Allenes from ynals

Article abstract of DOI:10.1016/S0040-4039(01)00252-0

Hydrosilylation of ynals with triethylsilane is catalyzed by RhCl(PPh₃)₃ and leads to α-triethylsilyl enals. Addition of aryl Grignard reagents takes place to give secondary allylic alcohols which are easily converted to 1,3-disubstituted allenes.

Full text of DOI:10.1016/S0040-4039(01)00252-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2605–2608
Allenes from ynals
Marcus A. Tius* and Subrata K. Pal
Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, HI 96822, USA
Received 19 January 2001; revised 5 February 2001; accepted 6 February 2001
Abstract—Hydrosilylation of ynals with triethylsilane is catalyzed by RhCl(PPh3)3 and leads to a-triethylsilyl enals. Addition of
aryl Grignard reagents takes place to give secondary allylic alcohols which are easily converted to 1,3-disubstituted allenes.
© 2001 Elsevier Science Ltd. All rights reserved.
An interest in the chemistry and synthesis of allenes1
led us to consider the approach which is summarized in
Scheme 1. If the catalytic hydrosilylation reaction of
ynal 1 produced isomeric a-silyl enals 2 or 3, both
geometrical isomers could be converted to the same
allene 5 through intermediate 4, the product of Grig-
nard addition to the aldehyde carbonyl group. A num-
ber of elimination processes could then be considered
for the conversion of 4 to 1,3-disubstituted allenes 5.
The proposed approach is a member of a general family
of reactions in which elimination of X and Y from
vinylic precursor 6 leads to allene 5. The most common
mechanism for the allene-forming step is either a reduc-
tive elimination of XꢀY,2 or a nucleophilic addition to
X, followed by elimination of Y−.3 Where X is halogen,
metal–halogen exchange to form a vinyl metal interme-
diate, followed by expulsion of Y−, also has been used
to produce allenes.4,5
In early work, Chan and co-workers described a syn-
thesis of terminal allenes from the adducts of a-
triphenylsilylvinyllithium
with
aldehydes
and
ketones.3d,e In a conceptually related study, Konoike
and Araki described a synthesis of chiral, non-racemic
allenes from a-tri-n-butylstannyl allylic alcohols.3b Our
goals were to develop a convenient means to access a
series of 1,3-disubstituted allenes, and to avoid the use
of tin-containing intermediates. Our results are dis-
cussed in what follows.
Journet’s excellent method furnished products 1 in high
yield.6 Although the catalytic hydrosilylation of enones
and enoates is very well developed, we were not aware
of a specific example of the hydrosilylation of an a,b-
acetylenic aldehyde.7 In the event, hydrosilylation of
3-cyclohexylprop-2-ynal (1, R1=cyclohexyl) took place
during 4 h at 45°C in the presence of 1 mol%
SiEt3
Et3SiH
catalyst
CHO
SiEt3
SiEt3
CHO
R1
R1
R1
CHO
R1
CHO
2
3
1
7
R2MgX
SiEt3
X
Y
R1
R1
R1
•
R2
5
R2
R2
OH
6
4
Scheme 1.
Keywords: allenes; elimination reactions; reduction; rhodium and compounds.
* Corresponding author. Tel.: 808-956-2779; fax: 808-956-5908; e-mail: tius@gold.chem.hawaii.edu
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00252-0

