Palladium-catalysed borylsilylation of alkynes and borylsilylative carbocyclization of diynes and an enyne compound

Article abstract of DOI:10.1039/a702802d

Addition reactions or addition-carbocyclization reactions of a borylsilane with alkynes, α,ω-diynes or an enyne compound proceed efficiently in the presence of palladium catalysts, P(OCH₂)₃CEt being the ligand of choice.

Full text of DOI:10.1039/a702802d

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Palladium-catalysed borylsilylation of alkynes and borylsilylative
carbocyclization of diynes and an enyne compound
Shun-ya Onozawa, Yasuo Hatanaka and Masato Tanaka*
National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan
Addition reactions or addition–carbocyclization reactions of
a borylsilane with alkynes, a,w-diynes or an enyne com-
pound proceed efficiently in the presence of palladium
catalysts, P(OCH2)3CEt being the ligand of choice.
than PPh3 in the platinum-8 or palladium-catalysed reactions9 of
unsaturated carbon compounds with disilanes or digermanes.
The reaction of oct-1-yne with 1 was also better when catalysed
by the Pd2(dba)3–4PMe3 system (70% yield, 80 °C, 2 h) than by
the Pd2(dba)3–4PPh3 system (0% yield, 80 °C, 2 h vs. 18%
yield, 110 °C, 7 h). However, as the foregoing typical
experiment exemplifies, etpo is superior to the other phosphines
as the ligand. In view of the decreasing trend of performance
etpo > PMe3 > PPh3, the ligand effect in the borylsilylation is
concluded to be similar to our previous observation made in the
double silylation of acetylenes9a,b and quinones9c with disilanes
and double germylation of acetylenes, dienes and alkenes.9d
Addition reactions of inter-heteroatom bonds such as Si–Si and
Sn–Sn to unsaturated carbon bonds are extremely useful for
one-step generation of two heteroatom–carbon bonds.1 Similar
chemistry starting with B–B bonds is also rapidly emerging.2
An obvious extension is addition reactions of bonds comprising
two different elements.3 In a previous paper we disclosed
palladium-catalysed addition of borylstannanes to alkynes
(borylstannylation) leading to efficient synthesis of 1-boryl-
2-stannyl alkenes.4 a,w-Diynes also react with borylstannanes,
undergoing borylstannylative carbocyclization reactions to
afford 1-borylmethylene-2-stannylmethylene cycloalkanes in
high yields.5a 1,4-Addition of conjugated dienes also proceeds
smoothly when Pd(etpo)2 catalyst [etpo = 4-ethyl-2,6,7-trioxa-
1-phosphabicyclo[2.2.2]octane, P(OCH2)3CEt] is used.5b As
regards corresponding borylsilylation, however, only unsuc-
cessful results have been reported by Buynak and Geng.6 While
we were studying borylsilylation as a useful extension of the
borylstannylation, Ito and co-workers briefly presented success-
Me
N
+
H13C6
B
SiMe2Ph
N
Me
i
H
13C6
H13C6
Me
Me
+
PhMe2Si
Me
B
N
N
B
SiMe2Ph
Me
2b
N
N
ful examples of addition reactions of
a borylsilane
2a
(4,4,5,5-tetramethyl-2-dimethylphenylsilyl-2-bora-1,3-dioxa-
pentane) to alkynes, mainly using the 1,1,3,3-tetramethylbutyl
isocyanide–palladium diacetate catalyst system.7 Their commu-
nication prompted us to report our own results.
L = etpo 92% (2a:2b >99:1)
L = PMe3 70% (2a:2b >99:1)
