Palladium-catalyzed selenoacylation of allenes leading to the regioselective formation of functionalized allyl selenides

Article abstract of DOI:10.1021/ol801329q

(Chemical Equation Presented) Palladium-catalyzed regio- and stereoselective selenoacylation of allenes with selenol esters proceeded to produce functionalized allyl selenides with the acyl moiety at the inner carbon and the SePh group at the terminal car

Full text of DOI:10.1021/ol801329q

ORGANIC
LETTERS
2008
Vol. 10, No. 18
3957-3960
Palladium-Catalyzed Selenoacylation of
Allenes Leading to the Regioselective
Formation of Functionalized Allyl
Selenides
Masashi Toyofuku,† Erika Murase,† Shin-ichi Fujiwara,*,‡ Tsutomu Shin-ike,‡
Hitoshi Kuniyasu,† and Nobuaki Kambe*,†
Department of Applied Chemistry, Graduate School of Engineering, Osaka UniVersity,
Suita, Osaka 565-0871, Japan, and Department of Chemistry, Osaka Dental
UniVersity, Hirakata, Osaka 573-1121, Japan
fujiwara@cc.osaka-dent.ac.jp; kambe@chem.eng.osaka-u.ac.jp
Received June 12, 2008
ABSTRACT
Palladium-catalyzed regio- and stereoselective selenoacylation of allenes with selenol esters proceeded to produce functionalized allyl selenides
with the acyl moiety at the inner carbon and the SePh group at the terminal carbon in high yields. A mechanism accounting for the observed
regio- and stereoselectivities is proposed based on the results of DFT calculations.
During the past 20 years, allenes have been shown to be
versatile building blocks in organic synthesis.1 It is known
that a variety of heteroatom compounds such as organotin,
-borane, and -selenium species add to allenes in the presence
of transition metal catalysts such that an allene double bond
inserts into R3Sn-H, R3Si-BR′2, and RSe-SeR′ bonds.2
There are fewer known examples, however, of allene
insertion into carbon-heteroatom bonds:3,4 Chatani and co-
workers reported Ni or Pd catalyzed cyanosilylation of
allenes with trimethylsilyl cyanide (NC-SiMe3).3a Hua and
Tanaka reported Rh catalyzed chloroesterification of allenes
with chloro carbonates (ROC(O)-Cl).3b Shirakawa, Hiyama,
and co-workers described Ni catalyzed acylstannylation and
alkynylstannylation of allenes with acyl stannanes and
alkynyl stannanes (RC(O)-SnR′3 and RCt C-SnR′3,
respectively).3c-e In these cases, vinylic heteroatom products
like 1 were obtained as the major products rather than allylic
compounds 2 (eq 1). Here, we disclose the first example of
carbothiolation and carboselenation of allenes that proceeds
with reverse regioselectivity leading to 2 (eq 2).
† Osaka University.
‡ Osaka Dental University.
(1) For recent reviews, see: (a) Ma, S. Chem. ReV. 2005, 105, 2829. (b)
Ma, S. Acc. Chem. Res. 2003, 36, 701. (c) Modern Allene Chemistry; Krause,
N., Hashmi, A. S. K., Eds.; Wiley-VCH: Weinheim, 2004.
(2) For recent examples, see: (a) Kamiya, I.; Nishinaka, E.; Ogawa, A.
Tetrahedron Lett. 2005, 46, 3649. (b) Kazmaier, U.; Lucas, S.; Klein, M.
J. Org. Chem. 2006, 71, 2429. (c) Ohmura, T.; Suginome, M. Org. Lett.
2006, 8, 2503. (d) Ohmura, T.; Taniguchi, H.; Suginome, M. J. Am. Chem.
Soc. 2006, 128, 13682. (e) Kodama, S.; Nishinaka, E.; Nomoto, A.; Sonoda,
M.; Ogawa, A. Tetrahedron Lett. 2007, 48, 6312. (f) Abe, Y.; Kuramoto,
K.; Ehara, M.; Nakatsuji, H.; Suginome, M; Murakami, M.; Ito, Y.
Organometallics 2008, 27, 1736. For others, see refs 1a and 1c.
(3) (a) Chatani, N.; Takeyasu, T.; Hanafusa, T. Tetrahedron Lett. 1986,
27, 1841. (b) Hua, R.; Tanaka, M. Tetrahedron Lett. 2004, 45, 2367. (c)
Shirakawa, E.; Nakao, Y.; Hiyama, T. Chem. Commun. 2001, 263. (d)
Shirakawa, E.; Nakao, Y.; Tsuchimoto, T.; Hiyama, T. Chem. Commun.
2002, 1962. (e) Nakao, Y.; Shirakawa, E.; Tsuchimoto, T.; Hiyama, T. J.
Organomet. Chem. 2004, 689, 3701. (f) Saso, H.; Ando, W. Chem. Lett.
1988, 1567. (g) Seyferth, D.; Shannon, M. L.; Vick, S. C.; Lim, T. F. O.
Organometallics 1985, 4, 57.
(4) For recent radical reactions, see: (a) Tsuchii, K.; Imura, M.; Kamada,
N.; Hirao, T.; Ogawa, A. J. Org. Chem. 2004, 69, 6658. (b) Mei, Y.-Q.;
Liu, J.-T.; Liu, Z.-J. Synthesis 2007, 739.
10.1021/ol801329q CCC: $40.75
Published on Web 08/21/2008
2008 American Chemical Society

