Boraformylation and Silaformylation of Allenes

Article abstract of DOI:10.1002/anie.201611314

The boraformylation of allenes with B₂(pin)₂and a formate ester as boron and formyl source, respectively, proceeds in the presence of a copper catalyst. The reaction selectively affords the corresponding β-boryl β,γ-unsaturated aldeh

Full text of DOI:10.1002/anie.201611314

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International Edition: DOI: 10.1002/anie.201611314
German Edition: DOI: 10.1002/ange.201611314
Borylation
Hot Paper
Boraformylation and Silaformylation of Allenes
Tetsuaki Fujihara,* Ayumi Sawada, Tatsuya Yamaguchi, Yosuke Tani, Jun Terao, and
Yasushi Tsuji*
Abstract: The boraformylation of allenes with B2(pin)2 and
a formate ester as boron and formyl source, respectively,
proceeds in the presence of a copper catalyst. The reaction
selectively affords the corresponding b-boryl b,g-unsaturated
aldehydes in good to high yields. Furthermore, the silaformy-
lation of allenes was achieved with a formate ester and
À
PhMe2Si B(pin) as the silicon source.
Scheme 1. a) Boraformylation and b) silaformylation of allenes.
O
rganoboronic acids and esters are versatile boron func-
tional groups in organic synthesis, especially for cross-
coupling reactions.[1] Thus various methods for the prepara-
tion of organoboron compounds have been explored, such as
the hydroboration of carbon–carbon unsaturated bonds[2] and
reactions of boron electrophiles with organolithium or
Grignard reagents.[1a] Being more common in organic syn-
thesis, silicon functional groups have been employed in
a variety of highly efficient transformations.[3]
formylation of alkenes under CO/H2 pressure.[6] We found
that the boraformylation of allenes could be achieved with
B2(pin)2 and a formate ester as boron and formyl source,
respectively, in the presence of a copper catalyst. Formate
esters have been used as formyl sources in Claisen-type
condensation reactions.[7] To the best of our knowledge, this is
the first report on the boraformylation of unsaturated
substrates. Furthermore, the silaformylation of allenes was
The simultaneous incorporation of two different func-
À
À
tional groups onto C C unsaturated bonds is one of the most
achieved for the first time by employing PhMe2Si B(pin) as
capable approaches for the preparation of complex mole-
cules. In particular, allenes are valuable reaction partners as
they can be converted into a structurally diverse array of
products.[4] However, such dual allene functionalizations
usually provide a range of regio- and stereoisomers. Thus
the selective formation of desired products is a highly
important task.
the silicon source along with a formate ester (Scheme 1b).
First, the reaction of 3-methyl-1,2-nonadiene (1a),
B2(pin)2, and hexyl formate was conducted at 508C in the
presence of CuOAc (3.0 mol%) and a ligand (4.0 mol%) in
toluene (Table 1).[8] Impressively, the steric properties of the
ligand strongly affected the reaction. PPh3, dppe, xantphos,
and dppbz[9] were ineffective in promoting the catalytic
reaction, yielding the desired b-boryl b,g-unsaturated alde-
hyde (2a) in trace amounts (entries 1–4). However, Xy-dppbz
produced 2a in 9% yield (entry 5). Finally, DTB-dppbz[10] and
DTBM-dppbz selectively afforded 2a in yields of 99% and
96%, respectively (entries 6 and 7). As for N-heterocyclic
carbene (NHC) ligands, IPr gave 2a in 32% yield (entry 8)
whereas IMes did not enable the formation of this compound
(entry 9). Benzyl formate (instead of hexyl formate) success-
fully afforded 2a in 80% yield (entry 10) whereas the
utilization of phenyl formate considerably decreased the
yield (entry 11).
