2’-Phenyl position, deuterium exchange

Ring protons of 1,2,3-thiadiazoles are known to undergo rapid deuterium exchange under basic conditions. It has been reported that even weak bases such as phenolate can extract the proton at the 5-position of 4-phenyl-l,2,3-thiadiazole <1999J(P1)1473>. 

A quinone methide makes a surprising appearance as an intermediate of the photochemical exchange of deuterium from D20 into the 2 -position of 2-phenylphenol (13).27 29 No photochemical deuterium exchange is observed for reactions of the related phenylanisole 10 or phenylphenols 11 and 12, and deuterium exchange at the 4 -phenyl position of 13 is very much slower than... 

Upon treatment with O-dcuteromethanol/sodium methoxide or O-deutero-ter .-butanol/potassium tert.-butoxide in perdeuterotetrahydrofuran at 50° 4-phenyl-2//-l,3-ditellurole exchanged the hydrogen atom at the 4-position for deuterium one hundred-times faster than the hydrogen atoms in the 2-position. The exchange in the 4-position was complete after 1.5 h, the exchange in the 2- and 4-positions after one week1. 

Supporting evidence for the mechanism comes from the observation that the bromination and iodination proceed at the same rates. The deuterium exchange is also comparable in absolute rate. Very extensive work with the optically active sec-butyl phenyl ketone, C2H5—CH(CH3)COC6H6, has shown that the acid-catalyzed iodination, bromination, and inversion have identical rates. The base-catalyzed, OD, rates of deuteration and inversion have also been shown to be equal. If the enol and enolate ion can be considered to be planar about the a carbon atom, then these results provide very strong support for the slow enolization step. In fact it is difficult to find any other reasonable interpretation of the data. The enol mechanism is also compatible with the well-known susceptibility of H atoms, in the alpha position to one or more C==0 groups, to substitution reactions. 

Proton magnetic resonance techniques have been used for the measurement of rates of hydrogen-deuterium exchange of pyrazine (in CHsOD-CHsONa at 164.6") (591) for a study of protonation of pyrazine (1472) for analysis of the reaction mixture from quatemization of 2-substituted pyrazines with methyl iodide (666) for elucidation or confirmation of the structures of alkylpyrazines obtained by alkylation of pyrazines with aldehydes and ketones in the presence of a solution of an alkali or alkaline earth metal in liquid ammonia, or a suspension of these metals in other solvents (614) for a study of changes in chemical shifts produced on ionization of 2-methyl and 2-amino derivatives of pyrazine in liquid ammonia (665) for characterization of methoxymethylpyrazines (686) for the determination of the position of the A -oxide function in monosubstituted pyrazine V-oxides and the analysis of V-oxidation reactions (838) for a study of the structure of the cations of fV-oxides of monosubstituted pyrazines (1136) and for the determination of the structure of the products of peroxyacetic and peroxysulfuric acid iV-oxidation of phenyl- and chlorophenylpyrazines (733b). 

The recent example of talaromycins A and B (Fig. 2.40) produced by the tox-icogenic fungus Talaromyces stipitatus illustrate the strategic utilization of sophisticated spectroscopic techniques in structure determination (239). These two toxins were isolated as a mixture and were extremely difficult to resolve. Due to the loss of water in the positive CI-MS, the molecular ions of these diastereomers were detected only by NCI-MS, with deuterium exchange NCI-MS confirming the presence of two exchangeable protons. The coupling connectivities of the individual diastereomers were mapped with a COSY spectrum (Fig. 2.40). The purified compounds were ultimately separated after formation of their phenyl boronic acid esters (Fig. 2.41). 

General.—Aliphatic seleno-esters RCSeOEt condense with o-phenyl-enediamine, o-aminophenol, or o-aminothiophenol to yield the benzazoles (510 X = NH, O, or S, respectively). Cyclic voltammetry of the salts (511 X, Y = O, S, or Se R = SEt or SeEt) results in electrochemical reduction to dimers. The rates of base-catalysed hydrogen-deuterium exchange in the 2-position of compounds (510 X = 0, S, or Se R = H) have been determined. Successive treatment of 1,8-dibromonaphthalene with butyl-lithium, sulphur, butyl-lithium, and selenium gives naphtho[l,8-Cif]-l,2- selenathiole (512 X = S, Y = Se) the tellurathiole (512 X = S, Y = Te) and the telluraselenole (512 X = Se, Y = Te) were prepared similarly. ... 

Homogeneous catalytic activation of C—H bonds has also been recently summarized.60915 Thus, RuHCl(PPh3)3 catalyzes the exchange of ortho hydrogens on the phenyl groups of triphenylphosphine with deuterium gas. Under similar conditions, TaH5 (dmpe)2 [where dmpe = (CH3)2PCH2CH2P(CH3)2] catalyzes the deuterium substitution in the meta and para positions.609 Ex-... 

In principle, the equilibrations shown in Figure 8 can result in the selective formation of H2, HD or D2, but the reactions are difficult to control. The problem is further exacerbated by hydrogen exchange with suitable ligands. As shown in Equation (8) phenyl groups on eg phosphine) ligands can undergo or//zo-metallation with coordinatively-unsaturated metal sites. The evidence that such pathways operate is the incoiporation of deuterium into the or//zo-positions of the phenyl residue. 

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