Boration of an α,β-enone using a diboron promoted by a copper(I) - Phosphine mixture catalyst

Article abstract of DOI:10.1016/S0040-4039(00)01161-8

The Cu(I)-catalyzed boration of an α,β-enone using a diboron is described. Combination of a Cu(I) salt and tributylphosphine is an effective catalyst system, though the individual use of Cu(I) and tributylphosphine is not active for the reaction. (C) 2000

Full text of DOI:10.1016/S0040-4039(00)01161-8

Tetrahedron Letters 41 (2000) 6821±6825
Boration of an a,b-enone using a diboron promoted by a
copper(I)±phosphine mixture catalyst
Hajime Ito,a Hiroshi Yamanaka,b Jun-ichi Tateiwab and Akira Hosomib,*
aInstitute for Molecular Science, Research Center for Molecular Materials, Myodaiji, Okazaki 444-8585, Japan
bDepartment of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba,
Ibaraki 305-8571, Japan
Received 24 May 2000; revised 5 July 2000; accepted 7 July 2000
Abstract
The Cu(I)-catalyzed boration of an a,b-enone using a diboron is described. Combination of a Cu(I) salt and
tributylphosphine is an e€ective catalyst system, though the individual use of Cu(I) and tributylphosphine is
not active for the reaction. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: copper and compounds; boron and compounds; enones; catalysts.
The addition of the compounds possessing an M±M and M±M0 bond (M, M0 = Si, B, Sn, Ge,
S) to C±C multiple bonds catalyzed by a transition metal has been an extensively studied topic
during the past decade.1 For example, Miyaura and co-workers reported a number of studies for
the Pt- and Pd-catalyzed reactions of diborons accompanied with B±B bond cleavage.2 While
much interest has been focused on the group 10 metals as a versatile catalyst for these reactions,
few examples using other metals have been reported.3 We recently found the conjugate addition
of Si±Si compounds to a,b-enones catalyzed by a Cu(I) salt.4 This result indicates that the Cu(I)
salt would be a new promising catalyst in this area. Here we describe an unprecedented reaction
of a diboron with an a,b-enone by means of a Cu(I) catalyst.5
We ®rst examined the reaction between bis(pinacolato)diboron 1a and chalcone 2a in the
.
presence of a catalytic amount of (CuOTf)2 C6H6 (0.05 equiv., 0.10 equiv. for an a,b-enone as a
Cu(I) salt) at room temperature (Table 1). However, no reaction products were detected (entry 1).
.
Next, a mixture of a catalytic amount of (CuOTf)2 C6H6 (0.05 equiv.) and tributylphosphine
(0.11 equiv.) was employed. By this modi®ed catalyst system, the reaction was dramatically
improved to give boration product 3a in good yield after hydrolysis (entry 2).6 It is noteworthy
that the reaction temperature of this Cu(I) catalysis is lower than that of the Pt-catalyzed
reaction.7 Addition of a phosphine is essential for the reaction (entry 2) and a less donative
* Corresponding author. Tel: +81-298-53-4237; fax: +81-298-53-6503; e-mail: hosomi@chem.tsukuba.ac.jp
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01161-8

6822
phosphine resulted in moderate yield (entry 3). In the absence of a Cu(I) salt, the addition reaction
scarcely proceeded (entries 4, 5). The ratio of a phosphine ligand to a Cu(I) salt and its chelation
ability a€ected the activity of the catalyst. Excess use of tributylphosphine decreased the yield of
product (entry 6). A bidentate phosphine, such as 1,3-bis(diphenylphosphino)propane, also
retarded the addition reaction, and higher temperature was needed to obtain the product (entry
7). Copper(I) chloride, a more easily accessible Cu(I) salt than air-sensitive CuOTf, was also
e€ective in the presence of tributylphosphine, although a slightly longer reaction time was needed
(entries 8, 9). DMF was a good solvent for the promotion of the reaction and a less polar solvent
(THF, CH2Cl2, toluene) was also useful (entries 10±13).
Table 1
Cu(I)-catalyzed reactions of a diboron 1a to an a,b-enone 2a under various conditions
The generality of this Cu(I) salt-catalyzed addition was demonstrated by the reaction with
various a,b-enones (Table 2). Although the less-hindered substrates gave good yields (entries 1±5),
the tetra-substituted enone 2g furnished the product in a disappointing yield.

