C O M M U N I C A T I O N S
Table 1. Reaction of o-Methoxyacetophenone (10) with
respectively. A sequential double alkylation using 6 and benzyl-
boronate afforded 16 in 72% yield. The use of alkenylboronates
provided alkyl-alkenyl coupling products, such as 17 and 18, in
high yields. For the present coupling reaction, vinylsilanes were
highly reactive, but allyl(trimethyl)silane and phenyltrimethylsilyl
acetylene were less reactive under these reaction conditions.
We described herein the first direct observation of the oxidative
addition of an aryl carbon-oxygen bond in an aromatic ether to a
transition metal complex. The structure of 3 was determined from
1H and 31P NMR and X-ray data. C-H bond cleavage with the
ruthenium complex is the kinetically favored process, but aryl C-O
bond cleavage is the thermodynamically favorable one. Chemose-
lective C-C bond formation using vinylsilanes and organoboronates
can be achieved on the basis of the difference in the reactivity of
C-H and C-O bonds toward the ruthenium complex. Further
studies of the reactivity of these carbon bonds and the scope of the
reaction will be the subject of future work.
Vinylsilanes and Organoboronatesa
a Reaction conditions: 10 (0.5 mmol), vinylsilane (1 mmol), organobo-
ronate (1 mmol), 1 (0.04 mmol), toluene (0.5 mL), reflux.
Acknowledgment. This work was supported, in part, by
PRESTO, JST, and by a Grant-in-Aid for Scientific Research on
Priority Areas “Advanced Molecular Transformations of Carbon
Resources” from the Ministry of Education, Culture, Sports, Science
and Technology, Japan. S.U. acknowledges Research Fellowships
of J.S.P.S. for Young Scientist.
is noteworthy that, although the bond dissociation energy of a C-H
bond in benzene (BDE (C6H5-H) ) 113 kcal/mol) is larger than
that of the aryl C-O bond in anisole (BDE (C6H5-OCH3) ) 101
kcal/mol,14 C-H bond cleavage took place preferentially.
Interestingly, complex 8 remained unchanged, even after reaction
at room temperature for 2 days. When the reaction mixture was
heated at 80 °C for 3 h, 8 completely disappeared and a new deep-
red species 9 appeared at 33.16 ppm in the 31P NMR spectrum
which was almost the same as that of 3. These results indicate that
the hydride complex 8 is the kinetic product and the aryloxy
complex 9 is the thermodynamic one.
Supporting Information Available: X-ray crystallographic files
in CIF format for 3 and text providing experimental procedures and
spectroscopic data for all new compounds. This material is available
References
On the basis of this difference in reactivity between C-H and
C-O bonds, we attempted to develop highly chemoselective
selective sequential C-C bond formations using vinylsilanes and
organoboronates. Thus, alkylation should take place preferentially
at the C-H bond, and coupling with the organoboronate should
proceed at the C-O bond. When the reaction of 2′-methoxyac-
etophenone (10) with 6 and phenylboronate 4 was carried out in
refluxing toluene, alkylation at the C-H bond and a phenylation
at the C-O bond proceeded selectively to give 11 in 93% yield
(eq 2). Neither dialkylated nor diphenylated acetophenones were
produced. These observations indicate the following important
features of this coupling reaction: (1) the C-H/olefin coupling
reaction proceeds faster than the C-H/R-B(OR′)2 coupling because
phenylation of the C-H bond can also take place under these
reaction conditions;15 (2) once the carbon-oxygen bond is cleaved,
transmetalation between the Ru-OMe intermediate and the phen-
ylboronate occurred rapidly as evidenced by the fact that no product
formed via â-hydride elimination from the Ru-OCH3 intermediate
was observed.10
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