Communication
In conclusion, we have developed a palladium-catalyzed in-
termolecular C(sp3)–C(sp) cross-coupling of diarylmethyl carbo-
nates and terminal alkynes. The reaction proceeds smoothly
under external base-free conditions, and CO2 and MeOH are
thus the sole byproducts. Moreover, the stereoinvertive, enan-
tiospecific reaction with optically active carbonates can pro-
vide unique access to the chiral alkynyl-substituted diarylme-
thanes, which are difficult to prepare by other means. Further
studies on related decarboxylative couplings are currently un-
derway and will be reported in due course.
Experimental Section
Typical procedure for Pd-catalyzed coupling of methyl car-
bonates and terminal alkynes
The synthesis of 3aa is representative (Table 1, entry 13): Pd(OAc)2
(5.6 mg, 0.025 mmol) and dppb (21.3 mg, 0.050 mmol) were placed
in a 20 mL two neck flask, which was filled with nitrogen. 1,4-Diox-
ane (2.0 mL) was added to the flask, and suspension was stirred for
10 min. A solution of methyl (2-naphthyl)(phenyl)methyl carbonate
(1a; 146.2 mg, 0.50 mmol) and triisopropylsilylacetylene (2a;
45.6 mg, 0.25 mmol) in 1,4-dioxane (1.0 mL) was then added to the
flask, and the suspension was stirred for 6 h at 1208C. The resulting
mixture was quenched with water (20 mL) and then extracted
three times with ethyl acetate (20 mL x 3). The combined organic
layer was dried over sodium sulfate. Concentration in vacuo and
subsequent purification by column chromatography on silica gel
with hexane as an eluent gave triisopropyl[3-(naphthalen-2-yl)-3-
phenylprop-1-yn-1-yl]silane (3aa; 77.7 mg, 0.19 mmol) in 78%
yield.
Scheme 4. Enantiospecific cross-coupling of enantioenriched diarylmethyl
carbonates.
lyst (Cp = cyclopentadienyl),[15] the enantiospecific cross-cou-
pling of (S)-1a and 2a occurred with inversion of configuration
to form the optically active alkynylated product (R)-3aa in
84% yield with 88:12e.r.[16] Other silylacetylenes 2b and 2c
also took part in the reaction, and the enantioenriched cross-
coupling products (R)-3ab and (R)-3ac were obtained with
91:9 and 89:11e.r., respectively. The methoxy- and chloro-sub-
stituted diaryl methyl carbonates (S)-1b and (S)-1d underwent
the same chirality transfer to afford (R)-3bb and (R)-3 db with
similar levels of stereochemical fidelity. Although similar stereo-
specificity is observed in the nickel-catalyzed cross-coupling of
optically active diarylmethanol derivatives with Mg, Zn, and B-
based sp3 and sp2 carbon nucleophiles,[17] application to sp
carbon nucleophiles still remains underdeveloped. Thus, the
present stereospecific palladium catalysis can provide a unique
approach to the alkynylated chiral tertiary stereocenters, which
are relatively difficult to construct by conventional methods.[18]
On the other hand, from the mechanistic point of view, the ob-
served stereochemical outcome suggests the stereoinvertive
SN2-like oxidative addition and stereoretentive reductive elimi-
nation pathways (Scheme 1).[19] Additionally, while preliminary,
the catalytic stereoinduction in the reaction of racemic 1a’
with 2a was also achieved with a Pd(OAc)2/(S,S)-BDPP catalyst
system (Scheme 5).[20] Further mechanistic studies on the ste-
reochemical course and evaluations of chiral ligands for the
asymmetric catalysis are ongoing in our laboratory.
Acknowledgements
This work was supported by JSPS KAKENHI Grant Nos.
15K13696 (Grant-in-Aid for Exploratory Research) and
15H05485 (Grant-in-Aid for Young Scientists (A)) to K.H. and
24225002 (Grant-in-Aid for Scientific Research (S)) to M.M.
Keywords: carbonates
· cross-coupling · decarboxylative
coupling · palladium · stereospecificity
[1] a) A. de Meijere, F. Diederich, Metal-Catalyzed Cross-Coupling Reactions,
2nd ed., Wiley-VCH, Weinheim, 2004; b) J. Tsuji, Palladium Reagents and
Catalysts, 2nd ed., Wiley, Chichester, 2004; c) N. Miyaura, Cross-Coupling
Reactions: Top. Curr. Chem. 219, Springer, Berlin, 2002; d) J.-P. Corbet, G.
[2] For selected reviews and accounts, see: a) D. Alberico, M. E. Scott, M.
2007, 36, 200; c) L. C. Campeau, D. R. Stuart, K. Fagnou, Aldrichimica
Synthesis 2009, 4087; j) X. Chen, K. M. Engle, D.-H. Wang, J.-Q. Yu,
k) L. Ackermann, R. Vicente, A. R. Kapdi, Angew. Chem. Int. Ed. 2009, 48,
Scheme 5. Catalytic enantioselective cross-coupling of racemic diarylmethyl
carbonate under Pd(OAc)2/(S,S)-BDPP catalysis.
Chem. Eur. J. 2015, 21, 16823 – 16827
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