gem-Silylborylation of an sp carbon: Novel synthesis of 1-boryl-1-silylallenes

Article abstract of DOI:10.1021/ol027367o

(Matrix presented) Novel synthesis of 1-boryl-1-silylallenes involving gem-silylborylation of 3-chloro- or 3-alkoxyalkyn-1-yllithiums with (dimethylphenylsilyl)-(pinacolato)borane has been established. The reaction proceeds via 1,2-migration of the silyl

Full text of DOI:10.1021/ol027367o

ORGANIC
LETTERS
2003
Vol. 5, No. 2
225-227
gem-Silylborylation of an sp Carbon:
Novel Synthesis of 1-Boryl-1-silylallenes
Masaki Shimizu,* Takuya Kurahashi, Hirotaka Kitagawa, and Tamejiro Hiyama
Department of Material Chemistry, Graduate School of Engineering, Kyoto UniVersity,
Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
shimizu@npc05.kuic.kyoto-u.ac.jp
Received November 27, 2002
ABSTRACT
Novel synthesis of 1-boryl-1-silylallenes involving gem-silylborylation of 3-chloro- or 3-alkoxyalkyn-1-yllithiums with (dimethylphenylsilyl)-
(pinacolato)borane has been established. The reaction proceeds via 1,2-migration of the silyl group from the negatively charged boron atom
of an intermediary borate complex to the terminal acetylenic carbon and is accelerated by the addition of chlorotrimethylsilane in the case that
methanesulfonyloxy is employed as a leaving group. Furthermore, axially enantioenriched products could be prepared from mesylates of
optically active propargylic alcohols.
Since gem-diorganometallics are versatile reagents in organic
synthesis,1 facile synthesis of those dimetalated compounds
is of great significance. Especially, reactions in which two
metals are introduced simultaneously into an organic mol-
ecule by utilizing an interelement compound are straight-
forward and highly efficient for the purpose. From this point
of view, we have developed a novel and efficient method
for the preparation of 1-boryl-1-silyl-1-alkenes and 1-boryl-
1-silyl-2-alkenes via gem-silylborylation of alkylidene-type
carbenoids and R-chloroallyllithiums with silylboranes,
respectively.2 These reactions proceed through formation of
an ate complex from a lithium carbenoid and a silylborane,
followed by 1,2-migration of the silyl group from a nega-
tively charged boron to the carbenoid carbon. To further
extend the scope of gem-silylborylation utilizing silylboranes,
we turned our attention to gem-silylborylation of an sp carbon
in terminal acetylenes, leading to allenyl organodimetallics.3
We report herein that treatment of 3-chloro- or 3-alkoxy-1-
alkyne 1 (X ) Cl or OR′) with a base generates the
corresponding alkynyllithium 2, which reacts with (dimeth-
ylphenylsilyl)(pinacolato)borane (3) to produce 1-boryl-1-
silylallenes 5 in moderate to good yields (Scheme 1).4,5 This
method allows us to prepare enantioenriched allenes 5 by
using optically active 3-mesyloxy-1-alkynes 1 (X ) OMs).
(1) Reviews on gem-diorganometallics: (a) Marek, I.; Normant, J.-F.
Chem. ReV. 1996, 96, 3241-3267. (b) Marek, I. Chem. ReV. 2000, 100,
2887-2900. (c) Matsubara, S.; Oshima, K.; Utimoto, K. J. Organomet.
Chem. 2001, 617-618, 39. (d) Marek, I.; Normant, J. F. In Organozinc
Reagents-A Practical Approach; Knochel, P., Jones, P., Eds.; Oxford
University Press: Oxford, 1999; pp 119-137. (e) Normant, J. F. Acc. Chem.
Res. 2001, 34, 640-644.