6823
Table 2
Reactions of a diboron 1a and a,b-unsaturated carbonyl compound 2 in the presence of a copper(I) catalyst
Oxidation of the product 3a to the corresponding b-hydroxy ketone 4a represents the utility of
the Cu(I)-catalyzed boration for the synthesis of b-hydroxy ketones (Scheme 1). Bis(catecholato)-
diboron 1b was also useful for the Cu(I)-catalyzed reaction, though the isolation of the
corresponding borylated ketone from the reaction mixture was not successful. One-pot reactions
of the copper(I)-catalyzed addition and oxidation gave the product 4a, 4d in moderate yield
(Scheme 2).
Scheme 1. Oxidation of 3a with hydrogen peroxide

6824
Scheme 2. Synthesis of b-hydroxyketone 4 by one-pot reaction of Cu(I)-catalyzed boration and oxidation
We also examined the reaction between equimolar amounts of CuCl, tributylphosphine and
diboron 1a in the absence of a,b-enones in THF under similar conditions; however, no products
were observed by GC. That is, in contrast to the similar reaction of Pt complex, in which B±B
bond cleavage proceeded with the oxidative addition of the diboron to the Pt metal, the copper(I)
salt itself did not consume the diboron in the absence of an a,b-enone.2b
We considered the coordination of the phosphine to Cu(I) to be the most important factor for
the reaction. If the dominant role of the phosphine is activation of the diboron by coordination to
the B atom, excess use of the phosphine should enhance the reaction. However, the opposite
results were obtained (Table 1, entries 5, 6). The 31P NMR measurements also supported this
assumption.8 The 31P NMR (109.3 MHz, DMF-d7) spectroscopy revealed that the chemical shift
of tributylphosphine (ꢀ ^30.7) was only slightly changed in the presence of an equimolar diboron
1a (ꢀ ^31.4). On the contrary, the mixture of tributylphosphine and CuCl exhibited a signi®cant
down-®eld shift (ꢀ ^14.2). These results suggest that the degree of the coordination of tributyl-
phosphine to Cu(I) is greater than that between tributylphosphine and the diboron 1a. We
assumed the complexation between a ligand and Cu(I) metal reduced the aggregation of the Cu(I)
salt in a solvent and this de-aggregation would enhance the catalytic activity.
In conclusion, we have described Cu(I) salt-catalyzed boration of an a,b-enone using a diboron
with cleavage of the B±B bond. These ®ndings disclose a new catalytic ability of the Cu(I) salt as
well as a new pathway for the synthesis of boron-containing compounds.
Acknowledgements
This work was partly supported by Grants-in-Aid for Scienti®c Research and Grants-in-Aid
for Scienti®c Research on Priority Areas (No. 706: Dynamic Control of Stereochemistry) from
the Ministry of Education, Japan and P®zer Inc.
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.
6. Representative procedure was described as follows: In a glove box, (CuOTf)2 C6H6 (25 mg, 0.05 mmol) was
weighed and placed in a two-necked round bottomed ¯ask (30 ml) equipped with a magnetic stirring bar, a rubber
septum and a re¯ux condenser. The reaction tube was removed from the glove box and connected with a nitrogen
line. After the addition of DMF (2.0 ml) and tributylphosphine (22 mg, 0.11 mmol), the mixture was stirred for 10
min. To another reaction vessel which was dried and ®lled with nitrogen, the catalyst solution (0.4 ml) was
transferred with a syringe and 1a (60.9 mg, 0.24 mmol) and 2a (41.6 mg, 0.2 mmol) were added. After the mixture
was stirred for 10 h at room temperature, water (1.0 ml) was added and the resulting mixture was extracted by
ethyl acetate (10 mlÂ3). The combined organic layer was dried over Na2SO4, ®ltered and concentrated. The
residue was puri®ed by column chromatography (SiO2) to give 3a (64.6 mg, 96%). The spectral data was identical
with the literature.7
7. Lawson, Y. G.; Lesley, M. J.; Marder, T. B.; Norman, N. C.; Rice, C. R. Chem. Commun. 1997, 2051±2052.
8. Phosphoric acid (85% solution in water) was used for external standard.