Palladium-catalyzed oxidative arylating carbocyclization of allenynes

Article abstract of DOI:10.1002/anie.201107592

Vinylallenes or cross-conjugated trienes are obtained selectively in the title reaction (see scheme). Two possible mechanisms are suggested to rationalize the formation of the different types of products. Control experiments indicate that p-benzoquinone (

Full text of DOI:10.1002/anie.201107592

Angewandte
Chemie
DOI: 10.1002/anie.201107592
Carbocyclization
Palladium-Catalyzed Oxidative Arylating Carbocyclization of
Allenynes**
Youqian Deng, Teresa Bartholomeyzik, Andreas K. ꢀ. Persson, Junliang Sun, and Jan-
E. Bꢁckvall*
Transition metal-catalyzed cyclizations of allenynes provide
efficient and atom-economical routes to polyunsaturated
carbo- and heterocycles.[1–5] In addition to cycloisomeriza-
tions[1] and intramolecular Pauson–Khand reactions,[2] cycli-
zations of allenynes in combination with a second coupling
partner have been studied previously. For instance, 1,7-
allenynes react under RhI catalysis with arylboronic acids to
afford arylated bicyclic compounds.[3] Nonoxidative Pd0-
catalyzed carbocyclizations of allenynes in the presence of
organoboronic acids[4] or bis(pinacolato)diboron (B2pin2)[5]
have also been demonstrated.
Much attention has recently been addressed towards an
oxidative counterpart of the nonoxidative carbocyclizations,[6]
thus allowing for new and different types of catalysts and
reaction conditions.[7–10] Our research group has been
involved in the development of various oxidative palladium-
catalyzed carbocyclizations of en- and dienallenes.[9,10] Very
Scheme 1. a) Oxidative Pd-catalyzed carbocyclization of enallenes[10]
b) Possible pathway for the carbocyclization of allenynes 1 based on
our previous studies of enallenes. E=CO2Me.
.
recently, we reported on the palladium-catalyzed oxidative
carbocyclization/borylation[10a] and carbocyclization/aryla-
tion[10b] of enallenes (Scheme 1a). To the best of our knowl-
edge, there is no report on the palladium-catalyzed oxidative
carbocyclization of allenynes.
allenynes in the presence of arylboronic acids or B2pin2 with
p-benzoquinone (BQ) as the oxidant.
With this notion in mind, we envisioned a carbocycliza-
tion/arylation or carbocyclization/borylation of allenynes with
simple PdII salts as the catalyst (Scheme 1b). We hypothe-
sized that interaction of allenynes 1 with PdII would form
intermediate A through nucleophilic attack on palladium by
the allene.[9,10] Subsequent cis insertion of the alkyne into the
generated vinyl–palladium bond would give intermediate B,
which may undergo transmetalation with an arylboronic acid
or B2pin2 to yield a carbocyclized product C. Thus, the
challenge involves regeneration of the catalyst, tuning of the
reactivity of the allene and alkyne moieties, and control of the
stereochemistry. Herein, we describe new patterns of PdII-
catalyzed oxidative carbocyclization of structurally diverse
In preliminary experiments we observed that treatment of
1a with Pd(OAc)2 (5 mol%), PhB(OH)2 (1.1 equiv), and BQ
(1.1 equiv) in THF at 508C gave the phenylated triene
product (Z)-2aa in 60% yield (determined by NMR spec-
troscopy; with anisole as the internal standard; see the
Supporting Information). In a solvent study, THF was found
to be the best solvent for this transformation. Other solvents
such as acetone, 1,4-dioxane, methanol, or toluene gave
significantly lower yields. An excess of BQ (1.3 equiv) did not
improve the yield of the reaction compared to the use of
1.1 equiv. When the amount of phenylboronic acid was
increased to 1.3 equiv the yield rose to 67%. A further
increase of the amount of phenylboronic acid (up to 2 equiv)
gave no additional improvement. Surprisingly, the yield of
(Z)-2aa could be increased to 75% simply by decreasing the
catalyst loading to 1 mol%. Finally, room temperature (258C)
was found to be the optimal temperature without loss in yield
(Scheme 2). It is interesting to note that in most cases trace
amounts of cycloisomerization products 3, such as 3a,[11] were
also detected in the crude reaction mixture.