(2) (a) Hata, T.; Kitagawa, H.; Masai, H.; Kurahashi, T.; Shimizu, M.;
Hiyama, T. Angew. Chem., Int. Ed. 2001, 40, 790-792. (b) Shimizu, M.;
Kitagawa, H.; Kurahashi, T.; Hiyama, T. Angew. Chem., Int. Ed. 2001, 40,
4283-4286. (c) Kurahashi, T.; Kitagawa, H.; Masai, H.; Hata, T.; Shimizu,
M.; Hiyama, T. Tetrahedron 2002, 58, 6381-6395. See also: (d) Shimizu,
M.; Kurahashi, T.; Hiyama, T. Yuki Gosei Kagaku Kyokai Shi 2001, 59,
1062-1069. (e) Shimizu, M.; Kurahashi, T.; Hiyama, T. Synlett 2001,
1006-1008.
(3) Reviews of allenyl organometallics: (a) Yamamoto, H. In Compre-
hensiVe Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon
Press: Oxford, 1991; Vol. 2, pp 81-98. (b) Masse, C. E.; Panek, J. S.
Chem. ReV. 1995, 95, 1293-1316. (c) Marshall, J. A. Chem. ReV. 1996,
96, 31-47. (d) Marshall, J. A. Chem. ReV. 2000, 100, 3163-3185.
(4) Preparations and reactions of 1-boryl-1-silylallenes: (a) Haruta, R.;
Ishiguro, M.; Furuta, K.; Mori, A.; Ikeda, N.; Yamamoto, H. Chem. Lett.
1982, 1093-1096. (b) Wang, K. K.; Nikam, S. S.; Ho, C. D. J. Org. Chem.
1983, 48, 5376-5377. (c) Yamamoto, Y.; Ito, W.; Maruyama, K. J. Chem.
Soc., Chem. Commun. 1984, 1004-1005. (d) Furuta, K.; Ishiguro, M.;
Haruta, R.; Ikeda, N.; Yamamoto, H. Bull. Chem. Soc. Jpn. 1984, 57, 2768-
2776. (e) Nikam, S. S.; Wang, K. K. J. Org. Chem. 1985, 50, 2193-2195.
(f) Yamamoto, Y.; Maruyama, K.; Komatsu, T.; Ito, W. J. Org. Chem.
1986, 51, 886-891. (g) Wang, K. K.; Wang, Z.; Gu, Y. G. Tetrahedron
Lett. 1993, 34, 8391-8394.
10.1021/ol027367o CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/28/2002

Table 1. gem-Silylborylation of 1a
Scheme 1. Novel Synthesis of 1-Boryl-1-silylallenes via
gem-Silylborylation of an Acetylenic Carbon
Treatment of 1 with BuLi in THF at -110 °C was
followed by the addition of 3 at -110 °C, while 1 was
deprotonated in the presence of 3 at -110 °C when lithium
diisopropylamide (LDA) was employed as a base (Scheme
2).6 In both cases, the resulting solution was allowed to warm
Scheme 2. Reaction Procedure for the Preparation of 5
a To a solution of 1 (0.50 mmol) in THF (3 mL) was added BuLi (0.50
mmol) at -110 °C. After stirring for 2 min, the reaction mixture was treated
with 3 (0.50 mmol) and allowed to warm to room temperature before
quenching with saturated aqueous NH4Cl (1 mL). b Isolated yields after
column chromatography on silica gel. c Purified by GPC. d Pent ) n-C5H11
e Chlorotrimethylsilane (0.55 mmol) was added to the reaction mixture at
-110 °C after the addition of 3.
to room temperature before quenching with saturated aqueous
NH4Cl solution.
Workup and purification by column chromatography on
silica gel afforded 5 in moderate to good yields.7 The results
are summarized in Table 1. Alkynyllithium 2a prepared from
3-chloro-3-methyl-1-butyne (1a) with BuLi reacted with 3
to give 5a in 70% yield (entry 1). The yield of 5a was slightly
increased by changing the leaving group from chlorine to
OAc (entry 2). Deprotonation of 1c with BuLi and subse-
quent treatment with 3 gave allenylidenecyclohexane 5c in
53% yield (entry 3). Silylborylation of racemic R-monosub-
stituted propargylic chlorides 1d-f using BuLi gave 3-sub-
stituted allenes 5d-f in moderate to good yields (entries 5,
7, and 9). In the case of 1c-e, better yields were obtained
when LDA was used instead of BuLi (entries 4, 6, and 8).
