Lewis acid catalyzed trans-allylsilylation of unactivated alkynes

Article abstract of DOI:10.1021/ja970443b

The addition of different substituted allylsilanes 2 to unactivated alkynes 1 in the presence of catalytic mounts of HfCl₄ or the EtAlCl₂- TMSC1 catalyst system produced in high yields the silylated 1,4-dienes 3 regio- and stereosele

Full text of DOI:10.1021/ja970443b

J. Am. Chem. Soc. 1997, 119, 6781-6786
6781
Lewis Acid Catalyzed trans-Allylsilylation of Unactivated
Alkynes
Eiji Yoshikawa, Vladimir Gevorgyan, Naoki Asao, and Yoshinori Yamamoto*
Contribution from the Department of Chemistry, Graduate School of Science, Tohoku UniVersity,
Sendai 980-77, Japan
ReceiVed February 10, 1997X
Abstract: The addition of different substituted allylsilanes 2 to unactivated alkynes 1 in the presence of catalytic
amounts of HfCl4 or the EtAlCl2-TMSCl catalyst system produced in high yields the silylated 1,4-dienes 3 regio-
and stereoselectively. The exclusive trans manner of addition was confirmed by analysis of crude reaction mixtures
by 1H NMR and capillary GLC methods. Good agreement of relative reactivities of reaction of various allylsilanes
2a-e toward phenylacetylene (1a) in the presence of HfCl4 with the relative reaction rates of 2a-e with carbenium
ions supported the involvement of cationic species 11 as a reaction intermediate. The mechanisms for the HfCl4 and
EtAlCl2-TMS catalyzed trans-allylsilylation of alkynes are proposed.
Table 1. Allylsilylation of Alkynes in the Presence of the
EtAlCl2-TMSCl Catalyst System
Introduction
Since the first example of carbometalation demonstrated by
Ziegler and Ba¨hr,1 a number of additions of organometallics to
carbon-carbon multiple bonds have been reported.2 The
allylmetalation of activated alkynes, such as alkynyl ketones
(Michael acceptors) and alkynols (functionally substituted
alkynes), in both intramolecular and intermolecular versions
proceeds smoothly with various allylmetals.2,3 However, the
allylmetalation of simple unactivated alkynes is not easy, and
only a limited number of allylmetals can serve for this
purpose.2,4 Recently we have communicated the first example
of trans-allylsilylation of unactivated alkynes 1 in the presence
of the EtAlCl2-TMSCl catalyst system (eq 1).5 Now we report
the HfCl4-catalyzed trans-allylsilylation of unactivated alkynes,
which is more efficient than the EtAlCl2-TMSCl-catalyzed
allylsilylation, along with a detailed study on the EtAlCl2-
TMSCl catalyst system.
entry
1
R1
R2
3
product yield %a
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1f
Ph
H
H
H
H
H
H
Me
3a
3b
3c
3d
3e
3f
93
95
p-CH3C6H4
PhCH2
CH3(CH2)5
CH3(CH2)9
1-cyclohexenyl
Ph
57b,c
90b
85b,d
73b,e
88
1g
3g
a Isolated yield, except for where otherwise indicated. b Yield
determined by 1H NMR with p-xylene as an internal standard.
c PhCH2(CH2dCHCH2)CdCH2 (4c) was produced in 13% yield.
d CH3(CH2)9(CH2dCHCH2)CdCH2 (4e) was produced in 5% yield.e 1-
(1,4-Pentadien-2-yl)cyclohexene (4f) was produced in 5% yield.
Results and Discussion
Allylsilylation of Alkynes Catalyzed by the EtAlCl2-
TMSCl Catalyst System. As we have previously shown,5 the
X Abstract published in AdVance ACS Abstracts, July 1, 1997.
(1) Ziegler, K.; Ba¨hr, K. Chem. Ber. 1928, 61, 253.
addition of allyltrimethylsilane (2a) to unactivated alkynes 1 is
catalyzed by the EtAlCl2-TMSCl catalyst system to give the
corresponding trans-silylated 1,4-dienes 3 in good to high yields
(eq 1, Table 1). The reaction of phenylacetylene (1a) with
allyltrimethylsilane (2a) catalyzed by EtAlCl2 in the presence
of 20 equiv of TMSCl gave the trans-carbosilylation product
3a regio- and stereoselectively in 93% yield (entry 1). Neither
the stereoisomer of 3a (cis addition product) nor the regioisomer
of 3a was produced.6 The reaction of 4-ethynyltoluene (1b)
gave 3b in 95% yield (entry 2), whereas the addition to
3-phenyl-1-propyne (1c) afforded 3c in 57% yield along with
the desilylated product 4c as a byproduct (entry 3). Very trace
amounts of desilylated products were also detected by 1H NMR
of the crude products in entries 1 and 2. Reactions of 1-octyne
(1d) and 1-dodecyne (1e) gave 3d and 3e, respectively, in high
yields (entries 4 and 5). The trans-allylsilylation of the enyne
1f and internal acetylene 1g also proceeded smoothly to give
the corresponding alkenylsilanes 3f and 3g, respectively, in high
(2) For reviews, see: (a) Normant, J. F.; Alexakis, A. Synthesis 1981,
841 (organo Li, Mg, Zn, B, Al, and Cu compounds). (b) Oppolzer, W.
Angew. Chem., Int. Ed. Engl. 1989, 28, 38 (stoichiometric organo Li, Mg,
and Zn and catalytic Ni, Pd, Pt compounds). (c) Negishi, E. Pure Appl.
Chem. 1981, 53, 2333 (organo Al/Ti and Al/Zr systems). (d) Knochel, P.
ComprehensiVe Organometallic Chemistry II; Able, E. W., Stone, F. G.
