While hundreds of papers have been published on frustrated Lewis pairs (FLPs) – the combination of bulky Lewis acids and bases which cannot form adducts – few of these use aluminum-based Lewis acids. The research outlined in this thesis expands the small molecule activation chemistry of FLPs to include Al.Combinations of bulky phosphines and AlX3 (X = halide or C6F5) with CO2 leads to the rapid activation to form the complexes R3P(CO2)(AlX3)2 (R = otol, Mes). Subsequent treatment with ammonia borane (AB) results in the rapid reduction of the CO2 fragment to methanol after water quench. Subsequent reactivity studies have established that AB adducts of AlX3, which react with CO2, are key intermediates in this chemistry.
Further studies with Mes3P(CO2)(AlX3)2 revealed that these can reduce exogenous CO2 to CO, along with the generation of Mes3P(C(OAlX2)2O)(AlX3) and [Mes3PX][AlX4]. Detailed experimental and theoretical mechanistic investigations outline a possible mechanism involving direct CO2 insertion into free AlX3, followed by nucleophilic attack by PMes3 resulting in the expulsion of CO.
Reactions with olefins were also investigated. While addition products of the type R3P(CH2CH2)AlX3 could be obtained with ethylene, C–H bond activation occurred with bulkier olefins. The resulting allyl species underwent subsequent C–C bond forming reactions with other olefins or CO2.
Hydrogen was also activated using PR3/AlX3 FLPs to form species of the general formula, [R3PH][(H)(AlX3)2] (X = I, C6F5). These were found to reduce unactivated olefins, generating the redistributed products [R3PH][AlX4] and RAlX2 (R = alkyl). Attempts to circumvent this redistribution and favour alkyl protonation, thus generating a catalytic hydrogenation catalyst, are also discussed.
Finally, the activation of N2O was also examined. While addition products could be formed, unexpected aromatic or benzylic C–H bond activation chemistry occurred in the presence of excess Al. A radical reaction pathway is proposed
| Identifer | oai:union.ndltd.org:TORONTO/oai:tspace.library.utoronto.ca:1807/35906 |
| Date | 09 August 2013 |
| Creators | Menard, Gabriel |
| Contributors | Stephan, Douglas |
| Source Sets | University of Toronto |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
Page generated in 0.0575 seconds