Effects of silyl substituents on the palladium-catalyzed asymmetric synthesis of axially chiral (allenylmethyl)silanes and their S E2′ chirality transfer reactions

Article abstract of DOI:10.1002/ejoc.201101634

A series of axially chiral 4-substituted-1-silyl-2,3-butadienes [(allenylmethyl)silanes] were synthesized from 3-bromo-5-silylpenta-1,3-dienes by a Pd-catalyzed asymmetric reaction with a soft nucleophile. The optically active (allenylmethyl)silanes react with an acetal in the presence of TiCl ₄ to give the enantiomerically enriched 1,3-diene derivatives through an S_E2′ pathway. Effects of the silyl groups on the enantioselectivity of the asymmetric allene synthesis and the subsequent S _E2′ chirality transfer reaction were studied. It was found that as the steric bulk of the silyl groups in the 3-bromo-5-silylpenta-1,3-dienes was increased from -SiMe₃ to -SiiPr₃, the enantioselectivity of the two enantioselective processes also improved.

Full text of DOI:10.1002/ejoc.201101634

FULL PAPER
DOI: 10.1002/ejoc.201101634
Effects of Silyl Substituents on the Palladium-Catalyzed Asymmetric Synthesis
of Axially Chiral (Allenylmethyl)silanes and Their SE2Ј Chirality Transfer
Reactions
Masamichi Ogasawara,*[a] Yonghui Ge,[a] Atsushi Okada,[a] and Tamotsu Takahashi*[a]
Keywords: Allenes / Chirality / Electrophilic substitution / Palladium
A series of axially chiral 4-substituted-1-silyl-2,3-butadienes
[(allenylmethyl)silanes] were synthesized from 3-bromo-5-
silylpenta-1,3-dienes by a Pd-catalyzed asymmetric reaction
with a soft nucleophile. The optically active (allenylmethyl)-
silanes react with an acetal in the presence of TiCl4 to give
the enantiomerically enriched 1,3-diene derivatives through
an SE2Ј pathway. Effects of the silyl groups on the enantio-
selectivity of the asymmetric allene synthesis and the subse-
quent SE2Ј chirality transfer reaction were studied. It was
found that as the steric bulk of the silyl groups in the 3-
bromo-5-silylpenta-1,3-dienes was increased from -SiMe3
to -SiiPr3, the enantioselectivity of the two enantioselective
processes also improved.
sequence provides convenient access to optically active
(allenylmethyl)silanes 3 and dienyl derivatives 5, the
enantioselectivity of both steps shows rooms for further im-
provement. To solve this problem, we were interested in
modifying the silyl groups in 1 and 3. We envision that the
enantioselectivity of both steps could be improved with
bulkier silyl groups. Here we report the substituent effects
of the silyl substituents on the two enantioselective reac-
tions, the Pd-catalyzed asymmetric synthesis of (allenyl-
methyl)silanes[10] and SE2Ј chirality transfer reactions.[11]
Introduction
Allenes are important and useful compounds in synthetic
organic chemistry.[1,2] Introduction of proper substituents
at the appropriate positions of an allenic skeleton induces
unique axial chirality in the allenic molecule.[3] Optically
active forms of such axially chiral allenes have been utilized
as chiral synthons that are capable of transferring their ax-
ial chirality onto newly formed stereogenic centers in the
products.[4–8] The majority of successful examples of such
axis-to-center chirality transfer reactions are categorized
into either intramolecular cyclization reactions[4] and/or
transformation of allenylmetal species into the correspond-
ing propargyl derivatives.[5] Indeed, examples of transfer-
ring the allenic axial chirality onto a stereogenic carbon
atom which is derived from the incoming intermolecular
reactant have been extremely rare.[6–8]
Recently, we developed a Pd-catalyzed reaction for the
preparation of various multisubstituted allenes,[9a–9h] and
the use of an appropriate chiral Pd catalyst enables axially
chiral allenes of high enantiopurity to be furnished.[7,9i–9l]
The reaction was applied to the asymmetric synthesis of
axially chiral (allenylmethyl)silanes 3 (Scheme 1), and the
obtained enantiomerically enriched 3 were applied in the
SE2Ј reaction to produce diene derivatives 5 containing a
stereogenic carbon atom.[7] The second step of this se-
quence was a rare example of the intermolecular axis-to-
center chirality transfer reaction.[6–8] Although the reaction
Scheme 1. Pd-catalyzed asymmetric synthesis of axially chiral
(allenylmethyl)silane 3am and axis-to-center chirality transfer in
5mx.[7]
Results and Discussion
Preparation of 3-Bromo-5-silylpenta-1,3-dienes
[a] Catalysis Research Center and Graduate School of Life Science,
Hokkaido University,
Preparation of 3-bromo-5-trimethylsilylpenta-1,3-diene
(1a) was reported by Parrain and Santelli in 2000.[12] The
starting compound of their method was 1,4-bis(trimethyl-
silyl)-2-butene (6a), which can be easily obtained by the Ti-
Kita-ku, Sapporo 001-0021, Japan
Fax: +81-11-706-9150
E-mail: ogasawar@cat.hokudai.ac.jp
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101634.
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 1656–1663

Pd-Catalyzed Synthesis of Axially Chiral (Allenylmethyl)silanes
catalyzed coupling of vinyl Grignard reagent and chlorotri- responding (allenylmethyl)silanes 3 were excellent and
methylsilane as reported by Terao and Kambe.[13] The ranged from 81 to 96%. The steric characteristics of the
whole sequence is applicable to the preparation of the tri- silyl substituents in 1 show no or negligible effects on the
ethylsilyl and dimethylphenylsilyl analogues, and com- chemical yields of 3. Even with the sterically most de-
pounds 1b and 1c were prepared in 59 and 48% yield, manding silyl group, -SiiPr3, in 1d, the Pd-catalyzed reac-
respectively (Scheme 2, top).
tion afforded the desired (allenylmethyl)silanes in good
yields (Table 1, Entries 4, 8, 11). All the allenic products are
stable and can be purified by standard silica gel chromatog-
raphy.
