β-borylallylsilanes as a new tool for convenient synthesis of alkenylboranes [1]

Full text of DOI:10.1021/ja0058381

J. Am. Chem. Soc. 2001, 123, 4601-4602
4601
Communications to the Editor
Table 1. Lewis Acid-Promoted Reactions of â-Borylallylsilanes
with Acetalsa
â-Borylallylsilanes as a New Tool for Convenient
Synthesis of Alkenylboranes
Michinori Suginome,* Yutaka Ohmori, and Yoshihiko Ito*
Department of Synthetic Chemistry and
Biological Chemistry, Graduate School of Engineering
Kyoto UniVersity, Kyoto 606-8501, Japan
ReceiVed NoVember 30, 2000
Allylsilane is one of the most useful building blocks for
nucleophilic allylation in organic synthesis.1 The usefulness of
the allylsilanes, which are synthetically accessible with regio- and
stereo-defined forms, arises from high regio- and stereoselectivi-
ties in the allylation reactions. Furthermore, stability of the
allylsilanes, which is due to the covalent character of their
silicon-carbon bond, may enable selective carbon-carbon bond-
forming reactions with functional groups, which are not tolerable
by means of anionic allylmetal reagents.
In our recent exploitation of the synthetic application of
silylboranes,2,3 we found that palladium-catalyzed silaboration of
allenes proceeded in good yields to give â-borylallylsilanes 1 (eq
1).4,5 Notably, the addition took place at the more substituted CdC
a Allylsilane 1, acetal 2 (1.2 equiv), and Lewis acid (1.2 equiv) were
reacted in CH2Cl2 at -78 °C for 3 h unless otherwise noted. b Isolated
yield. c The reaction was conducted at -78 °C for 0.5 h and -20 °C
for 2.5 h. d 1.5 equiv of 2c and AlCl3 were used.
was attained with AlCl3, although higher temperature (-20 °C)
was required.8 Under the reaction conditions using TiCl4, reactions
of 1a with an acetal 2b derived from R-branched aldehyde (entry
4) similarly gave the corresponding homoallyl ether 3ab in an
almost quantitative yield. In the reaction with 2-methoxytetra-
hydropyran (2c), AlCl3 provided 2-substituted THP 3ac in slightly
higher yield than TiCl4 (entry 5). Reaction of benzaldehyde
dimethyl acetal (2d) with 1a may deserve some comments. Use
of TiCl4 as a Lewis acid at -78 °C resulted in facile allylation
followed by chloro-demethoxylation to give a homoallyl chloride
4ad in 69% yield without formation of the expected homoallyl
ether 3ad (eq 3). However, the chloro-demethoxylation was
bond of terminal allenes in a highly regioselective manner with
exclusive B-C bond formation at the central sp carbon of the
allene. With these new organometallic compounds in our hands,
our interest has been focused on its synthetic utilization.6 Herein,
we disclose new organometallic synthons (1), which promotes
nucleophilic allylation in the presence of Lewis acid to lead to
the formation of functionalized alkenylboranes,7 including cyclic
ones, whose synthesis is not trivial.
Initially, we examined the reactions of simple â-borylallyl-
silanes 1a-c with acetals in the presence of Lewis acids (eq 2).
These allylsilanes were readily prepared by silaboration of the
corresponding allenes in good yield with high regioselectivity.4
It was our pleasure to find that the reaction of 1a with
propionaldehyde diethyl acetal (2a) proceeded in the presence of
TMSOTf, AlCl3, and TiCl4 (1.2 equiv each) to give boryl-
substituted homoallyl ether 3aa (Table 1, entries 1-3). In
particular, TiCl4 exhibited the highest activity for the reaction to
give 3aa in high yield at -78 °C for 3 h. The comparable yield
almost completely suppressed by alternative use of AlCl3 at -78
°C, affording homoallylic ether 3ad in good yield (entry 6). In
the presence of TiCl4, unsubstituted and cyclohexyl-substituted
â-borylallylsilanes 1b and 1c also reacted with acetal 2a, giving
alkenylboranes 3 in high yields (entries 7 and 8). It is noteworthy
that, in all the reactions, only (E)-alkenes 3 were obtained without
being accompanied by any possible (Z)-isomers.
