Ruthenium-Catalyzed Monoalkenylation of Aromatic Ketones by Cleavage of Carbon-Heteroatom Bonds with Unconventional Chemoselectivity

Article abstract of DOI:10.1002/anie.201503641

Ruthenium-catalyzed selective monoalkenylation of ortho C-O or C-N bonds of aromatic ketones was achieved. The reaction allowed the direct comparison of the relative reactivities of the cleavage of different carbon-heteroatom bonds, thus suggesting an unc

Full text of DOI:10.1002/anie.201503641

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201503641
German Edition: DOI: 10.1002/ange.201503641
Synthetic Methods
Ruthenium-Catalyzed Monoalkenylation of Aromatic Ketones by
Cleavage of Carbon–Heteroatom Bonds with Unconventional
Chemoselectivity**
Hikaru Kondo, Nana Akiba, Takuya Kochi, and Fumitoshi Kakiuchi*
Abstract: Ruthenium-catalyzed selective monoalkenylation of
À
À
ortho C O or C N bonds of aromatic ketones was achieved.
The reaction allowed the direct comparison of the relative
reactivities of the cleavage of different carbon-heteroatom
bonds, thus suggesting an unconventional chemoselectivity,
where smaller, more-electron-donating groups are more easily
À
cleaved. Selective monofunctionalization of C O bonds in the
À
presence of ortho C H bonds was also achieved.
T
ransition-metal-catalyzed carbon–carbon bond-formation
by selective cleavage of unreactive bonds has been exten-
sively investigated because it allows transformations with
unconventional chemoselectivity.[1,2] Our group has reported
ruthenium-catalyzed functionalizations of aromatic ketones
with organoboronates through the cleavage of carbon–hydro-
gen and carbon–heteroatom bonds by oxidative addition.[3–8]
In these reactions, simple carbonyl groups, which are useful
for further transformations, effectively function as directing
groups by forming a chelate and selectively convert the
unreactive bonds at the ortho-positions of the aryl ring into
carbon–carbon bonds. In many cases, however, selective
monofunctionalization is difficult for substrates bearing more
than one ortho carbon–hydrogen or carbon–oxygen bond.[9]
For example, the reaction of 2’,6’-dimethoxyacetophenone (1)
with arylboronates in the presence of a catalytic amount of
[RuH2(CO)(PPh3)3] gave the diarylation products as major
À
Scheme 1. Ruthenium-catalyzed C O functionalization of aromatic
À
ketones possessing multiple ortho C O bonds.
À
products, and it is indicated that the second C O bond
cleavage is faster than the dissociation of the monoarylation
product from the catalyst (Scheme 1a).[5a]
Herein we report that selective monoalkenylation of
ortho carbon–heteroatom bonds of aromatic ketones was
achieved for the ruthenium-catalyzed reaction with alkenyl-
boronates (Scheme 1b). This work allowed direct comparison
of the relative ease of the cleavage of different carbon–
heteroatom bonds, and it is suggested that smaller, more-
electron-donating groups are more easily cleaved (Sche-
me 1c), a phenomenon which is unconventional for reactions
proceeding by oxidative addition to transition metals. Selec-
À
tive monofunctionalization of C O bonds in the presence of
À
ortho C H bonds is also found to be possible for benzophe-
none derivatives (Scheme 1d).
À
First, we examined the ruthenium-catalyzed C O alke-
nylation[5a,11] of 1 with the styrylboronate 2a (Table 1). When
the reaction of 1 was performed with 2 equivalents of 2a in
the presence of 4 mol% of [RuH2(CO)(PPh3)3] at 1408C in p-
[*] H. Kondo, N. Akiba, Dr. T. Kochi, Prof. Dr. F. Kakiuchi
Department of Chemistry, Faculty of Science and Technology
Keio University
À
xylene for 8 hours, alkenylation of the C O bond proceeded
to give the monoalkenylation product 3a in 57% yield
(NMR) along with 6% yield (NMR) of dialkenylation
product 4a (entry 1). The observed selectivity toward the
3-14-1 Hiyoshi, Kohoku-ku, Yokohama, 223-8522 (Japan)
E-mail: kakiuchi@chem.keio.ac.jp
Prof. Dr. F. Kakiuchi
JST, ACT-C, Kawaguchi, Saitama, 332-0012 (Japan)
À
monoalkenylation is in contrast with the C O arylation of
1,[5a] which mainly affords diarylation products (Scheme 1a).