[*] Dr. Y. Deng, T. Bartholomeyzik, A. K. ꢀ. Persson,
Prof. Dr. J.-E. Bꢁckvall
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University, 10691 Stockholm (Sweden)
E-mail: jeb@organ.su.se
Prof. Dr. J. Sun
Structural Chemistry and Berzelii Centre EXSELENT on Porous
Materials, Stockholm University (Sweden)
The scope of boronic acids was studied using allenyne 1a
(Scheme 3). The electronic properties of the arylboronic acid
had little effect on the reaction. Phenylboronic acids with
alkyl substituents in the para or meta position reacted
smoothly under the standard conditions to give the products
(2ab, 2ac, and 2ae) in 73–79% yield. Sterically demanding
[**] Financial support from the European Research Council (ERC AdG
247014), the Swedish Research Council, and the Berzelii Centre
EXSELENT is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107592.
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alization. Acetyl and formyl groups were also well tolerated
under the reaction conditions (2ah and 2ai, respectively).
Nitro- or trifluoromethyl-substituted electron-deficient aryl-
boronic acids reacted slower and afforded the corresponding
products 2aj and 2ak in 58 and 50% yield, respectively. It is
worth noting that (E)-styrylboronic acid was a good reaction
partner as well (affording 2al). Furthermore, benzene-1,4-
diboronic acid reacted smoothly with 1a to afford product
2am (Scheme 4).
Scheme 2. Oxidative phenylating carbocyclization of allenyne 1a.
E=CO2Me.
We also investigated the aryl-substituted alkynes 1b–d
(Scheme 3). Despite the higher steric hindrance, these
substrates reacted smoothly when using 5 mol% of Pd(OAc)2
Scheme 4. Carbocyclization of 1a with benzene-1,4-diboronic acid.
E=CO2Me.
in refluxing THF to afford the corresponding arylated
products in good to high yields. The configuration of the
cross-conjugated triene products 2bb (Z) and 2da (E) was
established by NOESY 1H NMR measurements.
By altering the substituent on the alkyne from H or aryl
groups to alkyl groups, new products were obtained (Table 1).
Substrate 1e with a methyl group (R = H, Table 1) on the
triple bond afforded the expected triene product 2ea in 50%
yield along with the unexpected vinylallene product 4ea in
17% yield. Interestingly, reaction of allenyne 1 f having an
ethyl group on the triple bond proceeded with reversed
selectivity affording the vinylallene product 4 fa as the major
product in 65% yield, while the amount of triene (2 fa)
decreased to 10%. Pentyl-substituted substrate 1g displayed
a similar selectivity. When both methyl groups on the terminal
carbon atom of the allene moiety of 1g were replaced by
pentamethylene (forming the cyclohexylidene group of 1h),
the reaction gave the vinylallene product 4ha in 67% yield
together with only trace amounts of triene product 2ha.
Furthermore, it was found that the scope with respect to the
arylboronic acid coupling partner was broad. Arylboronic
acids with alkyl or vinyl substituents reacted smoothly under
the standard conditions (products 4hb and 4hc or 4hd).
Halogen substituents at the meta or para position, including
sensitive aryl iodides which would serve as further functional
handles, were compatible with the reaction conditions (4he–
4hh).
Scheme 3. PdII-catalyzed oxidative arylating carbocyclization of 1a–1d.
E=CO2Me. [a] Pd(OAc)2 (2 mol%), 508C. [b] Pd(OAc)2 (5 mol%),
reflux.
(ortho-methylphenyl)boronic acid required elevated temper-
ature (508C) and increased catalyst loading (2 mol% of
Pd(OAc)2) for full conversion and acceptable yield (2ad).
Notably, the procedure tolerated a range of functional groups
as phenyl substituents, including vinyl and bromo groups (2af
and 2ag, respectively), which are useful for further function-
A range of aryl substituents with varying electronic
properties was tolerated, from electron-rich methoxy to
electron-deficient formyl and nitro groups (products 4hi,
4hj, and 4hk, respectively). Even benzene-1,4-diboronic acid
worked and afforded bis(vinylallene) product 4hl. It is worth
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Table 1: PdII-catalyzed oxidative arylating carbocyclization of 1e–1h.[a]
attention to the borylation of allenynes (Scheme 5). By
increasing the catalyst loading to 2 mol% the reaction of 1a
with B2pin2 at 508C gave complete conversion affording the
Entry
1
Allenyne
Boronic acid
PhB(OH)2
Products, Yields [%]
1e
2ea, 50
4ea, 17
2
3
PhB(OH)2
PhB(OH)2
1 f
2 fa, 10
2ga, 9
4 fa, 65
4ga, 67
Scheme 5. PdII-catalyzed oxidative borylating carbocyclization of alle-
nynes 1. E=CO2Me.