Herein, we report on the boraformylation of allenes by
simultaneous incorporation of boryl and formyl functional
groups onto a C C double bond of an allene to afford b-boryl
b,g-unsaturated aldehydes with high regioselectivity (Sche-
me 1a). The formyl moiety is a potent functional group that
can be further converted to yield useful compounds. It is
frequently formed by oxidation of primary alcohols,[5] reduc-
tion of carboxylic acids,[5] and, in industry, by the hydro-
À
[*] Prof. Dr. T. Fujihara, A. Sawada, T. Yamaguchi, Prof. Dr. Y. Tani,
Prof. Dr. J. Terao, Prof. Dr. Y. Tsuji
With DTBM-dppbz as the ligand, various allenes were
employed as substrates under the optimized reaction con-
ditions (Table 2). 1,1-Dialkyl-substituted allenes (1a–1d)
provided the corresponding b-boryl b,g-unsaturated alde-
hydes (2a–2d) in good to high yields after isolation (entries 1–
4). In some cases, the addition of sodium laurate[11]
(20 mol%) increased the catalytic activity. For example, 2e
was isolated in 78% yield when the reaction of 1e was
conducted in the presence of sodium laureate (entry 5)
whereas a GC yield of 65% was determined in the absence
of sodium laurate. With sodium laurate, allenes bearing
several groups, such as terminal olefin (1e), chlorophenyl
(1 f), iodophenyl (1g), silyloxy (1h), acetal (1i), ester (1j), or
carbamate (1k) moieties, provided the corresponding prod-
Department of Energy and Hydrocarbon Chemistry
Graduate School of Engineering, Kyoto University
Kyoto 615-8510 (Japan)
E-mail: tfuji@scl.kyoto-u.ac.jp
ytsuji@scl.kyoto-u.ac.jp
Prof. Dr. Y. Tani
Present address: Department of Chemistry
Graduate School of Science, Osaka University
Toyonaka, Osaka 560-0043 (Japan)
Prof. Dr. J. Terao
Present address: Department of Basic Science
Graduate School of Art and Sciences
The University of Tokyo, Tokyo 153-8902 (Japan)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201611314.
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Table 2: Boraformylation of various allenes.[a]
Table 1: Ligand Effect on the copper-catalyzed boraformylation of 1a.[a]
Entry
1
Allene 1
Product 2
Yield [%][b]
87
Entry
Ligand
Formate R=
Yield [%][b]
1
2
3
4
5
PPh3
dppe
xantphos
dppbz
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
PhCH2
Ph
trace
2
trace
trace
9
99
96
32
0
Xy-dppbz
2
84
64
6
7
DTB-dppbz
DTBM-dppbz
IPrCuCl/tBuOK
IMesCuCl/tBuOK
DTBM-dppbz
DTBM-dppbz
8[c]
9[d]
10
11
3[c]
80
31
[a] Reaction conditions: 1a (0.30 mmol), B2(pin)2 (0.36 mmol,
1.2 equiv), formate (0.33 mmol, 1.1 equiv), CuOAc (3.0 mol%), ligand
(4.0 mol%), toluene (0.45 mL), 508C, 6 h. [b] Determined by GC
analysis. [c] A mixture of IPrCuCl (3.0 mol%) and tBuOK (10 mol%) was
used. [d] A mixture of IMesCuCl (3.0 mol%) and tBuOK (10 mol%) was
used.
4
46
78
74
5[d]
6[d]
7[d]
8[d]
9[d]
52
73
90
ucts (2e–2k) in moderate to good yields (entries 5–11). A
monosubstituted allene, 4,4-dimethyl-1,2-hexadiene (1l), did
not afford the corresponding boraformylation product, even
after full consumption of the starting material.
10[d]
56
45
The present procedure was amenable to a gram-scale
reaction. Thus, starting from 0.83 g (6.0 mmol) of 1a, 1.3 g of
2a were obtained, which corresponds to a yield of 72%
(Scheme 2a). Suzuki–Miyaura coupling[1a] of 2a with 4-tert-
butyliodobenzene smoothly afforded the desired product in
83% yield after isolation (Scheme 2b). Coupling with
b-bromostyrene also provided the corresponding diene in
58% yield (Scheme 2c). Horner–Wadsworth–Emmons reac-
tion[12] of 2a enabled the isolation of the olefinated product in
high yield (Scheme 2d).