Table 1. Palladium-catalyzed synthesis of (allenylmethyl)silanes 3.[a]
Entry Bromodiene 1 Nu-H 2
Temp [°C] Yield of 3 [%][b]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1a
1b
1c
1d
1a
1b
1d
2m
2m
2m
2m
2n
2n
2n
2n
2o
2o
2o
23
23
23
23
23
23
23
23
40
40
40
91 (3am)
94 (3bm)
88 (3cm)
92 (3dm)
87 (3an)
89 (3bn)
83 (3cn)
96 (3dn)
84 (3ao)
92 (3bo)
81 (3do)
Scheme 2. Preparation of 3-bromo-5-silylpenta-1,3-dienes 1a–d.
For the preparation of triisopropylsilyl derivative 1d,
some modifications of the synthetic scheme are required.
The Ti-catalyzed reaction is not applicable to reactions with
bulky chlorosilanes, and the reaction of vinylmagnesium
chloride with iPr3SiCl failed to give (E/Z)-1,4-bis(triiso-
9
10
11
propylsilyl)-2-butene (6d). It was found, however, that 6d [a] The reaction was carried out with 1 (0.50 mmol) and 2
(0.55 mmol) in THF in the presence of NaH and a Pd catalyst
could be easily obtained in 85% yield by metathesis homo-
(2 mol-%) generated from [PdCl(π-allyl)]2 and dpbp. [b] Isolated
yield after silica gel chromatography.
dimerization of commercially available allyltriisopropyl-
silane in the presence of the second generation Grubbs cata-
lyst[14] in refluxing dichloromethane. Thermolysis of gem-
dibromocyclopropane, which was prepared from 6d, af-
Substituent Effects on Pd-Catalyzed Asymmetric Synthesis
of (Allenylmethyl)silanes 3
forded an E/Z mixture of 3-bromo-1,5-bis(triisopropylsilyl)-
penta-1,3-diene (7d) through elimination of HBr, and de-
sired debromosilylation product 1d was not obtained. The
TIPS group on the sp2 carbon atom in 7d was selectively
removed by acidic treatment (1% CF3CO2H in CH2Cl2),
and 1d was obtained in 53% overall yield from 6d
(Scheme 2, bottom). All bromosilyldienes 1a–d were easily
purified by standard silica gel chromatography and ob-
tained as the (Z)-isomers exclusively.
The asymmetric synthesis of axially chiral (allenylmeth-
yl)silanes 3 was examined under reaction conditions iden-
tical to those reported for the reaction between 1a and
2m.[7] That is, with a palladium catalyst (10 mol-%) gener-
ated from Pd(dba)2 and an appropriate chiral phosphane
ligand in THF at 40 °C. Products (R)-3 were obtained in
good enantioselectivity (up to 90%ee, Table 2).
Among various chiral phosphane ligands examined, (R)-
segphos[16] showed the best performance in terms of
enantioselectivity. The reaction of 1a with 2m with the use
of the Pd/(R)-segphos species afforded (R)-3am in 75%
Palladium-Catalyzed Reactions of 3-Bromo-5-silylpenta-1,3-
dienes with Nucleophiles
All 3-bromo-5-silylpenta-1,3-dienes 1a–d obtained as in yield with 79%ee (Table 2, Entry 1). The use of (R)-binap
Scheme 2 are excellent precursors to (allenylmethyl)silane and (R)-tBu-segphos[17] decreased the enantioselectivity of
derivatives (Table 1). A series of 4-substituted-1-silyl-2,3- the same reaction to 43 and 74%ee, respectively (Table 2,
butadienes 3 were synthesized from 1 and appropriate pro- Entries 2 and 3). Interestingly, a clear correlation was ob-
nucleophile 2 in the presence of NaH (1.0 equiv. with re- served between the enantioselectivity of the Pd-catalyzed
spect to 2) and a Pd catalyst (2 mol-%), which was gener- reaction and the steric bulk of the silyl groups in 1. The
ated in situ from [PdCl(π-allyl)]2 and dpbp,[15] in THF.[7] enantioselectivity in 3bm, 3cm, and 3dm, which were ob-
With various malonate-based pronucleophiles 2m–o, the re- tained by using Pd/(R)-segphos-catalyzed reactions with
action proceeded cleanly, and the isolated yields of the cor- 2m/NaH as a nucleophile, was 80, 80, and 87%ee, respec-
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M. Ogasawara, Y. Ge, A. Okada, T. Takahashi
FULL PAPER
Table 2. Palladium-catalyzed asymmetric synthesis of (allenyl- The (E) configuration of the internal double bond in 5mx
methyl)silanes 3.[a]
was assigned on the basis of 1H NMR nOe experiments. An
nOe was detected between MeOCH- and NuCH2CH=C-,
whereas no nOe was observed between NuCH2CH=C- and
1
CH2=CH-. Other conjugated dienes 5 showed similar H
NMR behavior. The exclusive formation of the (E)-diene
indicates that the electrophile approaches the central car-
bon of the allene from the side opposite to the sterically
demanding NuCH2- substituent (Scheme 3). The same ste-
reochemical outcome was reported in the protodesilylation
of (allenylmethyl)silanes.[11a]
Table 3. TiCl4-promoted SE2Ј reaction of (allenylmethyl)silanes 3
with RCH(OMe)2 4.[a]
Entry
1
2
Base
P-P
Yield of 3
ee of
[%][b]
3 [%][c,d]
1
2
3
4
5
6
7
8
1a 2m
1a 2m
1a 2m
1b 2m
1c 2m
1d 2m
1a 2n CsOtBu
1d 2n CsOtBu
1a 2o CsOtBu
1b 2o CsOtBu
1d 2o CsOtBu
NaH
NaH
NaH
NaH
NaH
NaH
segphos
binap
75 (3am)
79 (3am)
74 (3am)
75 (3bm)
83 (3cm)
85 (3dm)
70 (3an)
91 (3dn)
72 (3ao)
75 (3bo)
85 (3do)
79 (R)
43 (R)
74 (R)
80 (R)
80 (R)
87 (R)
81 (R)
90 (R)
62 (R)
80 (R)
87 (R)
tBu-segphos
segphos
segphos
segphos
segphos
segphos
segphos
segphos
segphos
Entry
3
ee
of 3 [%]
Acetal Yield of
4
ee of
Chirality
transfer
[%][d]
5 [%][b]
5 [%][c]
9
10
11
1
2
3
4
5
6
7
3am
3bm
3dm
3an
79
80
87
81
90
79
87
4x
4x
4x
4x
4x
4y
4y
87 (5mx)
71 (5mx)
71 (5mx)
97 (5nx)
95 (5nx)
70 (S)
71 (S)
88
89
92
62
83
26
17
80 (S)
50[e] (S)
75[e] (S)
[a] The reaction was carried out with 1 (0.50 mmol) and 2
(0.55 mmol) in THF in the presence of base (0.55 mmol) and a Pd
catalyst (10 mol-%) generated from Pd(dba)2 and a chiral phos-
phane. [b] Isolated yield after chromatography on alumina. [c] De-
termined by chiral HPLC analysis with a chiral stationary phase
column (Chiralpak AD-H for 3am, 3cm, 3ao, 3bo, and 3do; Chi-
ralcel OD-H for 3bm, 3dm, 3an, and 3dn). [d] The absolute configu-
ration was deduced by the Lowe–Brewster rule (ref.[18]).