A., Wilkinson, G.; Eds.; Pergamon Press: Oxford, 1995; Vol. 11, p 159
(organo Li, Mg, Zn, B, Al, Cu, Hg/Pd, Ni, and Mn compounds). (e) Knochel,
P. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon Press: Oxford, 1991; Vol. 4, p 865. (f) Yamamoto, Y.; Asao,
N. Chem. ReV. 1993, 93, 2207 (organo Li, Mg, Zn, B, and Al compounds).
(3) Araki, S.; Imai, A.; Shimizu, K.; Yamada, M.; Mori, A.; Butsugan,
Y. J. Org. Chem. 1995, 60, 1841 (allylindiums).
(4) (a) Takai, K.; Yamada, M.; Odaka, H.; Utimoto, K.; Fujii, T.;
Furukawa, I. Chem. Lett. 1995, 315 (allyl-Ta). (b) Takahashi, T.; Kotora,
M.; Kasai, K.; Suzuki, N. Tetrahedron Lett. 1994, 35, 5685 (allyl-Zr). (c)
Chatani, N.; Amishiro, N.; Morii, T.; Yamashita, T.; Murai, S. J. Org. Chem.
1995, 60, 1834 (allyl-Zn). (d) Molander, G. A. J. Org. Chem. 1983, 48,
5409 (allyl-Zn). (e) Miller, J. A.; Negishi, E. Tetrahedron Lett. 1984, 25,
5863 (allyl-Al). (f) Negishi, E.; Miller, J. A. J. Am. Chem. Soc. 1983, 105,
6761 (allyl-Zn). (g) Eishi, J. J.; Boleslawski, M. P. J. Organomet. Chem.
1987, 334, C1 (allyl-Ti). (h) Yeon, S. H.; Han, J. S.; Hong, E.; Do, Y.;
Jung, I. N. J. Organomet. Chem. 1995, 499, 159 (allyl-Si).
(6) Jung and co-workers reported cis-allylsilylation of phenylacetylene
(ref 4h). However, their stereochemical assignment of reaction product was
not correct (see ref 5, note 7).
(5) Asao, N.; Yoshikawa, E.; Yamamoto, Y. J. Org. Chem. 1996, 61,
4874.
S0002-7863(97)00443-5 CCC: $14.00 © 1997 American Chemical Society

6782 J. Am. Chem. Soc., Vol. 119, No. 29, 1997
Yoshikawa et al.
Scheme 1. Proposed Mechanism for the EtAlCl2-TMSCl Catalyzed trans-Allylsilylation of Unactivated Alkynes
Table 2. Lewis Acid Catalyzed Addition of Allyltrimethylsilane
to 1d
yields (entries 6 and 7). The use of other Lewis acids, such as
AlCl3, AlBr3, and HfCl4 in combination with TMSCl, also gave
the allylsilylation product; however, EtAlCl2 afforded the best
yields of 3. The trans-carbosilylation was unambiguously
confirmed by the stereochemistry of the allylation product 3g.
Thus, irradiation of methyl protons attached to the double bond
of 3g enhanced both methylene protons of the allylic position
(5.7% NOE) and a vinyl proton at the C-5 position (1.4% NOE),
whereas irradiation of protons of the TMS group did not
noticeably enhance these protons.
Lewis
acid
temp
(°C)
yield
ratio
(3d + 4d, %)a 3d:4db
A plausible mechanism for the EtAlCl2-catalyzed trans-
allylsilylation is shown in Scheme 1. The coordination of
EtAlCl2 to 1 would produce the π-complex 5. Allyltrimethyl-
silane would attack the electron-deficient triple bond from the
side opposite to the Lewis acid to produce the aluminum-ate
complex 6 stereoselectively. Transmetalation of aluminum
halide by the trimethylsilyl group would afford 3 and regenerate
the catalyst. On the other hand, the coupling between the chloro
and silyl group would produce Me3SiCl and the alkenylalumi-
num derivative 7, which would afford the minor byproduct 4
upon hydrolysis. An excess amount of TMSCl is needed to
drive the equilibrium over in favor of replacing aluminum with
silicon. In fact, in the reaction of 1-octyne, the use of 1 equiv
of TMSCl gave a 31:69 ratio of 3d:4d, and the ratio changed
to 5:95 in the absence of TMSCl. Furthermore, when the
reaction of 1a was carried out in the absence of TMSCl and
quenched with D2O, the deuterated alkene (Z)-Ph(CH2d
CHCH2)CdCHD was obtained, clearly indicating that an
alkenylaluminum intermediate is involved.7 To clarify the
intramolecular Vs intermolecular character of the last silylation
step (6 to 3), a competition experiment with another trapping
agent was performed. Thus, the use of triethylchlorosilane
(TESCl) instead of TMSCl afforded the triethylsilyl analogue
3n in moderate chemical yield (Scheme 1). The test experi-
ments indicated that no reaction took place between 1a and
allyltriethylsilane in the presense of TESCl (10 equiv). Taken
together, these two facts unambigously support the intermo-
lecular mechanism of the silylation step, and serve as an
additional support for ate-complex 6 as a key intermediate in
this trans-allylsilylation reaction (Scheme 1).
entry
solvent
1
2
3
4
5
6
7
EtAlCl2 toluene -78 f 0
20
40
50
9
5:>95c
24:76
15:85
>95:5d
e
AlCl3
AlBr3
HfCl4
HfCl4
HfCl4
HfCl4
toluene -78 f 0
toluene -78 f 0
toluene -78 f 0
hexane
CH2Cl2 -78 f 0
CH2Cl2
-78 f 0
trace
50
>95:5d
>95:5d
0
88 f
a The yield was determined by 1H NMR with p-xylene as an internal
standard. b The ratio was determined by 270 MHz 1H NMR. c 3d was
not detected by H NMR. d 4d was not detected by H NMR. e Not
1
1
determined. f Isolated yield.
a more synthetically useful catalyst system for this unprec-
edented Lewis acid catalyzed trans-allylsilylation reaction. Thus,
we examined the addition of allyltrimethylsilane (2a) to 1-octyne
(1d) in the presence of various Lewis acids (eq 3, Table 2).