2606
M. A. Tius, S. K. Pal / Tetrahedron Letters 42 (2001) 2605–2608
1
RhCl(PPh3)3 (Wilkinson’s catalyst) in neat triethylsilane
by H NMR at 300 MHz, after filtration through Celite
followed by concentration. On the other hand, hydro-
to give 2 (R1=cyclohexyl) as the only product detected
Table 1. Allenesa
1H NMR
H-C(sp2)
13C NMR
C(sp)
IR
C=C=C
X
1
1950 cm-1
204.1
6.15 (dd, J = 6.3, 2.9 Hz, 1H)
5.56 (dd, J = 6.3, 6.3 Hz, 1H)
•
SiEt3
X = OH, 56%
X = Cl, 95%
88%
Ph
Ph
X
1950 cm-1
204.4
6.21 (dd, J = 6.3, 2.9 Hz, 1H)
5.61 (dd, J = 6.3, 6.3 Hz, 1H)
2
•
SiEt3
X = OH, 55%
X = Cl, 98%
Me
85%
Me
X
3
1950 cm-1
1950 cm-1
1950 cm-1
204.8
204.5
204.2
6.34 (dd, J = 6.4, 3.1 Hz, 1H)
5.52 (dd, J = 6.4, 6.4 Hz, 1H)
•
SiEt3
X = OH, 52%
X = Cl, 98%
85%
X
Cl
Cl
6.10 (dd, J = 6.3, 2.9 Hz, 1H)
5.60 (dd, J = 6.3, 6.3 Hz, 1H)
4
•
Cl
Cl
SiEt3
X = OH, 50%
X = Cl, 95%
83%
X
5
6.12 (dd, J = 6.3, 3.1 Hz, 1H)
5.58 (dd, J = 6.3, 6.3 Hz, 1H)
•
SiEt3
X = OH, 55%
X = Cl, 98%
90%
X
6
7
1945 cm-1
1950 cm-1
205.2
206.6
6.12 (dt, J = 6.4, 3.1 Hz, 1H)
5.57 (dt, J = 6.4, 6.4 Hz, 1H)
nC6H13
•
nC6H13
SiEt3
X = OH, 50%
X = Cl, 98%
86%
Ph
Ph
6.19 (dt, J = 6.5, 2.9 Hz, 1H)
5.62 (dt, J = 6.5, 6.5 Hz, 1H)
X
nC6H13
•
nC6H13
SiEt3
86%
X = OH, 48%
X = Cl, 97%
1945 cm-1
205.6
6.83 (dt, J = 6.6, 2.9 Hz, 1H)
5.61 (dt, J = 6.6, 6.6 Hz, 1H)
8
X
nC6H13
nC6H13
nC6H13
•
nC6H13
SiEt3
X = OH, 45%
X = Cl, 98%
82%
X
1950 cm-1
1950 cm-1
9
205.4
204.4
6.01 (dd, J = 6.6, 2.9 Hz, 1H)
5.59 (dd, J = 6.6, 6.6 Hz, 1H)
F
•
nC6H13
F
SiEt3
82%
X = OH, 47%
X = Cl, 96%
OMe
OMe
X
10
6.08 (dd, J = 6.5, 3.1 Hz, 1H)
5.53 (dd, J = 6.5, 6.5 Hz, 1H)
•
nC6H13
SiEt3
55%
X = OH, 48%
X = Cl, 75%
a Percentages refer to isolated yields. Alcohols and allenes were characterized by 1H and 13C NMR, IR and mass spectrometry.