Additionally, a mesyloxy group was found to be a good
leaving group for the present transformation. gem-Silyl-
borylation of (()-1g proceeded via deprotonation with BuLi
or LDA, giving rise to (()-5e in 51% or 57% yield,
respectively (entries 10 and 11). Noteworthy is that the
addition of chlorotrimethylsilane to the reaction mixture
starting from 1g accelerated the gem-silylborylation and
enhanced the yield of 5f to 75% (entry 12). In this case, the
reaction went to completion below -78 °C, whereas without
chlorotrimethylsilane it was essential to warm to room
temperature to consume 3. Chlorotrimethylsilane is consid-
ered to play the role of a Lewis acid for promoting
elimination of a mesyloxy group (Figure 1). In all cases, no
isomerization of 5 to propargylboranes was observed.8
(5) Pioneering studies on the preparation of allenylboranes from prop-
argylic chlorides or acetates and organoboranes: (a) Leung, T.; Zweifel,
G. J. Am. Chem. Soc. 1974, 96, 5620-5621. (b) Midland, M. M. J. Org.
Chem. 1977, 42, 2650-2651. Preparation of allenylzinc reagents via 1,2-
migration of a carbonaceous substituent in alkynylzincates was reported.
(c) Harada, T.; Katsuhira, T.; Osada, A.; Iwazaki, K.; Maejima, K.; Oku,
A. J. Am. Chem. Soc. 1996, 118, 11377-11390.
(6) Treatment of 1 with LDA followed by the addition of 3 resulted in
no production of 5 at all; the reason is not clear at present.
(7) Alkylidene-type carbenoids reacted smoothly with in situ generated
(trimethylsilyl)(pinacolato)borane to give the corresponding 1-boryl-1-
trimethylsilyl-1-alkenes in good yields (ref 2c), whereas silylborylation of
2 with the trimethylsilylborane failed.
Figure 1. Acceleration of gem-silylborylation by Me3SiCl.
Org. Lett., Vol. 5, No. 2, 2003
226

To explore the possibility of asymmetric synthesis of
1-boryl-1-silylallenes, we next carried out the gem-silyl-
borylation of optically active mesylates (S)-1g and (S)-1h.9
After several experiments,10 we found that treatment of (S)-
1g or (S)-1h with LDA in THF at -110 °C in the presence
of 3 followed by addition of chlorotrimethylsilane gave (R)-
5e or (R)-5f in 75% or 67% yield with >74% ee (vide infra)
or 70% ee (Scheme 3).11 The fact that central S chirality
Absolute configurations of 5e and 5f were deduced by
chemical transformation to known alcohols 7e and 7f
(Scheme 4). Thus, 5e or 5f reacted with cyclohexane-
Scheme 4. Stereochemical Assignment of (R)-5e and -5fa,b
Scheme 3. Preparation of Axially Enantioenriched
gem-Silylborylallenes 5
a A toluene solution (2 mL) of 5 (0.30 mmol) and cyclohexan-
ecarbaldehyde (0.45 mmol) was stirred at -20 °C for 24 h before
quenching with water (10 mL). bTo a solution of 6 (0.25 mmol) in
THF (2 mL) was added a 1 M THF solution of Bu4NF (0.25 mmol).
The resulting solution was stirred at 0 °C for 6 h. cDetermined by
GC analysis using Chiral-DEX CB column.
a Estimated after transformation of 5e into known compound 7e
(vide infra). bDetermined by HPLC analysis using Daicel AD
column.
carbaldehyde in toluene at -20 °C to yield with high anti
diastereoselectivity 6e or 6f, which was desilylated to give
7e or 7f, respectively. Since the negative sign of specific
rotations (-1.78 (c 0.93, CHCl3) for 7e; -3.21 (c 1.37,
CHCl3) for 7f) is attributed to the (1R,2S)-enantiomer,
absolute configurations of 5e and 5f are assigned R.12 No
loss of optical purity in the reaction of (R)-5e indicates that
this propargylation is perfectly stereospecific. Hence, the ee
of 5e was estimated to be >74% as shown in Scheme 3.
