Communications
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[15] For further details on our optimization study see the Supporting
Information.
um(IV) intermediate D, which would undergo fast reductive
elimination to give E.[26] The pinBPdOAc formed would then
eliminate pinBOAc with formation of Pd0. Although we favor
the pathway via PdII/Pd0 we cannot rule out a PdIV/PdII
pathway at present.[27]
In conclusion, we have developed a palladium(II)-cata-
lyzed oxidative borylative carbocyclization of enallene com-
pounds. The protocol gives alkylboronates in good yields and
requires only minimal amounts of reactants. Formally this
reaction constitutes a 1,2-carboborylation of olefins under
oxidative conditions and it occurs with cis addition to the
olefin. We believe that this approach can be applied to other
similar palladium(II)-catalyzed oxidation processes. Current
studies are focused on extending the use of diboronates in
palladium-catalyzed oxidations with the purpose to quench
organopalladium intermediates.
Experimental Section
Catalytic oxidative carboborylation: Pd(OAc)2 (0.45 mg, 0.002 mmol,
1 mol%), B2pin2 (51 mg, 0.20 mmol), and BQ (26 mg, 0.24 mmol)
were added to a solution of enallene 1a (0.1m, 56 mg, 0.20 mmol) in
toluene. The reaction mixture was then heated at 408C for 10 h. The
solvent was evaporated and the residue was purified by flash column
chromatography on silica gel (eluent: pentane/diethyl ether (v/v)
50:1) to give the alkylboronate 2a as a white solid (62 mg, 77%).
1H NMR (CDCl3, 400 MHz): d = 5.60 (d, J = 2.6 Hz, 1H), 4.92 (m,
1H), 4.87 (brs, 1H), 3.70 (s, 3H), 3.67 (s, 3H), 3.54 (ddd, J = 7.1, 6.3,
2.6 Hz, 1H), 3.03 (m, 1H), 1.91 (s, 3H), 1.62 (m, 2H), 1.45–1.26 (m,
5H), 1.17 (s, 6H), 1.16 ppm (s, 6H). 13C NMR (CDCl3, 100 MHz): d =
170.8, 170.3, 151.8, 140.7, 123.6, 114.1, 82.8 (2C), 68.5, 52.5, 52.0, 46.1,
43.7, 24.8 (2C), 24.6 (2C), 23.2, 23.0, 22.8, 22.3 ppm. HRMS (ESI): m/
z calcd for C22H33BO6Na [M+Na] + : 427.2262; found: 427.2267.
Oxidation of borylated carbocycle 2a: H2O2 (35% aq, 0.64 mmol,
56 mL), and NaOH (3m aq, 0.39 mmol, 0.13 mL) were added to a
solution of alkylboronate 2a (52 mg, 0.13 mmol) in THF (3 mL). The
reaction mixture was stirred at RT for 3 h and was subsequently
extracted with CH2Cl2 (3 ꢁ 15 mL). The organic layer was washed by
brine, dried over MgSO4, and concentrated. The residue was purified
by flash column chromatography on silica gel (eluent: pentane/ethyl
acetate (v/v) 5:1) to give the product 4 as a white solid (32 mg, 84%).
1H NMR (CDCl3, 400 MHz): d = 5.85 (brs, 1H), 5.14 (brs, 1H), 5.10
(brs, 1H), 4.15 (brs, 1H), 3.76 (s, 3H), 3.74 (s, 3H), 3.22 (m, 1H), 3.12
(dd, J = 8.5, 4.0 Hz, 1H), 2.46 (d, J = 5.0 Hz, 1H), 1.96 (s, 3H), 1.99–
1.92 (m, 1H), 1.87–1.83 (m, 1H), 1.74–1.65 (m, 1H), 1.46–1.36 (m,
2H), 1.34–1.28 ppm (m, 1H). 13C NMR (CDCl3, 100 MHz): d = 172.4,
171.0, 149.1, 137.7, 127.0, 115.7, 68.2, 65.2, 52.9 (2C), 48.2, 41.2, 30.6,
23.1, 20.8, 15.6 ppm. HRMS (ESI): m/z calcd for C16H22O5Na
[M+Na]+: 317.1359; found: 317.1358.
Received: December 20, 2010
Revised: April 7, 2011
Published online: May 17, 2011
Keywords: borylation · carbocyclization · enallenes · oxidation ·
.
palladium
[16] The reactions can be run in air and solvents can be used directly
without any purification or drying.
[17] Formation of products arising from palladium(0) catalysis would
indicate a slow reoxidation process. In these cases 1.2 equivalent
of BQ seems sufficient to avoid this pathway.
[1] For reviews involving carbocyclization reactions, see: a) E. M.
Beccalli, G. Broggini, M. Martinelli, S. Sotocarnola, Chem. Rev.
6158
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Angew. Chem. Int. Ed. 2011, 50, 6155 –6159