11[d]
[a] Reaction conditions: 1 (0.30 mmol), B2(pin)2 (0.36 mmol, 1.2 equiv),
hexyl formate (0.33 mmol, 1.1 equiv), CuOAc (3.0 mol%), DTBM-dppbz
(4.0 mol%), toluene (0.45 mL), 508C, 6 h. [b] Yields of isolated products
are given. [c] 16 h. [d] Sodium laurate (20 mol%) was added.
Formate esters, especially aryl formates, are known to
decompose to liberate CO,[13] thereby enabling a boraformy-
lation reaction with the CO that is evolved in situ. However,
hexyl formate was not converted when the reaction shown in
Table 2, entry 1 was conducted without both B2(pin)2 and 1a.
Furthermore, the boraformylation product (2a) was not
obtained upon replacement of hexyl formate with a mixture
of CO (1 atm) and hexanol [Eq. (1)]. Thus CO is not involved
in the present allene boraformylation.
With respect to the silaformylation (silylformylation)[14] of
À
unsaturated C C bonds, Matsuda et al. reported a rhodium-
catalyzed reaction of alkynes employing hydrosilanes under
CO pressure [Eq. (2)].[14a,b] Intramolecular alkene silaformy-
2
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Scheme 3. The effect of ligands on the silaformylation. Reaction
À
conditions: Reaction conditions: 1b (0.20 mmol), PhMe2Si B(pin)
(0.24 mmol, 1.2 equiv), hexyl formate (0.22 mmol, 1.1 equiv), CuOAc
(4.0 mol%), ligand (5.0 mol%), THF (0.20 mL), 608C, 4 h. [a] Deter-
mined by GC analysis. [b] Yield of isolated product.
Scheme 4. Silaformylation of various allenes. Reaction conditions:
Scheme 2. Derivatization of a boraformylation product.
À
1 (0.20 mmol), PhMe2Si B(pin) (0.24 mmol), hexyl formate
(0.22 mmol), CuOAc (4.0 mol%), DTBM-dppbz (5.0 mol%), THF
(0.20 mL), 608C, 4 h. Yields of isolated products are given. [a] 15 h.
lation also proceeds under CO pressure (1000 psi).[14h] How-
ever, there is no precedent for allene silaformylation. The
silaformylation of allenes was found to proceed with a silyl-
borane and a formate ester as silyl and formyl source,
respectively, in the presence of a copper catalyst (Scheme 1b).
Note that toxic CO was not required in the present
À
silaformylation. When the reaction of 1b with PhMe2Si
B(pin) and hexyl formate was conducted in THF at 608C, the
silaformylation product (3b) was obtained in 91% GC yield
and isolated in 86% yield (Scheme 3a). Less bulky ligands
such as Xy-dppbz and dppbz were not effective (Sche-
me 3b,c), which is in good agreement with the ligand effect
observed in the boraformylation (Table 1).
Various allenes gave the corresponding b-silyl b,g-unsa-
turated aldehydes in good to high yields via the silaformyla-
tion reaction (Scheme 4). Functional groups, such as terminal
olefin (3e), chlorophenyl (3 f), iodophenyl (3g), acetal (3i),
and ester (3j) moieties, were tolerated in the reaction. The
monosubstituted allene 1l yielded the corresponding silafor-
mylation product (3l) in moderate yield. Gram-scale silafor-
mylation of 1b (4.0 mmol) was also possible and provided 3b
(1.0 g) in 95% yield.
Scheme 5. Possible catalytic cycle.
The bora- and silaformylation reactions follow similar
catalytic cycles (Scheme 5). It is known that boryl copper
species (Ab) are generated by the reaction of B2(pin)2 with Cu
catalytic cycles (step 0). Ab and Asi subsequently add across
the terminal double bond of an allene (1), generating b-boryl
(Bb)[17] or b-silyl (Bsi)[18] allyl copper intermediates (step 1).
