Palladium-catalyzed alkoxycarbonylation of (Z)-2-en-4-yn carbonates leading to 2,3,5-trienoates

Article abstract of DOI:10.1021/ol102989q

Pd(0)-catalyzed carbonylation of (Z)-2-en-4-yn carbonates in the presence of a balloon pressure of CO in an alcohol donates vinylallenyl esters with an exclusively E-configuration and in high yields. The fact that no such reactivity could be observed with

Full text of DOI:10.1021/ol102989q

ORGANIC
LETTERS
2011
Vol. 13, No. 4
748–751
Palladium-Catalyzed Alkoxycarbonylation
of (Z)-2-En-4-yn Carbonates Leading
to 2,3,5-Trienoates
€ €
G. Eray Akpınar, Melih Kus-, Muhammed Uc-uncu, Erman Karakus-, and Levent Artok*
Department of Chemistry, Faculty of Science, Izmir Institute of Technology, Urla 35430,
Izmir, Turkey
leventartok@iyte.edu.tr
Received December 9, 2010
ABSTRACT
Pd(0)-catalyzed carbonylation of (Z)-2-en-4-yn carbonates in the presence of a balloon pressure of CO in an alcohol donates vinylallenyl esters
with an exclusively E-configuration and in high yields. The fact that no such reactivity could be observed with E-configured enyne carbonates may
indicate that the reaction is promoted via the cooperative coordination of palladium with both alkynyl and carbonate moieties.
As a part of our initiated systematic studies involving the
transition metal catalyzed reactions of conjugated enynes
with a leaving group in the allylic position we have conducted
Pd(0)-catalyzed carbonylation reactions of 2-en-4-yn car-
bonate reagents in the presence of an alcohol (alkoxy-
carbonylation). It is well-known that the Pd(0)-catalyzed
alkoxycarbonylation of allylic compounds usually leads
to β,γ-unsaturated esters through the involvement of a
π-allylpalladium complex (I) (Scheme 1).1
Scheme 1. Alkoxycarbonylation of Allylic Compounds
(1) (a) Kiji, J.; Okano, T.; Higashimae, Y.; Fukui, Y. Bull. Chem. Soc.
Jpn. 1996, 69, 1029. (b) Tsuji, J.; Sato, K.; Okumoto, H. J. Org. Chem. 1984,
49, 1341. (c) El Houssame, S.; El Firdoussi, L.; Allaoud, S.; Karim, A.;
Castanet, Y.; Mortreux, A. J. Mol. Catal. A 2001, 168, 15. (d) Shimizu, I.;
Maruyama, T.; Makuta, T.; Yamamoto, A. Tetrahedron Lett. 1993, 34, 2135.
(2) σ-Allenylpalladium complexes were isolated and characterized:
(a) Elsevier, C. J.; Kleijn, H.; Ruitenberg, K.; Vermeer, P. J. Chem. Soc.,
Chem. Commun. 1983, 1529. (b) Elsevier, C. J.; Kleijn, H.; Boersma, J.;
Vermeer, P. Organometallics 1986, 5, 716.
(3) (a) Tsuji, J.; Sugiura, T.; Minami, I. Tetrahedron Lett. 1986, 27,
731. (b) Tsuji, J.; Mandai, T. J. Organomet. Chem. 1993, 451, 15 and
references therein. (c) Arzoumanian, H.; Choukrad, M.; Nuel, D. J. Mol.
Catal. 1993, 85, 287. (d) Marshall, J. A.; Wolf, M. A.; Wallace, E. M. J. Org.
Chem. 1997, 62, 367. (e) Marshall, J. A.; Wolf, M. A. J. Org. Chem. 1996,
61, 3238. (f) Trieu, N. D.; Elsevier, C. J.; Vrieze, K. J. Organomet. Chem.
1987, 325, C23. (g) Imada, Y.; Alper, H. J. Org. Chem. 1996, 61, 6766.
(h) Tsuji, J.; Mandai, T. Angew. Chem., Int. Ed. 1995, 34, 2589. (i) Ma, S.
Eur. J. Org. Chem. 2004, 1175 and references therein. (j) Knight, J. G.;
Ainge, S. W.; Baxter, C. A.; Eastman, T. P.; Harwood, S. J. J. Chem. Soc.,
Perkin Trans. 1 2000, 3188. (k) Piotti, M. E.; Alper, H. J. Org. Chem. 1997,
62, 8484.
On the other hand, the analogous reaction of alkynes
containing a leaving group in the propargylic position
proceeds via a σ-allenylpalladium2 (II) as an intermediate
to produce allenic esters (Scheme 2).3
Interestingly however, the analogous experiments with
Z-configured enyne carbonates revealed that, even though
these conjugated enynes bear the leaving group on the
allylic carbon atom, they act in a similar way as the pro-
pargylic reagents do, with their Pd(0)-catalyzed reactions
proceeding through a 1,5-substitution to afford ester function-
alized vinylallene structures.
r
10.1021/ol102989q
Published on Web 01/10/2011
2011 American Chemical Society