As mentioned above,5 the addition of allyltrimethylsilane to
1d in the presence of only EtAlCl2 gave a 5:>95 mixture of
3d:4d with 20% overal yield (entry 1). The reaction in the
presence of AlCl3 provided a 24:76 mixture of 3d and 4d in
40% yield (entry 2). The use of AlBr3 gave rise to a combined
yield of 3d and 4d up to 50%; however, the ratio of products
was also shifted in favor of the desilylated 4d (entry 3).
Although allylsilylation in the presence of HfCl4 gave a very
low combined yield (9%), the only silylated adduct 3d was
detected as a reaction product (entry 4). We found this result
promising and briefly optimized HfCl4-catalyzed reaction condi-
tions (entries 5-7). Thus, the HfCl4-catalyzed allylsilylation
of 1d in dichloromethane at 0 °C afforded 3d as a single reaction
product in 88% isolated yield (Table 2, entry 7).8 We further
examined the HfCl4-catalyzed addition of allyltrimethylsilane
to various alkynes 1a-j (eq 3). The results are summarized in
Table 3. Addition of allyltrimethylsilane (2a) to alkyl- (entries
3-5), aryl- (entries 1, 2, 7, and 8), and alkenyl-substituted (entry
6) alkynes 1a-h in the presence of 50 mol % HfCl4 proceeded
smoothly, giving regio- and stereoselectively the corresponding
allylsilylated products 3a-h in good to excellent yields.9
Allylsilylation of Alkynes Catalyzed by HfCl4. Although
trans-allylsilylation of unactivated alkynes catalyzed by an
EtAlCl2-TMSCl catalyst system has importance from a mecha-
nistical point of view as the first example of the trans-
carbosilylation of alkynes, the synthetic utility of this method-
ology is limited by the fact that a great excess of TMSCl is
needed. Thus, it is doubtful that this method could be applied
for preparative scale syntheses. This prompted us to search for
(8) Other Lewis acids such as ZrCl4, TiCl4, SnCl4, BF3‚Et2O, ZnCl2,
and B(C6F5)3 did not catalyze the addition reaction of allyltrimethylsilane
to 1d.
(9) It is worth noting that 20 mol % of HfCl4 also catalyzed these
reactions; however, the yields of 3 in these cases were usually slightly lower.
(7) Negishi reported cis-allylalumination of unactivated acetylenes in the
presence of Zr catalyst (ref 4e). The present addition proceeds in a trans
manner. Accordingly, it seems that the allylaluminium species is not
involved as a reactive intermediate.

trans-Allylsilylation of UnactiVated Alkynes
J. Am. Chem. Soc., Vol. 119, No. 29, 1997 6783
Table 3. HfCl4-Catalyzed Addition of Allyltrimethylsilane to
Alkynes 1a-ja
without the chlorosilane (Table 4, entry 1, footnote c), the
HfCl4-TMSCl procedure was not applied to other substrates
due to obvious synthetic inconvenience (mentioned above). The
addition of allyl- (2a, entry 1), E- and Z-crotyl- (2b and 2c,
entries 2 and 3), methallyl- (2d, entry 4), and prenyltrimethyl-
silane (2e, entry 5) to phenylacetylene (1a) proceeded smoothly,
affording the corresponding adducts 3a,k-m in excellent
chemical yields (90-96%). Replacement of the trimethylsilyl
group in 2a with triethylsilyl (2f), dimethylphenylsilyl (2g), and
methydiphenylsilyl (2h) groups caused a slight decrease in the
chemical yields of allylsilylated products 3n-p, as well as a
noticeable elongation of reaction times (Table 4, entries 6-8).
It should be pointed out that in all cases only γ-addition products
3 were formed, and the formation of R-adducts was not detected
by analyses of crude reaction mixtures by 1H NMR and capillary
GLC (Table 4, eq 4).
entry
alkyne
R1
R2
product
yield(%)b
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
1i
Ph
H
H
H
H
H
H
Me
Et
TMS
H
3a
3b
3c
3d
3e
3f
3g
3h
3i
95
97
73
87
86
42
90
p-CH3C6H4
PhCH2
CH3(CH2)5
CH3(CH2)9
1-cyclohexenyl
Ph
Ph
H
t-Bu
This kind of regiochemistry is not surprising. The γ-addition
of different substituted allylsilanes to various electrophiles has
been extensively studied during the past two decades and well-
documented.10 The γ-regioselectivity of this reaction has been
explained by the intermediate formation of carbenium ions,
which are hyperconjugatively stabilized by the carbon-silicon
bond in the â-position.11 Furthermore, the recent kinetic study
on the reaction of carbenium ions with various allylsilanes
accomplished by Mayr provided the methodology for quantita-
tive determination of the nucleophilicity of the allylsilane
element (eq 5).12 Attack of the carbenium cation 8 at the
γ-position of the allylsilicon compound 2 is rate determining
82
65c,d
10b
10
1j
3j
a All reactions were carried out in CH2Cl2 at 0 °C with 50 mol % of
HfCl4.9 b Isolated yield, except for where otherwise indicated. c Yield
1
was determined by H NMR with p-xylene as an internal standard.
d The allyltrimethylsilane was added slowly Via syringe pump in order
to avoid its dimerization.4h
Table 4. HfCl4-Catalyzed Addition of 2a-h to Phenylacetylenea
and leads to formation of the â-silicon-stabilized carbenium ion
9, which subsequently transforms into product 10 Via elimination
of the silicon group (eq 5).12
In order to elucidate whether the relative reactivities of
different substituted allylsilanes in the HfCl4-catalyzed allylsi-
lylation of alkynes are similar to those toward carbenium ions
reported by Mayr,12 we determined relative reactivities for
addition of allylsilanes 2a-e to phenylacetylene (1a) based on
the measurement of half-reaction times13 (eq 4, Figure 1).