3dn
3am
3dm
89 (5my) 21 (nd)[f]
36 (5my) 15 (nd)[f]
[a] The reaction was carried out with 3 (0.35 mmol), 4 (0.53 mmol),
and TiCl4 (0.53 mmol) in CH2Cl2 for 1 h. [b] Isolated yield after
silica gel chromatography. [c] Determined by chiral HPLC analysis
with a chiral stationary phase column (Chiralcel OD-H). [d] Chiral-
ity transfer = (ee of 5)/(ee of 3)ϫ100. [e] The ee value was deter-
mined on the diol obtained by LiAlH4 reduction of 5nx. [f] Abso-
lute configuration was not determined.
tively (Table 2, Entries 4–6). Likewise, the enantioselectivity
of 81%ee for 3an, which was obtained from 1a and 2n/
CsOtBu, could be improved to 90%ee for 3dn under iden-
tical conditions (Table 2, Entries 7 and 8). The same trend
was observed for reactions of 1a–d with 2o/CsOtBu; the ee
values for 3ao, 3bo, and 3do were 62, 80, and 87%ee,
respectively (Table 2, Entries 9–11). In all the cases exam-
ined here, the isolated yields of the allenic species were
within the range of 70 to 91%, and no apparent deviation
was seen with regard to the choice of the silyl groups.
All the allenic products shown in Table 2 are levorota-
tory, and their absolute configurations were deduced to be
(R) by the Lowe–Brewster rule.[18] Because all the allenes
are not crystalline, X-ray crystal structure analysis could
not be used to determine their absolute configurations.
Scheme 3. Stereochemistry producing (E)-dienes in the SE2Ј reac-
tions between 3 and 4.
Diene products 5 possess a newly formed stereogenic car-
bon atom, and axis-to-center chirality transfer takes place
during the transformation.[6,7] The chirality transfer is
highly effective in the reactions with pivalaldehyde dimethyl
acetal (4x). Allene 3am (79%ee) containing a Me3Si group
gave diene 5mx in 87% yield with 70%ee. The chirality
Substituent Effects on Enantioselective SE2Ј Chirality
Transfer Reactions
Axially chiral (allenylmethyl)silanes (R)-3 obtained above transfer efficiency of this transformation was calculated to
were applied to the TiCl4-promoted SE2Ј reaction with an be 88% (Table 3, Entry 1). With the bulkier silyl groups in
appropriate acetal. The results are summarized in Table 3. 3, the chirality transfer becomes more effective. Et3Si-allene
In all cases, the reactions proceeded cleanly and dienes 5 3bm with 80%ee afforded diene 5mx in 71%ee (89% chiral-
were obtained with the (E) configuration exclusively.[7,11a] ity transfer; Table 3, Entry 2). The chirality transfer was
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Pd-Catalyzed Synthesis of Axially Chiral (Allenylmethyl)silanes
further improved to 92% in the reaction of 3dm (87%ee),
which contains an iPr3Si substituent, and 4x gave 5mx with
80%ee (Table 3, Entry 3). The effect of the silyl group is
more evident for the reactions of 3an/3dn with 4x. Whereas
Me3Si-allene 3an (81%ee) afforded diene 5nx with 50%ee
and with 62% chirality transfer (Table 3, Entry 4), iPr3Si
analogue 3dn (90%ee) gave 5nx with 75%ee and 83%
transfer (Table 3, Entry 5). On the other hand, the SE2Ј re-
actions with phenylacetaldehyde dimethyl acetal (4y)
showed inadequate chirality transfer in all cases. The reac-
tion between 3am (79%ee) and 4y gave diene 5my with
21%ee with a chirality transfer of only 26% (Table 3, En-
try 6). Similarly, 5my obtained from 3dm (87%ee) and 4y
was as low as 15%ee (17% transfer; Table 3, Entry 7).
The absolute configurations of 5mx and 5nx were deter-
mined as follows. 1,3-Dienes 5mx and 5nx were converted
into tBuCH(OMe)CO2Me (6) by successive oxidation and
diazomethane treatment (Scheme 4), and the absolute con-
figuration of both 5mx and 5nx was determined to be (S)
by correlation with an authentic sample of (S)-6, which was
prepared from commercially available (S)-tBuCH(OH)
CO2H.[7]
Scheme 5. Stereochemistry producing (S)-dienes in the SE2Ј reac-
tions between 3 and 4x.
