Synthesis and Reactions N-Heterocyclic Carbene BoranesBolt, Daniel (2019) Synthesis and Reactions N-Heterocyclic Carbene Boranes. Doctoral Dissertation, University of Pittsburgh. (Unpublished)
AbstractThe development of a new method of synthesizing a liquid N-heterocyclic carbene (NHC) borane reagent is described, and a survey of reactivity is completed to demonstrate its effectiveness as a reagent in organic synthesis. In addition, new radical and ionic reactions to prepare novel substituted NHC-boranes are studied. Chapter 1 describes the preparation of 1-butyl-3-methylimidazol-2-ylidene borane (BuMe-Imd-BH3), a liquid NHC-borane from readily-available materials. The facile synthesis of this reagent is accomplished via a solvent-free reaction of a commercially-available ionic liquid precursor with sodium borohydride, followed by filtration through a silica plug. A survey of radical and ionic reactions showed that the BuMe-Imd-BH3 reagent is a capable reagent in organic synthesis as a radical hydrogen donor and as a hydride donating reducing agent. We also demonstrate that BuMe-Imd-BH3 can participate in rhodium-catalyzed B–H insertion reactions and can be used to access Suzuki-Miyaura chemistry. Chapter 2 describes the preparation of NHC-boryl oxalates, which are themselves ligated forms of 1,3,2-dioxaborolane-4,5-dione, an unreported heterocycle. NHC-boryl oxalates are formed via direct reaction of NHC-boranes and oxalic acid in acetonitrile. X-ray crystallography confirmed our proposed structure of a cyclic oxalate species bound to boron. We showed that other organic diacids were unable to produce isolable products upon reaction with NHC-boranes, with the exception of phthalic acid, which produced an NHC-boryl phthalate product. The NHC-boryl oxalates can participate in electrophilic fluorination reactions with Selectfluor and rhodium-catalyzed B–H insertion reactions, producing novel substituted NHC-boryl oxalate products. In Chapter 3, the reaction of NHC-boranes with dibenzoyl peroxide (DBP) and radical initiators is studied. NHC-boranes and benzoyl peroxide are generally not mutually stable, and the addition of DBP to an NHC-borane causes ionic benzoyloxylation, forming NHC-boryl benzoate products. We found that the inclusion of a radical initiator such as azobisisobutyronitrile (AIBN) or di-tert-butyl hyponitrite (TBHN) increases the rate of the benzoyloxylation reaction, and the use of TBHN results in di-benzoyloxylated NHC-borane products. After examining possible mechanisms for this transformation, we hypothesized that NHC-boranes can react with benzoic acid when a radical initiator is included. Indeed, reactions of NHC-boranes with carboxylic acids and radical initiators gave NHC-boryl carboxylate products. Share
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