(10) (a) Colvin, E. W. Silicon in Organic Synthesis; Butterworth:
London, 1981. (b) Weber, W. P. Silicon Reagents for Organic Synthesis;
Springer: Berlin, 1983. (c) Fleming, I. In ComprehensiVe Organic
Chemistry; Barton, D. H. R., Ollis, W. d., Eds.; Pergamon Press: Oxford,
1979; Vol. III, p 541. (d) Sakurai, H. Pure Appl. Chem. 1982, 54, 1. (e)
Schinzer, D. Synthesis 1988, 263. (f) Hosomi, Acc. Chem. Res. 1988, 21,
200. (g) Fleming, I.; Dunogues, J.; Smithers, R. Org. React. (N.Y.) 1989,
37, 57. (h) Eaborn, R.; Bott, W. In Organometallic Compounds of the Group
IV Elements; Mac-Diarmid, A. G., Ed.; Marcel Dekker: New York, 1968;
Vol. 1, Part 1.
(11) (a) Wierchke, S. G.; Chandrasekhar, J. J. Am. Chem. Soc. 1985,
107, 1496. (b) White, J. C.; Cave, R. J.; Davidson, E. R. J. Am. Chem. Soc.
1988, 110, 6308. (c) Ibrahim, M. R.; Jorgensen, W. L. J. Am. Chem. Soc.
1989, 111, 819. (d) Eaborn, C. J. Chem. Soc., Chem. Commun. 1972, 1255.
(e) Lambert, J. B. Tetrahedron 1990, 46, 2677. (f) Apeloig, Y. In The
Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.;
John Wiley & Sons: Chichester, 1989; Part 1, p 196. (g) McGibbon, G.
A.; Brook, M. A.; Terlouw, J. K. J. Chem. Soc., Chem. Commun. 1992,
360. (h) Dallaire, C.; Brook, M. A. Organometallics 1990, 9, 2873. (i)
Kresge, A. J.; Tobin, J. B. Angew. Chem., Int. Ed. Engl. 1993, 32, 721. (j)
Gabelica, V.; Kresge, A. J. J. Am. Chem. Soc. 1996, 118, 3838. (k) Zhang,
W.; Stone, J. A.; Brook, M. A.; McGibbon, G. A. J. Am. Chem. Soc. 1996,
118, 5764.
a All reactions were carried out in CH2Cl2 at 0 °C with 50 mol % of
HfCl4. b Isolated yield. c In the presence of 10 equiv of TMSCl the
reaction was completed in 20 min.
Desilylated products 4 were not detected by 1H NMR and
capillary GLC analyses of crude reaction mixtures. An
exceptionally low yield of the allylsilylation of tert-butylacet-
ylene (1j) (entry 10) might be explained by steric factors. The
reverse regiochemistry of the addition of 2a to trimethylsilyl-
acetylene (1i) is in good agreement with the stabilization of a
cation â to silicon in the cationic intermediate 13, thus, directing
nucleophilic attack of the allyl group to the terminal carbon
atom (Vide post).
We further examined the HfCl4-catalyzed addition of different
substituted allylsilanes 2a-h to phenylacetylene (1a) (eq 4,
Table 4). Although the reaction between 1a and 2a in the
presence of 10 equiv of TMSCl proceeded more rapidly than
(12) Hagen, G.; Mayr, H. J. Am. Chem. Soc. 1991, 113, 4954.

6784 J. Am. Chem. Soc., Vol. 119, No. 29, 1997
Yoshikawa et al.
Figure 1. (a) Determined by capillary GLC with hexadecane as an internal standard. See also ref 13. (b) Relative reaction constants from ref 12.
An ) p-MeO-C6H4.
Scheme 2. Proposed Mechanism for the HfCl4-Catalyzed
trans-Allylsilylation of Unactivated Alkynes
of alkynes, coordination of HfCl4 to the triple bond of 1 would
form zwitterionic intermediate 10, which would attack the
double bond of allylsilane 2 at the γ-position affording
carbenium cation 11 trans-selectively. The elimination of the
silyl group from 11 would form ate-complex 12, and the
subsequent transmetalation of hafnium halide with silicon would
produce 3 and regenerate the catalyst. This mechanism nicely
explains the reverse regiochemistry upon allylsilylation of
trimethysilylacetylene (1i), previously mentioned (Table 3, eq
3). The coordination of HfCl4 to 1i would form another
regioisomer of 10, the cationic intermediate 13, due to â-silicon
stabilization11 (eq 6).
Subsequent reaction of zwitteronic 13 with 2a Via a similar
transformation pathway as 10 (Scheme 2) would produce 3i
(eq 6).
We found that relative reactivities of most allylsilanes bearing
trimethylsilyl groups (2a-e) in the HfCl4-catalyzed addition to
1a (Figure 1, part a) are in good agreement with relative
reactivities of the same allylsilanes 2a-e toward diarylcarbe-
nium ion 812 (Figure 1, part b). This finding encouraged us to
consider the intermediacy of some cationic species analogous
to 8 and 9 in the HfCl4-catalyzed allylsilylation of alkynes, and
allowed us to propose the plausible mechanism for this reaction
shown in Scheme 2.
Although the relative reactivities of most allyltrimethylsilanes
2a-e in the HfCl4-catalyzed addition to 1a are in good
agreement with relative reactivities of the same allylsilanes 2a-e
toward diarylcarbenium ion 8 reported by Mayr,12 the relative
reactivities of allylsilanes bearing more bulky groups at the
silicon atom are completely different from Mayr’s observations.
