affords the corresponding 1,4-diketones. The involvement of
the water free potassium phenyltrifluoroborate in the reaction
indicates that the alcohol can act as a proton donor. Although
alcohols are weak acids compared to water, the protonation
occurs efficiently. The reaction rates obtained with potassium
phenyltrifluoroborates and with phenylboronic acid are
similar indicating the involvement of a probable identical
intermediate in the rate limiting step of the reaction.
Experimental
For a typical catalytic run
A 50 mL stainless steel autoclave equipped with a magnetic
stirrer bar was charged with potassium phenyltrifluoroborate
(1.5 mmol), [Rh(cod)Cl]2 (0.0075 mmol; 1% Rh) and PPh3
(0.03 mmol). The solids were degassed by three vacuum
nitrogen cycles. A mixture of methyl vinyl ketone (3 mmol)
and MeOH (10 mL; distilled over Mg) was thus added
under nitrogen. The autoclave was pressurized to 20 bar with
CO and the mixture was agitated and warmed at 80 1C for
18 h. After cooling to room temperature, the reactor was
vented and 145 mL of undecane used as internal standard
were added to the yellow methanol solution before analysis
by GC.
Fig. 1 Reaction profiles obtained with phenylboronic acid and
potassium phenyltrifluoroborate at 80 1C (1.5 mmol of boron deriva-
tive, 3 mmol of methyl vinyl ketone, 0.25% [Rh(cod)Cl]2, 1.5% PPh3,
20 bar CO, 50 mL MeOH).
contrast more difficult to carbonylate and a large amount of
the non-carbonylated product 4 is for example obtained from
potassium o-tolyltrifluoroborate. This by-product formation
largely accounts for the low yield in derivative 3; however, the
selectivity can be further improved by using higher carbon
monoxide pressures. The same trend is observed with phenyl
vinyl ketone (Table 3; entries 7 to 10) and methyl vinyl ketone
(Table 3; entries 1 to 6). The products obtained from phenyl
vinyl ketone are easily isolated as they are readily crystallized
from methanol upon cooling to room temperature whereas
those obtained from methyl vinyl ketone have to be purified by
silica gel column chromatography.
Notes and references
z To the best of our knowledge, there is no example in the literature of
carbonylation reactions involving potassium aryltrifluoroborate salts.
The kinetic profiles of the reaction with phenylboronic acid
and potassium phenyltrifluoroborate at 80 1C under 20 bar of
CO in 50 mL MeOH also showed a strong similarity (Fig. 1).
Although the reaction with the phenylboronic acid needs a
short induction period which is not observed with potassium
phenyltrifluoroborate, the overall reaction rates are identical
in both cases. This indicates that a common intermediate is
probably involved in the rate limiting step.
1 (a) S. Darses and J.-P. Genet, Eur. J. Org. Chem., 2003, 4313;
(b) G. A. Molander and N. Ellis, Acc. Chem. Res., 2007, 40, 275;
(c) H. A. Stefani, R. Cella and A. S. Vieira, Tetrahedron, 2007, 63,
3623; (d) S. Darses and J.-P. Genet, Chem. Rev., 2008, 108, 288.
2 (a) R. A. Batey, A. N. Thadani and D. V. Smil, Org. Lett., 1999, 1,
1683; (b) M. Pucheault, S. Darses and J.-P. Genet, Tetrahedron
Lett., 2002, 43, 6155; (c) M. Pucheault, S. Darses and J.-P. Genet,
Eur. J. Org. Chem., 2002, 3552; (d) L. Navarre, S. Darses and
J.-P. Genet, Eur. J. Org. Chem., 2004, 69.
3 J. M. Murphy, C. C. Tzschucke and J. F. Hartwig, Org. Lett.,
2007, 9, 757.
4 G. A. Molander and N. M. Ellis, J. Org. Chem., 2006, 71, 7491.
5 G. A. Molander and D. E. Petrillo, J. Am. Chem. Soc., 2006, 128,
9634.
Additional experiments indeed showed that the overall
kinetics of the reaction under these conditions is dependent
on the methyl vinyl ketone and rhodium concentrations but
not on the organoboron concentration, suggesting that the
transmetallation step is not rate determining. This is also
consistent with the fact that the two overall reaction rates
are similar. The same intermediate possessing a Rh–COAr
moiety is probably obtained from both organoboron reagents,
and the reaction of this intermediate with the enone is
suggested to be the slowest step of the catalytic cycle.
6 G. A. Molander and M. Ribagorda, J. Am. Chem. Soc., 2003, 125,
11148.
7 G. A. Molander and R. Figueroa, Org. Lett., 2006, 8(1), 75.
8 M. Sauthier, Y. Castanet and A. Mortreux, Chem. Commun., 2004,
1520.
9 H. Chochois, M. Sauthier, E. Maerten, Y. Castanet and
A. Mortreux, Tetrahedron, 2006, 62, 11740.
¨
10 (a) O. Aksin, N. Dege, L. Artok, H. Turkmen and B. C
¸
etinkaya,
¨
Chem. Commun., 2006, 3187; (b) J. Dheur, M. Sauthier,
Y. Castanet and A. Mortreux, Adv. Synth. Catal., 2007, 349(16),
We have found that potassium aryltrifluoroborates can be
efficiently used in the 1,4-carbonylative addition of organo-
boron derivatives to methyl vinyl ketone and phenyl vinyl
ketone. The reaction proceeds with good yields and efficiently
¨
2499; M. Ku-s, O. A. Artok, F. Ziyanak and L. Artok, Synlett,
2008, 2587.
11 F. Menard, C. F. Weise and M. Lautens, Org. Lett., 2007, 9, 5365.
ꢀc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009
New J. Chem., 2009, 33, 969–971 | 971