Pt-Catalyzed [1,2]- versus [1,3]-Acyloxy Migration
COMMUNICATION
faster than the hydrolysis of allenes 11 and 16. Otherwise,
enone products would be obtained from the hydrolysis of al-
lenes at 808C as shown in Table 2.
Although alkynes preferentially undergo [1,3]-acyloxy mi-
gration, trace amounts of [1,2]-OAc-shifted products were
also obtained from 20c and 20d with major [1,3]-OAc-shift-
ed products 22c and 22d in Table 2. In this context, 23a and
23b were next used to examine whether [1,2]- versus [1,3]-
acyloxy migration could be controlled even in electronically
unbiased internal alkynes (Scheme 3).[12] The reactions of
23a and 23b yielded a mixture of products 25 and 27a/27b
from [1,3]-acyloxy migration and [1,2]-acyloxy migration, re-
spectively. Acetate 23a gave a 1:3 to 1:4 ratio of 25 (from
a [1,3]-OAc shift and hydrolysis) and 27a (from a [1,2]-OAc
shift followed by [1,2]-H shift) with the same catalyst system
(5 mol% PtCl2 under CO) in dry toluene at 808C. The addi-
tion of a small amount of water increased the yield of 25 by
promoting the hydrolysis of allene 24a. Interestingly, the
use of para-nitrobenzoate gave 27b as a major product via
the [1,2]-shift of benzoate and a [1,2]-H shift. It is likely that
the use of sterically hindered ester retards the hydrolysis of
24b, reducing the formation of enone compound 25, thus
giving 27b as a major product.
Scheme 4. Steric effect of the tertiary alkyl group at the propargylic
center.
vided products 32a/32b in excellent yield even at room tem-
perature.
In addition to the reaction temperature and the substitu-
ent pattern around the alkyne, the nature of the catalyst
also affected the reaction course. Au-catalyzed reactions
provided different results from Pt-catalyzed reactions. Al-
though 7a yielded a mixture of [1,2]- and [1,3]-acyloxy-shift-
ed products with PtCl2 at room temperature, and only [1,2]-
shifted product 8a at 608C as shown in Table 1 and
Scheme 5, the same substrate underwent [1,3]-acyloxy mi-
gration to give only compound 33 in 72% yield after hydrol-
ysis with 5 mol% (PPh3)AuCl/AgSbF6 (Scheme 5). Similarly,
the reaction of 34 with 5 mol% AuCl3 exclusively provided
the [1,3]-OAc shift-hydolysis product 36 in 88% yield at
room temperature, whereas the reaction with PtCl2 gave 1,3-
diene 38 by [1,2]-acyloxy migration followed by a [1,2]-H
shift.[13] These discrepancies are likely the result of faster hy-
drolysis of allenes 9a and 35 to 1,3-enone 33 and 36 in Au-
catalyzed reactions.
Scheme 3. Steric effect of sterically demanding propargylic ester for the
selectivity of [1,2]- versus [1,3]-acyloxy migration.
The substituent pattern around the alkyne affects the re-
action course as well. The reaction of 23a/23b with a linear
alkyl group at the propargylic position generated a mixture
of [1,3]- and [1,2]-acyloxy-migrated products, as shown in
Scheme 3. However, the reaction of 28a/28b with a branched
alkyl group at the same position gave only 1,3-diene 32a/
32b through [1,2]-acyloxy migration followed by [1,2]-H
shift, regardless of the kind of ester (Scheme 4). This sug-
gests that the extra alkyl group at the propargylic center of
29a/29b should slow down the deacylation whereby the for-
mation of enone 30 was prohibited. Moreover, the [1,2]-H
À
shift of the tertiary C H of 31a/31b is faster than that of
À
the corresponding secondary C H, resulting in the efficient
formation of trisubstituted alkene 32a/32b. It was recog-
À
nized that reactions of 23a/23b with a secondary C H bond
required a higher reaction temperature (808C) in Scheme 3,
Scheme 5. Effect of the nature of catalysts for the selectivity of [1,2]-
versus [1,3]-acyloxy migration.
À
whereas reactions of 28a/28b with a tertiary C H bond pro-
Chem. Eur. J. 2012, 18, 4495 – 4498
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4497