838
Chemistry Letters Vol.35, No.8 (2006)
(Z)-1,2,3,4-Tetraboryl-2-butene:
A Reagent for Stereoselective One-pot Triple Aldehyde Addition
Masaki Shimizu,ꢀ Katsuhiro Shimono, and Tamejiro Hiyama
Department of Material Chemistry, Kyoto University, Kyoto University Katsura, Nishikyo-ku, Kyoto 615-8510
(Received May 8, 2006; CL-060531; E-mail: shimizu@npc05.kuic.kyoto-u.ac.jp)
(Z)-1,2,3,4-Tetraboryl-2-butene reacted with three mole-
Table 1. Stereoselective triple aldehyde addition of 1 leading
to 2
cules of 2,3-dialkylidene-1,5-alkanediol as a single stereo-
isomer. Preparation and triple addition of the butene can be
effected in a single operation.
Entry
R
Product
Yield/%a
1
2
3
4
5
6
7
8
9
2-Naphthyl
4-C6H5-C6H4
4-CF3-C6H4
4-MeO-C6H4
3-MeO-C6H4
2-MeO-C6H4
3,5-(MeO)2-C6H3
C6H5(CH2)2
C2H5
2b
2c
2d
2e
2f
2g
2h
2i
63
86
63
80
69
82
57
83
71
73
Dimetalated compounds have emerged as versatile reagents
for an efficient construction of complex molecules,1,2 because
the compounds, in principle, allow us to perform multiple car-
bon–carbon and carbon–heteroatom bond formations in one-
pot and can also act as precursors of polyfunctional organo-
metallic reagents.3 In view that allylmetal compounds are ex-
tremely useful reagents for carbon–carbon bond formation with
excellent regio- and stereocontrol,4 ꢀ-metalated allylmetals
constitute an especially attractive class of dimetalated reagents
for stereoselective domino and sequential reactions.5,6 Indeed,
Flamme and Roush demonstrated ꢀ-borylated allylic boranes
to react two molecules of aldehydes in one-pot, providing both
syn- and anti-1,5-alkanediols with high enantioselectivities,
respectively.5c Further appropriate installation of metals into
ꢀ-metalated allylmetals are quite intriguing not only to maxi-
mize the potential of the allylmetals as reagents for domino re-
actions but also to shed a light on a new synthetic methodology
utilizing polymetalated compounds, if such reagents can be easi-
ly prepared and handled as well as perform multiple carbon–
carbon bond formation with high regio- and stereocontrol in
all steps. We recently reported facile and stereoselective synthe-
sis of (Z)-1,2,3,4-tetrakis(pinacolato)boryl-2-butene (1), involv-
ing Pt-catalyzed 1,4-diborylation of 2,3-diboryl-1,3-butadiene
(3) with bis(pinacolato)diboron (4).7 We envisioned that the
tetraborylated reagent 1 could perform multiple aldehyde addi-
tion because 1 can be regarded as double hybrid of the ꢀ-bo-
rylated allylic boranes. We report herein that 1 undergoes triple
aldehyde addition in one-pot, affording 2,3-dialkylidene-1,5-
alkanediols 2 in good yields as a single stereoisomer (eq 1). In
addition, one-pot preparation–triple addition of 1 is also demon-
strated.
2j
2k
10b
PhCH2OCH2
b
aIsolated yield. The reaction proceeded at 80 ꢁC.
hyde addition are summarized in Table 1. Various kinds of aro-
matic aldehydes were applicable to the reaction in good yields
(Entries 1–7), while the addition to such aliphatic aldehydes as
3-phenylpropanal and propanal also proceeded in good yields
(Entries 8 and 9). Benzyloxyacetaldehyde was found to react
smoothly at 80 ꢁC (Entry 10). Noteworthy is that all products
2b–2k were isolated as a single stereoisomer, indicating that
each step of the whole transformation proceeded in a highly
stereoselective manner (for mechanism, see vide infra). As a
consequence, we succeeded one-pot conversion of four C–B
bonds into two C–C bonds and a C=C bond with perfect 1,5-
remote and olefinic stereocontrol.8
The stereochemical outcome is reasonably explained by
assuming 6-membered cyclic transition states which is well
accepted for allylation of allylic borane reagents (Scheme 1).4
Reagent 1 would react with RCHO via T1e in which substituent
R adapted an equatorial position in favor of T1a with axial-posi-
tioned R, giving rise to 5syn.0 The second allylation with 5syn
would proceed via T2 over T2 to produce 6anti, because 1,3-di-
axial repulsion between H and CH(OB)R was much severe than
those between H and CH2B. Third RCHO would be allylated
with 6anti via T3 to generate 7anti which should cause ꢁ-elimina-
tion of the remaining boryl and boroxy groups in a syn-fashion to
give 2 in preference to the fourth addition due probably to the
steric hindrance around the boryl group. Even when one equiv.
of RCHO was employed, neither 1:1 nor 1:2 adduct was detected
during the reaction, indicating that the first addition was consid-
ered to be a rate-determining step of the sequence.
R
OH
OH
B
B
3 RCHO
(1)
toluene
100 °C
R
R
B
B
1
2
B: pinacolatoboryl
single stereoisomer !
A toluene solution of 1 and 4 equiv. of benzaldehyde was
heated at 100 ꢁC for 14 h, giving rise to 2a (R ¼ Ph) in 86% yield
as a single stereoisomer which turned out to be a 1:3 adduct (See
Supporting Information). When the same reaction was carried
out in toluene at 80 ꢁC or in 1,2-dichloromethane or 1,4-dioxane
at 100 ꢁC resulted in decrease of the isolated yields (30–71%)
of 2a, respectively.
Furthermore, the triple addition can be performed in con-
junction with the preparation of 1 (eq 2). Thus, a solution of 3
(1.0 equiv.), 4 (1.4 equiv.), RCHO (3.5 equiv.), and Pt(PPh3)4
(3 mol %) in toluene was heated at 100 ꢁC for 11–17 h, affording
2a–2e, and 2g as a single stereoisomer, respectively, in accept-
able yields at once.7b In other words, sequential stereoselective
formation of two carbon–boron bonds and three carbon–carbon
Representative examples of this stereoselective triple alde-
Copyright Ó 2006 The Chemical Society of Japan
Shimizu, Masaki
