Synthesis and reactivity of five-membered palladalactones from arylallenes and carbon dioxide: Relevance to catalytic lactone synthesis

Article abstract of DOI:10.1021/om400089n

A bis(tri-n-butylphosphine)palladium ((nBu₃P)₂Pd) complex was shown to catalyze the cyclization of arylallenes with carbon dioxide to yield six-membered lactones. The stoichiometric reaction of a (Me ₃P)₂Pd comp

Full text of DOI:10.1021/om400089n

Note
pubs.acs.org/Organometallics
Synthesis and Reactivity of Five-Membered Palladalactones from
Arylallenes and Carbon Dioxide: Relevance to Catalytic Lactone
Synthesis
Jun-Chul Choi,* Kouichi Shiraishi, Yasumasa Takenaka, Hiroyuki Yasuda, and Toshiyasu Sakakura*
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki
305-8565, Japan
S
* Supporting Information
ABSTRACT: A bis(tri-n-butylphosphine)palladium
((nBu3P)2Pd) complex was shown to catalyze the cyclization
of arylallenes with carbon dioxide to yield six-membered
lactones. The stoichiometric reaction of a (Me3P)2Pd complex
with 1 equiv of arylallenes, followed by treatment with CO2,
resulted in the formation of five-membered palladalactones,
the structures of which were fully characterized by X-ray crystallography, NMR, IR, HRMS, and elemental analysis.
ynthetic chemists have long dreamed of using carbon
dioxide as a starting material for chemical syntheses.
such as Me3P, tBu3P, Cy3P, Ph3P, and Ph2CH2CH2PPh2,
resulted in poor yields. The lactone produced contained
isomeric products at each CC double bond, although the
selectivity for the production of lactone A was 80%.5 Higher
pressures and lower temperatures decreased the selectivity of A.
The reaction of 3-phenyl-1-propyne with CO2 in the presence
of a catalytic amount of (nBu3P)2Pd complex did not give A.
The reaction path via the isomerization of phenylallene to the
corresponding acetylene was negligible under the present
reaction conditions.
Stoichiometric Reaction. A possible mechanism for the
lactone synthesis is shown in Scheme 1. The thermolysis of (η5-
C5H5)Pd(η3-C3H5), a convenient precursor for palladium(0),
in the presence of 1 equiv of arylallene resulted in the
generation of a new complex assignable to (Me3P)2Pd(η2-
CH2CCHAr) (2a, Ar = C6H5; 2b, Ar = C6H4OCH3-p). A
large high-field shift in the CCH2 1H NMR signal, to δ 2.66
ppm, strongly supported the formation of a coordination
complex with palladium. The arylallene appeared to be
horizontally oriented and coplanar with the P−Pd−P plane,
because the two phosphorus atoms were not equivalent,
according to the 31P{1H} NMR spectrum. On the other hand,
the reaction of Pd(0) with excess arylallene resulted in a new
complex assignable to palladacyclopentane (3). The formation
of a similar metallacyclopentane was reported in the context of
reactions of allenes with transition-metal complexes.6
S
Carbon−carbon bond formation using CO2 is both particularly
attractive and challenging.1 Although Ni- or Pd-catalyzed six-
membered lactone syntheses from butadienes and acetylenes
have been well-documented,2 few examples of reactions
between CO2 and allenes have been reported.3 The lactones
obtained from these reactions have been used in a variety of
reactions, including hydrogenation, hydroformylation, hydro-
aminomethylation, hydroamination, alcoholysis, hydration,
hydrosilation, oxidation, and polymerization reactions.4 Sasaki
reported a low yield for the Ni-catalyzed reaction of
unsubstituted allene and CO2.3b Tsuda et al. obtained lactones
in a moderate yield for a specific substrate (methoxyallene)
using a palladium catalyst coordinated by P−N ligands or
Cy3P.3c Mechanistic investigations of these systems have not
yet been conducted. We report here the preparation of six-
membered lactones from arylallenes and CO2 in the presence
of (nBu3P)2Pd catalysts. We identified several possible catalytic
intermediates on the basis of the results obtained from the
stoichiometric reactions.
