Ruthenium(II) complex catalyzed o-borylation of alcohols with vinylboronates

Article abstract of DOI:10.1055/s-0029-1217732

A new, highly effective method for the selective synthesis of boryl ethers via O-borylation of alcohols with vinylboronates catalyzed by the Ru-H complexes [Ru(CO)ClH(PPh3)3] and [Ru(CO)ClH(PCy3)2] is described. The mechanism of catalysis of this new de-e

Full text of DOI:10.1055/s-0029-1217732

LETTER
2433
Ruthenium(II) Complex Catalyzed O-Borylation of Alcohols with
Vinylboronates
O-Borylation of
A
o
lcohols
w
ith
g
V
inylboron
d
a
tes an Marciniec,* Jędrzej Walkowiak
Department of Organometallic Chemistry, Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznań, Poland
Fax +48(61)8291508; E-mail: marcinb@amu.edu.pl
Received 28 May 2009
of alcohols owing to its thermal stability.9 On the other
Abstract: A new, highly effective method for the selective synthe-
hand, boronic esters have been attractive intermediates in
the synthesis of organic derivatives containing B–C
sis of boryl ethers via O-borylation of alcohols with vinylboronates
catalyzed by the Ru-H complexes [Ru(CO)ClH(PPh3)3] and
[Ru(CO)ClH(PCy3)2] is described. The mechanism of catalysis of
bonds, which are of great importance in Suzuki transfor-
this new de-ethenative borylation is supported by equimolar reac- mations.10 Alkoxyboranes are also used in the synthesis of
nanocomposites and nanotubes.11
tion of Ru–B complex with alcohol.
Key words: homogeneous catalysis, vinylboronates, ruthenium,
coupling reactions, boryl ethers
For this reason, a variety of O-borylation methods have
been developed for preparing boryl ethers via esterifica-
tion.12 Those processes are, however, restricted by the ne-
cessity to remove water, and by the instability of the
The silylative coupling of olefins with vinyl-silicon com-
pounds, which we have developed over the last two de-
cades, generally takes place in the presence of complexes
(Ru, Rh, Ir) containing or generating M–H and M–Si (sil-
icometallics) bonds, and has become a very valuable syn-
thetic tool for the preparation of vinyl-substituted
organosilicon reagents and polymers.1 However, we have
recently found that this mode of reactivity is general and
also exhibited by vinyl derivatives of other p-block ele-
ments, for example boron2 and germanium.3 It has also
been extended to catalytic activation by such vinylmetal-
loids (mainly vinylsilicon) as ≡Csp-H,4 =CAr-H5 and even
the O–H bond of alcohols6 and silanols,7 indicating a new
general role of vinylmetalloids as metalloid agents and
hydrogen acceptors.1a
substrates in when contact with moisture. Common meth-
ods used for preparing boryl ethers involve reactions of
chloroboranes as well as boranes or sodium tetraboro-
hydrides with alcohols, however, under these conditions,
corrosive HCl or explosive H2 are evolved respectively.13
Herein, we present a new catalytic reaction that involves
activation of the O–H bond of alcohols by vinylboronates.
The reaction occurs in the presence of ruthenium-hydride
complexes, yielding boryl ethers with elimination of eth-
ylene (Equation 2).
O
O
R
O
O
[Ru-H], toluene
– ethylene
+
H OR
B
O
B
Equation 2
O-Borylation of silanols with vinylboronates proceeding
in the presence of a ruthenium complex, leads to the for-
mation of boryl silyl ethers (borasiloxanes) with evolution
of ethylene (Equation 1).8
The borylation reactions were first examined in the pres-
ence of [Ru(CO)ClH(PCy3)2] (I), which is known to be
the most active catalyst for metallative coupling of olefins
and alkynes by vinylsilanes,1,4 vinylgermanes3,4b,c and
vinylboronates2 in both open and closed (sealed am-
poules) systems in toluene (60–80 °C, under argon).
O
O
SiR3
O
O
O
O
[Ru-H], toluene
– ethylene
+
H
OSiR3
B
O
B
B
A two or three-fold excess of vinylboronate was used to
achieve either quantitative or very high yields of the boryl
ethers. Under such conditions, a competitive homo-cou-
pling of the vinylboronates was observed to give bis(bor-
yl)ethenes as by-products (Equation 3 and Table 1). The
latter could be effectively separated by column chroma-
tography.
O
O
O
B
O
O
O
B
,
B
=
Equation 1
The aim of this work was to make use of the new role of
vinyl-boronates as hydrogen acceptors in the O-boryla-
tion of alcohols, which can be useful in borylation strate-
gies because the boryl group is suitable for the protection
In order to eliminate formation of the side-product, anoth-
er catalyst [Ru(CO)ClH(PPh3)3] (II) was used, which has
been recently recognized to be inactive in the homo-cou-
pling of vinylboronates.2a Under such conditions, the
above process leads to the exclusive formation of boryl
ethers, without any vinylboronate homo-coupling prod-
ucts (Table 2).
SYNLETT 2009, No. 15, pp 2433–2436
x
x
.x
x
.2
0
0
9
Advanced online publication: 27.08.2009
DOI: 10.1055/s-0029-1217732; Art ID: G18409ST
© Georg Thieme Verlag Stuttgart · New York

