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Colorimetric and Fluorescent Screening Hydrosilation of Alkenes and Imines

Crabtree and coworkers used the disappearance of color to monitor the activity of 12 potential catalysts for hydrosilation reactions [6]. The group used alkenes and imines with ferrocenyl and pyrimidyl substituents at either end as colored re-actants. On hydrosilation the color is bleached, because conjugation between the electron donor and acceptor groups is reduced. The bleaching is easily observed by eye and can be recorded quantitatively by means of a digital camera (Figure 5.4.1). By use of this method, the known Wilkinson catalyst was identified in a proof-of-concept experiment. A palladacycle, [Pd{(o-tolyl)2PC6H4}OAc]2, usually used in Heck reactions, was also found to be catalytically active.

Fig. 5.4.1. Bleaching of reactive dyes.

This method might suffer from lack in generality, because a reactive dye was used as a substrate. Control reactions proved that the relative order of reactivity of the identified hydrosilation catalysts is, however, identical when conventional un-saturated systems instead of the reactive dyes are employed.

Heck Reactions

Fig. 5.4.1. Bleaching of reactive dyes.

2a X = CHSiHPh2 (light yellow) 2b X = NHSiHPh2 (light yellow)

The formation of a fluorescent product anchored to a solid support was used by Hartwig's group to monitor the capacity of 45 different phosphine and diphos-

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