Our group reported that the reaction of 2’-methoxyacetophe-
[**] This work was supported, in part, by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science
and Technology (Japan), JST, ACT-C, and the Asahi Glass Founda-
tion.
À
À
none, which has both ortho C H and C O bonds, with
alkenylboronates in the presence of a catalytic amount of
À
[RuH2(CO)(PPh3)3] provides the C H alkenylation product
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503641.
selectively, and C O alkenylation was not observed.[4c] In this
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Table 1: Ruthenium-catalyzed monoselective C O alkenylation of the
acetophenone derivative 1 with 2a.[a]
Entry Ru cat.
Additive Conv. [%][b] Yield [%][b]
3a
4a
1
2
[RuH2(CO)(PPh3)3]
–
–
63
85
57
6
80
5
3
4
5
6
7
8
9
10
[RuCl(TMS)(CO)(PPh3)2]
[RuH(OAc)(CO)(PPh3)2] (6)
[RuH2(CO)(PPh3)3]
5
–
–
36
72
61
63
82
90
35
69
55
58
76
83
80
81
1
3
5
3
6
7
6
5
À
Scheme 2. Ruthenium-catalyzed monoselective C O alkenylation of
CsF
CsF
CsF
CsF
Cs2CO3 87
K2CO3 88
1 with alkenylboronates (2). Reaction conditions: 1 (0.5 mmol), 2
(1.0 mmol), 6 (0.02 mmol), CsF (0.04 mmol), p-xylene (0.5 mL),
1408C, 8 h. Yields of the isolated products are shown. [a] Performed
for 24 h. [b] Performed for 40 h. [c] Styrene (2 equiv) was used as an
additive. [d] E/Z=91:9.
[RuCl(TMS)(CO)(PPh3)2]
6
6
6
[a] Reaction conditions: 1 (0.5 mmol), 2a (1.0 mmol), Ru cat.
(0.02 mmol), additive (0.04 mmol), p-xylene (0.5 mL), 1408C, 8 h.
[b] Determined by 1H NMR analysis. TMS=trimethylsilyl.
a longer reaction time but provided the monoalkenylation
product 3 f in 52% yield. The reaction with alkyl-substituted
alkenylboronates gave high yields of the monoalkenylation
products 3g–i when styrene was used as an additive.[13]
The ruthenium-catalyzed alkenylation was also conducted
À
case, coordination of the alkenyl group from the C H
alkenylation product is thought to disrupt the interaction of
À
with the anthrone derivative 7, which possesses an ortho C O
À
the C O bond with the metal center. In the alkenylation of
bond on each of the two aromatic rings (Scheme 3). The
1 with 2a, therefore, similar coordination of the alkenyl group
À
in 3a may prevent the second C O bond cleavage (Table 1).
Screening of the catalyst was then examined. The reaction
À
with the catalyst 5, which is effective in the C H-selective
alkenylation of 2’-methoxyacetophenone, gave 3a in 80%
yield (entry 2). In search of more convenient catalysts, the
reaction with other carbonyl ruthenium(II) complexes having
two triphenylphosphines ligands[12] was examined, and the use
of [RuCl(TMS)(CO)(PPh3)2] and [RuH(OAc)(CO)(PPh3)2]
(6) was found to give 3a in 35 and 69% yields, respectively
(entries 3 and 4). Additives were then investigated for the
À
C O monoalkenylation, and addition of CsF was found to be
effective for some catalysts.[4e] While the use of CsF as an
additive did not improve the product yield (entries 5 and 6),
increase of the yield was observed for the reaction in the
presence of [RuCl(TMS)(CO)(PPh3)2] and 6 (entries 7 and 8).
