.
Angewandte
Communications
acetoxyallene 3 (for a detailed mechanism, see the Supporting
Information). However, with this mechanism one would not
obtain any significant change of the ratio between vinylallene
3 and dimers with dideuterated species [D2]-1a compared to
nondeuterated 1a, since with this mechanism the ratio
between the competing pathways leading to 3 and dimers
would be determined in the first step without any possible
isotope effect (see the Supporting Information). The low ratio
of 1.3:1 between [D1]-3aa and dimers from [D2]-1a (Sche-
me 5b) therefore rules out this mechanism. In contrast, the
two mechanisms proposed in Scheme 6 (via intermediates B
and C, respectively) are in agreement with the results
observed in Equations (1)–(4) and Scheme 5.
Scheme 5. The control reactions of allenynes [D6]-1a (a) and [D2]-1a
(b).
In summary, we have developed a novel palla-
dium-catalyzed oxidative carbocyclization of alle-
nynes in the presence of various carboxylic acids,
providing access to potentially synthetically useful
acyloxylated vinylallenes. During this carbocycliza-
À
À
tion a new C C bond, a new C O bond, and a new
allene structure are formed. Furthermore, an aero-
bic version of this transformation using a catalytic
amount of BQ was developed to enhance the utility
of this method. According to the results of deute-
rium labeling experiments, we propose that the
reaction of the allenynes proceeds through compet-
À
ing propargylic and allylic C H bond cleavage
pathways or via a pallada(IV)cyclopentene inter-
mediate with competing b-eliminations. Further
studies on the mechanism and synthetic application
of this reaction are ongoing.
Scheme 6. Plausible mechanisms for the palladium-catalyzed oxidative acetoxyla-
tion/carbocyclization of allenyne 1.
Experimental Section
Typical experimental procedure for palladium-catalyzed
On the basis of these experimental findings, we propose
oxidative acyloxylation/carbocyclization of allenyne 1: To a mixture
of BQ (26.2 mg, 0.24 mmol) and Pd(OAc)2 (2.4 mg, 0.01 mmol) were
added 1b (69.5 mg, 0.20 mmol) and HOAc (0.4 mL) at room temper-
ature. The reaction was stirred at 608C for 17 h. After full
consumption of starting material 1b, as monitored by TLC, the
reaction was cooled to room temperature, diluted with Et2O (20 mL),
and quenched with H2O (5 mL). The organic phase was separated and
the aqueous phase was extracted with Et2O (2 ꢀ 20 mL). The
combined organic layers were washed with H2O and dried over
anhydrous Na2SO4. Evaporation and column chromatography on
silica gel (pentane/ethyl acetate = 10:1) afforded 3ba (53.3 mg, 66%)
as a liquid; 1H NMR (400 MHz, CDCl3): d = 5.74 (d, J = 1.2 Hz, 1H),
5.34–5.24 (m, 1H), 3.73 (s, 3H), 3.72 (s, 3H), 3.18 (d, J = 3.6 Hz, 2H),
2.46–2.38 (m, 1H), 2.36–2.24 (m, 1H), 2.08–1.98 (m, 2H), 1.97 (s, 3H),
1.66–1.46 (m, 7H), 1.45–1.18 (m, 5H), 0.89 ppm (t, J = 7.2 Hz, 3H);
13C NMR (100 MHz, CDCl3): d = 198.1, 170.8, 170.7, 169.1, 149.4,
125.3, 104.2, 95.0, 79.7, 63.5, 52.9, 52.8, 36.6, 34.8, 33.6, 31.2, 29.2, 25.4,
22.1, 21.6, 21.52, 21.50, 13.9 ppm; HRMS (ESI): calc. for C23H32NaO6
[M+Na]+: 427.2091; found: 427.2091.
the mechanism shown in Scheme 6. p-Complex formation of
1 with Pd(OAc)2 to give chelate A and subsequent rearrange-
À
ment involving a propargylic C H bond cleavage would
produce vinylpalladium intermediate B. Intramolecular vinyl-
palladation of the allene moiety would generate (p-allyl)pal-
ladium intermediate D, which is attacked by an acetate
nucleophile (coordinated or external)[14] to give 3. Competing
[4d]
À
allene attack in A through allylic C H bond cleavage and
subsequent alkyne insertion would generate intermediate E.
Reaction of E with another molecule of allenyne 1 through
insertion of the vinyl–Pd bond of E into the allene moiety of
1 would give the p-allyl species F, which would yield dimers
(4, 6, and 7; for details, see the Supporting Information). Also,
a mechanism involving a pallada(IV)cyclopentene[7] inter-
mediate C could be possible, which would generate inter-
mediates D and E through b-H elimination and subsequent
loss of HOAc leading to product 3 and dimeric by-products,
respectively. Although b-H elimination in electron-deficient
PdIV intermediates is considered to be less likely,[15] b-H
elimination from less electron-deficient PdIV intermediate C
may occur.
Received: October 30, 2012
Revised: January 15, 2013
Published online: February 5, 2013
One could also consider a mechanism through acetoxy-
=
palladation of the terminal C C double bond of the allene,
followed by insertion of the alkyne into the newly generated
vinyl–Pd bond and subsequent b-H elimination to give
Keywords: allenynes · carboxylic acids · cyclization · oxidation ·
palladium
.
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 3217 –3221