RESULTS AND DISCUSSION
■
Catalytic Reaction. Phenylallene reacted with CO2 (50
bar) at 80 °C in the presence of a catalytic amount of a
(nBu3P)2Pd complex to yield a set of six-membered lactones,
with 3,5-dibenzyl-2-pyrone (A) as the major isomer, in a
moderate yield (eq 1). The use of other phosphine ligands,
Figure 1 shows the molecular structure of 3a, which assumed
a square-planar coordination geometry around the Pd center
such that the two PMe3 ligands were located at the cis
positions. The C−C bond lengths in the five-membered
chelated ring (C4−C5 = 1.490(2) Å, C5−C5* = 1.484(3) Å)
were similar to the bond length (1.49(2) Å) reported
previously for rhodacyclopentane, which was prepared by the
Received: February 1, 2013
Published: May 28, 2013
© 2013 American Chemical Society
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Note
Scheme 1. Possible Mechanism for the Formation of A
structures of 4a,b, which were similar, as expected. The Pd
center assumed a square-planar coordination geometry in which
the two PMe3 groups were located at the cis positions. The
much shorter Pd1−P2 bond lengths (2.2262(3) Å (4a),
2.2211(17) Å (4b)) in comparison to the Pd1−P1 bond length
(2.3448(4) Å (4a), 2.3470(17) Å (4b)) most likely arose from
the influence of a Pd1−O1 bond in the trans position. The
Pd1−C7 bond length (2.0662(13) Å (4a), 2.067(6) Å (4b))
was slightly shorter than the bond length (2.086(2) Å) reported
previously for a palladalactone prepared by the reaction of
(Me3P)2Pd(styrene) with diketene.7 Although the closely
related nickelalactone was reported to form from allene and
CO2, the structure was proposed only on the basis of IR and
elemental analysis.8
The six-membered lactones (A) were presumably produced
through reductive elimination from the seven-membered
palladalactone (5) formed via 3 or 4. The acetylene insertion
into the five-membered nickelalactone was reported in the
context of a six-membered lactone synthesis from 2 mol of
acetylene and CO2.9 The formation of 5 through 4 was
consistent with the regioselectivity of the catalytic reaction,
which yielded the 3,5-disubstituted 2-pyrones.
In summary, we have demonstrated that (nBu3P)2Pd
complexes catalyze the cyclization between arylallenes and
carbon dioxide to yield six-membered lactones. The catalysis
presumably proceeds via the five-membered palladalactones,
which are isolable from a stoichiometric reaction of (Me3P)2Pd,
arylallene, and CO2.
Figure 1. ORTEP drawing of palladacyclopentane 3a. Hydrogen
atoms are omitted for simplicity; thermal ellipsoids are drawn at the
50% probability level. Atoms with asterisks are crystallographically
equivalent to the atoms labeled with the same number without
asterisks. Selected bond lengths (Å) and angles (deg): Pd1−P1 =
2.3200(5), Pd1−C4 = 2.0973(16), C4−C5 = 1.490(2), C5−C5* =
1.484(3), C5−C6 = 1.355(3); P1−Pd1−P1* = 97.27(2), Pl−Pd1−C4
= 90.25(5), Pl−Pd1−C4* = 171.77(5), C4−Pd1−C4* = 82.45(9),
Pd−C4−C5 = 110.19(11), C4−C5−C6 = 128.73(16), C4−C5−C5*
= 111.37(11).
EXPERIMENTAL SECTION
■
General Procedures. All manipulations of air-sensitive materials
were carried out under an argon atmosphere using standard Schlenk
techniques or in a glovebox. Solvents were purified by conventional
means and were distilled immediately prior to use. (η5-C5H5)Pd(η3-
C3H5) was synthesized according to the literature method.10 The H,
1
13C{1H}, and 31P{1H} NMR spectra were recorded on a JEOL-
LA400WB superconducting high-resolution spectrometer (400 MHz
for 1H). 31P{1H} NMR spectra were referenced to external 85%
phosphoric acid. IR spectra were recorded on a Shimadzu FTIR-8500
spectrometer. The elemental analyses were carried out using a CE-EA
1110 automatic elemental analyzer. Reaction mixtures were analyzed
by GC using GL Science TC-1 (60 m) capillary columns on a
Shimadzu GC-2010 gas chromatograph equipped with a flame
ionization detector (FID). GC-MS data were obtained using a
Shimadzu GC-17A gas chromatograph connected to a GCMS-QP
5000 mass spectrometer.
reaction of RhCl(PMe3)3 with arylallenes.6c The single 31P{1H}
NMR signal at δ −21.95 ppm indicated a symmetrical structure.
