Highly Efficient “Inverse” Frustrated Lewis Pair Catalysts for Hydrogenation Reactions

Frustrated Lewis Pairs (FLPs) are sterically encumbered Lewis acid and Lewis base that do not form a classical Lewis acid-base adduct due to unfavorable repulsive interactions (frustration). The unquenched reactivity of FLP was successfully utilized to activate the small molecules such as H2, CO, CO2 and N2O. FLPs are first “metal-free” systems that were utilized to activate the strong H-H bond. The concept of FLPs pioneered by Douglas Stephan, represents an innovative concept in sustainable chemistry. It has a potential of replacing the expensive, less abundant and highly toxic metals in their classical domain, the catalytic hydrogenation of unsaturated organic molecules. FLPs have been successfully utilized as hydrogenation catalysts for unsaturated organic molecules such as imines, carbonyl compounds, alkenes and alkynes. However, the Lewis acid component used in the FLP chemistry was mostly limited to highly acidic and highly fluorinated B(C6F5)3 or RB(C6F5)2 related systems. Also, the use of strong Lewis acids limits the substrate scope of donor containing imines and carbonyl compounds as it binds with substrate irreversibly. So, the use of weak Lewis acids could be advantageous in FLP mediated hydrogen cleavage and catalytic hydrogenation of unsaturated organic molecules as they have lesser binding affinities toward donor atoms than strong Lewis acids. In this dissertation, we have utilized the moderate to weak Lewis acids such as BPh3, 9-BBN, 9-aryl-BBN, 9-alkyl-BBN and BEt3 for hydrogen cleavage in combination with Verkade’s superbase [N{CH2CH2N(iPr)}3P] and phosphazene base [(pyrr)3P=NtBu] via “Inverse” FLP approach. Also, the combinations of Phosphazene base [(pyrr)3P=NtBu] and various 9-aryl-BBNs have been used as highly efficient catalysts for hydrogenation of imines and carbonyl compounds. The “Inverse” FLP approach has advantages over the existing FLP catalysts as they are more tolerant to donor atoms in the substrates. Also, the binuclear Lewis acid-base complex of Verkade’s base/[Al(C6F5)3]2 has been used as an efficient catalyst for dimerization of terminal alkynes to gem-enynes selectively with no E- or Z-isomers.