In summary, we have demonstrated that 1-boryl-1-silyl-
allenes can be efficiently synthesized from readily available
3-chloro- or 3-alkoxyalkyn-1-yllithiums with silylborane in
moderate to good yields. In addition, the present method is
applicable to the preparation of enantioenriched 1-boryl-1-
silylallenes. Studies on synthetic application of the gem-
dimetalated allenes are in progress.
transferred into axial R chirality can be reasonably explained
by assuming that 1,2-migration of the silyl group with
elimination of a mesyloxy group proceeds in anti SN2′ fashion
as exemplified in Figure 1. These results are the first
demonstration of asymmetric gem-silylborylation of orga-
nolithium reagents.
(8) Zweifel, G.; Backlund, S. J.; Leung, T. J. Am. Chem. Soc. 1978,
100, 5561-5562.
(9) Examples of axially optically active allenylmetallic compounds.
Allenylboranes: (a) Matsumoto, Y.; Naito, M.; Uozumi, Y.; Hayashi, T. J.
Chem. Soc., Chem. Commun. 1993, 1468-1469. Allenylsilanes: (b) Buckle,
M. J. C.; Fleming, I. Tetrahedron Lett. 1993, 34, 2383-2386. (c) Marshall,
J. A.; Maxson, K. J. Org. Chem. 2000, 65, 630-633. (d) Shepard, M. S.;
Carreira, E. M. J. Am. Chem. Soc. 1997, 119, 2597-2605. (e) Marshall, J.
A.; Adams, N. D. J. Org. Chem. 1997, 62, 8976-8977. (f) Han, J. W.;
Tokunaga, N.; Hayashi, T. J. Am. Chem. Soc. 2001, 123, 12915-12916.
R-Silylallenyltitaniums: (g) Okamoto, S.; An, D. K.; Sato, F. Tetrahedron
Lett. 1998, 39, 4551-4554. (h) Song, Y.; Okamoto, S.; Sato, F. Org. Lett.
2001, 3, 3543-3545. Allenylzincs: (i) Marshall, J. A.; Adams, N. D. J.
Org. Chem. 1999, 64, 5201-5204. Preparation of enantioenriched alle-
nylzinc reagents by kinetic resolution of the racemates is reported. (j)
Poisson, J.-F.; Normant, J. F. J. Am. Chem. Soc. 2001, 123, 4639-4640.
Allenylindiums, -bismuths, and -tins: (k) Marshall, J. A.; Grant, C. M. J.
Org. Chem. 1999, 64, 8214-8219.
Acknowledgment. This work was supported by Grants-
in-Aid for COE Research on Elements Science, no. 12CE2005,
and for Young Scientists (B), no. 14750678, from Ministry
of Education, Culture, Sports, Science, and Technology,
Japan. Japan Society for the Promotion of Science (JSPS) is
highly acknowledged for the financial support to T.K. as a
predoctoral research student.
(10) Reaction of (S)-1g and 3 with Me3SiCl in Et2O resulted in the
production of (R)-5e in 64% yield with 60% ee, and (R)-5e formed in THF
without the addition of Me3SiCl in 51% yield with 70% ee.
(11) With monitoring of the specific rotation of 5e in solution such as
THF, toluene, or CH2Cl2 with time, no racemization of 5e was confirmed.
(12) Propargylation of aldehydes with allenylboranes is assumed to
proceed via six-membered cyclic transition states. For example: Haruta,
R.; Ishiguro, M.; Ikeda, N.; Yamamoto, H. J. Am. Chem. Soc. 1982, 104,
7667-7669.
Supporting Information Available: Detailed descrip-
tions of experimental procedures and spectral data for new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
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