Complex 3b yielded NMR spectra similar to those obtained
from 3a and was proposed to assume the same five-membered
palladacycle structure.
Treatment of the (Me3P)2Pd(η2-CH2CCHAr) complex
(2) with CO2 instantaneously resulted in the generation of a
five-membered palladalactone (4). This is the first reported
isolation of a CO2-derived palladalactone. The structure was
Preparation of 3,5-Dibenzyl-2-pyrone (A). In a stainless steel
autoclave (20 cm3 inner volume), carbon dioxide (10 bar) was added
to a mixture of CH3CN (5 mL), (η5-C5H5)Pd(η3-C3H5) (47 mg, 0.22
1
fully elucidated by elemental analysis, IR, H NMR, 13C{1H}
NMR, and 31P{1H} NMR. Figure 2 shows the X-ray crystal
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Note
Figure 2. (a) ORTEP drawing of palladalactone 4a. Hydrogen atoms are omitted for simplicity; thermal ellipsoids are drawn at the 50% probability
level. Selected bond lengths (Å) and angles (deg): Pd1−P1 = 2.3448(4), Pd1−P2 = 2.2262(3), Pd1−O1 = 2.0742(9), Pd1−C7 = 2.0662(13), O1−
C9 = 1.2991(17), O2−C9 = 1.2329(17), C7−C8 = 1.5005(18), C8−C9 = 1.5146(18), C8−C10 = 1.342(2); P1−Pd1−P2 = 101.948(13), Pl−Pd1−
O1 = 84.15(3), Pl−Pd1−C7 = 166.75(4), P2−Pd1−O1 = 172.60(3), P2−Pd1−C7 = 91.29(4), O1−Pd1−C7 = 82.70(5), Pd1−O1−C9 =
114.94(8), Pd1−C7−C8 = 106.06(9), O1−C9−O2 = 122.87(13), O1−C9−C8 = 114.89(11), O2−C9−C8 = 122.24(12), C7−C8−C9 =
116.23(12), C7−C8−C10 = 126.30(12), C9−C8−C10 = 117.21(12). (b) ORTEP drawing of palladalactone 4b. Hydrogen atoms are omitted for
simplicity; thermal ellipsoids are drawn at the 50% probability level. One of the two crystallographically independent molecules is shown. Selected
bond lengths (Å) and angles (deg): Pd1−P1 = 2.3470(17), Pd1−P2 = 2.2211(17), Pd1−O1 = 2.091(4), Pd1−C7 = 2.067(6), O1−C9 = 1.262(7),
O2−C9 = 1.293(7), C7−C8 = 1.443(8), C8−C9 = 1.473(7), C8−C10 = 1.403(7); P1−Pd1−P2 = 99.18(6), Pl−Pd1−O1 = 91.64(13), Pl−Pd1−C7
= 171.08(17), P2−Pd1−O1 = 168.92(13), P2−Pd1−C7 = 87.39(17), O1−Pd1−C7 = 82.1(2), Pd1−O1−C9 = 110.8(3), Pd1−C7−C8 = 103.3(4),
O1−C9−O2 = 120.2(5), O1−C9−C8 = 118.1(5), O2−C9−C8 = 121.7(5), C7−C8−C9 = 114.1(5), C7−C8−C10 = 128.2(5), C9−C8−C10 =
116.5(5).