The bulky tBu substituent in 4x is important for the ef-
fective chirality transfer in the SE2Ј reaction. With the steri-
cally more compact acetal 4y, the chirality transfer effi-
ciency in 5my was considerably diminished due to ineffec-
tive discrimination between the two enantiotopic faces in
the electrophile. Because the relative steric proportions be-
tween -CH2Ph and -OMe are not so different, effective chi-
rality transfer in the present SE2Ј reaction is inherently diffi-
cult with 4y, and thus, the positive effects from the bulkier
silyl groups in 3 are not visible using 4y.
The conversion of achiral bromodienes 1 into centrally
chiral 5 is the process of the two subsequent enantioselec-
tive reactions. Whereas bulkier silyl groups induce higher
enantioselectivity in both enantioselective reactions, final
products 5 of much higher optical purity are obtained start-
ing with 1d, which contains a SiiPr3 group, without any
appreciable loss in the chemical yields. Starting substrates 1
with various silyl groups provide identical products 5 in the
end of the two-step reaction sequence, because the silyl
groups are the leaving groups in the SE2Ј reaction.
Scheme 4. Determination of the absolute configurations of 5mx
and 5nx.
The stereochemical pathway for the reaction of (R)-3
with 4x to give (S)-(E)-5x is shown in Schemes 3 and 5. On
the basis of the anti stereochemistry observed for the SE2Ј
reactions of allylsilanes,[19] the electrophile tBuCH=O+Me,
which is generated in situ from 4x and TiCl4, approaches
the C=C bond in 3 from the side opposite to the silyl group.
Subsequently, nucleophilic attack takes place onto the Re
face of the electrophile. Steric interactions between the elec-
trophile and (R)-3 are minimized in the relative orientation
of the two reactants, which is illustrated in Scheme 5 (bot-
tom left). In this synclinal-like transition state,[19d] the bulk-
iest substituent in the electrophile (tBu group) occupies the
sterically least congested position, whereas the smallest one
(methine hydrogen) possesses a position adjacent to the
=CHCH2Nu hydrogen in (R)-3, which sticks out toward the
electrophile. An antiperiplanar-like transition state and a
second synclinal-like transition state (Scheme 5, bottom
right), which would lead to (R)-5x, are unfavorable due to
steric repulsions between =CHCH2Nu and the OMe group
or between =CHCH2[Si] and the tBu substituent. The bulk-
ier silyl substituent in (R)-3 further destabilizes one of the
disfavored transition states (Scheme 5, far bottom right),
which leads to better chirality transfer with (R)-3dm and
(R)-3dn.
Conclusions
The substituent effects of the silyl groups were studied in
the palladium-catalyzed asymmetric synthesis of (allenyl-
methyl)silanes and their SE2Ј chirality transfer reactions. It
is found that as the steric bulk of the silyl groups in the 3-
bromo-5-silylpenta-1,3-dienes increased from -SiMe3 to
-SiiPr3, the enantioselectivity of the two enantioselective
processes also improved remarkably.
Experimental Section
General Methods: All anaerobic and/or moisture-sensitive manipu-
lations were carried out with standard Schlenk techniques under
predried nitrogen or with glove box techniques under prepurified
argon. 1H NMR and 13C NMR were measured with a JEOL JNM-
ECX400 spectrometer; chemical shifts are reported in ppm down-
field of internal tetramethylsilane. Tetrahydrofuran and hexane
(homogenized with tetraglyme) were distilled from benzophenone-
ketyl under an atmosphere of nitrogen prior to use. The second
generation Grubbs catalyst,[14] 1,4-disilyl-2-butenes (6a–c),[13] 3-
Eur. J. Org. Chem. 2012, 1656–1663
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M. Ogasawara, Y. Ge, A. Okada, T. Takahashi
FULL PAPER
bromo-5-trimethylsilylpenta-1,3-diene (1a),[12] [PdCl(π-allyl)]2,[20]
H), 6.08 (t, J = 8.2 Hz, 1 H), 6.31 (dd, J = 10.5, 16.5 Hz, 1 H) ppm.
13C NMR (101 MHz, CDCl3): δ = 11.4, 16.4, 18.7, 115.3, 123.9,
dpbp,[15] Pd(dba)2,[21] (R)-segphos,[16] (S)-tBu-segphos,[17] tBuCH-
(OMe)2,[22] and (MeO)2CHCH2CH(CO2Me)2
were prepared as 133.4, 136.2 ppm. HRMS (EI): calcd. for C14H27BrSi 302.1065;
[23]
reported. All other chemicals were obtained from commercial
sources and used without additional purification.
found 302.1074.
Palladium-Catalyzed Synthesis of (Allenylmethyl)silanes 3: Prepara-
tion of racemic (allenylmethyl)silanes 3 was conducted according
to a reported procedure.[7] The reaction conditions and results are
summarized in Table 1. A mixture of [PdCl(π-allyl)]2 (1.8 mg,
10 μmol/Pd), dpbp (5.7 mg, 11 μmol), and 1 (0.50 mmol) was dis-
solved in THF (5 mL), and the solution was added to a mixture of
2 (0.55 mmol) and NaH (0.55 mmol) by cannula under an atmo-
sphere of nitrogen. The mixture was stirred at the appropriate tem-
perature for 12 h, then filtered through a short pad of SiO2 to re-
move the precipitated inorganic salts. The silica gel pad was washed
with a small amount of Et2O (3ϫ), and the combined solution was
evaporated to dryness under reduced pressure. The yellow residue
was chromatographed on silica gel to give allene 3 in pure form.
Allenes 3am,[7] 3ao,[9g] 3bo,[9g] and 3do[9g] were characterized by
comparison of their spectroscopic data with those reported pre-
viously. The characterization data of the other allenic products are
listed below.