Indeed, even more bulky allyltriethylsilane (2f) and allyldi-
methyl(tert-butyl)silane (2i) reacted with a carbenium ion
slightly faster than allyltrimethylsilane (2a) (krel ) 3.13 × 102,
2.04 × 102, and 1.87 × 102 for 2f, 2i, and 2a, respectively).12
In contrast we found that the triethylsilyl analogue 2f reacted
almost two times slower than its trimethylsilyl counterpart 2a
(t1/2 ) 72 and 39 min for 2f and 2a, respectively),13 whereas
the most hindered 2i did not react with 1a at all. Although our
measurements of relative reactivities are a rough estimation,
these results indicate the dramatic effect of the bulkiness of
substituents at the silyl moiety on the overall reaction rate.
Initially, we considered the transmetalation of 12 with a silyl
group as a slow step, which could be dependent upon the size
As we have previously proposed for the Lewis acid catalyzed
hydro-14 and allylstannation15 and hydro-16 and allylsilylation5
(13) Although the so-called “half-reaction time” data are very ap-
proximate, they are only useful for the rough estimation of relative
reactivities. For example, see: Fleming, I.; Langley, J. A. J. Chem. Soc.,
Perkin Trans. 1 1981, 1412.
(14) (a) Asao, N.; Liu, J.-X.; Sudoh, T.; Yamamoto, Y. J. Org. Chem.
1996, 61, 4568. (b) Asao, N.; Liu, J.-X.; Sudoh, T.; Yamamoto, Y. J. Chem.
Soc., Chem. Commun. 1995, 2405.
(15) Asao, N.; Matsukawa, Y.; Yamamoto, Y. J. Chem. Soc., Chem.
Commun. 1996, 1513.
(16) Asao, N.; Sudo, T.; Yamamoto, Y. J. Org. Chem. 1996, 61, 7654.

trans-Allylsilylation of UnactiVated Alkynes
J. Am. Chem. Soc., Vol. 119, No. 29, 1997 6785
(m, 2H), 3.21 (ddd, J ) 1.5, 2.7, 7.0 Hz, 2H), -0.19 (s, 9H). 13C
NMR (67.9 MHz, CDCl3) δ 157.4 (C), 135.9 (CH), 144.0, 127.9, 127.7,
127.0 (Ar), 116.4 (CH2d), 46.7 (CH2), 0.0 (Si-CH3). IR (neat) 3078,
2953, 2897, 1639, 1610, 1595, 1490, 1248, 839 cm-1. MS (EI) m/z
216 (M+, 3), 201 (M+ - CH3, 36), 73 (100). HRMS calcd for C14H20-
Si 216.1334, found 216.1290.
(Z)-2-(p-Tolyl)-1-(trimethylsilyl)-1,4-pentadiene (3b): 1H NMR
(CDCl3, 270 MHz) δ 7.17-7.03 (m, 4H), 5.83 (ddt, J ) 7.0, 9.3, 17.8
Hz, 1H), 5.56 (t, J ) 1.3 Hz, 1H), 5.06-4.99 (m, 2H), 3.14 (ddd, J )
1.3, 2.7, 7.0 Hz, 2H), 2.35 (s, 3H), -0.17 (s, 9H). 13C NMR (67.9
MHz, CDCl3) δ 157.4 (C), 136.6, 136.1 (CH), 141.1, 128.4, 127.8,
127.6 (Ar), 116.3 (CH2d), 46.7, 21.2 (CH2), 0.1 (Si-CH3). IR (neat)
2953, 1601, 1510, 1246, 841 cm-1. MS (EI) m/z 230 (M+, 12), 215
(M+ - CH3, 28), 73 (100). HRMS calcd for C15H22Si 230.1491, found
230.1491.
of a silyl group, thus affecting the overall reaction rate. In order
to clarify the formation of 12 in the reaction of phenylacetylene
(1a) with bulky 2i we quenched the reaction mixture with H2O.
In the case where the ate-complex 12 is present in the reaction
mixture, hydrolysis should unavoidably lead to desilylated
product 4. The experiment indicated no traces of 4 were formed.
Instead, a major portion of starting 1a was recovered, thus
discounting the proposed rationale. Although the dramatic affect
of steric hindrance of the silyl group on the apparent nucleo-
philicity of allylsilanes is not completely understood, the
observed regio- and streochemistry of the HfCl4-catalyzed
allylsilylation of alkynes is reasonably explained by the proposed
mechanism (Scheme 2).
(Z)-2-Benzyl-1-(trimethylsilyl)-1,4-pentadiene (3c): 1H NMR (CDCl3,
270 MHz) δ 7.32-7.16 (m, 5H), 5.76 (ddt, J ) 7.0, 10.3, 16.8 Hz,
1H), 5.46 (s, 1H), 5.04-4.89 (m, 2H), 3.51 (s, 2H), 2.66 (dd, J ) 1.1,
7.0 Hz, 2H), 0.16 (s, 9H). 13C NMR (67.9 MHz, CDCl3) δ 155.1 (C),
136.5, 128.7 (CH), 139.5, 128.3, 126.8, 126.1 (Ar), 116.3 (CH2d), 43.0,
41.6 (CH2), 0.4 (Si-CH3). IR (neat) 3028, 2953, 1639, 1614, 1495,
1248, 839 cm-1. MS (EI) m/z 230 (M+, 5), 215 (M+ - CH3, 33), 73
(100). HRMS calcd for C15H22Si 230.1576, found 230.1534.
(E)-2-Hexyl-1-(trimethylsilyl)-1,4-pentadiene (3d): 1H NMR (CDCl3,
270 MHz) δ 5.88-5.73 (m, 1H), 5.19 (d, J ) 1.1 Hz, 1H), 5.05-5.00
(m, 1H), 2.18 (dd, J ) 1.1, 7.0 Hz, 2H), 2.14-2.08 (m, 2H), 1.38-
1.29 (m, 8H), 0.89 (t, J ) 7.3 Hz, 3H), 0.09 (s, 9H). 13C NMR (67.9
MHz, CDCl3) δ 158.0 (C), 136.9, 124.3 (CH), 115.9 (CH2d), 43.5,
36.3, 31.6, 29.6, 29.1, 22.7 (CH2), 14.1 (CH3), 0.4 (Si-CH3). IR (neat)
3078, 2957, 2928, 2858, 1637, 1610, 1431, 1248, 837 cm-1. MS (EI)
m/z 224 (M+, 2), 209 (M+ - CH3, 7), 73 (100). HRMS calcd for
C14H28Si 224.1961, found 224.1976.