Reaction of 1a with propionaldehyde also proceeded in the
presence of TiCl4 at -78 °C to form boryl-substituted homoallylic
alcohol 5 selectively (eq 4). Attempts at isolation of 5 by column
(1) (a) Fleming, I.; Dunogues, J.; Smithers, R. Org. React. 1989, 37, 57.
(b) Panek, J. S. In ComprehensiVe Organic Synthesis; Trost, B. M., Ed.;
Pergamon: Oxford, 1991; Vol. 1, p 579.
(2) (a) Suginome, M.; Ito, Y. Chem. ReV. 2000, 100, 3221. (b) Suginome,
M.; Matsuda, T.; Ito, Y. J. Am. Chem. Soc. 2000, 122, 11015. (c) Suginome,
M.; Fukuda, T.; Ito, Y. Organometallics 2000, 19, 719. (d) Suginome, M.;
Nakamura, H.; Matsuda, T.; Ito, Y. J. Am. Chem. Soc. 1998, 120, 4248.
(3) For the convenient preparation of silylboranes, see: Suginome, M.;
Matsuda, T.; Ito, Y. Organometallics 2000, 19, 4647.
(4) (a) Suginome, M.; Ohmori, Y.; Ito, Y. Synlett 1999, 1567. (b) Suginome,
M.; Ohmori, Y.; Ito, Y. J. Organomet. Chem. 2000, 611, 403.
(5) For closely related synthesis of â-borylallylsilanes, see: Onozawa, S.-
y.; Hatanaka, Y.; Tanaka, M. Chem. Commun. 1999, 1863.
chromatography, however, failed due to its partial conversion to
a five-membered cyclic boronate through intramolecular B-O
bond formation with loss of the pinacol group. An acetyl-protected
(6) For an example of the synthesis of â-borylallylsilane, see: Rivera, I.;
Soderquist, J. A. Tetrahedron Lett. 1991, 32, 2311.
(7) For the synthetic utility of alkenylboranes, see: (a) Matteson, D. S.
Stereodirected Synthesis with Organoboranes; Springer, Berlin, 1995. (b)
Miyaura, N.; Suzuku, A. Chem. ReV. 1995, 95, 2457.
(8) No reaction took place in the presence of AlCl3 or TMSOTf, at
-78 °C.
10.1021/ja0058381 CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/24/2001

4602 J. Am. Chem. Soc., Vol. 123, No. 19, 2001
Communications to the Editor
Scheme 1
Scheme 2
homoallyl alcohol 6 was obtained in good yield on acetylation
of the crude reaction mixture in the presence of pinacol. Unlike
the TiCl4-catalyzed reaction, TMSOTf-catalyzed reaction of 1a
with propionaldehyde afforded a cyclization product, in which
two molecules of propionaldehyde were incorporated. A single-
crystal X-ray analysis revealed a tricyclic structure 7, which may
be formed through Prins-type cyclization followed by intra-
molecular Friedel-Crafts reaction (eq 5). The formation of the
Table 2. TMSOTf-Promoted Cyclization of â-Borylallylsilanes 10
and 11 with Aldehydesa
entry
allylsilane
aldehyde (R)
product
% yieldb
1
2
3
4
5
6
7c
8
9
10 (n ) 1)
n-Hex
Cy
t-Bu
Ph
n-Hex
Cy
t-Bu
Ph
12a
12b
12c
12d
13a
13b
13c
13d
13e
89
89
50
34
87
83
85
94
85
10
10
10
11 (n ) 2)
11
11
11
11
BnOCH2
a For entries 1-4, allylsilane 10, aldehyde (1.0 equiv), and TMSOTf
(0.5 equiv) were reacted in CH2Cl2 at -78 °C for 1 h. For entries 5-9,
11, aldehyde (1.1 equiv), and TMSOTf (1.1 equiv) were employed at
-78 °C unless otherwise noted. b Isolated yield. c At -40 °C.
ring formation. Thus, prim-, sec-, and tert-alkyl aldehydes as well
as aromatic aldehyde afforded 2-substituted 4-boryloxacyclohept-
4-enes 13a-d in high yields (entries 6-9). Seven-membered
cyclic ether 13e bearing a benzyloxymethyl group was synthesized
in high yield by reaction of 11 with benzyloxyacetaldehyde (entry
9).
single stereoisomer 7 may suggest highly regio- and stereo-
selective cyclization via six-membered chairlike conformation as
shown in eq 5.
