Gold(i)-catalysed [1,3] O→C rearrangement of allenyl ethers

Article abstract of DOI:10.1039/c3cc49629e

A simple and rapid access to the α-substituted acryl aldehydes has been provided by developing a gold-catalysed [1,3] rearrangement of the allenyl ethers importantly with a record turnover frequency of 4600 h^-1 (at 0.05 mol% of the catalyst concentration) in homogeneous gold(i) catalysis. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc49629e

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Gold(I)-catalysed [1,3] O-C rearrangement
of allenyl ethers†‡
Cite this: Chem. Commun., 2014,
50, 2152
Chandrababu Naidu Kona and Chepuri V. Ramana*
Received 19th December 2013,
Accepted 2nd January 2014
DOI: 10.1039/c3cc49629e
www.rsc.org/chemcomm
A simple and rapid access to the a-substituted acryl aldehydes has trace amounts of [1,3] rearrangement products has been noticed
been provided by developing a gold-catalysed [1,3] rearrangement during [3,3] Claisen rearrangement of propargyl vinyl ethers and allyl
of the allenyl ethers importantly with a record turnover frequency vinyl ethers by Toste and Krafft.11,12 On the other hand, in the case
of 4600 hꢀ1 (at 0.05 mol% of the catalyst concentration) in homo- of the reactions involving allenyl ethers and gold-complexes, Cui and
geneous gold(I) catalysis.
co-workers have recently reported the gold-catalysed addition of
alcohols at the C1 of allenyl(p-methoxybenzyl) ether (1c).13 Indeed,
During the last decade, gold-complexes enabled organic synthesis 1c has been selected as a starting point for the projected [Au]-catalysed
with a remarkable reactivity and have the ability to catalyse diverse [1,3] rearrangement by considering the fact that electron-donating
organic transformations.1 In particular, the selective activation of groups on the aryl ring will stabilize the intermediate benzylcation
allenes by gold-complexes, followed by the subsequent inter and formed. We reasoned that carrying out the reaction in aprotic solvents
intramolecular nucleophilic additions and [3,3]-sigmatropic rearran- would facilitate the reaction in the requisite direction (Fig. 1).
gements (such as Claisen and Cope rearrangements), has received
To start in this direction, the allenyl ethers 1a–1c, having,
substantial attention.2,3 Surprisingly, the utilization of allene units and respectively, the decyl, benzyl and PMB units as R groups were
the gold catalysts in the [1,3] O-C rearrangement has been less selected as representative substrates for looking at the scope and
explored. The [1,3] rearrangement reaction of vinyl ethers constitutes limitations inter alia to learn about how the stability of the in situ
an important C–C bond forming reaction and has attracted consider- generated carbocation will influence the outcome of the [1,3]
able attention over the last two decades.4,5 Lewis acids, in general, rearrangement. The exploratory experiments were carried out
have been employed as catalysts for this reaction. Recently, the employing 2 mol% of catalyst in dichloromethane as the solvent.
complexes of Pd, Co, Ir and Ru have been shown to be effective for The reactions with Au[III] salts ended up with the hydrolysis of the
this purpose.6 The [1,3] rearrangement reactions involving the Lewis allenyl ethers 1a–1c. In the case of the Au[I]-complexes, when
acid catalysts are generally postulated to proceed through the hetero-
lytic cleavage of the O–R bond of the vinyl ether and via the formation
of an intermediate ion-pair comprising the carbocationic species R+
and an enolate counterpart. The success of this reaction depends
upon a careful choice of Lewis acids, as well as the selection of
appropriate R groups that can stabilize the transient carbocation.7
Considering the prerequisite of an ion-pair mechanism for the
success of a ‘‘[1,3] rearrangement’’ and the formation of ion pairs with
the catalytically active cations in the Au[I]-catalyzed reactions,8 we
envisioned that the [1,3] rearrangement of the allenyl ethers would
constitute a general protocol for the synthesis of C2-substituted acryl
aldehyde derivatives.9,10 Coming to the gold-catalysis, the formation of
Division of Organic Chemistry, CSIR-National Chemical Laboratory,
Dr. Homi Bhabha Road, Pune-411008, India. E-mail: vr.chepuri@ncl.res.in;
Fax: +91 20 2590 2629; Tel: +91 20 2590 2577
† Dedicated to Professor Ganesh Pandey on the occasion of his 60th birthday.
Fig. 1 Gold(I) catalysed [1,3] rearrangement of vinyl ethers and the
proposed synthesis of C2-substituted acryl aldehydes via allenyl ethers.
‡ Electronic supplementary information (ESI) available: Characterization data
and spectra of all new compounds. See DOI: 10.1039/c3cc49629e
2152 | Chem. Commun., 2014, 50, 2152--2154
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Table 2 Scope of [Au]-catalyzed [1,3] rearrangement reaction
employed alone, the starting materials were recovered intact. As
expected, the combination of the Au(I)-complexes with the additive
AgSbF6 resulted in the quantitative conversion of 1c within 5 minutes
at 0 1C in dichloromethane and 2-(4-methoxybenzyl)acrylaldehyde (2c)
was obtained in excellent yield. Under similar conditions, the allenyl
ethers 1a and 1b hydrolysed immediately after the addition of
the catalyst. Changing either the ligand on the Au[I]-complex or the
counter anion did not provide any promising results with the