mmol), tri-n-butylphosphine (89 mg, 0.44 mmol), sodium acetate
(AcONa) (18 mg, 0.22 mmol), and phenylallene (0.50 g, 4.30 mmol)
at room temperature. The initial pressure was adjusted to 50 bar at 80
°C, and the autoclave was heated at that temperature for 12 h. After
cooling, biphenyl (50 mg) was added to the reaction mixture as an
internal standard for the GC analysis. The yield of 3,5-dibenzyl-2-
pyrone (A) was determined by GC (0.45 g (38%), TON = 7.4). 3,5-
Dibenzyl-2-pyrone (A) was purified using a Japan Analytical Industry
(400 MHz in THF-d8 at −60 °C): δ 1.24 (d, 9H, J(PH) = 7 Hz,
P(CH3)3), 1.27 (d, 9H, J(PH) = 7 Hz, P(CH3)3), 2.10 (m, 2H,
CH2), 6.36 (m, 1H, CH−), 6.91 (1H, para), 7.11 (2H, meta),
7.47 (2H, ortho). 31P{1H} NMR (160 MHz in THF-d8 at −60 °C): δ
−26.8 (d, J(PP) = 28 Hz), −23.6 (d, J(PP) = 28 Hz).
Complex 2b was prepared analogously (85% yield) as a yellow
powder. This powder was used without further purification (95%
1
1
purity, as determined by H NMR). H NMR (400 MHz in toluene-
d8): δ 1.10 (br, 18H, P(CH3)3), 2.66 (br, 2H, CH2), 3.41 (s, 3H,
−OCH3), 6.84 (br, 1H, CH−), 6.91 (1H, meta), 7.84 (2H, ortho).
31P{1H} NMR (160 MHz in toluene-d8): δ −27.2 (br), −23.9 (br).
Preparation of 3a,b. A mixture of (η5-C5H5)Pd(η3-C3H5) (0.17g,
0.80 mmol) and trimethylphosphine (0.13 g, 1.68 mmol) in THF (10
mL) was stirred for 0.5 h at room temperature. To the mixture was
added phenylallene (0.37g, 3.20 mmol) at room temperature. The
reaction mixture was then stirred at room temperature for 24 h under
an argon atmosphere. Volatiles were removed under vacuum, and the
residual solid was washed with hexane (3 × 4 mL) and dried in vacuo.
Recrystallization from toluene−hexane afforded 3a as pale yellow
1
Co. Lc-250HS recycling preparative HPLC. H NMR (400 MHz in
acetone-d6): δ 3.68 (s, 2H, C6H5-CH2), 3.72 (s, 2H, C6H5-CH2), 7.03
(s, 1H), 7.12−7.36 (m, 10H, Ph), 7.46 (s, 1H). 13C{1H} NMR (100
MHz in acetone-d6): δ 35.53, 37.01, 119.63, 127.19, 127.36, 129.17,
129.42, 129.48, 129.58, 129.74, 139.45, 139.93, 142.37, 147.62, 162.52
(CO). IR (KBr, cm−1): 1714 (ν(CO)). HRMS (FAB): calcd for
C19H16O2 (MH+) 276.1150, found 276.1151.
Preparation of 2a,b. A mixture of (η5-C5H5)Pd(η3-C3H5) (0.51g,
2.40 mmol) and trimethylphosphine (0.37 g, 4.80 mmol) in diethyl
ether (25 mL) was stirred for 0.5 h at room temperature. To the
mixture were added diethyl ether (5 mL) and phenylallene (0.28 g,
2.40 mmol) at room temperature. The reaction mixture was then
stirred at room temperature for 3 h under an argon atmosphere.
Removing the volatiles under vacuum and washing the residual solid
with hexane (3 × 3 mL) gave 2a (0.70 g, 78% yield) as a yellow
powder. This powder was used without further purification (97%
1
crystals (0.25 g, 64%). H NMR (400 MHz in CD2Cl2): δ 1.31 (d,
18H, J(PH) = 7 Hz, P(CH3)3), 2.33 (m, 4H, CH2−), 6.05 (s, 2H, 
CH−), 7.09 (2H, para), 7.27 (4H, meta), 7.49 (4H, ortho). 31P{1H}
NMR (160 MHz in CD2Cl2): δ −21.95. Anal. Calcd for C24H34P2Pd:
C, 58.72; H, 6.98. Found: C, 59.06; H, 6.71. Mp: 141−143 °C dec.