(E/Z)-1,4-Bis(triisopropylsilyl)-2-butene (6d): Allyltriisopropylsilane
(5.47 g, 27.6 mmol) was dissolved in CH2Cl2 together with the sec-
ond generation Grubbs catalyst (0.59 g, 0.69 mmol, 2.5 mol-%) un-
der an atmosphere of nitrogen. The solution was heated at reflux
for 12 h with stirring. The solution was cooled to room tempera-
ture, then filtered through a short pad of SiO2. After removing the
solvent under reduced pressure, vacuum transfer of the residue gave
6d as a mixture of two isomers (E/Z = 72:28) in 85% yield. 1H
NMR (400 MHz, CDCl3): δ = 0.99–1.10 (m, 42 H of both isomers),
1.51 (d, J = 6.0 Hz, 4 H of E-isomer), 1.54 (d, J = 6.4 Hz, 4 H of Z-
isomer), 5.30–5.35 (m, 2 H of both isomers) ppm. 13C{1H} NMR
(101 MHz, CDCl3, E-isomer): δ = 11.1, 15.3, 18.8, 125.3 ppm.
13C{1H} NMR (101 MHz, CDCl3, Z-isomer): δ = 10.2, 11.3, 18.8,
123.9 ppm. HRMS (EI): calcd. for C22H48Si2 368.3295; found
368.3293.
3-Bromo-5-silylpenta-1,3-dienes (1a–c):[10] To a suspension of dis-
ilylbutene 6 (10 mmol) and KOtBu (3.4 g, 30 mmol) in dry hexane
(20 mL) was added bromoform (7.6 g, 30 mmol) slowly at 0 °C.
The mixture was stirred for 1 h at 0 °C and 1 h at room tempera-
ture. The mixture was then filtered through Celite, and the filtrate
was washed with saturated aqueous NaCl. The aqueous phase was
extracted with diethyl ether, and the combined organic layer was
dried with MgSO4. After removing the solvent, the crude di-
bromocyclopropane was heated at 100–150 °C (bath temperature)
under vacuum in a distillation assembly to afford a crude product,
which was with the eliminated bromosilane and bromoform. The
pure product was obtained by a second distillation of the crude
product. Bromodiene (Z)-1a, obtained in 71% isolated yield, was
characterized by comparison of its spectroscopic data with those
reported previously.[10] The characterization data of (Z)-1b and (Z)-
1c are listed below.
Dimethyl 2-Methyl-2-[5-(triethylsilyl)penta-2,3-dienyl]propane-1,3-
dioate (3bm): 1H NMR (400 MHz, CDCl3): δ = 0.54 (q, J = 8.0 Hz,
6 H), 0.94 (t, J = 8.0 Hz, 9 H), 1.31–1.32 (m, 1 H), 1.33–1.34 (m,
1 H), 1.44 (s, 3 H), 2.53 (d, J = 2.3 Hz, 1 H), 2.55 (d, J = 2.3 Hz,
1 H), 3.721 (s, 3 H), 3.724 (s, 3 H), 4.86–4.92 (m, 1 H), 5.02–5.09
(m, 1 H) ppm. 13C NMR (101 MHz, CDCl3): δ = 3.2, 7.4, 12.7,
19.8, 36.2, 52.5, 52.6, 54.0, 84.5, 87.4, 172.38, 172.45, 206.3 ppm.
HRMS (EI): calcd. for C17H30O4Si 326.1913; found 326.1918.
Dimethyl 2-[5-(Dimethylphenylsilyl)penta-2,3-dienyl]-2-methylprop-
1
ane-1,3-dioate (3cm): H NMR (400 MHz, CDCl3): δ = 0.30 (s, 6
H), 1.39 (s, 3 H), 1.51–1.53 (m, 1 H), 1.54–1.55 (m, 1 H), 2.47–2.49
(m, 2 H), 3.70 (s, 3 H), 3.71 (s, 3 H), 4.84–4.91 (m, 1 H), 5.00–5.07
(m, 1 H), 7.33–7.37 (m, 3 H), 7.49–7.52 (m, 2 H) ppm. 13C NMR
(101 MHz, CDCl3): δ = –3.32, –3.25, 17.2, 19.8, 36.2, 52.5, 52.6,
54.0, 84.8, 86.9, 127.9, 129.1, 133.7, 138.5, 172.3, 172.4, 206.6 ppm.
HRMS (EI): calcd. for C19H26O4Si 346.1600; found 346.1605.
(Z)-3-Bromo-5-triethylsilylpenta-1,3-diene (1b): B.p. (bath temp.) =
1
60–65 °C (0.2 Torr). Yield: 59%. H NMR (400 MHZ, CDCl3): δ
Dimethyl 2-Methyl-2-[5-(triisopropylsilyl)penta-2,3-dienyl]propane-
1
= 0.58 (q, J = 7.8 Hz, 6 H), 0.96 (t, J = 7.8 Hz, 9 H), 1.86 (d, J =
8.7 Hz, 2 H), 5.06 (d, J = 10.6 Hz, 1 H), 5.42 (d, J = 16.5 Hz, 1
H), 6.03 (t, J = 8.7 Hz, 1 H), 6.31 (dd, J = 10.6, 16.5 Hz, 1 H) ppm.
13C NMR (101 MHz, CDCl3): δ = 3.5, 7.3, 18.8, 115.1, 123.6,
132.8, 136.0 ppm. HRMS (EI): calcd. for C11H21BrSi 260.0596;
found 260.0591.
1,3-dioate (3dm): H NMR (400 MHz, CDCl3): δ = 1.01–1.08 (m,
21 H), 1.41–1.42 (m, 1 H), 1.43–1.44 (m, 1 H), 1.44 (s, 3 H), 2.54
(d, J = 1.8 Hz, 1 H), 2.56 (d, J = 1.8 Hz, 1 H), 3.718 (s, 3 H), 3.724
(s, 3 H), 4.85–4.91 (m, 1 H), 5.09–5.15 (m, 1 H) ppm. 13C NMR
(101 MHz, CDCl3): δ = 10.6, 11.0, 18.8 (unresolved diastereotopic -
SiCHMe2), 19.8, 36.0, 52.5, 52.6, 54.0, 84.6, 88.2, 172.39, 172.44,
206.3 ppm. HRMS (EI): calcd. for C20H36O4Si 368.2383; found
368.2387.
