New platinum-catalysed dihydroalkoxylation of allenes

Article abstract of DOI:10.1002/adsc.201000342

A new platinum-catalysed dihydroalkoxylation of alienes is described to give aliphatic acetals by an unexpected attack of two molecules of methanol to the terminal carbon of the aliene moiety. Deuteration experiments suggest an unprecedented formal 1,3-dipolar addition of methanol to a zwitterionic platinum carbene as the key step. The first platinum-catalysed intermolecular carbonbased nucleophile addition to alienes is also reported.

Full text of DOI:10.1002/adsc.201000342

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DOI: 10.1002/adsc.201000342
New Platinum-Catalysed Dihydroalkoxylation of Allenes
Marꢀa Paz MuÇoz,a,* Marꢀa C. de la Torre,a and Miguel A. Sierrab
a
Instituto de Quꢀmica Orgꢁnica General (IQOG), CSIC, Juan de la Cierva 3, 28006 Madrid, Spain
Fax : (+34)-91-564-4853; phone: (+34)-91-562-2900 ext: 437; e-mail: paz.munoz@iqog.csic.es
Facultad de Ciencias Quꢀmicas, Universidad Complutense de Madrid, Avda. Complutense s/n, 28040 Madrid, Spain
b
Received: May 3, 2010; Published online: September 7, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000342.
Abstract: A new platinum-catalysed dihydroalkoxy-
lation of allenes is described to give aliphatic ace-
tals by an unexpected attack of two molecules of
methanol to the terminal carbon of the allene
moiety. Deuteration experiments suggest an unpre-
cedented formal 1,3-dipolar addition of methanol to
a zwitterionic platinum carbene as the key step.
The first platinum-catalysed intermolecular carbon-
based nucleophile addition to allenes is also report-
ed.
In the single example describing the formation of
dimethyl acetals from allenes, the Au(I)-catalysed se-
quential addition of two molecules of methanol to the
central carbon atom of allene is reported.[3] However,
to the best of our knowledge the double addition of
two molecules of MeOH to the terminal carbon of an
allene is unknown to date.
Here we report that the intermolecular reaction of
allenes with alcohols in the presence of catalytic
amounts of PtCl2 follows a different reaction mode to
the one reported for Au(I), leading to an unexpected
aliphatic acetal formation by attack of two molecules
of methanol to the terminal carbon of the allenic
system and thus complete reduction of the allene is
observed (Scheme 1, b).[4]
Keywords: acetals; allenes; hydroalkoxylation;
mechanism; platinum
Commercially available cyclohexylallene 1a was
chosen as model substrate to study this reaction.
Transition metal-catalysed hydroalkoxylation of al- Thus, heating 1a in refluxing MeOH (2a) in the pres-
lenes has received much attention in the last years, ence of 5 mol% of PtCl2, led to the dimethyl acetal
and both the intra-[1] and intermolecular[2] versions 3aa in 81% along with traces of the corresponding al-
have been reported. For the intermolecular version, dehyde 4a[5] (Scheme 2).[6]
Au(I) complexes have proven to be the most active
Aldehyde 4a could be obtained as the single reac-
catalysts giving allylic ethers regioselectively by tion product when the reaction was carried out under
attack of the oxygen nucleophile to the terminal or in- similar conditions using a 3:1 mixture of THF:H2O at
ternal carbon of the allenic system depending on the 708C (Entry 1, Table 1).
reaction conditions (Scheme 1, a).
The reaction was extended to the formation of dif-
ferent acetals by reaction of 1a with other alcohols.
The reactivity pattern for alcohols was found to be
Scheme 1. Au(I)- vs. Pt(II)-catalysed hydroalkoxylation of
allenes.
Scheme 2. Pt-catalysed reaction of cyclohexylallene in
MeOH.
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Table 1. Pt-catalysed reaction of cyclohexylallene (1a) and
different alcohols (2).[a]
Table 2. Pt-catalysed reaction of different monosubstituted
allenes (1) in MeOH (2a).[a]
Entry
2, ROH (equiv.)
Products 3yx[b]
(% isolated yield)
Entry 1, Allenes
Products 3yx[b]
(% isolated yield)
1
2
3
4
5
6
7
2b, H2O
4a (67)
3ac (49), 4a (18)
3ad (61)
2c, EtOH[c]
2d, n-BuOH[c]
2e, i-PrOH[c]
1
2
3
4
5
6
1b, R’=CH2C
1b, R’=CH2CACHTUNGTRENNNUG
1c, R’=CH2-phthalimide 3ca (78)
1d, R’=n-hexyl
1e, R’=Ph
G
3ba (85), 4b (15)
3be (20)[c]
4a’ (75)
2f, t-BuOH[c]
4a (63)
2g, PhCH2OH (3)
2h, OHCH2CH2CH2OH (2)
3ag (54), 5[7] (34)
3ah (75)
3da (29), 6d (39)
3ea (61), 6e (21), 7e (5)
3fa (32), 6f (30), 8f (2)
1f, R’=tolyl
[a]
Reaction conditions: 1a (0.2 mM), PtCl2 (5 mol%), THF
(2 mL), ROH (2–4 equiv.), 708C, 20 h. Conversion
>99%.