Complex 3b was prepared analogously (50% yield) as colorless
1
1
purity, as determined by H NMR). H NMR (400 MHz in toluene-
d8): δ 1.09 (br, 18H, P(CH3)3), 2.64 (br, 2H, CH2), 6.85 (br, 1H,
CH−), 7.10, 7.33, 7.89 (br, 5H, C6H5−). 31P{1H} NMR (160 MHz
in toluene-d8): δ −27.1 (br), −24.0 (br). 1H NMR (400 MHz in THF-
d8): δ 1.23 (br, 18H, P(CH3)3), 2.18 (br, 2H, CH2), 6.34 (br, 1H,
CH−), 6.87 (1H, para), 7.07 (2H, meta), 7.42 (2H, ortho). 31P{1H}
1
crystals. H NMR (400 MHz in toluene-d8): δ 0.80 (d, 18H, J(PH) =
7 Hz, P(CH3)3), 2.72 (br, 4H, CH2−), 3.40 (s, 3H, −OCH3), 6.64 (s,
2H, CH−), 6.88 (4H, meta), 7.68 (2H, ortho). 31P{1H} NMR (160
MHz in toluene-d8): δ −22.30.
Preparation of 4a,b. An excess amount of CO2 (0.19 g, 4.28
mmol) was vacuum-transferred into a diethyl ether (5 mL) solution of
1
NMR (160 MHz in THF-d8): δ −28.1 (br), −24.6 (br). H NMR
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Organometallics
Note
2a (0.40 g, 1.07 mmol), and the mixture was stirred for 12 h at room
temperature. The resulting solid product was collected by filtration,
washed with hexane, and dried in vacuo. Recrystallization from
(5) See the Supporting Information for more details concerning
structural identification of lactone A.
(6) (a) Barker, G. K.; Green, M.; Howard, J. A. K.; Spencer, J. L.;
Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1978, 1839. (b) Winchester,
W. R.; Gawron, M.; Palenik, G. J.; Jones, W. M. Organometallics 1985,
4, 1894. (c) Choi, J.-C.; Sarai, S.; Koizumi, T.; Osakada, K.;
Yamamoto, T. Organometallics 1998, 17, 2037.
(7) (a) Osakada, K.; Doh, M.-K.; Ozawa, F.; Yamamoto, A.
Organometallics 1990, 9, 2197. (b) Kakino, R.; Nagayama, K.;
Kayaki, Y.; Shimizu, I.; Yamamoto, A. Chem. Lett. 1999, 685.
(8) Hoberg, H.; Oster, B. W. J. Organomet. Chem. 1984, 266, 321.
(9) (a) Hoberg, H.; Schaefer, D. J. Organomet. Chem. 1982, 238, 383.
1
CH2Cl2−THF gave 4a as colorless crystals (0.36 g, 80%). H NMR
(400 MHz in CD2Cl2): δ 1.31 (d, 9H, J(PH) = 9 Hz, P(CH3)3), 1.33
(d, 9H, J(PH) = 9 Hz, P(CH3)3), 2.53 (br, 2H, CH2−), 6.89 (br, 1H,
CH−), 7.13 (1H, para), 7.23 (2H, meta), 7.37 (2H, ortho). 13C{1H}
NMR (100 MHz in CD2Cl2): δ 14.54 (d, J(CP) = 21 Hz, P(CH3)3),
16.76 (d, J(CP) = 33 Hz, P(CH3)3), 29.64 (br, CH2−), 124.64 (br,
−CCH), 126.07, 127.75, 128.84, 138.01, 143.04 (br, −CCH),
180.40 (CO). 31P{1H} NMR (160 MHz in CD2Cl2): δ −18.93 (d,
J(PP) = 42 Hz), −5.11 (d, J(PP) = 42 Hz). IR (KBr, cm−1): 1629
(ν(CO)). Anal. Calcd for C16H26O2P2Pd: C, 45.89; H, 6.26. Found: C,
45.59; H, 6.00. Mp 164−165 °C dec.