(Z)-3-Bromo-5-dimethylphenylsilylpenta-1,3-diene (1c): B.p. (bath
temp.) = 80–90 °C (0.2 Torr). Yield: 48%. 1H NMR (400 MHz,
CDCl3): δ = 0.34 (s, 6 H), 2.07 (d, J = 8.7 Hz, 2 H), 5.07 (d, J =
10.1 Hz, 1 H), 5.43 (d, J = 16.0 Hz, 1 H), 5.98 (t, J = 8.7 Hz, 1 H),
6.29 (dd, J = 10.1, 16.0 Hz, 1 H), 7.35–7.39 (m, 3 H), 7.51–7.54
(m, 2 H) ppm. 13C NMR (101 MHz, CDCl3): δ = –2.8, 22.9, 115.6,
124.5, 128.0, 129.4, 132.0, 133.6, 136.1, 138.2 ppm. HRMS (EI):
calcd. for C13H17BrSi 280.0283; found 280.0286.
Diethyl 2-Phenyl-2-[5-(trimethylsilyl)penta-2,3-dienyl]propane-1,3-
dioate (3an): 1H NMR (400 MHz, CDCl3): δ = –0.02 (s, 9 H), 1.17–
1.20 (m, 2 H), 1.245 (t, J = 7.3 Hz, 3 H), 1.249 (t, J = 7.1 Hz, 3
H), 2.99 (d, J = 2.8 Hz, 1 H), 3.01 (d, J = 2.8 Hz, 1 H), 4.17–4.27
(m, 4 H), 4.91–5.00 (m, 2 H), 7.26–7.35 (m, 3 H), 7.42–7.44 (m, 2
H) ppm. 13C NMR (101 MHz, CDCl3): δ = –1.9, 14.1 (unresolved
diastereotopic -CO2CH2CH3), 17.6, 36.7, 61.6, 61.7, 63.0, 85.0,
87.2, 127.5, 128.1, 128.4, 136.7, 170.3, 170.4, 206.3 ppm. HRMS
(EI): calcd. for C21H30O4Si 374.1913; found 374.1915.
(Z)-3-Bromo-5-triisopropylsilylpenta-1,3-diene (1d): Thermolysis of
the dibromocyclopropane, which was obtained from 6d as above,
gave 1,5-bis(triisopropylsilyl)-3-bromopenta-1,3-diene (7d). Crude
7d was dissolved in CH2Cl2 containing 1% CF3CO2H, and the
solution was stirred at room temperature for 1 h. After removing
all the volatiles under vacuum, 1d was purified by vacuum distil-
Diethyl
2-Phenyl-2-[5-(triethylsilyl)penta-2,3-dienyl]propane-1,3-
dioate (3bn): 1H NMR (400 MHz, CDCl3): δ = 0.50 (q, J = 8.0 Hz,
6 H), 0.91 (t, J = 8.0 Hz, 9 H), 1.22–1.24 (m, 2 H), 1.24 (t, J =
7.1 Hz, 3 H), 1.25 (t, J = 7.1 Hz, 3 H), 2.94–3.05 (m, 2 H), 4.19–
4.27 (m, 4 H), 4.92–4.97 (m, 2 H), 7.26–7.35 (m, 3 H), 7.42–7.44
(m, 2 H) ppm. 13C NMR (101 MHz, CDCl3): δ = 3.2, 7.4, 12.5,
1
lation. B.p. (bath temp.) = 110–120 °C (0.2 Torr). Yield: 53%. H
NMR (400 MHz, CDCl3): δ = 1.02–1.11 (m, 21 H), 1.92 (d, J =
8.2 Hz, 2 H), 5.06 (d, J = 10.5 Hz, 1 H), 5.42 (d, J = 16.5 Hz, 1
1660
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 1656–1663

Pd-Catalyzed Synthesis of Axially Chiral (Allenylmethyl)silanes
14.1 (unresolved diastereotopic -CO2CH2CH3), 36.5, 61.61, 61.64,
62.9, 85.0, 87.3, 127.5, 128.1, 128.4, 136.7, 170.3, 170.4, 206.1 ppm.
HRMS (EI): calcd. for C24H36O4Si 416.2383; found 416.2390.
(R)-3ao: HPLC (Daicel Chiralpak AD-H, hexane/iPrOH = 100:1,
0.5 mL/min): tR = 29.7 min, tS = 32.8 min. [α]2D5 = –56.0 (c = 1.03,
CHCl3 for a sample of 62%ee).
Diethyl
2-[5-(Dimethylphenylsilyl)penta-2,3-dienyl]-2-phenylprop- (R)-3bo: HPLC (Daicel Chiralpak AD-H, hexane/iPrOH = 200:1,
1
ane-1,3-dioate (3cn): H NMR (400 MHz, CDCl3): δ = 0.248 (s, 3 0.5 mL/min): tR = 54.3 min, tS = 62.8 min. [α]2D5 = –87.2 (c = 1.01,
H), 0.251 (s, 3 H), 1.23 (t, J = 7.3 Hz, 3 H), 1.24 (t, J = 7.3 Hz, 3
H), 1.42–1.45 (m, 2 H), 2.92–2.95 (m, 2 H), 4.17–4.25 (m, 4 H),
4.91–4.97 (m, 2 H), 7.25–7.40 (m, 8 H), 7.46–7.49 (m, 2 H) ppm.
