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Development of Frustrated Lewis Pair Chemistry

Final Report Summary - FLPCHEM (Development of Frustrated Lewis Pair Chemistry)

Frustrated Lewis pairs (FLPs) feature Lewis acid/base (LA/LB) pairs that are effectively hindered from neutralizing Lewis adduct formation by steric bulk. The coexistence of active LA/LB pairs in solution offers chances of finding and developing novel cooperative reactions. Metal-free dihydrogen activation has been a most prominent case. FLP chemistry has been significantly developed and brought to new issues with this project. This includes the development of new FLP systems, more reactive and more robust in H2-activation and their utilization for finding a variety of novel FLP reactions. A variety of new intramolecular FLPs were obtained by hydroboration pathways using Piers’ borane [HB(C6F5)2]. A planar-chiral ferrocene based P/B FLP was prepared highly enantiomerically enriched and used for asymmetric catalytic imine hydrogenation. Thermally robust intramolecular FLPs were obtained by attaching the P/B pair in 1,2-position at cyclohexadienyl and phenylene frameworks. These were obtained in short synthetic sequences making use of our advanced variant of the 1,1-carboboration reaction. The new very convenient variants of the 1,1-carboboration reaction were also used for generating thermally robust heterocycles (e.g. phosphole systems) having the components of active P/B FLPs attached to them. Very active intramolecular FLPs have been obtained by shutting off the usual residual internal phosphane/borane interaction that is typical for many vicinal FLP systems. This has e.g. been achieved by using suitable organic frameworks, e.g. norbornane, for attaching the FLP Lewis acid and base components in exo- and endo-positions. We have also prepared N/B FLP systems by the hydroboration route. The course of the reaction of a trimethylene-bridged example with triplet dioxygen was examined. A hydrogen splitting product was used for carrying out a B(C6F5)3 Lewis acid catalyzed trans-hydridoborate addition to unfunctionalized terminal alkynes. Many of our P/B FLPs were used in small molecule binding and activation, most prominently of CO, NO, isonitriles, CO2, SO2 and related heterocumulenes. Some vicinal FLPs showed a surprising metal-reminiscent coordination behavior leading to the unique 1,1-adduct formation with CO, NO or isonitriles. We have substantially developed carbon monoxide chemistry. Several reactive vicinal P/B FLPs served as templates making the realization of the untypical CO reduction by BH boranes to the formyl stage possible. This led to new formyl boranes and –borates and their further transformation. Several of our new P/B FLP stabilized 2-formylboranes reacted further with dihydrogen without the aid of a catalyst leading to a further reduction of the CO derived unit to CH2 at the FLP framework. The 2-formylborane moiety could be coupled with a nitric oxide (NO) molecule. The reaction is thought to involve radical reaction steps and the intermediate generation of nitroxyl (HNO). Further NO chemistry has been developed via the unique persistent FLPNO radicals and examples of the utilization of radicals and radical pathways in FLP chemistry have been found. CO2 and SO2 addition to new FLP types have been observed. Examples of stable, thermodynamically favored methylene phosphonium systems, the rare P-analogues of the ubiquitous iminium ions, have been prepared by specific FLP pathways. FLP chemistry has been used for finding novel chemical reaction types. Typical examples are the phospha-Stork and phospha-Claisen reactions found in this project and the beginning of a disclosure of reactions involving CH activation under FLP conditions. We think that the FLPCHEM project in general has made an impact on the currently visible “Renaissance” of main group element chemistry and that it has specifically contributed significantly to the transition of FLP chemistry from a promising emerging field to an established widely visible chemical research area.