Regioselective cyclizations utilizing a gold-catalyzed [3,3] propargyl ester rearrangement

Article abstract of DOI:10.1002/anie.201203923

Switch-Au-roo: A new strategy for the regioselective synthesis of unsaturated carbocycles by chemoselective activation of a Rauhut-Currier zwitterion surrogate, formed from the Au-catalyzed [3,3] sigmatropic rearrangement of propargylic esters, has been achieved. By reversing the regiochemistry of the propargyl ester the synthesis of either the endo- or exocyclic enones is feasible.

Full text of DOI:10.1002/anie.201203923

Angewandte
Chemie
DOI: 10.1002/anie.201203923
Synthetic Methods
Regioselective Cyclizations Utilizing a Gold-Catalyzed [3,3] Propargyl
Ester Rearrangement**
John W. Cran* and Marie E. Krafft
Unsaturated carbocycles of the general configuration
1 (Scheme 1) constitute a fundamental building block in
organic synthesis owing to their inherent flexibility for further
elaboration. Such compounds have traditionally been
accessed through the intramolecular Rauhut–Currier (RC)
reaction utilizing trialkyl-phosphine- and tertiary-amine-
based catalysts [Eq. (1); Scheme 1].[1–3] The efficacy of this
method has been demonstrated through its use as a key step in
natural product syntheses by several groups.[4]
group [FG; Eq. (2); Scheme 1] for one of the unsaturated
carbonyl groups, capable of activation by an appropriate
catalyst to reveal latent reactivity analogous to the nucleo-
philic zwitterionic intermediate proposed for the RC reaction
[FG*; Eq. (2); Scheme 1].[5] Such an intermediate would then
be primed for Michael addition to the second a,b-unsaturated
carbonyl group, thus allowing either regioisomer to be cleanly
accessed provided the catalyst possessed the necessary
chemoselectivity to discriminate between the surrogate and
the unmodified unsaturated carbonyl group.
Herein we report the successful utilization of the gold-
catalyzed [3,3] rearrangement of propargyl esters for this
purpose.[6,7] Propargyl carboxylates have been reported by
several groups[8] to undergo a gold-catalyzed [2,3] rearrange-
ment by a 1,2-acyl migration to give alkenyl gold carbenoids
of type A (Scheme 2). Subsequently, Zhang[6] showed that
Scheme 1. Chemoselective formation of zwitterion surrogate.
However, utilization of this protocol can be hindered by
the lack of regioselectivity demonstrated by the nucleophilic
catalyst for unsymmetrical substrates where R1 and R2 differ.
Although this problem can be overcome by ensuring a suffi-
cient difference in electrophilicity between the two unsatu-
rated systems, this feature inherently limits the reaction scope.
Additionally, there remains no way of inducing the less
reactive unsaturated carbonyl group to preferentially react
with the catalyst to access the regioisomerically disfavored
cycloadduct. Conceptually, we envisaged a solution to this
problem might be found by introducing a surrogate functional
Scheme 2. Proposed gold-catalyzed propargyl ester rearrangements.
propargyl carboxylates can also undergo [3,3] rearrangment
through either consecutive 1,2-acyl migrations or a 1,3-acyl
migration to give the a-vinyl gold oxocarbenium intermediate
C via allene B. Interconversion between the species is thought
to be reversible[9] and the equilibrium is determined largely by
the steric and electronic nature of the substituents.[10]
The interception of the intermediates B or C with various
nucleophiles and electrophiles has since been reported.[7a,10,11]
In particular, Zhang and co-workers have shown that the
postulated a-vinyl gold intermediate C has sufficient nucle-
ophilic character to undergo intermolecular addition to soft
electrophiles such as NIS.[11h,12] As such, we hypothesized that
the intermediates C1 or D1 (Scheme 3), could serve as a RC
zwitterion equivalent and intramolecularly add to suitably
appended a,b-unsaturated carbonyl groups, which as Michael
acceptors, are also soft electrophiles.