2434
B. Marciniec, J. Walkowiak
LETTER
O
O
B
O
O
O
[Ru(CO)ClH(PCy3)2]
– ethylene
B
O
R
H
O
B
+
+
R
O
B
O
O
(A)
(B)
Equation 3
Table 1 O-Borylation of Alcohols with 2-Vinyl-1,3-dioxaborinane,
Catalyzed by [Ru(CO)ClH(PCy3)2]
Table 2 O-Borylation of Alcohols with 2-Vinyl-1,3-dioxaborinane
Catalyzed by [Ru(CO)ClH(PPh3)3]
Entry
Alcohola
Conversion of Selectivity A/B
Entry ROH
Molar ratio Conversion Isolated
[ROH]:[B]a of alcohol yield
alcohol (%)b
(%)b
(%)c
100
100
96
(%)
1
2
100
83:17
OH
OH
1
2
3
4
1:3
OH
82
85
56
76:24
82:18
88:12
1:2
78
1:2b
1:1.5
3
4
93
OH
OH
5
1:3
100
71
OH
O
6
7
8
1:3
1:3
1:3
98
84
0
75
–
a Reaction conditions: [Ru]:[ROH]:[B] = 2 × 10–2:1:3, toluene
(0.5M), 24 h, 80 °C; [Ru] = [Ru(CO)ClH(PCy3)2].
b Determined by GC and GC-MS.
OH
OH
–
OH
In some cases, O-borylation of alcohols by vinylboronates
proceeded quantitatively and many of the boryl ethers
were isolated and characterized spectroscopically (1H
NMR, 13C NMR, 11B NMR, GC-MS); unisolated com-
9
1:3
100
67
OH
10
1:2
99
98
87
pounds were identified by GC-MS spectroscopy.14 It is in- 11
teresting to note that, in the reaction of vinylboronate with
1:2b
1:1.5
–
–
12
pent-4-en-1-ol (entry 7), the product of O-borylation was
obtained exclusively and no trace of the trans-alkene bo-
rylation product was observed.
13
1:5
82
58
OH
The same reaction with allyl alcohol (entry 8) did not pro-
ceed at all, presumably because the strong simultaneous
p-allyl and O–H interaction of the substrate with the ru-
thenium center blocks the subsequent possible reactions
14
1:3
70
53
–
15
1:3b
of trans- vs. O-borylation.
16
1:5
88
61
OH
On the basis of the results obtained here and on our previ-
ous reports on Ru-H catalyzed metallation of alkenes,
alkynes and silanols by vinylsilanes, vinylgermanes and
vinylboronates, the insertion–elimination mechanism of
catalysis is also proposed for this new process. While the
insertion of vinylboronates into ruthenium–hydride bond
has already been reported,2a in order to provide evidence
for the second-half of the catalytic cycle, a stoichio-
metric reaction of alcohol with the ruthenium–boryl
complex was examined. Thus, the reaction of equimolar
17
18
1:3
76
72
–
1:3b
OH
19
1:5
54
O
20
1:3
48
OH
21
1:3
100
73
NC
amounts
of
the
ruthenium–boryl
complex
a Reaction conditions: [Ru-H]:[ROH] = 2 × 10–2:1, toluene (0.5 M),
80 °C; [Ru-H] = [Ru(CO)ClH(PPh3)3].
[Ru(BO2C6H4)(CO)Cl(PCy3)2] (III) (prepared according
to the previously reported procedure2,15) and cyclohex-
anol, was carried out at 60 °C and the progress of the re-
action was monitored by 1H NMR and GC-MS.16
b Reaction conducted at 60 °C.
c Determined by GC and GC-MS.
Synlett 2009, No. 15, 2433–2436 © Thieme Stuttgart · New York