substrates 1a and 1b. Controlled experiments revealed that, with
the silver salts [5–10 mol%] AgOAc, AgOTf and AgNTf2, only the
hydrolysis of the allenyl ether 1c was observed.14 With AgSbF6, the
reaction was sluggish and the acryl aldehyde 2c was obtained in
moderate yields. These experiments clearly demonstrate that the
active catalyst involved in the [1,3] rearrangement was the in situ
generated cationic [Au]-complex and that the weakly coordinating
counter anion favours the rearrangement (Table 1).15
As the reaction with 2 mol% of the catalyst was found to be almost
instantaneous, we next examined the optimal concentration of the
catalyst required at ambient temperature [see Table E3, ESI,‡ for
complete details]. Controlled experiments were conducted with the
allenyl ether 1c at different concentrations of the catalyst, varied from
0.0125–0.05 mol%. Out of all the concentrations, the reaction with
0.05 mol% catalyst at 0 1C (5 min duration, S/C = 2800) was found to
be optimal for C–C bond formation and gave the required rearranged
product 2c in 97% yield (on 1 g scale) with the highest TOF
(4600 hꢀ1).16 For lower concentrations like 0.0125 mol%, the reaction
was sluggish at rt and, when refluxed, the reaction proceeded within
12 h (80% conversion), after which there was no further conversion of
1c, providing 2c in 90% isolated yield (S/C = 9072).
Table 2 reveals the scope of the current reaction. All the reactions The rearrangement of the (4-(N,N-dimethylamino)phenyl)-methanol
were carried out by employing 0.05 mol% of the catalyst. The allenyl ethers 1n–1p also proceeded smoothly and delivered the
C1-secondary allenyl ethers of the (4-methoxyphenyl)methanol with corresponding rearranged products 2n–2p in 92–95% yield. Although
n-butyl 1d (on 1 g scale), phenyl 1e (1 g scale) and benzyl 1f the simple benzyl allenyl ethers 1b and 1v are not compatible,
substitutions underwent the [1,3] rearrangement smoothly and gratifyingly, the diphenylmethanol allenyl ether 1r gave the rearranged
provided the corresponding acryl aldehydes 2d–2f in excellent yield. product 2r in 87% yield, revealing that the stabilization of the inter-
A similar trend was observed with the substrates having methoxy mediate carbocation is important. As expected upon the stabilization of
group(s) at either ortho and/or meta and/or para (2g–2m and 2q) the intermediate carbocation, the rearrangement of allenyl ethers of
positions, which revealed that the presence of the methoxy substituent 1-(naphthalen-2-yl)ethan-1-ol 1x, 1-(naphthalen-1-yl)ethan-1-ol 1y
is important, but that it is not necessary for the substituent to be at and (tetrahydrofuran-2-yl)methanol 1z was found to be unsuccessful.
the para position. Similarly, the [1,3] rearrangement of allenyl ether of The successful synthesis of the 2,3-disubstituted acryl aldehydes
electron-rich 6-methoxy-1,2,3,4-tetrahydronaphthalen-1-ol (1s) was facile. 2t and 2u (isolated as an inseparable E/Z mixture) reveals the
applicability of this methodology for the [1,3] rearrangement of
the C1-substituted allenyl ethers.
Table 1 Catalyst optimization (see Table E2 of ESI for full details)
Next, the feasibility of the rearrangement with other electron rich-
and heterocyclic systems was examined. As shown in Table 2, the
rearrangement of ferrocenyl methanol allenyl ether 1a0 provided the
corresponding acrylaldehyde 2a0. The [1,3] rearrangement of allenyl
ethers having various heterocyclic units was also facile under these
conditions. The acryl aldehydes having N-methylpyrrole (2b0),
N-benzylpyrrole (2d0), thiophene (2c0), benzofuran (2e0), benzo-
thiophene (2f0) and N-benzylindole (2g0) structural units have
been prepared in excellent yields by employing 0.05 mol% of
the catalyst. However, under these conditions, the pyridine
derived allenyl ether 1h0 was found to remain intact.
Entry
Substrate
Catalyst
Additive Yield
1–6
7–9
1a or 1b or 1c AuCl3 or AuBr3
1a or 1b or 1c AuCl(PPh3)
—
—
AgSbF6
AgSbF6
AgNTf2
Hydrolysis
No reaction
Hydrolysis
91%
10 and 11 1a or 1b
12
13
AuCl(PPh3)
AuCl(PPh3)
Coming to the mechanism of the reaction, two possible
modes for the activation of allenyl ether were expected – through
1c
1a or 1b or 1c AuCl(PPh3)
Hydrolysis
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the allene unit.9 In the case of the reaction of 1c with
Au(PPh3)NO3, it has been proposed by Cui and co-workers that
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(Fig. 2).13 As a control, when allenyl ether 1c was exposed to
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hydrolysis of the C–O bond occurred through the allenyl ether
activation by the [Au]-complex.
In summary, the first report on the [Au]-catalysed [1,3] O-C
rearrangement of allenylethers leading to the C2-substituted
acryl aldehydes was documented. The reaction is facile even
with 5 ꢁ 10ꢀ2 mol% of catalyst, which we believe is the lowest
catalyst loading that has been reported in the area of homo-
geneous gold-catalysis.
We thank CSIR (India) for funding this project under the
12FYP ORIGIN program and a research fellowship to KCN.
We thank Mr Mahesh Patil and Mr Mahesh Shinde for the
synthesis of some intermediate allenes.
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2154 | Chem. Commun., 2014, 50, 2152--2154
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