We developed a method to elucidate the complete structure of triacylglycerols by means of high-energy collisional activation tandem mass spectrometry (MS/MS). Both ESI- and FAB-produced [M + NH4]+ and [M + met.]+ ions (where met. = Li, Na, and Cs) of triacylglycerols undergo charge-remote and charge-driven fragmentations. We emphasize the study of fragment ions from ESI-produced [M + NH4]+ and [M + Na]+ ions and FAB-produced [M + Na]+ ions. ESI-produced [M + NH4]+ ions fragment to produce four types of ions, [M + NH4 - RnCOONH4]+, [RnCO + 128]+, [RnCO + 74]+, and RnCO+ ions, from which the carbon number and the degree of unsaturation of each acyl group are obtained. In addition, three series of ions are produced by charge-remote fragmentations (CRFs), and analysis of their patterns gives the position and the number of double bonds on the acyl groups. Information about the position of each acyl group on the glycerol backbone, however, is not provided by collisionally activated dissociation of [M + NH4]+ ions. On the other hand, ESI- and FAB-produced [M + Na]+ ions fragment to form eight types of ions (named A-J ions) that, like those produced by CRF, are highly structurally informative. The absence of certain series members also carries useful structural information. Interpretation of these patterns enables one to obtain the number of carbons, degrees of unsaturation, and location of double bonds, as well as the positions of acyl groups on the glycerol backbone.
This study represents the first example an environmentally benign, sustainable, and practical synthesis of substituted quinolines and pyrimidines using combinations of 2-aminobenzyl alcohols and alcohols as well as benzamidine and two different alcohols, respectively. These reactions proceed with high atom efficiency via a sequence of dehydrogenation and condensation steps that give rise to selective C-C and C-N bond formations, thereby releasing 2 equiv of hydrogen and water. A hydride Mn(I) PNP pincer complex recently developed in our laboratory catalyzes this process in a very efficient way. A total of 15 different quinolines and 14 different pyrimidines were synthesized in isolated yields of up to 91 and 90%, respectively.
We
have prepared and structurally characterized a new class of
Fe(II) PNP pincer hydride complexes [Fe(PNP-iPr)(H)(CO)(L)]n (L = Br–, CH3CN, pyridine, PMe3, SCN–, CO, BH4–; n = 0, +1) based on
the 2,6-diaminopyridine scaffold where the PiPr2 moieties of the PNP ligand are connected to the pyridine
ring via NH and/or NMe spacers. Complexes [Fe(PNP-iPr)(H)(CO)(L)]n with labile ligands (L
= Br–, CH3CN, BH4–) and NH spacers are efficient catalysts for the hydrogenation of
both ketones and aldehydes to alcohols under mild conditions, while
those containing inert ligands (L = pyridine, PMe3, SCN–, CO) are catalytically inactive. Interestingly, complex
[Fe(PNPMe-iPr)(H)(CO)(Br)], featuring
NMe spacers, is an efficient catalyst for the chemoselective hydrogenation
of aldehydes. The first type of complexes involves deprotonation of
the PNP ligand as well as heterolytic dihydrogen cleavage via metal-alkoxide
cooperation, but no reversible aromatization/deprotonation of the
PNP ligand. In the case of the N-methylated complex the mechanism
remains unclear, but obviously does not allow bifunctional activation
of dihydrogen. The experimental results complemented by DFT calculations
strongly support an insertion of the C=O bond of the carbonyl
compound into the Fe–H bond.
Herein, we describe an efficient coupling of alcohols and amines catalyzed by well-defined isoelectronic hydride Mn(I) and Fe(II) complexes, which are stabilized by a PNP ligand based on the 2,6-diaminopyridine scaffold. This reaction is an environmentally benign process implementing inexpensive, earth-abundant non-precious metal catalysts, and is based on the acceptorless alcohol dehydrogenation concept. A range of alcohols and amines including both aromatic and aliphatic substrates were efficiently converted in good to excellent isolated yields. Although in the case of Mn selectively imines were obtained, with Fe-exclusively monoalkylated amines were formed. These reactions proceed under base-free conditions and required the addition of molecular sieves.
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