relation: http://d-scholarship-dev.library.pitt.edu/33821/
title: New Reactions with N-Heterocyclic Carbene Boranes and Amidine Boranes, and the Study of Initiators in the Radical Hydrostannation of Propargyl Silyl Ethers
creator: McFadden, Timothy R.
description: The development of new reactions of N-heterocyclic carbene (NHC) boranes and amidine-boranes is described.  Additionally, the mechanism of the AIBN- and triethylborane-initiated radical hydrostannation of propargyl silyl ethers is studied.   Chapter 1 describes the reaction of NHC-boranes with dimethyl acetylenedicarboxylate (DMAD) to form alkenylboranes and boriranes. These products arise through formal single and double trans-selective hydroborations. The reaction conditions were optimized and applied to produce a small library of NHC-borane derived products. Alkenylboranes were formed as the major product from the reaction of N,N-dialkyl NHC-boranes with DMAD, while boriranes were the major product formed from the reactions of N,N-diaryl NHC-boranes. Scope and limitation studies were performed to show that borirane-formation is specific to the reaction of NHC-boranes and borohydrides with DMAD. Boriranes were not formed from the reaction of NHC-boranes with any other alkynes or from the reaction of amine-borane complexes with DMAD.  Control experiments showed that the products could not be interconverted. The mechanism was probed by deuterium-labeling experiments.   Chapter 2 describes the synthesis and characterization of amidine-boranes.  These ligated borane complexes can be prepared in good yield from the reaction of  heterocyclic amidines, such as 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) or 1,2-dimethylimidazole, with borane. These complexes are air- and water-stable solids at room temperature and do not decomplex in solution at elevated temperature. The amidine-boranes are demonstrated to be reactive towards acids and halogens, in addition to being competent aldehyde, ketone, and imine reducing agents. Compared to other ligated boranes, DBU-borane is a more reactive hydride donor.   In Chapter 3, the azobisisobutyronitrile (AIBN)- and triethylborane (Et3B)/oxygen (O2)-initiated hydrostannation of propargyl silyl ethers to form alkenylstannanes is studied extensively. When Et3B/O2 is used to initiate the reaction, the resulting alkenylstannane is formed with high Z-selectivity, while little selectivity is observed when the reaction is initiated by AIBN. Recent publications asserted that the difference in selectivity is derived from differing reaction mechanisms of the two initiators. The role of initiator, temperature, and reaction time are probed to demonstrate that both AIBN and Et3B/O2 initiated the reaction by the same mechanism, but at differing efficiencies, leading to different product ratios.
date: 2018-06-28
type: University of Pittsburgh ETD
type: PeerReviewed
format: application/pdf
language: en
identifier: http://d-scholarship-dev.library.pitt.edu/33821/1/trmcfadden_etd_2018_1.pdf
identifier:   McFadden, Timothy R.  (2018) New Reactions with N-Heterocyclic Carbene Boranes and Amidine Boranes, and the Study of Initiators in the Radical Hydrostannation of Propargyl Silyl Ethers.  Doctoral Dissertation, University of Pittsburgh.    (Unpublished)