Next, Bb and Bsi react with a formate ester at the g-position
to provide Cb and Csi via a six-membered-ring transition state
compounds.[15] Silyl copper species (Asi) are also afforded
[16]
À
upon reaction of PhMe2Si B(pin) with copper compounds.
Thus Ab and Asi must be key catalyst species involved in the
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e) I. Fleming, J. Dunogues, R. Smithers, Org. React. 1989, 37, 57 –
575.
(step 2). b-Elimination from Cb and Csi produces
2
(Y= B(pin)) and 3 (Y= SiMe2Ph), which extrude alkoxy
copper species D (step 3). It has been reported that ketone-
s,[17e,18b] aldehydes,[17e,18b,d] and imines[17f,g] react with Bb and
Bsi to give the corresponding allylated alcohols and amines.
In the present reaction, the alkoxy moiety of the formate is
efficiently removed as an alkoxy copper species (D) by copper
species bearing rather bulky ligands, such as DTBM-dppbz
and DTB-dppbz. Finally, s-bond metathesis of D with
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À
B2(pin)2 and PhMe2Si B(pin) regenerates the active catalyst
species Ab and Asi, respectively (step 4).
In conclusion, bora- and silaformylation reactions of
allenes have been described for the first time. For the
boraformylation, B2(pin)2 and formate esters were good
boron and formyl sources, respectively, in the presence of
a copper catalyst. The corresponding b-boryl b,g-unsaturated
aldehydes were selectively obtained in good to high yields. In
the silaformylation, b-silyl b,g-unsaturated aldehydes were
[7] a) R. P. Mariella, J. Am. Chem. Soc. 1947, 69, 2670 – 2672;
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[8] See the Supporting Information for details.
[9] dppe = 1,2-bis(diphenylphosphino)ethane, xantphos = 4,5-bis-
(diphenylphosphino)-9,9-dimethylxanthene, dppbz = 1,2-bis(di-
phenylphosphino)benzene,
thylphenyl)phosphino)benzene, DTB-dppbz = 1,2-bis(bis(3,5-
di-tert-butylphenyl)phosphino)benzene, DTBM-dppbz = 1,2-
Xy-dppbz = 1,2-bis(bis(3,5-dime-
À
provided when PhMe2Si B(pin) was employed as the silicon
source in combination with a formate ester. Further studies on
the reaction mechanisms and enantioselective variants are in
progress.
bis(bis(3,5-di-tert-butyl-4-methoxyphenyl)phosphino)benzene,
IPr= 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene, IMes =
1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene.
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Acknowledgements
[11] Whereas sodium propionate could also be used in the reaction,
sodium laurate was preferred owing to its less hygroscopic
nature.
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This work was supported by JSPS KAKENHI Grant Number
25708017, a Grant-in-Aid for Young Scientists (A), and JSPS
KAKENHI Grant Number JP16H01020 (Precisely Designed
Catalysts with Customized Scaffolding) from MEXT, Japan.
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Conflict of interest
The authors declare no conflict of interest.
Keywords: allenes · borylation · copper · formate esters ·
silylation
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[18] For selected examples for b-silylallyl copper intermediates
Manuscript received: November 18, 2016
À
generated from allenes and Cu Si species (Asi), see: a) Y.
Final Article published: && &&, &&&&
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Communications
Borylation
T. Fujihara,* A. Sawada, T. Yamaguchi,
Y. Tani, J. Terao, Y. Tsuji*
&&& — &&&
Boraformylation and Silaformylation of
Allenes
Boron or silicon? Boraformylation and
silaformylation reactions of allenes with
formate esters smoothly proceed in the
presence of a copper catalyst using B2-
and silicon sources, respectively. These
reactions selectively afford the corre-
sponding b-boryl and b-silyl b,g-unsatu-
rated aldehydes in good to high yields
with excellent regioselectivity.
À
(pin)2 and PhMe2Si B(pin) as the boron
6
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