Particularly the highest yield of 83% was obtained for the
reaction using 6 and CsF (entry 8). Other additives such as
Cs2CO3 and K2CO3 also gave 3a in high yields but did not
exceed the yield achieved with CsF.
À
Scheme 3. Ruthenium-catalyzed monoselective C O alkenylation of
the anthrone derivative 7 with the alkenylboronates 2.
reaction with 2a proceeded to give the monoalkenylation
product 8a in 68% yield upon isolation.[14,15] Similarly, the
reaction with the methoxy-substituted styrylboronate 2b
afforded 8b in 65% yield within 5 hours. Higher product
selectivity was observed for the reaction with the alkenylbor-
onate 2e to provide 8e and the dialkenylation product 9e in
61 and 11% yields, respectively.
À
The C O monoalkenylation products formed using var-
ious alkenylboronates can be isolated in high yields
(Scheme 2). In addition to 3a, b-styrylboronates having
electron-donating groups such as methoxy and methyl
groups, and electron-withdrawing groups such as chloro and
trifluoromethyl groups were reacted with 1 to afford the
corresponding monoalkenylation products 3b–e in yields of
72–75%. The reaction with a-styrylboronate required
À
The high selectivity of the C O alkenylation to give
monoalkenylation products allows direct comparison of the
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Table 2: Ruthenium-catalyzed chemoselective C O alkenylation of the
tigate the reactivity of substrates with two different alkoxy
acetophenone derivatives 10 with 2a.[a]
leaving groups having different basicities, the reaction was
conducted with an acetophenone having methoxy and 2,2,2-
trifluoroethoxy groups (10g), and the major product was the
monoalkenylation product formed by cleaving the more-
electron-donating methoxy group (11g; entry 9). The reaction
of a substrate with ethoxy and 2,2,2-trifluoroethoxy groups,
which have similar steric bulk but different electron-donating
À
ability, also gave the alkenylation product arising from Ar
OEt bond cleavage as the major product (entry 10).
Entry 10
R1O R2O
Conv. [%] Yield [%]
À
The results of the C O alkenylation with acetophenone
11
12
derivatives having two different oxygen functional groups
suggest that smaller, more-electron-donating groups are more
easily cleaved in this reaction. The lower reactivity of the
bonds with better leaving groups is not commonly observed
for bond-cleavage reactions by oxidative addition. The reason
for the observed trend is unclear, but it can be explained by
considering that smaller, more-electron-donating groups
coordinate to the metal center more easily to increase the
chances of being cleaved by oxidative addition, as observed
1
10a MeO PhO
10a MeO PhO
72[b]
85[b]
79[b]
84[b]
55[b] (11a) 7[b] (12a)
71[b] (11a) 8[b] (12a)
59[b] (11b) 8[b] (12a)
70[b] (11c) 9[b] (12a)
66[b] (11d) 11[b] (12a)
50[e] (11e) 27[e] (12a)
68[e] (11 f) 5[e] (12a)
80[e] (11 f) 4[e] (12a)
61[e] (11g) 4[e] (12a)
45[e] (11g) 13[e] (12b)
2[c]
3[c]
4[c]
5[c]
6
10b MeO 4-CF3C6H4O
10c MeO 4-MeC6H4O
10d MeO 4-MeOC6H4O 82[b]
10e MeO EtO
82[d]
73[d]
84[d]
77[d]
62[d]
7
10 f MeO iPrO
10 f MeO iPrO
10g MeO CF3CH2O
10h EtO CF3CH2O
8[f]
9
10
À
for our previously reported aromatic C N bond cleavage by
a ruthenium center.[6c]
[a] Reaction conditions: 10 (0.5 mmol), 2a (1.0 mmol), 6 (0.02 mmol),
CsF (0.04 mmol), p-xylene (0.5 mL), 1408C, 8 h. [b] Determined by
1H NMR analysis. [c] Used 3 equiv of 2a, 6 mol% of 6, and 12 mol% of
CsF. [d] Determined based on the amount of 10 recovered by isolation.