(E)-2-Decyl-1-(trimethylsilyl)-1,4-pentadiene (3e): 1H NMR (CDCl3,
270 MHz) δ 5.81 (ddt, J ) 7.0, 9.2, 18.0 Hz, 1H), 5.20 (t, J ) 1.1 Hz,
1H), 5.05-4.98 (m, 2H), 2.81 (ddd, J ) 1.1, 2.6, 7.0 Hz, 2H), 2.13-
2.08 (m, 2H), 1.39-1.27 (m, 16H), 0.88 (t, J ) 7.0 Hz, 3H), 0.09 (s,
9H). 13C NMR (67.9 MHz, CDCl3) δ 158.0 (C), 136.9, 124.3 (CH),
115.9 (CH2d), 43.5, 36.2, 31.9, 29.9, 29.6, 29.3, 29.1, 22.7 (CH2), 14.1
(CH3), 0.4 (Si-CH3). IR (neat) 3078, 2926, 2855, 1637, 1610, 1466,
1247, 837 cm-1. MS (EI) m/z 280 (M+, 3), 265 (M+ - CH3, 6), 73
(100). HRMS calcd for C18H36Si 280.2586, found 280.2593.
Conclusion
We are now in position to effectively prepare (regio- and
stereoselectively in high to excellent yields) various types of
silylated 1,4-dienes Via the Lewis acid catalyzed allylsilylation
of unactivated alkynes. The resulting silylated 1,4-dienes are
not easily available Via the previously known methodologies
and may be useful as building blocks in organic synthesis.
Formation of carbocation intermediates such as 10 (and
aluminum analogue) followed by allyl transfer from the side
opposite Hf (or Al) can explain the regio- and stereoselectivity
of the Lewis acid catalyzed allylsilylation reaction.
Experimental Section
General Information. 1H and 13C NMR spectra were recorded on
a JEOL JNM-GSX-270 (270 MHz) instrument. IR spectra were
recorded on a Shimadzu FTIR-8200A spectrometer. High-resolution
mass spectra were recorded on a Hitachi M-2500S spectrometer.
Capillary GLC analysis was performed on a SHIMADZU GC-14A
(CPB20-M25-025 column). Column chromatography was carried out
employing Merck silica gel (Kieselgel 70-230 mesh), and analytical
thin layer chromatography (TLC) was performed on 0.2-mm precoated
silica gel plates (Kieselgel 60 F254). All manipulations were conducted
under an argon atmosphere following standard Schlenk techniques.
Anhydrous solvents were purchased from Kanto Chemicals. The
following allylsilanes were prepared according to known procedures:
(E)-, and (Z)-crotyltrimethylsilane (b, c),17 (2-methylallyl)trimethylsilane
(2d),18 (3,3-dimethylallyl)trimethylsilane (2e),19 allyltriethylsilane (2f),10a
allyldimethylphenylsilane (2g),10a allylmethyldiphenylsilane (2h),10a and
allyldimethyl(tert-butyl)silane (2i).10a All other compounds used were
commercially available and purchased from Aldrich.
EtAlCl2-TMS-Catalyzed Preparation of 3 (General Procedure).
EtAlCl2 (0.52 mL, 0.96 M in hexane, 0.5 mmol) was added to a mixture
of allyltrimethylsilane 2 (0.19 mL, 1.2 mmol) and TMSCl (2.54 mL,
20 mmol) at -47 °C followed by addition of alkyne 1 (1.0 mmol).
The reaction mixture was stirred for 2 h at -47 °C, quenched with
diethylamine (2.0 mL, 19.4 mmol), and then allowed to warm to 0 °C.
NaHCO3(aq) was added and the mixture was extracted (Et2O-H2O),
dried (Na2SO4), and concentrated. Product 3 was purified by column
chromatography (silica gel).
(Z)-2-(1-Cyclohexenyl)-1-(trimethylsilyl)-1,4-pentadiene (3f): 1H
NMR (CDCl3, 270 MHz) δ 5.78 (ddt, J ) 7.0, 9.2, 17.8 Hz, 1H), 5.47
(quint, J ) 1.8 Hz, 1H), 5.19 (s, 1H), 5.01-4.97 (m, 2H), 2.88 (ddd,
J ) 1.5, 2.6, 7.0 Hz, 2H), 2.07-1.98 (m, 4H), 1.68-1.56 (m, 4H),
0.04 (s, 9H). 13C NMR (67.9 MHz, CDCl3) δ 160.7, 140.5 (C), 136.6,
124.8, 124.3 (CH), 115.6 (CH2d), 43.2, 27.0, 25.0, 22.6, 22.0 (CH2),
0.5 (Si-CH3). IR (neat) 2929, 1645, 1597, 1442, 1244, 916, 840 cm-1
.
MS (EI) m/z 220 (M+, 1), 205 (M+ - CH3, 20), 73 (100). HRMS
calcd for C14H24Si 220.1648, found 220.1682.
