ORGANOMETALLIC GAS-PHASE ION CHEMISTRY AND CATALYSIS: INSIGHTS INTO THE USE OF METAL CATALYSTS TO PROMOTE SELECTIVITY IN THE REACTIONS OF CARBOXYLIC ACIDS AND THEIR DERIVATIVES

Carboxylic acids are valuable organic substrates as they are widely available, easy to handle, and exhibit structural and functional variety. While they are used in many standard synthetic protocols, over the past two decades numerous studies have explored new modes of metal-mediated reactivity of carboxylic acids and their derivatives. Mass spectrometry-based studies can provide fundamental mechanistic insights into these new modes of reactivity. Here gas-phase models for the following catalytic transformations of carboxylic acids and their derivatives are reviewed: protodecarboxylation; dehydration; decarbonylation; reaction as coordinated bases in C-H bond activation; remote functionalization and decarboxylative C-C bond coupling. In each case the catalytic problem is defined, insights from gas-phase studies are highlighted, comparisons with condensed-phase systems are made and perspectives are reached. Finally, the potential role for mechanistic studies that integrate both gas- and condensed-phase studies is highlighted by recent studies on the discovery of new catalysts for the selective decomposition of formic acid and the invention of the new extrusion-insertion class of reactions for the synthesis of amides, thioamides, and amidines. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

A phosphine-mediated domino sequence of salicylaldehyde with but-3-yn-2-one: rapid access to chromanone

Chromanone is a privileged structure with a wide range of unique biological activities.

1,5-Phosphonium betaines from N-triflylpropiolamides, triphenylphosphane, and active methylene compounds

N-Phenyl-N-(trifluoromethylsulfonyl)propiolamides react with triphenylphosphane in the presence of various active methylene compounds CH₂XY in a 1:1:1 molar ratio to furnish 1-phosphonium-5-oxabetaines, Ph₃P⁺-C(R)=CH-C(O^-)=CXY. These betaines are formed preferentially, but not exclusively, as E-diastereoisomers with respect to the vinylic double bond. In some cases, separation of the two diastereoisomers was achieved by fractionating crystallization. Structure determination by X-ray diffraction analysis revealed marked conformational differences around the CH-C(O^-) single bond of E and Z-isomers and extended charge delocalization in the anionic part.

Phosphine-Catalyzed α-Umpolung-Aldol Reaction for the Synthesis of Benzo[ b]azapin-3-ones

A novel phosphine-catalyzed intermolecular cyclization between 2-sulfonamidobenzaldehyes and ynones is reported. This methodology serves as a conduit for the construction of benzo[ b]azepin-3-ones in good to excellent yields under mild conditions. The resulting 2-benzylidene moieties are formed exclusively in the E-configuration. Mechanistically, this unusual annulation occurs through a phosphine-catalyzed α-umpolung addition, followed by an aldol reaction. One of the benzo[ b]azepin-3-one products was converted to the core structure of 3-amino-[ a]benzazepin-2-one-1-alkanoic acids, many of which function as angiotensin-converting enzyme inhibitors.

Phosphine Organocatalysis

The hallmark of nucleophilic phosphine catalysis is the initial nucleophilic addition of a phosphine to an electrophilic starting material, producing a reactive zwitterionic intermediate, generally under mild conditions. In this Review, we classify nucleophilic phosphine catalysis reactions in terms of their electrophilic components. In the majority of cases, these electrophiles possess carbon-carbon multiple bonds: alkenes (section 2), allenes (section 3), alkynes (section 4), and Morita-Baylis-Hillman (MBH) alcohol derivatives (MBHADs; section 5). Within each of these sections, the reactions are compiled based on the nature of the second starting material-nucleophiles, dinucleophiles, electrophiles, and electrophile-nucleophiles. Nucleophilic phosphine catalysis reactions that occur via the initial addition to starting materials that do not possess carbon-carbon multiple bonds are collated in section 6. Although not catalytic in the phosphine, the formation of ylides through the nucleophilic addition of phosphines to carbon-carbon multiple bond-containing compounds is intimately related to the catalysis and is discussed in section 7. Finally, section 8 compiles miscellaneous topics, including annulations of the Hüisgen zwitterion, phosphine-mediated reductions, iminophosphorane organocatalysis, and catalytic variants of classical phosphine oxide-generating reactions.

Mechanistic features of the copper-free Sonogashira reaction from ESI-MS

The mechanism of the Sonogashira reaction in methanol was studied in detail using pressurized sample infusion electrospray ionization mass spectrometry (PSI-ESI-MS). Several key intermediates were identified and their structures were assigned by MS/MS studies. Cationic and anionic charged-tagged substrates were employed to look into the mechanism of this reaction from variety of angles. A reverse kinetic isotope effect was observed in which the reaction rate is accelerated in deuterated solvents (kH/kD = 0.6). The reaction was found to be zero order with respect to the aryl iodide and first order with respect to the phenylacetylene. A Hammett parameter of ρ = 1.4 indicates that the reaction is more favorable for aryl iodides with para EWGs. No evidence of product inhibition, dimerization of palladium catalyst, or agglomeration were observed. However, catalyst decomposition was inferred from a non-zero intercept in the plot of catalyst loading versus reaction rate. Monitoring the reaction by PSI-ESI-(-)MS on neutral and negatively charged substrates at variety of concentrations and conditions did not reveal any detectable anionic palladium complexes. Likewise no evidence of carbopalladation and relevant intermediates in the absence of a base was observed.