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Format:
Online
Author:
Bange, Christine Anne
Dept./Program:
Department of Chemistry
Year:
2018
Degree:
PhD
Abstract:
Catalytic hydrophosphination has enormous potential in the selective preparation of value-added organophosphines, despite the challenge of the reaction. This dissertation aims to address the hurdles in catalytic hydrophosphination with respect to substrate scope, selectivity, and reaction conditions using K⁵-N,N,N,N,C- (Me₃SiNCH₂CH₂)₂NCH₂CH₂NSiMe₂CH₂]Zr (1). Compound 1 readily engages with a suite of primary phosphines. These are challenging substrates for this reaction, but 1 readily provides high conversions with these substrates. Increasingly large primary phosphines, including chiral phosphines, undergo catalysis with 1. Furthermore, a variety of underreported unsaturated substrates can be functionalized in catalytic hydrophosphination with 1. Alkynes are underreported substrates, but 1 showed not only catalytic reactivity with internal alkynes, but also the first example of a double hydrophosphination with these substrates. Almost entirely absent from catalytic hydrophosphination are unactivated alkenes, yet 1 catalyzes them with TON and TOF that now rival those of styrenes. Additionally, a new tandem inter- and intramolecular diene hydrophosphination was reported to give cyclic phosphine products. The selectivity in catalytic hydrophosphination 1 in all processes is novel in many regards. In alkyne hydrophosphination, vinyl phosphines or double hydrophosphination products could be isolated as secondary phosphines, depending on reaction conditions. For alkenes, secondary or tertiary phosphines can be formed by modification of the reaction stoichiometry. Isolated secondary phosphines were further elaborated into chiral tertiary phosphines. Catalytic hydrophosphination with a chiral, air-stable primary phosphine gave chiral secondary phosphine products. Efforts to synthesize a chiral ligand to close the gap on catalysts (and therefore substrates) for asymmetric hydrophosphination are discussed. Catalysis with 1 proceeds under photolysis. Direct irradiation of 1 by ultraviolet or visible light during alkene hydrophosphination substantially enhanced catalytic activity. For example, previous reports of styrene hydrophosphination with 1 showed TON = 18 and TOF = 1.5 h-1. Under irradiation, the process is substantially more efficient (TON = 20 and TOF = 60 h-1) and the substrate scope is expanded. Computational and spectroscopic data indicate that photoexcitation results in a charge transfer in the active catalyst, which appears to accelerate catalysis by promoting substrate insertion based on a linear freeenergy relationship. The impressive substrate scope, mild conditions, and increased catalytic activity from photoexcitation, rather than heat, are among the best reported for the reaction. Identification of a photoexcitation event that promotes substrate insertion may enable enhanced reactivity from other metal catalysts for this transformation.