73 research outputs found
Letter: Formation and fragmentation reactions of platinum-terpyridine complexes of glycine and cysteine and their methyl derivatives via electrospray ionization mass spectrometry
Neighboring group versus cis-elimination mechanisms for side chain loss from protonated methionine, methionine sulfoxide and their peptides
A Focus Honoring Helmut Schwarz's Election to the National Academy of Sciences
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Role of the sulfhydryl group on the gas phase fragmentation reactions of protonated cysteine and cysteine containing peptides
AbstractThe gas phase fragmentation reactions of protonated cysteine and cysteine-containing peptides have been studied using a combination of collisional activation in a tandem mass spectrometer and ab initio calculations [at the MP2(FC)/6-31G∗//HF/6-31G∗ level of theory]. There are two major competing dissociation pathways for protonated cysteine involving: (i) loss of ammonia, and (ii) loss of the elements of [CH2O2]. MS/MS, MS/MS of selected ions formed by collisional activation in the electrospray ionization source as well as ab initio calculations have been carried out to determine the mechanisms of these reactions. The ab initio results reveal that the most stable [M + H − NH3]+ isomer is an episulfonium ion (A), whereas the most stable [M + H − CH2O2]+ isomer is an immonium ion (B). The effect of the position of the cysteine residue on the fragmentation reactions of the [M + H]+ ions of all the possible simple dipeptide and tripeptide methyl esters containing one cysteine (where all other residues are glycine) has also been investigated. When cysteine is at the N-terminal position, NH3 loss is observed, although the relative abundance of the resultant [M + H − NH3]+ ion decreases with increasing peptide size. In contrast, when cysteine is at any other position, water loss is observed. The proposed mechanism for loss of H2O is in competition with those channels leading to the formation of structurally relevant sequence ions
Dimethylcuprate-Catalyzed Decarboxylative Coupling of Allyl Acetate
Allylic alkylation of organocuprates represents an important
class of C–C bond forming reactions. A drawback of these reactions
is their stoichiometric nature. Although metal-catalyzed decarboxylative
coupling reactions offer new opportunities for formation of C–C
bonds, catalytic allylic alkylation reactions of organocuprates have
not been previously reported. Here, multistage mass spectrometry experiments
performed on ion trap mass spectrometers in conjunction with electronic
structure calculations are used to demonstrate that the dimethylcuprate
anion, [CH<sub>3</sub>CuCH<sub>3</sub>]<sup>−</sup>, can catalyze
decarboxylative coupling of allyl acetate in the gas phase via a simple
two-step catalytic cycle. In step 1, [CH<sub>3</sub>CuCH<sub>3</sub>]<sup>−</sup> undergoes a cross-coupling reaction with allyl
acetate to yield [CH<sub>3</sub>CuO<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup> as the major product ion. Step 2 involves subjecting
the product ion to collision-induced decarboxylation to re-form [CH<sub>3</sub>CuCH<sub>3</sub>]<sup>−</sup>, thereby closing the
catalytic cycle. Since the details of step 2 have been previously
described (Rijs, N.; Khairallah, G. N.; Waters, T.; O’Hair, R. A. J. J. Am. Chem. Soc. 2008, 130, 1069−1079), here the focus is on step 1. Comparison
of the reactivity of dimethylcuprate toward allylic susbtrates reveals
that while allyl acetate reacts around 70 times more slowly than allyl
iodide, it is more selective for cross-coupling. This is rationalized
by DFT calculations, which reveal that an increase in kinetic barrier
is responsible for both reactivity trends. The lowest energy path
was found to involve a stepwise π-oxidative addition proceeding
via an η<sup>2</sup>-C<sub>3</sub>H<sub>5</sub>O<sub>2</sub>CCH<sub>3</sub> intermediate and extrusion of a leaving group (LG)
anion, followed by a reductive elimination where this LG is recomplexed