[*] Dr. J. W. Cran, Prof. M. E. Krafft
Department of Chemistry and Biochemistry
Florida State University, Tallahassee, FL 32306 (USA)
E-mail: john_cran@hotmail.com
[**] We wish to thank Dr. Virginie Maggiotti for running NOE and
decoupling experiments, Dr. Eli Ron and Dr. Phong Vu for useful
discussions, and the NSF and the MDS Research Foundation for
their financial support.
Accordingly, we synthesized the propargyl pivolate 3a
(Scheme 4),[13] with the expectation that it would undergo
rearrangement and cyclization to give the cyclic enone 4a
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203923.
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
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Table 1: Optimization study.
Entry [M]
X
Solvent
t [min] Yield [%][a]
1
2
AuCl
AuCl
ClO4
ClO4
MeNO2
MeNO2
10
10
89
71–86
(10 mol%)
3
AuCl
ClO4
ClO4
–
MeNO2[b] 60
trace
4
5
6
7
8
9
10
11
12
13
14
15
16
AuCl
AuCl
AuCl3
CH2Cl2
10
10
10
10
10
180
60
10
10
60
60
60
60
35
32
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
–
83[c]
84
PPh3AuCl
PtCl2
ClO4
ClO4
–
–
–
ClO4
ClO4
ClO4
–
33
0
PivOH[d]
TsOH[e]
HClO4
–
25
46[c]
0
Scheme 3. Proposed mechanism for cyclic enone formation.
–
trace
<10
72[c]
0
NHCAuCl
PPh3AuNTf2
(Ph3PAu)3OBF4
–
[a] Yield of isolated product. Unreacted starting material accounted for
the majority of the mass balance. [b] 99% distilled MeNO2 with 1%
deionized water. [c] No starting material present. [d] Used 40 mol%
catalyst. [e] Used 20 mol% catalyst. NHC=1,3-Bis(2,6-diisopropyl-
phenyl)imidazol-2-ylidene, Tf=trifluoromethanesulfonyl, Ts=4-toluene-
sulfonyl.
Scheme 4. Synthesis of substrate 3a. PCC=pyridinium chlorochro-
mate, Piv=pivaloyl, TBAF=tetra-n-butylammonium fluoride,
TBS=tert-butyldimethylsilyl, THF=tetrahydrofuran.
gold(I) catalysts examined gave distinctly contrasting results
(entries 15 and 16).
With the optimized results in hand the reaction scope was
studied (Table 2). In general, yields were high with the
reactions complete in less than 1 hour. Significantly, it was
found that alkyl substituents and electron-rich and electron-
poor aryl substituents were tolerated at both the R1 and R2
(Table 1). To our satisfaction, subjecting 3a to a range of
commercially available gold and platinum catalysts revealed
that the strategy was feasible (Table 1). Optimum reaction
conditions were found by employing AuClO4 (formed in situ
from AuI and AgClO4) in dry MeNO2 (entry 1). A 2:1 ratio of
Ag/Au was found to give higher yields and shorter reaction
times than a 1:1 ratio (entry 2). When using wet MeNO2
virtually no product was formed (entry 3), thus suggesting
that while exogenous water is presumably needed for release
of the catalyst too much is detrimental for the reaction. AuCl3
was also found to be an active catalyst although the
conversion was not as clean as with AuCl (entry 6). AuCl
without AgClO4 was found to catalyze the reaction, although
gold(III) formed from disproportionation might be the active
Table 2: Cyclization reactions.
Entry Substrate
n
R1
R2
Product Yield [%][a]
1
2
3
4
5
6
7
8
3b
3c
3d
3e
3 f
3g
3a
3h
3i
3j
3k
3l
3m
3n
3o
1
1
1
1
1
1
2
2
2
2
2
1
2
1
2
Ph
Me
Ph
nBu
nBu
H
Ph
Ph
Me
Ph
Ph
Me
Ph
Me
Me
Ph
Me
Ph
Me
Me
4b
4c
4d
4e
4 f
4g
4a
4h
4i
4j
4k
4l
4m
4n
4o
92
93[b]
85
75
79
0[c]
96
94
66[b]
71
0[c]
91
73
84
65
catalyst (entry 5).[14] AgClO4 was found to be inactive as
[15]
a catalyst (entries 12 and 13), while PtCl2
catalyzed the
reaction, but at a slower rate (entry 8). Pivalic acid (entry 9),
a reaction by-product formed through loss of the pivolate, was
found to be inert, while the more acidic TsOH (entry 10)
catalyzed the reaction but much less efficiently than gold.