[e] Yield of isolated product. [f] Performed for 24 h.
The alkenylation of an acetophenone derivative, possess-
ing both a methoxy and dimethylamino group (13), with 2a
proceeded to give the monoalkenylation product 3a in 90%
yield upon isolation (Scheme 4). The result essentially
À
relative reactivity of the unreactive C O bonds in this
reaction. The reaction of several acetophenone derivatives
possessing two different oxygen functional groups (10) at the
ortho positions were examined (Table 2). First, acetophenone
with methoxy and phenoxy groups (10a) were employed as
substrates for the reaction under the standard reaction
conditions (entry 1). Although phenoxide is less basic and
generally considered as a better leaving group than meth-
oxide, the ruthenium-catalyzed reaction gave the monoalke-
À
nylation product formed by Ar OMe bond cleavage (11a),
which still has a phenoxy group, as a major product. The yield
of 11a was improved to 71% by increasing the amounts of 6,
CsF, and 2a to 6 mol%, 12 mol%, and 3 equivalents,
respectively (entry 2). The reaction of 2’-methoxyacetophe-
nones possessing various aryloxy groups with para substitu-
ents (10b–d) also delivered the alkenylation products 11b–d
À
Scheme 4. Selective ruthenium-catalyzed C N alkenylation in the pres-
À
ence of an ortho C O bond.
showed that completely selective cleavage of the more-
electron-donating dimethyamino group over the methoxy
(resulting from Ar OMe bond cleavage) as major products in
group occurred in this reaction.[6a,12] The selective C N
À
À
À
59–70% yields (entries 3–5). The C O alkenylation was then
alkenylation was also observed for the reaction with the
alkenylboronates 2b and 2e to give 3b (73%) and 3e (65%),
respectively.
Another important feature of the C O monoalkenylation
is that it may allow the selective conversion of one C O bond
examined with acetophenones having two different alkoxy
groups. The reaction of an acetophenone derivative having
both methoxy and ethoxy groups (10e) proceeded to give the
À
À
À
À
alkenylation products, for both Ar OMe and Ar OEt bond
cleavage, in 50 and 27% yields, respectively (entry 6). In this
case, the larger size of ethoxy group relative to the methoxy
group appears to become an important factor in determining
the product selectivity, because methoxide and ethoxide have
in highly oxygenated aromatic compounds without damaging
À
À
other C O bonds, and even C H bonds at positions ortho to
carbonyl groups. Therefore, the reaction may be applied for
the synthesis of polyoxygenated aromatic compounds, which
are found in the structures of many naturally occurring
molecules.
The C O monoalkenylation was examined with 2,4,6-
trimethoxybenzophenenone derivatives (14; see Scheme 5),
which can be readily prepared in high yields from 1,3,5-
À
similar basicity. Higher product selectivity toward Ar OMe
bond cleavage was observed for the reaction with a substrate
having both methoxy and isopropoxy groups (10 f) to give 11 f
in 68% yield (entry 7). The yield of 11 f was improved to 80%
by performing the reaction for 24 hours (entry 8). To inves-
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Please note: Minor changes have been made to this manuscript since
its publication in Angewandte Chemie Early View. The Editor.