1g
borylated product 5a in 62% yield. Aryl-substituted sub-
strates 1b, 1c, and 1d could also successfully be used to give
borylated products 5b (73%), 5c (75%), and 5d (89%),
respectively. Surprisingly, the borylation of methyl-substi-
tuted substrate 1e afforded the triene product 5e selectively
in 90% yield with < 2% of allenic products analogous to 4.
We believe that the mechanism of this borylation reaction
involves the pathway depicted in Scheme 1b. The configu-
ArB(OH)2
4
5
6
7
1h
1h
1h
1h
Ar=Ph
Ar=p-MeC6H4
Ar=p-tBuC6H4
Ar=p-(CH2 CH)- 2hd, <2
C6H4
2ha, <2
2hb, 2
2hc, 3
4ha, 67
4hb, 68
4hc, 69
4hd, 60
=
À
8[b]
9
10
11
12
13
14
1h
1h
1h
1h
1h
1h
1h
Ar=p-IC6H4
2he, 2
4he, 60
4hf, 72
4hg, 73
4hh, 62
4hi, 75
4hj, 85
4hk, 80
ration of the C C double bond in these triene products was
further established by X-ray diffraction studies of 5b and
5e.[15] It is interesting to note that the borylation in the
oxidative reactions only occurs at the alkyne in contrast to the
corresponding nonoxidative reactions, where borylation oc-
curred at the allene.[5]
Ar=p-BrC6H4
Ar=m-BrC6H4
Ar=p-ClC6H4
Ar=m-OMeC6H4 2hi, 3
Ar=p-CHOC6H4 2hj, <2
Ar=m-NO2C6H4 2hk, <2
2hf, <2
2hg, <2
2hh, <2
To gain further insight into the mechanism some control
experiments were carried out (for details see the Supporting
Information). When a mixture of stoichiometric amounts of
Ar=
p-B(OH)2C6H4
1
15[c]
1a, Pd(OAc)2, and PhB(OH)2 was monitored by H NMR
spectroscopy, only trace amounts of 2aa were observed, the
ratio of 2aa/1a being 1/222 after 1.5 h. After addition of
10 mol% of BQ this ratio was dramatically increased to 1/2.8
within 1 h. After 14 h the starting material was fully con-
sumed. This result clearly indicates that BQ plays an
important role as a ligand in addition to its role as an oxidant
for the reaction.[16]
4hl, 50
[a] E=CO2Me. Conditions unless otherwise noted: Pd(OAc)2 (1 mol%),
boronic acid (1.3 equiv), BQ (1.1 equiv), room temperature. [b] Pd-
(OAc)2 (5 mol%), boronic acid (2 equiv). [c] Benzene-1,4-diboronic acid,
1h (2.6 equiv), Pd(OAc)2 (2 mol%), BQ (2.2 equiv), 508C.
Moreover, when enyne 6 was used, no carbocyclization
was observed under the standard conditions (Scheme 6).[17]
This shows that the allene moiety is crucial for the oxidative
transformation.
noting that the PdII-catalyzed oxidative carbocyclization
provides an efficient route to cyclic vinylallene derivatives,
known to be versatile precursors of hydrindenones[12] and
retinoids.[13]
Although formation of triene product 2 in principle can be
explained by the mechanism outlined in Scheme 1b, the
formation of vinylallene product 4 cannot. The reaction giving
2 and 4 should begin with the formation of ArPdOAc[18] from
ArB(OH)2 and Pd(OAc)2.[19] Insertion of the alkyne into the
Organoboronates, a synthetically useful class of com-
À
pounds, can be used for the construction of C C bonds, for
example through Suzuki–Miyaura cross-coupling reactions.[14]
Thus, after realization of the arylation, we turned our
Ar Pd bond[20] in D can generate E (Scheme 7a). In the next
À
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The order of rates of the alkynes in the control experiment
therefore does not seem to support mechanism (a).
In the lower pathway in Scheme 7a, the b elimination
À
from the vinyl Pd intermediate formed from F (through
insertion) should be slow.[21a] In the control experiment above,
Scheme 6. Reaction of enyne 6 with phenylboronic acid.
À
where 4-octyne readily inserted into Ar Pd, we observed no
b elimination to give allene, not even under the conditions of
the catalytic reaction at 508C for 20 h.