Employing HClO4 (entry 11) resulted in considerable decom-
position. The presence of a PPh3 ligand on gold had no
beneficial effect (entry 7), and the presence of an NHC ligand
on gold rendered the catalyst insoluble in MeNO2, thereby
resulting in a low yield (entry 14). Surprisingly, the cationic
9
nBu
nBu
H
10
11
12
13
14
15
p-F3CC6H4
p-MeOC6H4 Me
Ph
Ph
p-MeC6H4
p-ClC6H4
[a] Yield of isolated product. [b] Run for 2 h. [c] Only starting material
recovered.
2
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Table 3: Cyclization reactions.
positions. Consequently, the method allowed the synthesis, in
high yields, of cycloadducts that would either be regiochemi-
cally disfavored or not cleanly isolable by the traditional RC
reaction. In this regard, of particular note are entries 2, 5, 10,
13, and 15 where the nature of R1 would render the
corresponding enone the least reactive towards PR3 or NR3,
thus leading predominantly to the opposite regioisomer. Also
notable are the examples shown in entries 4 and 9 where little
discrimination between the two alkyl enones would be
expected. Interestingly, when R1 = H (entries 6 and 11) no
reaction was observed, thus corroborating previous reports
that secondary propargylic aliphatic esters undergo neither
1,3 nor 1,2-migration efficiently if the alkyne is terminal.[16]
The synthesis of exocyclic enones (Scheme 5), a class of
compounds hitherto not accessed by the traditional RC
approach, was also examined by switching the regiochemistry
of the propargyl ester motif.
Entry Substrate
n
R1
R2 Product Yield [%][a]
E/Z[b]
1
2
3
4
5
6
5a
5b
5c
5d
5e
5 f
1
1
1
1
2
2
Ph
Ph
Me
Ph
6a
6b
6c
6d
6e
6 f
95
87
>20:1
>20:1
9:1
>20:1
>20:1
–
PhCH2CH2 Ph
78
iPr
Ph
Ph
Ph
Ph
Me
62[c]
67
complex
mixture
1
[a] Yield of isolated product. [b] Ratio determined using H NMR
spectroscopy. Stereochemistry confirmed by NOE using 1H NMR
spectroscopy. [c] Used 25 mol% AuCl, 50 mol% AgClO4, 3.5 h.
center and the iPr group in the a-vinyl gold intermediate C2
(Scheme 5). For six-membered rings, a reasonable yield was
obtained when R1 and R2 were phenyl substituents (entry 5),
however, changing R2 to a methyl group led to a complex
mixture of products (entry 6).
1
No intermediates were observable by H NMR spectro-
scopy or TLC for either series of substrates, thus suggesting
that the initial rearrangement of the propargyl pivolate is the
rate-determining step.[17] For substrates 3a–o, it was unclear
whether the key nucleophilic intermediate was the allene D1
(Scheme 3) or the a-vinyl gold oxocarbenium ion C1 as both
gold catalysts and strong protic acids catalyzed the reaction.
In the event that D1 is the cyclization precursor, wherein an
integral role for gold is suggested by its superior activity
compared with protic acids, a Michael addition catalyzed by
the pivalic acid by-product cannot be ruled out.
Scheme 5. Proposed mechanism for formation of exocyclic enones.
For compounds 5a–f, the absence of the Z stereoisomer
suggested that the allene D2 was not the progenitor to
cyclization, otherwise the enone would be expected to
approach from the least hindered face, thus resulting in the
formation of the Z product (Scheme 7). Furthermore, the
predominance of the E stereoisomer is indicative of a cis
relationship between R1 and Au in C2 (Scheme 5). This
configuration is expected to be more stable than if R1 is cis to
the oxocarbenium group because of the reduced steric
crowding as a consequence of the long Au–C(sp2) bond.[11h]
Notably, in the presence of TsOH, 5a failed to undergo
cyclization under the standard reaction conditions after
1 hour. As only D2 (Scheme 5) can be the cyclization
precursor in this case, this provides strong evidence against
its involvement in the gold-catalyzed reaction. The Z isomer
Accordingly, substrates 5a–f were synthesized (Scheme 6)
and the substrates were subsequently subjected to our
standard reaction conditions (Table 3). Overall the expected
exocyclic enones were obtained, almost exclusively, as the
E isomers in high yields. Five-membered rings formed
efficiently when phenyl or alkyl substituents were present at
R1 (entries 1–3). When R1 = iPr, a marked reduction in
reactivity was observed (entry 4);
a substoichiometric
amount of the gold catalyst (25 mol%) was required for
complete conversion within 3.5 hours, most likely a result of
the steric hindrance from an interaction between the gold
Scheme 6. Synthesis of substrates 5a–f.
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
Scheme 7. Approach of the enone.
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was initially expected via a less sterically crowded transition
state (Scheme 7), however, it appears that the endo nature of
the nucleophilic enol makes cyclization unfavorable. In
contrast the a-vinyl gold species can undergo a favored 5-
exo-trig cyclization. Furthermore, the larger orbitals associ-
ated with Au and the longer C–Au bond length, likely reduce
conformational strain in the transition state, thus aiding
cyclization.
To further elucidate the reaction mechanism, particularly
regarding the mechanistic ambiguity concerning substrates
3a–o, the enantioenriched propargyl pivolates 3a and 5a were
synthesized and subjected to the standard reaction conditions
(Scheme 8). Strikingly, complete racemization was observed
Received: May 21, 2012
Revised: June 21, 2012
Published online: && &&, &&&&
Keywords: carbocycles · gold · rearrangement · regioselectivity ·
.
synthetic methods
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in both cases. Regarding 3a, this likely rules out a reaction
pathway involving the chiral allene intermediate D1 and
indicates the presence of either the achiral intermediates A or
C1 as significant components of the precyclization equilibri-
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excess would be expected.[7c,18] It should be noted however
that D1 could still be the direct precursor to cyclization after
racemization via A or C1. In the case of 5a, the observed
racemization adds weight to a mechanism proceeding via the
achiral a-vinyl gold intermediate C2 (Scheme 5). When
considered alongside the inertness of 5a towards TsOH,
and the isolation of 6a–e predominately as the E stereoiso-
mers, this data amounts to compelling evidence for C2 as the
cyclization precursor.
In conclusion, we have demonstrated a concise strategy
for the regioselective synthesis of unsaturated carbocycles,
utilizing the gold-catalyzed chemoselective formation of
a nucleophilic RC zwitterion equivalent. In particular this
approach allows access to regioisomers which are disfavored
in the traditional RC reaction. The protocol has also been
extended to the synthesis of exocyclic enones. Mechanistically
our results suggest the intermediacy of the a-vinyl gold
oxocarbenium species C1 and C2. Regarding the substrates
3a–o, either C1 or D1 could be the progenitor to cyclization,
and for substrates 5a–f the case for C2 as the precursor is
persuasive. Further studies, to more accurately determine the
mechanistic details and to assess the reaction scope, including
an asymmetric variant, are ongoing.
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Communications
Synthetic Methods
Switch-Au-roo: A new strategy for the
regioselective synthesis of unsaturated
carbocycles by chemoselective activation
of a Rauhut–Currier zwitterion surrogate,
formed from the Au-catalyzed [3,3] sig-
matropic rearrangement of propargylic
esters, has been achieved. By reversing
the regiochemistry of the propargyl ester
the synthesis of either the endo- or
exocyclic enones is feasible.
J. W. Cran,* M. E. Krafft
&&&& — &&&&
Regioselective Cyclizations Utilizing
a Gold-Catalyzed [3,3] Propargyl Ester
Rearrangement
6
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