13C NMR (101 MHz, CDCl3): δ = –3.4, –3.3, 14.1 (unresolved dia-
stereotopic -CO2CH2CH3), 16.8, 36.5, 61.6, 61.7, 62.9, 85.3, 86.8,
127.5, 127.8, 128.1, 128.4, 129.1, 133.7, 136.7, 138.5, 170.3, 170.4,
206.5 ppm. HRMS (EI): calcd. for C26H32O4Si 436.2070; found
436.1956.
CHCl3 for a sample of 80%ee).
(R)-3do: HPLC (Daicel Chiralpak AD-H, hexane/iPrOH = 200:1,
0.5 mL/min): tR = 21.7 min, tS = 24.8 min. [α]2D7 = –109 (c = 0.95,
CHCl3 for a sample of 87%ee).
TiCl4-Promoted SE2Ј Reaction of (Allenylmethyl)silanes 3 with
RCH(OMe)2 4: The reaction was performed according to a re-
ported method[7] with slight modifications. The reaction conditions
and results are summarized in Table 3. A typical procedure is given
for the preparation of (E)-dimethyl 2-[3-(1-methoxy-2,2-dimeth-
ylpropyl)penta-2,4-dienyl]-2-methylpropane-1,3-dioate (5mx): Un-
der a nitrogen atmosphere, to a solution of tBuCH(OMe)2 (4x,
92.6 mg, 0.7 mmol) in CH2Cl2 (2 mL) was added a CH2Cl2 solution
of TiCl4 (1.0 m, 0.7 mL, 0.7 mmol). The solution was cooled to
–78 °C and to this was added a CH2Cl2 solution (2 mL) of (allenyl-
methyl)silane (R)-3am (99.6 mg, 0.35 mmol, 78% ee) by cannula.
The solution was stirred at this temperature for 1 h and then
quenched with water. The reaction mixture was extracted with di-
ethyl ether (3ϫ), and the combined organic solution was washed
Diethyl
2-Phenyl-2-[5-(triisopropylsilyl)penta-2,3-dienyl]propane-
1,3-dioate (3dn): 1H NMR (400 MHz, CDCl3): δ = 1.02 (br., 21 H),
1.24 (t, J = 7.3 Hz, 3 H), 1.25 (t, J = 7.4 Hz, 3 H), 1.31 (d, J =
2.5 Hz, 1 H), 1.33 (d, J = 2.5 Hz, 1 H), 2.94–3.05 (m, 2 H), 4.16–
4.28 (m, 4 H), 4.90–5.03 (m, 2 H), 7.25–7.34 (m, 3 H), 7.41–7.43
(m, 2 H) ppm. 13C NMR (101 MHz, CDCl3): δ = 10.3, 11.0, 14.1
(unresolved diastereotopic -CO2CH2CH3), 18.8 (unresolved dia-
stereotopic -SiCHMe2), 36.3, 61.61, 61.64, 62.9, 85.1, 88.0, 127.5,
128.1, 128.4, 136.7, 170.3, 170.4, 206.1 ppm. HRMS (EI): calcd.
for C27H42O4Si 458.2852; found 458.2834.
Palladium-Catalyzed Asymmetric Synthesis of (Allenylmethyl)sil- with saturated aqueous NaCl and dried with anhydrous MgSO4.
anes 3: Asymmetric synthesis of (allenylmethyl)silanes (R)-3 was
conducted according to the reported method.[7] The reaction condi-
tions and the results are summarized in Table 2. A typical pro-
cedure is given for the preparation of (R)-3am: A mixture of
Pd(dba)2 (28.8 mg, 50.0 μmol), (R)-segphos (34.2 mg, 56.0 μmol),
and 2m (101 mg, 0.600 mmol) was dissolved in THF (5 mL). After
stirring the solution at 40 °C for 15 min, (Z)-1a (110 mg,
0.500 mmol) was added by syringe. The mixture was stirred at this
temperature for 24 h, then cooled to room temperature and filtered
through a short pad of alumina to remove the precipitated inor-
ganic salts. The alumina pad was washed with a small amount of
Et2O (3ϫ), and the combined solution was evaporated to dryness
under reduced pressure. The residue was purified by chromatog-
raphy on alumina (hexane/Et2O = 4:1) to afford allene (R)-3am
(107 mg, 75%) as a colorless oil.
After removal of the solvent, the residue was chromatographed on
silica gel (hexane/Et2O = 4:1) to give diene 5mx as a colorless oil.
The enantiopurity of 5mx and 5my was determined by HPLC
analysis with a chiral stationary phase column. The HPLC analysis
conditions and the specific optical rotation data are listed below.
(S)-5mx: HPLC (Daicel Chiralcel OD-H, double columns con-
nected in series, hexane/iPrOH = 500:1, 0.8 mL/min): tS = 71.3 min,
tR = 74.6 min. [α]2D0 = –1.1 (c = 1.01, CHCl3 for a sample of
80%ee).
5my: HPLC (Daicel Chiralcel OD-H, hexane/iPrOH = 200:1,
1.0 mL/min): t1 = 68.1 min, t2 = 74.1 min. [α]2D0 = –10.4 (c = 0.99,
CHCl3 for a sample of 26%ee).
For the determination of the enantiopurity of (S)-5nx, the com-
pound was derivatized by LiAlH4 reduction in THF into the corre-
sponding diol, which was analyzed by chiral HPLC (Daicel Chi-
ralcel OD-H, double columns connected in series, hexane/iPrOH =
50:1, 1.0 mL/min): tS = 182.6 min, tR = 193.4 min.
The enantiopurity of 3 was determined by HPLC analysis with a
chiral stationary phase column. The HPLC analysis conditions and
the specific optical rotation data are listed below.
(R)-3am: HPLC (Daicel Chiralpak AD-H, hexane/iPrOH = 200:1,
0.3 mL/min): tR = 23.2 min, tS = 29.4 min. [α]2D0 = –60.0 (c = 0.99,
CHCl3 for a sample of 79%ee).
