab vs. 10ac). On the other hand, yields decreased
when bulky secondary and tertiary alcohols were employed, in
particular with tert-butanol (10ae). Cyclic alcohols could also
be used as alcohol partners with maintained good yields of
the a,b-unsaturated esters (10af and 10ag). The product 10ah
Allenynes with internal alkynes were used for the formation
of tetrasubstituted a,b-unsaturated esters. As expected, aro-
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Table 2. Scope of the Pd-catalyzed carbocyclization–carbonylation of alle-
Table 3. Scope of the alcohol partners in Pd-catalyzed carbocyclization–
nynes and enallenes.
carbonylation of allenynes and enallenes.
Entry
1
Allenyne
Product
Yield [%][a]
74[b]
2
3
56[c]
66[c]
4
5
59[d,e]
58[d]
6
47[d]
7
8
9
85[b]
63[c]
71[d]
[a] The alcohol was used as solvent. [b] DCE was used as solvent together
with 5.0 equiv of alcohol. [c] The yield corresponds to NMR yield using
mesitylene as the internal standard. [d] 1.0 equiv of hydroquinone (HQ)
was used as alcohol partner in DCE. [e] 3:1 mixture of 10al and 15al.
(For 15al see Scheme 4).
is particularly interesting as allylcarboxylates were often used
as electrophiles in metal-catalyzed allylation reactions.[16] Good
yields were obtained when using either 1- or 2-phenylethanol
in the carbocyclization–carbonylation of the allenyne 9a (10ai
and 10aj). Phenols were found to react with allenyne 9a to
give the corresponding phenolic esters 10al–10an in moder-
ate yields. Enallene 12a also reacted nicely with other aliphatic
alcohols to give the products 13aa, 13ab, and 13ao in good
to excellent yields.
[a] Isolated yield of the product after column chromatography.
[b] 1 mol% of Pd(OAc)2 was used. [c] 5 mol% of Pd(OAc)2 was used.
[d] Reaction was done at 558C using 5 mol% of Pd(OAc)2. [e] Reaction
with EtOH gave the corresponding ethyl ester (10db) in 60% yield.
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When alkyl-substituted allenynes 9g and 9h were used for
the carbonylation reaction, the product outcome changed
completely and the major product obtained was the vinylallen-
ic ester 14 (Scheme 3). The formation of these products could
Scheme 3. PdII-catalyzed carbocyclization–carbonylation of alkyl-substituted
allenynes.
be explained by the previously proposed propargylic CÀH acti-
vation[11,12] (cf Scheme 1b) affording a vinyl–palladium inter-
mediate, which undergoes vinylpalladation with the allene
moiety to give intermediate 5 (Scheme 1b). Subsequent car-
bonylation and reaction with methanol would give the vinylal-
lenic esters 14 (Scheme 3).
Interestingly, a control experiment carried without any alco-
hol partner resulted in the formation of 15al as the major
product along with 10al in a combined NMR yield of 57%
(Scheme 4). A highly reducing carbon monoxide atmosphere
Scheme 5. Proposed mechanism for the domino carbocyclization–carbonyla-
tion of allenynes and enallenes ([O]=BQ or O2/ETMs, E=CO2Me).
or 12. Nucleophilic attack of the allene moiety on PdII gener-
ates dienyl–PdII species 18, which undergoes carbopalladation
with the alkyne/alkene unit to give 19. Insertion of carbon
monoxide leads to acyl–PdII complex 20, and subsequent reac-
tion with the alcohol provides
the product and Pd0. Reoxida-
tion of Pd0 to PdII by either ben-
zoquinone or molecular oxygen
completes the catalytic cycle.
In summary, we have reported
the first Pd-catalyzed oxidative
domino carbocyclization–carbon-
Scheme 4. Double carbocyclization–carbonylation with in situ generated hydroquinone.
ylation reaction of allenynes and
enallenes. Moderate to good
together with the palladium catalyst might result in the forma-
tion of small amounts of hydroquinone (HQ) from BQ. The HQ
may add as the alcohol partner to give ester and Pd0. The reox-
idation of Pd0 to PdII by BQ will then generate more HQ, neces-
sary for the next cycle.
yields were obtained for various allenynes and enallenes, and
a wide range of alcohols could be used as coupling partners in
the carbonylation reaction. The use of 1 mol% of Pd catalyst at
room temperature under 1 atm of CO is noteworthy. The aero-
bic version of this transformation allows for a decrease of ben-
zoquinone to catalytic amounts, while using the environmen-
tally friendly O2 as terminal oxidant.
To increase the synthetic utility of the carbocyclization–car-
bonylation, the reaction was also tested under aerobic bio-
mimetic oxidative conditions using catalytic amounts of benzo-
quinone. We have previously reported such aerobic procedures
for a wide range of palladium-catalyzed oxidative reactions in
which molecular oxygen was used as the terminal oxidant to-
gether with electron-transfer mediators (ETMs) in catalytic
amounts.[6b,c,17,18] Under the optimized biomimetic conditions
consisting of 5 mol% of [Co(salophen)] and 20 mol% of BQ as
ETMs for the reoxidation of Pd0 to PdII, 9a afforded the a,b-un-
saturated carbocyclic ester 10aa in 72% yield. The aerobic
conditions were also applied to two other substrates 9a and
12a, which afforded 10af and 13aa in 55 and 83% yield, re-
spectively (see Supporting Information).
Acknowledgements
Financial support from European Research Council (ERC AdG
247014), the Swedish Research Council, and the Wenner-Gren
Foundation (postdoctoral fellowship to C.M.R.V.) is gratefully
acknowledged.
Keywords: carbocyclization · carbonylation · homogeneous
catalysis · oxidation · palladium
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Received: March 20, 2014
Published online on May 14, 2014
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