Chemistry Reference
In-Depth Information
Other well-known but more recent examples of encapsulated metal catalysts
are the palladium-containing polyurea microcapsules of Steven Ley's group at
Cambridge (commercialized as PdEnCat
TM
; Bremeyer et al. 2002; Ley et al. 2002,
2003; Ramarao et al. 2002; Yu et al. 2003; Lee et al. 2005; Baxendale et al.
2006). The preparation was based on the interfacial polymerization of isocyanates
by Herbert Scher discussed earlier (Scher 1981; Scher et al. 1998). To make the
capsules, palladium acetate and PMPPI were dissolved in dichloroethane, which
was then dispersed in an aqueous continuous-phase surfactant cocktail. Simple
stirring at 800 rpm generated the emulsion, yielding capsules ranging from 50 to
250 mm (Ramarao et al. 2002). The resulting Pd(II)-embedded microcapsules were
active in various palladium-catalyzed cross-coupling reactions, such as carbonyla-
tions and the Heck, Suzuki, and Stille reactions (Ley et al. 2002; Ramarao et al.
2002). With the capsules packed into a high-performance liquid chromatography
column, the catalyst also carried out the Suzuki reaction in continuous flow,
in organic solvent and supercritical CO
2
(Lee et al. 2005), and with microwave
assistance (Baxendale et al. 2006).
The palladium(II) in the microcapsules can also be reduced directly to Pd(0).
Typical of metal nanoparticle catalysis, the researchers found that the type of redu-
cing agent affected the size of the nanoparticles and thus controlled the catalytic
activity (Narayanan and El-Sayed 2003). Therefore, using hydrogen gas to reduce
the palladium, the resulting nanoparticles were greater than 5 nm in size and inactive.
Using formic acid as the reducing agent, however, gave particles of 2 nm or less that
were quite active. The Pd(0)-containing microcapsules were then found to catalyze
the H
2
hydrogenation of alkenes, alkynes, imines, and nitro groups (Bremeyer
et al. 2002) and the transfer hydrogenation of aryl ketones and benzyl alcohols
(Yu et al. 2003). Finally, the microcapsules catalyzed the ring-opening hydrogenoly-
sis of benzylic epoxides (Ley et al. 2003).
Catalyst leaching from the capsules appears to occur, and it is quite solvent depen-
dent. The capsules are catalyst reservoirs rather than truly site isolating the catalyst
(Broadwater and McQuade 2006). The catalyst is recyclable (20 times for hydrogen-
ation in cyclohexane), however, and capsules are easily isolated for reuse by simple
filtration through a 20-mm frit (Bremeyer et al. 2002).
Organocatalysts.
Sometimes it is desired that an encapsulated catalyst never leave
the confines of the capsule because other reagents or catalysts in the reaction mixture
would then interfere with it, or vice versa. In this case, the organocatalyst must be
attached to a much larger molecule that cannot diffuse out of the shell or to the
inside of the shell itself.
Our research group has developed a multicatalyst system around an encapsulated
amine catalyst (Kobaˇlija and McQuade 2006) that promotes nitroalkene synthesis.
Like the Royer catalysts, these microcapsules are based on a PEI shell. Unlike that
system, however, templating is accomplished with a methanol in cyclohexane
(“oil-in-oil”) emulsion in an interfacial polymerization, with the PEI cross-linked
