Catalytic Stereoselective 1,3-Enyne Carboboration, Hydroalkynylation, and Hydrothiolation Reactions

Abstract

Chapter 1. Senphos–Palladium-Catalyzed cis-Carboboration of Internal 1,3-Enynes with Carbon–Bound Boron Enolates: Reaction Development and Mechanistic Analysis. A new family of carbon-bound boron enolates (C–boron enolates) that are created through a kinetically controlled halogen exchange process between B–chlorocatecholborane and silylketene acetals is presented. These C–boron enolates are demonstrated to activate 1,3-enynes substrates in the presence of a Senphos-Pd complex to achieve carboboration reaction of an alkyne unit. This carboboration reaction produced highly substituted dienyl boron building blocks in high site-, regio-, and diastereoselectivity. A combined experimental and computational study of this carboboration reaction by Density-Functional Theory (DFT) calculations, 31P NMR study, kinetic study, Hammett analysis and Arrhenius/Eyring analysis will also be described. Mechanistic study supports a syn outer-sphere oxidative addition mechanism featuring a Pd-π-allyl intermediate followed by coordination-assisted rearrangement instead of the conventional inner-sphere β-migratory insertion mechanism. Chapter 2. trans-Hydroalkynylation of Internal 1,3-Enynes Enabled by Cooperative Catalysis. A trans-hydroalkynylation reaction of internal 1,3-enynes enabled by a cooperative catalysis system that comprises of Senphos–Pd complex, tris(pentafluorophenyl)borane, copper(I) bromide, and 2,2,6,6-tetramethylpiperidine, is described. The tris(pentafluorophenyl)borane as Lewis acid catalyst is shown to promote the reaction involving the emerging outer-sphere oxidative reaction step. This hydroalkynylation reaction affords the cross-conjugated dieneynes that serve as versatile synthons for organic synthesis. The photophysical properties of these cross-conjugated dieneynes depend on the position of electron donor/acceptor substituents along the conjugation path, as characterized by UV–vis absorption and emissions spectroscopy. Chapter 3. Senphos–Palladium/B(C6F5)3-Catalyzed trans-Hydrothiolation of 1,3-Enynes: Reaction Development and Mechanistic Analysis. A trans-hydrothiolation reaction of 1,3-enynes enabled by a cooperative catalysis system that comprises of Senphos–Pd complex and tris(pentafluorophenyl)borane is detailed. The tris(pentafluorophenyl)borane is shown to alter the reaction pathway, leading to a trans-addition product over cis-addition one. Experimental mechanistic study that includes 31P NMR, kinetic study, kinetic isotope effect (KIE) study, Hammett analysis, is consistent with a cooperative activation mechanism that features an outer-sphere protonation step.