L = PPh3 0%
A representative procedure for the reaction of alkynes was as
follows. A mixture of Pd2(dba)3 (0.005 mmol; dba = dibenzyli-
deneacetone) and etpo (0.02 mmol), i.e. Pd2(dba)3–etpo catalyst
system in which P/Pd = 2, in C6D6 (0.2 ml) was heated at 80 °C
for 5 min, while the colour of the solution was changing from
red to green. 1,3-Dimethyl-2-dimethylphenylsilyl-2-bora-
1,3-diazacyclopentane 1 (0.2 mmol), oct-1-yne (0.3 mmol) and
1,4-dioxane (20 ml; internal standard for NMR analysis) were
added to the resulting solution and the mixture was heated at
80 °C in a sealed NMR tube. NMR and GC–MS analyses of the
reaction mixture after 2 h indicated complete consumption of 1
and adduct 2a† being formed in 92% yield (Scheme 1). Note
that the regioselectivity was > 99% and that only a trace of
isomer 2b was detected by NMR analysis. Isolation of the
product was simple; evaporation of the reaction mixture,
addition of hexane (2 ml) to the residue, filtration and
distillation of the filtrate afforded nearly pure 2a in 84% isolated
yield, which exhibited satisfactory spectral and analytical data.
Observation of an 8% NOE between the allylic protons and the
vinylic proton suggested that selective cis-addition of the B–Si
bond had taken place.
Unlike for the borylstannylation, Pd–PPh3 complexes were
not as efficient catalysts; the reaction of oct-1-yne with 1 did not
proceed under the same conditions when either Pd(PPh3)4 or
PdCl2(PPh3)2 was used as the catalyst. The former did catalyse
the reaction at higher temperatures, but the yields were still
unsatisfactory, e.g. only 78% yield (2a/2b > 99:1) even after
heating at 110 °C for 24 h. However the latter, which catalysed
the borylstannylation at room temperature, was totally inactive
even at 110 °C. We have reported that PMe3 performs better
Scheme 1 Reagents and conditions: i, Pd2(dba)3 (2.5 mol%), ligand
(L/Pd = 2), C6D6, 80 °C, 2 h
Phenylacetylene appeared to be slightly less reactive towards
1 than oct-1-yne, but the reaction did proceed smoothly at
110 °C in the presence of the Pd2(dba)3–4etpo catalyst system to
give 97% yield of 3† as the sole product in 2 h (Scheme 2).
However, the reaction of dimethyl acetylenedicarboxylate
resulted in a complicated mixture.
Borylsilylative carbocyclization of hepta-1,6-diyne (0.3
mmol) with 1 (0.2 mmol) was also catalysed smoothly by the
Pd2(dba)3–4etpo system in C6D6 at 110 °C to afford 4 in 81%
yield.† Configurations at the silylmethylidene and boryl-
methylidene moieties of 4 were confirmed by 19.3 and 18.2%
NOEs between the vinylic and allylic protons. A byproduct 5
coming from simple addition of 1 to one of the two acetylenic
bonds without carbocyclization was also formed in 18% yield
(Scheme 3). The Pd2(dba)3–4PMe3 system was as active, but
less selective (84% total yield after 2 h at 110 °C,
4:5 = 30:70). The Pd2(dba)3–4PPh3 system was less active
and even less selective (88% total yield after 7 h at 110 °C,
4:5 = 26:74). PdCl2(PPh3)2 was totally inactive at 110 °C.
Me
Ph
Me
N
i
+
Ph
B
SiMe2Ph
PhMe2Si
B
N
N
N
Me
3 97%
Me
Scheme 2 Reagents and conditions: i, Pd2(dba)3 (2.5 mol%), etpo (etpo/
Pd = 2), C6D6, 110 °C, 2 h
Chem. Commun., 1997
1229