Table 2. Alkoxycarbonylation of Enyne Carbonate 1aa
Scheme 2. Alkoxycarbonylation of Propargylic Compounds
entry
ROH
isolated yield (%)
The reaction of the (Z)-2-en-4-yn 1a with MeOH at just
50 °C under a balloon pressure of CO in the presence of 1
mol % Pd(PPh3)4 yielded exclusively the related E-config-
ured vinylallenyl ester molecule (3aa) with high yield
(Table 1, entry 1). The separate addition of a PPh3 ligand
and palladium in the form of Pd2(dba)3-CHCl3 or Pd-
(OAc)2 with a respective ligand to palladium ratio of 4/1
afforded lower yields (entries 2 and 3). Nevertheless, low-
ering the ligand-to-Pd ratio to 2/1 provided the optimal
catalyst activity (entries 4-6). However, the conversion of
1a could not proceed to completion when performing the
reaction at room temperature even at prolonged reaction
times (entry 7).
1
MeOH
EtOH
90 (3aa)
85 (3ab)
91 (3ac)
85 (3ad)
91 (3ae)
0 (3af)
2
3
PrOH
4
BuOH
i-PrOH
t-BuOH
5b
6
a 1a, 0.3 mmol; ROH, 5 mL; CO, balloon pressure; 1 mol % Pd,
2 mol % PPh3. b 16 h.
unsuitable because the enyne 1a was recovered as unreacted
when it was carbonylated in tert-butyl alcohol (entry 6).
The scope of the carbonylation method was also sur-
veyed for a range of (Z)-2-en-4-yn carbonates.4 The meth-
od is fully suitable for enynes bearing a methyl, secondary
alkyl, cyclohexyl, or phenyl group on the alkynyl moiety
(R1) thereby providing the corresponding 2,3,5-trienoate
products 3ba-3da in high yields when carbonylated in
methanol for 4 h under the established conditions (Table 3,
entries 1-3).
Table 1. Optimization of Methoxycarbonylation Reaction of
Enyne Carbonate 1aa
The presence of a highly bulky tert-butyl group on the
alkynyl terminus, however, required a relatively longer
reaction period (20 h) for complete conversion and gave
rise to the product 3ea in a moderate yield (64%) (entry 4).
The methodology also tolerated well the enyne carbonates
of which the R2 position is occupied by a H, butyl, or
phenyl group, affording the desired products 3fa-3ha in
around 80-82% yields (entries 5-7).
entry
Pd
PPh3 (%)
yield (%)b
1
Pd(PPh3)4
-
4
85
2
Pd2(dba)3-CHCl3
Pd(OAc)2
74
3
4
67
It appears that the enyne reactivity is more severely
influenced by the size of the R3 group. Varying the R3
group from methyl to butyl reduced the reactivity of the
enyne carbonate remarkably; it took nearly 16 h for com-
plete conversion of 1i and to produce the corresponding
vinylallenyl ester 3ia at a yield of 81% (entry 8). An even
more drastic effect could be observed with the substrate in
which R3 was the isopropyl group; the reaction of 1j was
highly sluggish, and its conversion was incomplete when
the reaction temperature employed was 50 °C (entry 9).
Nevertheless, elevation of the reaction temperature to
65 °C shortened the reaction period for the complete
conversion of 1j to give the product 3ja at a yield of 71%.
The methoxycarbonylation of an enyne carbonate of a
primary alcohol 1k interestingly resulted in the formation
of a nonseparable mixture of an ester functionalized 2-en-
4-yn (4ka) byproduct and the desired vinylallene product
3ka (entry 10).
4
Pd2(dba)3-CHCl3
Pd(OAc)2
2
89
5
2
83
6
Pd(OAc)2
1
76
7d
8
Pd2(dba)3-CHCl3
Pd(PPh3)2Cl2
2
74 (22)c
0
-
a 1a, 0.3 mmol; MeOH, 5 mL; CO, balloon pressure. b Determined by
1H NMR using dimethyl sulfoxide as an internal standard. c Unreacted
reactant. d Rt, 30 h.
The compound Pd(PPh3)2Cl2 displayed no catalytic activ-
ity;when present, the enyne 1awasrecovered unchanged at
the end of the reaction (entry 8).
The carbonylative reaction of 1a with MeOH, EtOH, or
primary saturated C3 and C4 alcohols, under the optimal
reaction conditions, that is at 50 °C, a balloon pressure of
CO, in 5 mL of alcohol, used in combination with Pd2-
(dba)3-CHCl3 (1 mol %)/PPh3 (2 mol %) gave rise to the
corresponding 2,3,5-trienoate products (3aa-3ad) just
within 4-5 h in the range of 85-91% isolated yields
(Table 2, entries 1-4). A high yield of 2,3,5-trienoate
product 3ae could also be obtained with a bulky alcohol,
isopropyl alcohol, albeit at a prolonged reaction time (16 h)
(entry 5). In contrast tert-butyl alcohol appears to be
(4) The preparation of Z-enyne carbonates, such as the one contain-
ing a phenyl group in the R3 position, or in the case where two methyl
groups are on the allylic carbon (enyne carbonate of a tertiary alcohol),
failed owing to their decomposition in the course of modification of the
hydroxyl group to the carbonate.
Org. Lett., Vol. 13, No. 4, 2011
749