(Z)-3-Phenyl-2-(trimethylsilyl)-2,5-hexadiene (3g): 1H NMR (CDCl3,
400 MHz) δ 7.27-7.21 (m, 3H), 7.09-7.07 (m, 2H), 5.72 (ddt, J )
6.4, 10.0, 17.0 Hz, 1H), 5.00-4.93 (m, 2H), 3.17 (d, J ) 6.4 Hz, 2H),
1.83 (s, 3H), -0.25 (s, 9H). 13C NMR (67.9 MHz, CDCl3) δ 148.6,
145.7, 134.8, 133.6, 128.9, 127.6, 126.5, 115.0, 40.4, 17.8, 0.1. IR
(neat) 3076, 2952, 2895, 1952, 1891, 1815, 1637, 1593, 1247, 912,
835, 762, 702 cm-1. MS (EI) m/z 230 (M+, 29), 215 (M+ - CH3, 26),
73 (100). Anal. Calcd for C15H22Si: C, 78.19; H, 9.62. Found: C,
77.910; H, 9.374.
HfCl4-Catalyzed Preparation of 3 (General Procedure). Allyl-
trimethylsilane 2 (0.16 mL, 1.0 mmol) was added to a suspension of
HfCl4 (80 mg, 0.25 mmol) in dichloromethane (1 mL) at 0 °C followed
by addition of alkyne 1 (0.5 mmol). The reaction temparature was
kept at 0 °C and monitored by GLC analysis. After completion of the
reaction diethylamine (0.5 mL) was added followed by addition of
NaHCO3(aq). Then mixture was extracted (Et2O-H2O), dried (Na2-
SO4), and concentrated. Product 3 was purified by column chroma-
tography (silica gel).
(Z)-4-Phenyl-3-(trimethylsilyl)-3,6-heptadiene (3h): 1H NMR
(CDCl3, 270 MHz) δ 7.29-7.24 (m, 3H), 7.12-7.08 (m, 2H), 5.73
(ddt, J ) 6.6, 10.3, 16.9 Hz, 1H), 5.01-4.93 (m, 2H), 3.20 (dt, J )
6.6, 1.5 Hz, 2H), 2.33 (q, J ) 7.5 Hz, 2H), 1.08 (t, J ) 7.5 Hz, 3H),
-0.21 (s, 9H). 13C NMR (67.9 MHz, CDCl3) δ 149.1, 145.2, 139.9,
135.7, 129.2, 127.5, 126.5, 115.3, 40.0, 24.9, 15.2, 0.6. IR (neat) 2957,
2898, 1943, 1886, 1745, 1590, 1485, 1443, 1249, 836 cm-1. MS (EI)
m/z 244 (M+, 13), 229 (M+ - CH3, 12), 73 (100). HRMS calcd for
C16H24Si 244.1646, found 244.1641.
(Z)-2-Phenyl-1-(trimethylsilyl)-1,4-pentadiene (3a): 1H NMR
(CDCl3, 270 MHz) δ 7.30-7.25 (m, 3H), 7.17-7.13 (m, 2H), 5.83
(ddt, J ) 7.0, 9.2, 17.9 Hz, 1H), 5.59 (t, J ) 1.5 Hz, 1H), 5.02-4.98
1
(Z)-1,1-Bis(trimethylsilyl)-1,4-pentadiene (3i): H NMR (CDCl3,
(17) Tzeng, D.; Weber, W. P. J. Org. Chem. 1981, 46, 265.
(18) Yanami, T.; Akira, M. M.; Koshi, Y. J. Org. Chem. 1980, 45, 607.
(19) Hosomi, A.; Sakurai, H. Tetrahedron Lett. 1978, 2589.
270 MHz) δ 6.57 (t, J ) 7.0 Hz, 1H), 5.84 (ddt, J ) 6.6, 10.6, 16.5
Hz, 1H), 5.08-5.01 (m, 2H), 3.03-2.69 (m, 2H), 0.17 (s, 9H), 0.09

6786 J. Am. Chem. Soc., Vol. 119, No. 29, 1997
Yoshikawa et al.
(s, 9H). 13C NMR (67.9 MHz, CDCl3) δ 154.1, 141.8, 136.4, 115.4,
39.8, 1.8, 0.3. IR (neat) 2955, 1562, 1249, 991, 945, 914, 879, 839,
761, 684, 617 cm-1. MS (EI) m/z 244 (M+, 13), 229 (M+ - CH3, 12),
73 (100). HRMS calcd for C11H24Si2 212.1415, found 212.1432.
(Z)-2-(tert-Butyl)-1-(trimethylsilyl)-1,4-pentadiene (3j): 1H NMR
(CDCl3, 270 MHz) δ 5.80 (ddt, J ) 7.0, 10.3, 16.9 Hz, 1H), 5.24 (s,
1H), 5.06-4.96 (m, 2H), 2.89 (ddd, J ) 1.1, 2.6, 7.0 Hz, 2H), 1.13 (s,
9H), 0.15 (s, 9H). 13C NMR (67.9 MHz, CDCl3) δ 165.7, 138.5, 123.8,
115.6, 41.8, 38.0, 30.4, 2.2. IR (neat) 2954, 1440, 1388, 1251, 843
cm-1. MS (EI) m/z 196 (M+, 1), 181 (M+ - CH3, 8), 73 (100). HRMS
calcd for C12H24Si 196.1645, found 196.1633.
NMR (67.9 MHz, CDCl3) δ 165.1, 147.2, 142.2, 130.0, 126.9, 126.4,
125.8, 111.0, 44.4, 26.7, -0.2. IR (neat) 2964, 1585, 1245, 867, 837
cm-1. MS (EI) m/z 244 (M+, 15), 229 (M+ - CH3, 8), 73 (100). Anal.
Calcd for C16H24Si: C, 78.61; H, 9.90. Found: C, 78.729; H, 9.982.
