Chemistry Letters Vol.37, No.6 (2008)
641
Since the interface area should increase with increasing stirring
rate, the product ratio of the branched to linear product (R)
also increases. The catalyst activity in the interface should
be higher than that in water phase, since nucleophile exsit in
hexane phase.
Such a phenomenon displays a novel aspect in the chemical
reaction and raises the importance of mechanical mixing in
controlling the reaction selectivity of biphasic catalyses. Diverse
applications toward many chemical reactions is expected.
Further mechanistic studies are required to clarify the origin of
the present novel interface effect.
2
00 rpm
400 rpm
Water
750 rpm
Hexane
1500 rpm
Figure 2.
Nu
Nu
+
Hexane
Water
References and Notes
Water
Pd
1
a) Aqueous-Phase Organometallic Catalysis, Concepts and
Applications, ed. by B. Cornils, W. A. Herrmann, Wiley-
VCH, Weinheim, 1998, and references cited therein. b)
Applied Homogeneous Catalysis with Organometllic
Compounds, ed. by B. Cornils, W. A. Herrmann, VCH,
Weinheim, 1996, Vols. 1 and 2.
L
L
+
Pd Ln
-
Nu = HSPh or SPh
Figure 3.
reagent in the dark significantly inhibited this isomerization.
Thus, only minor apparent isomerization of 1,1-dimethylallyl
phenyl sulfide to 3-methyl-2-butenyl phenyl sufide was observed
under the reaction conditions.
In order to get further insights of this mechanism, apparent
mixing conditions of the reaction solution in the flask were vis-
ually analyzed (Figure 2). Under the slow stirring rate below
2
3
4
J. Tsuji, Palladium Reagents and Catalysts, Wiley, Chiches-
ter, 1995.
500 rpm, where low selectivity for the branched product was
observed, the clear interface of water and hexane was always
observed. However, when the stirring speed was raised more
than 750 rpm, small lumps of solvents started to appear. Under
the fast stirring conditions, more than 1000 rpm by our appara-
5
6
8
tus, two phases could not be apparently distinguished and
the system was filled with innumerable small solvent lumps.
This fact does not mean a difference of microscopic interface
structure at a molecular level, but simply indicates the increase
of interface area by mechanical stirring.
Typical procedure for catalytic allylation as follows. In
a 25-mL Schlenk tube, Pd(OAc)2 (4.5 mg, 0.020 mmol)
and trisodium salt of tris(m-sulfonatophenyl)phosphine
(TPPTS, 56.5 mg, 0.100 mmol) were placed under nitrogen
atmosphere. Degassed pure water (4 mL) was introduced,
A possible mechanism for this allylation is initial formation
of allylpalladium(II) intermediate, which is formed from allylic
ꢁ
and the mixture was stirred vigorously at 30 C for
0
alcohol and Pd species, followed by the reaction with nucleo-
several minutes, then the aqueous solution became a clear
yellow solution. Allylic alcohol (0.100 mL, 1.00 mmol)
was added via a syringe. After about 10 min stirring, ben-
zenethiol (0.100 mL, 1.00 mmol) and 2,6-di-tert-butylphenol
(0.0206 g, 0.1 mmol) in hexane (4 mL) were added. Biphenyl
(0.1545 g, 0.999 mmol) was added as an internal standard
to the solution. After stirring the organic layer was analyzed
by GLC.
phile such as thiol or thiolate anion to give corresponding
sulfide. We propose from the microscopic point of view that
the water-soluble allylpalladium(II) intermediate in the water/
hexane interface should have a highly oriented structure as
1
shown in Figure 3, where the allyl moiety exists as ꢀ -3-meth-
yl-2-butenyl (or 2-butenyl)palladium(II). Namely the more hy-
9
drophilic TPPTS ligand side of the allylpalladium intermediate
should stay in the water phase, but the hydrophobic allyl moiety
side is facing toward the hexane phase. The allyl ligand may
7
8
The reaction in the absence of 2,6-di-tert-butylphenol gave
a 65:35 mixture of 1,1-dimethylallyl phenyl sulfide and
1
2
exist in an ꢀ -fashion probably due to effective invasion to the
hexane phase or possible steric congestion by water solvation,
3-methyl-2-butenyl phenyl sulfide.
Reactions were performed in the test tube type Schlenk
flask, whose inside diameter is 2.0 cm, using a standard
1.5 cm long teflon-covered magnetic stirring bar with an
outside diameter of 0.5 cm.
Orientation of solvent molecules in the water/hydrophobic
interface is theoretically proposed to be well-organized and
have charged structure. Allylpalladium catalyst in the
and thus the 3-methyl-2-butenyl structure would be favored
0
in comparison with the 1,1-dimethylallyl. SN2 reaction of the
allyl ligand with thiol or thiolato anion would selectively give
branched product. The allylation reaction within water medium
9
3
may also take place, where an ꢀ -allylpalladium(II) intermediate
3
exists. In this case the less congested carbon of the ꢀ -allyl moi-
ety may selectively react to give mainly less substituted product
3
as expected in allylation via an ꢀ -allylpalladium intermediate.
Komiya, Sanshiro