Table 3. Methoxycarbonylation of 2-En-4-yne Carbonatesa
Scheme 3. Methoxycarbonylation of E-Enyne Carbonate 1l
carbonate functionality (A) should facilitate the initiation
of the reaction cycle (Scheme 4). As analogy to the Pd(0)-
catalyzed reactions of propargylic reagents, the Pd(0)-cata-
lyzed alkoxycarbonylation of Z-enyne carbonates should
involve the formation of a σ-allenylpalladium(II) complex
(C) by elimination of the leaving group and double bond
migration.5
Scheme 4. Reaction Mechanism of Pd(0)-Catalyzed Alkoxy-
carbonylation of Z-2-En-4-yn Carbonates
The formation of the σ-allenylpalladium intermediate
from propargylic reagents has been based on two unver-
ified paths: (1) the oxidative addition of Pd(0) species and
the subsequent shift of Pd to the far alkynyl carbon, or
(2) via SN20 substitution by the attack of the Pd(0)
species to the alkynyl carbon.2,6 Also for our case, it
is uncertain whether the oxidative pathway or 1,5-SN200
type Pd(0)-substitution is responsible for the formation
of the vinylallenypalladium (C) intermediate. None-
theless, the marked effect on reactivity observed here
for the Z-enyne carbonates with respect to the size of
their allylic cosubstituent (R3) may imply the former. It
should also be noted that the formation of (Z)-methyl
4-methyldec-3-en-5-ynoate (4ka) from the methoxy
carbonylation of 1k can be ascribed to an allylpalladium(II)
intermediate.1
a 1a, 0.3 mmol; MeOH, 5 mL; CO, balloon pressure; 1 mol % Pd,
2 mol % PPh3. b Isolated yield. c Incomplete conversion. d 65 °C.
The reaction cycle should finalize by the reaction of the
intermediate C with CO and alcohol to generate the
product 3 and a catalytically active Pd(0) species.
Evidently, the method would be successful only if the
enyne carbonate substrate had a Z-configuration, because
an E-configured enyne carbonate E-1l led to a complex
mixture (containing only small amounts of the desired
vinylallenes) when subjected to the Pd(0)-catalyzed meth-
No center-to-axis chirality transfer selectivity was
demonstrated with the our established method; the
respective methoxycarbonylation of a nonracemic enyne
1
(5) The isolation of the vinylallenylpalladium species by adopting the
methods applied for propargylic reagents (see ref 2) resulted in decom-
position in all our attempts.
oxycarbonylation reaction as determined by H NMR
analysis (Scheme 3).
(6) (a) Keinan, E.; Bosch, E. J. Org. Chem. 1986, 51, 4006. (b) Mandai,
T.; Yoshikazu, T.; Matsuoka, S.; Tsuji, J. Tetrahedron Lett. 1993, 34, 7615.
(c) Mandai, T.; Matsumoto, T.; Tsujiguchi, Y.; Matsuoka, S.; Tsuji, J.
J. Organomet. Chem. 1994, 473, 343.
Whencomparing the results from the reactions of E- and
Z-configured enyne carbonates, it can be considered that
the dual coordination of Pd species to a triple bond and the
750
Org. Lett., Vol. 13, No. 4, 2011

carbonate (R,Z)-1b (94.5% ee) led to a racemic mixture of
the product.
Acknowledgment. Dedicated to Prof. Bekir C-etinkaya
€
on the occasion of his retirement. We thank Ms. I. Ozc-elik
€
of Izmir Institute of Technology, Mr. S. Gunnaz of Ege
University, and Dr. W. Hiller of Dortmund University of
In summary we have revealed for the first time that the
Pd(0)-catalyzed carbonylation of (Z)-2-en-4-yn carbonates
in an alcohol medium lead to high yields of vinylallenyl esters.
Z-Conjugated enyne reagents with a leaving group in the
allylic position seem to reveal similar characteristic reactiv-
ities toward transition metal catalyzed reactions with respect
to the propargylic reagents. The studies involving the synth-
esis of enantioenriched vinylallenes, and the application of
Z-enynes with different types of leaving groups toward various
transition metal catalyzed reactions are currently underway.
€
Technology for NMR analyses, Dr. H. Ozgener of Izmir
Institute of Technology for FTIR analyses, and the En-
vironmental Research Center for GC-MS analyses.
Supporting Information Available. Detailed experimen-
tal procedures, and compound characterization data.
This material is available free of charge via the Internet
at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 4, 2011
751