to the copper center. Finally, DFT calculations were used to shed
light on the role of leaving groups LG = CH<sub>3</sub>CO<sub>2</sub><sup>–</sup>, I<sup>–</sup>, Br<sup>–</sup>,
Cl<sup>–</sup> in the allylic alkylation. While selectivity
is indeed achieved at the cost of reactivity, the LG effects were
more complex than can be accounted for by a simple consideration of
the anion proton affinity (APA) of the LG
Mechanism of Deoxygenation and Cracking of Fatty Acids by Gas-Phase Cationic Complexes of Ni, Pd, and Pt
Deoxygenation and subsequent cracking of fatty acids are key steps in production of biodiesel fuels from renewable plant sources. Despite the fact that multiple catalysts, including those containing group 10 metals (Ni, Pd, and Pt), are employed for these purposes, little is known about the mechanisms by which they operate. In this work, we utilized tandem mass spectrometry experiments (MSn) to show that multiple types of fatty acids (saturated, mono-, and poly-unsaturated) can be catalytically deoxygenated and converted to smaller hydrocarbons using the ternary metal complexes [(phen)M(O2CR)]+], where phen = 1,10-phenanthroline and M = Ni, Pd, and Pt. The mechanistic description of deoxygenation/cracking processes builds on our recent works describing simple model systems for deoxygenation and cracking, where the latter comes from the ability of group 10 metal ions to undergo chain-walking with very low activation barriers. This article extends our previous work to a number of fatty acids commonly found in renewable plant sources. We found that in many unsaturated acids cracking can occur prior to deoxygenation and show that mechanisms involving group 10 metals differ from long-known charge-remote fragmentation reactions
Dimethylcuprate-Mediated Transformation of Acetate to Dithioacetate
Dithiocarboxylic
acids, RCS<sub>2</sub>H, and their esters, RCS<sub>2</sub>R′,
are useful reagents that can be synthesized by
the reaction of carbon disulfide with organometallic reagents. Here
the coinage-metal-mediated transformation of acetate to dithioacetate
is explored in the gas phase using multistage mass spectrometry experiments
in a linear ion trap mass spectrometer in conjunction with density
functional theory (DFT) calculations. The ion–molecule reactions
between coinage-metal dimethylmetalate anions [CH<sub>3</sub>MCH<sub>3</sub>]<sup>−</sup> (M = Au, Ag, Cu), formed via double decarboxylation
of the metal acetate anions, [CH<sub>3</sub>CO<sub>2</sub>MO<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup>, and carbon disulfide, were examined.
Only [CH<sub>3</sub>CuCH<sub>3</sub>]<sup>−</sup> reacts with
CS<sub>2</sub> with a reaction efficiency of 0.8% of the collision
rate to yield the adduct [CH<sub>3</sub>CuS<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup> (77.5%) as well as CH<sub>3</sub>CS<sub>2</sub><sup>–</sup> (22.5%). Collision-induced dissociation (CID)
of the adduct [CH<sub>3</sub>CuS<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup> gives CH<sub>3</sub>CS<sub>2</sub><sup>–</sup> as the major
product, with a small amount of [CuS<sub>2</sub>CCH<sub>2</sub>]<sup>−</sup> being formed via loss of methane. DFT calculations
reveal the following. (i) [CH<sub>3</sub>CuCH<sub>3</sub>]<sup>−</sup> reacts via oxidative addition to form a CuÂ(III) intermediate, followed
by reductive elimination of CH<sub>3</sub>CS<sub>2</sub><sup>–</sup>, which is captured by Cu to form [CH<sub>3</sub>CuS<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup>. This energetic adduct can fragment via
loss of CH<sub>3</sub>CS<sub>2</sub><sup>–</sup> or can be
collisionally cooled by the helium bath gas used in the experiments.
(ii) Loss of CH<sub>4</sub> from [CH<sub>3</sub>CuS<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup> also involves a CuÂ(III) intermediate
and results in formation of the metalladithiolactone [CuÂ(CH<sub>2</sub>CS<sub>2</sub>)]<sup>−</sup>
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