Table 1. Pd(0)-Catalyzed Selenoacylation of Allenesa
When a toluene solution (0.3 mL) containing selenol ester
4a (R′ ) nHex, 0.4 mmol), cyclohexylallene 3a (R ) cHex,
1.2 equiv), Pd2(dba)3·CHCl3 (2.5 mol %), and PPh3 (10 mol
%) was heated at reflux for 12 h, selenoacylation product,
allyl selenide 5a5 (corresponds to 2 in eq 2), was obtained
in 70% yield (Table 1, entry 1) with excellent regio- and
stereoselectivity.6 In this reaction, the selenol ester adds to
the terminal C-C double bond of the allene exclusively with
the acyl moiety at the inner carbon and the SePh group at the
terminal carbon to form an allyl selenide. Regioisomers such
as vinyl selenide 1 or its stereoisomers were not detected by
1H NMR analysis of the crude product.
Table 1 summarizes the results obtained using several
selenol esters and allenes. When n-octylallene 3b was
employed, allyl selenide 5b was formed when Pd(PPh3)4 was
used instead of Pd2(dba)3·CHCl3 and PPh3 as a catalyst
(entry 2).6 Although the reaction of phenylallene 3c with 4a
gave allyl selenide 5c in 90% yield (entry 3) in 5 h, a small
amount of E isomer was also detected.7 Next, selenoacylation
of allenes having an electron-donating or -withdrawing group
was examined. For example, allene 3d having a benzyloxy
group can be subjected to selenoacylation using 4a giving
rise to allyl selenide 5d in 87% yield (entry 4); however,
when ethyl 2,3-butadienoate (R ) CO2Et) was employed
under similar reaction conditions, almost all of 4a was
recovered and the expected allyl selenide was not obtained
probably due to oligomerization of the allene.8,9 Selenol ester
4b bearing a cyclohexyl group also underwent selenoacy-
lation with 3c to give 5e; however, the stereoselectivity
diminished in this case (entry 5). In all cases, no decarbo-
nylative product was formed.10 Next, selenoesterification and
a Conditions: 4 (0.4 mmol), 3 (0.48 mmol), Pd2(dba)3·CHCl3 (2.5 mol
%), PPh3 (10 mol %), toluene (0.3 mL), reflux. b Isolated yields (NMR
c
1
yields). Determined by H NMR. “>98/2” means no minor isomer was
detected by NMR. d 5 mol % of Pd(PPh3)4 was used as catalyst.
selenocarbamoylation of allene precursors were examined.
When the reaction of selenocarbonate 4c with phenylallene
3c was carried out under similar conditions, the expected
allyl selenide 5f was obtained in high yield with perfect regio-
and stereoselectivity (entry 6).11 In contrast, although sele-
nocarbamoylation of 3c with carbamoselenoate 4d proceeded
to give 5g in high yield, the E isomer was formed
predominantly (entry 7).12 Existence of an alkyne unit did
not affect the reaction. For example, the reaction of cyclo-
hexylallene 3a with selenol ester 4e bearing a terminal
alkynyl group proceeded to give selenoacylation product 5h
selectively (eq 3).
(5) For synthetic utilities of allyl selenides, see: (a) Tanaka, K.; Horiuchi,
H.; Yoda, H. J. Org. Chem. 1989, 54, 63. (b) Krief, A.; Colaux, C.; Dumont,
W. Tetrahedron Lett. 1997, 38, 3315. (c) Takada, H.; Oda, M.; Miyake,
Y.; Ohe, K.; Uemura, S. Chem. Commun. 1998, 1557. (d) Nishiyama, Y.;
Kishimoto, Y.; Itoh, K.; Sonoda, N. Synlett 1999, 611. (e) Bourland, T. C.;
Carter, R. G.; Yokochi, A. F. T. Org. Biomol. Chem. 2004, 2, 1315. (f)
Yamashita, K.; Takeda, H.; Kashiwabara, T.; Hua, R.; Shimada, S.; Tanaka,
M. Tetrahedron Lett. 2007, 48, 6655. (g) Waetzig, S. R.; Tunge, J. A. Chem.
Commun. 2008, 3311. For recent total synthesis of natural products using
transformation of allyl selenides, see: (h) Albert, B. J.; Sivaramakrishnan,
A.; Naka, T.; Koide, K. J. Am. Chem. Soc. 2006, 128, 2792. (i) Albert,
B. J.; Sivaramakrishnan, A.; Naka, T.; Czaicki, A.; Koide, K. J. Am. Chem.
Soc. 2007, 129, 2648.
(6) For screening of phosphine ligands, see Supporting Information.
(7) Oxidation of isolated 5c with m-CPBA followed by hydrolysis
resulted in the formation of allyl alcohol 11 in 80% yield.
(10) Hirai, T.; Kuniyasu, H.; Kato, T.; Kurata, Y.; Kambe, N. Org. Lett.
2003, 5, 3871.
(11) Pd-catalyzed addition of thiocarbonate to alkynes was reported. Hua,
R.; Takeda, H.; Onozawa, S.; Abe, Y.; Tanaka, M. J. Am. Chem. Soc. 2001,
123, 2899.
(12) Selenocarbamoylation of 1-octyne with 4d proceeded to give
ꢀ-selenoacrylamide in 40% yield: Toyofuku, M.; Fujiwara, S.; Shin-ike,
T.; Kuniyasu, H.; Kambe, N. J. Am. Chem. Soc. 2005, 127, 9706.
(8) Ethyl 2,3-butadienoate was reported to oligomerize in the presence
of PPh3. See: Zhang, C.; Lu, X. J. Org. Chem. 1995, 60, 2906.
(9) Selenoacylation of R,R-disubstituted allenes such as vinylidenecy-
clohexane with 4a did not proceed at all.
3958
Org. Lett., Vol. 10, No. 18, 2008