The absolute configurations of both 5mx and 5nx were determined
as follows.[7] Diene 5 was treated with a mixture of K2CO3/NaIO4/
KMnO4 (6 equiv./16 equiv-/2.3 equiv.) in tBuOH/H2O (1:2) for
12 h. By this treatment, both 5mx and 5nx were converted into
tBuCH(OMe)CO2H. The crude carboxylic acid was treated with
etherial CH2N2, and then the obtained methyl ester was purified
by vacuum transfer. The absolute configuration of the ester from
both 5mx and 5nx, tBuCH(OMe)CO2Me, was determined to be
(S) by comparison of retention time on the chiral HPLC with that
of an authentic sample (S)-tBuCH(OMe)CO2Me, which was pre-
pared from commercially available (S)-tBuCH(OH)CO2H (Ald-
rich). The methyl ester tBuCH(OH)CO2Me was too volatile to be
purified in a small scale, and thus determination of the absolute
configuration by the signs of optical rotation was abandoned.
(R)-3bm: HPLC (Daicel Chiralcel OD-H, hexane/iPrOH = 1000:1,
0.5 mL/min): tS = 25.7 min, tR = 28.4 min. [α]2D6 = –62.4 (c = 0.89,
CHCl3 for a sample of 80%ee).
(R)-3cm: HPLC (Daicel Chiralpak AD-H, hexane/iPrOH = 400:1,
0.5 mL/min): tR = 23.9 min, tS = 27.2 min. [α]2D5 = –60.2 (c = 0.97,
CHCl3 for a sample of 80%ee).
(R)-3dm: HPLC (Daicel Chiralcel OD-H, hexane/iPrOH = 1000:1,
0.5 mL/min): tS = 24.0 min, tR = 26.1 min. [α]2D6 = –132 (c = 1.00,
CHCl3 for a sample of 87%ee).
(R)-3an: HPLC (Daicel Chiralcel OD-H, hexane/iPrOH = 400:1,
0.5 mL/min): tR = 25.8 min, tS = 30.8 min. [α]2D6 = –57.8 (c = 1.01,
CHCl3 for a sample of 81%ee).
Dienes 5mx and 5my were characterized by comparison of its spec-
troscopic data with those reported previously.[7] The characteriza-
tion data of 5nx and the diol derived from 5nx are listed below.
(R)-3dn: HPLC (Daicel Chiralcel OD-H, hexane/iPrOH = 1000:1,
1.0 mL/min): tS = 22.7 min, tR = 26.4 min. [α]2D5 = –118 (c = 0.86,
CHCl3 for a sample of 90%ee).
(S)-(E)-Diethyl 2-[3-(1-Methoxy-2,2-dimethylpropyl)penta-2,4-di-
enyl]-2-phenylpropane-1,3-dioate (5nx): 1H NMR (400 MHz,
Eur. J. Org. Chem. 2012, 1656–1663
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1661

M. Ogasawara, Y. Ge, A. Okada, T. Takahashi
FULL PAPER
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CDCl3): δ = 0.79 (s, 9 H), 1.239 (t, J = 7.3 Hz, 3 H), 1.242 (t, J =
7.3 Hz, 3 H), 3.04 (s, 3 H), 3.25 (dd, J = 6.9, 16.1 Hz, 1 H), 3.34
(dd, J = 7.6, 16.1 Hz, 1 H), 3.41 (s, 1 H), 4.16–4.28 (m, 4 H), 5.17
(d, J = 11.4 Hz, 1 H), 5.25 (d, J = 18.3 Hz, 1 H), 5.42 (t, J =
7.1 Hz, 1 H), 6.53 (dd, J = 11.4, 18.3 Hz, 1 H), 7.24–7.33 (m, 3 H),
7.40–7.42 (m, 2 H) ppm. 13C NMR (101 MHz, CDCl3): δ = 14.0
(unresolved diastereotopic -CO2CH2CH3), 26.8, 34.2, 35.7, 56.6,
61.7 (unresolved diastereotopic -CO2CH2CH3), 62.5, 89.7, 116.1,
126.1, 127.6, 128.2, 128.4, 134.0, 136.6, 137.5, 170.5 (unresolved
diastereotopic -CO2CH2CH3) ppm. HRMS (EI): calcd. for
C24H34O5 402.2406; found 402.2406. C24H34O5 (402.53): calcd. C
71.61, H 8.51; found C 71.54, H 8.59. [α]2D5 = –6.2 (c = 1.02, CHCl3
for a sample of 75%ee).
[4]
(S)-(E)-2-[3-(1-Methoxy-2,2-dimethylpropyl)penta-2,4-dienyl]-2-
phenylpropane-1,3-diol: 1H NMR (400 MHz, CDCl3): δ = 0.74 (s,
9 H), 2.52 (br., 2 H), 2.61 (dd, J = 7.4, 15.1 Hz, 1 H), 2.69 (dd, J
= 7.8, 15.1 Hz, 1 H), 3.00 (s, 3 H), 3.40 (s, 1 H), 3.91–3.94 (m, 2
H), 4.06–4.09 (m, 2 H), 5.14 (d, J = 11.4 Hz, 1 H), 5.22 (d, J =
17.4 Hz, 1 H), 5.25 (t, J = 7.8 Hz, 1 H), 6.56 (dd, J = 11.4, 17.4 Hz,
1 H), 7.19–7.23 (m, 1 H), 7.30–7.35 (m, 4 H) ppm. 13C NMR
(101 MHz, CDCl3): δ = 26.8, 32.0, 35.7, 47.9, 56.7, 68.2, 68.3, 89.5,
115.7, 126.7, 126.8, 127.1, 128.8, 134.3, 137.0, 141.2 ppm. HRMS
(FAB): calcd. for C20H30NaO3 [M + Na]+ 341.2093; found
341.2103.
[5]
[6]
[7]
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Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Research
on Priority Areas “Advanced Molecular Transformations of Car-
bon Resources” from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
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Received: November 11, 2011
Published Online: January 30, 2012
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