Beilstein J. Org. Chem. 2013, 9, 1724–1729.
for formal enyne cross metathesis (f-EYCM) [5]. These exam- seemed to retard the reaction and a less bulky PPh3 was chosen
ples allow for an effective alkyne–alkene coupling under mild as the optimal ligand (Table 1, entries 8–13). Further optimiz-
reaction conditions (rt) with as little as 1.5 ~ 2 equiv of an ation with regard to reactants stoichiometry was conducted. An
excess component.
increased rate was observed when the amount of allyl ethers
increased up to 3 equivalents. However, an increase in the
amount of sulfonylacetylene (1) was less effective (Table 1,
entries 14–18). Finally, SbF6– turned out to be an optimal
counter-anion for cationic [Au(PPh3)]+ (Table 1, entries 19–21).
A control experiment with AgSbF6 as the only catalyst led to no
reaction (Table 1, entry 22). Apparently, unlike allyl alcohols,
sterically bulkier allyl ethers do not undergo O-attack on the
alkyne in the presence of Ag-catalyst [7].
Figure 1: Donor- and acceptor-substituted alkynes for Au-catalyzed
intermolecular reactions.
With the above optimized conditions in hand, the scope of the
carboalkoxylation of sulfonylacetylene was examined (Table 2).
The alkoxy group in the ethers 2 had an impact on the effi-
Expanding upon the intermolecular coupling reactions of ciency of the current tandem carboalkoxylation. The reaction of
readily available alkenes with alkynes would significantly methyl ether 2a was accompanied by a side product 4
enhance the synthetic utility of gold catalysis and therefore (R1 = Me) resulting from a premature dissociation of the allyl
should find fruitful applications. While it has been known for a cation fragment before the rearrangement, decreasing the yield
long time that allyl alcohols undergo intermolecular alkoxyla- of desired 3a (Table 2, entry 1). However, 2b having sterically
tion-[3,3]-sigmatropic rearrangement under Ag(I) or Au(I) bulky secondary (IPr) or primary alkoxy groups underwent
catalysis [7,8], allyl ethers that are less nucleophilic due to smooth reactions (Table 2, entries 2 and 3). It is reasonable to
steric reasons react more slowly and have not been known to assume that a bulky group R1 would decelerate the initial
undergo similar reactions until recently. In our previous work O-attack on the alkyne. However, once the Au-bound oxonium
[9], it was shown that ester-substituted alkynes underwent an ion (A in Scheme 1) is formed, the resulting rearrangement
efficient intermolecular carboalkoxylation with allyl ethers via a seems to be facilitated by the presence of a bulky substituent at
tandem conjugate addition and a [3,3]-sigmatropic rearrange- R1.
ment [10-12]. Preliminary results in the above studies [5,9]
have demonstrated that a polarizing effect of the sulfonyl The substituents on the allyl unit also affected the reaction
substituent on the alkyne is highly effective in promoting the significantly. A cyclohexyl group as γ-substituent (R2) led to a
reaction under a mild condition with relatively low amount of slower reaction, delivering 3d only in 54% yield, with a
excess reactants. We report herein the details of our investi- concomitant decrease in the ratio of [3,3]- versus [1,3]-
gation on the intermolecular reactions of alkynyl sulfones with rearrangement products, while primary alkyl groups as R2 were
allyl ethers aimed at definition of the substrate scope and at well accommodated (Table 2, entries 4–6). These indicated that
elucidation of the competitive [1,3], and [3,3]-rearrangement a steric crowding in the proposed [3,3]-sigmatropic rearrange-
pathways and their respective mechanisms.
ment transition state (Path A in Scheme 1) resulted in a slug-
gish reaction, but affected the competitive [1,3]-rearrangement
less severely. It is noteworthy that an unsubstituted (R2, R3 = H)
Results and Discussion
At the outset, the effect of ligand, counter-anion and solvent in allyl ether 2g afforded 3g in a good yield (Table 2, entry 7),
the Au-catalyzed coupling of p-toluenesulfonylacetylene (1) unlike the reactions with propiolates [9] where only ~21% of
with an allyl ether 2 was examined (Table 1). When Au(L)SbF6 carboalkoxylation product was obtained. However, a competi-
(L = di-t-butyl-o-biphenylphosphine, JohnPhos) formed in situ tion experiment using 3h having two different allyl groups
was used as catalyst, the reaction was more efficient in chlori- showed that the more electron-rich allyl unit migrated exclu-
nated solvents rather than polar aprotic or aromatic hydro- sively (Table 2, entry 8), clearly indicating that an electron-rich
carbon solvents (Table 1, entries 1–7). Contrary to the previous R2 substituent accelerated the [3,3]-sigmatropic rearrangement.
[4 + 2] cycloaddition, formal enyne cross metathesis or [2 + 2] In the presence of R3 (α-)substituent (2i–k), however, both the
cycloaddition [4,5] where JohnPhos ligand showed the best rate and the yield of the reaction was significantly compro-
performance, the optimal ligand for the current carboalkoxyla- mised and the reaction was accompanied by the extensive for-
tion was different. While the role of electron density of the mation of either 4 or 5 (Table 2, entries 9–13). It is interesting
ligand was less obvious, the steric bulk on the ligand clearly to note that the ratio of [3,3]- versus [1,3]-rearrangement prod-
1725
Jun, Jungho