We also carried out experiments with a large excess of
PhB(OH)2 or a large excess of a 1:1 mixture of PhB(OH)2 and
B2pin2. With mechanism (a) one would expect some diary-
lation or arylation–borylation product, whereas with mecha-
nism (b) only monoarylation or monoborylation is expected.
Reaction of 1e with 3.0 equiv of PhB(OH)2 or 1.3 equiv of
PhB(OH)2 and 1.3 equiv of B2pin2 in the presence of 5 mol%
of Pd(OAc)2 and 1.1 equiv of BQ gave no detectable amounts
of diarylation or arylation–borylation product. This observa-
tion is better explained by mechanism (b).
In conclusion, we have developed an unprecedented PdII-
catalyzed oxidative arylating or borylating carbocyclization of
allenynes, which provides access to trienes and vinylallenes
not accessible with other metals. We propose that the
borylating carbocyclization proceeds through allene attack
on PdII whereas arylating carbocyclization may involve
arylpalladation of a triple/double bond or formation of
a pallada(IV)cyclopentene intermediate. Structural differ-
ences in the starting material will determine whether trienes 2
or vinylallenes 4 are formed. Further studies on the scope,
mechanism, and synthetic application of these reactions are
underway in our laboratory.
Scheme 7. Proposed mechanisms for the palladium-catalyzed oxidative
arylating carbocyclization of allenyne 1. R.E.=reductive elimination.
Experimental Section
Typical experimental procedure for the palladium-catalyzed oxida-
tive arylating carbocyclization of allenyne 1: To a mixture of
PhB(OH)2 (32.0 mg, 0.26 mmol), BQ (24.0 mg, 0.22 mmol), and
Pd(OAc)2 (0.5 mg, 0.002 mmol) were added 1a (47.5 mg,
0.20 mmol) and THF (1 mL) at room temperature (RT). The reaction
was stirred at RT for 20 h. After the reaction was complete as
monitored by TLC, evaporation and column chromatography on
silica gel (pentane/ethyl acetate = 30/1) afforded (Z)-2aa (47.7 mg,
76%) as a liquid; 1H NMR (400 MHz, CDCl3): d = 7.20–7.11 (m, 5H),
6.53 (s, 1H), 6.19 (s, 1H), 4.93–4.91 (m, 1H), 4.73–4.71 (m, 1H), 3.77
(s, 6H), 3.37 (d, J = 1.6 Hz, 2H), 1.40–1.38 ppm (m, 3H); 13C NMR
(100 MHz, CDCl3): d = 170.7, 149.7, 140.0, 139.9, 137.2, 134.2, 129.1,
127.3, 126.6, 122.5, 115.8, 62.5, 52.9, 42.7, 21.4 ppm; HR-MS (ESI):
calc. for C19H20NaO4 [M+Na]+: 335.1254; found: 335.1258.
À
step, the allene can insert into the new Pd C bond.
Subsequent b elimination would give 2. Alternatively, inser-
À
tion of the allene into the Ar Pd bond in D would give F.
Insertion of the alkyne into the vinyl Pd bond in F and
À
subsequent b elimination would produce 4.[21]
The formation of 2 and 4 can also be explained by the
cyclopalladation mechanism proposed in Scheme 7b. First,
interaction of allenyne 1 and ArPdOAc in the presence of BQ
would form pallada(IV)cyclopentene intermediate G regio-
selectively.[4a,22,23] This dicyclopentene-fused intermediate G
may be kinetically preferred. b Hydride elimination involving
one of the gem-dimethyl groups and loss of HOAc would give
intermediate H, which on reductive elimination would give
cross-conjugated triene 2. The alternative b hydride elimina-
tion followed by reductive elimination of HOAc would
generate allene intermediate I,[21] which after subsequent
reductive elimination would give vinylallene 4.
When 1 equiv of Pd(OAc)2 and 1 equiv of PhB(OH)2 were
mixed with of 4-phenyl-1-butyne, 1-phenyl-1-butyne, or
4-octyne in [D8]THF, H NMR monitoring showed that all
three alkynes undergo facile arylpalladation, however in the
order 4-phenyl-1-butyne > 4-octyne > 1-phenyl-1-butyne. To
explain the observed ratio between 2 and 4 the mechanism in
Received: October 27, 2011
Revised: December 22, 2011
Published online: January 27, 2012
Keywords: alkynes · allenes · cyclization · oxidation · palladium
.
1
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