(Z)-2-Phenyl-1-triethylsilyl-1,4-pentadiene (3n): 1H NMR (CDCl3,
270 MHz) δ 7.28-7.24 (m, 3H), 7.17-7.14 (m, 2H), 5.83 (ddt, J )
17.6, 9.5, 7.0 Hz, 1H), 5.53 (d, J ) 1.3 Hz, 1H), 5.06-4.98 (m, 2H),
3.17 (ddd, J ) 7.0, 1.3, 1.3 Hz 2H), 0.81 (t, J ) 7.9 Hz, 6H), 0.30 (q,
J ) 7.9 Hz, 9H). 13C NMR (67.9 MHz, CDCl3) δ 158.1, 144.3, 136.1,
127.8, 127.6, 127.0, 124.7, 116.3, 47.1, 7.5, 4.5. IR (neat) 2952, 2873,
1595, 1491, 1458, 1236, 737, 700 cm-1. MS (EI) m/z 258 (M+, 2),
229 (M+ - C2H5, 100). Anal. Calcd for C17H26Si: C, 78.99; H, 10.14.
Found: C, 77.155; H, 10.329.
(Z)-2-Phenyl-1-(dimethylphenylsilyl)-1,4-pentadiene (3o): 1H NMR
(CDCl3, 270 MHz) δ 7.45-7.42 (m, 2H), 7.33-7.30 (m, 3H), 7.23-
7.20 (m, 3H), 7.11-7.07 (m, 2H), 5.92-5.79 (m, 1H), 5.77 (t, J ) 1.5
Hz, 1H), 5.07-5.00 (m, 2H), 3.18 (ddd, J ) 7.0, 2.6, 1.1 2H), 0.00 (s,
6H). 13C NMR (67.9 MHz, CDCl3) δ 158.8, 143.6, 140.3, 135.7, 133.6,
128.5, 127.9, 127.6, 127.0, 125.7, 116.6, 46.8, -1.3. IR (neat) 3068,
2954, 1953, 1887, 1826, 1593, 1427, 1247, 1112, 837, 700 cm-1. MS
(EI) m/z 278 (M+, 77), 263 (M+ - CH3, 27), 135 (100). Anal. Calcd
for C19H22Si: C, 81.95; H, 7.96. Found: C, 81.945; H, 8.063.
(Z)-3-Methyl-2-phenyl-1-(trimethylsilyl)-1,4-pentadiene (3k): 1H
NMR (CDCl3, 270 MHz) δ 7.30-7.24 (m, 3H), 7.11-7.07 (m, 2H),
5.85 (ddd, J ) 16.3, 11.2, 7.0 Hz, 1H), 5.58 (d, J ) 1.1 Hz, 1H),
5.00-4.93 (m, 2H), 3.14 (quint, 1H), 1.11 (d, J ) 7.0 Hz, 2H), -0.22
(s, 9H). 13C NMR (67.9 MHz, CDCl3) δ 143.5, 142.1, 128.6, 127.4,
126.7, 126.0, 113.5, 48.2, 18.8, 0.0. IR (neat) 2964, 2896, 1637, 1608,
1593, 1247, 914, 871, 702 cm-1. MS (EI) m/z 230 (M+, 3), 215 (M+
- CH3, 7), 73 (100). HRMS calcd for C15H22Si 230.1489, found
230.1480. Anal. Calcd for C15H22Si: C, 78.19; H, 9.62. Found: C,
77.961; H, 9.776.
1
(Z)-4-Methyl-2-phenyl-1-(trimethylsilyl)-1,4-pentadiene (3l): H
NMR (CDCl3, 270 MHz) δ 7.28-7.26 (m, 2H), 7.17-7.13 (m, 3H),
5.62 (t, J ) 1.3 Hz, 1H), 4.77 (m, 1H), 4.65 (m, 1H), 3.12 (s, 1H),
1.71 (s, 3H), -0.17 (s, 9H). 13C NMR (67.9 MHz, CDCl3) δ 156.4,
143.9, 142.9, 129.3, 127.9, 127.6, 126.8, 112.9, 51.4, 22.1, 0.1. IR
(neat) 2925, 2854, 1639, 1608, 1465, 1247, 891, 837 cm-1. MS (EI)
m/z 230 (M+, 3), 215 (M+ - CH3, 7), 73 (100). HRMS calcd for
C15H22Si 230.1490, found 230.1496. Anal. Calcd for C15H22Si: C,
78.19; H, 9.62. Found: C, 78.054; H, 9.849.
(Z)-2-Phenyl-1-(diphenylmethylsilyl)-1,4-pentadiene (3p): 1H NMR
(CDCl3, 270 MHz) δ 7.47-7.41 (m, 4H), 7.34-7.27 (m, 6H), 7.17-
7.12 (m, 3H), 7.05-7.02 (m, 2H), 6.02 (t, J ) 1.3 Hz, 1H), 5.88 (ddt,
J ) 17.6, 9.5, 7.0 Hz, 1H), 5.11-5.03 (m, 2H), 3.25 (ddd, J ) 7.0,
2.6, 1.3 2H), 0.06 (s, 3H). 13C NMR (67.9 MHz, CDCl3) δ 160.3,
143.2, 138.3, 135.6, 134.5, 128.8, 127.8, 127.6, 127.1, 123.7, 123.6,
116.8, 46.9, -3.3. IR (neat) 3067, 3049, 3020, 2178, 1950, 1886, 1824,
1593, 1489, 1427, 1109, 793, 729, 700 cm-1. MS (EI) m/z 340 (M+,
69), 325 (M+ - CH3, 12), 197 (100). Anal. Calcd for C24H24Si: C,
84.65; H, 7.10. Found: C, 84.950; H, 7.269.
(Z)-3,3-Dimethyl-2-phenyl-1-(trimethylsilyl)-1,4-pentadiene (3m):
1H NMR (CDCl3, 270 MHz) δ 7.30-7.26 (m, 3H), 7.07-7.04 (m,
2H), 5.96 (dd, J ) 17.6, 10.6 Hz, 1H), 5.02 (dd, J ) 10.6, 1.5 Hz,
1H), 4.98 (dd, J ) 17.6, 1.5 Hz, 1H), 1.16 (s, 6H), -0.25 (s, 9H). 13
C
JA970443B