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Me
B
to give 8† in 84% yield (Scheme 5). The 1H NMR spectrum of
the reaction mixture displayed a weak singlet at d 6.48,
suggesting the formation of a small quantity (9%) of a
byproduct, which might have come from plain addition of 1 to
the triple bond.
Partial financial support from the Science and Technology
Corporation (JST) through the CREST program is gratefully
acknowledged. We thank Dr T. Sakakura for valuable discus-
sions in the initial stages of the research.
N
N
+
SiMe2Ph
Me
i
SiMe2Ph
Me
SiMe2Ph
Me
N
N
B
B
+
N
N
Me
Footnotes
Me
4
5
* E-mail: mtanaka@ccmail.nimc.go.jp
† Selected spectral and/or physical data for the products 1H NMR (C6D6),
270 MHz; J/Hz) for 2a: bp 135–140 °C at 2.3 3 1023 Torr; dH 0.43 (s, 6 H,
SiCH3), 0.86 [t, J 6.8, 3 H, CH2(CH2)4CH3], 1.14–1.53 [m, 8 H,
CH2(CH2)4CH3], 2.28 [t, J 7.2, 2 H, CH2(CH2)4CH3], 2.51 (s, 6 H, NCH3),
2.89 (s, 4 H, NCH2), 6.36 (s, 1 H, NCH), 7.10–7.33 (m, 3 H, arom),
7.50–7.65 (m, 2 H, arom). For 3: dH 0.40 (s, 6 H, SiCH3), 2.49 (s, 6 H,
NCH3), 2.84 (s, 4 H, NCH2), 6.56 (s, 1 H, NCH), 6.95–7.35 (m, 8 H, arom),
7.50–7.60 (m, 2 H, arom). For 4: bp 128–132 °C at 2.8 3 1023 Torr
(mixture of 4 and 5); dH 0.38 (s, 6 H, SiCH3), 1.46–1.68 (m, 2 H), 2.31–2.41
(m, 4 H), 2.46 (s, 6 H, NCH3), 2.83 (s, 4 H, NCH2), 5.40 (s, 1 H, NCH), 5.62
(s, 1 H, NCH), 7.12–7.31 (m, 3 H, arom), 7.42–7.60 (m, 2 H, arom). For 6:
L = etpo, L/Pd = 2, 2 h, 98% (4:5 = 81:19)
L = etpo, L/Pd = 1, 2 h, 97% (4:5 = 87:13)
L = PMe3,L/Pd = 2, 2 h, 84% (4:5 = 30:70)
L = PPh3, L/Pd = 2, 7 h, 98% (4:5 = 26:74)
Scheme 3 Reagents and conditons: i, Pd2(dba)3 (2.5 mol%), ligand
(L/Pd = 2 or 1), C6D6, 110 °C
A decrease in the quantity of etpo relative to palladium is
envisioned to be favourable to minimize the formation of
byproduct 5 if simultaneous coordination of both acetylenic
bonds to palladium is a prerequisite for the cyclization leading
to 4. Indeed, the reaction of hepta-1,6-diyne with 1 at etpo/Pd
= 1 under otherwise identical conditions resulted in a slight
increase in the selectivity for 4; the 4:5 ratio was 87:13,
obtained in a 97% combined yield. The reaction of octa-
1,7-diyne with 1 was slightly less reactive and much less
selective for the cyclization product 6,† with 7 as the major
product (Scheme 4). The selectivity was not appreciably
improved even when etpo/Pd = 1, indicating the necessity for
a new design of catalyst to circumvent the difficulty of
coordination of both acetylenic bonds.
dH 0.40 and 0.43 (both s, 6 H, SiCH3), 1.45–1.73 (m, 4 H), 2.20–2.40 (m,
4 H), 2.55 (s, 6 H, NCH3), 2.83–2.97 (m, 4 H, NCH2), 5.30 (s, 1 H, NCH),
5.47 (s, 1 H, NCH), 7.05–7.26 (m, 3 H, arom), 7.54–7.63 (m, 2 H, arom). For
8: dH 0.24 (s, 3 H, SiCH3), 0.26 (s, 3 H, SiCH3), 0.82 (dd, J 11.9 and 14.6,
1 H, SiCH2), 0.90 (t, J 7.1, 3 H, OCH2CH3), 0.91 (t, J 7.1, 3 H, OCH2CH3),
1.32 (dd, J 2.0 and 14.6, 1 H, SiCH2), 1.80–1.93 (m, 1 H, CH2), 2.50 (s, 6
H, NCH3), 2.74–3.07 (m, 7 H, NCH2, CH2 and CH), 3.14 (d, J 17.5, 1 H,
CH2), 3.32 (d, J 17.5, 1 H, CH2), 3.85–4.03 (m, 4 H, OCH2CH3), 5.43 (s, 1
H, NCBH), 7.13–7.27 (m, 3 H, arom), 7.45–7.53 (m, 2 H, arom).
References
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Bateson and M. B. Mitchell, Academic Press, New York, 1994, pp. 129–
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Finally an enyne compound, 4,4-bis(ethoxycarbonyl)hept-
6-en-1-yne, also smoothly underwent similar cyclization with 1
Me
N
+
B
SiMe2Ph
N
Me
i
3 T. Ishiyama, K.-i. Nishijima, N. Miyaura and A. Suzuki, J. Am. Chem.
Soc., 1993, 115, 7219; L.-B. Han, N. Choi and M. Tanaka, J. Am. Chem.
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7 M. Suginome, H. Nakamura, T. Matsuda and Y. Ito, presented at the 72nd
Spring Annual Meeting of the Chemical Society of Japan, Abstract
2F226, Tokyo, March 27–30th, 1997.
Me
SiMe2Ph
Me
B
SiMe2Ph
B
N
N
+
N
N
Me
Me
6 17%
7 62%
Scheme 4 Reagents and conditions: i, Pd2(dba)3 (2.5 mol%), etpo (etpo/
Pd = 2), C6D6, 110 °C, 2 h
Me
EtO2C CO2Et
N
+
B
SiMe2Ph
N
Me
i
8 T. Hayashi, A. M. Kawamoto, T.-a. Kobayashi and M. Tanaka, J. Chem.
Soc., Chem. Commun., 1990, 563.
SiMe2Ph
Me
EtO2C
EtO2C
9 (a) H. Yamashita, M. Catellani and M. Tanaka, Chem. Lett., 1991, 241;
(b) H. Yamashita and M. Tanaka, Chem. Lett., 1992, 1547; (c)
T. Hayashi, H. Yamashita, T. Sakakura, Y. Uchimaru and M. Tanaka,
Chem. Lett., 1991, 245; (d) H. Yamashita, N. P. Reddy and M. Tanaka,
Macromolecules, 1993, 26, 2143.
N
B
N
Me
8 84%
Received in Cambridge, UK, 24th April 1997; Com.
7/02802D
Scheme 5 Reagents and conditions: i, Pd2(dba)3 (2.5 mol%), etpo (etpo/
Pd = 2), C6D6, 110 °C, 2 h
1230
Chem. Commun., 1997