LETTER
O-Borylation of Alcohols with Vinylboronates
2435
Figure 1 1H NMR spectrum of the stoichiometric reaction of complex (III) with cyclohexan-1-ol: a) after 2 h; b) after 24 h.
After 24 hours, the 1H NMR spectra confirmed the forma-
tion of the Ru-H complex (triplet signal d = –24.3 ppm,
JH–P = 20.3 Hz) of high intensity and a still weak triplet at
d = –5.09 ppm (Figure 1b).
O
B
O
O
[RuII]-H
R'
B
O
O
While the former signal provides evidence for recovery of
the Ru-H complex (I), the latter can be attributed to the
Ru(IV)-H bond of the intermediate formed by the oxida-
tive addition of the alcohol to Ru–B bond [with formation
of (RO)Ru(IV)–H bond].
H
O
O
H
H
[RuIV
]
[RuII]
B
O
B
R'O
H
H
O
During the stoichiometric reaction, the color of the reac-
tion mixture changed from pale-yellow [the ruthenium–
boryl complex (III)], through blue [Ru(IV)-H complex,
after 2 h], to yellow-greenish [Ru(II)-H (I), after 24 h].
O
[RuII]–
B
ethylene
R'OH
O
Scheme 1 Proposed mechanism for the O-borylation reaction of
According to the spectroscopic literature data on hydride
complexes of ruthenium, including also alkoxy and aryl-
oxy ligands,17 higher oxidation states of ruthenium in such
hydride complexes lead to an increase in the value of 1H
NMR chemical shifts of the corresponding proton. Elec-
tron-accepting alkoxy substituents in Ru(IV)-H complex-
es are also responsible for downfield-shifting the Ru-H 1H
NMR signal.
alcohols with vinylbronates
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
Therefore, the stoichiometric reaction provides convinc-
ing evidence for the proposed mechanism of oxidative ad-
dition of the alcohol to ruthenium–boron bonds, followed
by reductive elimination of the boryl ether and regenera-
tion of the Ru-H complex (Scheme 1).
Financial support from the Ministry of Science and Higher Educa-
tion (Poland); Grant PBZ KBN 118/T09/17 is gratefully acknowl-
edged. J. W. wishes to acknowledge a grant from the Operational
Programme for Human Resources, Action 8.2., co-financed by the
EU European Social Fund and the Polish State.
In conclusion, we have developed a new catalytic route
for O-borylation of alcohols with vinylboronates in which
vinylboronate acts as both borylating agent and hydrogen
acceptor, to form boryl ether with evolution of ethylene.
Further applications of this protocol to the borylation
(metallation) of other heteroatom–hydrogen bonds are un-
der investigation.
References and Notes
(1) For recent reviews, see: (a) Marciniec, B. Acc. Chem. Res.
2007, 40, 943. (b) Marciniec, B. Coord. Chem. Rev. 2005,
249, 2374.
Synlett 2009, No. 15, 2433–2436 © Thieme Stuttgart · New York