3yx: y refers to the allene and x to the alcohol used.
Neat alcohol, no THF used.
[a]
Reaction conditions: 1 (0.2 mM), PtCl2 (5 mol%), MeOH
(2 mL), 708C, 20 h. Conversion >99%.
3yx: y refers to the allene and x to the alcohol used.
Reaction carried out in i-PrOH.
[b]
[c]
[b]
[c]
similar as in the Au-catalysed hydration of alkynes.[3b] (6y) formed from the attack of methanol (or water)
Results are shown in Table 1. to the more substituted carbon of the allenic system
The hydroalkoxylation reaction is compatible with was also obtained. The diisopropyl acetal 3be could
different types of alcohols. Primary alcohols, including be isolated from the reaction of 1b in i-PrOH without
EtOH, n-BuOH, secondary alcohols (i-PrOH), terti- evidence of trimerization in this case (Entry 2,
ary alcohols (t-BuOH), benzyl alcohol, and propane- Table 2).
diol all yielded the corresponding acetals 3ax, togeth-
Results compiled in Table 2 show the compatibility
er with variable amounts of free and trimeric alde- of the dihydroalkoxylation of allenes with aliphatic,
hydes 4a or 4a’.[8] The combined yield in aldehyde-de- aromatic, esters and imide groups.[12]
rived compounds was good to excellent in the cases
Opposite to monosubstituted allenes, disubstituted
tested (Table 1). However, the use of alcohols having allenes yield no aliphatic acetals (Figure 1).[13] Thus,
unsaturated motives (propargylic alcohol) rendered geminal disubstituted allene 1g led to decomposition,
polymeric reaction mixtures, while phenol or acetic while 1h formed traces of acetophenone under the
acid[9] gave no reaction.[10]
same conditions (probably from a metathesis-like re-
The substitution in the allene was next investigated action). Similarly, 1,3-disubstituted allene 1i gave a
using MeOH as the alcohol and PtCl2 as the catalyst complex reaction mixture, from which no identifiable
(Table 2 and Figure 1). Monosubstituted allenes[11] products could be isolated.
formed the corresponding dimethyl acetals in moder-
Although Au(I) and Pt(II) complexes often catalyse
ate to good yields (entries 1 and 3–6, Table 2). In similar reactions, there are a few examples of diver-
some cases (entries 4, 5, 6, Table 2), the ethyl ketone gent reactivity of these two metals.[14] This seems to
be the case of the reaction of simple allenes and alco-
hols. Thus, in the presence of Au(I) catalysts, allyl
ethers are obtained via vinyl-Au intermediates,[2] but
in the presence of Pt(II) catalysts, terminal aliphatic
acetals are produced.
To test the involvement of allyl ethers as intermedi-
Figure 1. Disubstituted allenes tested.
ates in the formation of aliphatic acetals under our
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New Platinum-Catalysed Dihydroalkoxylation of Allenes
Pt-catalysed conditions, the isolated allyl ether 9e ob- ionic Pt carbene[16] or an all-carbon 1,3-dipole (C,
tained from the reaction of 1e in MeOH under Scheme 4).[17,18] This Pt complex (A$B$C) would
Au(I)[2e] catalysis, was then reacted with PtCl2 under react with the alcohol in a 1,3-dipole manner or step-
the conditions used in this work (Scheme 3). On the wise via a vinyl-Pt complex D, followed by protona-
outset, 9e was recovered unreacted and no acetal was tion, to give Pt-carbene E. This new intermediate ex-
detected, suggesting different reaction pathways in periences a 1,2-H shift promoted by the highly elec-
the reactions of allenes with each of these metals.
trophilic carbene carbon and the alkoxide moiety to
yield intermediate F, which would lead to the final
product by addition of a new alcohol molecule, fol-
lowed by protodemetalation of G, or alternatively re-
ductive elimination upon protonation of the metallic
center (H, Scheme 4).[19,20]
Proof to support this proposal was obtained by
deuteration experiments. Thus, reaction of 1b and 1e
with 5 mol% PtCl2 in MeOD gave the corresponding
2,3-d2-acetals with one deuterium added to the central
carbon (from the final protodemetalation), and the
other one to the internal more substituted carbon of
the allenic system (from the first addition of MeOH)
Scheme 5).[21,22]
In our first attempt to extend this methodology to
carbon-based nucleophiles,[23] 3ai was obtained as the
only product, by a remarkable and unprecedented Pt-
catalysed bis-indolization on the terminal carbon of
Scheme 3. Possible intermediates in the Pt(II)-catalysed re-
action of allenes in MeOH.