trimethoxybenzene and benzoyl chloride derivatives. The
reaction of 2,4,6-trimethoxybenzophenenone (14a), which
À
À
has two C O bonds and two C H bonds at positions ortho to
the carbonyl group, with 2a in the presence of 6, CsF, and
Keywords: boron · chemoselectivity · ruthenium ·
substituent effects · synthetic methods
À
styrene gave the corresponding C O monoalkenylation
À
product 15a in 60% yield along with 22% of the C O
How to cite: Angew. Chem. Int. Ed. 2015, 54, 9293–9297
Angew. Chem. 2015, 127, 9425–9429
dialkenylation product 16a (Scheme 5).[16] The reaction of the
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[1] Representative reviews of C H functionalization: a) V. Ritleng,
C. Sirlin, M. Pfeffer, Chem. Rev. 2002, 102, 1731; b) F. Kakiuchi,
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[2] Representative reviews and accounts of C O functionalization:
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À
[4] Related examples of our [RuH2(CO)(PPh3)3]-catalyzed C H
functionalization: a) F. Kakiuchi, T. Kochi, E. Mizushima, S.
Murai, J. Am. Chem. Soc. 2010, 132, 17741; b) S. Hiroshima, D.
Matsumura, T. Kochi, F. Kakiuchi, Org. Lett. 2010, 12, 5318; c) S.
Ueno, T. Kochi, N. Chatani, F. Kakiuchi, Org. Lett. 2009, 11, 855;
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e) F. Kakiuchi, Y. Matsuura, S. Kan, N. Chatani, J. Am. Chem.
Soc. 2005, 127, 5936. See also Ref. [3].
À
Scheme 5. Selective ruthenium-catalyzed C O alkenylation in the pres-
À
ence of ortho C H bonds. [a] The reaction was performed in styrene/
À
p-xylene (1.2:1) at 1308C for 4 h. THF=tetrahydrofuran.
[5] C O functionalization: a) F. Kakiuchi, M. Usui, S. Ueno, N.
Chatani, S. Murai, J. Am. Chem. Soc. 2004, 126, 2706; b) S. Ueno,
E. Mizushima, N. Chatani, F. Kakiuchi, J. Am. Chem. Soc. 2006,
128, 16516.
pentamethoxybenzophenone 14b also provided 15b in 57%
yield. The use of the methoxy-substituted styrylboronate 2b
for the reaction with 14b gave 15c in 68% yield. This
alkenylated benzophenone derivative (15c) as well as its
reduction product 17, which was quantitatively obtained by
the reduction of 15c with LiBHEt3, have been used as
common intermediates in Snyder’s synthesis of various
resveratrol-based natural products.[17]
À
[6] C N functionalization: a) S. Ueno, N. Chatani, F. Kakiuchi, J.
Am. Chem. Soc. 2007, 129, 6098; b) T. Koreeda, T. Kochi, F.
Kakiuchi, J. Am. Chem. Soc. 2009, 131, 7238; c) T. Koreeda, T.
Kochi, F. Kakiuchi, Organometallics 2013, 32, 682; d) T. Kore-
eda, T. Kochi, F. Kakiuchi, J. Organomet. Chem. 2013, 741 – 742,
148.
À
[7] C F functionalization: K. Kawamoto, T. Kochi, M. Sato, E.
Mizushima, F. Kakiuchi, Tetrahedron Lett. 2011, 52, 5888.
In summary, we described the selective ruthenium-cata-
À
À
[8] Recently, Snieckus and co-workers reported on C H, C O, and
À
À
lyzed monoalkenylation of either ortho C O or C N bonds of
aromatic ketones. A novel catalyst system consisting of 6 and
CsF was identified as for this reaction and various alkenyl
groups were introduced to the aromatic rings. The reaction
enabled the direct comparison of the relative reactivities of
different carbon–heteroatom bonds, and it was suggested that
smaller, more-electron-donating groups are more easily
cleaved, a phenomenon which is unconventional for reactions
proceeding by oxidative addition. Exceedingly chemoselec-
tive C N alkenylation in the presence of an ortho C O bond
was also observed. Selective monofunctionalization of C O
bonds can also be achieved in the presence of ortho C H
bonds. Studies on the synthetic applications of the selective
monoalkenylation of carbon–heteroatom bonds are under-
way.