2436
B. Marciniec, J. Walkowiak
LETTER
(2) (a) Marciniec, B.; Jankowska, M.; Pietraszuk, C. Chem.
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Dudziec, B. Organometallics 2007, 26, 5188.
2-vinyl-1,3,2-dioxaborinane (1–2.5 mmol) were added and
the ampoule was heated at 60–80 °C for 24 h. Alcohol
conversion was determined by GC and GC-MS. After the
reaction, the solvent and excess borane were removed under
vacuum, and the crude product was purified by column
chromatography (silica gel modified with HMDS; hexane–
EtOAc).
2-Butoxy-1,3-dioxaborinane: 1H NMR (300 MHz, CDCl3):
d = 1.10 (t, J = 7.2 Hz, 3H, CH3), 1.78 (q, J = 7.0 Hz, 2H,
CH2CH3), 1.92 (quin, J = 5.5 Hz, 2H, BOCH2CH2CH2O),
2.16 (brH2CH2CH3), 3.85 [t, J = 4.6 Hz, 2H, CH2 (CH2)2CH3],
4.03 (t, J = 5.5 Hz, 4H, BOCH2CH2CH2O). 13C NMR (75
MHz, CDCl3): d = 15.8 (CH3), 20.3 (CH2CH3), 27.2
(BOCH2CH2CH2O), 33.2 (CH2CH2CH3), 62.7
(c) Marciniec, B.; Ławicka, H.; Dudziec, B. J. Organomet.
Chem. 2008, 693, 235. (d) Dudziec, B.; Marciniec, B.
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[M+ – 15], 129 (2), 115 (100), 103 (29), 85 (22), 71 (11), 56
(14). Anal. Calcd for C7H15BO3: C, 53.21; H, 9.57. Found:
C, 53.01; H, 9.44.
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(14) O-Borylation of alcohols by vinylboronates; general
procedure: The ruthenium catalyst [Ru(CO)ClH(PPh3)3]
(II) (2 mol%) was dissolved in toluene and placed in a glass
ampoule. The reagents and dodecane as internal standard
(5% by volume all components), alcohol (0.5 mmol) and
3-(1¢,3¢,2¢-Dioxaborinan-2¢-yloxy)propanonitrile: 1H
NMR (300 MHz, CDCl3): d = 1.99 (quin, J = 5.5 Hz, 2H,
BOCH2CH2CH2O), 2.60 (t, J = 6.0 Hz, 2H, CH2C≡N), 3.89
(t, J = 6.0 Hz, 2H, CH2CH2C≡N), 4.03 (t, J = 5.5 Hz, 4H,
BOCH2CH2CH2O). 13C NMR (75 MHz, CDCl3): d =
26.3 (CH2C≡N), 27.1 (BOCH2CH2CH2O), 57.9
(BOCH2CH2C≡N), 62.8 (BOCH2CH2CH2O), 117.6 (C≡N).
11B NMR (96 MHz, CDCl3): d = 28.7. MS (EI): m/z (%) =
156 (6) [M+ + 1], 140 (7), 125 (10), 115 (100), 98 (25), 85
(23), 71 (10), 54 (28). Anal. Calcd for C6H10BNO3: C, 58.74;
H, 9.31; N, 9.04. Found: C, 58.88; H, 9.23; N, 9.18.
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(16) Experimental procedure for the stoichiometric
experiment: In an NMR tube,
[Ru(BO2C6H4)(CO)Cl(PCy3)2] (III) (10 mg, 0.012 mmol)
and cyclohexan-1-ol (1.6 mg, 0.016 mmol) and toluene-d8
(0.6 mL) were placed under argon. The reaction was carried
out at 80 °C and monitored by 1H NMR.
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