Deuterium-quenching experiments

It was predicted, a priori, that isocomene could be formed simply by treating 4 with methyllithium, followed by exposure of the resultant tertiary carbinol to acid.2b However, many attempts to effect the addition of a variety of nucleophilic methyl derivatives to ketone 4 were unsuccessful. It was revealed by deuterium quenching experiments that ketone 4 undergoes ready enolization in the presence of nucleophilic methyl derivatives. Despite its hindered nature, however, the ketone carbonyl in 4 reacts with methylenetri-... 

In the case of 4-piperidinosulfonylpyridine (469), higher amounts of LDA are detrimental, leading to dimetalation (e.g., I2 or PhCHO quenches led to mixtures of respective 3-substituted and 3,5-disubstituted products which favored the latter at longer metalation times). A 3,5-dilithio intermediate was ruled out by deuterium incorportion experiments. In fact, it appeared that the reaction of LDA and the electrophiles are slower than C-5 deprotonation of the monosubstituted product 470. Scheme 141 summarizes results of LDA metalation experiments on the piperidyl sulfonamide 469 leading to products 470 and 471, which support some of the previous facts (87JOC1133). 

Fluorescence quenching experiments (with diquat dichloride), deuterium kinetic isotope effects, and the effects of triplet state quenchers have suggested a mechanism involving primary electron transfer to singlet excited paraquat [(184) and (185)]. 

At first, 1-chlorocyclopropyl phenyl sulfoxide (93) was synthesized from the olefin 92 in three steps in good yield. Treatment of sulfoxide 93 with 2.5 eq of /-PrMgCl in THF at —78 °C for 5 min followed by quenching with CD3OD afforded the deuteriated chlorocy-clopropane (95) in 78% yield with high deuterium incorporation. From this experiment, it was proved that the intermediate of the reaction is a magnesium cyclopropylidene species 94 (equation 26). 

The IR spectra were recorded with a ReactIRe4000 equipped with a DiComp ATR probe (ASI Applied Systems, Mettler Toledo). The following steps of the experiments have been performed (1) THF was cooled to -75°C (2) the spectral baseline was reset to zero and the spectra recording was started (3) the substrate was added (4) the base was added dropwise and (5) the reaction was quenched by deuterium oxide. 

Figure 10.2 Schematic representation of the experimental workflow used in mass spectrometry-based experiments to determine the pK of histidine residues in proteins. The first step of the protocol involves incubation of the protein in a series of D,0 buffers at different pH values for at least 2 days before the hydrogen exchange reactions are quenched. The protein samples from each hydrogen exchange reaction are then digested with proteolytic enzyme(s), and the proteolytic fragments are analyzed by LC-TSI-MS to determine the deuterium content cjf histidine-ccjntaining peptides...

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