(b) Hoberg, H.; Schaefer, D.; Bruckhart, G.; Kruger, C.; Romao, M. J.
̈
̃
J. Organomet. Chem. 1984, 266, 203.
Complex 4b was prepared analogously (85% yield) as pale yellow
(10) Tatsuno, Y.; Yoshida, T.; Otsuka, S. Inorg. Synth. 1979, 19, 220.
(11) Sheldrick, G. M. SHELXL-97, Program for the Solution and the
1
crystals. H NMR (400 MHz in CD2Cl2): δ 1.35 (d, 9H, J(PH) = 9
Hz, P(CH3)3), 1.40 (d, 9H, J(PH) = 9 Hz, P(CH3)3), 2.58 (br, 2H,
CH2−), 3.77 (s, 3H, −OCH3), 6.84 (d, 2H, J(HH) = 8 Hz, meta), 6.91
(br, 1H, CH−), 7.40 (d, 2H, J(HH) = 8 Hz, ortho). 13C{1H} NMR
(100 MHz in CD2Cl2): δ 14.49 (d, J(CP) = 20 Hz, P(CH3)3), 16.65
(d, J(CP) = 32 Hz, P(CH3)3), 29.21 (CH2−), 55.76 (−OCH3), 113.15
(meta), 125.10 (−CCH), 131.14 (ortho), 130.38 (ipso), 140.88
(−CCH), 157.95 (para), 180.53 (CO). 31P{1H} NMR (160 MHz in
CD2Cl2): δ −9.09 (d, J(PP) = 39 Hz), −5.26 (d, J(PP) = 39 Hz). IR
(KBr, cm−1): 1610 (ν(CO)). Anal. Calcd for C17H28O3P2Pd: C, 45.50;
H, 6.29. Found: C, 45.38; H, 6.04. Mp 142 °C dec.
Refinement of Crystal Structures; Universitat Gottingen, Gottingen,
Germany, 1997.
̈
̈
̈
Crystal Structure Determination. Data collection was performed
on a Bruker SMART APEX-II CCD diffractometer (Mo Kα radiation,
graphite monochromator). The crystal data, data collection, and
refinement parameters are summarized in Table S1 (Supporting
Information). The structures were solved using direct methods and
standard difference map techniques and were refined by full-matrix
least-squares procedures on F2 using the SHELX-97 program.11
Hydrogen atoms on carbon were included in the calculated positions.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, tables, and CIF files giving NMR spectra of A and
crystallographic data for 3a and 4a,b. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: junchul.choi@aist.go.jp (J.-C.C.); t-sakakura@aist.go.
jp (T.S.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This study was partially supported by Ministry of Education,
Culture, Sports, Science and Technology of Japan.
REFERENCES
■
(1) (a) Aresta, M.; Dibenedetto, A. Dalton Trans. 2007, 2975.
(b) Sakakura, T.; Choi, J.-C.; Yasuda, H. Chem. Rev. 2007, 107, 2365.
(c) Omae, I. Coord. Chem. Rev. 2012, 256, 1384.
(2) (a) Carbon Dioxide as a Source of Carbon: Biochemical and
Chemical Uses; Aresta, M., Forti, G., Eds.; ; D. Reidel: Dordrecht, The
Netherlands, 1987. (b) Carbon Dioxide Activation by Metal Complexes;
Weller, M. G., Ed.; Wiley-Interscience VCH: Weinheim, Germany,
1988.
(3) (a) Dohring, A.; Jolly, P. W. Tetrahedron Lett. 1980, 21, 3201.
̈
(b) Sasaki, Y. J. Mol. Catal. 1989, 54, L9. (c) Tsuda, T.; Yamamoto, T.;
Saegusa, T. J. Organomet. Chem. 1992, 429, C46. (d) Zimmer, R.;
Dinesh, C. U.; Nandanan, E.; Khan, F. A. Chem. Rev. 2000, 100, 3067.
(4) Behr, A.; Henze, G. Green Chem. 2011, 13, 25.
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