À
C N functionalizations of benzaminde derivatives using our
ruthenium catalyst systems: a) Y. Zhao, V. Snieckus, Adv. Synth.
Catal. 2014, 356, 1527; b) Y. Zhao, V. Snieckus, J. Am. Chem.
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16, 3200.
À
[9] Selected examples of C H monofunctionalizations: a) T. Satoh,
Y. Kawamura, M. Miura, M. Nomura, Angew. Chem. Int. Ed.
Engl. 1997, 36, 1740; Angew. Chem. 1997, 109, 1820; b) S. Oi, S.
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Chem. Soc. 2010, 132, 8270; i) B. Xiao, Y.-M. Li, Z.-J. Liu, H.-Y.
Yang, Y. Fu, Chem. Commun. 2012, 48, 4854; j) H.-X. Dai, J.-Q.
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also Ref. [4a,b].
[14] When the reaction of 7 with 4 equivalents of 2a was performed
for 36 h, the dialkenylation product 9a was obtained in 99%
yield.
[15] The lower monoselectivity for the reaction of 7 can be attributed
to the difference in the lone pairs of the carbonyl oxygen atom
À
À
[10] Catalytic C O alkenylation of ethers. Allyl ethers: a) R.
used for the C O bond cleavage. In the cleavage of the second
À
Matsubara, T. F. Jamison, J. Am. Chem. Soc. 2010, 132, 6880;
Benzyl ethers: b) M. R. Harris, M. O. Konev, E. R. Jarvo, J. Am.
Chem. Soc. 2014, 136, 7825; See also Ref. [5].
C O bond in 7, the lone pair of the carbonyl oxygen atom used
to bind to the metal is different from the one used for the second
À
C O bond cleavage and is further from the introduced alkene
À
À
[11] Higher reactivity of C N bonds than C O bonds in the
moiety, and thus may be less effective in disturbing the
À
ruthenium-catalyzed reactions has been indicated in Ref. [8c].
interaction of the second C O bond with the metal center.
À
À
[12] This C O alkenylation is considered to proceed by coordination
[16] To examine the tolerance of C Br bonds, 2-bromophenyl 2,4,6-
of the carbonyl oxygen atom of the substrate to the ruthenium,
trimethoxyphenyl ketone was used instead of 14a for the
À
À
oxidative addition of the C O bond, transmetalation, and
reductive elimination, which is a similar mechanism to the
reaction but did not give any C O alkenylation product.
[17] a) S. A. Snyder, A. L. Zografos, Y. Lin, Angew. Chem. Int. Ed.
2007, 46, 8186; Angew. Chem. 2007, 119, 8334; b) S. A. Snyder,
S. P. Breazzano, A. G. Ross, Y. Lin, A. L. Zografos, J. Am. Chem.
Soc. 2009, 131, 1753; c) S. A. Snyder, A. Gollner, M. I. Chiriac,
Nature 2011, 474, 461; d) S. A. Snyder, S. B. Thomas, A. C.
Mayer, S. P. Breazzano, Angew. Chem. Int. Ed. 2012, 51, 4080;
Angew. Chem. 2012, 124, 4156.
À
previsouly reported C O arylation. In the alkenylation, the
product may coordinate to the ruthenium through both the
carbonyl oxygen atom and the introduced alkene moiety, after
the reductive elimination. The strong binding of the product may
inhibit the coordination of the incoming substrate, and the
lowering of the equivalents of PPh3 ligands from three to two
may be effective for creating an open coordination site for the
substrate.
[13] The role of the styrene additive is unclear at this point, but this
additive may be effective in stabilizing the low-valent catalyti-
cally active species to prevent the catalyst decomposition.
Received: April 21, 2015
Published online: June 19, 2015
Angew. Chem. Int. Ed. 2015, 54, 9293 –9297
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 9297