The new synthetic access to the cyclic alkenylboranes seems
to be highly attractive, since their preparation has been less
explored than that for acyclic counterparts.11 Synthetic utility of
the cyclic alkenylborane 12a thus prepared was demonstrated by
some transformations including oxidation, Suzuki-Miyaura
coupling,7b and Rh-catalyzed conjugate addition12 that led to the
formation of useful synthetic intermediates 14-17 (Scheme 2).
In summary, we established the synthetic utility of â-boryl-
allylsilanes as a convenient tool for the delivery of â-borylallyl
groups through C-C bond formation. Although a similar delivery
of the â-borylallyl group with a â-borylallylborane prepared by
diboration of allenes was briefly documented,13 the present
approach may greatly expand the scope of the â-borylallylation
chemistry. Further synthetic exploitation of more elaborated
â-borylallylsilanes are now undertaken in this laboratory, directing
toward stereoselective organic synthesis.
We finally turned our attention to the reactions of â-boryl-
allylsilanes bearing siloxyalkyl group, which may be applicable
to the synthesis of cyclic alkenylboranes through an acetalization-
cyclization sequence with aldehydes.9 Allylsilanes 10 and 11,
which possessed a siloxyalkyl side chain (n ) 1 and 2), were
prepared by silaboration of siloxyalkyl-substituted allenes 8 and
9 and subjected to the reaction with aldehydes in the presence of
TMSOTf at -78 °C (Scheme 1). In the reaction of 10 with
primary and secondary alkyl aldehydes, 6-substituted 4-boryloxa-
cyclohex-3-enes 12a and 12b were obtained in high yields (Table
2, entries 1 and 2). Pivalaldehyde and benzaldehyde afforded the
corresponding cyclic alkenylboranes 12c and 12d, respectively,
although concurrent elimination of the adjacent silyl and siloxy
groups resulted in low yields of the desired products (entries 3
and 4).10 The formation of seven-membered cyclic alkenylborane
13 starting from 11 was more effective than the six-membered
Supporting Information Available: Experimental procedures, char-
acterization data for the new compounds, and details of a single-crystal
X-ray analysis of 7 (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
(9) (a) Marko´, I. E.; Mekhalfia, A. Tetrahedron Lett. 1992, 33, 1799. (b)
Marko´, I. E.; Bayston, D. J. Tetrahedron Lett. 1993, 34, 6595. (c) Marko´, I.
E.; Bayston, D. J. Tetrahedron 1994, 50, 7141. (d) Marko´, I. E.; Bailey, M.;
Murphy, F.; Declercq, J. P.; Tinant, B.; Feneau-Dupont, J.; Krief, A.; Dumont,
W. Synlett 1995, 123. (e) Mohr, P. Tetrahedron Lett. 1993, 34, 6251. (f)
Oriyama, T.; Ishiwata, A.; Sano, T.; Matsuda, T.; Takahashi, M.; Koga, K.
Tetrahedron Lett. 1995, 36, 5581. (g) Sano, T.; Oriyama, T. Synlett 1997,
716. Asymmetric synthesis: (h) Suginome, M.; Iwanami, T.; Ito, Y. J. Org.
Chem. 1998, 63, 6096. (i) Suginome, M.; Iwanami, T.; Ito, Y. Chem. Commun.
1999, 2537. (j) Huang, H.; Panek, J. S. J. Am. Chem. Soc. 2000, 122, 9836.
(10) Buta-1,3-dien-2-ylpinacolborane, formed by the elimination reaction,
JA0058381
(11) For recent progress in the synthesis of cyclic alkenylboranes via Ru-
catalyzed ring-closing metathesis, see: Renaud, J.; Ouellet, S. G. J. Am. Chem.
Soc. 1998, 120, 7995.
(12) Sakai, M.; Hayashi, H.; Miyaura, N. Organometallics 1997, 16, 6,
4229.
(13) Ishiyama, T.; Kitano, T.; Miyaura, N. Tetrahedron Lett. 1998, 39, 2357.
1
was detected by H NMR.