M. A. Tius, S. K. Pal / Tetrahedron Letters 42 (2001) 2605–2608
2607
OMe
OMe
OMe
HO
nC6H13
Et3SiO
nC6H13
KH, THF
0 oC to rt
(1)
nC6H13
•
SiEt3
8
9 10% yield
10 70% yield
silylation of non-2-ynal (1, R1=n-hexyl) under the
same conditions led in quantitative yield to an E,Z
mixture of a-triethylsilyl enals 2 (67%) and 3 (10%),
along with b-triethylsilyl product 7 (23%). The high
degree of stereo- and regiospecificity of the first
hydrosilylation is ascribed to the steric bias imparted on
the first reaction by the cyclohexyl group. The regio-
selectivity is important, since b-triethylsilyl enal 7 does
not lead to allene. Consequently, we initiated a limited
study to determine conditions that would minimize the
proportion of 7 in the product. Hexachloroplatinic
acid, Karstedt’s catalyst7a and two palladium pincer
catalysts8 were examined in combination with triethyl-
silane and 1 (R1=n-hexyl). Results in all cases were
inferior to those obtained with RhCl(PPh3)3.
gave a chromatographically inseparable mixture of
allene and 1,3-dienes (ca. 1/1).
In conclusion, conditions have been developed and
optimized for the hydrosilylation of ynals.9 In the case
of 3-cyclohexylprop-2-ynal, the hydrosilylation is regio-
and stereospecific. The products of the hydrosilylation
have been used for the synthesis of a series of 1,3-aryl
substituted allenes. The a-triethylsilyl enals 2 are poten-
tially versatile synthetic intermediates, as are the prod-
ucts of sequential CꢀC and CꢀO hydrosilylation.
Acknowledgements
Addition of a series of Grignard reagents to the crude
product mixtures led to allylic alcohols 4, which were
characterized, following purification by flash column
chromatography (Table 1). A series of simple and sub-
stituted aromatic Grignard reagents were examined.
The elimination step to form the allenes was carried out
by analogy with Chan’s pioneering work.3d Treatment
of the allylic alcohols with thionyl chloride in CCl4/pyr-
idine at 0°C led in excellent yield to allylic chlorides.
Exposure of the chlorides to 2 equiv. of tetra-n-butyl-
ammonium fluoride in DMSO at rt for 24 h led in high
yield to the allenes shown in Table 1.
Acknowledgement is made to Sea Grant (Institutional
Grant No. NA36RG0507) for generous support. We
thank Dr. David Morales-Morales and Professor Craig
Jensen for making the pincer catalysts available to us.
References
1. For example, see: Harrington, P. E.; Tius, M. A. Org.
Lett. 2000, 2, 2447–2450 and references cited therein.
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5. Where Y=H and X is a leaving group, base catalyzed
processes have also been used to form allenes. See: (a)
Brummond, K. M.; Gesenberg, K. D.; Kent, J. L.;
Kerekes, A. D. Tetradehron Lett. 1998, 39, 8613–8616; (b)
Langer, P.; Do¨ring, M.; Seyferth, D. J. Chem. Soc., Chem.
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Tetrahedron Lett. 1998, 39, 6427–6428.
Several interesting observations were made during the
execution of this work. Exposure of 2 (R1=n-hexyl) to
excess triethylsilane and 1 mol% Wilkinson’s catalyst
for 8–9 h at 45°C led to the formation of the triethylsi-
lyl ether of 2-triethylsilyl-2-nonen-1-ol. This reaction
promises to be useful for the conversion of ynals to
2-triethylsilyl primary allylic alcohols. A single attempt
to perform a KH-mediated Peterson olefination on 8
(Eq. (1)) led in low yield to the desired allene 9. The
major product 10, which was isolated in 70% yield,
resulted from C-to-O migration of the triethylsilyl
group. A similar silicon migration, mediated by fluoride
anion, has been described by Chan and
Mychajlowskij.3e A cyclic, fluoride-bridged transition
state was postulated for their process, involving the
development of a strong hydrogen bond to fluoride.
However, a bimolecular mechanism can reasonably be
postulated for both the KH and the fluoride mediated
reactions.
The reaction sequence appears to be limited to aro-
matic Grignard reagents. For example, when the
adduct of 2 (R1=n-hexyl) with n-butyllithium was
exposed to thionyl chloride, a mixture of allylically
rearranged chlorides was formed. Treatment of the
chloride mixture with tetra-n-butylammonium fluoride
7. (a) Johnson, C. R.; Raheja, R. K. J. Org. Chem. 1994, 59,

2608
M. A. Tius, S. K. Pal / Tetrahedron Letters 42 (2001) 2605–2608
2287–2288; (b) Chan, T. H.; Zheng, G. Z. Tetrahedron Lett.
1993, 34, 3095–3098; (c) Revis, A.; Hilty, T. K. J. Org.
Chem. 1990, 55, 2972–2973.
RhCl(PPh3)3 was added. The mixture was heated to 50°C
for 10 min and to the orange homogeneous mixture was
added 200 mg (1.47 mmol) 3-cyclohexylprop-2-ynal. The
reaction was stirred at 45°C for 5 h, at which time TLC
indicated that some unreacted starting material remained.
The mixture was cooled, diluted with ether, filtered through
Celite and concentrated to give 370 mg (99% yield) of crude
(2E)-3-cyclohexyl-2-(triethylsilyl)prop-2-enal:Rf=0.70(2.5%
8. See: (a) Morales-Morales, D.; Grause, C.; Kasaoka, K.;
Redo´n, R.; Cramer, R. E.; Jensen, C. M. Inorg. Chim. Acta
2000, 300–302, 958–963; (b) Ohff, M.; Ohff, A.; van der
Boom, M. E.; Milstein, D. J. Am. Chem. Soc. 1997, 119,
11687–11688; (c) Rimml, H.; Venanzi, L. M. J. Organomet.
Chem. 1983, 259, C6–C7.
1
EtOAc in hexane); H NMR (300 MHz, CDCl3): l 10.30
9. Hydrosilylation of 3-cyclohexylprop-2-ynal (entry 1). Into a
(s, 1H), 6.59 (d, J=9.8 Hz, 1H), 3.08 (m, 1H), 0.88 (t, J=8.0
Hz, 9H), 0.65 (q, J=8.0 Hz, 6H).
dry flask 513 mg (4.41 mmol) triethylsilane and ca. 1 mol%
.
.