Mechanistic Studies of the Formation and Reactions of Organometallic Intermediates in the Gas Phase.

Abstract

Organic synthesis focuses on constructing molecules through strategic bond transformations at various types of carbon atoms. The 2010 Nobel Prize recognized palladium-catalyzed cross- coupling reactions, which involve the transmetalation process. While Suzuki reactions are well- studied, fundamental data (e.g., thermochemistry) on transmetalation remain scarce. This thesis brings a mechanistic lens into aspects of the formation and reactions of organometallic intermediates relevant to elementary steps in catalytic cross-coupling reactions, utilizing various mass spectrometry-based approaches and DFT calculations. Gas-phase ion–ion reactions between tris(1,10-phenathroline) transition metal dications and the tetraphenylborate anion, presented in chapter 2, were used for the first time to investigate the formation of ion-pairs in the gas-phase and their role as precursors to transmetalation reactions. Initial formation of outer sphere ion-pair [(phen)3M][BPh4]+ is highly exothermic and the excess energy can fuel loss of a phen ligand to form the inner sphere ion-pair [(phen)2M][BPh4]+, which is a key precursor to transmetalation to form the organometallic cations [(phen)nMPh]+. The gas-phase ion–ion reaction approach was extended, presented in chapter 3, to an investigation into the formation of ion-pairs derived from a f-block metal and their role as precursors to transmetalation reactions. Electron transfer is the dominant pathway and produces the ytterbium dication [Yb(II)(L)3]2+ (6b). One of the simplest cationic copper(II) aryl complexes with a bidentate chelating ligand, presented in chapter 4, [(phen)Cu(Ph)]+, has been prepared in the gas phase through three routes: transmetalation of [(phen)3Cu][BPh4]+, desulfination of [(phen)Cu(O2SPh)]+ and decarboxylation of [(phen)Cu(O2CPh)]+. The bimolecular reactivity of these organocopper(II) cations towards a range of organic substrates was evaluated for their ability to undergo C–H, C–C, C–I, and C–S bond forming reactions. The aim of the work presented chapter 5 in this chapter was to translate the organocopper [(phen)Cu(Ar)]+ observed in the gas-phase to the solution phase using the concept of pseudo gas phase conditions pioneered by Prof. Krossing. Chapter 6 explored the gas-phase decarboxylation of alkali metal complexes in the gas-phase using the fixed charge ligand [4-(CH3)3N(C6H4)CO2] consisting of a benzoate substituted with a cationic quaternary ammonium group at the para-position. CID of the carboxylate complexes [4-(CH3)3N(C6H4)CO2M]+ (M = Li, Na, K, Rb and Cs) produced the mono nuclear organoalkali cations [4-(CH3)3N(C6H4)M]+ in all cases, which is consistent with DFT calculations which reveal that loss of the alkali cation via bond heterolysis is higher in energy than decarboxylation. Additionally, other surprising product cations were observed and these were rationalised as products of a SN2 reaction between the alkali hydroxide (MOH) and the quaternary cation (CH3)3N(C6H5)+ within the exit channel ion-neutral complex via a roaming mechanism. Chapter 7 covers a theoretical study to establish a methide affinity (MA) scale for permethylated cations, neutrals, and anions from elements of both the s and p blocks. The calculated MA values shows that the cations are mostly positive indicating the stability of neutrals towards methide loss. However, the MA values of the anions are all negative, indicating that the dianions thermodynamically unstable with respect to methide loss. Three systems Me4Ca2−, Me4Sr2−, and Me4Ba2− calculated to be kinetically stable and thus might be observable in the gas phase.