Recoverable, Soluble Polymer-Supported Organic Catalysts
305
Scheme 2.
Jacobsen developed a fully organic catalyst for the Strecker reaction
using a thiourea based chiral Brønsted acid that turned out to be extremely chemically active, stereoselective, and with broad application.
The optimization of the catalyst structure was realized through a series
of modifications of the salen-based structure carried out on an insoluble
polystyrene support, and using the principles of combinatorial chemistry for finding out the best aminoacid, diamine, and diamine-amino
acid linker combination. The screening of three successive libraries led
to the identification of the supported thiourea catalyst 10a as the single
best form from which the non-supported counterpart 10b was derived.
At a loading as low as 1 mol%, 10b promoted the hydrocyanation of
N-allyl or -benzyl imines derived from aromatic and aliphatic aldehydes
and of some ketones in very high yield and almost complete stereoselectivity (see Scheme 2). It is interesting to note that the soluble and the
resin-bound catalysts performed equally well. Recovery and recycling
of the supported catalyst was shown to occur without any erosion of
chemical or stereochemical efficiency over ten reaction cycles.
305
Scheme 2.
Jacobsen developed a fully organic catalyst for the Strecker reaction
using a thiourea based chiral Brønsted acid that turned out to be extremely chemically active, stereoselective, and with broad application.
The optimization of the catalyst structure was realized through a series
of modifications of the salen-based structure carried out on an insoluble
polystyrene support, and using the principles of combinatorial chemistry for finding out the best aminoacid, diamine, and diamine-amino
acid linker combination. The screening of three successive libraries led
to the identification of the supported thiourea catalyst 10a as the single
best form from which the non-supported counterpart 10b was derived.
At a loading as low as 1 mol%, 10b promoted the hydrocyanation of
N-allyl or -benzyl imines derived from aromatic and aliphatic aldehydes
and of some ketones in very high yield and almost complete stereoselectivity (see Scheme 2). It is interesting to note that the soluble and the
resin-bound catalysts performed equally well. Recovery and recycling
of the supported catalyst was shown to occur without any erosion of
chemical or stereochemical efficiency over ten reaction cycles.