Angewandte
Chemie
with the anisyl sulfone substrate 5e, the double oxidation
lecular cyclopropanation. The reaction scope was then
À
displayed a much improved yield, while the C H insertion
examined. As shown (8b–e), the aryl sulfones 5 with different
aryl groups, including furan-2-yl (8d), underwent the reaction
smoothly. The terminal substituents of the alkyne could be
either a phenyl (8e) or a cyclopropyl (8 f). Our attempt to
vary the styrene led to successful reaction with 4-bromostyr-
ene (8g) and 4-tert-butylstyrene (8h), but in the case of
4-methoxystyrene, polymerization occurred during the reac-
tion. Other electron-rich alkenes, such as ethyl vinyl ether,
were also not suitable.
With a-methylstyrene as a solvent, a stepwise, formal
[3+2] cycloaddition occurred, thus affording the dihydrofuran
9 in a serviceable yield (Table 3).[5a] This divergent reactivity
can be understood by considering that the benzylic position of
the styrene can better accommodate positive charge and has
increased steric hindrance, both of which disfavor the
expected concerted cyclopropanation reaction.
product was formed in less than 5% yield (entry 4). Similarly,
a cyclohexyl group at the alkyne terminus was allowed
(entry 5). In addition, the alkyne terminus could accommo-
date a cyclopropyl (entry 6), a phenyl (entry 7), and a furan-2-
yl (entry 8) group, the last two of which allowed regiospecific
generation of diaryl-substituted a-oxo gold carbenes by
simply using appropriate aryl alkynyl sulfone substrates.
Besides phenyl-based sulfones, a trans-b-styryl sulfone (5j;
entry 9) and a furan-2-yl sulfone (5k; entry 10) underwent the
reaction without incident, thus affording the diketone prod-
ucts 7j (75%) and 7k (87%), respectively. Of note, in some
cases (entries 5,6, and 8–10), 2,6-dichloropyridine N-oxide
(6a) was more efficient than 6c.
While these results are consistent with acyl gold carbene
ꢀ
intermediates of type B, the fact that the C C bond is
oxidized into a symmetric 1,2-dicarbonyl moiety prevented us
from unequivocally establishing the regiochemistry of these
carbenes, despite the difficulty in formulating an alternative
mechanism for the formation of its isomer A. Liu and co-
workers[14] previously demonstrated that that carbenes
related to B, which are generated by intramolecular alkyne
oxidation, can undergo cyclopropanation reactions with
styrenes. When the oxidation of the alkynyl sulfone 5a was
performed in the presence of styrene, the intended cyclo-
propanation reaction indeed occurred, and the cyclopropyl
ketone 8a was formed selectively in 80% yield upon isolation
(Table 3). This result offers strong support for the regiospe-
cific generation of gold carbenes of the type B, and reveals
that the pyridine byproduct does not interfere the intermo-
The success with the b-styryl sulfone 5j (Table 2, entry 9)
encouraged us to examine other alkenyl sulfones. As shown in
Equation (1), the 2-methylprop-1-en-1-yl sulfone 5l under-
went the gold-catalyzed oxidative desulfonylation, but the
isolated product was the trans-dienone 10, instead of the
corresponding diketone. Our attempt to trap the alkenyl acyl
gold carbene intermediate with styrene was futile. This result
highlights the facile nature of the carbene to undergo E1-type
elimination under the reaction conditions, as outlined in the
equation. With the cyclohexen-1-yl sulfone 5m as the
substrate, the expected dienone product 11 was isolated,
along with the bicyclic furan 12 [Eq. (2)]. The formation of
the latter product should involve a 4p electrocyclic ring
closure, which is in line with the related work reported by the
group of Liu[5b] and ourselves.[15]
Table 3: Cyclopropanation with styrenes: Scope.[a]
8a (80%)
8d (57%)
8b (77%)
8e (76%)
8c (51%)
To further investigate the desulfonylative carbene rear-
rangement, we surmised that instead of an aryl or alkenyl
group an alkoxy might behave in a similar manner, thereby
leading to the generation of a new type of donor/acceptor-
8 f (54%)[b]
8g (83%)
8h (67%)
9 (57%)[b]
[a] All reactions run in vials. The oxidant was introduced by syringe pump
over a 12 h period, and the product d.r.>95:5. [b] 6b was used as the
oxidant, and a-methylstyrene as solvent.
Angew. Chem. Int. Ed. 2015, 54, 11775 –11779
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim