Gold-catalyzed [2+2+1] cycloaddition of 1,6-diyne carbonates and esters with aldehydes to 4-(cyclohexa-1,3-dienyl)-1,3-dioxolanes

Article abstract of DOI:10.1002/chem.201303685

A synthetic method to stereoselectively prepare 4-(cyclohexa-1,3-dienyl)-1, 3-dioxolanes in good to excellent yields by gold(I)-catalyzed [2+2+1] cycloaddition of 1,6-diyne carbonates and esters with aldehydes is described. The cascade process involves 1,2-acyloxy migration followed by cyclopropenation and cycloreversion. This leads to an unprecedented [2+2+1] cycloaddition of the resulting alkenylgold carbenoid species, examples of which are extremely rare, with two aldehyde molecules at catalyst loadings as low as 1 mol %. The usefulness of this cycloisomerization chemistry was further demonstrated by the transformation of one example to the corresponding phenol. Copyright

Full text of DOI:10.1002/chem.201303685

DOI: 10.1002/chem.201303685
Full Paper
&
Synthetic Methods
Gold-Catalyzed [2+2+1] Cycloaddition of 1,6-Diyne Carbonates
and Esters with Aldehydes to 4-(Cyclohexa-1,3-dienyl)-1,3-
dioxolanes
Weidong Rao and Philip Wai Hong Chan*[a]
Abstract: A synthetic method to stereoselectively prepare 4-
(cyclohexa-1,3-dienyl)-1,3-dioxolanes in good to excellent
yields by gold(I)-catalyzed [2+2+1] cycloaddition of 1,6-
diyne carbonates and esters with aldehydes is described.
The cascade process involves 1,2-acyloxy migration followed
by cyclopropenation and cycloreversion. This leads to an un-
precedented [2+2+1] cycloaddition of the resulting alkenyl-
gold carbenoid species, examples of which are extremely
rare, with two aldehyde molecules at catalyst loadings as
low as 1 mol%. The usefulness of this cycloisomerization
chemistry was further demonstrated by the transformation
of one example to the corresponding phenol.
Introduction
Over the last decade, gold-catalyzed alkyne cycloisomerizations
involving an in situ generated gold carbenoid species have
become an important addition to the synthetic chemist’s tool-
box for the assembly of architecturally challenging com-
pounds.[1–16] An illustrative example of this has been the
number of impressive works describing the construction of
various synthetically valuable products from cyclopropane-sub-
stituted gold carbenoid intermediates derived from 1,n-enynes
(Scheme 1).[2–7] In the case of transformations with an aldehyde
or ketone as the nucleophile, this has provided an expedient
approach to oxaheterocyclic compounds.[6,7] Included in this
has been an intramolecular variant of the reaction applied to
the synthesis of the natural products englerin A and B.[7] How-
ever, the analogous cycloadditions of an in situ formed alkenyl-
gold carbenoid adduct, resulting from [2,3]-sigmatropic rear-
rangement of a propargylic carbonate or ester, with a carbonyl
compound is not known.[8] This is surprising given that the
presence of a vinyl instead of a cyclopropane moiety in the pu-
tative organogold intermediate might be expected to exhibit
different modes of reactivity and thus the possibility of access
to a wider scope of cycloisomerization products. In this context
and as part of an ongoing program examining the utility of
gold catalysis in heterocyclic synthesis,[9] we were drawn to ex-
amining the AuI-catalyzed cycloisomerization chemistry of 1,6-
diyne carbonates and esters 1 in the presence of an aldehyde
Scheme 1. Exploiting the reactivity of in situ formed cyclopropylgold carbe-
noid species.
[Scheme 2, Eq. (1)].[10,11] In doing so, we report herein our
discovery that the resultant putative gold carbenoid species I
formed in situ was susceptible to an unprecedented [2+2+1]
cycloaddition with aldehydes. This provided an expedient and
chemoselective route to 4-(cyclohexa-1,3-dienyl)-1,3-dioxolanes
2, a key building block in organic synthesis, in good to excel-
lent yields. To our knowledge, this type of dipolar cycloaddi-
tion had been previously only achieved by RhII,II-, CuI- or CuII-
catalyzed reactions of a-diazo esters with aldehydes.[17,18] More-
over, supported by experimental evidence and ONIOM-
(QM:QM’) [ONIOM(QM:QM’)=our own n-layered intergrated
molecular orbital and molecular mechanics(quantum mechan-
ics:quantum mechanics’)] calculations, the present gold carbe-
noid intermediate approach has only been documented once
before in which it was shown to undergo Nazarov cyclization
[a] Dr. W. Rao, Prof. Dr. P. W. H. Chan
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University
Singapore 637371 (Singapore)
Fax: (+65)6791-1961
E-mail: waihong@ntu.edu.sg
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303685.
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complex G as catalyst (entries 7, 8, and 11). No reac-
tion was detected in control experiments catalyzed
by Ph3PAuNTf2 or AuCl based on TLC and 1H NMR
analysis of the crude mixtures (entries 9 and 10). Like-
wise, control reactions with PtCl2 or AgSbF6 as the
catalyst were found to lead to the recovery of the
substrate in near quantitative yield (entries 12 and
13). On the basis of the above results, reaction of 1a
with n-heptanal (10 equiv) in the presence of 5 mol%
of NHC-AuI complex A and 4 ꢁ MS in dichlorome-
thane at 08C for 30 min was deemed to provide the
optimum conditions.
Scheme 2. Cycloisomerizations of 1,6-diyne carbonates and esters involving an in situ
formed gold carbenoid species.[2–7]
To define the scope of the present procedure, we
next turned our attention to the reactions of a variety
of 1,6-diyne carbonates and esters and aldehydes
to give 2,4a-dihydro-1H-fluorenes 3 [Scheme 2, Eq. (2)].[9a,12]
Prior to this and the present work, there have been only three
other examples describing the posited migration of
a gold carbenoid moiety to an alkyne to form
a second alkenylgold carbenoid species.[13] Addition-
ally, there has been only one reported instance of
this type of reactivity in which [2+2+2] cycloaddi-
tion of 1,6-enynes (Z=C(CO2Me)2 in Scheme 1) with
two molecules of the aldehyde that was demonstrat-
ed to afford 1,3-dioxanes in low to good yields.[6b]
(Table 2). These experiments showed that with of NHC-AuI
complex A as catalyst, the reaction conditions proved to be
Table 1. Optimization of the reaction conditions.[a]
Results and Discussion
Table 1 summarizes our efforts to establish the reac-
tion conditions for the gold-catalyzed cycloaddition
of 1,6-diyne ester 1a with n-heptanal.[19] This initially
revealed that treating 1 equivalent of the starting
ester and aldehyde (5 equiv) with 5 mol% of NHC-
AuI (NHC=N-heterocyclic carbene) complex A as the
catalyst and 4 ꢁ molecular sieves (MS) in dichlorome-
thane at room temperature for 30 min gave 2a as
a single diastereomer along with 3a in 62 and 8%
yield, respectively (entry 1). The structure and stereo-
chemistry of the 1,3-cyclohexadiene adduct was con-
firmed by NMR analysis and X-ray crystal structure
determination of a closely related analogue (vide
infra).[20] Our studies subsequently showed an in-
crease in the amount of the n-heptanal from five to
ten equivalents resulted in an increase in product
yield from 62 to 68% (entry 2). By lowering the reac-
tion temperature from room temperature to 08C,
a further increase in product yield from 68 to 72%
yield was observed (entry 3). In contrast, an inspec-
tion of entries 4–11 in Table 1 showed that repeating
the reaction with other gold(I) and gold(III) com-
plexes in place of A was found to be markedly less
effective. The analogous reactions with the NHC-
gold(I) complexes B, C, and D as catalyst gave the
product in lower yields or as a mixture with the tricy-
clic adduct (entries 4–6).[21] A similar outcome was
found when NHC-gold(I) complex A was replaced
with the AuI phosphine complexes E and F, and AuIII
Catalyst
Reaction Time [h]
Yield [%][b]
2a
3a
1[c]
2
A
A
A
B
C
D
E
0.5
0.5
0.5
20
2
20
0.5
24
24
24
20
24
24
62
68
72
23
61
55
11
19
8
–
–
22
–
–
57
52
–
–
–
3[d]
4
5
6
7
8
F
[e]
9
Ph3PAuNTf2
AuCl
G
PtCl2
AgSbF6
–
–
[e]
10
11
12
13
7
[e]
–
–
–
[e]
–
[a] All reactions were performed with 0.2 mmol of 1a and 2 mmol of n-heptanal in
the presence of 5 mol% of catalyst and 4 ꢁ MS (100 mg) at room temperature. PNB=
p-nitrobenzyl. [b] Isolated yield. [c] Reaction performed with 5 equiv of n-heptanal.
[d] Reaction performed at 08C. [e] No reaction detected based on TLC and 1H NMR
analysis of the crude reaction mixture.
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general with a variety of substituted 4-(cyclohexa-1,3-dienyl)-
1,3-dioxolanes afforded in 40–84% yield from [2+2+1] cyclo-
addition of the corresponding substrates 1a–t and aldehydes.
Reactions of 1a with acetaldehyde, propionaldehyde, isobutyr-
aldehyde, pivalaldehyde and n-hexanal were found to proceed
well and furnish 2b–f in 71–81% yield. The [2+2+1] cycloaddi-
tion of 1a with aldehydes with a pendant carbocyclic (cyclo-
propanal and cyclohexanal) or alkene (4-pentenal) or ether (4-
(benzyloxy)butanal)) group were found to well tolerated under
the reaction conditions to give 2g–j in 61–79% yield. The dia-
stereochemistry and structure of 2g was also confirmed by X-
ray crystallography (Figure 1).[20] The presence of a Bz (1b), Ac
(1c), Piv (1d), Cbz (1e), and Alloc (1 f) instead of a PNB migrat-
ing group was found to have little influence on the course of
the reaction, with [2+2+1] cycloaddition of these substrates
with either acetaldehyde or propionaldehyde providing 2k–o
in 58–84% yield. Similarly, reactions of starting 1,6-diyne esters
containing other aryl motifs or a cyclopropane, cyclohexene, n-
hexane or thiophene group at the alkyne or ben-
zoate carbon center (1g–t) with propionaldehyde or
cyclohexanal were found to give the corresponding
Table 2. [2+2+1] Cycloaddition of 1,6-diyne carbonates and esters 1b–t with alde-
hydes catalyzed by A.[a]
1,3-dioxolanes 2p–2d in 40–79% yield. In all the
above transformations, the cycloisomerization pro-
cess was additionally shown to occur in a highly se-
lective manner with the [2+2+1] cycloadduct being
obtained as a single diastereo- and regioisomer.
Other than a number of unknown byproducts, no
cyclopropane adducts that could be obtained from
cyclopropanation of the putative gold carbenoid
species in substrates containing an alkene moiety, as
1
in 1i, o, q, v, and a, were detected by H NMR mea-
surement of the crude reaction mixtures.[4] Likewise,
no oxacyclic side-products resulting from participa-
tion of the organogold intermediate in [3+2] cyclo-
addition with an aldehyde were observed by
1H NMR analysis of the crude reaction mixtures. This
was further corroborated by obtaining 2l as the
only cyclic adduct in 77% yield (1.5 g) on repeating
the large-scale [2+2+1] cycloaddition of 1c (1.4 g)
with propionaldehyde in the presence of 1 mol% of
A under the conditions described in Scheme 3.
A speculative mechanism for the present AuI-cata-
lyzed [2+2+1] cycloaddition of 1,6-diyne carbonates
and esters with aldehydes is illustrated in Scheme 4.
This could initially involve coordination of the ester-
eal alkyne moiety of 1 by the AuI catalyst to afford
the gold(I)-activated complex II.[8] As a consequence,
this triggers the syn 1,2-acyloxy migration process to
give the 1,3-dioxin-1-ium species III, which under-
goes cycloreversion to form the gold carbenoid in-
termediate IV. Further reaction of this newly formed
organogold complex with the remaining alkyne
moiety followed by activation of the ensuing cyclo-
propene compound V by the Lewis acidic catalyst
might then furnish the gold(I)-coordinated species
VI.[15] Electrophilic ring opening of the cyclopropene
motif in this bicyclic adduct would provide the
second gold carbenoid intermediate I and its gold-
stabilized allylic cation resonance form I’.[9a,13] Subse-
quent [2+2+1] cycloaddition of this metallocarbe-
noid species with two molecules of the aldehyde
would lead to the regeneration of the gold(I) cata-
lyst and formation of 2. The observed product ste-
reochemistry could be due to the cycloaddition oc-
curring via the transition state conformation shown
in Scheme 4 so as to minimize the possibility of un-
[a] All reactions were performed with 0.2 mmol of 1 and 2 mmol of aldehyde in the
presence of 5 mol% of catalyst and 4 ꢁ MS (100 mg) at 08C for 0.2–24 h. Values in pa-
rentheses denote isolated product yield. [b] Reaction performed with 4 mmol of acet-
aldehyde.
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favorable steric interactions between the R3 and R4 groups of
the substrates.[17,18] The formation of the product as a single
diastereomer along with the absence of the tetrahydroisoben-
zofuran adduct 4 resulting from a competitive [3+2] cycloaddi-
tion, as depicted in Scheme 4, suggests the 1,3-dioxolane ring
forming process might also occur in a concerted manner.
In this work, the conversion of the 1,3-cyclohexadiene to the
phenol was also examined (Scheme 5). Subjecting a dichloro-
methane solution containing 2l to 1.5 equivalents of 2,3-di-
chloro-5,6-dicyano-1,4-benzoquinone (DDQ) at room tempera-
ture for 4 h was found to afford the phenyl acetate 5l in 96%
yield. Further exposure of this aromatic compound to K2CO3 in
methanol at room temperature for 2 h followed by FeCl3
(10 mol%) in dichloroethane at 808C for 2 h then furnished 6l
in 83% yield. In view of the synthetic utility of phenols as
building blocks in organic synthesis, the present method also
provides a new route to this important member of the benze-
noid family of compounds.[16,22]
Figure 1. ORTEP drawing of 2g with thermal ellipsoids at 50% probability
levels.[20]
Conclusion
In summary, we have developed an efficient chemo- and dia-
stereoselective gold-catalyzed synthetic route to 4-(cyclohexa-
1,3-dienyl)-1,3-dioxolanes from [2+2+1] cycloaddition of 1,6-
diyne carbonates and esters with aldehydes. The approach
offers a potential scale-up strategy at a catalyst loading as low
as 1 mol% to the synthetically useful 1,3-cyclohexadiene motif
containing an equally valuable 1,3-dioxolane ring system, as
demonstrated by one example in an excellent yield. Added to
this, the utility of this chemistry was illustrated by the conver-
sion of one adduct to the corresponding phenol, another
useful building block in organic synthesis. Efforts ex-
ploring the scope and synthetic applications of the
present reaction are currently underway and will be
reported in due course.
Scheme 3. Large-scale [2+2+1] cycloaddition of 1c with propionaldehyde
catalyzed by A.
Experimental Section
To a solution of the 1,6-diyne carbonate or ester
1 (0.2 mmol), aldehyde (2 mmol) and 4 ꢁ MS (100 mg)
in CH2Cl2 (3 mL) was added gold(I) complex A (10 mmol)
in CH2Cl2 (1 mL) under an argon atmosphere at 08C.
The reaction mixture was stirred at 08C for a further
0.2–24 h and monitored by TLC analysis until comple-
tion. The solution was filtered through Celite, washed
with CH2Cl2 (5 mL), concentrated under reduced pres-
sure and purified by flash column chromatography on
silica gel (eluent: n-hexane/EtOAc/CH2Cl2 =50:1:1) to
give the 4-(cyclohexa-1,3-dienyl)-1,3-dioxolane product
2.
Acknowledgements
This work is supported by a College of Science Start-
Up Grant from Nanyang Technological University
Scheme 4. Proposed mechanism for the [2+2+1] cycloaddition of 1,6-diyne carbonates
and esters with aldehydes catalyzed by A. The possibility of a competing [3+2] cycload-
dition pathway is represented by the grey dotted lines.
and a Tier 2 Grant (MOE2013-T2-1-060) from the
Ministry of Education of Singapore. We thank Dr.
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Keywords: aldehydes · cycloaddition · gold · homogenous
catalysis · propargylic compounds
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Received: September 19, 2013
Published online on December 9, 2013
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