The presence of discrete alkynes (arising from the 1a (Scheme 6).[24]
isomerisation of the allene) as intermediates in this
Although MeOH does not incorporate to the final
reaction was also ruled out[15] by reaction of the com- product, it was essential to achieve high conversions,
mercially available 3-phenyl-1-propyne 10 under the supporting a mechanism similar to the one proposed
same conditions, which led to ketone 11 in 70% yield in Scheme 4, with the involvement of iminium inter-
as the only product (Scheme 3).
mediates of type F’ (Scheme 6) stabilized in a more
The formation of acetals may be explained through polar medium.[25]
coordination of the Pt catalyst to the central carbon
In conclusion, herein we describe a novel Pt-cata-
of the allene (A, Scheme 4), which can also be seen lysed dihydroalkoxylation of allenes to give aliphatic
as a s-allylic Pt cation (B, Scheme 4), and as a zwitter- acetals. The reaction showed an unprecedented and
Scheme 4. Proposed mechanism for the Pt-catalysed formation of acetals from allenes.
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nucleophile (2–4 equiv.) was added when needed, and the
reaction mixture heated at 708C for 20 h. The reaction was
then cooled to room temperature and filtered through
Celite. The solvent was evaporated and the crude purified
by flash column chromatography on silica gel using hexane/
Et2O as eluent.
Acetal 3aa: colourless oil; yield: 80%. 1H NMR
(300 MHz, CDCl3): d=4.29 (t, J=5.7 Hz, 1H), 3.27 (s, 6H),
1.69–1.54 (m, 6H), 1.21 (m, 6H), 0.84 (m, 2H); 13C NMR
(75 MHz, CDCl3): d=104.76, 52.39, 37.48, 33.23, 32.08,
29.76, 26.58, 26.28; HR-MS (ESI): m/z=155.1428, calcd. for
C10H19O (M+ꢀOMe): 155.1430.
More details and characterization of all the products can
be found in the Supporting Information.
Scheme 5. Pt(II)-catalysed reaction of allenes in MeOD.
Acknowledgements
The authors would like to thank the Spanish Government for
a Ramꢀn y Cajal Award to MPM. This work was supported
by the Spanish Government (Consolider-Ingenio 2010:
CSD2007-00006. MEC: CTQ2007-67730-C02-01/BQU) and
the Comunidad Autꢀnoma de Madrid (AVANCAT-CAM-
S2009/PPQ-1634).
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Dihydroalkoxylation of Allenes
A solution of allene (0.2–0.8 mmol) in dry THF (0.2 mM) or
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[18] Several structures have been proposed for the coordi-
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This cation can also be seen as a zwitterionic carbene
or an all-carbon 1,3-dipole as proposed in Scheme 4.
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[12] The intramolecular version of the Pt-catalysed hydro-
ACHTUNGTRENNUNG
A
[19] Styrene was reacted with 1a in the presence of MeOH
to trap carbene intermediate E in an alternative path-
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way to the 1,2-H migration. However, only dimethyl
acetal 3aa was obtained (80%). For intermolecular
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J. Am. Chem. Soc. 2003, 125, 5757–5766.
of drier MeOD. Formation of 7e is currently being in-
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[20] A mechanism involving vinyl ethers as intermediates
was also investigated and cannot be completely ruled
out. However, the reaction of 1e and 1 equiv. of MeOH
in THF gave only unreacted starting material and no
vinyl ether or other intermediate was detected.
[24] For a Pt-catalysed reaction of allenols in the presence
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S. Ma, Org. Lett. 2009, 11, 1213–1216. For synthesis of
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2005, 7, 5063–5065.
[25] Reaction in MeOD, gave 3ai in 60% with no deuterium
incorporation. Reaction of 1a and 3 equivalents of
indole 2i in THF under the same conditions with no
MeOH added, gave low conversion of the allene and
3ai was obtained in only 23%.
ꢀ
[21] A mechanism involving Pt H cannot be completely
ruled out. However, if this species gave scrambling
with MeOD, the observed complete deuterium incorpo-
ration in the final products would not be explained.
[22] Formation of ketones 6y could be explained by the re-
verse regioselectivity of the MeOH 1,3-addition or by
allene-alkyne isomerisation by 1,3-H migration to form
the internal alkyne, followed by hydration with traces
of water. This second hypothesis would be more proba-
ble due to the lack of ketone formation in the presence
2194
asc.wiley-vch.de
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 2189 – 2194