Communications
Table 1: Gold-catalyzed rearrangement of alkynylcyclopropanes 1 into 6-
alkynyl-1,3-cyclohexadienes 2,3.
Entry
R1
R2
2 (Yield [%])[a]
3 (Yield [%])[a]
Scheme 3. Gold-catalyzed rearrangement of alkynylcyclopropanes 1k,l
bearing an unbranched alkyl substituent. Tf=trifluoromethanesulfonyl,
Ts =4-toluenesulfonyl.
1
2
3
4
5
Ph
Ph
2a (75)
2b (88)
2c (73)
2d (70)[b]
2e+3e
(70, 1.4:1)[c]
2 f+3 f
–
–
–
–
–
p-ClC6H4
p-MeC6H4
p-MeOC6H4
p-ClC6H4
p-ClC6H4
p-MeC6H4
p-MeOC6H4
Ph
treatment of alkynylcyclopropane 1c (R1 = R2 = p-MeC6H4)
with a cationic gold(I) catalyst in the presence of excess of
MeOH resulted in the formation of 4-methoxybicyclo-
[3.2.1]octadiene 7 (85% yield) as a separable endo/exo
mixture (Scheme 4a). On the other hand, bulkier alcohols
(iPrOH, tBuOH) reacted with 1a,g leading selectively to
tricyclo[3.2.1.02,7]octenes 8a,b (Scheme 4b).[17,18]
A tentative mechanism to account for these transforma-
tions is depicted in Scheme 5. First, we propose that
regioisomers 2/3 originate specifically from complexes 1-
6
7
p-ClC6H4
p-MeOC6H4
p-ClC6H4
–
–
(62, 3:1)[b,c]
2 f+3 f
p-MeOC6H4
(55, 2.4:1)[b,c]
–
8[d]
9
10
11
p-MeOC6H4
Ph
Ph
p-CNC6H4
cPr
tBu
3g (>45)
2h (54)
2i (47)
–
–
–
2j (66)[b]
ꢁ
Ph-C C-
Ph
[a] Yields of isolated products. [b] Around 5% of gem-disubstituted
cyclohexadiene 5 was detected in the crude reaction mixture. See
Ref. [15]. [c] Isolated as a mixture. The selectivity was determined by
1H NMR spectroscopic analysis. [d] Reaction conditions: PtCl4
(5 mol%), CO (1 atm), 708C, 5 h. 3g was further dehydrogenated
yielding 4g in 45% overall yield. See Ref. [16].
isomers (entries 2–4).[15] Unexpectedly, variable mixtures of
regioisomers 2/3 were formed from heterosubstituted cyclo-
propanes (R1 ¼ R2; entries 5–6). Thus, inseparable mixtures of
2e/3e (1.4:1; R1/R2 = p-ClC6H4/Ph) and 2 f/3 f (3:1; R1/R2 = p-
ClC6H4/p-MeOC6H4) were obtained. Interestingly, the regioi-
someric cyclopropane 1 f’ (R1/R2 = p-MeOC6H4/p-ClC6H4,
entry 7) yielded a mixture of regioisomers 2 f/3 f in a ratio
of 2.4:1. Thus, the regioselectivity appeared to be dependent
on the electronic demand of the aryl groups. Accordingly, it
was found that cyclohexadiene 3g was exclusively formed
(PtCl4, 5 mol%, CO, 708C, 5 h) from 1g having aryl groups of
opposite electronic nature (R1 = p-MeOC6H4; R2 = p-
CNC6H4; entry 8). As a result of its low stability, compound
3g was dehydrogenated to form 4g (DDQ, 808C; 45% overall
yield from 1g).[16] Importantly, a general discrimination
between phenyl and alkyl groups was discovered (entries 9–
10). Thus, cyclopropyl- and tBu-substituted substrates 1h,i
yielded 2h and 2i, respectively, as single isomers (47–54%
yield). On the other hand, the replacement of the alkynyl unit
with a diynyl unit (1j; R1 = phenylethynyl; entry 11) resulted
in the chemo- and regioselective formation of the butadiynyl-
substituted adduct 2j in a satisfactory yield.
Surprisingly, a completely different transformation oc-
curred from cyclopropane derivatives having a primary alkyl
substituent (Scheme 3). Stirring 1k,l in the presence of a
gold(I) catalyst at ambient temperature provided stereose-
lectively the bicyclic structures 6a,b. It is also noteworthy that
the process tolerated both amino and alkene functionalities.
Based on the assumption that cationic species might be
involved, further experiments were conducted in the presence
of an alcohol as the nucleophile (Scheme 4). Thus, the
Scheme 4. Gold-catalyzed rearrangement of alkynylcyclopropanes 1 in
the presence of alcohols. Tol=tolyl.
Au+/1’-Au+, which in turn, result from the gold(I)-catalyzed
equilibration of 1. Such a reversible process can be explained
by the 6-endo-dig pC–C attack (a) in 1/1’, thus resulting in the
cationic species I/II that equilibrate into III. Then, a retro 6-
endo cyclization from III provides both alkynylcyclopropanes
1-Au+ (cleavage A) and 1’-Au+ (cleavage B).[19] The proposed
intermediate species III and I/II were trapped with methanol
and bulky alcohols, respectively (compounds 7,8; Scheme 4).
In the same way, intermediates III arising from 1k,l (R2 =
CH2-R) underwent rapid proton elimination to form 6a,b
(Scheme 3).
Then, the formation of 2/3 from 1-Au+/1’-Au+, could be
explained by the irreversible 3-exo-dig nucleophilic attack by
the sC–C bond (b)[20] that results in the allylic cationic species
IV/V. The latter intermediates would then provide 2/3
ꢀ
through metal elimination and cleavage of the C C bond.
The kinetic selectivity toward 2/3 can be rationalized in terms
2108
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 2107 –2110