Scheme 2. Computational Calculations for Allene Insertion
The reaction of a thiol ester 4f with 3c gave the
thioacylation product 5i with good regio- and stereoselectivity
albeit in a low yield under similar conditions (eq 4).13
A plausible reaction pathway for allyl selenide formation
involving a π-allylpalladium14 intermediate is shown in
Scheme 1, which explains the regioselectivity of the products.
Scheme 1. Proposed Pathway Leading to Allyl Selenide 5
to the side of the acyl group in A.15 Subsequent allene
insertion may occur in the acyl-Pd bond via counterclock-
wise rotation of the allene to give a σ-allylpalladium species
B, which is the precursor of the π-allylpalladium intermedi-
ates D. Other possible pathways, i.e., formation of C, E, and
F are unlikely since TS2, TS3, and TS4 are less stable than
TS1 and C, E, and F are also less stable than B. Accordingly,
formation of B via TS1 will be both kinetically and
thermodynamically the most favored pathway. In the forma-
tion of B and F, five-membered chelation by the intramo-
lecular coordination of carbonyl oxygen to Pd could be
important for structural stabilities (>20 kcal/mol more stable
than C and E). In addition, the carbonyl group and the CdC
unit are conjugated in B while not conjugated in F. These
factors may be crucial for the predominant formation of the
intermediate B in this system. Reductive elimination from
B affords the allyl selenide products. In this step, the PPh3
ligand may be essential based on Kurosawa’s study of [Pd(π-
allyl)(SPh)]2 reactions with PPh3.16 In fact, addition of 4a
to 3c was sluggish (<5%) without the PPh3 ligand (compare
with entry 3 in Table 1).6
This catalytic process is initiated by oxidative addition of
the acyl-Se bond of selenol ester 4 to Pd(0), affording the
acyl-Pd-Se complex 6. Insertion of the coordinated allene
into the acyl-Pd bond generates σ-allylpalladium 8 (in
equilibrium with π-allylpalladium 8′) having an acyl group
at the central carbon of the allene unit prior to reductive
elimination which produces allyl selenide 5.
To shed light on the mechanism of the reaction and on
the regio- and stereoselectivity of the process, DFT calcula-
tions were performed with methylallene and MeC(O)SeMe
as substrates and PH3 as the ligand for the Pd. Complex A
shown in Scheme 2 is the optimized model for the allene
coordinated intermediate (corresponds to 7 in Scheme 1). It
is noteworthy that the axis of the Pd-coordinated allene leans
As for the stereoselectivity of the products, the (Z)-adduct
of EtC(O)SePh to 3a (model of (Z)-5a) is calculated to be
(13) Although starting material 4f remained, yield of 5i was not improved
when the reaction time was prolonged to 10 h.
(15) It is proposed that insertion of coordinated olefins occurs via rotation
of the CdC axis toward the coordination plane of the metal center. For
examples, see: (a) Michalak, A.; Ziegler, T. Organometallics 1999, 18, 3998.
(b) Lin, B.-L.; Liu, L.; Fu, Y.; Luo, S.-W.; Chen, Q.; Guo, Q.-X.
Organometallics 2004, 23, 2114.
(14) Carbopalladation of allene to form the π-allylpalladium intermediate
is well known as a key step for various reactions using allene and Pd catalyst:
Mandai, T. In ref 1c; pp 925-972.
Org. Lett., Vol. 10, No. 18, 2008
3959

2.6 kcal/mol more stable than the corresponding (E)-adduct.
Similarly, the (Z)-adduct of MeOC(O)SePh to 3a (model of
(Z)-5f) is calculated to be 2.1 kcal/mol more stable than the
corresponding (E)-adduct. However, (Z)-5g is 0.5 kcal/mol
less stable than (E)-5g.17 These results do not conflict with
the observed stereoselectivities in entries 1, 6, and 7 (Scheme
3). In addition, when the isolated product (E)-5g was
afforded allyl-metal species like 5.18 The authors proposed
transmetalation between bismetal reagents and the π-al-
lylpalladium intermediate like 8′ prior to reductive elimina-
tion of the allyl-metal product. Thus, we examined the
possibility of selenoacylation of allenes using diselenides as
bismetal reagents. When the mixture of acyl chloride 9 (0.40
mmol), phenylallene 3c (0.48 mmol), and (PhSe)2 (0.40
mmol) was heated at reflux for 5 h with Pd(0)-PPh3 catalyst,
only bisselenation product 10 was obtained,2a and no
selenoacylation product such as 5c was detected probably
due to faster oxidative addition of (PhSe)2 to Pd(0) over that
of acyl chloride 9 (eq 6).
Scheme 3
.
Computational Calculations for Stabilities of
Products
In summary, we report that 1,2-addition of selenol esters
onto allenes proceeds with excellent regioselectivity and high
stereoselectivity in the presence of Pd(0)-PPh3 catalyst,
producing functionalized allyl selenides. A reaction pathway
accounting for the observed regio- and stereoselectivity is
proposed based on the results of DFT calculations.
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research on Priority Areas “Advanced
Molecular Transformations of Carbon Resources” from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan. M.T. expresses his special thanks for a JSAP
Research Fellowship for Young Scientist for financial support
and The Global COE (center of excellence) Program “Global
Education and Research Center for Bio-Environmental
Chemistry” of Osaka University.
subjected to the same reaction conditions as in entry 7,
isomerization ensued to give the Z/E (28/72) mixture of 5g,
while isomerization did not proceed in the presence of a
catalytic amount of PPh3 without Pd (eq 5).
Supporting Information Available: Experimental pro-
cedures and characterization data of new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
These results suggest that the stereoselectivities observed
reflect the relative thermodynamic stabilities of the products.
Furthermore, the evidence that Pd(0) inserts into the C-Se
bond of allyl selenides16 indicates that the last step of Scheme
1 (8 to 5) is reversible, and Z/E isomerization proceeds via
the π-allylpalladium intermediates 8′. Cheng and co-workers
revealed that Pd-catalyzed three-component assembly of
allenes, acyl chlorides, and bismetal reagents such as (BPin)2
OL801329Q
(17) Since the regioisomer of 5g shown below is calculated to be ca. 5
kcal/mol less stable than (Z)-5g, formation of this isomer will be
thermodynamically unfavored. In addition, reverse oxidative addition and
reductive elimination leading to 5g will proceed from the result of eq 5.
(16) Miyauchi, Y.; Watanabe, S.; Kuniyasu, H.; Kurosawa, H. Orga-
nometallics 1995, 14, 5450.
(18) Yang, F.-Y.; Shanmugasundaram, M.; Chuang, S.-Y.; Ku, P.-J.;
Wu, M.-Y.; Cheng, C.-H. J. Am. Chem. Soc. 2003, 125, 12576.
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Org. Lett., Vol. 10, No. 18, 2008